Stereoselective synthesis of 8-oxabicyclo[3.2.1]octane-2,3,4,6,7-pentols and total asymmetric synthesis of 2,6-anhydrohepturonic acid derivatives and of beta-C-manno-pyranosides suitable for the construction of (1 -> 3)-C,C-linked trisaccharides

Enantiomerically pure (+)-(1S4S,5S 6S)-6-endo-(benzyloxy)-5-exo-{[(tert-butyl)dimethylsilyl]oxy}-7-oxa-bicy clo[2.2.1]heptan-2-one ((+)-5) and its enantiomer (-)-5, obtained readily from the Diets-Alder addition of furan to 1-cyanovinyl acetate. can be converted with high stereoselectivity into 8-oxabicyclo[3.2.1]octane-2,3.4.6,7-pentol derivatives (see 23- 28 in Scheme 2). A precursor of them, (1R,2S,4R,5S,6S,7R,8R)-7-endo-(benzyloxy)-8-exo-hydroxy-3,9-dioxatricycl o[4.2.1.0(2.4)]non-.5-endo-yl benzoate ((-)-19), is transformed into (1R,2R,5S, 6S,7R,8S)-6-exo,8-endo-bis(acetyloxy)-2-endo-(benzyloxy)-4-oxo-3,9-dioxa bicyclo[3,3.1]non-7-endo-yl benzoate ((-)-43) (see Scheme 5). The latter is the precursor of several protected 2,6-anhydrohepturonic acid derivatives such as the diethyl dithioacetal (-)-57 of methyl 3,5-di-O-acetyl-2,6-anhydro-4-O-benzoyl-D-glycero-D-galactohepturonate (see Schemes 7 and 8). Hydrolysis of (-)-57 provides methyl 3,5-di-O-acetyl-2,6-anhydro-4-O-benzoyl-D-glycero-D-galacto-hepturonate 48 that undergoes highly diastereoselective Nozaki-Oshima condensation with the aluminium enolate resulting from the conjugate addition of Me2AlSPh to (1S,5S,6S,7S)-7-endo..]oxy}-8-oxabicyclo[3.2,1]oct-3-en-2-one ((-)-13) derived from (benzyloxy)-6-exo-1[ (tert-butyl)dimethylsityl]oxy}-8-oxabicyclo[3.2.1]oct-3-en-2-one (( (+)-5 (Scheme 12). This generates a beta -C-mannopyranoside, i.e., methyl (7S)-3,5- di-O- acetyl-2,6-anhydro-4-O-benzoyl-7-C-[(1R,2S,3R,4S,5R,6S,7R)-6-endo-(benzy loxy)-7-exo-{[(tert-butyl)dimethylsilyl]oxy}-4-endo-hydroxy-2- exo-(phenylthio)-8-oxabicyclo[3.2.1]oct-3-endo-yl-L-glycero -D-manno-heptonate ((-)-70 see Scheme12), that is converted into the diethyl dithioacetal (-)-75 of methyl 3-O-acetyl-2.6-anhydro-4,5-dideoxy-4-C-{[methyl (7S)-3,5,7- tri- O -acetyl-2,6-anhydro-4- O -benzoyl-L -glycero-D-manno- heptonate] -7- C-yl] -5- C- (phenylsulfonyl)-L-glycero-D-galacto-hepturonate (76 see Scheme 13). Repeating the Nozaki-Oshima condensation to enone (-)-13 and the aldehyde resulting from hydrolysis of (-)-75, a (1 - 3)-C.C-linked trisaccharide precursor (-)-77 is obtained.