Reductive Oxa Ring Opening of 7-Oxabicycloheptan-2-ones. Synthesis of C-α-Galactosides of Carbapentopyranoses

Article abstract of DOI:10.1021/jo00131a007

Photoinduced electron transfer from Et3N 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-α-D-galactopyranosides of carbapentapyranoses.Radical α-D-gaactosidation of (+/-)- (1RS,4RS,5RS,6RS)-6-endo-methoxy-3-methylidene-5-exo-(phenylseleno)-7-oxabicyclo<2.2.1>hept-2-one ((+/-)/51) followed by seleno-Pummerer rearrangement and reduction with Bu3SnH gave (+)-(1R,2S,3S,4R,6S)-((+)-58) and (+)-(1S,2R,3R,4S,6S)-3-endo-methoxy-5-oxo-6-endo-<(2',3',4',6'-tetra-O-acetyl-α-D-galactopyranosyl)methyl>-7-oxabicyclo <2.2.1>hept-2-endo-yl acetate ((+)-59), which were separated by column chromatography.Irradiation (254 nm) in the presence of Et3N gave (+)-(1S,2R,3R,6R)- ((+)-60) and (+)-(1R,2S,3S,6S)-2-hydroxy-6-methoxy-4-oxo-3-,(2',3',4',6'-tetra-O-acetyl-α-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-O-acetyl-α-D-galactopyranosyl)methyl>cyclohexa-1,3,4-triyl triaceate ((+)-64).