Ring-closing metathesis in flavonoid synthesis, part 3: isoflavenes

Isoflav-3-enes with natural substitution patterns were prepared in two steps from readily available starting materials by means of a one-pot Wittig methylenation - etherification - Wittig methylenation, followed by ring-closing metathesis catalyzed by the Grubbs second generation catalyst. This included the preparation of a series of five novel 1-[(2-phenylallyl)oxy]-2-vinylbenzenes in good yields (61 - 89%). Olefin metathesis thus affords a common strategy for the synthesis of all three flavonoid subclasses, i.e. the flavonoids, neoflavonoids and isoflavonoids.