Synthesis of peptides containing 1,2,3,4-tetrahydroquinoline-2-carboxylic acid. Part 2. Crystal and molecular structure of a 1H,3H,5H-oxazolo[3,4-a]quinolin-3-one derivative obtained from a linear tripeptide

Abstract

Treatment of N-benzyloxycarbonyl-(S)-alanyl-(S)-phenylalanyl-(R)-1,2,3,4-tetrahydroquinoline-2-carboxylic acid (1a) with acetic anhydride–sodium acetate gives a cyclization product (4a) containing the 1H,3H,5H-oxazolo[3,4-a]quinoline system. The structure of the bromo-derivative (4b) was examined by crystallographic and spectroscopic methods. Crystals are monoclinic, space group P2₁, a= 9.835, b= 26.018, c= 10.856 Å, β= 93.78°, Z= 4. The structure has been refined to R 0.08 for 3 615 reflections. The two molecules found in the asymmetric unit assume slight different conformations. Some spectroscopic data for the new peptide are described in relation to the conformations found in the crystals. The mechanism of the cyclization is discussed and related to the Dakin–West reaction.