Stereoselective Synthesis of Bicyclic Lactones from Furans

Significance

Functionalized ortho-esters undergo condensation with trimethylsiloxyfuran derivatives under sub-stoichiometric Lewis acid conditions to give butenolides which, using tris(trimethylsilyl)silane (TTMSS), a ‘green’ version of the corresponding tin hydride, participate in radical-induced ring closure to afford bicyclo[3.n.0]lactones. The two-step sequence is not limited to ­dioxolane-containing ortho-esters as trialkoxy ortho-esters also participate to give products in good to high yields. In the second step, up to three contiguous stereocenters with high diaste­reoselectivity (>95%) are formed.