The 13C-{1H}-n.m.r. spectra (acidic D,O, pD -1) of N-methyl-l-deoxynojirimycin
(1,5-dideoxy-1,5-methyliminium-D-glucitol), an inhibitor of processing cx-D-glucosidases involved in glycopeptide biosynthesis, showed two isomers (-11:l ratio) differing in the stereochemistry of the N+DCH3 group. Assignments of carbon absorbances were made by lH-r3C heterocorrelation 2D n.m.r. spectroscopy, and DEPT techniques.
For the minor (axial N-methyl group) species, the chemical shifts of the ring carbons in 1,4 position to the methyl group (and of the methyl carbon atom itself) were shifted characteristically upfield relative to the absorbances for the corresponding carbon atoms in the major (equatorial Nmethyl) species. Two N-'DCZf, singlets, in an -11: 1 ratio, were also noted in the slow-exchange limit (SEL) 'H-n.m.r. spectrum recorded for a solution in acidic aqueous medium (pD -1). Vicinal proton-proton coupling constants were consistent with a chair conformation for the SEL cationic major species, and for the free base at the fast exchange limit. The 5hydroxymethyl gcauche-gauche rotamer of the cationic equatorial N-methyl diastereomcr (and of the free base) was found to be strongly preponderant [>90%] in aqueous solution.