Rearrangement to other heterocyclic species

Many examples are known of rearrangement of azoles involving scrambling of the ring atoms to give a new isomeric azole molecule. Different mechanisms are involved. 

For isoxazoles the first step is the fission of the weak N—O bond to give the diradical (60) which is in equilibrium with the vinylnitrene (61). Recyclization now gives the substituted 27/-azirine (62) which via the carbonyl-stabilized nitrile ylide (63) can give the oxazole (64). In some cases the 2H-azirine, which is formed both photochemically and thermally, has been isolated, in other cases it is transformed quickly into the oxazole (79AHC(25)147). 

MNDO results suggest that the activation energies are similar for the gas phase thermal isomerization of isoxazole to oxazole via either a nitrile ylide or a keteneimine, through an azirine intermediate. The first step is rate limiting, which is in good agreement with experimental results (90JPO611). 

The photorearrangement of pyrazoles to imidazoles is probably analogous, proceeding via iminoylazirines (isomerization enthalpy =42 kJ mol ) (82AHC(30)239) indazoles similarly rearrange to benzimidazoles (67HCA2244). 3-Pyrazolin-5-ones (65) are photochemically converted into imidazo-lones (66) and open-chain products (67) (70AHC(ll)l). The 1,2- and 1,4-disubstituted imidazoles are interconverted photochemically. 

Irradiation of isothiazole gives thiazole in low yield. In phenyl-substituted derivatives an equilibrium is set up between the isothiazole (68) and the thiazole (70) via intermediate (69) (72AHC(l4)l). 

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Photochemical fragmentation Equilibria with open-chain compounds Rearrangement to other heterocyclic species Polymerization... 

The photolysis of 1,2,4-oxadiazoles in the presence of sulfur nucleophiles has been shown to afford 1,2,4-thiadiazoles. N—S bond formation between the ring species and the sulfur nucleophile is thought to account for the observed products.96 A review has appeared which includes an account of the rearrangement of 1,2,3-thiadiazoles to other heterocycles such as 1,2,3-triazoles and 1,2,3,4-thiatriazoles.97... 

A variety of biochemical pathways are known which may lead to reactive quinoid derivatives. They include dihydroxylation of aromatic or heterocyclic compounds and epoxide formation and hydrolysis to -diphenolic compounds (Booth and Boyland 1957) o- and p-hydroxylations of phenols or arylamines (In-SCOE et al. 1965 Miller et al. 1960 Booth and Boyland 1957) and rearrangement of -hydroxyarylamines to o-aminophenols (Miller and Miller 1960). It now appears that aromatic hydroxylations proceed via highly reactive arene oxides, i.e., compounds in which a formal aromatic double bond has undergone epoxidation. Depending on the compound, arene oxides may give rise to other electrophilic reactive species, including quinoid structures, but react as such readily with nucleophiles and thus provide a basis for understanding covalent attachment of aromatic hydrocarbon derivatives to protein and nucleic acids (Jerina and Daly 1974). 

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