Nitrile ylids generation

Photolysis of the azirines 68 in the presence of DEAZD gives 1,2,4-triazolines (69, R = Et) via cycloaddition to the nitrile ylid.114 The nitrile ylid generated thermally from 70 gives 1,2,4-triazolines (69, R = Me, R1 = R2 = CF3) (Scheme 7).11S The cycloadditions proceed in good yield, and the triazolines 69 are readily converted into aromatic 1,2,4-triazoles. 

Nitrile ylids generated in this way can be trapped with methanol108 Thus, on irradiation in methanol, the azirine 137 was converted to the methoxy-imine (138). The intramolecular equivalent reaction leading to the oxazoline 139 has also been observed on irradiation of the 2//-azirine (140 R = OH),... 

Similar cycloadditions between thiirene dioxides and 1,3-dipoles generated in situ give heterocycles which result from either loss of sulfur dioxide or from the three-membered ring opening of the initially formed adduct (e.g. 174). Such cycloadditions with nitrilium imides (173a) and nitrile ylids (173b) are illustrated in equation 69175. 

Azirinylcyclopropenes, e.g. 213, are converted quantitatively into the corresponding pyridine derivatives, e.g. 215, on pyrolysis (or photolysis) via nitrile ylides, e.g. 214. The independently generated isomer 216 (equation 72) gives the same pyridine and can be diverted to adduct 217 in the presence of dipolarophile. Insights into the detailed sequence of events involved in the conversion of ylids such as 214 and 216 into pyridine have been... 

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