Synthesis of functionalised furans and pyrroles through annulation reactions of 4-pentynones.

A new approach to 4-pentynones through palladium-catalysed coupling reaction of the ready available 2-propynyl ketones with aryl iodides and/or vinyl triflates is proposed. Annulation reactions of both 2-propynyl ketones and 4-pentynones gave functionalised furans using potassium tert-butoxide in DMF and functionalised pyrroles in the presence of benzylamine or ammonia, respectively in good to high yields. The methodology has been extended to the preparation of 17 beta-hydroxyandrost-4-en[3,2-b](5-methyl)furan and to 17 beta-hydroxyandrost-4-en[3.2-b](1-benzyl-5-methyl)pyrrole. A different reaction pattern was observed when the 4-pentynones were treated with sodium methoxide in MeOH. The influence of the reaction medium on the outcome of the annulation reaction in the case of one 2-pentyn-1,6-dione (heteroannulation vs. carbocyclization) is also shown. (C) 1998 Published by Elsevier Science Ltd. Ail rights reserved.