Total synthesis of forskolin — Part II

Abstract

The further elaboration of the key-intermediate 5 into forskolin 1 has been achieved via two different routes. Key features of this new total synthesis are: 1) the stereospecific formation of the 6β, 7β, 8α-triol via the BF 3Et 2O assisted opening of the epoxy carbamate 8; 2) use of the 8α, 11-di- t-butylsilylene ketal for the specific protection of the 6β, 7β-diol, from the tetrol 10 A ; two new sequences for the formation of the dihydro-γ-pyrone ring in high overall yield from 23; 4) the stereoselective divinyl cuprate conjugate α-addition on the dihydro-γ-pyrone 16 or 28, in the presence of BF 3Et 2O, with the stereochemistry required for forskolin synthesis.