SYNTHESIS OF THE ZIEGLER KEY INTERMEDIATE AND RELATED PRECURSORS FOR THE SYNTHESIS OF FORSKOLIN AND ERIGEROL

The Ziegler key intermediate 2, previously used in three total syntheses of forskolin (1), has been synthesized from enone 5a. Starting from 5a, two sequences to 2 have been developed (Schemes I and II). The key step in both sequences is the early and stereoselective introduction of the C-6, C-7 oxygen functional groups present in the natural product. This constitutes a new formal total synthesis of forskolin. The preparation of the key intermediate 18, a diastereomer of 2, potentially useful for the synthesis of analogues of 1 and for the synthesis of the highly oxygenated labdane diterpene erigerol (3), starting also from 5a, is described. © 1990, American Chemical Society. All rights reserved.