2/f-Azirines

Diels-Alder cycloaddition of 2/f-azirines 23 with cyclopentadienones provides 3//-azepines 25 in excellent yields by electrocyclic ring opening, with concomitant loss of carbon monoxide, of the initially formed, nonisolable cycloadducts 24, followed by a [1,5]-H shift in the resulting 2//-azepines.31 108... 

In an earlier survey of the cycloaddition chemistry of diazo compounds with imines (5), it was noted that diazoalkanes react particularly well with both electron-deficient acyclic and strained cyclic imines, such as 2/f-azirines, producing... 

Phenyl- 2/f-azirine 3-Phenyl-2 H-azirine 2,3,4,5,6,7,8 a-Octa-hydroquinoline ... 

On pyrolysis of isoxazoles, decarbonylation has also been observed. Carbon labeling experiments have indicated that the formation of 2,3-diphenyl-2/f-azirine (86) and 2-phenylindole from 3,5-diphenylisoxazole takes place as shown in Scheme 22 thus, 3,5-13C-labeled starting material gave 2-[2-13C]phenylindole containing 51% of the original 13C.205... 

The formation of 820 was rationalized by a stepwise mechanism. The initial addition of the fulvene to the activated 2/f-azirine generates the zwitterionic intermediate 822 which then cyclizes to [2]pyrindine 820 via intermediate 823 (Scheme 203). 

As has been shown in Section 4.11.4.3, 1,2,3-triazole and A -l,2,3-triazolines can be converted either thermally or photochemically into three-membered 2/f-azirines and aziridines, respectively, with evolution of nitrogen (for reviews see B-67MI41101, B-70MI41101, B-76PH403). In the following section some additional applications of greater preparative interest are described. 

Thermolysis in toluene under reflux of 2-benzoyl-2-halo-2/f-azirines led to 4-haloisoxazoles in high yields (Equation 95), while the reaction of halogenated cyclopropanes with the nitrosyl cation appears to be a feasible and effective method for preparing 5-chloroisoxazoles (Equation 96) <20028605, 1997JOC5229>. 

Benzonitrile-p-nitrobenzylide (69) reacts in situ with 2/f-azirines 62 to form 1,3-diazabicyclo[3.1.0]hex-3-enes 70 in low to moderate yields. Other... 

The major thermal reaction of 2/f-azirines involves C(2)—N bond cleavage as shown in Equation (6) to form vinyl nitrenes (47). Cleavage of the C—C bond to produce iminocarbenes (48), diradical... 

Aziridinations of vinylsilanes <89TL2703> and vinylstannanes <89CC836> have been reported. Treatment of the resultant aziridines with caesium fluoride caused elimination to the 2/f-azirine, as shown in Scheme 59. 

UV Irradiation of an 3-aryl-2-azidoacrylic ester (2-azidocinnamate) results in elimination of nitrogen and formation of a 2/f-azirine, which undergoes spontaneous electrocyclic ring opening to give a reactive intermediate which can react as an azomethine ylide dipole or an alk-oxycarbonyl(benzylidenimino)carbene. In favorable cases, the carbene form can be trapped by intramolecular [l-t-2] cycloaddition to an alkene double bond to give a dihydro-cyclopropa[c]isoquinoline, e.g.l, and 2 (Houben-Weyl, Vol. E19b, pi 166). 

A number of examples of imidazole synthesis from azirines and aziridines have been included under other classifications (see Sections 3.02.8.1.l(iii) and 3.02.8.1.2(i),(ii)). 2/f-Azirines are intermediates in the rearrangement of oximino p-toluenesulfonates to aminoketones in the presence of bases, and with 2-acylphenols these azirines form imidazolones. 4//-Imidazoles (244) are formed when a 2-amino-3,3-dimethylazirine reacts with an acetamide (Scheme 181) <90CHE719>. 

When treated with heterocycles possessing acidic NH groups, 2-monosubstituted 3-amino-2/f-azirines ring expand (Scheme 182) <90HCA2275,92HCA1251,92HCA2270). 

Thio-2-thiazoline (86a), which is in tautomeric equilibrium with (86b), reacts with 3-(dimethyl-amino)-2/f-azirines in acetonitrile at ambient temperature giving rise to a mixture of imidazo[4,3-... 

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//-... 

The reactions of triethyl phosphite with 5-amino- (31) and 5-alkoxy- (32) isoxazoles have been extensively investigated. In both cases aziridinyl phosphonates were obtained, the best yields being in the presence of a proton donor. Similar reactions with the isomeric 2 f-azirines (33) and (34)... 

Isomeric nitrile methylides are the primary products of irradiation of 3-phenyl-2/f-azirines. These ylides continue to be of interest as synthetic intermediates because of the ease with which they undergo 1,3-dipolar addition to a variety of unsaturated species. Irradiation in benzene of 2,2-dimethyl-3-phenyl- or... 

Reactions of 2,3-di(methoxycarbonyl)-2/f-azirine with keto-stabilized and ester-stabilized ylides give, respectively, substituted pyrroles and iminophos-phoranes (76). A similar reaction of the ketene-stabilized ylide (77) with 3.3-dimethyl-2-phenyl-l-azirine forms the oxaphospholene (78). ... 

A typical high-yield synthesis of a vinyl substituted 2/f-azirine is shown by the conversion of the azide 624 into 625". Similar behaviour is seen for the isomeric azidoeneynes 626 and 627 that are converted into the alkynyl substituted azirine 628 while irradiation of the azidoallylphosphonates 629 yields the corresponding azirines 630 and 631 is transformed into 632 at 0 This route to azirines has been extended to the synthesis of the bis-2/f-azirines 633 and 634, which are formed by irradiation of the bis azides 635 and 636", respectively. The formation of these products arises by sequential loss of nitrogen. Thus the bis azide 635 is converted to 637 prior to the ultimate formation of the final product 633. Methylene azirines such as 638 are formed on photoelimination of nitrogen from the azides 639. In this instance, however, the presence of the azirine derivative was established by trapping experiments with HCN when the adducts 640 are formed . 

In some instances the vinyl azide does not undergo the conversion to the 2/f-azirine and instead yields a nitrene, which adds to alkenes and dienes. This is seen with the azide 653 which, on irradiation through Pyrex, affords the corresponding nitrene that adds to alkenes stereospecifically to yield aziridines. Stereo specific addition to dienes is exemplified by the irradiation of the quinone azide 654 in the presence of trans,trans-hQX3,-2,4-diene. Aziridines do not result from this process and instead the stereoisomeric indole derivatives 655 and 656 are obtained. This process has been developed as a route to... 

Ylides can be quenched " with electron-deficient alkenes to yield A -pyrrolines, 225. They also react, albeit more slowly, with less activated alkenes such as diethyl maleate or 2,3-dimethyl-2-butene. In the absence of reactants, the ylide forms a 2/f azirine (226), but on irradiation " this reverts to the ylide, 227. 

The triazoles are numbered to indicate the relative positions of the nitrogen atoms, tetrazole and pentazole are unambiguous names. 1,2,3-Triazoles are surprisingly stable, when one considers that they contain three directly-linked nitrogen atoms, but on flash vacuum pyrolysis at 500 °C they do lose nitrogen to give 2. f-azirines,... 

Coe, P.L. Cook, M.I. Reactions of tetrafluoroethene oligomers. Part 16. The synthesis and reactions of perfluoro[2,3-dimethyl-2-(3-methylpentan-3-yl)]-2/f-azirine a source of 2,3-bis(trifluoromethyl)aziridine derivatives. J. Chem. Soc., Perkin Trans. 1 2000, 1537. 

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