Asymmetric synthesis of L-azetidine-2-carboxylic acid and 3-substituted congeners--conformationally constrained analogs of phenylalanine, naphthylalanine, and leucine

Biocatalytic One-Carbon Ring Expansion of Aziridines to Azetidines via a Highly Enantioselective [1,2]-Stevens Rearrangement

We report enantioselective one-carbon ring expansion of aziridines to make azetidines as a new-to-nature activity of engineered "carbene transferase" enzymes. A laboratory-evolved variant of cytochrome P450BM3, P411-AzetS, not only exerts unparalleled stereocontrol (99:1 er) over a [1,2]-Stevens rearrangement but also overrides the inherent reactivity of aziridinium ylides, cheletropic extrusion of olefins, to perform a [1,2]-Stevens rearrangement. By controlling the fate of the highly reactive aziridinium ylide intermediates, these evolvable biocatalysts promote a transformation which cannot currently be performed using other catalyst classes.

A Unified Approach to Phytosiderophore Natural Products

This work reports on the concise total synthesis of eight natural products of the mugineic acid and avenic acid families (phytosiderophores). An innovative "east-to-west" assembly of the trimeric products resulted in a high degree of divergence enabling the formation of the final products in just 10 or 11 steps each with a minimum of overall synthetic effort. Chiral pool starting materials (l-malic acid, threonines) were employed for the outer building blocks while the middle building blocks were accessed by diastereo- and enantioselective methods. A highlight of this work consists in the straightforward preparation of epimeric hydroxyazetidine amino acids, useful building blocks on their own, enabling the first synthesis of 3''-hydroxymugineic acid and 3''-hydroxy-2'-deoxymugineic acid.

Stereoselective Access to Azetidine-Based α-Amino Acids and Applications to Small Peptide Synthesis

Non-natural azetidine-based amino acids (Aze) present interesting features in protein engineering. A simple organometallic route toward unsaturated carboxylic acid precursors is presented. Subsequent metal-catalyzed asymmetric reduction allowed for the synthesis of a new library of 2-azetidinylcarboxylic acids, which were finally employed in the formation of small peptide chains.

Direct anti-selective catalytic asymmetric Mannich-type reactions of alpha-ketoanilides for the synthesis of gamma-amino amides and azetidine-2-amides

Breaking with convention: A homodinuclear nickel complex derived from a biphenyldiamine-based Schiff base catalyzed an anti-selective Mannich-type reaction of alpha-ketoanilides (see scheme) to afford unique building blocks for the synthesis of azetidine-2-amides and alpha-hydroxy gamma-amino amides. This approach stands in contrast to conventional Mannich-type reactions for the synthesis of beta-amino carbonyl compounds. o-Ns = o-nitrobenzenesulfonyl.

Amino acid-azetidine chimeras: synthesis of enantiopure 3-substituted azetidine-2-carboxylic acids

Azetidine-2-carboxylic acid (Aze) analogs possessing various heteroatomic side chains at the 3-position have been synthesized by modification of 1-9-(9-phenylfluorenyl) (PhF)-3-allyl-Aze tert-butyl ester (2S,3S)-1. 3-Allyl-Aze 1 was synthesized by regioselective allylation of alpha-tert-butyl beta-methyl N-(PhF)aspartate 13, followed by selective omega-carboxylate reduction, tosylation, and intramolecular N-alkylation. Removal of the PhF group and olefin reduction by hydrogenation followed by Fmoc protection produced nor-leucine-Aze chimera 2. Regioselective olefin hydroboration of (2S,3S)-1 produced primary alcohol 23, which was protected as a silyl ether, hydrogenated and N-protected to give 1-Fmoc-3-hydroxypropyl-Aze 26. Enantiopure (2S,3S)-3-(3-azidopropyl)-1-Fmoc-azetidine-2-carboxylic acid tert-butyl ester 3 was prepared as a Lys-Aze chimera by activation of 3-hydroxypropyl-Aze 26 as a methanesulfonate and displacement with sodium azide. Moreover, orthogonally protected azetidine dicarboxylic acid 4 was synthesized as an alpha-aminoadipate-Aze chimera by oxidation of alcohol 26. These orthogonally protected amino acid-Aze chimeras are designed to serve as tools for studying the influence of conformation on peptide activity.

Practical Asymmetric Preparation of Azetidine-2-carboxylic Acid

[reaction: see text] Facile and straightforward syntheses of both enantiomers of azetidine-2-carboxylic acid are described. The syntheses depart from inexpensive chemicals and allow for the production, in five to six steps, of practical quantities of each enantiomer. Synthetic highlights include the construction of the azetidine ring using an intramolecular alkylation and the use of optically active alpha-methylbenzylamine as chiral auxiliary.

Stereoselective synthesis of conformationally constrained ω-amino acid analogues from pyroglutamic acid

Bicyclic lactams derived from pyroglutamic acid provide a useful scaffold for synthesis of conformationally restricted analogues of lysine, ornithine and glutamine, as well as an Ala-Ala dipeptide analogue. Amino alcohol and carboxylic acid derivatives are accessible from a common intermediate. In this strategy, the bicyclic lactam system not only controls, but also facilitates the determination of the stereochemistry of the synthetic intermediates.

Azetidinic amino acids: stereocontrolled synthesis and pharmacological characterization as ligands for glutamate receptors and transporters

A set of ten azetidinic amino acids, that can be envisioned as C-4 alkyl substituted analogues of trans-2-carboxyazetidine-3-acetic acid (t-CAA) and/or conformationally constrained analogues of (R)- or (S)-glutamic acid (Glu) have been synthesized in a diastereo- and enantiomerically pure form from beta-amino alcohols through a straightforward five step sequence. The key step of this synthesis is an original anionic 4-exo-tet ring closure that forms the azetidine ring upon an intramolecular Michael addition. This reaction was proven to be reversible and to lead to a thermodynamic distribution of two diastereoisomers that were easily separated and converted in two steps into azetidinic amino acids. Azetidines 35-44 were characterized in binding studies on native ionotropic Glu receptors and in functional assays at cloned metabotropic receptors mGluR1, 2 and 4, representing group I, II and III mGlu receptors, respectively. Furthermore, azetidine analogues 35, 36, and 40 were also characterized as potential ligands at the glutamate transporter subtypes EAAT1-3 in the FLIPR Membrane Potential (FMP) assay. The (2R)-azetidines 35, 37, 39, 41 and 43 were inactive in iGlu, mGlu and EAAT assays, whereas a marked change in the pharmacological profile at the iGlu receptors was observed when a methyl group was introduced in the C-4 position, compound 36 versus t-CAA. At EAAT1-3, compound 35 was inactive, whereas azetidines 36 and 40 were both identified as inhibitors and showed selectivity for the EAAT2 subtype.

Total synthesis of polyoximic acid

The structure and stereochemistry of polyoximic acid, a degradation product of polyoxins, was originally designated as trans-3-ethylidene-L-azetidine-2-carboxylic acid. However, total synthesis revealed that the correct structure was in fact cis-3-ethylidene-L-azetidine-2-carboxylic acid, which was confirmed by X-ray crystallography. The synthesis of the trans-isomer was also done and its identity was confirmed by X-ray analysis as well. The key step for constructing the four-membered ring was a rhodium catalyzed carbenoid insertion into the N—H bond of a beta-amino acid derivative. The stereoselectivity of the exo-double bond was controlled by conducting a Horner-Emmons-Wadsworth or a Wittig reaction to generate the trans- and cis-isomers, respectively. Weinreb's amide was used as a latent methyl group for the separation of trans and cis mixtures. The double bond stereochemistry of polyoximic acid in the parent polyoxin was also confirmed to be cis by extensive 2D NMR studies.Key words: diazoinsertion, azetidine, olefination.