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Synlett 2004(14): 2579-2581  
DOI: 10.1055/s-2004-834790
LETTER
© Georg Thieme Verlag Stuttgart · New York

Microwave Enhanced Tandem 5-exo Cyclization/Claisen Rearrangement Reactions: A Convenient Route to Cycloheptanoid Ring Systems

Caitlin E. McIntosh, Isamir Martínez, Timo V. Ovaska*
Connecticut College, Department of Chemistry, 270 Mohegan Avenue, New London, CT 06320, USA
Fax: +1(860)4392477; e-Mail: tvova@conncoll.edu;
Further Information

Publication History

Received 29 July 2004
Publication Date:
20 October 2004 (online)

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Abstract

The use of microwave irradiation was found to significantly accelerate a one-pot reaction sequence involving a base-catalyzed 5-exo cyclization of appropriately substituted 4-alkyn-1-ols and subsequent Claisen rearrangement of the intermediate 2-methylene tetrahydrofuran derivatives. In most cases, the resulting ­cycloheptanoid ring systems were produced in a matter of minutes when subjected to microwave irradiation. The use of solvent-free conditions was also found to be compatible with the reaction ­sequence.

Key words

5-exo cyclization - Claisen rearrangement - microwave irradiation - solvent-free

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CEM Discover Focused MicrowaveTM Synthesis System was used for all reactions.

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Typical Experimental Procedure: A 10 mL flame-dried microwave tube was charged with a solution of the appropriate acetylenic alcohol (typically 0.5-1.0 M concentration) in anhyd DMF or phenetole. To this was added ca 10 mol% of MeLi in Et2O and the tube was placed in the microwave oven for the specified time. The product mixture was then either chromatographed on silica gel directly (when phenetole was used as the solvent) or following removal of the solvent and aqueous work-up (when DMF was used as the solvent).

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All previously reported compounds gave spectroscopic and analytical results consistent with their assigned structures.