Reactions of 2,7-dimethyloxepin with dimethyldioxirane and methyl(trifluoromethyl)dioxirane: Ring opening and probable observation of the intermediate '2,3-epoxyoxepin'

摘要

The reactions of dimethyldioxirane (7) and methyl(trifluoromethyl)dioxirane (8) with 2,7-(4) both yielded Z,Z-3,5-octadiene 2,7-dione (Z,Z-6) as their initial stable products. This is the first published reaction of a dioxirane with an isolable pure oxepin. Reaction of the dienedione Z,Z-6 with one mole equivalent of either 7 or 8 yielded the corresponding monoepoxide. Treatment of this monoepoxide with another equivalent of 8 yielded the corresponding di-epoxides (probably meso and R,S). The suggestion of Murray and co-workers that dioxiranes may model some of the reactivities of monooxygenases and their rapid epoxidations of alkenes under neutral conditions at low temperatures suggested their use. Our initial attempts to directly observe the putative intermediate 1,3-dimethyl-2,8-dioxabicyclo-[5.1.0]octa-3,5-diene ["2,7-dimethyl-2,3-oxepin" (2)] at low temperatures (ca. -60 degrees C) yielded promising but inconclusive results when dimethyldioxirane (7) was employed. The epoxidation reaction was sufficiently slow that it only occurred measurably above -30 degrees C, a temperature at which thermal ring opening to the dienedione is competitive. Initial reactions with the much more reactive methyl trifluoromethyl dioxirane (8) led to immediate ring opening at temperatures as low as -80 degrees C. Since 8 is known to isomerize to methyl trifluoroacetate and since water is present, a trace of trifluoroacetic acid was suspected of catalyzing the ring opening of 2, Thus, inclusion of either suspended Na2HPO4 or miscible 2,6-di-t-butylpyridine at -80 degrees C yielded an intermediate stable up to ca. 0 degrees C, which is likely to be the 2,3-epoxyoxepin 2. [References: 38]