With azetes

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... 

In view of the uneven attention which azetidines, azetines and azetes have received and because of their lack of chemical similarity, they are treated separately in this chapter. Furthermore, because of the considerable literature on azetidin-2-ones, these have been dealt with in their own right, rather than as derivatives of azetidine. 

There are dubious claims for the displacement of the A7-fluorine in perfluoroazetine (240) (60DOK(l3l)l338) by ethanol and for the formation of an azete by reaction of the bicyclic azetine (241) with base. An attempt to produce azete by elimination of p-toluenesulfinic acid from azetine (237 Z = Ts) failed (7UOC435). 

Unlike the situation with unfused azetes, there is considerable support from reactivity studies for the existence of benzazetes which can, therefore, be regarded as firmly established. 

The benzodiazepine fused azetes (353 R = Et, Pr) have been claimed as useful tranquilizers and sedatives (76BRP1448895). However, their stability and formation from benzodiazepines (352) by reaction with an aldehyde and base is hard to reconcile with the proposed structure. 

Azetes, which are stabilized by rert-butyl groups, react with trifluoroaceto-nltrile to give pyrimidines [233] (equation 51)... 

There is only one reported synthesis of a bcnzannulated 1,2,4-triazocine system. 2-Phenylbenz-azete (1), which can be generated from 4-phenyl-l,2,3-benzotriazine by pyrolysis, is reacted with tetrazine 2 to give a [4 + 2] adduct, which loses nitrogen and finally results in the eight-membered-ring derivative after valence isomerization.2... 

The efficacy of RF in stabilizing small rings is well illustrated by the fact that the azete from trifluoro-l,2,3-triazine is considerably more reactive. Trapping experiments were unsuccessful and a polymer was isolated at room temperature. The dimer (41) forms an observable anion with CsF, which confirmed the endo structure [87CC1699 90JCS(P1)975, 90JCS(P1)983], In contrast, trifluoro-l,2,4,-triazine is resistant to vapor phase photoysis and flow pyrolysis [87JCS(P1) 1251]. 

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). 

The scheme for treating these subjects is as follows. We compare typical aromatic and antiaromatic compounds. Similar to the comparison between benzene and cyclobutadiene, we concern ourselves here with pyridine (34) and azete (57). Such an approach provides, apart from other advantages,... 

The title compounds may be classified into two groups, viz., the hetero-substituted annulenes containing a type Y heteroatom, for example, pyridine and azete, and the derivatives of monocyclic conjugated ions with heteroatoms of type X (two 7r-electrons) or Z (zero 7r-electrons) (Table VI), such as pyrrole (42)-borole (98) or borirene (99)-2-azirine (100). 

It turns out that cyclobutadiene is not a perfect square (two bonds are longer than the others), but it is essentially planar. Not surprisingly, it is very unstable and dimerizes extremely readily. It only exists at very low temperatures either in a matrix with an inert solvent (where the molecules are kept apart), or at room temperature as an inclusion compound in a suitable host molecule. Azacyclobutadiene (azete) is also extremely unstable, for similar reasons. 

Chromans with defined stereochemistry are accessible by manipulation of dihydrochromeno[3,2-b]azete-2,8-diones 26 which result from the cyclisation of azetidine-2-carboxylic chlorides 25 <99T5567> and also from chroman-4-ones via an initial enantioselective reduction by BHj in the presence of Coreyls oxazaborolidine <99T7555>. 

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). 

The only four-membered heterocycles with two endocyclic double bonds are the highly reactive azetes approaches to their synthesis are discussed in CHEC 5.09.5.3.1 and CHEC-II 1.18.5.6. 

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... 

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