Synthesis of (Z) isomers of benzoheterocyclic derivatives of combretastatin A-4: a comparative study of several methods

Several methods for the preparation of (Z) trimethoxystyrene derivatives 1 were investigated. After finding that the Wittig reaction led to unsatisfactory results, three different strategies were considered: a Suzuki coupling with a stereodefined monobromoalkene, a combination of hydrosilylation/vinylsilane hydrolysis and a palladium-catalyzed semi-hydrogenation step, using DMF/KOH as the hydrogen source. Our studies led us to prepare a series of diarylacetylene derivatives via a Sonogashira coupling reaction, and also to find out a copper-free basic cyclization leading to benzo[b]thiophenes. The final choice for the synthesis method of 1 strongly depends on the target compound but the semi-hydrogenation, which avoids the use of a toxic tin reducing agent, should be generally preferred.