3- Pentanone 4 + 3 cycloaddition reactions

Cyclopropylidene-l,3-dioxanes or -1,3-dioxolanes are ketene acetals and examples of bis-donor-substituted methylenecyclopropanes which undergo facile cycloaddition with electron-deficient alkenes 12. Thus, spiro[2.3]pentanone acetals 13 were obtained after reaction in an aromatic solvent either at room temperature or 40 C. Upon subsequent aqueous workup, hydrolytic cleavage of a C-C bond of the cyclobutane ring took place rather than the expected acetal hydrolysis (Table 7). This cycloaddition was found to proceed in a stepwise fashion as reaction of the dimethyl ( )-but-2-enedioate gave a cisj tram mixture of cycloadducts on further heating, this ratio changed. 

The unusual incorporation of the cyano group of l-butyl cyano ketene was observed in the reaction of an azomethine with excess of r-butyl cyanoketene resulting in pyrimido[6,l-d][l,3,5] dioxazine (29). It was said to proceed via an intermediate adduct (98), which further underwent [2 + 2 + 2] cycloaddition in the presence of two molecules of ketene (Scheme 10) <78CC820). 4-Isothiocyanato-4-methyl-2-pentanone and A -(2-hydroxyethyl)hydrazine in refluxing ethanol led to... 

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