Synthesis of 4,4-disubstituted 2-aminocyclopentanecarboxylic acid derivatives and their incorporation into 12-helical beta-peptides

An enantioselective synthetic route is reported for trans-2-aminocyclopentanecarboxylic acids (ACPC) bearing geminal side chain pairs at the 4-position. Beta-peptides containing the 4,4-disubstituted ACPC residues adopt the 12-helical conformation, as demonstrated by 2D NMR analysis in aqueous solution. These 4,4-disubstituted ACPC residues display functional groups, including acidic and hydrogen bond donating groups, in a geometrically defined fashion, which should be useful for the design of beta-peptides for specific applications. [structure: see text]     

Synthesis and Conformational Characterisation of Hexameric β‐Peptide Foldamers by Using Double POAC Spin Labelling and cw‐EPR

A selected set of terminally protected β‐hexapeptides, each containing two nitroxide‐based (3R,4R)‐4‐amino‐1‐oxyl‐2,2,5,5‐tetramethylpyrrolidine‐3‐carboxylic acid (POAC) residues combined with four (1S,2S)‐2‐aminocyclopentane‐1‐carboxylic acid (ACPC) residues, was synthesised by using solution methods and was fully characterised. The two POAC residues are separated in the sequences by different numbers of intervening ACPC residues. The conformational features of the doubly spin‐labelled β‐hexapeptides were examined in chloroform by FTIR absorption and continuous‐wave electron paramagnetic resonance spectroscopic techniques. In particular, the biradical exchange coupling (J) between two POAC residues within each peptide indicates unambiguously that the secondary structure overwhelmingly adopted is the 12‐helix. Taken together, these results support the view that POAC is an excellent β‐amino acid for exploring this type of helical conformation in doubly labelled β‐peptides.     

Accommodation of alpha-substituted residues in the beta-peptide 12-helix: expanding the range of substitution patterns available to a foldamer scaffold

Beta-amino acid oligomers composed exclusively of homochiral trans-2-aminocyclopentanecarboxylic acid (ACPC) residues and/or related pyrrolidine-based residues are known to favor a specific helical secondary structure that is defined by 12-membered ring C=O(i)- -H-N(i+3) hydrogen bonds ("12-helix"). The 12-helix is structurally similar to the familiar alpha-helix and therefore represents a source of potential alpha-helix-mimics. The 12-helix will be most useful in this regard if this conformational scaffold can be employed to arrange specific sets of protein-like side chains in space. Here we examine whether the 12-helix tolerates insertion of acyclic beta-amino acid residues bearing a substituent in the alpha-position ("beta(2)-residues"). Seventeen homologous beta-peptide heptamers have been prepared in which one to four beta(2)-residues reside among ACPC and/or pyrrolidine residues. Circular dichroism comparisons suggest that beta(2)-residues have a lower 12-helical propensity than do residues preorganized by a five-membered ring, as expected, but that beta-peptides containing beta(2)-residues at one or two of the seven positions retain a significant preference for 12-helix formation. These results indicate that a limited number of beta(2)-residues can be used to introduce side chains at specific positions along the surface of a 12-helix.     

Halogenierte Pyridine II. Reaktionen der 3-Halogenmethyl-pyridine

Halogenated pyridines II. Reactions of 3-halogenomethyl-pyridines . The facile syntheis of 3-halogenomethyl-pyridines provides an easy access to a variety of new 2, 6-di- and 2, 5, 6-trihalogented pyridines with various functional groups in the 3-position of the pyridine ring. Substitution reactions of the 3-halogenomethyl groups, their Friedel Crafts reactions, and nitrations of the halogenated pyridine ring are reported.     

K2CO3促进苯并呋喃单元3-位与硫酚构建C-S键

本文在前期工作基础上进一步考察了2-位支链酮苯并呋喃单元在3位与硫酚构建C-S键的反应。研究发现,以K₂CO₃作碱,苯并呋喃单元2-位官能团为不同链长的脂肪酮时能高效地合成各种不同取代基的3-芳硫基苯并呋喃衍生物,所得产物经核磁共振、HRMS等方法进行了表征。该反应具有条件温和、底物适用范围广、操作简单、产率高、环境友好等优点,可为含有芳香烃硫基取代的天然产物及复杂药物分子的合成提供潜在的合成途径。     

ELECTRONIC EFFECTS OF POLYFLUORINATED SUBSTITUENTS ON THE POLYMERIZATION AND THE PROPERTIES OF POLYTHIOPHENES

Three series of polythiophenes containing fluoroalkoxy and fluoroether substituentswere prepared by electrochemical polymerization. The effect of substituents with fluoro-alkoxy or ether functional groups on the electrochemical polymerization of thiophene mono-mers and properties of the obtained polymers were analyzed. The introduction of a flu-oroether functional group at the 3-position of the thiophene ring leads to an increase ofthe oxidation potential of the monomer and to a decrease of the conductivity of the re-sulting polymers, even with the use of a CH_2 group as spacer. Conversely, the presenceof an oxygen atom directly at the 3-position of the thiophene ring, which offsets the nega-tive withdrawing effect of fluoroalkyl groups, facilitates the synthesis of highly conductingpolythiophenes.     

Synthesis of Computationally Designed 2,5(6)-Benzimidazole Derivatives via Pd-Catalyzed Reactions for Potential E. coli DNA Gyrase B Inhibition

A pharmacophore model for inhibitors of Escherichia coli’s DNA Gyrase B was developed, using computer-aided drug design. Subsequently, docking studies showed that 2,5(6)-substituted benzimidazole derivatives are promising molecules, as they possess key hydrogen bond donor/acceptor groups for an efficient interaction with this bacterial target. Furthermore, 5(6)-bromo-2-(2-nitrophenyl)-1H-benzimidazole, selected as a core molecule, was prepared on a multi-gram scale through condensation of 4-bromo-1,2-diaminobenzene with 2-nitrobenzaldehyde using a sustainable approach. The challenging functionalization of the 5(6)-position was carried out via palladium-catalyzed Suzuki–Miyaura and Buchwald-Hartwig amination cross-coupling reactions between N-protected-5-bromo-2-nitrophenyl-benzimidazole and aryl boronic acids or sulfonylanilines, with yields up to 81%. The final designed molecules (2-(aminophen-2-yl)-5(6)-substituted-1H-benzimidazoles), which encompass the appropriate functional groups in the 5(6)-position according to the pharmacophore model, were obtained in yields up to 91% after acid-mediated N-boc deprotection followed by Pd-catalyzed hydrogenation. These groups are predicted to favor interactions with DNA gyrase B residues Asn46, Asp73, and Asp173, aiming to promote an inhibitory effect.     

Trifluoromethoxy substituted anilines: metalation as the key step for structural elaboration

Trifluoromethoxy-substituted anilines undergo hydrogen/lithium permutation ("metalation") with optional site selectivity depending on the N-protective group employed. N-tert-Butoxycarbonyl-2- and -4-(trifluoromethoxy)aniline react with tert-butyllithium at the nitrogen-adjacent 6- and 2-position affording, after electrophilic trapping, products 1-6. In contrast, deprotonation of the para isomer occurs at the oxygen-neighboring 3-position, giving rise to the acid 12, when the amino group is carrying two trimethylsilyl groups. sec-Butyllithium attacks 3-trifluoromethoxy-N-mono(trimethylsilyl)aniline at the 2-position, but 3-trifluoromethoxy-N,N-bis(trimethylsilyl)aniline at the 4-position to provide respectively the acids 10 and 11 after carboxylation. The synthesis of two new benzodiazepines illustrates (19 and 22) the preparative potential of the aniline functionalization mediated by organometallic reagents.     

Synthesis of 2,3-substituted thienylboronic acids and esters

A noncryogenic protocol for the synthesis of 2-substituted 3-thienylboronic acids and esters as well as 3-substituted 2-thienylboronic acids and esters has been developed. Electrophiles were introduced regiospecifically in the 2-position of 2,3-dibromothiophene and in the 3-position of 2-bromo-3-iodothiophene by halogen-magnesium exchange followed by quenching with electrophiles. Palladium-catalyzed borylation of the 2,3-substituted halothiophenes with pinacolborane and P(t-Bu)3 as ligand for Pd produced 9 and 10. The borylation protocol was tolerated by a range of functional groups; however, strongly electron-withdrawing substituents decreased the stability of the thienylboronic acids and esters.     

High selectivity from configurational match/mismatch in carbon-hydrogen insertion reactions of steroidal diazoacetates catalyzed by chiral dirhodium(II) carboxamidates.

Diazo decomposition of steroidal diazoacetates, where the point of attachment is the 3-position of the steroid A-ring, catalyzed by chiral dirhodium(II) carboxamidates results in products from carbon-hydrogen insertion in high yield and selectivities. Use of S-configured catalysts shows a distinctive preference for insertion into the 3-position to form beta-lactone products. The R-configured catalysts direct insertion preferentially to the equatorial C-H bond at the 2-position. Substituents or functional groups at the 5/6-position prevent C-H insertion from taking place at the 4-position. Even in the best case with the 5/6-positions fully saturated, however, insertion into the 3-position remains competitive with insertion into the 4-position. Corresponding 3-substituted phenyldiazoacetates give only beta-lactone products, and selectivity here is highest with chiral dirhodium(II) prolinate catalysts. A model is presented to explain these results. Overall, this methodology is versatile for functionalization of the steroid A-ring at positions 2 and 3.     

基于GC/Q-TOF MS技术鉴定S-zorb汽油中噻吩类化合物

结合气相色谱分离技术和MS/MS串联质谱筛查技术,通过选定目标母离子进行碰撞诱导解离,获取精确质量子离子信息,实现汽油馏分中噻吩类化合物的痕量筛查分析.选取5种不同碳数取代噻吩类化合物作为标准物绘制定量标准曲线,建立了基于气相色谱-四极杆串联飞行时间质谱(GC/Q-TOF MS)直接测定汽油中噻吩类化合物的方法.利用该...     

2,3-Disubstituted indoles from olefins and hydrazines via tandem hydroformylation-Fischer indole synthesis and skeletal rearrangement

The tandem hydroformylation-Fischer indolisation protocol is used in the synthesis of 2,3-disubstituted indoles. After hydroformylation of selected olefins to form alpha-branched aldehydes in a one-pot procedure these are condensed with phenylhydrazine to give hydrazones. Upon acid-promoted [3,3]-sigmatropic rearrangement indolenine intermediates with quaternary centres in the 3-position are formed, which, after selective Wagner-Meerwein-type rearrangement of one of the substituents from the 3- to the 2-position, lead to 2,3-disubstituted indoles. Several olefins, bearing substituents with various functional groups, as well as cyclic olefinic systems are investigated.     

利用脱镁叶绿酸甲酯的化学修饰合成红紫素酰亚胺衍生物

A series of novel purpurin-18 imide derivatives exhibiting long wavelength absorption and amphiphilicity wereobtained from methyl pheophorbide-a(MPa)by modification of the peripheral functional groups.The vinyl groupat 3-position was oxidized with OsO_4 and NaIO_4 to form the formyl group and the Grignard reaction of this alde-hyde with the alkyl magnesium bromide was carried out to give the corresponding 3-(1-hydroxylalkyl)pheophor-bide-a.The E-ring of these chlorines was converted into anhydride ring to give purpurin derivatives by air oxidation.The trans-3~2-alkyl purpurin derivatives were obtained by dehydration of hydroxyl group at 3~1-position.TheN-hydroxyl purpurin imide was generated by treatment of the anhydride ring with bydroxylamine hydrochloride.The 3~1-alkylacyl-N-hydroxyl purpurin imides were obtained by oxidation of hydroxyl group at 3~1-position.Theacylation of N-hydroxyl group was completed to afford N-acyloxy purpurin imides.The photocytoxicity of severalcompounds in vitro were tested.     

Remarkable wavelength-dependent photoreactions of the bis(diazo) ketone having inequivalent diazo groups: studies in fluid solutions and in low-temperature matrixes

The photoreactions of the bis(diazo) ketone 11, which has two inequivalent diazo groups, have been investigated in solutions at room temperature and in matrixes at 12 K. Irradiation of 11 in benzene containing methanol gave a mixture of the spironorcaradiene 13 and the diazo ketone 17 as primary isolable photoproducts. The former 13 originated from the diazo ketene 20, which was formed from the initial extrusion of N(2) from the 2-position of 11, while the latter product 17 was derived from the diazo ketene 24 which was generated by the initial decomposition of the diazo group at the 4-position of 11. The product distribution was remarkably dependent upon the excitation wavelength: 13 was predominantly obtained in the photolysis with light of >350 nm, while the irradiation with long-wavelength light (>420 nm) exclusively gave 17. The consistent wavelength effects were observed in photoreactions in an Ar matrix at 12 K. The irradiation of 11 matrix-isolated in Ar with light of >350 nm afforded 20 in preference to the isomer 24 as the first-formed intermediates, while 24 was mainly obtained in the long-wavelength irradiation (>420 nm). On the basis of these experimental data, we conclude that the short-wavelength irradiation of 11 causes a preferential cleavage of the diazo group at the 2-position and that the selective extrusion of N(2) from the 4-position is practically achieved by the irradiation with long-wavelength light. The reason for the selective cleavage of the two inequivalent diazo groups of 11 is discussed on the basis of theoretical calculations with the PM3 CI method.     

Derivation of fluorine-containing pyridinedicarboxylates. III Regio-selective anion chemistry at the 2- and 4-position

In contrast of Part II of the series, 4-alkyl-2-(difluoromethyl)-6-(trifluoromethyl)-3,5-pyridinedicarboxylates, 1 , were reacted with various strength bases to effect regio-selective reaction of electrophiles at either the benzylic carbanion of the 4-position, or at the 2-(difluoromethyl)carbanion. Weaker bases up to and including potassium butoxide or lithium bis(trimethylsilylamide) effected reaction of 1 at the 4-position to produce 2 and 3 by Stobbe-type condensations of aldehydes and ketones. In similar manner carbon disulfide, carbon dioxide, alkyl halides, silyl halides, and hexachloroethane produced the highly functionalized derivatives 4-10. In contrast, use of lithium diisopropylamide and like bases selectively effected carbanion formation at the 2-position to form, with the cited electrophiles and others, substitution products 11. The latter were further derived to the highly functional materials, 12-19.     

A general method for acylation of indoles at the 3-position with acyl chlorides in the presence of dialkylaluminum chloride

[reaction--see text] Indoles are selectively acylated at the 3-position in high yields on treatment with a wide variety of acyl chlorides in CH(2)Cl(2) in the presence of diethylaluminum chloride or dimethylaluminum chloride. The reaction proceeds under mild conditions and is applicable to indoles bearing various functional groups without NH protection.     

Synthesis of 3/8-carbonylated chlorophyll derivatives and regiodependent reductivity of their carbonyl substituents

Methyl pyropheophorbide-d possessing a formyl group at the 3-position and its regioisomer having 8-CHO were prepared and their reactivities with a reductant were determined by the ¹H NMR technique: 3-CHO>8-CHO. The regioselective reduction of a synthetic 3,8-diformyl-chlorin also supported the higher reactivity in 3-CHO than in 8-CHO. Regiodependent reduction of the corresponding acetyl-chlorins confirmed that carbonyl groups at the 3-position in chlorophyllous pigments were reduced more rapidly than those at the 8-position. From the reports that reactions of 3-CHO with amines were preferable to those of 7-CHO, the CO functional groups on the pyrrole A-ring of chlorophylls are more reactive than those on the B-ring.     

C-3-吡唑啉基取代的叶绿素类二氢卟吩衍生物的合成

以脱镁叶绿酸-a甲酯为起始原料,通过对E-环、3-位碳碳双键和20-位meso-氢的结构修饰,在二氢卟吩色基上构建和引进不同的官能团,并进一步利用其3-位乙烯基与重氮甲烷的1,3-偶极环加成反应,完成一系列C-3-吡唑啉基取代的叶绿素类二氢卟吩衍生物的合成.同时,讨论了叶绿素衍生物的1,3-偶极环加成反应的的区域选择性与立体选择性.所得新的叶绿素类二氢卟吩衍生物的化学结构均经UV,IR,1H NMR及元素分析得以证实.     

On the structure and torsional potential of trifluoromethoxybenzene: an ab initio and density functional study

The torsional potential of trifluoromethoxybenzene around the aryl–O bond was investigated with the aid of large-scale ab initio calculations performed at the Møller–Plesset second order (MP2) level, with several post-MP2 methods, and with a hybrid density functional method (B3LYP). Contrary to several recent reports, we do not find substantial qualitative differences between MP2 and B3LYP results, provided sufficiently large basis sets are used. The results are confronted with analogous MP2 and B3LYP data for methoxybenzene, for hypothetical anions as obtained by deprotonation at the para-position, and for ethylbenzene. The trends in the calculated torsional potentials, barrier heights and energy differences between conformers are discussed and correlated with selected structural parameters.     

Modification of 1-substituents in the 2-azabicyclo[2.1.1]hexane ring system; approaches to potential nicotinic acetylcholine receptor ligands from 2,4-methanoproline derivatives

Successful nucleophilic substitution at a methylene attached to the bridgehead (1-) position of the 2-azabicyclo[2.1.1]hexane ring system opens the way to construction of novel derivatives having a wider range of functional groups attached to the 1-position via a methylene "spacer" (including the beta-amino acid homologue of 2,4-methanoproline) and provides access to epibatidine analogues containing heterocyclic substituents and also to further homologation at the 1-position. Displacements with loss of a nucleofuge (e.g., loss of mesylate anion from the 1-mesyloxymethyl derivative) require thermal activation but proceed without the rearrangement initially anticipated in such a strained bicyclic ring system. A novel tricyclic carbamate intermediate 17 has been isolated; nucleophilic attack on 17 leads directly to the isolation of N-deprotected substitution products (with concomitant decarboxylation).