Y-Dehydro-a-amino acid

The first family is represented by peptides based on a, 3-dehydro a-amino adds in which a C=C bond confers rigidity and electronic Y-conjugation on the system (Section 11.1). Peptides rich in these amino acids are widespread in nature, particularly as antibiotics, and possess interesting conformational properties. 1-11 In addition to the E and Z configurational isomers, occurring when two different atoms (substituents) on CP are present, fully-extended, characteristically flat conformations or p-tums and 310/a-helices may be induced by u,p-dehydro a-amino acids. 

Side-chain brominated amino acid derivatives have been exploited in stereo-controlled syntheses of dehydro, cyclopropyl and hydroxy amino acids [39-41], The biochemical hydroxylation of amino acids often involves radical oxidation, where the regioselectivity is analogous to that of bromination. Consequently bromination followed by hydrolysis provides a convenient method for synthesis of the natural products [41], Alternatively, dimethyldioxirane has been used for the direct synthesis of y-hydroxyleucine derivatives through oxygen atom insertion into the yC-H bond [42],... 

Ranu et al. have developed a mild and efficient method for the synthesis of a-dehydro-p-amino esters and nitriles by nucleophilic addition of amines to MBH acetates in water at room temperature without using any basic, acidic or metal catalyst. The reactions yielded only y-addition products, providing, with high stereoselectively, ( )-isomers in the case of MBH adducts bearing a carboxylic ester moiety and (Z)-isomers for adducts containing CN functionality (Scheme 3.123). An interesting stereoselective transformation of MBH adducts into cinna-mylamines via treatment with DMF-DMA has been described by Kim and coworkers (Scheme 3.124). ... 

Chromic acid in acetone converted lycocernuine to dehydrolyco-cernuine 1700,1635,1410 cm i) which was nonbasic but which was believed to be an aminoketone rather than a y-lactam. The evidence for the -amino ketone structure was based on UV- and ORD-data and the fact that the compound was readily reduced by borohydride to lycocernuine (J02). In the NMR-spectrum of the dehydro compound the low field quartet of lycocernuine was shifted to 6.09 8 and a new signal appeared at 3.72 S (1 H) which was attributed to a methine hydrogen situated between nitrogen and the carbonyl group (—CO—CH—NC ). 

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