Preparative acetonation of pyranoid, vicinal trans-glycols under kinetic control: methyl 2,3:4,6-di-O-isopropylidene-α- and -β-d-glucopyranoside
✍ Scribed by Jean-Louis Debost; Jacques Gelas; Derek Horton; Ole Mols
- Publisher
- Elsevier Science
- Year
- 1984
- Tongue
- English
- Weight
- 442 KB
- Volume
- 125
- Category
- Article
- ISSN
- 0008-6215
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✦ Synopsis
Previous papers in this series have documented' the value of Z-alkoxypropenes for kinetically controlled acetonation of sugars. Aldohexoses are readily converted into the corresponding 4,6-0-isopropylidenealdohexopyranose?; those aldohexoses having O-2 and O-3 &-disposed may be preparatively transformed by a larger proportion of the reagent into 2,3:4,6-di-O-isopropylidenealdohexopyranoses',3.
Vicinal truns-glycols in simple pyranoid rings do not generally undergo ap preciable conversion into isopropylidene acetals (rruns-fused 1 ,Idioxolanes) under conventional reaction-conditions4. Fo~ation of a trans-fused, 1,3-dioxolane ring tends to decrease the dihedral angle between the C-O bonds on adjacent carbon atoms, and thereby increase the puckering of the pyranose chair. Even slight distortion in this sense entails a steep energy-barrier, because of aggravation of 1,3diaxial interactions. Because of this added energy, this type of ring fusion does not generally result under the~odynami~lly controlled conditions of acetonation. However, kinetically controlled acetalation offers potential for effective formation of 1,3-dioxolane rings trans-fused to pyranoid rings. In this paper it is shown, with methyl cy-and /3-D-glucopyranosides (1 and 5) as the examples, that acetonation with 2-alkoxypropenes affords initially, and predictably', the 4,6-isopropylidene acetals (2 and 6, respectively). Use of an excess of the reagent permits conversion of these glycosides in high yields into the respective 2,3:4,6-diisopropylidene acetals (4 and 8) in which the dioxolane ring (2,3-acetal) spans the truns-glycol group at C-2 and C-3. Detailed n.m.r.-spectral comparisons of the 4,6-acetals (2 and 6), their 2,3-diacetates (3 and 7) and the 2,3:4,6-diacetals (4 and 8) suggest that the "c,(D) conforma tion of the parent aldopyranosides (1 and 5) does not undergo major distortion by introduction of the vicinal, trans-acetal bridge.