Stereocontrolled asymmetric synthesis of alpha-hydroxy-beta-amino acids. A stereodivergent approach

Stereodivergent routes in organic synthesis: carbohydrates, amino acids, alkaloids and terpenes

The natural occurrence of enantiomers and diastereomers is often encountered.

Cross-Aldol Reaction of Activated Carbonyls with Nitrosocarbonyl Intermediates: Stereoselective Synthesis toward α-Hydroxy-β-amino Esters and Amides

A practical and flexible strategy toward α-hydroxy-β-amino esters and amides, which are important biological motifs, based on an organocatalytic cross-aldol reaction of in situ-generated nitrosocarbonyl intermediates followed by hydrogenation is presented. The protocol features operational simplicity, high yields, a wide substrate scope, and high regio- and diastereoselectivity profiles. The utility of this method was showcased through the synthesis of bestatin analogues and indole formation.

Toward an ab initio potential energy surface for paclitaxel: A C-13 isoserine side chain conformational study

(S)-3-Methyl-3-butenyl-(2R,3S)-N-benzoyl-3-phenylisoserinate is used as a model of the C-13 side chain, an essential subunit for the cytotoxicity of the diterpenoid paclitaxel, a chemotherapeutic drug used in the treatment of cancer. The potential energy surface (PES), calculated using a density functional theory method (DFT) and refined with MP2 single-point energy calculations, based on B3LYP geometries, was evaluated. Twelve intramolecular hydrogen bond patterns were identified for 103 in vacuo conformers. The most stable subset of these structures was found to have cooperative NH ⋯ OH ⋯ OC(O) motifs and six minima of importance that lie within 1.2kcal/mol of each other. The oxygen atoms of the ester groups effectively compete with the 2'-oxygen as a proton acceptor of NH to form stable internal hydrogen bonded structures. Additionally, the conventional OH ⋯ OC(N) hydrogen bond, which is represented by almost one third of the located minima, donates a number of stable conformers. However, the PES of the conformationally flexible model is highly dependent on the polarity of the environment. For example, the OH ⋯ OC(N) feature dominates over the cooperative motif in water. The side chain of the experimental T-taxol shaped structure agrees nicely with the respective theoretical lowest energy minimum. The π-π interactions of the phenyl rings and ethylene moiety of this structure are also discussed.

A chiral auxiliary cleavable by ring-closing alkene metathesis — Efficient synthesis of chiral nonracemic cycloalkenes

p-Menthane-3-carboxaldehyde is a readily available chiral auxiliary used to prepare cycloalkenes and heterocycles bearing a chiral tertiary or quaternary carbon of high enantiomeric purity. The auxiliary is available in both enantiomeric forms and is inexpensive and recyclable. It is cleaved by a ring-closing alkene metathesis reaction directly yielding the cycloalkene.Key words: chiral auxiliary, cleavage reaction, cyclization, ring-closing alkene metathesis, enantioenriched cycloalkenes.

Acyclic Chiral Amines and Amino Acids as Inexpensive and Readily Tunable Catalysts for the Direct Asymmetric Three-Component Mannich Reaction

The direct three-component asymmetric Mannich reaction catalyzed by acyclic chiral amines or amino acids is presented. Simple acyclic chiral amines and amino acids--such as alanine-tetrazole (9), alanine, valine, and serine-catalyzed the three-component asymmetric Mannich reactions between unmodified ketones, p-anisidine, and aldehydes with high chemo- and stereoselectivity, furnishing the corresponding Mannich bases with up to >99 % ee. This study demonstrates that the whole range of amino acids in nature, as well as nonproteogenic amino acid derivatives, can be considered in the design and tuning of novel, inexpensive organocatalysts for the direct asymmetric Mannich reaction.

Multiple Component Approaches to C-Glycosyl β-Amino Acids by Complementary One-Pot Mannich-Type and Reformatsky-Type Reactions

The development of new methods for the preparation of C-glycosyl beta-amino acid libraries with chemical and stereochemical diversity levels was investigated and the results are described herein. Two complementary one-pot three-component Mannich-type and Reformatsky-type synthetic strategies have been developed for the construction of chiral 3-amino propanoate fragments (eventually bis-substituted at C-2) directly linked to the anomeric carbon of pyranose and furanose residues. Both methods involved as the initial step the coupling of a sugar aldehyde to p-methoxybenzylamine but differed in the nucleophile (a d(2) synthon equivalent) which was successively added: a ketene silyl acetal (Mannich route) or a bromozinc enolate (Reformatsky route). Individual C-glycosyl beta-amino esters were isolated as single 3R diastereoisomers in fair to excellent yield (60-90%) and their structure assigned by NMR spectroscopy (Riguera protocol) supported by X-ray crystallography. A tentative explanation of the observed stereochemical outcome based on transition-state models is provided. A preliminary study on the synthesis of alpha,alpha-difluoro C-glycosyl beta-amino acids via a more traditional Reformatsky route is also reported.

Enantioselective Nitrile Anion Cyclization to Substituted Pyrrolidines. A Highly Efficient Synthesis of (3S,4R)-N-tert-Butyl-4-Arylpyrrolidine-3-Carboxylic Acid

[reaction: see text] A practical asymmetric synthesis of N-tert-butyl disubstituted pyrrolidines via a nitrile anion cyclization strategy is described. The five-step chromatography-free synthesis of (3S,4R)-1-tert-butyl-4-(2,4-difluorophenyl)pyrrolidine-3-carboxylic acid (2) from 2-chloro-1-(2,4-difluorophenyl)-ethanone achieved a 71% overall yield. The cyclization substrate was prepared via a catalytic CBS asymmetric reduction, t-butylamine displacement of the chlorohydrin, and a conjugate addition of the hindered secondary amine to acrylonitrile. The key nitrile anion 5-exo-tet cyclization concomitantly formed the pyrrolidine ring with clean inversion of the C-4 center to afford 1,3,4-trisubstituted chiral pyrrolidine in >95% yield and 94-99% ee. Diethyl chlorophosphate and lithium hexamethyldisilazide were shown to be the respective optimum activating group and base in this cyclization. The trans-cis mixture of the pyrrolidine nitrile undergoes a kinetically controlled epimerization/ saponification to afford the pure trans-pyrrolidine carboxylic acid target compound in >99.9% chemical and optical purity. This chemistry was also shown to be applicable to both electronically neutral and rich substituted phenyl substrates.

The direct catalytic asymmetric mannich reaction

The direct catalytic asymmetric addition of unmodified carbonyl compounds to preformed or in situ-generated imines has emerged as a promising new route to optically enriched alpha- and beta-amino acid derivatives, beta-lactams, and 1,2- and gamma-amino alcohols. The direct catalytic asymmetric Mannich reactions are mediated by small organometallic and organic amine catalysts that can achieve levels of selectivity similar to those possible with natural enzymes. The different small-molecule catalysts described here are complementary in their applications. They also complement each other in syn or anti selectivity of the direct asymmetric Mannich reaction. In this Account, we highlight the recent developments in and contributions to this research.

Synthesis of All Four Stereoisomers of 3-Amino-2-hydroxybutanoic Acids

All four stereoisomers of 3-amino-2-hydroxybutanoic acids were been obtained as single enantiomers via stereospecific reactions from D-gulonic acid gamma-lactone and D-glucono-delta-lactone.

A highly enantioselective amino acid-catalyzed route to functionalized alpha-amino acids

The development of syntheses providing enantiomerically pure alpha-amino acids has intrigued generations of chemists and been the subject of intense research. This report describes a general approach to functionalized alpha-amino acids based on catalytic asymmetric synthesis. Proline catalyzed Mannich-type reactions of N-PMP-protected alpha-imino ethyl glyoxylate with a variety of unmodified ketones to provide functionalized alpha-amino acids in high yields with excellent regio-, diastereo-, and enantioselectivities. Study of seven examples yielded six with product ee values of > or = 99%. In reactions involving ketone donors where diastereoisomeric products could be formed, two adjacent stereogenic centers were created simultaneously upon carbon-carbon bond formation with complete syn-stereocontrol. Significantly, this methodology utilizes readily available and rather inexpensive starting materials, does not require any preactivation of substrates or metal ion assistance, and can be carried out on a gram scale under operationally simple reaction conditions. The keto-functionality present in the products provides a particularly attractive site for versatile modifications. This study compliments and extends our bioorganic approach to asymmetric synthesis to a versatile synthon class. Given that we have shown that a variety of optically active amino acids can be synthesized with proline catalysis, where an L-amino acid begets other L-amino acids, our results may stimulate thoughts concerning prebiotic syntheses of optically active amino acids based on this route.