ChemInform Abstract: Specific Synthesis and Stereochemical Assignment of the Diastereomeric 3,5‐O‐Benzylidene‐1,2‐O‐isopropylidene‐α‐D‐glucofuranose Isomers.

Abstract

Pyridinium tosylate-catalyzed acetal exchange between benzaldehyde dimethyl acetal and 6-O-(tert-butyldiphenylsilyl)-1,2-O-isopropylidene-alpha-D-glucofu ranose was investigated as an alternative to the original procedure of Brigl and Gruner (condensation of a D-glucose triol with benzaldehyde under zinc halide catalysis) for synthesis of 3,5-O-benzylidene-1,2-O-isopropylidene-alpha-D-glucofuranose. The two routes afford opposite benzylidene diastereoisomers: the traditional procedure leads to the thermodynamically favored isomer (phenyl and C-6 trans), whereas the new sequence gives the cis compound. The orientations and conformations of these isomers were determined after conversion into the corresponding 6-iodides 5 and 7. X-Ray crystallography revealed that the 1,3-dioxane ring of the trans isomer 7 exists in the expected chair, O-inside conformation. In contrast, a combination of n.m.r. spectroscopy and molecular-mechanics calculations demonstrated that the same ring of cis diastereomer 5 does not adopt the alternative chair, H-inside conformation; instead, it exists in a specific twist form.