Regioselective base-induced ethereal bridge openings of 7-oxabicyclo[2.2.1]heptane-2,3-dicarboxylic derivatives; Synthesis of squalestatin core analogues

The Diels-Alder adduct of furfuryl alcohol and maleic anhydride was converted into dimethyl 5-exo,6-exo-dihydroxy-1{[tris (isopropyl)silyloxy] methyl} -7-oxabicyclo[2.2.1]heptane-2-exo,3-exo-dicarboxylate (12). Regioselective monosilylation of diol 12 gave the 5-O-(tert-butyl)dimethylsilyl ether 18. Under basic conditions the 7-oxanorbornane system of 18 was isomerized selectively into a 4,5,6-trihydroxy-6-(hydroxymethyl)cyclohex-2-ene-1,2-dicarboxylic derivative that was converted into a doubly-branched hepturonic acid derivative (24), potential precursor for the preparation of squalestatin core analogues.