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

Abstract

A successful synthesis of pterocarpans 1, based on a "disfavored" 5-endo-trig radical cyclization reaction, has been accomplished. The radical precursor 4-(2'-bromoaryloxy)-2H-chromene 8 was synthesized in six steps, starting from aryl propynyl ether 2. On treatment with tributyltin hydride in refluxing benzene, aryl enol ether 8 underwent radical cyclization to furnish the pterocarpan 1. A deuterium label study was performed to prove the occurrence of 5-endo-trig radical cyclization. This methodology was extended further 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.