Enantiomerically Pure Tetrahydropyrimidinones in Asymmetric Synthesis: Preparation of a Protected α-Methylasparagine Derivative and Corresponding Dipeptides

Methyl ester 5a, available in enantiomerically pure form from the amino acid asparagine via a one-pot cyclization/protection sequence, followed by esterification, can be effectively deprotonated with LDA/DMPU/LiCl. Treatment with MeI affords the corresponding alkylated adduct in enantiomerically pure form, from which α-methylaspartic acid is obtained. Variation of the amine protection group allows for the isolation of a protected carboxylic acid/free amine derivative of α-methylasparagine. The utility of H-MeAsn-OMe is demonstrated in the formation of dipeptides.