Synthesis of pterocarpans by means of a "disfavored" 5-endo-trig radical cyclization reaction

A successful synthesis of pterocarpans 1, based on a "disfavored" 5-endo-tr ig radical cyclization reaction, has been accomplished. The radical precurs or 4-(2 ' -bromoaryloxy)-2H-chromene 8 was synthesized in six steps, starti ng from aryl propynyl ether 2. On treatment with tributyltin hydride in ref luxing benzene, aryl enol ether 8 underwent radical cyclization to furnish the pterocarpan 1. A deuterium label study was performed to prove the occur rence of 5-endo-trig radical cyclization, This methodology was extended fur ther to synthesize a class of hitherto unknown pterocarpans 13. A novel and stereoselective route towards the synthesis of aryl ethers of bromohydrins by means of the Mitsunobu reaction is also described.