3-cyclopropyl- and 3-tert-butyl-substituted propyne iminium salts as dienophiles in diels-alder reactions

Starting from cyclopropylacetylene and 3,3-dimethyl-but-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-l,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.