CASCADE RADICAL CYCLIZATIONS VIA BIRADICALS GENERATED FROM (Z)-1,2,4-HEPTATRIEN-6-YNES

On heating in refluxing benzene, acyclic enyne-allene 4 underwent intr amolecular transformations in a sequence with an initial Myers cycloar omatization to form alpha,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-qui nodimethane 7, which in turn was captured in an intramolecular Diels-A lder 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 furni shed 18 having the fused 5,6,6,5-ring system. Tetracycle 19 substitute d with an angular methyl group was obtained from 17. In the cases of 2 4 and 34, a predominant 1,5-hydrogen shift of an allylic hydrogen to t he benzenoid radical center of biradicals 26 and 35 followed by a homo lytic coupling produced spiro derivatives 31 and 40, respectively. On the other hand, a preferential 7-endo ring closure of biradicals 41 an d 49, derived from 25 and 48, gave predominantly the tricyclic derivat ives 46 and 54, respectively. Similarly, cascade radical cyclizations via biradicals generated from enyne-allenes 67-70 produced the bicycli c spiro compounds 77-80 having a four-membered ring.