Cascade Radical Cyclizations via Biradicals Generated from (Z)-1,2,4-Heptatrien-6-ynes

Abstract

On heating in refluxing benzene, acyclic enyne−allene 4 underwent intramolecular transformations in a sequence with an initial Myers cycloaromatization to form α,3-didehydrotoluene biradical 5 followed by a 5-exo cyclization of the benzenoid radical center in 5 to produce 6. Biradical 6 then decayed through a 1,5-hydrogen shift to furnish o-quinodimethane 7, which in turn was captured in an intramolecular Diels−Alder reaction to afford 8 having the tetracyclic steroidal skeleton in a single step from 4 in 50% isolated yield. Similarly, acyclic enyne−allene 16 having one of the tethers shortened by one carbon atom furnished 18 having the fused 5,6,6,5-ring system. Tetracycle 19 substituted with an angular methyl group was obtained from 17. In the cases of 24 and 34, a predominant 1,5-hydrogen shift of an allylic hydrogen to the benzenoid radical center of biradicals 26 and 35 followed by a homolytic coupling produced spiro derivatives 31 and 40, respectively. On the other hand, a preferential 7-endo ring closure of biradicals 41 and 49, derived from 25 and 48, gave predominantly the tricyclic derivatives 46 and 54, respectively. Similarly, cascade radical cyclizations via biradicals generated from enyne−allenes 67−70 produced the bicyclic spiro compounds 77−80 having a four-membered ring.