This paper addresses the problem of preparing novel species such as the cyclopropa -fused tropylium ion and cyclopropa -fused tropones and tropolones. Addition of dibromocarbene and dichlorocarbene to the adduct (21) derived from dimethyl 3,3-dimethylcyclopropene-1,2- dicarboxylate and buta-1,3-diene yields the anti-tricyclo [5.1.0.03,5] octyl derivatives (22) and (23) which after hydrolysis and halodecarboxylation were transformed into (1α,3β,5β,7α)- 1,4,4,7-tetrabromo-8,8-dimethyltricyclo[5.1.0.03,5]octane (26), (1α,3β,5β,7a)-1,7-dibromo-4,4- dichloro-8,8-dimethyltricyclo 1.0.03,5]octane (27) and (1α,3β,5β,7α)-4,4-dichloro-1,7-diiodo-8,8-dimethyltricyclo[5.1.0.03,5]octane (28). All of these halides were remarkably stable towards dehydrohalogenation, and this is attributed to the rigidity of the tricyclic ring system. The reaction of (1α,3β,5β,7α)-1,7-diiodo-8,8-dimethyltricyclo[5.1.0.03,5]octane (38) with potassium t- butoxide results in deiodination rather than dehydroiodination and gives a cyclopropene which was trapped with isobenzofuran as adducts (42) and (43). Addition of dibromocarbene to the adducts (48) and (49) derived from dimethyl and di -t-butyl 3,3-dimethylcyclopropene-1,2-dicarboxylate and 2-trimethylsilyloxybuta-1,3-diene followed by ring expansion gave the bicyclo [5.1.0] octyl derivatives (56) and (57). Acidic hydrolysis of the di -t-butyl ester (57) afforded diacid (59), which on iododecarboxylation gave 4-bromo- 1,7-diiodo-8,8-dimethylbicyclo[5.1.0]oct-4-en-3-one (60). Treatment of (60) with triethylamine afforded the title compound (62), the first example of a cyclopropa-fused tropone.
5H-Cycloprop[f]isobenzofuran (6) and the sulfur analogue 5H-cyclopropa[f][2] benzothiophen (18) have been prepared by a sequence of reactions involving trapping of 1,2-dibromocyclopropene with 3,4-dimethylidenetetrahydrofuran and 3,4-dimethylidenetetrahydrothiophen followed by sequential dehydrogenation and di-dehydrobromination. Both cyclopropa -fused heterocycles, like their parents, have limited stability. Several other 5,6-methylene-bridged and 5,6-disubstituted isobenzofurans (32) have been generated and characterized as their adducts with dimethyl fumarate . Second-order rate constants for the reaction of dimethyl fumarate with isobenzofuran, 5H-cycloprop[f] isobenzofuran as well as the series of substituted derivatives have been measured. The reactivity span is only one order of magnitude suggesting that π-bond fixation (the Mills-Nixon effect) does not play a significant role in determining the reactivity of (6).
The application of known methodology has provided 2,3-dimethylidene-2,3-dihydro-1,4-benzodioxin (12) as the essential precursor for preparation of the cyclopropa-fused dibenzodioxin 3,8-dioxa-1H-cyclopropa [b] anthracene (14). Cycloproparene (14) is both unstable and acid-sensitive, and readily dimerizes to a mixture of the dihydrotetraoxaheptacene (15) and its heptaphene analogue (16).