Glycosylidene Carbenes. Part 16. Glycosidation of methyl 6‐O‐trityl‐α‐D‐altropyranoside

Abstract

Hydrogen bonding of the triol 4 in chlorinated solvents was studied by IR (CH,CI, and CC1,) and 'H-NMR spectroscopy (CDCl,), 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 n-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)glucal18. In agreement with the predictions, glycosidation in non-coordinating solvents gave the 1,2-, 1,3-, and 1 ,Clinked disaccharides in decreasing relative amounts. GlyCOSiddtiOn in THF proceeded with a lower degree of regioselectivity and led preferentially to thep -o-anomers, except for the minor, l,.?-linked disaccharides, where THF had only a weak influence on stereoselectivity at room temperature and led to a slight increase of the c( -D-anomer at -8OO.Introduction. -In the glycosidation of alcohols by glycosylidene-derived diazirines, the intermediate alkoxy(alky1)carbenes are protonated by the OH group involved in the weakest intramolecular H-bond, to generate an ion pair where the oxy anion is initially located in the a-plane of the oxycarbenium cation. Combination of these ions yields glycosides. For this, the ions must reorient themselves, so that the oxy anion can attack in the 7c-plane of the oxycarbenium cation [ 1-31. Alternatively, the oxycarbenium ion can be attacked by another, suitably oriented oxy-anion group, generated by H-transfer from a second OH group to the initially generated oxy anion. This alternative appears to be realized in the glycosidation by the diazirine 1 of the a-D-allopyranoside 2 [l]. In chlorinated solvents, this diol forms the two H-bonded tautomers 2a and 2b in a ratio of ca. I : 1. Its glycosidation in non-coordinating solvents is unusual; regioselectivity does not reflect the relative strength of the H-bonds and the ratio of the tautomers, and the stereoselectivity differs from the one which has been observed for all other alcohols so far investigated. This result was explained by a preferred protonation of the carbene by HO-C(2) in 2a and HO-C(3) in 2b, involved in the weakest H-bonds. Protonation by HO-C(3) leads to glycosidation by the oxy anion derived from the same OH group, but protonation by HO-C(2) is followed by H-transfer from HO-C(3) to -0-C(2) and glycosidation by -0-C(3), located in the n-plane of the cation. One can, however, argue that the regioselectivity is due to a more rapid protonation of the intermediate carbene by

1H-NMR Analysis of Intra- and Intermolecular H-Bonds of Alcohols in DMSO: Chemical Shift of Hydroxy Groups and Aspects of Conformational Analysis of Selected Monosaccharides, Inositols, and Ginkgolides

“…50%. A comparison of the 1 H-NMR data of the a-d-allopyranoside 12 [27] and the a-daltropyranoside 13 [28] in CDCl 3 and (D 6 )DMSO should allow a comparison of the persistence of various intramolecular H-bonds in (D 6 )DMSO solution with their relative strength, as deduced above from IR data. In (D 6 )DMSO, the bifurcated Hbond of 12 is mostly retained, as evidenced by J(2,OH) 9.5 Hz.…”

“…All J(H,OH) of the tritylated a-d-altroside 13 are rather large, evidencing a system of cooperative H-bonds involving HOÀC(4), HOÀC(3), and the MeO group besides a weak H-bond between HOÀC(2) and the ring O-atom [28]. The a-d-allopyranoside 12 is characterised by a large J(2,OH) of 11.7 Hz and a smaller J(3,OH) of 6.5 Hz [27].…”

“…According to this parameter, intramolecular H-bonds in a six-membered ring, as for HOÀC(3) of 13, are stronger than H-bonds in five-membered rings. The strength of H-bonds forming fivemembered rings decreases from bifurcated H-bonds, as in 9, 10, and 12 and H-bonds from an equatorial OH to a vicinal axial OR group, as in 7 and 13 3 ), to H-bonds from an axial OH to a vicinal equatorial OR group, as in 11b and 12b, and further to H-bonds from an equatorial OH to a vicinal equatorial OR group, as in 6a and 6b [24] [27] [28] [31] [32]. The relative strength of the intramolecular H-bonds in 5 ± 13 may be assigned on the basis of the Dn Ä values.…”

1H-NMR Analysis of Intra- and Intermolecular H-Bonds of Alcohols in DMSO: Chemical Shift of Hydroxy Groups and Aspects of Conformational Analysis of Selected Monosaccharides, Inositols, and Ginkgolides

“…A comparison of the 1 H-NMR data of the a-d-allopyranoside 12 [27] and the a-daltropyranoside 13 [28] in CDCl 3 and (D 6 )DMSO should allow a comparison of the persistence of various intramolecular H-bonds in (D 6 )DMSO solution with their relative strength, as deduced above from IR data. In (D 6 )DMSO, the bifurcated Hbond of 12 is mostly retained, as evidenced by J(2,OH) 9.5 Hz.…”

“…All J(H,OH) of the tritylated a-d-altroside 13 are rather large, evidencing a system of cooperative H-bonds involving HOÀC(4), HOÀC(3), and the MeO group besides a weak H-bond between HOÀC(2) and the ring O-atom [28]. All J(H,OH) of the tritylated a-d-altroside 13 are rather large, evidencing a system of cooperative H-bonds involving HOÀC(4), HOÀC(3), and the MeO group besides a weak H-bond between HOÀC(2) and the ring O-atom [28].…”