Azomethine ylide photochemical generation

For the examples reported so lar, usually in the presence of die alkene the azomethine ylide is generated in situ by photochemical disrotatory ring-opening of the aziridine (254 nm) in dioxane yielding pyrrolidines. (Obviously, thermal conrotatoiy ring-opening of aziridines demands conditions, which oppose an efficient stereocontrol of the subsequent cycloaddition with a,P-unsaturated carboxylic ester derivatives.)... 

Azomethine ylides (Section 4.03.6.1.1) have been generated from a wide variety of aziridines using both thermal and photochemical methods. With carbon-carbon unsaturated dipolarophiles, pyrrolines or pyrrolidines are obtained. With hetero double bonds, however, ring systems of interest to this discussion result. 

From a synthetic viewpoint, iV-arylvinylamines are not appropriate as azomethine ylide precursors because the hydrogen shift from the intermediary ylides must be extremely accelerated by the rearomatization of the fused dihydro benzo moiety. Zaima and Mitsuhashi were the first to succeed with synthetic applications of the above photochemical generation method of cyclic azomethine ylides (83JHC1). The substrate employed in their work is bis(l-methoxycarbonylvinyl)amine. Irradiation of this divinylamine in carbon tetrachloride at 18°C in the presence of acetylenedicarboxylates produces excellent yields of 7-azabicyclo[2.2.1]hept-2-ene-l,2,3,4-tetracarboxylates 172, which correspond to the cycloadducts of the expected azomethine ylide intermediate 171. Heating the bicyclic cycloadducts 172 at 90-120°C induces a smooth cycloreversion eliminating a molecule of ethene to give pyrrole-2,3,4,5-tetracarboxylates in quantitative yields. The azomethine ylide 171 can be trapped also with olehnic dipolarophiles, such as maleates and fuma-rates, to furnish stereospecifically 7-azabicyclo [2.2.1] heptane-1,2,3,4-tetra-carboxylates 173 and 174, respectively (84JHC445). 

Thermolysis or photolysis of suitably substituted aziridines is a very convenient method for the generation of azomethine ylides. Thermal ring opening of aziridines involves a conrotatory pathway like that of a cyclopropyl anion with which it is isoelectronic. As predicted by the orbital symmetry rules, the ring opening of aziridines follows disrotatory mode under the photochemical conditions. The stereochemical consequences of these two modes of ring opening result in the formation of tra/is-azomethine ylide on thermolysis of cw-dicarboxylic ester and of cis-azomethine ylide on photolysis (Scheme 5.17). The rra j-dicarboxylic acid ester behaves in the expected manner. 

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