The preparation of a stable 8-oxatricyclo[4,3,0,0^7,9]nona-2,4-diene and an evaluation of its potential as a source for the photochemical generation of oxiren

Abstract

The synthesis of 2-methyl-3,4,5-triphenyl-8-oxatricyclo[4,3,0,0^7,9]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 -78º, to yield (14) as a crystalline solid, m.p. 128º. 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.