INFLUENCE OF REACTANT STRUCTURE AND SOLVENT ON GALACTOSIDE SYNTHESES FROM GALACTOSYL SULFONATES

The glycosidation of some 2-O-allyl- and 2-O-benzyl-D-galactopyranosyl sulfonates was studied as a function of alcohol, solvent, leaving group at C-1, and substituent groups at C-3, -4, and -6. In the 2,3,4-tri-O-benzyl series, no combinations of these parameters lead to high α-stereoselectivity, and high β-stereoselectivity was observed only in methanolyses with the trifluoromethanesulfonate leaving-group. With 2,3-di-O-benzyl-4,6-di-O-(N-phenylcarbamoyl)-D-galactopyranosyl p-toluene-sulfonate, high α-specificity (>90%) was achieved on methanolysis and, with the corresponding trifluoromethanesulfonate, high β-stereoselectivity. Results with 2-O-allyl- and 2-O-benzyl-3,4-di-O-(N-phenylcarbamoyl)-6-O-acetyl-D-galactopyranosyl sulfonates were comparable to each other. In general methane- and p-toluene-sulfonates in solvents of the ether type gave high α-stereoselectivity (>90%) with a number of alcohols of widely different structures, including saccharides. Thus, appropriately substituted D-galactopyranosyl sulfonates, reacting overnight with an equivalent amount of alcohol, formed α-glycosides in high yield and steric purity, as has previously been observed with related glucose derivatives. The nature of the substituents at C-3, C-4, and C-6 has a profound influence on the steric course of the glycosidation reaction. © 1978.