Electron deficient enolate geometry

Only the E,3E stereoisomer of the 2-azadiene was found to be reactive under the reaction conditions. Consequently, each of the piperidine products was found to possess the cis 2-phenyl, 5-methyl relative stereochemistry. Moreover, the cycloaddition products derived from reactions of enol ethers possessed the all-cis 2-phenyl-4-alkoxy-5-methyl stereochemistry necessarily derived from exclusive endo cycloaddition. Dienophile geometry is maintained during the course of the [4 + 2] cycloaddition, and no products derived from a potential stepwise, addition-cyclization reaction were detected. Representative neutral and electron-deficient dienophiles failed to undergo cycloaddition. A related boron trifluoride etherate-catalyzed [4 + 2] cycloaddition of simple 2-aza-l,3-butadienes with carbonyl compounds provides 5,6-dihydro-2//-l,3-ox-azines and appears to proceed preferentially through an endo [4 + 2] transition state although evidence supporting a stepwise, addition-cycli-zation was occasionally detected.630... 

In contrast with the photochemical cycloaddition reaction of two alkenes, the [2+2] cycloaddition of a ketene and an alkene occurs under thermal conditions. The ketene is formed typically from an acid chloride and a mild base such as EtsN, or from an a-halo-acid chloride and zinc. Cycloaddition with an alkene occurs stereospecifically, such that the geometry of the alkene is maintained in the cyclobutanone product. The regioselectivity is governed by the polarization of the alkene, with the more electron-rich end of the alkene forming a bond to the electron-deficient central carbon atom of the ketene. Thus, the product from cycloaddition of dimethylketene with the enol ether Z-171 is the cyclobutanone m-172, whereas with -171, the isomer trans-lll is formed (3.116). ... 

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