Concise asymmetric synthesis of beta-hydroxy alpha-amino acids using the sulfinimine-mediated asymmetric Strecker synthesis: phenylserine and beta-hydroxyleucine

Thirtieth Anniversary of the Discovery of Laxaphycins. Intriguing Peptides Keeping a Part of Their Mystery

Lipopeptides are a class of compounds generally produced by microorganisms through hybrid biosynthetic pathways involving non-ribosomal peptide synthase and a polyketyl synthase. Cyanobacterial-produced laxaphycins are examples of this family of compounds that have expanded over the past three decades. These compounds benefit from technological advances helping in their synthesis and characterization, as well as in deciphering their biosynthesis. The present article attempts to summarize most of the articles that have been published on laxaphycins. The current knowledge on the ecological role of these complex sets of compounds will also be examined.

A Unified Strategy for the Asymmetric Synthesis of Highly Substituted 1,2-Amino Alcohols Leading to Highly Substituted Bisoxazoline Ligands

A general procedure for the asymmetric synthesis of highly substituted 1,2-amino alcohols in high yield and diastereoselectivity is described that uses organometallic additions of a wide range of nucleophiles to tert-butylsulfinimines as the key step. The addition of organolithium reagents to these imines follows a modified Davis model. The diastereoselectivity for this reaction depends significantly on both the nucleophile and electrophile. These highly substituted 1,2-amino alcohols are used to synthesize stereochemically diverse and structurally novel, polysubstituted 2,2'-methylene(bisoxazoline) ligands in high yields.

Approaches to Obtaining Fluorinated α-Amino Acids

Fluorine does not belong to the pool of chemical elements that nature uses to build organic matter. However, chemists have exploited the unique properties of fluorine and produced countless fluoro-organic compounds without which our everyday lives would be unimaginable. The incorporation of fluorine into amino acids established a completely new class of amino acids and their properties, and those of the biopolymers constructed from them are extremely interesting. Increasing interest in this class of amino acids caused the demand for robust and stereoselective synthetic protocols that enable straightforward access to these building blocks. Herein, we present a comprehensive account of the literature in this field going back to 1995. We place special emphasis on a particular fluorination strategy. The four main sections describe fluorinated versions of alkyl, cyclic, aromatic amino acids, and also nickel-complexes to access them. We progress by one carbon unit increments. Special cases of amino acids for which there is no natural counterpart are described at the end of each section. Synthetic access to each of the amino acids is summarized in form of a table at the end of this article with the aim to make the information easily accessible to the reader.

Diastereoselective Synthesis of Unnatural Amino Acids by Alkylation of α- tert-Butanesulfinamide Auxiliary-Bound Enolates

A new chiral auxiliary for the diastereoselective alkylation of amino ester enolates that takes advantage of chiral information stored on the enolate side of the amino ester substrate has been developed. Chiral α-sulfinamido esters were alkylated under basic conditions in good yields (up to 90%) and good to high diastereoselectivities (generally >6:1) to provide unnatural mono- and α,α-disubstituted amino acid derivatives. This auxiliary allowed for the ready conversion of ester functionality without the need for esoteric reagents. Furthermore, the auxiliary is easily removed to provide enantiopure amino acids. Computational studies revealed that a chelated transition state governs electrophile addition from the convex face of a transient bicyclic intermediate. This method allows ready access to enantioenriched natural and unnatural amino acids.

Stereocontrolled synthesis of syn-β-Hydroxy-α-amino acids by direct aldolization of pseudoephenamine glycinamide

β-Hydroxy-α-amino acids figure prominently as chiral building blocks in chemical synthesis and serve as precursors to numerous important medicines. Reported herein is a method for the synthesis of β-hydroxy-α-amino acid derivatives by aldolization of pseudoephenamine glycinamide, which can be prepared from pseudoephenamine in a one-flask protocol. Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide in the presence of LiCl followed by addition of an aldehyde or ketone substrate affords aldol addition products that are stereochemically homologous with L- or D-threonine, respectively. These products, which are typically solids, can be obtained in stereoisomerically pure form in yields of 55-98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction with sodium borohydride. This new chemistry greatly facilitates the construction of novel antibiotics of several different classes.

Asymmetric synthesis of β-hydroxy-α-amino phosphonic acid derivatives via organocatalytic direct aldol reaction of α-isothiocyanato phosphonates with aldehydes

α-Isothiocyanato phosphonates are first used as nucleophiles to react with aldehydes for the asymmetric synthesis of β-hydroxy-α-amino phosphonic acid derivatives. The process is catalyzed by a quinine-derived thiourea via cascade aldol/cyclization reaction, affording a wide range of protected β-hydroxy-α-amino phosphonates containing adjacent quaternary-tertiary stereocenters in up to 93% yield, up to 81% ee, and >99:1 dr. This work represents the first example of α-isothiocyanato phosphonates serving as nucleophiles that are used in the catalytic asymmetric synthesis.

Expedient synthesis of threo-beta-hydroxy-alpha-amino acid derivatives: phenylalanine, tyrosine, histidine, and tryptophan

An expedient synthesis of enantiomerically pure threo-beta-hydroxy-alpha-amino acid derivatives of phenylalanine, tyrosine, histidine, and tryptophan is described. The NBS-mediated radical bromination of the N,N-di-tert-butoxycarbonyl protected alpha-amino acids and subsequent treatment with silver nitrate in acetone provided the trans-oxazolidinones predominantly. Cesium carbonate catalyzed hydrolysis then generated the beta-hydroxy amino acid derivatives in excellent overall yield.

Stereoselective synthesis of beta-hydroxyphenylalanines using imino 1,2-Wittig rearrangement of hydroximates

The imino 1,2-Wittig rearrangement of hydroximates containing a furan ring provides a novel method for the synthesis of beta-hydroxy-alpha-amino acids. Upon treatment with LDA, hydroximates smoothly underwent the rearrangement to give Z-2-hydroxyoxime ethers in good yield, which were converted into both cis- and trans-oxazolidinones with high stereoselectivity. The cis- and trans-oxazolidinones were stereoselectively converted into erythro- and threo-beta-hydroxyphenylalanines, respectively, via the oxidative cleavage of a furan ring, ring-opening of oxazolidinone, and deprotection.

Synthesis of Enantiopure Sulfinimines (Thiooxime S-Oxides) Catalyzed by Yb(OTf)3 from p-Toluenesulfinamide and Aldehydes in Mild Reaction Conditions

Enantiomerically pure sulfinimines as important building blocks in the asymmetric synthesis of amine derivatives are prepared in good to excellent yields from chiral p-toluenesulfinamide with aromatic, heteroaromatic, and aliphatic aldehydes. The unprecedented feature of the reported procedure is that the formation of the sulfinimines was achieved by the catalytic action of Yb(OTf)(3) in THF at room temperature. The reaction conditions were also applicable to Ellman's sulfinimines.

The direct organocatalytic asymmetric mannich reaction: unmodified aldehydes as nucleophiles

The unprecedented application of unmodified aldehydes as nucleophilic donors in direct catalytic asymmetric Mannich-type reactions is disclosed in a full account. Our efforts in broadening the applicability of chiral pyrrolidine-based catalysts in direct asymmetric Mannich-type reactions led to the highly diastereo- and enantioselective and concise synthesis of functionalized alpha- and beta-amino acids, beta-lactams, and amino alcohols.

Highly diastereoselective strecker reaction of enolizable aliphatic sulfinimines

The reaction of chiral sulfinimines 1c-g derived from aliphatic aldehydes with TMSCN in the presence of CsF gave alpha-amino nitriles in high diastereoselectivity and yield. alpha,beta-Diamino acid derivatives were also obtained in high diastereoselectivity from the reaction of 2-aziridinesulfinimines 1h and 1i followed by ring-opening of the products with thiophenol. The presence of hydrogen at the alpha-position of the C=N double bond is crucial in this TMSCN addition reaction.

Photo aldol reactions with 5-methoxyoxazoles: Highly regio- and diastereoselective synthesis of α-amino β-hydroxy carboxylic acid derivatives

A versatile route to derivatives of α-amino β-hydroxy carboxylic acids, either with tertiary (from a corresponding glycine equivalent) or a quaternary α-carbon center is described. The key reaction is the cycloaddition of electronically excited carbonyl compounds (aromatic and aliphatic aldehydes, respectively) to oxazoles. To make the products hydrolytically more labile, a methoxy substituent at position C-5 was introduced leading to the formation of cycloadducts with an orthoester substructure. The photocycloaddition, either with triplet excited (aromatic) carbonyls or with singlet excited (aliphatic) carbonyls, led to the formation of mixtures of endo- and exo-diastereoisomers with moderate to very high exo-selectivities. The 4-unsubstituted 5-methoxyoxazole 1 (glycine equivalent) gave the [2 + 2]-adducts 3a–3f with aldehydes 2a–2f in high yields and excellent diastereoselectivities. Hydrolysis of these compounds resulted in the α-amino β-hydroxy esters 4a–4f with preferred erythro (S*,S*) configuration. As an extension of this process, 4-alkylated 5-methoxyoxazoles 5a–5f were applied as alkene components and the corresponding cycloadducts with benzaldehyde 6a–6f were obtained. Again, the exo-diastereoisomers were formed as the major products in diastereoselectivities from 73:27 (exo:endo) up to >95:5. Hydrolysis of these adducts resulted in the formation of α-amino β-hydroxy esters 7a–7f with like (S*,S*) configuration of the major diastereoisomers.Key words: photo aldol reaction, oxazoles, oxetanes, photocycloaddition, amino acids, hydroxy acids.

Diastereoselective aldolization of alpha-aminonitriles. Diastereoselective synthesis of beta-amino alcohols and beta,gamma-diamino alcohols

Aldolization performed by addition of lithiated N-benzyl-N-tert-butylaminoacetonitrile to aldehydes provides diastereomerically pure anti-beta-hydroxy-alpha-aminonitriles. They are transformed into syn,anti-protected beta,gamma-diamino alcohols by a two-step procedure, involving addition of a Grignard reagent and reduction. The cleavage of the N-tert-butyl group is achieved by a simple acidic treatment. The application of this methodology to chiral, nonracemic aldehydes is studied. Starting from D-isopropylideneglyceraldehyde, an anti, anti, syn, anti-(2R,3S,4S,5R,6R)-diaminotriol is prepared in acceptable yield and with a good level of diastereoselectivity.

Asymmetric synthesis of polyhydroxy alpha-amino acids with the sulfinimine-mediated asymmetric strecker reaction: 2-amino 2-deoxy L-xylono-1,5-lactone (polyoxamic acid lactone)

Polyhydroxylated sulfinimines derived from protected 1,2-O-isopropyliden-L-threoses undergo the sulfinimine-mediated Strecker syntheses to give alpha-amino nitriles in good yield and de. A double stereodifferentiation effect was not observed and the diastereoselectivity is controlled by the absolute configuration of the sulfinyl group. Hydrolysis of the amino nitriles afforded the lactone rather than polyoxamic acid.

A facile synthesis of enantiopure 2-aziridinesulfinimines and their highly diastereoselective reactions with phosphite anions

Two novel enantiopure 1-benzyl-2-aziridinesulfinimines bearing a chiral group on both sides of the carbon-nitrogen double bond were synthesized from the condensation of racemic 1-benzyl-2-aziridinecarboxaldehyde and enantiopure p-toluenesulfinamide. The addition reaction with phosphite anion followed by the ring-opening reaction with thiophenol provided chiral alpha,beta-diamino-phosphonic acid derivatives. The addition reactions showed the operation of a double-stereodifferentiation effect. The possible transition states of the reaction were proposed, and high diastereoselectivities of the addition reactions of phosphite anions to both aziridinesulfinimines were realized in the presence or absence of zinc bromide.