GLYCOSYLIDENE CARBENES .19. REGIOSELECTIVE GLYCOSIDATION OF ALLYL 2-DEOXY-2-PHTHALIMIDO-D-ALLOPYRANOSIDES

The regio- and stereoselectivity of the glycosidation of the partially protected mono-alcohols 3 and 7, the diols 2 and 8, and the triol 4 by the diazirine 1 have been investigated. Glycosidation of the alpha-D-diol 2 (Scheme 2) gave regioselectively the 1,3-linked disaccharides 11 and 12 (80%, alpha-D/beta-D 9:1), whereas the analogous reaction with the beta-D-anomer 8 led to a mixture of the anomeric 1,3- and 1,4-linked disaccharides 13 (12.5%), 14 (16%), 15 (13%), and 16 (20.5%; Table 2). Protonation of the carbene by OH-C(4) of 2 is evidenced by the observation that the alpha-D-mono-alcohol 3 did not react with 1 under otherwise identical conditions, and that the beta-D-alcohol 7 yielded predominantly the beta-D-glucoside 18 (52%) besides 14% of 17. Similarly as for the glycosidation of the diol 2, the influence of the H-bond of HO-C(4) on the direction of approach of the carbene, the role of HO-C(4) in protonating the carbene, and the stereoelectronic control in the interception of the ensuing oxycarbenium cation are evidenced by the reaction of the triol 4 with 1 (Scheme 3), leading mostly to the alpha-D-configurated 1,3-linked disaccharide 19 (41%), besides its anomer 20 (16%), and some 4-substituted beta-D-glucoside 21 (9%). No 1,6-linked disaccharides could be detected. In agreement with the observed reactivity, the H-1-NMR and IR spectra reveal a strong H-bond between HO-C(3) and the phthalimido group in the alpha-D-, but not in the beta-D-allosides. The different H-bonds in the anomeric phthalimides are in keeping with the results of molecular-mechanics calculations.