Unusual reduction of a lactone carbonyl in a Bu3SnCl and Na(CN)BH3 mediated radical cyclization of 3-(o-bromophenoxymethyl)coumarins

3-Chloromethyl coumarin was treated with different substituted 2-bromophenols in the presence of anhydrous potassium carbonate in refluxing acetone to afford a number of 3-(2-bromophenoxymethyl)coumarins 3a-f in 80-95% yield. These were then refluxed with tributyltin chloride and sodium cyanoborohydride in benzene under nitrogen, in the presence of a catalytic amount of azobisisobutyronitrile (AIBN) for 7-10 h to give spiro[chroman-3,3′-(2′H)-benzofurans] 4a-f in 60-75% yields.