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]heptan-2-ones can generate the corresponding 3-hydroxycyclohexanone derivatives. The method has been applied to the synthesis of C-alpha-D-galactopyranosides of carbapentopyranoses. Radical alpha-D-galactosidation of (+/-)-(1RS,4RS,5RS, 6RS)-6-endo-methoxy-3-methylidene-5-exo-(phenylseleno)-7-oxabicyclo[2.2.l]hept-2-one ((+)-51) followed by seleno-Pummerer rearrangement and reduction with Bu(3)SnH gave (+)- (1R,2S,3S,4R,6R)-((+)-58) and (+)-(1S,2R,3R,4S,6S)-3-endo-methoxy-5-oxo-6-endo-[(2',3',4',6'-tetra-0-acetyl-alpha-D-galactopyranosyl)methyl]-7-oxabicyclo[2.2.1]hept-2-endo-yl acetate ((+)-59), which were separated by column chromatography. Irradiation (254 nn) in the presence of Et(3)N gave (+)-(1S,2R,3R,6R)-((+)-60) and (+)-(1R,2S,3S,6S)-2-hydroxy-6-methoxy-4-oxo-3-[(2',3',4',6'-tetra-0-acetyl-alpha-D-galactopyranosyl)methyl]cyclohexyl acetate (+)-61, respectively. NaBH4 reduction and acetylation provided (+)-(1S,2S,3R,4R,5R)-((+)-62) and (+)-(1R,2R,3S,4S,5S)-5-methoxy-2-[(2',3',4',6'-tetra-0-acetyl-alpha-D-galactopyranosyl)methyl]cyclohexa-1,3,4-triyl triacetate ((+)-64).