Enantiomerically Pure 7-Oxabicyclo[2.2.1]Hept-5-En-2-Yl Derivatives as Synthetic Intermediates .16. Total Asymmetric-Synthesis of 6-Deoxy-5-C-Methyl-D-Ribo-Hexose Structure in Aqueous-Solution

(1R,2R,4R)-2-exo-Cyano-7-oxabicyclo[2.2.1]hept-5-en-2-yl (1S)-camphanate [1, Diels Alder adduct of furan to 1-cyanovinyl (1S)-camphanate, a ''naked sugar''] was transformed with high stereoselectivity into (1S,4S,5R,6R,7R)-6-exo,7-exo-(isopropylidenedioxy)-4-exo-methyl-2,8-diox abicyclo[3.2.1]octan-3-one [(-)-4] and (1S,5R,6R,7R)-6-exo,7-exo-(isopropylidenedioxy)-4,4-dimethyl 2,8-dioxabicyclo[3.2.1] octan-3-one [(-)-51. Reaction of (-0-5 with Me3SiCH2Li gave 5,7-dideoxy-5,5-dimethyl-2,3-)-isopropylidene-CD-ribo-heptofuranos-6-ulo se [(+)-8, which was then converted into 6-deoxy-5-C-methyl-D-ribo-hexose (10 steps from furan, 59% overall yield) and derivatives. N.m.r. studies established that branching of 6-deoXy-D-allose by a methyl group at C-5 favours the furanose versus the pyranose form of the carbohydrate in aqueous solution.