DIASTEREOSELECTIVE HYDROGENATIONS OF ALPHA-ALKYL -TETRA-O-ACETYL-BETA-D-GLUCOPYRANOSYLOXY)METHYLENE CARBONYL-COMPOUNDS - NEW ROUTE TO STEREOPURE ALPHA-ALKYL ALPHA-OXYMETHYL CARBONYL-COMPOUNDS

Wittig condensation of the stabilised phosphoranes 9, 10 and 26 with f ormyl-2,3,4,6-tetra-O-acetyl-beta-D-glucopyranose 11 leads to the viny logous carbonates 12, 13 and 22, The salts 27-30 and 44, prepared from the corresponding carbonyl compounds, ethyl formate and sodium methox ide, react with acetobromoglucose 21 to give compounds 22-25 and 43. T he vinylogous esters/carbonates 12, 13, 22-25 and 43 undergo stereosel ective catalytic hydrogenations under mild conditions to give mainly t he dihydro derivatives 14, 15, 31-34 and 16, Although the selectivity for re-face addition is modest (ranging from 85: 15 to 67:33), it is p ossible to isolate the dihydro derivatives 15 and 31-33 in acceptable yields (ranging from 71 to 49%) simply by fractional crystallisation, Acidic hydrolysis of compound 31 provides (alpha S)-alpha-hydroxymethy l-gamma-butyrolactone 39 in high yield with an ee of similar to 96%. A model to account for the role of the 2,3,4,6-tetra-O-acetyl-beta-D-gl ucopyranosy unit in the stereoinduction process is presented.