2- alkene-1,6-dione cyclopropane

The reaction of electron-rich alkenes with diazo compounds containing two electron-withdrawing groups does not always result in cyclopropane formation. Instead, a [3+2] cycloaddition can occur, presumably by means of a stepwise process [1,2]. One report has appeared which demonstrates that the N-amido-prolinate 3 can catalyze a [3+2] cycloaddition between diazocyclohexane-1,3-dione (39) and 2,3-dihydrofuran (40) to form the tricyclic system 41 with very high enantioselectivity [Eq. ( 21)] [8]. 

Under analogous conditions, 2,2-dibromo-l,3-diones react with alkenes to give 4,5-dihy-drofurans rather than cyclopropanes (see Houben-Weyl, Vol. E19b, pl225). A notable exception is the case of ethyl 2,2-dibromo-3-oxobutanoate, where predominant cyclopropanation of styrene was observed to give cis- and trans-9. ... 

The carbenoid reaction between a-diazo ketones and simple alkenes or styrenes leads to acylcyclopropanes. (For the enantioselective cyclopropanation of styrene with 2-diazo-5,5-dimethylcyclohexane-l,3-dione, see Section 1.2.1.2.4.2.6.3.2.). With ketene acetals, 2,3-dihyd-rofurans are obtained. In contrast, l-acyl-2-oxycyclopropanes or 2-oxy-2,3-dihydrofurans can be formed in reactions with enol ethers and enol acetates the result depends strongly on the substitution pattern of both reaction partners.Whereas simple diazo ketones usually lead to cyclopropanes (Table 15), 3-diazo-2-oxopropanoates and 2-diazo-l,3-dicarbonyl compounds, such as 2-diazoacetoacetates, 3-diazopentane-2,4-dione, and 2-diazo-5,5-dimethylcy-clohexane-1,3-dione, yield 2,3-dihydrofurans and occasionally acyclic structural isomers thereof when reacted with these electron-rich oxy-substituted alkenes. 

Photochemical ring contraction of various substrates, e.g. cyclobutane-1,3-diones and 1 -pyrazolin-4-ones - gives either isolable cyclopropanes or cyclopropanones as intermediates which are usually intercepted as adducts or which fragment to alkenes and carbon monoxide. The photoextrusion process most often leads to fragmentation products. 

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