Cyclopropanes, -sigmatropic opening

Copper-catalyzed cyclopropanation of benzene and its derivatives by a diazoacetic ester yields a norcaradiene 230 which undergoes spontaneous ring opening to cyclo-heptariene 231. At the temperatures needed for successful cyclopropanation, sigmatropic H-shifts leading to conjugated isomers of cycloheptatriene carboxylates cannot be avoided. The situation is complicated by the formation of regioisomers upon cyclopropanation of substituted benzenes, and separation of the cycloheptatriene isomers may became tedious if not impossible. 

In a single case, thermal treatment of enone 36 induced a 1,3-sigmatropic silyl shift which led to the formation of the rearranged product 37 (Eq. 8.3) [40], So far, only one example of a ring-opening reaction of a cyclopropane 38 has been reported which furnished tetramethoxy-substituted allene 39 (Eq. 8.4) [41],... 

Photolysis of diazomethane in benzene gives cycloheptatriene (1, X = Y = The nor-caradiene intermediate 2, although detected by UV spectroscopy, cannot be isolated in this case it readily undergoes a [3,3]-sigmatropic shift with ring opening of the cyclopropane moiety, as do most cw-l,2-divinylcyclopropane systems. ... 

The cyclopropanation of dienes with 17 is not very selective and results in the formation of a 4 1 ratio of cis- and irans-l,2-divinylcyclopropanes 18. The c/.v-isomers 18 immediately undergo a Cope rearrangement to yield 19 under the reaction conditions, whereas the more stable trans-isomers can be isolated. However, the Pww-compounds 18 (R1 = CH3) arc rapidly transformed to ring-opened trienes 20 via a [1,5] sigmatropic hydrogen shift on distillation or attempted chromatography. 

The divinylcyclopropane unit is converted to a cycloheptadiene in this 3,3-sigmatropic rearrangement. Interestingly, this rearrangement also opens one ring (cyclopropane) and forms a new ring (cycloheptadiene). 

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