Total synthesis of (+/-)-kellermanoldione: stepwise cycloaddition of a functionalized diene and allenoate.

The total synthesis of the diterpene kellermanoldione 1 is reported. Stepwise [4 + 2] cycloaddition of the ketal diene 8 and the allenoate 3 afforded the exo adduct 10x as the major product. It was converted into 1 via six steps, among them a key nonconjugative hydrolysis of a gamma-methylene silyl enol ether.