Comparison of stereospecific conformations for threo and erythro beta-hydroxy-N-acetyl-tryptophanamide

Different isomers of beta-hydroxy-N-acetyl-tryptophanamide are obtained depending upon the preparative route. The threo (from enzymatic oxidation) and erythro (from synthetic reduction) forms are distinguished further by their conformation, particularly by the severely restricted rotation about the C beta-C gamma bond, which appears to be independent of the presence of the hydroxyl group on C beta. The torsional angle X21 about the C beta-C gamma bond is + 112 degrees for the threo molecule and -3 degrees for the erythro molecule. Threo-beta-hydroxy-N-acetyl-L-tryptophanamide (C13H15N3O3) crystallizes in space group P21 with cell parameters a = 13.572(4) A, b = 6.296(1) A, c = 13.796(4) A and beta = 148.10(2) degrees. Erythro-beta-hydroxy-N-acetyl-DL-tryptophanamide (C13H15N3O3.1/2C6H6) crystallizes in space group P21/c with cell parameters a = 23.178(15) A, b = 7.202(3) A, c = 9.218(4) A and beta = 91.13(5) degrees.