摘要:
Treatment of acetylenic esters with low-valent tantalum (TaCl5 and Zn) in DME and benzene produces tantalum-alkyne complexes (not isolated), which react with carbonyl compounds regioselectively at the alpha-position of the esters to give Z isomers of trisubstituted alpha,beta-unsaturated esters in a stereoselective manner. In contrast, tantalum-alkyne complexes derived from acetylenic amides react with carbonyl compounds at the beta-position of the amides predominantly.