Glycosylidene Carbenes. Part 15. Synthesis of disaccharides from allopyranose‐derived vicinal 1,2‐diols. Evidence for the protonation by a H‐bonded hydroxy group in the σ‐plane of the intermediate carbene, followed by attack on the oxycarbenium ion in the π‐plane

The α‐ D ‐ allo ‐diol 9 possesses an intramolecular H‐bond (HOC(3) to OC(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 HOC(2) with both OC(1) and OC(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 HOC(3) and OC(4). In solution, the β‐ D ‐anomer 12 is also a mixture of tautomers, 12a and presumably a dimer. The H‐bonding in 9 and 12 is evidenced by their IR and 1 H‐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 σ‐plane) and the combination of the thereby formed ions (π‐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 β‐ D ‐anomers ( Table 4 ). Glycosidation of the α‐ D ‐anomer 9 gave mostly the 1,3‐linked disaccharides 16 and 17 (α‐ D β‐ D ) along with the 1,2‐linked disaccharides 14 and 15 (α‐ D < β‐ D , 1,2‐/1,3‐linked glycosides ca. 1:4), except in THF and at low temperature, where the β‐ D ‐configurated 1,2‐linked disaccharide 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 α‐ D /β‐ 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.