水銀トリフラートとオレフィン環化反応

抄録

During a trip to Indonesia, I came across a tropical fruit, duku. By extraction of the fruit skin, a hair growth active triterpene lansic acid (1) was isolated. In our planned total synthesis of 1, an effective olefin cyclization agent was required, and mercuric triflate (2) was bone. Then we carried out syntheses of a variety of polycyclic terpenoids and investigated the mechanism of biomimetic olefin cyclization. Upon cyclization of geranylgeranyl acetate, we detected chair/boat/chair cyclization products along with chair/chair/chair cyclization in 1 : 5 ratio. On the basis of the observation that the mercuric triflate is stable but still reactive enough in the presence of water, we trapped each cationic intermediate with water by cyclization in aqueous media. This is the first experimental evidence of the stepwise mechanism of olefin cyclization that takes place via conformationally flexible cationic intermediates. In this review we also deal with an approach into the mystery of the steroid biosynthesis, and a synthetic study toward the taxane skeleton by means of mercuric triflate-induced olefin cyclization.