Cycloaddition/rearrangement cyclopropanes, preparation

Dihydrospiro[cyclopropane-l,5 -isoxazoles] 41 can be prepared by cycloaddition of nitrile oxides to methylenecyclopropanes 40. Thermolysis of the spiro-compounds led, in general, to the formation of mixtures of enaminones 42 and pyridones 43. However, the proportion of the enaminone 42 was much reduced when the rearrangements were carried out by flash-vacuum pyrolysis at 400 °C, and good yields of pyridones 43 were isolated (see Section 1. A.5.2.3.1 ... 

Methylenecyclopropcmes. The preparation of 6-methylenebicyclo[3,l,0]hexane (122) from tosylhydrazides (120) and (121) involves methylene-cyclopropane rearrangement. The high-temperature isomerization of equilibrium mixtures of ethylidene-cyclopropane and methylmethylene cyclopropane to isoprene is thought to proceed by way of a biradical intermediate which suffers 1,4-hydrogen transfer. The tetra-cyclononane (267) does not undergo [ 2 -I- 2j cycloaddition, but affords (268) by way of geometrical isomerization and ene reaction. ... 

In an extension of previous work, it has been found that Pd(0)-catalysed intramolecular cyclization of allylic acetates (21) can be used to prepare the chrysanthemic acid analogues (22). The potentially useful cw-cyclopropane (23) can be simply obtained by base-induced addition of cyanoacetate to ethyl 2-bromo-3,3-dimethylacrylate followed by decarboxylation oddly, a similar reaction using malonate fails to give a cyclopropane. Optically pure dichloro cw-chrysanthemic acid (26) has been obtained by a Favorskii rearrangement of the chiral cyclobutanone (25) prepared from the keten (24) by sequential [2 + 2]cycloaddition, cine-rearrangement, and resolution (Scheme 3). ... 

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