Taxol Semisynthesis: A Highly Enantio- and Diastereoselective Synthesis of the Side Chain and a New Method for Ester Formation at C-13 Using Thioesters

Citations Over TimeTop 13% of 1997 papers

Abstract

A very simple, new, and straightforward approach to the Paclitaxel (Taxol) and Docetaxel (Taxotere) side chains has been developed using the imine addition reaction of thioester-derived boron enolates bearing chiral ligands. The addition reaction was studied extensively, using a combination of different thioesters (ROCH2COSPh, ROCH2COSt-Bu), oxygen protecting groups (R = Bn, TBDMS, COPh, EE, TMS), chiral boron ligands [derived from both (−) and (+)-menthone], imines (PhCHNSiMe3, PhCHNCOPh), and in the presence or in the absence of additional Lewis acids (BF3−OEt2, Et2AlCl, TiCl4). The side chain was assembled in a few steps with the correct relative (syn) and absolute stereochemistry (2R,3S). The stereochemical outcome of the boron-mediated reaction was rationalized using chair vs boat transition state structures. A new direct route for attachment of the side chains to the baccatin nucleus using thioester chemistry has also been developed. By treatment of a mixture of a thioester (8, 12, or 17) and protected baccatin III (2b, 2c) with LHMDS, the 13-O acylated compounds were obtained in high yield (up to 90%). Hydrolysis of 18b gave Paclitaxel (1a) in 80% yield.

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