Aziridines, from triazolines

Photodecomposition of A -l,2,3-triazolines gives aziridines. In cyclohexane the cis derivative (304) gives the cis product (305), whereas photolysis in benzene in the presence of benzophenone as sensitizer gives the same ratio of cis- and trans-aziridines from both triazolines and is accounted for in terms of a triplet excited state (70AHC(ll)i). A -Tetrazo-lines are photolyzed to diaziridines. 

Elimination of N2 from triazolines 83 occurred thermally ( 20 °C) as well as photochemically. Depending on the substituent pattern and the method used, different products were obtained, such as aziridines 84a and 84b, oximines 85, and 1,3-oxazoles 86 (143). 

Based on results presented in Scheme 37, Logothetis suggested that the thermal decomposition products from the olefinic azides in the scheme are derived from triazoline intermediates formed by an intramolecular cycloaddition reaction and not by fragmentation of the azido group to a nitrene.100 However, allyl azide and 4-azido-l-pentene do not undergo internal cycloaddition because of the strain in the corresponding triazoline they fail to give aziridines and imines upon thermolysis.100... 

The influence of both anti and syn substitution in the 10-position on the thermolysis of triazoline adducts from a number of 7-substituted norbornenes (Scheme 14) has been investigated.128 Both the anti- and syn-substituted triazolines give only the exo-aziridine upon photolysis but pyrolysis yields endo-aziridines from the syn- and exo-aziridines from the anti compounds. The syn substituents have a strong endo directing influence and no exo-aziridine is detected in any case. Ketone formation in some cases apparently results from hydrolysis of the imines during workup.128... 

Synthesis of triazolines or aziridines from azides by photodecomposition or flash vacuum pyrolysis of 1,2,3-triazolines. 

Photoelimination of nitrogen from triazolines can similarly be employed in the synthesis of aziridines. Thus, the triazolines (41) have been converted in this way... 

Examples of the photoelimination of nitrogen from triazolines and the subsequent formation of aziridines have been reported.20 21 In particular, triazoline ring-contraction has been investigated as a possible synthetic route to mitomycins.22 Irradiation of the triazolines (33), in the presence of a triplet quencher... 

Scheme 6.17 Aziridines and imines by elimination of nitrogen from triazolines...

Some applications of the triazoline route to aziridines from alkenes and azides are described below. 

Scheme 6.23) (see also Chapter 7). Under these conditions the intermediate triazoline is not isolable the Lewis acid appears to have the dual role of activating the enone and facilitating the decomposition of the triazoline. Triflic acid is a very efficient catalyst for promoting the formation of aziridines from azides and electron deficient alkenes such as but-3-en-2-one (Scheme 6.23). ° The acid appears to activate the alkene to nucleophilic attack by the azide it is not necessary to invoke triazolines as intermediates although they may be involved. 

Another conceptually unique approach in alkene aziridination has come from Johnston s labs. These workers shrewdly identified organic azides as nitrene equivalents when these compounds are in the amide anion/diazonium resonance form. Thus, when a range of azides were treated with triflic acid and methyl vinyl ketone at 0 °C, the corresponding aziridines were obtained, in synthetically useful yields. In the absence of the Bronsted acid catalyst, cycloaddition is observed, producing triazolines. The method may also be adapted, through the use of unsaturated imi-des as substrates, to give anti-aminooxazolidinones (Scheme 4.25) [32]. 

Photoelimination of nitrogen from 1,2,3-triazolines has been widely used as a synthetic route to aziridines the reaction has been reviewed.355 Recent applications include the formation of a new valence isomer (425) of azepine from the triazoline 426,356 and conversion of the triazoline 427 into the aziridine 428, a process with potential as a synthetic route to mitomycins.357... 

Acyl azides 268, derived from furan, thiophene and selenophene, add slowly at room temperature to the strained double bond of 5-methylenebicyclo[2.2.1]hept-2-ene. Two regioisomeric triazolines, 269 and 270, which form in the first step, are unstable and decompose with elimination of nitrogen to provide aziridine derivatives 271. Products 271 are isolated in good yield (73-85%). It is worthy to note that not only the terminal, unstrained double bond in the starting material, 5-methylenebicyclo[2.2.1]hept-2-ene, is unaffected, but also the typical dipolarophiles like esters of crotonic, propiolic and byt-2-ynoic acids do not react with azides 268 under these conditions (Scheme 39) <2002J(P1)1420>. 

Ejection of dinitrogen from the triazoline adducts to form the related aziridines was promoted by ultraviolet irradiation (300 nm, benzene) and usually proceeded in excellent yield. An exception was found in the irradiation of the triazoline substrate 59, where cleavage of the cyclobutane ring occurred as the dominant reaction pathway to form the pyridazino norbomadiene 61 (and secondary photoproducts derived therefrom), together with the triazole-4,5-diester 62. A role for the pyridazine ring and the 2-pyridyl substituents in stabilising the diradical intermediate 60 has been proposed for this abnormal outcome (Scheme 8). 

Finally (pathway e, Scheme 5.56), triazoline 103 formed by cyclo addition of azide to glycal 1 can be photolytically converted into a 1,2-aziridine intermediate 104, from which 2-benzylamino-2-deoxy-P-glucosides can be formed on addition of an alcohol and catalytic scandium triflate [176]. 

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