Stereoselective Construction of a Highly Functionalized Taxoid ABC-Ring System: the C2-C9 Oxa-Bridge Approach

The goal of this investigation is to assemble the 20‐carbon unit 1 of the taxoid diterpene skeleton with a high level of stereocontrol by means of a three‐reaction sequence developed in this laboratory. The strategy involves seven C–C bond‐forming operations together with eighteen functional group transformations, circumventing the stereoselectivity issue altogether. Furthermore, there is no isomer formation and hence no need for chromatographic separation. A temporary oxa‐bridge (C2/C9) was used as a problem‐solving approach. The key step in the planned sequence was based on achieving the last C–C bonding between C11 and C12, following a successful C11 functionalization. X‐ray analyses of 8b, 17, 18, 19, 20, and 21, together with extensive use of 800 MHz 1H (200 MHz 13C) NMR spectra, support the suggested structures. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)