1,2-Annulated Sugars: Synthesis of Polyhydroxylated 2,10-Dioxadecalins with β-manno Configuration

A new synthetic route to 1,2-pyran-annulated O-glycosides (pyranopyrans) with beta-manno configuration is presented. Starting from a benzyl-protected allyl beta-2-uloside obtained by C2-oxidation from the corresponding allyl glucoside, vinylation and ring-closing metathesis provided a bicyclic alkene as a key substrate. Optimized reaction conditions involving the 2-uloside were required to circumvent the formation of the unwanted 3,2-enolone. Subsequent stereoselective alkene dihydroxylations were investigated and are presented. Diastereoselective catalytic osmoylation afforded the bicyclic beta-mannoside with syn-diol configuration. In contrast, a sequence featuring a directed vanadium-catalyzed alkene epoxidation and epoxide opening provided the corresponding 2,10-dioxadecalin target compound with trans-diol configuration. X-ray crystallography was employed to study the molecular structure of the target compound with syn-diol configuration. In addition, the molecular packing and hydrogen-bonding interactions of the hydroxy-rich molecule were examined.