Acetylenic Vinyllithiums: Consecutive Cycloisomerization-[4 + 2] Cycloaddition Reactions.
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.
Bailey
William F.
WF
Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-4060, and the Department of Chemistry, Connecticut College, New London, Connecticut 06320-4196.
Wachter-Jurcsak
Nanette M.
NM
Pineau
Mark R.
MR
Ovaska
Timo V.
TV
Warren
Rachel R.
RR
Lewis
Carl E.
CE