Novel Dienes and Dienophiles, VI. On the Chemical Behavior of 2‐Ethynyl‐1,3‐butadiene

The behavior of 2‐ethynyl‐1,3‐butadiene ( 1 ) as the diene component in Diels‐Alder additions has been studied using a selection of representative double and triple bond dienophiles. The latter include maleic anhydride ( 5a ), diethyl fumarate ( 6 ), benzoquinone ( 7 ), methyl acrylate ( 11 ), juglone ( 20 ), 1‐methylcyclopropene ( 23 ), dimethyl acetylenedicarboxylate ( 26 ), propiolic aldehyde ( 32 ), 4‐phenyl‐1,2,4‐triazolin‐3,5‐dione ( 35 ) and diethyl azodicarboxylate ( 37 ). [2 + 4] Cycloadducts were formed in all cases in varying yields. The addition is accompained by thermal dimerization of 1 which leads to 1,4‐diethynyl‐4‐vinyl‐1‐cyclohexene ( 43 ) and 1,6‐diethynyl‐1,5‐cyclooctadiene ( 39 ). The mechanism of this dimerization is discussed. In a competition experiment towards maleic anhydride ( 5a ), diene 1 was shown to be ca. five times less reactive than isoprene.