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