Cyclic peptides. VI. Asymmetric hydrogenation of dehydroalanine or dehydroaminobutanoic acid residue in cyclodipeptides.

Several cyclo(-L-aminoacyl-deltaAla-) (aminoacyl = Ala, Val, Leu, Phe, Pro and Lys (epsilon-Ac)) were prepared by tosylation and successive detosylation of cyclo(-L-aminoacyl-L-Ser-), which were synthesized via the Nitecki and Fischer methods. Hydrogenation of the double bond of dehydroalanine residues in cyclodipeptides was carried out using Pd black in methanol at 1-atm pressure and room temperature. The degree of asymmetric hydrogenation was assessed by determining the amounts of L- and D-alanine by a modified Manning and Moore procedure. When L-valine was used as a chiral source, L-alanine residue with chiral induction of 98.4% was derived from cyclo(-L-Val-deltaAla-). L-Amino acids other than L-proline also were effective in inducing remarkable asymmetric hydrogenation. Hydrogenation of alpha,beta-dehydro-alpha-aminobutanoic acid residues in cyclodipeptides produced L-alpha-aminobutanoic acid residues with effective chiral induction to the same extent as observed with dehydroalanine residues. Optically pure l-alanine was prepared from cyclo(-L-Lys(epsilon-Ac)-deltaAla-) via asymmetric hydrogenation. A mechanism of chiral induction is discussed.