CYCLIC PEPTIDES

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Abstract
Several cyclo (‐l‐aminoacyl‐ΔAla‐) (aminoacyl = Ala, Val, Leu, Phe, Pro and Lys (∍‐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‐ΔAla‐). l‐Amino acids other than l‐proline also were effective in inducing remarkable asymmetric hydrogenation. Hydrogenation of α, β‐dehydro‐α‐aminobutanoic acid residues in cyclodipeptides produced l‐α‐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(∍‐Ac)‐ΔAla‐) via asymmetric hydrogenation. A mechanism of chiral induction is discussed.