A-D-amino acids

The products of the conjugate addition of (f )-4-phenyl-2-oxazolidinone to nitroalkenes are converted into D-a-amino acids with high enantiomeric purity (Eq. 4.30).36... 

Fig. 9.6 Mirror image behaviour of enantiomeric molecules. The left-handed L-a-amino acid is converted to the right-handed D-a-amino acid by reflection... 

Addition of various organometalic reagents to chiral nitrones, derived from L-erythrulose, proceeds with variable diastereoselectivity, depending on Lewis acids as additives. ZnBr2 facilitates the attack at the Si face of the C=N bond, whereas Et2AlCl makes the attack at the Re face more preferable. The obtained adducts can be transformed into derivatives of /V - h y d r o x y - u. u - d i s u b s t i t u t e d -a-amino acids, with their further conversion into a,a-disubstituted a-amino acids (193, 202). 

Naturally air-fused DKPs are thermodynamically more stable, compared with their trawr-fused counterparts. This seems logical considering their biosynthetic origin, usually from two proteinogenic L-a-amino acids. Some other cis- and rntwr-functional DKPs are derived from nonproteinogenic D-a-amino acids. Naturally... 

Azumamide E is a cyclotetrapeptide natural product isolated from marine sponge Mycale izuensis. Like most cyclopeptide HDAC inhibitors, these compounds contain D-a-amino acids (D-Phe, D-Tyr, D-Ala, D-Val) which relieve ring strain (Fig. 7). However, they are unique from other natural cyclopeptide HDAC inhibitors with a retroenantio arrangement. 

Bender, D. A., Amino Acid Metabolism. New York John Wiley and Sons, 1985. 

In Sect. 2.3 we showed that when glycine crystals are grown at the air/aqueous solution interface in the presence of DL-a-amino acids, only one of its enan-tiotopic faces, e.g. (010), is exposed to the solution and so it picks up (together with glycine) only the D-a-amino acids, thus converting the centrosymmetric host glycine into chiral mixed crystals. By symmetry, crystals exposing their (0-10) face towards the solution occlude only the L-enantiomers, Scheme 5. 

D-Asparagine (D-a -amino acid) From I-Asparagine racemization Sweet... 

Boesten, W.H.J. and Cals, M.J.H. 1987. Process for the enzymatic hydrolysis of d-a-amino-acid amides. US Patent 4 705-52. 

D-a-Amino acids. Rapoport et al. have developed a general synthesis of unnatural D-a-amino acids with L-serine as the chiral eduet. The method involves aminoacylation... 

Figure 22-6. The hydrogen bond association of the L-diamide phase in its antiparaUel P-sheet conformation with (A) N-TFA-L-a-amino acid alkyl ester and (B) A-TFA-d-a-amino acid alkyl ester. (Reprinted from reference 35, with permission.)...

If a mixture of L- and D-alanine were caused to polymerize, nearly all the polymer molecules would have different structures because their sequences of D-alanine and L-alanine monomer units would differ. To create polymers with definite structures for particular roles, there is only one recourse to build all polypeptides from one of the optical isomers so that the properties will be reproducible from molecule to molecule. Nearly all naturally occurring a-amino acids are of the L form, and most earthly organisms have no use for D-a-amino acids in making polypeptides. Terrestrial life could presumably have begun equally well using mainly D-amino acids (all biomolecules would be mirror images of their present forms). The mechanism by which the established preference was initially selected is not known. 

Figure 19-26. Production of L-and D-a-amino acids by kinetic resolution of a-amino acids amides (DSM).

The L-a-amino acid moiety of ACM is not absolutely required, since other aminocyclohexadienes with a D-a-amino acid or without functionality in the side chain are also efficient inhibitors. In all cases, the rate of inactivation is higher with the cis than with the trans isomers." Very similar results were obtained with the M. tuberculosis enzyme that was inhibited by ACM and another analog (14) irreversibly, with the kinetic... 

D)-a-amino-acids (549) (Scheme 43). An alternative route from... 

D-a amino acid i Figure 1.3 Structures of a-amino acids the monomeric building blocks of proteins. 

Alkylation of protected glycine derivatives is one method of a-amino acid synthesis (75). Asymmetric synthesis of a D-a-amino acid from a protected glycine derivative by using a phase-transfer catalyst derived from the cinchona alkaloids (8) has been reported (76). 

Bender, D. A. Amino Acid Metabolism, 2nd ed. New York John Wiley, 1985. [A general treatment of the topic, with a particularly good section on tryptophan metabolism.]... 

A process for the production of D-a-amino acids has been developed by Roche Diagnostics based on the enzyme D-hydantoinase [115]. The recombinant protein was covalently fixed onto a carrier and used for the synthesis of a broad range of natural and artificial D-amino acids (132, Scheme 41). Starting from racemic hydantoins d/l-130, the enzyme exclusively hydrolyzed d-130 to 131, and new d-130 was internally produced by continuous in situ racemization of l-130. The process worked especially well with 5-(p-hydroxyphenyl)- and 6-phenylhydantoin, affording the corresponding amino acids 132 in high yield and optical purity. The number of reuse cycles until 50% of the initial enzyme activity was reached was calculated to be as high as - 200. Unfortunately, this process has never been used in the production of D-amino acids, as diazotation was found to be too noxious and complicated. 

Figure 14. Negative or positive chirality arising from the interaction of the pyrrolinone-type chromophore with the carboxyl moiety of L-a-amino acid (or ester) or D-a-amino acid (or ester) (51 or 52, respectively). (From Toome and Wegrzynski, 1978. Reproduced with permission of Academic Press, Inc.)...

Poly(L-a-amino acids) generally form right-handed helices, whereas poly(D-a-amino acids) form left-handed helices. An exception is poly()8-benzyl-L-aspartate), which occurs as left-handed helices. The helical structure is retained in solvents such as dioxan or dimethyl formamide. But in dichloroacetic acid or hydrazine, coiled molecules are present. 

Proteins are naturally occurring copolymers of high complexity. They consist predominantly of a-amino acids, and, to a slight extent, of imino acids. The a-amino acids are mostly present in the L configuration, but some proteins also contain up to 15% D-a-amino acids. The a-amino acids commonly occurring in proteins are collected in Table 30-1. Besides these, about 200 uncommon a-amino acids are known, that is, those that are not components of proteins and do not take part in the usual metabolic processes. 

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