REDUCTIVE OXA RING-OPENING OF 7-OXABICYCLO[2.2.1]HEPTAN-2-ONES - SYNTHESIS OF C-ALPHA-GALACTOSIDES OF CARBAPENTOPYRANOSES

Photoinduced electron transfer from Et(3)N to 7-oxabicyclo[2.2.1]hepta n-2-ones can generate the corresponding 3-hydroxycyclohexanone derivat ives. The method has been applied to the synthesis of C-alpha-D-galact opyranosides of carbapentopyranoses. Radical alpha-D-galactosidation o f (+/-)-(1RS,4RS,5RS, 5-exo-(phenylseleno)-7-oxabicyclo[2.2.l]hept-2-o ne ((+)-51) followed by seleno-Pummerer rearrangement and reduction wi th Bu(3)SnH gave (+)- (1R,2S,3S,4R,6R)-((+)-58) and yranosyl)methyl]-7 -oxabicyclo[2.2.1]hept-2-endo-yl acetate ((+)-59), which were separate d by column chromatography. Irradiation (254 nn) in the presence of Et (3)N gave (+)-(1S,2R,3R,6R)-((+)-60) and -acetyl-alpha-D-galactopyrano syl)methyl]cyclohexyl acetate (+)-61, respectively. NaBH4 reduction an d acetylation provided (+)-(1S,2S,3R,4R,5R)-((+)-62) and ha-D-galactop yranosyl)methyl]cyclohexa-1,3,4-triyl triacetate ((+)-64).