Total synthesis of (+)-quassin.

A total synthesis of (+)-quassin from naturally occurring (S)-(+)-carvone is described. The total number of steps was 28, and the overall yield was about 2.6%. The synthetic strategy for the construction of the tetracyclic carbon framework was based on a C-->ABC-->ABCD ring annulation sequence, involving an aldol reaction, an intramolecular Diels-Alder reaction, and an intramolecular acylation as the key steps. Subsequent functionalization of ring A and ring C then afforded the target (+)-quassin.