Kc. Santhosh et al., Synthesis of pterocarpans by means of a "disfavored" 5-endo-trig radical cyclization reaction, EUR J ORG C, (18), 2001, pp. 3461-3466
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.