Oxaziranes isomerization

The rearrangement of phenyl- er -butyl nitrone to the isomeric oxazirane (9) occurred in 95% yield on irradiation in acetonitrile for 2 hr. Because 2-ieri-butyl-3-phenyloxazirane (9) can be reconverted into the more stable nitrone, the photochemical reaction involves the conversion of radiation energy into chemical energy. 

The oxaziranes are in the majority of cases distillable liquids with boiling points somewhat higher than the corresponding Sehiff s bases. During distillation, temperatures above 100 C should be avoided. In distinction to the isomeric nitrones, the less polar oxaziranes are usually noncrystalline. They have a characteristic unpleasant smell and are nonbasic. Attempts to force salt formation results in fission of the three-membered ring. ... 

The UV and IR spectra eliminate structures with a CN double bond. The isomerism of nitrones and oxaziranes thus cannot be a result of CIS or traris arrangement of substituents about a double bond. The carbon atoms of an oxazirane are still at the oxidation level of the carbonyl compound used in its syntheses. By acid hydrolysis, for example, 2-terf-butyl-3-phenyloxazirane (9) can be split into benzaldehyde and tert-butylhydroxylamine fEq. (8)]. ... 

The molecular refraction of oxaziranes is in accordance with the values calculated from the individual refractions. For the isomeric nitrones, exaltation occurs. ... 

Two pieces of chemical evidence support the three-membered ring formulation. The bifunctional oxazirane prepared from glyoxal, tert-butylamine, and peracetic acid (6) can be obtained in two crystalline isomeric forms. According to the three-membered ring formula there should be two asymmetric carbon atoms which should allow the existence of meso and racemic forms. A partial optical resolution was carried out with 2-7i-propyl-3-methyl-3-isobutyloxazirane. Brucine was oxidized to the N-oxide with excess of the oxazirane. It was found that the unused oxazirane was optically active. 

Further indications of the structure of the oxaziranes are obtained by reduction and isomerization. None of the reactions described in the following sections is incompatible with the three-membered ring structure. 

Most oxaziranes withstand temperatures of 100 C for a short time, e.g., on distillation. At higher temperatures isomerization and decomposition occur. Oxaziranes derived from aromatic aldehydes are here again differentiated from the alkyl-substituted oxaziranes. 

Oxaziranes are in a real sense active oxygen compounds and exhibit many reactions grossly analogous to those of organic peroxides. Thus they undergo one electron transfer reaction with ferrous salts and on pyrolysis they are converted to amides. Oxaziranes are also useful synthetic intermediates since in appropriate cases they may be isomerized to aromatic nitrones which are a convenient source of N-alkylhydroxylamines. The reaction of oxaziranes with peracids also provides a source of nitrosoal-kanes and is in many instances the method of choice for preparation of these compounds. ... 

Cis-trans isomerization of a,JV-diphenylnitrones can be photosensitized but does not result upon direct irradiation.457 Instead, oxazirane formation occurs. Presumably, then, the nitrone -> oxazirane photorearrangement proceeds from excited singlets. 

As mentioned above, the oxazirane ring ha limited stability. The products of its thermal decomposition vary considerably with the structure of the oxaoirane and also with the pyrolysis conditions. The products of thermal decomposition of the 3-arytoxaziraoes are the isomeric nitrones. This is apparently a general reaction and may be conveniently accomplished by beating solutions of the oxasiraun in... 

In this sequence a radical, possibly a btradical derived from unpairing the electrons of the oxazirane oxygen-nitrogen bond, abstracts the a-hydrogen atom of the A -alkyl group to form (XXI) which subsequently isomerizes to (XXII). Alternatively the formation of (XXII) may take place directly by a concerted, reaction. In either event the iminoaikoxy radical (XXII) carries the chain. The ammonia which is formed presumably comes from aldol-like condensations of the imine (XXIV). The fact that vapor-phase pyrolysis does not take this course simply reflects tbo low probability of a chain reaction in the vapor phase. 

Nitrones photoisomerize to oxaziranes (212,213). The oxaziranes often undergo thermal isomerization to amides (214,215). In certain cases the amide (or the enolic amide) is isolated directly from the irradiation. The nitrone Formula 472 photoisomerizes to Formula 473 (35%) (212). 

Hydration of the diazaoxepine (226) takes place at the less hindered carbon-nitrogen double bond to give the substituted propene (222). The hydroxypyrazine (126) arises from isomerization of the oxaziranes (224) and (225). Ultraviolet irradiation of 2,5-diphenylpyrazine... 

The a-hydroxy-nitrone (72) is isomerised (toluene-p-sulphonic acid-benzene) to (81). Reduction of (81) with NaBH4 gives the hydroxylamine (83). Incomplete reduction leads to the nitrone (82). Oxidation of (83) with Cu gives the isomeric nitrone (84). The photochemistry of the nitrones (82) and (84) as well as that of the steroidal oxazirans has been studied. Nitrone (82) gives the two oxazirans (85) and (86), while (84) gives a mixture of the oxazirans (87). Irradiation of the oxazirans (87) leads to an azetidine (88, R = Ac). 

Various routes to 1,1,3-trisubstituted 1//-isoindoles have been reviewed.5 Thus, the triphenyl derivative 53 is available from the reaction of benzo-phenone imine with diphenyldichloromethane or by the Friedel-Crafts reactions between benzene and l,l,3-trichloro-lf/-isoindole. Recently, another route, which involves heating the cyclotriphosphatriazene 54 with benzophenone, has been described.86 The nitrone derivative 55 of the isoindolenine 53 may be obtained by treating the isoindolenine with m-chloroperbenzoic acid. This nitrone has also been obtained by the isomerization of the oxaziran 56 in the presence of a Lewis acid (Scheme 11).87... 

Most oxazirancs withstand temperatures of 100 C for a short time, e.g., on distillation. At higher temperatures isomerization and decomposition occur. Oxaziranes derived from aromatic aldehydes are here again differentiated from the alkyl-substituted oxaziranes, a. Isomerization to Nitrones. 2-Cyclohexyl-3-phenyloxazirane iso-merizes on heating to 200 C. With further increase in temperature it is converted into iV-cyclohexylbenzaldoximc (70%) fEq. (24)]. ... 

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