Azirines, nitrile ylide structure 2+ 3 -cycloaddition reactions

The ring opening of 2//-azirines to yield vinylnitrenes on thermolysis, or nitrile ylides on photolysis, also leads to pyrrole formation (B-82MI30301). Some examples proceeding via nitrile ylides are shown in Scheme 92. The consequences of attempts to carry out such reactions in an intramolecular fashion depend not only upon the spatial relationship of the double bond and the nitrile ylide, but also upon the substituents of the azirine moiety since these can determine whether the resulting ylide is linear or bent. The HOMO and second LUMO of a bent nitrile ylide bear a strong resemblance to the HOMO and LUMO of a singlet carbene so that 1,1-cycloadditions occur to carbon-carbon double bonds rather than the 1,3-cycloadditions needed for pyrrole formation. The examples in Scheme 93 provide an indication of the sensitivity of these reactions to structural variations. 

Irradiation of phenyl-2//-azirines in the presence of carbon dioxide leads to the formation of the 3-oxazoline-5-one system121-123 and, in some cases, to the isomeric 2-oxazolin-5-one122 [Eq. (24)1. The azirines serve as incipient nitrile ylides, whose 1,3-dipolar structure permits cycloaddition to the dipolarophile C02123 [Eq. (25)1. The reverse reaction, photolytic extrusion of C02 from pseudoxazolones, is synthetically useful, since the dipolar nitrile ylide thus formed can be trapped with a variety of dipolarophiles. Thus, 2,2,4-triphenyl-3-oxazolin-5-one (48) is readily converted into the stabilized ylide (49)124 [Eq. (26)1, and the use of methyl acrylate,122 acrylonitrile,122 and dimethylacetylene dicarboxy-... 

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