Alkylidenecyclopropanes as Dienophiles

Examples of the use of alkylidenecyclopropanes as dienophiles are very limited in the literature. Moreover, the few examples reported deal with rather sophisticated substrates such as 2,2-difluoromethylenecyclopropane, diarylmethylene-cyclopropanaphthalenes, bicyclopropylidene and (diacylmethylene)cyclo-propanes. 

Although diarylmethylenecyclopropabenzenes react in [4 -h 2] cycloadditions at the cyclopropene bridge bond [10a], diarylmethylenecyclopropa[5]-naphthalenes 14 react readily across the exocyclic double bond [10b]. The 

The formation of compound 16 likely involves an initial [4 + 2] cycloaddition across the exocyclic double bond to give the highly strained spirocyclo-proparene 15 which relieves the ring strain by rearrangement with concomitant ring expansion to 16. 

Reaction rates and yields increase upon using a hydrophilic solvent. The cycloaddition of 10 to 14a is essentially complete in a few hours when performed in ethylene glycol and gave 16a in higher yield (62%) [10b]. 

The reactions of 14 with a-pyrone (17) were carried out at 110 °C and gave the rearranged compound 19 [10b] (Table 2). 

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Although 1,1-disubstituted-l,3-dienes are quite unreactive in the Diels-Alder reaction, allylidenecyclopropanes exhibit a good reactivity especially toward activated dienophiles. The strain present in the alkylidenecyclopropane moiety is responsible for the reactivity enhancement observed in these compounds. The literature concerning the parent diene and few analogs until 1984 has been thoroughly reviewed by Krief [31]. Since then, some other examples of variously-substituted allylidenecyclopropane reacting as dienes in [4 + 2] cycloadditions were published. 

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