Synthesis of topsentolides A2 and C2, and non-enzymatic conversion of the former to the latter

The first total synthesis of the marine-derived cytotoxin topsentolide A(2), which eventually culminated in its stereochemical determination, was accomplished in 17 steps from a known chiral alcohol. An improved synthesis of its congener, topsentolide C-2, from a synthetic intermediate of topsentolide A(2) was also performed by utilizing the Yamaguchi lactonization to construct its nine-membered lactone ring. Treatment of epoxide ring-containing topsentolide Ay with HCl/MeOH brought about its quantitative conversion into topsentolide C-2. (C) 2014 Elsevier Ltd. All rights reserved.