The alpha-D-allo-diol 9 possesses an intramolecular H-bond (HO-C(3) to O-C(1)) in solution and in the solid state (Fig. 2). In solution, it exists as a mixture of the tautomers 9a and 9b (Fig. 3), which possess a bifurcated H-bond, connecting HO-C(2) with both O-C(I) and O-C(3). In addition, 9a possesses the same intramolecular H-bond as in the solid state, while 9b is characterized by an intramolecular H-bond between HO-C(3) and O-C(4). In solution, the beta-D-anomer it is also a mixture of tautomers, 12a and presumably a dimer. The H-bonding in 9 and 12 is evidenced by their IR and H-1-NMR spectra and by a comparison with those of 3-8, 10, and 11. The expected regioselectivity of glycosidation of 9 and 12 by the diazirine 1 or the trichloroacetimidate 2 is discussed on the basis of the relative degree of acidity/nucleophilicity of individual OH groups, as governed by H-bonding. Additional factors determining the regioselectivity of glycosidation by 1 are the direction of carbene approach/proton transfer by H-bonded OH groups, and the stereoelectronic control of both the proton transfer to the alkoxy-alkyl carbene (in the sigma-plane) and the combination of the thereby formed ions (pi-plane of the oxycarbenium ion). Glycosidation of 9 by the diazirine 1 or the trichloroacetimidate 2 proceeded in good yields (75-94%) and with high regioselectivity. Glycosidation of 9 and 12 by 1 or 2 gave mixtures of the disaccharides 14-17 and 18-21, respectively (Scheme 2). As expected, glycosidation of 12 by 1 or by 2 gave a nearly 1:1 mixture of regioisomers and a slight preference for the beta-D-anomers (Table 4). Glycosidation of the alpha-D-anomer 9 gave mostly the 1,3-linked disaccharides 16 and 17 (alpha-D > beta-D) along with the 1,2-linked disaccharides 14 and 15 (alpha-D < beta-D, 1,2-/1,3-linked glycosides ca. 1:4), except in THF and at low temperature, where the beta-D-configurated 1,2-linked disaccharride 15 is predominantly formed. Similarly, glycosidation of 9 with 2 yielded mainly the 1,3-linked disaccharides (1,2-/1,3-linked products ca. 1:3 and alpha-D/beta-D ca. 1:4). Yields and selectivity depend upon the solvent and the temperature. The regioselectivity and the unexpected stereoselectivity of the glycosidation of 9 by 1 evidences the combined effect of the above mentioned factors, which also explain the lack of regio-complementarity in the glycosidation of 9 by 1 and by 2 (Scheme 3). THF solvates the intermediate oxycarbenium ion, as evidenced by the strong influence of this solvent on the regio- and stereoselectivity, particularly at low temperatures, where kinetic control leads to a stereoelectronically preferred axial attack of THF on the oxycarbenium ion.

The relation between H-bonding in diequatorial trans-1,2 and axial,equatorial cis-1,2-diols and the regioselectivity of glycosidation by the diazirine 1 was examined. H-Bonds were assigned on the basis of FT-IR and H-1-NMR spectra (Fig. 1). Glycosidation by 1 of the gluco-configurated diequatorial trans-2,3-diols 4-7 yielded the mono-glucosylated products 16/17/20/21 (69-89%); 1,2-/1,3-linked products 37-46:63-54), 24/25/28/29 (60-63% 1,2-/1,3-linked products 46-51:54-49), 32-35 (69-94%; 1,2-/1,3-linked products 45-52:55-48), and 36/37/ 40/41 (59-63%; 1,2-/1,3-linked products 52-59:48-41), respectively (Scheme 1, Table 3). The disaccharides derived from 4, 5, and 7 were characterized as their acetates 18/19/22/23, 26/27/30/31, and 38/39/42/43, respectively. Glycosidation of the galacto-configurated diequatorial 2,3-diols 8 and 9 and the manno-configurated diequatorial 3,4-diol 10 by 1 (Scheme 2, Table 3) also proceeded in fair yields to give the disaccharides 44-47 (69-80%; 1,2-/1,3-linked products ca. 1:1), 48-51 (51-61%; 1,2/-1,3-linked products 54-56:56-54), and 56/57/60/61 (71-80%; 1,3-/1,4-linked products 49-54:51-46), respectively. The 1,3-linked disaccharides 56/57 derived from the diol 10 were characterized as the acetates 58/59. The regio- and stereoselectivities of the glycosidation by 1 were much better for the alpha-D-manna-configurated axial,equatorial cis-2,3-diol 11 and the galacto-configurated axial,equatorial cis-3,4-diol 13 (1,2-/1,3-linked disaccharides ca. 3:7 for 11 and 1,3-/1,4-linked disaccharides ca. 4:1 for 13; Scheme 3, Table 4). The regio- and stereoselectivity for the beta-D-manno-configurated cis-2,3-diol 12 were, however, rather poor (1,2-/1,3-linked products 48:52). The 1,2-linked disaccharides 66/67 derived from 12 were characterized as the acetates 70/71. Koenigs-Knorr-type glycosidation of the cis-diols 11-13 by 2 or 3 proceeded with a similar regio- and a higher stereoselectivity (alpha-D > beta-D with the donor 2 and alpha-D < beta-D with the donor 3) than with 1, with the exception of 12 which did not react with 2. The regioselectivity of the glycosidations by 1 agrees fully with the H-bonding Scheme of the diols and with the hypothesis that the intermediate carbene is preferentially protonated by the most weakly H-bonded OH group. The regioselectivity of the glycosidation by 2 and by 3 is determined by a higher reactivity of the equatorial OH groups and by H-bonding. Several H-bonded and equilibrating isomers of a given diol may intervene in the glycosidation by 1, or by 2 and 3, resulting in the same regioselectivity. The low nucleophilicity of 12 and the low degree of regioselectivity in its reaction with 3 show that stereoelectronic effects may also profoundly influence the nucleophilicity of OH groups.