SYNTHESIS OF RACEMIC ALPHA-AMINO CARBOXAMIDES VIA LEWIS ACID-MEDIATED REACTIONS OF ALPHA-METHOXYGLYCINAMIDE DERIVATIVES WITH ALLYLSILANES - ENZYMATIC RESOLUTION TO OPTICALLY-ACTIVE ALPHA-AMINO-ACIDS

A short and expedient synthetic route to optically active, saturated and gamma,delta-unsaturated alpha-amino acids is reported. The key step is a BF3.OEt2-mediated reaction of allylsilanes with N-(alkoxycarbonyl)-alpha-methoxyglycinamides 11-15, leading to the corresponding gamma,delta-unsaturated alpha-aminocarboxamides. The genuine S(N)1-character of this process with iminium ion 6 as intermediate is proven in the case of the glycine ester 10. Thus, reaction of enzymatically resolved 10 with pi-nucleophiles leads to racemic products. The most useful iminium precursors are the N-methoxyamides 12-14 providing good yields of coupling products. The most convenient N-protective group is the allyloxycarbonyl group. Deprotection proceeds via a Pd(0)-catalyzed transprotection to the corresponding BOC-protected analogues. Four examples of the enzymatic resolution of alpha-amino carboxamides, by using an L-specific aminopeptidase from Pseudomonas putida, are described in detail. Most notably, secondary N-methoxyamides are good substrates for the enzyme to provide the desired a-amino acids in high optical purity.