The convergent total synthesis and antibacterial profile of the natural product streptothricin F

A convergent, diversity-enabling total synthesis of the natural product streptothricin F has been achieved. Herein, we describe the potent antimicrobial activity of streptothricin F and highlight the importance of a total synthesis that allows for the installation of practical divergent steps for medicinal chemistry exploits. Key features of our synthesis include a Burgess reagent-mediated 1,2-anti-diamine installation, diastereoselective azidation of a lactam enolate, and a mercury(ii) chloride-mediated desulfurization-guanidination. The development of this chemistry enables the synthesis and structure–activity studies of streptothricin F analogs.

The second ever total synthesis of streptothricin F and the first achieved through a diversity-enabling convergent route. The synthesis is achieved in 35 total steps, with a longest linear sequence of 19 steps, and 0.40% overall yield.     

Azine-N-oxides as effective controlling groups for Rh-catalysed intermolecular alkyne hydroacylation

Heterocycle-derived aldehydes are challenging substrates in metal-catalysed hydroacylation chemistry. We show that by using azine N-oxide substituted aldehydes, good reactivity can be achieved, and that they are highly effective substrates for the intermolecular hydroacylation of alkynes. Employing a Rh(i)-catalyst, we achieve a mild and scalable aldehyde C–H activation, that permits the coupling with unactivated terminal alkynes, in good yields and with high regioselectivities (up to >20 : 1 l:b). Both substrates can tolerate a broad variety of functional groups. The reaction can also be applied to diazine aldehydes that contain a free N-lone pair. We demonstrate conversion of the hydroacylation products to the corresponding azine, through a one-pot hydroacylation/deoxygenation sequence. A one-pot hydroacylation/cyclisation, using N-Boc propargylamine, additionally leads to the synthesis of a bidentate pyrrolyl ligand.

Heterocycle-derived aldehydes are challenging substrates in metal-catalysed hydroacylation chemistry; using the N-oxide derivatives allows efficient reactions to be achieved.     

Enantioselective synthesis of α-amino ketones through palladium-catalyzed asymmetric arylation of α-keto imines

Chiral α-amino ketones are common structural motifs in natural products and pharmaceuticals, as well as important synthons in organic synthesis. Thus, establishing efficient methods for preparing compounds with these privileged scaffolds is an important endeavor in synthetic chemistry. Herein we disclose a new catalytic asymmetric approach for the synthesis of chiral α-amino ketones through a chiral palladium-catalyzed arylation reaction of in situ generated challenging α-keto imines from previously unreported C-acyl N-sulfonyl-N,O-aminals, with arylboronic acids. The current reaction offers a straightforward approach to the asymmetric synthesis of acyclic α-amino ketones in a practical and highly stereocontrolled manner. Meanwhile, the multiple roles of the chiral Pd(ii) complex catalyst in the reaction were also reported.

Chiral α-amino ketones are common structural motifs in natural products and pharmaceuticals, as well as important synthons in organic synthesis.     

Scalable synthesis and coupling of quaternary α-arylated amino acids: α-aryl substituents are tolerated in α-helical peptides

Quaternary amino acids are important tools for the modification and stabilisation of peptide secondary structures. Here we describe a practical and scalable synthesis applicable to quaternary alpha-arylated amino acids (Q4As), and the development of solid-phase synthesis conditions for their incorporation into peptides. Monomeric and dimeric α-helical peptides are synthesised with varying degrees of Q4A substitution and their structures examined using biophysical methods. Both enantiomers of the Q4As are tolerated in folded monomeric and oligomeric α-helical peptides, with the (R)-enantiomer slightly more so than the (S).

Both R and S enantiomers of Fmoc-protected amino acids bearing α-aryl substituents may be made on gram scale. Solid-phase synthesis leads to helical peptides unperturbed by the presence of these additional α-aryl groups.     

Electrochemical oxidative N–H/P–H cross-coupling with H2 evolution towards the synthesis of tertiary phosphines

Tertiary phosphines(iii) find widespread use in many aspects of synthetic organic chemistry. Herein, we developed a facile and novel electrochemical oxidative N–H/P–H cross-coupling method, leading to a series of expected tertiary phosphines(iii) under mild conditions with excellent yields. It is worth noting that this electrochemical protocol features very good reaction selectivity, where only a 1 : 1 ratio of amine and phosphine was required in the reaction. Moreover, this electrochemical protocol proved to be practical and scalable. Mechanistic insights suggested that the P radical was involved in this reaction.

A facile and novel electrochemical oxidative N–H/P–H cross-coupling method for obtaining tertiary phosphines(iii) was developed.     

Rh(iii)-Catalyzed [5 + 1] annulation of 2-alkenylanilides and 2-alkenylphenols with allenyl acetates

Herein, we report a mild and highly regioselective Rh(iii)-catalyzed non-oxidative [5 + 1] vinylic C–H annulation of 2-alkenylanilides with allenyl acetates, which has been elusive so far. The reaction proceeds via vinylic C–H activation, regioselective 2,3-migratory insertion, β-oxy elimination followed by nucleophilic cyclization to get direct access to 1,2-dihydroquinoline derivatives. The strategy was also successfully extended to C–H activation of 2-alkenylphenols for constructing chromene derivatives. In the overall [5 + 1] annulation, the allene serves as a one carbon unit. The acetate group on the allene is found to be crucial both for controlling the regio- and chemoselectivity of the reaction and also for facilitating β-oxy elimination. The methodology was scalable and also further extended towards late stage functionalization of various natural products.

A highly regioselective Rh(iii)-catalyzed non-oxidative [5 + 1] vinylic C–H annulation of 2-alkenylanilides and 2-alkenylphenols with allenyl acetates was described for accessing dihyroquinoline and chromene derivatives.     

Catalytic asymmetric hydrometallation of cyclobutenes with salicylaldehydes

Chiral, substituted cyclobutanes are common motifs in bioactive compounds and intermediates in organic synthesis but few asymmetric routes for their synthesis are known. Herein we report the Rh-catalyzed asymmetric hydrometallation of a range of meso-cyclobutenes with salicylaldehydes. The ortho-phenolic group promotes hydroacylation and can be used as a handle for subsequent transformations. The reaction proceeds via asymmetric hydrometallation of the weakly activated cyclobutene, followed by a C–C bond forming reductive elimination. A prochiral, spirocyclic cyclobutene undergoes a highly regioselective hydroacylation. This report will likely inspire the development of other asymmetric addition reactions to cyclobutenes via hydrometallation pathways.

Chiral, substituted cyclobutanes are common motifs in bioactive compounds and intermediates in organic synthesis but few asymmetric routes for their synthesis are known.     

Flow electrochemistry: a safe tool for fluorine chemistry

The heightened activity of compounds containing fluorine, especially in the field of pharmaceuticals, provides major impetus for the development of new fluorination procedures. A scalable, versatile, and safe electrochemical fluorination protocol is conferred. The strategy proceeds through a transient (difluoroiodo)arene, generated by anodic oxidation of an iodoarene mediator. Even the isolation of iodine(iii) difluorides was facile since electrolysis was performed in the absence of other reagents. A broad range of hypervalent iodine mediated reactions were achieved in high yields by coupling the electrolysis step with downstream reactions in flow, surpassing limitations of batch chemistry. (Difluoroiodo)arenes are toxic and suffer from chemical instability, so the uninterrupted generation and immediate use in flow is highly advantageous. High flow rates facilitated productivities of up to 834 mg h⁻¹ with vastly reduced reaction times. Integration into a fully automated machine and in-line quenching was key in reducing the hazards surrounding the use of hydrofluoric acid.

A scalable, efficient and safe electrochemical fluorination protocol is conferred. A broad range of iodine(iii) mediated transformations were performed in high yields without exposure to toxic HF.     

Catalytic defluorinative ketyl–olefin coupling by halogen-atom transfer

Ketyl–olefin coupling reactions stand as one of the fundamental chemical transformations in synthetic chemistry and have been widely employed in the generation of complex molecular architectures and natural product synthesis. However, catalytic ketyl–olefin coupling, until the recent development of photoredox chemistry and electrosynthesis through single-electron transfer mechanisms, has remained largely undeveloped. Herein, we describe a new approach to achieve catalytic ketyl–olefin coupling reactions by a halogen-atom transfer mechanism, which provides innovative and efficient access to various gem-difluorohomoallylic alcohols under mild conditions with broad substrate scope. Preliminary mechanistic experimental and computational studies demonstrate that this radical-to-polar crossover transformation could be achieved by sequentially orchestrated Lewis acid activation, halogen-atom transfer, radical addition, single-electron reduction and β-fluoro elimination.

A catalytic ketyl–olefin coupling reaction including sequentially orchestrated Lewis acid activation, halogen-atom transfer, radical addition, single-electron reduction and β-fluoro elimination has been developed.     

On-surface synthesis of organocopper metallacycles through activation of inner diacetylene moieties

The design of organometallic complexes is at the heart of modern organic chemistry and catalysis. Recently, on-surface synthesis has emerged as a disruptive paradigm to design previously precluded compounds and nanomaterials. Despite these advances, the field of organometallic chemistry on surfaces is still at its infancy. Here, we introduce a protocol to activate the inner diacetylene moieties of a molecular precursor by copper surface adatoms affording the formation of unprecedented organocopper metallacycles on Cu(111). The chemical structure of the resulting complexes is characterized by scanning probe microscopy and X-ray photoelectron spectroscopy, being complemented by density functional theory calculations and scanning probe microscopy simulations. Our results pave avenues to the engineering of organometallic compounds and steer the development of polyyne chemistry on surfaces.

The diacetylene skeletons of DNBD precursors are attacked on Cu(111) by copper adatoms resulting in the synthesis of organocopper metallacycles.     

Organocatalytic cycloaddition–elimination cascade for atroposelective construction of heterobiaryls

The first chiral phosphoric acid (CPA) catalyzed cycloaddition–elimination cascade reaction of 2-naphthol- and phenol-derived enecarbamates with azonaphthalenes has been established, providing a highly atroposelective route to an array of axially chiral aryl-C3-benzoindoles in excellent yields with excellent enantioselectivities. The success of this strategy derives from the stepwise process involving CPA-catalyzed asymmetric formal [3 + 2] cycloaddition and subsequent central-to-axial chirality conversion by elimination of a carbamate. In addition, the practicality of this reaction had been verified by varieties of transformations towards functionalized atropisomers.

An organocatalytic asymmetric cycloaddition–elimination cascade reaction of aryl enecarbamates with azonaphthalenes has been developed to access axially chiral heterobiaryls in excellent yields and enantioselectivities.     

Hexafluoroisobutylation of enolates through a tandem elimination/allylic shift/hydrofluorination reaction

The isobutyl side chain is a highly prevalent hydrophobic group in drugs, and it notably constitutes the side chain of leucine. Its replacement by a hexafluorinated version containing two CF₃ groups may endow the target compound with new and advantageous properties, yet this modification remains overlooked due to the absence of a general and practical synthetic methodology. Herein, we report the first general method to introduce the hexafluoroisobutyl group into ketoesters, malonates, 1,3-diketones, Schiff base esters and malononitrile. We demonstrated that the reaction occurs through an elimination/allylic shift/hydrofluorination cascade process which efficiently overcomes the usual fluoride β-elimination observed with α-CF₃-vinyl groups. We showed that with alkali metal bases, a pentafluorinated alkene is obtained predominantly, whereas the use of tetrabutylammonium fluoride (TBAF) allows hydrofluorination to occur. This tandem process represents a conceptually new pathway to synthesize bis-trifluoromethylated compounds. This methodology was applied to the multigram-scale synthesis of enantiopure (S)-5,5,5,5′,5′,5′-hexafluoroleucine.

Hexafluoroisobutylation of ketoesters, malonates, diketones, Schiff base esters and malononitrile is reported. The reaction involves an elimination/allylic shift/hydrofluorination cascade process that efficiently overcomes the usual S_N2′ mechanism.     

A general strategy for the synthesis of α-trifluoromethyl- and α-perfluoroalkyl-β-lactams via palladium-catalyzed carbonylation

β-Lactam compounds play a key role in medicinal chemistry, specifically as the most important class of antibiotics. Here, we report a novel one-step approach for the synthesis of α-(trifluoromethyl)-β-lactams and related products from fluorinated olefins, anilines and CO. Utilization of an advanced palladium catalyst system with the Ruphos ligand allows for selective cycloaminocarbonylations to give diverse fluorinated β-lactams in high yields.

β-Lactam compounds play a key role in medicinal chemistry, specifically as the most important class of antibiotics.     

Vinylazaarenes as dienophiles in Lewis acid-promoted Diels–Alder reactions

Described are the first examples of Lewis acid-promoted Diels–Alder reactions of vinylpyridines and other vinylazaarenes with unactivated dienes. Cyclohexyl-appended azaarenes constitute a class of substructures of rising prominence in drug discovery. Despite this, thermal variants of the vinylazaarene Diels–Alder reaction are rare and have not been adopted for synthesis, and Lewis acid-promoted variants are virtually unexplored. The presented work addresses this gap and in the process furnishes increased scope, dramatically higher yields, improved regioselectivity, and high levels of diastereoselectivity compared to prior thermal examples. These reactions provide scalable access to druglike scaffolds not readily available through other methods. More broadly, these studies establish a useful new class of dienophiles that, based on preliminary mechanistic studies, should be amenable to conventional strategies for enantioselective catalysis.

Vinyl-substituted azaarenes are rare and challenging substrates as dienophiles in Diels–Alder reactions; by employing Lewis acid activation, high yielding and highly selective cycloadditions with unactivated dienes are enabled.     

Distinctive features and challenges in catenane chemistry

From being an aesthetic molecular object to a building block for the construction of molecular machines, catenanes and related mechanically interlocked molecules (MIMs) continue to attract immense interest in many research areas. Catenane chemistry is closely tied to that of rotaxanes and knots, and involves concepts like mechanical bonds, chemical topology and co-conformation that are unique to these molecules. Yet, because of their different topological structures and mechanical bond properties, there are some fundamental differences between the chemistry of catenanes and that of rotaxanes and knots although the boundary is sometimes blurred. Clearly distinguishing these differences, in aspects of bonding, structure, synthesis and properties, between catenanes and other MIMs is therefore of fundamental importance to understand their chemistry and explore the new opportunities from mechanical bonds.

Catenane chemistry is closely associated with that of rotaxane and knot, and this perspective highlights their similarities and differences in various aspects including synthesis, structure and properties.     

‘Reverse biomimetic’ synthesis of l-arogenate and its stabilized analogues from l-tyrosine

l-Arogenate (also known as l-pretyrosine) is a primary metabolite on a branch of the shikimate biosynthetic pathway to aromatic amino acids. It plays a key role in the synthesis of plant secondary metabolites including alkaloids and the phenylpropanoids that are the key to carbon fixation. Yet understanding the control of arogenate metabolism has been hampered by its extreme instability and the lack of a versatile synthetic route to arogenate and its analogues. We now report a practical synthesis of l-arogenate in seven steps from O-benzyl l-tyrosine methyl ester in an overall yield of 20%. The synthetic route also delivers the fungal metabolite spiroarogenate, as well as a range of stable saturated and substituted analogues of arogenate. The key step in the synthesis is a carboxylative dearomatization by intramolecular electrophilic capture of tyrosine''s phenolic ring using an N-chloroformylimidazolidinone moiety, generating a versatile, functionalizable spirodienone intermediate.

l-Tyrosine provides a precursor for a practical synthesis of the unstable primary metabolite l-arogenate and some stabilised arogenate analogues.     

Selective desaturation of amides: a direct approach to enamides

C(sp³)–H bond desaturation has been an attractive strategy in organic synthesis. Enamides are important structural fragments in pharmaceuticals and versatile synthons in organic synthesis. However, the dehydrogenation of amides usually occurs on the acyl side benefitting from enolate chemistry like the desaturation of ketones and esters. Herein, we demonstrate an Fe-assisted regioselective oxidative desaturation of amides, which provides an efficient approach to enamides and β-halogenated enamides.

A novel and regioselective N-α,β-desaturation and dehydrogenative N-β-halogenation of amides was developed. This chemistry with high selectivity and broad substrate scope provides an efficient approach to enamides from simple amides.     

Palladium-catalyzed nucleomethylation of alkynes for synthesis of methylated heteroaromatic compounds

Herein, we disclosed a novel and efficient palladium-catalyzed nucleomethylation of alkynes for the simultaneous construction of the heteroaromatic ring and methyl group. The 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines were obtained in moderate to excellent yields. Notably, this methodology was employed as a key step for synthesis of a pregnane X receptor antagonist, zindoxifene, bazedoxifene and AFN-1252. The kinetic studies revealed that reductive elimination might be the rate-determining step.

A novel palladium-catalyzed nucleomethylation of alkynes is developed, affording 3-methylindoles, 3-methylbenzofurans and 4-methylisoquinolines in moderate to excellent yields.     

A direct-to-biology high-throughput chemistry approach to reactive fragment screening

Methods for rapid identification of chemical tools are essential for the validation of emerging targets and to provide medicinal chemistry starting points for the development of new medicines. Here, we report a screening platform that combines ‘direct-to-biology’ high-throughput chemistry (D2B-HTC) with photoreactive fragments. The platform enabled the rapid synthesis of >1000 PhotoAffinity Bits (HTC-PhABits) in 384-well plates in 24 h and their subsequent screening as crude reaction products with a protein target without purification. Screening the HTC-PhABit library with carbonic anhydrase I (CAI) afforded 7 hits (0.7% hit rate), which were found to covalently crosslink in the Zn²⁺ binding pocket. A powerful advantage of the D2B-HTC screening platform is the ability to rapidly perform iterative design–make–test cycles, accelerating the development and optimisation of chemical tools and medicinal chemistry starting points with little investment of resource.

A photoreactive fragment screening platform employing direct-to-biology high-throughput chemistry (D2B-HTC) for the rapid iterative synthesis and screening of libraries of photoaffinity bits.     

Rh(i)-catalyzed dimerization of ene-vinylidenecyclopropanes for the construction of spiro[4,5]decanes and mechanistic studies

Rh(i) complex catalyzed dimerization of ene-vinylidenecyclopropanes took place smoothly to construct a series of products containing spiro[4,5]decane skeletons featuring a simple operation procedure, mild reaction conditions, and good functional group tolerance. In this paper, the combination of experimental and computational studies reveals a counterion-assisted Rh(i)–Rh(iii)–Rh(v)–Rh(iii)–Rh(i) catalytic cycle involving tandem oxidative cyclometallation/reductive elimination/selective oxidative addition/selective reductive elimination/reductive elimination steps; in addition, a pentavalent spiro-rhodium intermediate is identified as the key intermediate in this dimerization reaction upon DFT calculation.

Rh(i) complex catalyzed dimerization of ene-vinylidenecyclopropanes has been demonstrated, and its reaction mechanism is revealed based on a series of mechanistic studies.