The synthesis of all four diastereoisomers of alpha,beta-dimethylphenylalanine (4) as well as those of alpha,beta-dimethyl-1 ,2,3,4,-tetrahydroisoquinoline-3-carboxylic acid (5 and 6) have been accomplished in high yield and high optical purity. Molecular mechanics calculations on the N-alpha-acetyl and N- methylcarboxamide derivatives of (3R,4R)-6 and (3R,4S)-5 indicate large and moderate energy stabilization for the gauche(-) but not the gauche(+) side-chain conformers of (3R,4S)-5 and (3R,4R)-6, respectively. By symmetry rules, the same holds for (3S,4R)-5 and (3S,4S)-6, respectively. Thus, these amino acids are potential building blocks for the topographical design of peptides (Kazmierski et al., J. Am. Chem. Sec. 1991, 113, 2275-2283) by providing acylated 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid derivatives in which a gauche(-) and not a gauche(+) side-chain conformation is energetically more stable for the L amino acid. Synthetic details and implications of these new amino acids for peptide and protein design are discussed.
