Total synthesis of forskolin - Part I

Abstract

A new total synthesis of forskolin 1 has been achieved and this note describes the synthesis of the transfused enone 18 as a key-intermediate. Key steps are: 1) the intramolecular Diels-Alder reaction of 17 to afford the tricyclic lactone 8 (48-56%); 2) a stereospecific and regiospecific one-pot conversion of 11(A) into 12 (94%) with LiBH4/BF3-THF at room temperature. The scheme allows a very early stereospecific introduction of the required 1α and 9α OH and their specific protection with respect to the end of forskolin synthesis.