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 cc-D-diol2 (Scheme 2) gave regioselectively the 1,3-linked disaccharides 11 and 12 (80%, c( -D /~-o 9: I), whereas the analogous reaction with the ,8-o-anomer 8 led to a mixture of the anomeric 1,3-and 1,Clinked 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 a -D-mono-alcohol3 did not react with 1 under otherwise identical conditions, and that thep-D-alcohol7 yielded predominantly theg-D-glucoside 18 (52%) besides 14% of 17. Similarly as for the glycosidation of the diol2, 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 a-D-configurated 1,3-linked disaccharide 19 (41 %),besides its anomer 20 (16%), and some 4-substituted/3-~-glucoside21(9%). No 1.6-linked disaccharides could be detected. In agreement with the observed reactivity, the 'H-NMR and IR spectra reveal a strong H-bond between HO-C(3) and the phthalimido group in the a -D-, but not in the -o-allosides. The different H-bonds in the anomeric phthalimides are in keeping with the results of molecular-mechanics calculations.Introduction. -The regioselectivity in the glycosidation of diols and triols by diazirine-derived glycosylidene carbenes is determined both by the protonation of the carbene and the interception of the ensuing oxycarbenium ion by an oxy anion or a OH group ([ 11 [2]; for reviews, see . Both processes are stereoelectronically controlled, protonation occurring in the a-plane of the carbene and nucleophilic attack in the n-plane of the oxycarbenium cation.The regioselectivity of the deprotonation of a diol or triol by the carbene is determined by the relative kinetic acidity of the individual OH groups, which depends mainly upon intramolecular H-bonds'). The regioselectivity of the C-0 bond formation depends upon the position of the carbene/oxycarbenium cation relative to the diol or triol unit [7-lo]. As illustrated in Fig.1, the carbene may be close in space to only one (the protonating) OH group, or close to two of them; in the first case, the regioselectivity of the deprotonation determines the regioselectivity of the C-0 bond formation, in the
