N-Diphenylmethylene-Protected Glycosyl Acceptors. Selective β-O-Glycosylation to Form Lactosyl-threo-Ceramides

A series of N-diphenylmethylene-protected sphingosine derivatives was synthesized (e.g 3a,b and 9a,b). These compounds are efficient glycosyl acceptors and were shown to undergo highly β-selective glycosylation using a modification of the Koenigs-Knorr reaction previously used in this laboratory for the synthesis of O-linked glycopeptides (none of the α-anomers were detected by either ¹H or ¹³C NMR).¹ The N-diphenylmethylene (Schiff base) protection imparts a favorable intramolecular hydrogen-bonding pattern (Ph2CN:→HO:). This intramolecular hydrogen-bonding enhances the nucleophilicity of the glycosyl acceptor relative to glycosyl acceptors with more conventional N-protection (i.e. Cbz, Boc, acyl etc.). The enhanced nucleophilicity allowed the glycosylation to be carried out under mild conditions (AgOTfl, CH₂Cl₂, rt overnight), and provided the corresponding β-glycosphingolipids 4a–c and 11a–d in excellent chemical yield (approximately 70%). After glycosylation, selective acid-catalyzed hydrolysis of the Schiff base protecting group was accomplished without cleavage of the glycosidic bond or the carbohydrate acetate protection. N-Acylation with palmitoyl chloride, followed by Zemplèn deacetylation (cat. NaOMe in MeOH), provided the two threo-β-lactosylceramide analogues 7a and 7b. These analogues possess the unnatural threo-configuration in the ceramide moiety and may prove useful in studies of the biosynthesis and cell surface composition of more complex glycosphingolipids.