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1.
Lizhu Zhang Cunbo Wei Jiawen Wu Dan Liu Yinchao Yao Zhuo Chen Jianxun Liu Chang-Jiang Yao Dinghua Li Rongjie Yang Zhonghua Xia 《Chemical science》2022,13(25):7475
Alkynes are widely used in chemistry, medicine and materials science. Here we demonstrate a transition-metal and photocatalyst-free inverse Sonogashira coupling reaction between iodoalkynes and (hetero)arenes or alkenes under visible-light irradiation. Mechanistic and computational studies suggest that iodoalkynes can be directly activated by visible light irradiation, and an excited state iodoalkyne acted as an “alkynyl radical synthetic equivalent”, reacting with a series of C(sp2)–H bonds for coupling products. This work should open new windows in radical chemistry and alkynylation method.A transition-metal and photocatalyst-free, photoinduced inverse Sonogashira coupling reaction was developed. Under visible-light irradiation, the excited state iodoalkyne acted as an “alkynyl radical synthetic equivalent”.Alkynes are among the most important class of compounds in organic chemistry. Because of their structural rigidity, special electronic properties and numerous methods available for the functionalization of the triple bond, alkynes are important tools and structural elements both in medicinal chemistry and materials sciences.1 Therefore, the development of a new methodology to introduce carbon–carbon triple bonds is of great importance in organic chemistry. The Sonogashira coupling reaction is typically used for the formation of C(sp)–C(sp2) bonds starting from hetero(aryl) halides and terminal alkynes.2 Recently, “inverse Sonogashira coupling” involving the direct alkynylation of unreactive C(sp2)–H bonds with readily available alkynyl halides has received growing interest in the development of a complementary strategy (Fig. 1a). Various main-group and transition metals have been developed to promote this transformation.3 In addition, a photomediated Sonogashira reaction without a photocatalyst was also developed by several groups (Fig. 1b).4Open in a separate windowFig. 1Models of alkynylation. (a) Conventional inverse Sonogashira reaction. (b) Photomediated Sonogashira reaction. (c) SOMOphilic alkynylation. (d) Photoinduced inverse Sonogashira reaction.In recent years, SOMOphilic alkylnylation (SOMO = singly occupied molecular orbital) has become an excellent method of introducing alkynyl groups (Fig. 1c).5 Based on photoredox and transition metal catalysis, numerous in situ generated radicals undergo α-addition and β-elimination to alkynyl reagents, like the broadly applicable ethynylbenziodoxolone (EBX) reagent. Various radical alkynylations were thus discovered by Li,6 Chen,7 Waser,8 and many other groups.9 However, extending the scope of radical precursors, more atom–economic reactions, and a deeper understanding of the mechanism in these transformations are still highly desirable.After the discovering of trityl radicals by Gomberg in 1900, the “rational” era of radical chemistry has since begun.10 Now, the development of radical reactions, especially those involving C(sp3) and C(sp2) radicals, enables rapid access to drug discovery, agrochemistry, materials science, and other disciplines.11 However, the C(sp) radical remains a baffling species. Due to their very high energy, short life time, and limited and harsh preparation methods, alkynyl radicals remain an elusive species, which just exists in some extreme environments, like outer-space and the petrochemical industry.12 Even though alkynyl radicals have been proposed as intermediates for some alkynylation methods, they were regarded as mysterious species and ignored by organic chemists for a long time.13 Recently, two approaches have been developed to aid the alkynyl radical generation step. In 2015, Hashmi and collaborators reported a [Au2(μ-dppm)2]2+ catalyzed free radical–radical C(sp)–C(sp3) bond coupling reaction between iodoalkynes and aliphatic amines.14 Under irradiation of sunlight, the dimeric gold complex was proposed to reduce the iodine acetylide to an alkynyl radical. In 2017, Li developed a transition-metal-free alkynylation reaction between iodoalkyne and 2-indolinone.15 Iodoalkynes could release alkynyl radicals under high temperature conditions. In 2019, we reported an Au(i) and Ir(iii) catalyzed alkynylative cyclization of o-alkylnylphenols with iodoalkynes, wherein the photosensitized energy transfer promoted the oxidative addition of a gold(i) complex with iodoalkynes.16 Based on our continuous interest in haloalkyne and photo-chemistry, we proposed that an iodoalkyne could be a potential “alkynyl radical precursor” under light irradiation. In this work, we uncovered a novel mode of transition-metal and photocatalyst-free, direct photoexcitation of iodoalkynes for the inverse Sonogashira coupling reaction with arenes, heteroarenes, and alkenes via an “alkynyl-radical type” transfer (Fig. 1d). 相似文献
2.
Melanie Heghmanns Andreas Rutz Yury Kutin Vera Engelbrecht Martin Winkler Thomas Happe Müge Kasanmascheff 《Chemical science》2022,13(29):8704
Correction for ‘The oxygen-resistant [FeFe]-hydrogenase CbA5H harbors an unknown radical signal’ by Melanie Heghmanns et al., Chem. Sci., 2022, 13, 7289–7294, https://doi.org/10.1039/D2SC00385F.The authors realized that incorrect references were cited following the sentence “In conjunction with the signal''s significant width, the frequency dependence clearly indicates spin–spin interaction between the F-clusters.” The correct references are shown below as ref. 1 and 2.Additionally ref. 36 and 37 were reversed in the reference list. The correct ref. 36 is shown below as ref. 3 and the correct ref. 37 is shown below as ref. 4.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. 相似文献
3.
Brendan J. Graziano Thais R. Scott Matthew V. Vollmer Michael J. Dorantes Victor G. Young Jr Eckhard Bill Laura Gagliardi Connie C. Lu 《Chemical science》2022,13(22):6525
Odd-electron bonds have unique electronic structures and are often encountered as transiently stable, homonuclear species. In this study, a pair of copper complexes supported by Group 13 metalloligands, M[N((o-C6H4)NCH2PiPr2)3] (M = Al or Ga), featuring two-center/one-electron (2c/1e) σ-bonds were synthesized by one-electron reduction of the corresponding Cu(i) ⇢ M(III) counterparts. The copper bimetallic complexes were investigated by X-ray diffraction, cyclic voltammetry, electron paramagnetic spectroscopy, and density functional theory calculations. The combined experimental and theoretical data corroborate that the unpaired spin is delocalized across Cu, M, and ancillary atoms, and the singly occupied molecular orbital (SOMO) corresponds to a σ-(Cu–M) bond involving the Cu 4pz and M ns/npz atomic orbitals. Collectively, the data suggest the covalent nature of these interactions, which represent the first examples of odd-electron σ-bonds for the heavier Group 13 elements Al and Ga.Hanging on by a thread. Formally zerovalent copper complexes with an Al(iii) or Ga(iii) support were investigated. The combined experimental and theoretical data corroborate the presence of an odd-electron σ-bond between Cu and the Group 13 center.Odd-electron σ-bonds, where the electrons are delocalized between two atoms, can occur as two-center/one-electron (2c/1e) or two-center/three-electron (2c/3e) interactions. Proposed by Pauling in 1931,1 odd-electron σ-bonds have garnered attention because of their fundamental importance to chemical bonding and their relationship to radical species generated during oxidative stress in biological systems.2–14 Examples of compounds exhibiting odd-electron bonding are typically homonuclear (like H2+, He2+, and alkali metal dimers) and transiently stable, limiting them to spectroscopic characterization.1,11,15–18The first solid-state structure of a formally one-electron σ-bond was a tetraphosphabenzene species (Fig. 1a) which was formed by the coupling of two diphosphirenyl radicals.19 Following this discovery, the formation of discrete 2c/1e σ-bonds, where the odd-electron is delocalized between two homonuclear main group centers, was reported for B·B and then extended to P·P.8,17,20 Of note, the first solid-state structure of a B·B compound was reported in only 2014 (Fig. 1b).21 Examples of 2c/1e σ-bonds between the heavier Group 13 congeners are even more lacking because of the greater propensity for their unpaired spins to couple, forming larger more stable clusters.8 To our knowledge, there are only three structurally characterized examples of odd-electron bonds for the heavy Group 13 atoms,22 and these examples are all homonuclear π-radicals (Fig. 1c).23–26Open in a separate windowFig. 1Select examples of structurally characterized molecules (a–d) featuring odd-electron bonds.Heteronuclear odd-electron σ-bonds are also rare. The Cu(TPB) complex, where TPB is a trisphosphinoborane, is the single structural example of a 2c/1e bond between heteroatoms (Fig. 1d).27 The authors described the bonding as Cu·B, where the unpaired electron is heavily polarized toward B. A theoretical study predicted that such a bond would also exist between Cu and Al, but no heavier analogues of Cu(TPB) have been synthesized to date.28 Furthermore, the heavier Group 13 elements by virtue of their lower electronegativity compared to B should facilitate greater covalent interactions with the Cu center.Hence, we sought to target formally zerovalent Cu complexes supported by Al(III) or Ga(III) as an extension of the previously reported isoelectronic nickelate species and Cu(TPB).29 Herein, we describe the synthesis, structure, spectroscopic characterization, and DFT calculations of cationic [CuML]+ complexes (L = [N((o-C6H4)NCH2PiPr2)3]3−; M = Al and Ga) as well as their one-electron reduced metalloradical counterparts that feature discrete 2c/1e bonds. 相似文献
4.
Natural systems produce various γ-dicarbonyl-bearing compounds that can covalently modify lysine in protein targets via the classic Paal–Knorr reaction. Among them is a unique class of lipid-derived electrophiles – isoketals that exhibit high chemical reactivity and critical biological functions. However, their target selectivity and profiles in complex proteomes remain unknown. Here we report a Paal–Knorr agent, 4-oxonon-8-ynal (herein termed ONAyne), for surveying the reactivity and selectivity of the γ-dicarbonyl warhead in biological systems. Using an unbiased open-search strategy, we demonstrated the lysine specificity of ONAyne on a proteome-wide scale and characterized six probe-derived modifications, including the initial pyrrole adduct and its oxidative products (i.e., lactam and hydroxylactam adducts), an enlactam adduct from dehydration of hydroxylactam, and two chemotypes formed in the presence of endogenous formaldehyde (i.e., fulvene and aldehyde adducts). Furthermore, combined with quantitative chemoproteomics in a competitive format, ONAyne permitted global, in situ, and site-specific profiling of targeted lysine residues of two specific isomers of isoketals, levuglandin (LG) D2 and E2. The functional analyses reveal that LG-derived adduction drives inhibition of malate dehydrogenase MDH2 and exhibits a crosstalk with two epigenetic marks on histone H2B in macrophages. Our approach should be broadly useful for target profiling of bioactive γ-dicarbonyls in diverse biological contexts.Natural systems produce various γ-dicarbonyl-bearing compounds that can covalently modify lysine in protein targets via the classic Paal–Knorr reaction.Synthetic chemistry methods have been increasingly underscored by their potential to be repurposed as biocompatible methods for both chemical biology and drug discovery. The most-known examples of such a repurposing approach include the Staudinger ligation1 and the Huisgen-based click chemistry.2 Moreover, bioconjugation of cysteine and lysine can be built upon facile chemical processes,3 while chemoselective labelling of other polar residues (e.g., histidine,4 methionine,5 tyrosine,6 aspartic and glutamic acids7,8) requires more elaborate chemistry, thereby offering a powerful means to study the structure and function of proteins, even at a proteome-wide scale.The classical Paal–Knorr reaction has been reported for a single-step pyrrole synthesis in 1884.9,10 The reaction involves the condensation of γ-dicarbonyl with a primary amine under mild conditions (e.g., room temperature, mild acid) to give pyrrole through the intermediary hemiaminals followed by rapid dehydration of highly unstable pyrrolidine adducts (Fig. S1†).Interestingly, we and others have recently demonstrated that the Paal–Knorr reaction can also readily take place in native biological systems.11–13 More importantly, the Paal–Knorr precursor γ-dicarbonyl resides on many endogenous metabolites and bioactive natural products.14 Among them of particular interest are isoketals15 (IsoKs, also known as γ-ketoaldehydes) which are a unique class of lipid derived electrophiles (LDEs) formed from lipid peroxidation (Fig. S2†)16 that has emerged as an important mechanism for cells to regulate redox signalling and inflammatory responses,17 and drive ferroptosis,18 and this field has exponentially grown over the past few years. It has been well documented that the γ-dicarbonyl group of IsoKs can rapidly and predominantly react with lysine via the Paal–Knorr reaction to form a pyrrole adduct in vitro (Fig. 1).15 Further, the pyrrole formed by IsoKs can be easily oxidized to yield lactam and hydroxylactam products in the presence of molecular oxygen (Fig. 1). These rapid reactions are essentially irreversible. Hence, IsoKs react with protein approximately two orders of magnitude faster than the most-studied LDE 4-hydoxynonenal (4-HNE) that contains α,β-unsaturated carbonyl to generally adduct protein cysteines by Michael addition (Fig. S3†).15 Due to this unique adduction chemistry and rapid reactivity, IsoKs exhibit intriguing biological activities, including inhibition of the nucleosome complex formation,19 high-density lipoprotein function,20 mitochondrial respiration and calcium homeostasis,21 as well as activation of hepatic stellate cells.22 Furthermore, increases in IsoK-protein adducts have been identified in many major diseases,23 such as atherosclerosis, Alzheimer''s disease, hypertension and so on.Open in a separate windowFig. 1The Paal–Knorr precursor γ-dicarbonyl reacts with the lysine residue on proteins to form diverse chemotypes via two pathways. The red arrow shows the oxidation pathway, while the blue one shows the formaldehyde pathway.Despite the chemical uniqueness, biological significance, and pathophysiological relevance of IsoKs, their residue selectivity and target profiles in complex proteomes remain unknown, hampering the studies of their mechanisms of action (MoAs). Pioneered by the Cravatt group, the competitive ABPP (activity-based protein profiling) has been the method of choice to analyse the molecular interactions between electrophiles (e.g., LDEs,24 oncometabolites,25 natural products,26,27 covalent ligands and drugs28–30) and nucleophilic amino acids across complex proteomes. In this regard, many residue-specific chemistry methods and probes have been developed for such studies. For example, several lysine-specific probes based on the activated ester warheads (e.g., sulfotetrafluorophenyl, STP;31N-hydroxysuccinimide, NHS32) have recently been developed to analyse electrophile–lysine interactions at a proteome-wide scale in human tumour cells, which provides rich resources of ligandable sites for covalent probes and potential therapeutics. Although these approaches can also be presumably leveraged to globally and site-specifically profile lysine-specific targets IsoKs, the reaction kinetics and target preference of activated ester-based probes likely differ from those of γ-dicarbonyls, possibly resulting in misinterpretation of ABPP competition results. Ideally, a lysine profiling probe used for a competitive ABPP analysis of IsoKs should therefore possess the same, or at least a similar, warhead moiety. Furthermore, due to the lack of reactive carbonyl groups on IsoK-derived protein adducts, several recently developed carbonyl-directed ligation probes for studying LDE-adductions are also not suitable for target profiling of IsoKs.33,34Towards this end, we sought to design a “clickable” γ-dicarbonyl probe for profiling lysine residues and, in combination with the competitive ABPP strategy, for analysing IsoK adductions in native proteomes. Considering that the diversity of various regio- and stereo- IsoK isomers15 (a total of 64, Fig. S2†) in chemical reactivity and bioactivities is likely attributed to the substitution of γ-dicarbonyls at positions 2 and 3, the “clickable” alkyne handle needs to be rationally implemented onto the 4-methyl group in order to minimize the biases when competing with IsoKs in target engagement. Interestingly, we reasoned that 4-oxonon-8-ynal, a previously reported Paal–Knorr agent used as an intermediate for synthesizing fatty acid probes35 or oxa-tricyclic compounds,36 could be repurposed for the γ-dicarbonyl-directed ABPP application. With this chemical in hand (herein termed ONAyne, Fig. 2A), we first used western blotting to detect its utility in labelling proteins, allowing visualization of a dose-dependent labelling of the proteome in situ (Fig. S4†). Next, we set up to incorporate this probe into a well-established chemoproteomic workflow for site-specific lysine profiling in situ (Fig. 2A). Specifically, intact cells were labelled with ONAyne in situ (200 μM, 2 h, 37 °C, a condition showing little cytotoxicity, Fig. S5†), and the probe-labelled proteome was harvested and processed into tryptic peptides. The resulting probe-labelled peptides were conjugated with both light and heavy azido-UV-cleavable-biotin reagents (1 : 1) via CuI-catalyzed azide–alkyne cycloaddition reaction (CuAAC, also known as click chemistry). The biotinylated peptides were enriched with streptavidin beads and photoreleased for LC-MS/MS-based proteomics. The ONAyne-labelled peptides covalently conjugated with light and heavy tags would yield an isotopic signature. We considered only those modified peptide assignments whose MS1 data reflected a light/heavy ratio close to 1.0, thereby increasing the accuracy of these peptide identifications. Using this criterium, we applied a targeted database search to profile three expected probe-derived modifications (PDMs), including 13 pyrrole peptide adducts (Δ273.15), 77 lactam peptide adducts (Δ289.14), and 557 hydroxylactam peptide adducts (Δ305.14), comprising 585 lysine residues on 299 proteins (Fig. S6 and S7†). Among them, the hydroxylactam adducts were present predominately, since the pyrrole formed by this probe, the same as IsoKs, can be easily oxidized when being exposed to O2. This finding was in accordance with a previous report where the pyrrole adducts formed by the reaction between IsoK and free lysine could not be detected, but rather their oxidized forms.37 Regardless, all three types of adducts were found in one lysine site of EF1A1 (K387, Fig. S8†), further confirming the intrinsic relationship among those adductions in situ.Open in a separate windowFig. 2Adduct profile and proteome-wide selectivity of the γ-dicarbonyl probe ONAyne. (A) Chemical structure of ONAyne and schematic workflow for identifying ONAyne-adducted sites across the proteome. (B) Bar chart showing the distribution of six types of ONAyne-derived modifications formed in situ and in vitro (note: before probe labelling, small molecules in cell lysates were filtered out through desalting columns).State-of-the-art blind search can offer an opportunity to explore unexpected chemotypes (i.e., modifications) derived from a chemical probe and to unbiasedly assess its proteome-wide residue selectivity.38,39 We therefore sought to use one of such tools termed pChem38 to re-analyse the MS data (see Methods, ESI†). Surprisingly, the pChem search identified three new and abundant PDMs (Fig. 1 and Table S1†), which dramatically expand the ONAyne-profiled lysinome (2305 sites versus 585 sites). Overall, these newly identified PDMs accounted for 74.6% of all identifications (Fig. 2B and Table S2†). Among them, the PDM of Δ287.13 (Fig. 1 and S7†) might be an enlactam product via dehydration of the probe-derived hydroxylactam adduct. The other two might be explained by the plausible mechanism as follows (Fig. 1). The endogenous formaldehyde (FA, produced in substantial quantities in biological systems) reacts with the probe-derived pyrrole adduct via nucleophilic addition to form a carbinol intermediate, followed by rapid dehydration to a fulvene (Δ285.15, Fig. S7†) and immediate oxidation to an aldehyde (Δ301.14, Fig. S7†). In line with this mechanism, the amount of FA-derived PDMs was largely eliminated when the in vitro ONAyne labelling was performed in the FA-less cell lysates (Fig. 2B and Table S3†). Undoubtedly, the detailed mechanisms underlying the formation of these unexpected PDMs require further investigation, and so does the reaction kinetics. Regardless, all main PDMs from ONAyne predominantly target the lysine residue with an average localization probability of 0.77, demonstrating their proteome-wide selectivity (Fig. S9†).Next, we adapted an ABPP approach to globally and site-specifically quantify the reactivity of lysine towards the γ-dicarbonyl warhead through a dose-dependent labelling strategy (Fig. 3A) that has been proved to be successful for other lysine-specific probes (e.g., STP alkyne).31 Specifically, MDA-MB-231 cell lysates were treated with low versus high concentrations of ONAyne (1 mM versus 0.1 mM) for 1 h. Probe-labelled proteomes were digested into tryptic peptides that were then conjugated to isotopically labelled biotin tags via CuAAC for enrichment, identification and quantification. In principle, hyperreactive lysine would saturate labelling at the low probe concentration, whereas less reactive ones would show concentration-dependent increases in labelling. For fair comparison, the STP alkyne-based lysine profiling data were generated by using the same chemoproteomic workflow. Although 77.5% (3207) ONAyne-adducted lysine sites can also be profiled by STP alkyne-based analysis, the former indeed has its distinct target-profile with 930 lysine sites newly identified (Fig. S10 and Table S4†). Interestingly, sequence motif analysis with pLogo40 revealed a significant difference in consensus motifs between ONAyne- and STP alkyne-targeting lysines (Fig. S11†).Open in a separate windowFig. 3ONAyne-based quantitative reactivity profiling of proteomic lysines. (A) Schematic workflow for quantitative profiling of ONAyne–lysine reactions using the dose-dependent ABPP strategy (B) Box plots showing the distribution of R10:1 values quantified in ONAyne- and STP alkyne-based ABPP analyses, respectively. Red lines showing the median values. ***p ≤ 0.001 two-tailed Student''s t-test. (C) Representative extracted ion chromatograms (XICs) showing changes in the EF1A1 peptide bearing K273 that is adducted as indicated, with the profiles for light and heavy-labelled peptides in blue and red, respectively.Moreover, we quantified the ratio (R1 mM:0.1 mM) for a total of 2439 ONAyne-tagged lysines (on 922 proteins) and 17904 STP alkyne-tagged lysines (on 4447 proteins) across three biological replicates (Fig. S12 and Table S5†). Strikingly, only 26.7% (651) of quantified sites exhibited nearly dose-dependent increases (R1 mM:0.1 mM > 5.0) in reactivity with ONAyne, an indicative of dose saturation (Fig. 3B and C). In contrast, such dose-dependent labelling events accounted for >69.1% of all quantified lysine sites in the STP alkyne-based ABPP analysis.31 This finding is in accordance with the extremely fast kinetics of reaction between lysine and γ-dicarbonyls (prone to saturation). Nonetheless, by applying 10-fold lower probe concentrations, overall 1628 (80.2%) detected lysines could be labelled in a fully concentration-dependent manner with the median R10:1 value of 8.1 (Fig. 3B, C, S12 and Table S5†). Next, we asked whether the dose-depending quantitation data (100 μM versus 10 μM) can be harnessed to predict functionality. By retrieving the functional information for all quantified lysines from the UniProt Knowledgebase, we found that those hyper-reactive lysines could not be significantly over-represented with annotation (Fig. S12†). Nonetheless, among all quantified lysines, 509 (25.1%) possess functional annotations, while merely 2.5% of the human lysinome can be annotated. Moreover, 381 (74.8%) ONAyne-labelled sites are known targets of various enzymatic post-translational modifications (PTMs), such as acetylation, succinylation, methylation and so on (Fig. S13†). In contrast, all known PTM sites accounted for only 59.6% of the annotated human lysinome. These findings therefore highlight the intrinsic reactivity of ONAyne towards the ‘hot spots’ of endogenous lysine PTMs.The aforementioned results validate ONAyne as a fit-for-purpose lysine-specific chemoproteomic probe for competitive isoTOP-ABPP application of γ-dicarbonyl target profiling. Inspired by this, we next applied ONAyne-based chemoproteomics in an in situ competitive format (Fig. 4A) to globally profile lysine sites targeted by a mixture of levuglandin (LG) D2 and E2, two specific isomers of IsoKs that can be synthesized conveniently from prostaglandin H2 (ref. 41) (Fig. S2†). Specifically, mouse macrophage RAW264.7 cells (a well-established model cell line to study LDE-induced inflammatory effects) were treated with 2 μM LGs or vehicle (DMSO) for 2 h, followed by ONAyne labelling for an additional 2 h. The probe-labelled proteomes were processed as mentioned above. For each lysine detected in this analysis, we calculated a control/treatment ratio (RC/T). Adduction of a lysine site by LGs would reduce its accessibility to the ONAyne probe, and thus a higher RC/T indicates increased adduction. In total, we quantified 2000 lysine sites on 834 proteins across five biological replicates. Among them, 102 (5.1%) sites exhibited decreases of reactivity towards LGs treatment (P < 0.05, Table S6†), thereby being considered as potential targets of LGs. Notably, we found that different lysines on the same proteins showed varying sensitivity towards LGs (e.g., LGs targeted K3 of thioredoxin but not K8, K85 and K94, Table S6†), an indicative of changes in reactivity, though we could not formally exclude the effects of changes in protein expression on the quantified competition ratios. Regardless, to the best of our knowledge, the proteome-wide identification of potential protein targets by IsoKs/LGs has not been possible until this work.Open in a separate windowFig. 4ONAyne-based in situ competitive ABPP uncovers functional targets of LGs in macrophages. (A) Schematic workflow for profiling LGs–lysine interactions using ONAyne-based in situ competitive ABPP. (B) Volcano plot showing the log2 values of the ratio between the control (heavy) and LGs-treated (light) channels and the −log10(P) of the statistical significance in a two-sample t-test for all quantified lysines. Potential targets of LGs are shown in blue (RC/T>1.2, P < 0.05), with the validated ones in red. (C) Bar chart showing the inhibitory effect of 2 μM LGs on the cellular enzymatic activity of MDH2. Data represent means ± standard deviation (n = 3). Statistical significance was calculated with two-tailed Student''s t-tests. (D) Pretreatment of LGs dose-dependently blocked ONAyne-labelling of MDH2 in RAW264.7 cells, as measured by western blotting-based ABPP. (E and F) LGs dose-dependently decreased the H2BK5 acetylation level in RAW 264.7 cells, as measured either by western blotting (E) or by immunofluorescence imaging (F). n = 3. For G, nuclei were visualized using DAPI (blue).We initially evaluated MDH2 (malate dehydrogenase, mitochondrial, also known as MDHM), an important metabolic enzyme that possesses four previously uncharacterized liganded lysine sites (K157, K239, K301 and K329, Fig. 4B) that are far from the active site (Fig. S14†). We found that LGs dramatically reduced the catalytic activity of MDH2 in RAW264.7 cells (Fig. 4C), suggesting a potentially allosteric effect. We next turned our attention to the targeted sites residing on histone proteins, which happen to be modified by functionally important acetylation, including H2BK5ac (Fig. 4B) that can regulate both stemness and epithelial–mesenchymal transition of trophoblast stem cells.42 We therefore hypothesized that rapid adduction by LGs competes with the enzymatic formation of this epigenetic mark. Immunoblotting-based competitive ABPP confirmed that LGs dose-dependently blocked probe labelling of H2B (Fig. 4D). Further, both western blots and immunofluorescence assays revealed that LG treatment decreased the level of acetylation of H2BK5 (average RC/T = 1.3, P = 0.007) in a concentration-dependent manner (Fig. 4E and F). Likewise, a similar competitive crosstalk was observed between acetylation and LG-adduction on H2BK20 (average RC/T = 1.2, P = 0.01) that is required for chromatin assembly43 and/or gene regulation44 (Fig. 4B and S15†). Notably, these findings, together with several previous reports by us and others about histone lysine ketoamide adduction by another important LDE, 4-oxo-2-noenal,11,45,46 highlight again the potentially important link between lipid peroxidation and epigenetic regulation. In addition to the targets validated as above, many other leads also merit functional studies considering diverse biological or physiologic effects of LGs in macrophages. 相似文献
5.
6.
Hlne Beucher Johannes Schrgenhumer Estíbaliz Merino Cristina Nevado 《Chemical science》2021,12(45):15084
A chelation-assisted oxidative addition of gold(i) into the C–C bond of biphenylene is reported here. The presence of a coordinating group (pyridine, phosphine) in the biphenylene unit enabled the use of readily available gold(i) halide precursors providing a new, straightforward entry towards cyclometalated (N^C^C)- and (P^C)-gold(iii) complexes. Our study, combining spectroscopic and crystallographic data with DFT calculations, showcases the importance of neighboring, weakly coordinating groups towards the successful activation of strained C–C bonds by gold.Pyridine and phosphine directing groups promote the C–C activation of biphenylene by readily available gold(i) halides rendering a new entry to (N^C^C)- and (P^C)-gold(iii) species.Activation of C–C bonds by transition metals is challenging given their inertness and ubiquitous presence alongside competing C–H bonds.1 Both the intrinsic steric hindrance as well as the highly directional character of the p orbitals involved in the σC–C bond impose a high kinetic barrier for this type of processes.2,3 Biphenylene, a stable antiaromatic system featuring two benzene rings connected via a four-membered cycle, has found widespread application in the study of C–C bond activation. Since the seminal report from Eisch et al. on the oxidative addition of a nickel(0) complex into the C–C bond of biphenylene,4 several other late transition metals have been successfully applied in this context.5 Interestingly, despite the general reluctance of gold(i) to undergo oxidative addition,6 its oxidative insertion into the C–C bond of biphenylene was demonstrated in two consecutive reports by the groups of Toste7a and Bourissou,7b respectively. The high energy barrier associated with the oxidation of gold could be overcome by the utilization of gold(i) precursors bearing ligands that exhibit either a strongly electron-donating character (e.g. IPr = [1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene])7a or small bite angles (e.g. DPCb = diphosphino-carborane).7b,8 In line with these two approaches, more sophisticated bidentate (N^C)- and (P^N)-ligated gold(i) complexes have also been shown to aid the activation of biphenylene at ambient temperature (Scheme 1a).7c,dOpen in a separate windowScheme 1(a) Previous reports on oxidative addition of ligated gold(i) precursors onto biphenylene. (b) This work: pyridine- and phosphine-directed C–C bond activation of biphenylene by commercially available gold(i) halides.In this context, we hypothesized that the oxidative insertion of gold(i) into the C–C bond of biphenylene could be facilitated by the presence of a neighboring chelating group.9 This approach would not only circumvent the need for gold(i) precursors featuring strong σ-donor or highly tailored bidentate ligands but also offer a de novo entry towards interesting, less explored ligand templates. However, recent work by Breher and co-workers showcased the difficulty of achieving such a transformation.10Herein, we report the oxidative insertion of readily available gold(i) halide precursors into the C–C bond of biphenylene. The appendage of both pyridine and phosphine donors in close proximity to the σC–C bond bridging the two aromatic rings provides additional stabilization to the metal center and results in a de novo entry to cyclometalated (N^C^C)- and (P^C)gold(iii) complexes (Scheme 1b).Our study commenced with the preparation of 5-chloro-1-pyridino-biphenylene system 2via Pd-catalyzed Suzuki cross coupling reaction between 2-bromo-3-methylpyridine and 2-(5-chlorobiphenylen-1-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane 1 (Scheme 2).11 To our delight, the reaction of 2 with gold(i) iodide in toluene at 130 °C furnished complex κ3-(N^C^C)Au(iii)–I 3 in 60% yield.12,13 Complex 3 was isolated as yellow plate-type crystals from the reaction mixture and its molecular structure was unambiguously assigned by NMR spectroscopy, high-resolution mass spectrometry (HR-MS) and crystallographic analysis. Complex 3 exhibits the expected square-planar geometry around the metal center, with a Au–I bond length of 2.6558(3) Å.14 The choice of a neutral weakly bound gold(i)-iodide precursor is key for a successful reaction outcome: similar reactions in the presence of [(NHC)AuCl + AgSbF6] failed to deliver the desired biscyclometalation adducts, as reported by Breher et al. in ref. 10. The oxidative insertion of gold(i) iodide into the four-membered ring of pyridino-substituted biphenylene provides a novel and synthetically efficient entry to κ3-(N^C^C)gold(iii) halides. These species have recently found widespread application as precursors for the characterization of highly labile, catalytically relevant gold(iii) intermediates,15a–d as well as for the preparation of highly efficient emitters in OLEDs.15e–g Previous synthetic routes towards these attractive biscyclometalated gold(iii) systems involved microwave-assisted double C–H functionalization reactions that typically proceed with low to moderate yields.15aOpen in a separate windowScheme 2Synthesis of complex 3via oxidative addition of Au(i) into the C–C bond of pyridine-substituted biphenylene. X-ray structures of complex 3 with atoms drawn using 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. Additional selected bond distances [Å]: N–Au = 2.126(2), C1–Au = 1.973(2), C2–Au = 2.025(2), Au–I = 2.6558(3) and bond angles [deg]: N–Au–I = 99.25(6), N–Au–C1 = 79.82(9), C1–Au–C2 = 81.2(1), C2–Au–I = 99.73(8). For experimental details, see ESI.†Encouraged by the successful results obtained with the pyridine-substituted biphenylene and considering the prominent use of phosphines in gold chemistry,6,16 we wondered whether the same reactivity would be observed for a P-containing system. To this end, both adamantyl- and tert-butyl-substituted phosphines were appended in C1 position of the biphenylene motif. Starting from 5-chlorobiphenylene-1-carbaldehyde 4, phosphine-substituted biphenylenes 5a and 5b could be accessed in 3 steps (aldehyde reduction to the corresponding alcohol, Appel reaction and nucleophilic displacement of the corresponding benzylic halide) in 64 and 57% overall yields, respectively.13 The reactions of 5a and 5b with commercially available gold(i) halides (Me2SAuCl and AuI) furnished the corresponding mononuclear complexes 7a–b and 8a–b, respectively (Scheme 3).13 All these complexes were fully characterized and the structures of 7a, 7b and 8a were unambiguously characterized by X-ray diffraction analysis.13 Interestingly, the nature of the halide has a clear effect on the chemical shift of the phosphine ligand so that a Δδ of ca. 5 ppm can be observed in the 31P NMR spectra of 7a–b (Au–Cl) compared to 8a–b (Au–I), the latter being the more deshielded. The Au–X bond length is also impacted, with a longer Au–I distance (2.5608(1) Å for 8a) compared to that measured in the Au–Cl analogue (2.2941(7) Å for 7a) (Δd = 0.27 Å).13Open in a separate windowScheme 3Synthesis and reactivity of complexes 7a–b, 8a–b, 9 and 10. X-ray structure of complexes 11b, 12 and 14 with atoms drawn using 50% probability ellipsoids. Hydrogen atoms have been omitted for clarity. For experimental details and X-ray structures see ESI.†Despite numerous attempts to promote the C–C activation in these complexes,10,13 all reactions resulted in the formation of highly stable cationic species 11a–b and 12, which could be easily isolated from the reaction media. In the case of cationic mononuclear-gold(i) complexes 11, a ligand scrambling reaction in which the chloride ligand is replaced by a phosphine in the absence of a scavenger, a process previously described for gold(i) species, can be used to justify the reaction outcome.17 The formation of dinuclear gold complex 12 can be ascribed to the combination of a strong aurophilic interaction between the two gold centers (Au–Au = 2.8874(4) Å) and the stabilizing η2-coordination of the metal center to the aromatic ring of biphenylene. Similar η2-coordinated gold(i) complexes have been reported but, to the best of our knowledge, only as mononuclear species.18Taking into consideration the observed geometry of complexes 7a–b in the solid state,13 the facile formation of stable cationic species 11 and 12 and the lack of reactivity of the gold(i) iodides 8a–b, we hypothesized that the free rotation around the C–P bond was probably restricted, placing the gold(i) center away from the biphenylene system and thus preventing the desired oxidative insertion reaction. To overcome this problem, we set out to elongate the arm bearing the phosphine unit with an additional methylene group, introduced via a Wittig reaction from compound 4 to yield ligand 6, prepared in 4 steps in 27% overall yield. Coordination with Me2SAuCl and AuI resulted in gold(i) complexes 9 and 10, respectively (Scheme 3). The structure of 9 was unambiguously assigned by X-ray diffraction analysis and a similar environment around the metal center to that determined for complex 7a was observed for this complex.13With complexes 9 and 10 in hand, we explored their reactivity towards C–C activation of the four-membered ring of biphenylene.19 After chloride abstraction and upon heating at 100 °C for 5 hours, ring opening of the biphenylene system was observed for complex 9. Interestingly, formation of mono-cyclometalated adduct 13 was exclusively observed (the structure of 13 was confirmed by 1H, 13C, 31P, 19F, 11B and 2D NMR spectroscopy and HR-MS).13 The solvent appears to play a major role in this process, as performing the reaction in non-chlorinated solvents resulted in stable cationic complexes similar to 11.13,20,21 The presence of adventitious water is likely responsible for the formation of the monocyclometalated (P^C)gold(iii) complex 13 as when the reaction was carried out in C2H4Cl2 previously treated with D2O, the corresponding deuterated adduct 13-d could be detected in the reaction media. These results showcase the difficulties associated with the biscyclometalation for P-based complexes as well as the labile nature of the expected biscyclometalated adducts. Interestingly though, these processes can be seen as a de novo entry towards relatively underexplored (P^C)gold(iii) species.22The C–C activation was further confirmed by X-ray diffraction analysis of the phosphonium salt 14, which arise from the reductive elimination at the gold(iii) center in 13 upon exchange of the BF4− counter-anion with the weakly coordinating sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF).13,23 The phosphorus atom is four-coordinate, with weak bonding observed to the distant counter-anion and a distorted tetrahedral geometry (C1–P–C2 = 95.05(17), C2–P–C3 = 112.1(1), C3–P–C4 = 116.6(1), C4–P–C1 = 107.4(2) deg). These results represent the third example in which the C(sp2)–P bond reductive elimination at gold(iii) has been reported.24Further, it is important to note that, in contrast to the reactivity observed for the pyridine-substituted biphenylene, neither P-coordinated gold(i) iodo complexes 8a, 8b nor 10 reacted to give cyclometalated products despite prolonged heating, which highlights the need for highly reactive cationized gold(i) species to undergo oxidative addition when phosphine ligands are flanking the C–C bond.13To get a deeper understanding on the observed differences in reactivity for the N- vs. P-based directing groups, ground- and transition-state structures for the oxidative insertion of gold(i) halides in C1-substituted biphenylenes were computed by DFT calculations. The reactions of Py-substituted 2 with AuI to give 3 (I) and those of P-substituted 7a (II) and 9 (III) featuring the cationization of the gold(i) species were chosen as models for comparative purposes with the experimental conditions (Fig. 1 and S1–S10 in the ESI†).25–27 The computed activation energies for the three processes are in good agreement with the experimental data. The pyridine-substituted biphenylene I exhibits the lowest activation barrier for the oxidative insertion process (ΔG‡ = 34.4 kcal mol−1). The reaction on the phosphine-substituted derivatives II and III proved to be, after cationization of the corresponding gold(i) halide complexes (II-BF4, III-BF4) higher in energy (ΔG‡ = 39.6 and 46.3 kcal mol−1 respectively), although the obtained values do not rule out the feasibility of the C–C activation process. The transition state between I and I′ exhibits several interesting geometrical features: (a) the biphenylene is significantly bent, (b) the cleavage of the C–C bond is well advanced (dC–C = 1.898 Å in TSIvs. dC–C = 1.504 Å in I), and (c) the two C and the I atoms form a Y-shape around gold with minimal coordination from the pyridine (dN–Au = 2.742 Å in TSIvs. dN–Au = 2.093 Å in I and 2.157 Å in I′, respectively). The transition-state structures found for the P-based ligands (TSII and TSIII) also show an elongation of the C–C bond and display a bent biphenylene. However, much shorter P–Au distances (dP–Au = 2.330 Å for TSII and 2.314 Å for TSIII) can be observed compared to the pyridine-based system, as expected due to the steric and electronic differences between these two coordinating groups. Analogously, longer C–Au distances were also found for the P-based systems (dC1–Au = 2.152 Å for TSIvs. 2.235 Å and 2.204 Å for TSII and TSIII; dC2–Au = 2.143 Å for TSIvs. 2.219 Å and 2.162 Å for TSII and TSIII), with a larger deviation of square planarity for Au in TSIII compared to TSII.28,29 These results suggest that, provided the appropriate distance to the C–C bond is in place, the strong coordination of phosphorous to the gold(i) center does not prevent the C–C activation of biphenylene but other reactions (i.e. formation of diphosphine gold(i) cationic species, protodemetalation) can outcompete the expected biscyclometalation process. In contrast, a weaker donor such as pyridine offers a suitable balance bringing the gold in close proximity to the C–C bond and enables both the oxidative cleavage as well as the formation of the double metalation product.Open in a separate windowFig. 1Energy profile (ΔG and ΔG‡ in kcal mol−1), optimized structures, transition states computed at the IEFPCM (toluene/1,2-dichloroethane)-B3PW91/DEF2QZVPP(Au,I)/6-31++G(d,p)(other atoms) level of theory for the C–C activation of biphenylene with gold(i) iodide from I and gold(i) cationic from II and III. Computed structures of the transition states (TSI, TSII and TSIII) and table summarizing relevant distances. 相似文献
7.
Julia Oktawiec Henry Z. H. Jiang Ari B. Turkiewicz Jeffrey R. Long 《Chemical science》2021,12(45):15170
Correction for ‘Influence of the primary and secondary coordination spheres on nitric oxide adsorption and reactivity in cobalt(ii)–triazolate frameworks’ by Julia Oktawiec et al., Chem. Sci., 2021, DOI: 10.1039/d1sc03994f.The authors regret that incorrect details were given for ref. 35, 37 and 59 in the original article. The correct versions of ref. 35, 37 and 59 are given below as ref. 1, 2 and 3, respectively.The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers. 相似文献
8.
Fang-Xin Wang Jia-Lei Yan Zhixin Liu Tingshun Zhu Yingguo Liu Shi-Chao Ren Wen-Xin Lv Zhichao Jin Yonggui Robin Chi 《Chemical science》2021,12(30):10259
The construction of an isoquinoline skeleton typically starts with benzene derivatives as substrates with the assistance of acids or transition metals. Disclosed here is a concise approach to prepare isoquinoline analogues by starting with pyridines to react with β-ethoxy α,β-unsaturated carbonyl compounds under basic conditions. Multiple substitution patterns and a relatively large number of functional groups (including those sensitive to acidic conditions) can be tolerated in our method. In particular, our protocol allows for efficient access to tricyclic isoquinolines found in hundreds of natural products with interesting bioactivities. The efficiency and operational simplicity of introducing structural complexity into the isoquinoline frameworks can likely enable the collective synthesis of a large set of natural products. Here we show that fredericamycin A could be obtained via a short route by using our isoquinoline synthesis as a key step.A concise approach for rapid assembly of multicyclic isoquinoline scaffolds from pyridines and β-ethoxy α,β-unsaturated carbonyl compounds was developed, which enabled the formal total synthesis of fredericamycin A. Isoquinolines and their derivatives are common structural motifs in numerous natural products. Among them, the analogues of isoquinolines fused with rings from the benzene side such as 8-hydroxyisoquinolin-1[2H]-one (Fig. 1a) have been found in hundreds of natural products with interesting bioactivities.1 For example, fredericamycin A and the related family members, isolated from Streptomyces griseus, show both antimicrobial and anti-tumor activities.2 Ericamycin is a natural product isolated in the culture of Streptomyces varius n. sp. with anti-staphylococcal activities.3 Due to the widespread presence of isoquinolines in both natural and synthetic molecules, numerous approaches have been developed to assemble this class of scaffolds.4 The dominated strategies reported to date focus on forming the new pyridine ring of isoquinolines (Fig. 1b, left part). Classic methods include Bischler–Napieralski isoquinoline synthesis,4a,b Pictet–Gams isoquinoline synthesis,4a and Pomeranz–Fritsch reaction.4a These reactions, proven to be useful since as early as 1893,5 have their own merits and limitations. For instance, high reaction temperature (e.g. reflux in toluene) and strong acids are typically required and thus functional group tolerance can become challenging. On the other side, the introduction of structural complexities and substitution patterns is constrained as the substrates have to be pre-settled to favor the formation of pyridine moieties. Here we report a new approach to prepare isoquinoline scaffolds by constructing a new benzene ring (Fig. 1b, right part).6 Our method starts with pyridine derivatives as the substrates to react with readily available β-ethoxy α,β-unsaturated carbonyl compounds. The reaction cascade involves five main plausible mechanistic processes (Michael addition, Dieckmann condensation, elimination, aromatization and in situ methylation) to furnish isoquinoline-based products with medium to good yields. The tricyclic isoquinoline-containing products might serve as formal common starting points for rapid total synthesis of a large number of natural products, such as those exemplified in Fig. 1a. In the present study, we demonstrate that starting from the tricyclic isoquinoline adduct 6a prepared using our method, fredericamycin A can be synthesized in 8 steps (Fig. 1c). Our strategy for isoquinoline assembly offers complementary and in certain cases better solutions not readily provided by the classic methods. We expect our method to find impressive applications in concise modular synthesis of complex natural products and molecular libraries, especially those bearing isoquinoline units fused with additional cyclic structures.Open in a separate windowFig. 1Isoquinoline analogues and their synthesis.Our design and initial studies are illustrated in Scheme 1.7 We first used pyridine 1a to react with α-substituted cycloenones (2a–2d), in the hope of obtaining isoquinoline 3a as the target product (Scheme 1a). The use of 2a and 2b was inspired by studies from Tamura, in which α-Br in 1,4-naphthoquinone was used as a leaving group to form an aromatic ring.8 Unfortunately, no product was formed and most of the starting materials were recovered. When SPh (2c) or SOPh (2d) was incorporated at the α site of the cycloenone, side products 4a and 4b were isolated respectively in moderate yields. The Michael products 4a and 4b could not be further transformed into our desired cyclic product 3a under various conditions. We then studied the use of β-substituted cycloenones (2e–2g) to react with 1a (Scheme 1b). No reactions were observed when 2e or 2f was used. To our delight, when the halogen of 2e/2f was replaced with a methoxy unit (OCH3, substrate 2g), an encouraging amount of annulation product 3a was detected (10% yield). A side product 5a was also obtained (5% yield) in this initial study and it couldn''t be further transformed into the annulation product 3a under various alkaline conditions. It is noteworthy that, while β-alkoxy cycloenones (specifically, only β-alkoxy cyclohexenones) have been used in Staunton–Weinreb annulation9 to prepare fused aromatic compounds, no examples for those containing a heterocyclic aromatic ring were reported.10 Even for the construction of an aromatic ring without any heteroatom, low yields (mostly ranging from 0 to 30%) often occurred for this type of annulation starting with β-alkoxy cycloenones,9 which severely hampered its usage in Staunton–Weinreb annulation for the total synthesis of natural products. Our initial results showcased the possibility of direct assembly of isoquinoline scaffolds from β-methoxy cyclopentenone for the first time, though also in a low yield of 10%.Open in a separate windowScheme 1Proposed routes and initial studies for isoquinoline synthesis.With the initial results in hand, we performed additional condition optimization (11 The β-methoxy cyclopentenone 2g could also react to give 6a in a lower yield of 65% (entry 3). Other bases [such as triethylenediamine (DABCO), diazabicyclo[5.4.0]undec-7-ene (DBU), 4-dimethylaminopyridine (DMAP), lithium bis(trimethylsilyl)amide (LiHMDS) and potassium bis(trimethylsilyl)amide (KHMDS)] gave poorer results with yields ranging from 0 to 42% (entry 4). When THF was changed to other solvents, lower yields (<41%) were obtained (entry 5). Revising the ratio of 1a to 2h from 1 : 1.5 to 1.5 : 1 delivered 6a in 39% to 54% yields (entries 6–8). Lower reaction temperature (e.g. −78 °C) could not improve the outcome of this cascade transformation, but gave 23% yield of 6a together with 16% yield of recovered starting material 2h (entry 9). Long exposure to low temperature in step 1 could also lead to a considerable amount of the undesired elimination product 5a (ca. 29% yield), which was decomposed under the following methylation conditions (step 2). No product was observed in the absence of the methoxy group in 1a as it could stabilize the transition state via the formation of a metallate complex (entry 10).Screening of conditionsa
Open in a separate windowaStandard conditions: 1a (0.2 mmol) and LDA (0.2 mmol) reacted in THF at −78 °C for 1 h; 2h (0.1 mmol) was added dropwise to the mixture before warming up to rt in 10 min. The reaction was quenched by the addition of saturated aqueous solution of NH4Cl after completion monitored by TLC. After the removal of solvents, the crude residue was treated directly with TBAB (0.2 eq.), NaOH (2.0 eq.) in water (1 mL), and Me2SO4 (4.0 eq.) in CH2Cl2 (1 mL).bIsolated yield.cRecovered starting material 2h: 16% yield.With the optimal reaction conditions in hand, we next examined the scope of the pyridine derivatives 1. As we can see from Scheme 2, substrates with the aliphatic substituents at C3 could afford the corresponding tricyclic isoquinoline products (6a and 6b) in acceptable yields. Besides, the incorporation of an aromatic ring at this site (6c–6j) also works well for this transformation, wherein electron-rich aromatic rings (6c–6g) could give higher yields than the corresponding electron-deficient ones (6h–6j). It should be noted that the relatively lower yield of 44% for 6h was partially due to the slow reaction rate as the recovered starting material was always detected in this transformation. When it comes to C4 substitution, the isoquinoline products with broad structural diversities such as alkyl (6k), alkenyl (6l–6n),12 alkynyl (6o), benzyl derivatives with different substituents on the phenyl ring (6p–6t), heteroaromatic ring (6u) and thioether (6v) could be obtained in 57–93% yields. Moreover, substrates bearing acid-hydrolyzable functionalities (6w) and with a relatively bulky secondary substituent (6x) also worked well under the optimized reaction conditions. Next, we examined the possibility of introducing a side chain at C5. To our delight, the substrate with an ethyl group instead of the methyl group on the aromatic ring reacted smoothly to deliver the corresponding isoquinoline 6y in 89% yield. Further study revealed that the exposure of the bicyclic substrate 5,6,7,8-tetrahydroisoquinoline derivative to the optimized reaction conditions could furnish the polycyclic product 6z in 92% yield. Finally, we relocated the nitrogen atom in the pyridine ring. The experimental results indicated that the substrate with nitrogen atom located at C3 can''t react to form the corresponding isoquinoline 6aa, possibly due to the mismatched dipole orientation. When the nitrogen atom was sited at the ortho-position of the methyl group in the aromatic ring, quinoline 6ab could not be detected either under the optimized reaction conditions. The control experiments showcased the decisive influence of the location of nitrogen atom in the aromatic ring on the reactivity of this cascade transformation.Open in a separate windowScheme 2Scope of pyridine derivatives.For the five-membered cycloenone derivatives 2 (Scheme 3), substrates with different substituents at the α′ position work well for this transformation (6ac–6ak),12 of which the incorporation of a quaternary carbon center (6aj) and a heteroatom (6ak) at this site was included. The introduction of an allyl group at the β′ position in cyclopentenone proved to be viable for this transformation, delivering 6al in 64% yield. More encouragingly, when the sterically hindered substrate with a quaternary carbon center located at the γ site was exposed to the optimized reaction conditions, the isoquinoline 6am was obtained in 65% yield. This is challenging, considering the fact that the reacting site is just adjacent to a sterically bulky all-carbon quaternary stereocenter. Bicyclic 3-ethoxy-1H-inden-1-one is also suitable for this cascade transformation, giving the tetracyclic 10H-indeno[1,2-g]isoquinolin-10-one derivative 6an in 89% yield. When it comes to six-membered cycloenone derivatives (6ao–6au), substrates with substituents at α′ and β′ positions all worked smoothly to provide the corresponding isoquinoline products in moderate to high yields. Notably, Kita reported a 5-step reaction sequence to get the tricyclic benzo[g]isoquinoline-derived product 6as starting from the 1a analogue in an overall yield of 22%.6b Using our developed method, 6as could be easily obtained in 53% yield from 1a. Unexpectedly, a side product 6av was isolated in moderate yield when it comes to the γ-substituted substrate. Further study revealed that cyclohept-2-en-1-one with a medium-sized ring (6aw), lactone (6ax), and lactam (6ay) all worked well for this annulation cascade, which significantly expanded the substrate scope of this powerful cascade transformation.Open in a separate windowScheme 3Scope of cycloenone derivatives and more.Finally, fredericamycin A was selected further as the target molecule to verify the flexibility of our method in the total synthesis of natural products, especially those containing 8-hydroxyisoquinolin-1[2H]-one units.13 Since its first isolation in 1981, fredericamycin A attracted much attention from the synthetic community due to its interesting chemical structure and significant anti-tumor activity.2,14,15 The synthetic route was inspired by the expeditious work from Bach.16a As shown in Scheme 4, we started our synthetic attempts with our developed multifold reaction sequence of pyridine 1a and β-ethoxy enone 2h, delivering the corresponding methyl ether 6a on a gram scale. To the best of our knowledge, this is the first example of isoquinoline synthesis directly starting from a pyridine derivative in a single step. The aromatic ketone 6a was subjected to a Mukaiyama aldol/pinacol rearrangement cascade with cyclobutene 7 to give spiro diketone 8 in 42% yield.7,16 After oxidation with DDQ, the pivotal synthon 9 was obtained in 62% yield.7 It should be noted that the addition of p-TsOH is necessary for this transformation as a sluggish reaction rate was detected in the absence of an acid. Meanwhile, a four-step access of phthalidyl chloride 10 was developed starting from a commercially available benzoic acid derivative.7,17 For the crucial Hauser–Kraus annulation18 between fragments 9 and 10, we found that the coupling product 11 was not stable and thus protected directly as the corresponding methyl ether. After extensive screening of reaction conditions,7 LiOtBu turned out to be the only efficient base for this annulation. Mechanistically, the intermolecular Michael addition of segments 9 and 10 was followed by successive transformations involving Dieckmann condensation of enolate V, extrusion of chloride anions from the diketone VI, and last aromatization of the advanced intermediate VII to afford the hexacyclic diphenol 11 with the full skeleton embedded in fredericamycin A. As far as we know, this is the first example of 3-halophthalide as the Hauser donor instead of the classic sulfonyl- or cyano-containing substrates in Hauser–Kraus annulation, as 3-halophthalide was previously reported not suitable for this annulation.18aIn situ methylation of the newly formed phenol hydroxyls delivered Kita''s intermediate 12 in 51% yield in 2 steps. A further 4-step sequence ensured the accomplishment of fredericamycin A.19 The overall synthetic route clearly showcased the power of ingenious introduction of multifold reaction cascades to realize the best performance from the point of step economy.Open in a separate windowScheme 4Formal synthesis of fredericamycin A. 相似文献
Entry | Variation from standard conditions | Yieldb (%) |
---|---|---|
1 | None | 72 |
2 | Without methylation | 14 |
3 | OCH3 instead of OEt in 2h | 65 |
4 | DABCO, DBU, DMAP, LiHMDS and KHMDS instead of LDA | 0–42 |
5 | Other solvents in step 1 | <41 |
6 | 1a : 2h = 1 : 1 | 39 |
7 | 1a : 2h = 1.5 : 1 | 54 |
8 | 1a : 2h = 1 : 1.5 | 42 |
9c | −78 °C for step 1 | 23 |
10 | H instead of OCH3 in 1a | 0 |
9.
Stefan Andrew Harry Michael Richard Xiang Eric Holt Andrea Zhu Fereshte Ghorbani Dhaval Patel Thomas Lectka 《Chemical science》2022,13(23):7007
We report a photochemically induced, hydroxy-directed fluorination that addresses the prevailing challenge of high diastereoselectivity in this burgeoning field. Numerous simple and complex motifs showcase a spectrum of regio- and stereochemical outcomes based on the configuration of the hydroxy group. Notable examples include a long-sought switch in the selectivity of the refractory sclareolide core, an override of benzylic fluorination, and a rare case of 3,3′-difluorination. Furthermore, calculations illuminate a low barrier transition state for fluorination, supporting our notion that alcohols are engaged in coordinated reagent direction. A hydrogen bonding interaction between the innate hydroxy directing group and fluorine is also highlighted for several substrates with 19F–1H HOESY experiments, calculations, and more. We report a photochemical, hydroxy-directed fluorination that addresses the prevailing challenge of high diastereoselectivity. Numerous motifs showcase a range of regio- and stereochemical outcomes based on the configuration of the hydroxy group.The hydroxy (OH) group is treasured and versatile in chemistry and biology.1 Its ubiquity in nature and broad spectrum of chemical properties make it an attractive source as a potential directing group.2 The exploitation of the mild Lewis basicity exhibited by alcohols has afforded several elegant pathways for selective functionalization (e.g., Sharpless epoxidation,3 homogeneous hydrogenation,4 cross-coupling reactions,5 among others6). Recently, we reported a photochemically promoted carbonyl-directed aliphatic fluorination, and most notably, established the key role that C–H⋯O hydrogen bonds play in the success of the reaction.7 Our detailed mechanistic investigations prompt us to postulate that other Lewis basic functional groups (such as –OH) can direct fluorination in highly complementary ways.8 In this communication, we report a hydroxy-directed aliphatic fluorination method that exhibits unique directing properties and greatly expands the domain of radical fluorination into the less established realm governing high diastereoselectivity.9Our first inclination that functional groups other than carbonyls may influence fluorination regiochemical outcomes was obtained while screening substrates for our published ketone-directed radical-based method (Scheme 1).8a In this example, we surmised that oxidation of the tertiary hydroxy group on substrate 1 cannot occur and would demonstrate functional group tolerance (directing to C11, compound 2). Surprisingly, the two major regioisomers (products 3 and 4) are derivatized by Selectfluor (SF) on C12 and C16 – indicative of the freely rotating hydroxyl directing fluorination. Without an obvious explanation of how these groups could be involved in dictating regiochemistry, we continued the mechanistic study of carbonyl-directed fluorination (Scheme 2A). We established that the regioselective coordinated hydrogen atom abstraction occurs by hydrogen bonding between a strategically placed carbonyl and Selectfluor radical dication (SRD).7 However, we noted that the subsequent radical fluorination is not diastereoselective due to the locally planar nature of carbonyl groups. Thus, we posed the question: are there other directing groups that can provide both regio- and diastereoselectivity? Such a group would optimally be attached to a sp3 hybridized carbon; thus the “three dimensional” hydroxy carbon logically comes to mind as an attractive choice, and Scheme 1 illustrates the first positive hint.Open in a separate windowScheme 1Observed products for the fluorination of compound 1.Open in a separate windowScheme 2(A) Proposed mechanism, (B) β-caryophyllene alcohol hypochlorite derivative synthetic probe, (C) isodesmic relation of transition states showing the general importance of the hydroxy group to reactivity (ωB97xd/6-31+G*), and (D) 1H NMR experiment with Selectfluor and various additives at different concentrations.We began our detailed study with a simple substrate that contains a tertiary hydroxyl group. Alcohol 5 was synthesized stereoselectively by the reaction of 3-methylcyclohexanone, FeCl3, and 4-chlorophenylmagnesium bromide;10 the 4-chlorophenyl substituent allows for an uncomplicated product identification and isolation (aromatic chromophore). We sought to determine optimal reaction conditions by examination of numerous photosensitizers, bases, solvents, and light sources (7 Although we utilize cool blue LEDs (sharp cutoff ca. 400 nm), CFLs (small amount of UVB (280–315 nm) and UVA (315–400 nm)) are useable as well.11 A mild base additive was also found to neutralize adventitious HF and improve yields in the substrates indicated ( Entry Sensitizer 19F yield 1 None 0% 2 Benzil 83% 3 Benzil, no base 63% 4 Benzil, K2CO3 68% 5 Benzil, CFL light source 75% 6 5-Dibenzosuberenone 15% 7 4,4′-Difluorobenzil 63% 8 9,10-Phenantherenequinone 71% 9 Perylene 8% 10 Methyl benzoylformate 42%