posted on 1996-11-06, 00:00authored byKung K. Wang, Zhongguo Wang, Anna Tarli, Peter Gannett
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.