Azetes, cycloadditions with

Kinetically stabilized azetes also show a high tendency for cycloaddition with a variety of other reagents. Cycloaddition of (47) with triplet oxygen produced a fully characterized dioxetan adduct (48), which decomposed at 25°C into t-butyl cyanide and the a-dione fragment (88AG(E)272). 

As expected, similar products of [2+2] cycloaddition are formed in the reaction of azete 126 with siloxy and ethoxyacetylene ... 

Phenylsydnone 89 is not restricted to [3+2] cycloaddition. Reaction of sydnone 89 and its derivatives with the substituted azete 90 gives isomeric l//-triazepines after extrusion of carbon dioxide (Equation 8). 

Pyridine 20 has been prepared by the silver- or acid-catalyzed isomerization of 1-aza-bicyclo[2.2.0]hexadiene 22, which in turn was obtained in a [2 + 2]-cycloaddition reaction of tris(to7-butyl)azete with hexafluorobut-2-yne. Irradiation of Dewar pyridine 22 gives a high yield of azaprismane 23, which isomerizes thermally to rearranged pyridine 24. Finally, pyridine 24 yields 1-aza Dewar pyridine 25 photolytically.34... 

Cycloaddition is the most common source of 1,2-oxazetidine skeletons, but sometimes it is also useful to construct 1,3-oxazetidine rings. Indeed, in contrast to the usual reactivity exhibited by mtinchnones, which react with multiple bond systems as azomethine ylides by a [3+2] cycloaddition process, azete 58 was found to react with miinchnone 59, under thermal conditions, to give an isomeric mixture of the oxazabicyclo[2.2.0]hexene (E/Z)-26,... 

The reaction of tristterl-buty 1)azete with trifluoroacetonitrile is a (4 + 2) cycloaddition followed by a retro (2 + 2) cycloaddition to afford 2-tri-fluoromethyl-4,5,6-tris(teri-butyl)pyrimidine [ 126]. 

Kinetically stabilized azetes (575) react with acceptor-substituted nitriles in a [4 -I- 2] cycloaddition step (Scheme 91). The initially formed Dewar pyrimidine (576) is subsequently isomerized to the pyrimidine (577). The sterically demanding substituents necessary for the stabilization of the azetes are thus introduced into the pyrimidine ring <90SL40i>. 

Analogously to siloxy- and alkoxyacetylenes (see Sections II.D.2 and III.C.4), alkynyl esters are good partners in [2+2] cycloaddition reactions. A typical example of such a reaction is the cycloaddition of benzoate 116 and tosylate 128 with azete 126 (equations 78... 

Azete 6 dimerizes thermally to the 1,3-diazetidine system 7, which can be isomerized to the 1,5-diazocine 7. Azete 6 shows numerous cycloaddition reactions, especially with activated acetylenes and phosphaalkines for its transformations to the pyridine valence tautomers see p. 364. 

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