Cyclopropa-Fusion to Seven-Membered Unsaturated Rings. The Synthesis of 4-Bromo-8,8-dimethylbicyclo[5.1.0]octa-1,4,6-trien-3-one, a Cyclopropa-Fused Tropone

Abstract

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 [5.1.0.0^3,5] 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[5.1.0.0^3,5]octane (26), (1α,3β,5β,7a)-1,7-dibromo-4,4- dichloro-8,8-dimethyltricyclo 1.0.0^3,5]octane (27) and (1α,3β,5β,7α)-4,4-dichloro-1,7-diiodo-8,8-dimethyltricyclo[5.1.0.0^3,5]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[5.1.0.0^3,5]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.