C3 - C5, C6 and C7 Ring Expansions

A mixture of cis- and trans-2-methylethynylcyclopropanes 48 and 49, prepared by the catalyzed (CuCl) addition of diazomethane to 3-penten-l-yne, upon heating to 530 °C underwent rearrangement to 3- and 4-methylenecyclopentenes 51 and 52, likely via the intermediate allylallene JO, and to 1,3-cyclohexadiene 54 and benzene 55, likely via the intermediate 1,3,5-hexatriene 53, Eq. (16) 26). 

The thermal isomerization of l-ethynyl-2-vinylcyclopropane 57 has been also reported. Thus, the dimer 59 was formed in the pyrolysis of the tosylhydrazone salt 

It is noteworthy that, contrary to 1-trimethylsiloxyvinylcyclopropanes which readily undergo thermal C3 - C5 ring expansion (vide infra, Sect. 5.5), the O-silylated 1-ethy-nylcyclopropanols 9 were recovered unchanged, on heating to 600 °C 28). 

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