Ketals, cyclopropenone vinylcarbene generation

An unusual reaction occurred when a mixture of cyclopropenone propane-1,3-diyl ketal and (cyclopropylmethylene)malononitrile (3) was heated to give 2-cyclopropyl-6,10-dioxa-spiro[4.5]dec-3-ene-l,l-dicarbonitrile (4) in very good yield.The reaction is complex and takes place due to generation of a singlet vinylcarbene. ... 

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