Synthesis of benzofuran derivatives-II: a new synthesis of visnagin

Starting with 2-methyl-5:7-dihydroxychromone, an allyl group is introduced into the 6-position by Claisen migration of a 5-allyl ether. The initial protection of the 7-hydroxyl is best effected by tosylation. If the tosyl group is removed before Claisen migration and the C-allyl compound subjected to ozonolysis and ring closure, norvisnagin is obtained in a poor yield and can be methylated to visnagin. In an alternative method the tosyl group is removed just before ozonolysis. For this purpose the previous methylation of the 5-position is advantageous. This route gives visnagin directly in good yield