GLYCOSYLIDENE CARBENES .16. GLYCOSIDATION OF METHYL 6-O-TRITYL-ALPHA-D-ALTROPYRANOSIDE

Hydrogen bonding of the triol 4 in chlorinated solvents was studied by IR (CH2Cl2 and CCl4) and H-1-NMR spectroscopy (CDCl3), and the regioselectivity of the glycosidation of the triol 4 by the diazirine 1 is predicted on the basis of two assumptions: preferred protonation of the intermediate glycosylidene carbene by the OH group involved in the weakest intramolecular H-bond, and attack in the pi-plane of the thereby generated oxycarbenium cation either by the reoriented oxy anion, or by a properly oriented vicinal OH group. Glycosidation led to the disaccharides 5-10 (Scheme) which were separated and characterized as their acetates 11-16, to the lactone azines 17 and to the 2(benzyloxy)glucal 18. In agreement with the predictions, glycosidation in non-coordinating solvents gave the 1,2-, 1,3-, and 1,4-linked disaccharides in decreasing relative amounts. Glycosidation in THF proceeded with a lower degree of regioselectivity and led preferentially to the beta-D-anomers, except for the minor, 1,4-linked disaccharides, where THF had only a weak influence on stereoselectivity at room temperature and led to a slight increase of the alpha-D-anomer at -80 degrees.