Derivatives of 2-amino-2-deoxy-D-gulose: synthetic and conformational studies
Methyl 2-benzamido-3,6-di-O-benzoyl-2-deoxy-4-O-methylsulphonyl-α-D-glucopyranoside (1) underwent two inversion reactions when treated with sodium methoxide to give an oxazoline derivative, methyl 2,3-dideoxy-3,2-(2-phenyl-1-oxa-3-azaprop-2-eno)-α-D-gulopyranoside (4) in high yield. The reaction proceeded via the galacto-3,4-epoxide (3) which underwent ring-opening with participation of the amide carbonyl group to give the gulo-oxazoline. Acid-catalysed opening of the oxazoline ring afforded, after N-acetylation, methyl 2-acetamido-3-O-benzoyl-2-deoxy-α-D-gulopyranoside (12), which was then de-O-benzoylated to give methyl 2-acetamido-2-deoxy-α-D-gulopyranoside (14).
The 1H n.m.r. data of various O-substituted derivatives of the oxazoline showed that they adopted non-chair conformations. As would be expected the 2-acylamino-2-deoxygulopyranosides obtained by cleavage of the oxazoline ring existed in the C14 conformation.