SYNTHESIS AND CONFORMATIONAL-ANALYSIS OF MURAMIC ACID DELTA-LACTAM STRUCTURES AND THEIR 4-O-(2-ACETAMIDO-2-DEOXY-BETA-D-GLUCOPYRANOSYL) DERIVATIVES, CHARACTERISTIC OF BACTERIAL SPORE PEPTIDOGLYCAN

1,6-Anhydro-4-O-benzyl-beta-muramic acid 1',2-lactam (2) was prepared by reduction of 1,6-anhydro-2-azido-4-O-benzyl-2-deoxy-3-O-[(R)-1-methoxycarbonylethyl]-beta-D-glucopyranose (1) followed by cyclisation. Debenzylation of 2 ( --> 3) and glycosylation of HO-4 with 3,4,6-tri-O-acetyl-2-deoxy-2-phthal-imido-beta-D-glucopyranosyl chloride afforded 75% of a beta-(1 --> 4)-linked disaccharide derivative (7). Removal of the Phth group from 7, then acetylation, and O-deacetylation yielded 4-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-amino-1,6-anhydro-3-O-[(R)-1-carboxyethyl]-2-deoxy-beta-D-glucopyranose 1',2-lactam (10) Acetolysis of the 1,6-anhydro ring in the 4-acetate (4) of 3 and the 3',4',6'-triacetate (9) of 10, with saponification of the products 5 and 11, afforded 2-amino-3-O-[(R)-1-carboxyethyl]-2-deoxy-D-glucopyranose 1',2-lactam (6) and 4-O-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-2-amino-3-O-[(R)-1-carboxyethyl]-2-deoxy-beta-D-glucopyranose 1',2-lactam (12), respectively. The structure of 12 corresponds to that of the disaccharide unit characteristic of the glycan chains of bacterial spore peptidoglycan. H-1 NMR spectroscopy indicated that the beta-D-glucopyranose ring in the 1.6-anhydro 1',2-lactam derivatives adopts the B(O,3) conformation. On cleavage of the 1,6-anhydro ring by acetolysis, the D-glucopyranose ring adopts the C-4(1) conformation. X-ray analysis of 2, 4, and 5 confirmed the proposed structures. Molecular mechanics and molecular dynamics simulations were used to follow the transformation of the B(O,3) conformation of the D-glucopyranose ring via transition states to the 4C1 form.