Azirines nitrile oxides

A variety of 1-azirines are available (40-90%) from the thermally induced extrusion (>100 °C) of triphenylphosphine oxide from oxazaphospholines (388) (or their acyclic betaine equivalents), which are accessible through 1,3-dipolar cycloaddition of nitrile oxides (389) to alkylidenephosphoranes (390) (66AG(E)1039). Frequently, the isomeric ketenimines (391) are isolated as by-products. The presence of electron withdrawing functionality in either or both of the addition components can influence the course of the reaction. For example, addition of benzonitrile oxide to the phosphorane ester (390 = C02Et) at... 

I.3.4.2.2. Nonaromatic Unsaturated Heterocycles Reactions of aromatic nitrile oxides with 1-azirines are followed by the ring opening of the latter to give 4-benzamidoisoxazoles 145 (314). The structure of 145 (R = 4-C1C6H4, Ar = Ar7 = Ph) was established by single-crystal X-ray analysis. A mechanism for the formation of 145 has been proposed, (see Scheme 1.29). 

Extensive work has been done to determine and understand the factors controlling diastereoselectivity in the cycloaddition of nitrile oxides to alkenes but very little is known about nitrile ylides in this regard. Work on their reactions with alkenes that are geminally disubstituted with electron-withdrawing groups (e.g., 187) has illustrated some of the difficulties in such studies. When the imidoyl chloride-base route was used to generate the nitrile ylides it was found that the products 188 epimerized under the reaction conditions. When the azirine route was used, the reaction was complicated by the photochemical isomerization of the dipolarophiles (96,97). Thus, in both cases, it proved impossible to determine the kinetic product ratio. 

Two research groups have independently observed that the pyrolysis of 4,5-dihydro-1,2,6-oxazaphospholes can be a useful synthesis of 1-azirines (99) in some cases.50-53 The oxazaphospholes (98) are readily prepared by the cycloaddition of nitrile oxides (96) to alkyl-idenephosphoranes (97). 

The 47r-electron system of nitrile oxides can also participate in 1,3-dipolar cycloaddition with 2H-azirines (71TL4831). For example. 

Cycloaddition reactions of the C(3)=N bond of azirines are common (Scheme 45) <71AHC(13)45, B-83MI 101-03,84CHEC-I(7)47>. Azirines can participate in [4 + 2] cycloadditions with dienes including cyclopentadienones, isobenzofurans, triazines, and tetrazines. They also participate in 1,3-dipolar cycloadditions with azomethine ylides, nitrile oxides, mesoionic compounds, and diazomethane. Cycloadditions with heterocumulenes, benzyne, and carbenes are known. Azirines also participate in other pericyclic reactions, such as ene reactions. 

Arenecarbonitrile oxides, generated by the joint action of acetyl chloride and sodium methox-ide on arylnitromethanes,7 react with symmetrical diaroylmethanes to yield transient 2/7-aziri-nes, which add a second molecule of the nitrile oxide to form 5-aroyl-3,6-diaryl-4//-1,2,4-oxa-diazines 1. The suggested course of the reaction is supported by the observation that 1,2,4-oxadiazines are formed from nitrile oxides and isolated azirines.8... 

It is known that 2/7-azirines can react with carbonitrile oxides to yield carbodiimides <71TL483I>. It has been found, however, that the 2-acyl-2/f-azirines (265), formed in situ by the addition of carbonitrile oxides to the enol tautomers of /1-diketones, are trapped by an excess of nitrile oxide at the bicyclic 1,3-dipolar cycloadducts (266), which promptly undergo ring expansion to 5-acyl-4//-... 

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