4640480866

2 A. DĄBROWSKA, W. WICZK, L. ŁANKIEWICZ

ABSTRACT

a-Amino acids are among the most important compounds in living or-ganisms. Amino acids are not only constituents of peptides and proteins but they play an important role in many reactions in living cells. There are many excellent methods of the asymmetric synthesis of oc-amino acids, but only some of them are versatile. In this work we have reviewed and presented the most versatile methods, giving the best Chemical yield and optical purity of the finał products.

One of the oldest and still useful synthetic routes to racemic a-amino acids is Strecker’s method [3], (Scheme 1). Modification of the method by Weinges et al. [8-11] (Scheme 2) allowed to obtain pure enantiomers of a-amino acids in high yields.

Next important group of the methods applies “glycine anion equivalent”. In this group, “bislactim-ether” method of Schollkopf et al. [13-21], (Scheme 3 and Tab. 1] has to be mentioned at first because of the high Chemical yield of the transformation and optical purity of the finał amino acid. The same type of methodology was employed by Seebach et al., in their method utilizing oxazoli-dinones or imidazolidinones [22-26] (Schemes 4, 5 and Tab. 2). Yield and enantiomeric excess (ee) of the finał amino acid in the method of Seebach et al., is very high (in most cases > 80%). The only inconvenience is connected with drastic conditions of finał hydrolysis. The similar, good results were also obtained in methods utilizing enolates obtained from oxazinones [29-31], (Schemes 6-8) and Tabs 3, 4).

Derivatives of the oxazinones were also applied as “ glycine cation equiva-lent” in the method in which “the chiral cation” reacts with nucleophiles [32, 33], (Scheme 9 and Tab. 5) with the high Chemical yield and ee of the finał product.

The similar procedures were used in an alkylation of a chiral Schiff base. The best yields and optical purity of the finał a-amino acid were obtained by Oppolzer et al. [35, 36], who applied a derivative of glycine attached to bor-nane-10,2-sultam (the derivative of camphor), (Schemes 13, 14).

The next group of synthetic procedures is based on a reaction of a ring opening of [ł-lactones obtained from serine [41-43], (Scheme 16) or threonine [44], and aziridines [45- 48], (Scheme 17). A nucleophile attack on the Jl-carbon of the substrate gives amino acid with modified side Chain. Despite the high yield and ee, a selection of possible nucleophiles is limitcd.

Another important methodology is called “electrophilic amination”. In this procedurę a chiral enolate obtained from N-acył-oxazolidinone [50, 52], (Scheme 18, 19) or N-acyl-sultam [51, 53], (Scheme 21) is aminated either by di-fćrt-butyl-azodicarboxylate (DBAD), [50, 51], trisyl azide [52], or 1-chloro-I-nitrozo-cyclohexane [53]. Yield of these reactions are high and ee cxceeds 95%.