During the course of our project directed towards the total synthesis of natural products with allelopathic activity, we intended to synthesize the biogenetically related helianane-type terpenoids, heliespirones A (1), C (2) and heliannuol E (3) based on the intramolecular Hosomi-Sakurai type reaction. Reaction of the substrates (5) possessing a benzoquinone and an allysilane moieties with TBSOTf in isobutyronitrile at -10℃ provided the unexpected rearranged products (11) with good yield and diastereoselectivity. This type of transformation has never been reported so far. Employing the rearranged product (11a) the first enantioselective total syntheses of heliespirones A (1) and C (2) were successfully completed. In addition, an alternative enantioselective total synthesis of heliannuol E (3) was also accoplished starting from 11a. It was demonstrated that our newly developed Lewis acid mediated rearrangement can be applied to the total synthesis of helianane-type sesquiterpenoids with allelopathic activity.
