The synthesis of
2-methyl-3,4,5-triphenyl-8-oxatricyclo[4,3,0,07,9]nona-2,4-diene
(14) is described, the first stable member of this ring system. Preparation of
(14) was achieved by reacting dibenzyl diazenedicarboxylate with
2-methyl-3,4,5-triphenylbicyclo[4,2,0]octa-2,4,7-triene (29) to form the adduct
(30a), which was selectively epoxidized at the cyclobutenyl double bond with
3-chloroperbenzoic to yield the epoxide (32a). Removal of the benzyl groups (H2/Pd)
from the hydrazo bridge of (32a), followed by oxidation (red mercuric oxide or
hydrogen peroxide) yielded the putative azo compound (33) which spontaneously
lost dinitrogen, even at -78�, to yield (14) as a crystalline solid, m.p. 128�.
While (14) has the potential to undergo 1,2-photoaromatization to yield oxiren
(4a), this is not realized and ring-opening to the cyclic epoxide (34) is the
major photochemical reaction observed under various ultraviolet irradiation
conditions. Isomerization of (14) into (34) also occurs under thermal
conditions. The 1H n.m.r, spectra of (14) as well as its trapping
with dienophiles is discussed.