Acetylenic vinyllithiums (2), which were generated from the corresponding acetylenic vinyl bromides (3) by low-temperature lithium-bromine exchange, cyclize on warming to give, following quench with water, isomerically pure conjugated bis-exocyclic 1,3-dienes (1) in good to excellent yield. Both five-membered and six-membered outer-ring dienes may be prepared: 5-exo closure of an acetylenic vinyllithium, which proceeds with total stereocontrol via syn-addition to give the E-isomer of a five-membered outer-ring diene, tolerates aryl-, silyl-, or alkyl-substituents at the distal acetylenic carbon; the corresponding 6-exo process is less facile and seems to be confined to substrates bearing an anion-stabilizing substituent, such as phenyl or trimethylsilyl, at the terminal acetylenic carbon. The highly reactive bis-exocyclic 1,3-dienes serve as precursors to polycyclic materials through subsequent Diels-Alder reaction with a wide variety of dienophiles. The consecutive exchange-cyclization-cycloaddition methodology, which can be conducted in one pot without isolation of intermediates, provides an efficient, operationally simple, and diastereoselective route to diverse polycyclic ring systems.

Acetylenic Vinyllithiums: Consecutive Cycloisomerization-(4 + 2) Cycloaddition Reactions.-Five-and six-membered bis-exocyclic 1,3-dienes such as (II) and (XX) are synthesized through cycloisomerization of a vinyllithium tethered to an acetylenic unit. The 5-exo closure proceeds with total stereocontrol to give the E-isomer of a five-membered outer-ring diene. Aryl-, silyl-, or alkyl-substituents are tolerated at the distal acetylenic carbon. The 6-exo process (e.g. (XIX) → (XX)) is less facile. The outer-ring dienes react stereoselectively with a wide range of dienophiles to give diastereoisomerically pure polycyclic ring systems in good to excellent yield. -(BAILEY, W. F.; WACHTER-JURCSAK, N. M.; PINEAU, M. R.; OVASKA, T. V.; WARREN, R. R.; LEWIS, C. E.; J. Org. Chem. 61 (1996) 23, 8216-8228; Dep. Chem., Univ. Conn., Storrs, CT 06269, USA; EN)