Novel Dienes and Dienophiles, VII Preparation and Chemical Behavior of 2,3‐Diethynyl‐1,3‐butadiene

The two 2,3‐diethynyl‐1,3‐butadiene derivatives 1b and 1c were prepared from 2,3‐dichloro‐1,3‐butadiene ( 26 ) and the Grignard reagents 19b and 19c by Kumada coupling. Desilylation of 1b provided the parent molecule 1a . The title compounds were used for the preparation of numerous new enediyne systems by [2 + 4] cycloaddition. The dienophiles employed include maleic anhydride ( 27a ), diethyl fumarate ( 29 ), tetracyanoethylene ( 31 ), diethyl azodicarboxylate ( 33 ), and dimethyl acetylenedicarboxylate ( 35 ). With p ‐quinones such as p ‐benzoquinone ( 44 ) as dienophiles the expected mono‐and bisadducts are formed which were dehydrogenated to the bis‐ and tetrakis(trimethylsilylethynyl)naphtho‐ and ‐anthraquinones 47 and 48 , respectively, by treatment with DDQ. Heating of 1b in THF at 65°C caused dimerization to the 1,5‐cyclooctadiene derivative 56 , which was desilylated to 1,2,5,6‐tetraethynyl‐1,5‐cyclooctadiene ( 59 ) by treatment with potassium fluoride in DMF. 1b is approximately 15 times less reactive than 2,3‐dimethyl‐1,3‐butadiene ( 54 ) towards maleic anhydride ( 27a ).