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A novel, facile approach to frondosin B and 5-epi-liphagal via a new [4 + 3]-cycloaddition.

A new [4 + 3]-cycloaddition between benzofuran allylic alcohols and dienes, promoted by camphorsulfonic acid, has been identified. A novel strategy which used this cycloaddition as a key step has been developed for the synthesis of 6,7,5-tricyclic skeleta, and syntheses toward frondosin B (1) and 5-epi-liphagal (2) have been achieved via short routes in good yields.