title: Specific synthesis and stereochemical assignment of the diastereomeric 3,5-O-benzylidene-1,2-O-isopropylidene-alpha-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 Grüner (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.
mesh_terms: D002236:Carbohydrate Conformation; D005947:Glucose / Q000031:analogs & derivatives* / Q000138:chemical synthesis; D006863:Hydrogen-Ion Concentration; D007202:Indicators and Reagents; D009682:Magnetic Resonance Spectroscopy; D013237:Stereoisomerism; D013816:Thermodynamics; D014961:X-Ray Diffraction
chemical_list: D007202:Indicators and Reagents; C052106:3,5-O-benzylidene-1,2-O-isopropylideneglucofuranose; D005947:Glucose