The seven-membered, aromatic a-tropolone ring 2 occurs naturally in three biosynthetically distinct classes: lI2 in the essential oils of Cupressae (eg, a-thujaplicin), in mold metabolites of the Penicillium family (eg, stipitatonic acid) and in the Colchicum alkaloids (eg, colchicine). The unique character of these seven-membered carbocycles has attracted considerable synthetic, biogenetic, and theoretical attention3 since the structure ...
