Synthesis of Vinylallenes by Conjugate 1,6‐, 1,8‐, 1,10‐ and 1,12‐Addition Reactions of Organocuprates with Acetylenic Michael Acceptors and Their Use as Dienes in Intermolecular Diels‐Alder Reactions

Various vinylallenes were synthesized by conjugate cuprate additions to acetylenic Michael acceptors. Thus, 1,6‐addition reactions with 2‐en‐4‐ynoates 1, 3 , and 5a , respectively, furnished vinylallenes 2, 4 , and 7 after regioselective electrophilic capture of the allenyl enolates formed. Likewise, 1,8‐addition to 2,4‐dien‐6‐ynoates 8a and 10 gave the vinylallenes 9 and 11 , whereas the 1,10‐addition of Me 2 CuLi to 2,4,6‐trien‐8‐ynoate 12 provided allene 13 , and the analogous 1,12‐addition to 2,4,6,8‐tetraen‐10‐ynoate 14 furnished the polyene 15 . These vinylallenes are valuable dienophiles in regio‐ and stereoselective Diels‐Alder reactions, as evidenced by the formation of the cycloaddition products 16–24 . In the presence of Lewis acids, vinylallene 4a presumably rearranges to a cyclopentadiene derivative which then forms the cycloadducts 25 and 26 .