Synthesis 2006(13): 2251-2259  
DOI: 10.1055/s-2006-942423
PAPER
© Georg Thieme Verlag Stuttgart · New York

3-Cyclopropyl- and 3-tert-Butyl-Substituted Propyne Iminium Salts as Dienophiles in Diels-Alder Reactions

Holger Gerster, Sigrid Espenlaub, Gerhard Maas*
Division of Organic Chemistry I, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany
Fax: +49(731)5022803; e-Mail: gerhard.maas@uni-ulm.de;
Further Information

Publication History

Received 7 May 2006
Publication Date:
24 June 2006 (online)

Abstract

Starting from cyclopropylacetylene and 3,3-dimethylbut-1-yne, and an imidoyl chloride 5, the alkynyl imines 6 were prepared and then converted into propyne iminium triflates 7 by N-methylation. These electron-deficient acetylenes were found to be reactive dienophiles in Diels-Alder reactions with cyclopentadiene, furan, 2,3-dimethylbuta-1,3-diene, and anthracene. Reduction of the iminium function, which is present in the cycloaddition products, generates tertiary amines. In this manner, norbornadiene (8, 9), 7-oxanorbornadiene (10, 11), cyclohexa-1,4-diene (12), benzene (13, 14), and dibenzobarrelene (15, 16) derivatives with a novel vicinal substitution pattern were obtained.

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6

We found, however, that the yield of (Me3Si)C≡CC(Ph)=NMe from (Me3Si)C≡CMgBr (ref. 5) increased from 73% to 79% in the presence of CuBr·Me2S (5 mol%).