Azomethine ylides, alkenyl cyclic

Open-chain C-alkenyl azomethine ylides 4.10.422 Cyclic C-alkenyl azomethine ylides... 

The mesoionic heterocycles called munchnones function as cyclic azomethine ylides in cycloaddition reactions.63 The alkenyl moiety can be attached at carbon or at nitrogen. 

Table VIII summarizes the cycloaddition reactions of azomethine ylides with maleimides and maleic anhydrides. The upper part of the table involves endo-selective cycloadditions and the lower part nonselective reactions. Endo selectivity is closely related to the substitution pattern of the azomethine ylides used. The azomethine ylides that show high endo selectivity to these cyclic olefins bear an ylide-stabilizing substituent of the carbonyl or cyano type (EWG) at one carbon and an aryl, heteroaryl, acyl, or 1-alkenyl substituent (R ) at the other. These key substituents must be present for high endo selectivity, although the substituent (R) on the ylide nitrogen is equally important.

Although very important for understanding the mechanism of the process, these reactions are of limited usefulness as synthetic methods. In the case of cyclic alkenes, c.g., AT-methyl maleinimidcs or maleic anhydride, high endo selectivity is achieved in the reaction with iV-un-substituted or A -metalated azomethine ylides bearing an electron-withdrawing substituent (carbonyl, cyano) at one carbon and an aryl, acyl, or 1-alkenyl group at the other. 

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