We report on the use of visible light to conduct carbene-transfer reactions of donor/acceptor diazoalkanes with cyclooctatetraene and polyunsaturated carbocycles to give the corresponding cyclopropanes in excellent yields with excellent stereoselectivities. This photochemical protocol proved to be superior to conventional metal-catalyzed cyclopropanation reactions and now provides platform chemicals containing a cyclic conjugated all-cis triene. The cyclopropane is an important structural feature to prevent 6π electrocyclization. Instead, the triene moiety can now be selectively functionalized in cycloaddition reactions.