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

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.

Key words

alkynes - imines - iminium salts - cycloadditions - Diels-Alder reactions

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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%).