Benzocyclobutadienes via Electrocyclizations of (Z,Z)-3,5-Octadiene-1,7-diynes Leading to Dimers with Unusual Polycyclic Structures

Abstract

(Z,Z)-3,5-Octadiene-1,7-diynes 15a−e were synthesized by condensation of enynyl aldehydes 12a−e with allenylborane 11 to furnish enynyl alcohols 14a−e followed by the elimination step of the Peterson olefination reaction. Desilylation of 15a with tetrabutylammonium fluoride (TBAF) followed by two consecutive electrocyclizations resulted in the formation of the corresponding benzocyclobutadiene 17a, leading to the four angular dimers 18a−d. The use of cyclopentadiene to capture 17a afforded the Diels−Alder adduct 19. Treatment of 15b with TBAF produced only one angular dimer 21. On the other hand, the presence of a phenyl substituent on the four-membered ring directed the dimerization of benzocyclobutadiene 17c toward the linear dimer 23, which then underwent a facile thermal rearrangement to dibenzocyclooctadiene 24. Interestingly, benzocyclobutadienes 17d and 17e having an alkenyl substituent on the four-membered ring dimerized via a formal [4 + 4] cycloaddition to the 1,5-cyclooctadienes 26 and 32, resp...