Ring contraction azirines

There are several examples of the formation of pyridazines from other heterocycles, such as azirines, furans, pyrroles, isoxazoles, pyrazoles or pyrans and by ring contraction of 1,2-diazepines. Their formation is mentioned in Section 2.12.6.3.2. 

Azcpincs under acid conditions reportedly117-225 yield aniline derivatives although ring contraction to pyridines is more usual. Thus, highly substituted 3//-azepines, e.g. 28, with a vacant 7-position, formed by cycloaddition of 2//-azirines with cyclopentadienones, on heating in acetic acid isomerize rapidly to the correspondingly substituted anilines 29.117... 

Strong acid hydrolysis of 1,2,6-thiadiazine 1,1-dioxides 239 or 240 results in ring contraction to afford the 1,2,5-thiadiazolinone 1,1-dioxides 241 in low yield <1996J(P2)293>. 3-Dialkylamino-2/7-azirines 242 suffer ring expansion with in situ-prepared iV-sulfonylamides 243 and carbamates to give both the 1,2,3-oxathiazoline 244 and the thiadiazoline 1,1-dioxide 245 (Equation 56). The oxathiazole 244 isomerizes quantitatively to the thermodynamically favored thiadiazoline 245 <1996J(P1)1629>. 

Themal ring contraction of 3,5-disubstituted isoxazoles (93) gives 1-azirines (94). 

At that time it was suggested that the isomerization occurred by way of an initial photo-ring contraction to an undetected 2-(A-methylimino)-2//-azirine intermediate (2) and subsequent ring expansion to the observed product, 1-methylimidazole (3). Although the intermediacy of acylazirines has been adequately demonstrated in the analogous isoxazole-to-oxazole phototransposition, such iminoazirines have not been detected in a pyrazole to imidazole isomerization, and thus, this mechanistic suggestion has never been experimentally substantiated. 

The ring contraction reaction of of isoxazoles 133 promoted by iron(II) catalysts affords acyl 27/-azirines 134 in nearly quantitative yield (Scheme 72) <1997T10911>. 

Thermal or photochemical treatment of isoxazoies 851 has been found to result in a ring-contraction reaction to produce acyl 277-azirines 852, which sometimes rearrange to form other heterocycles like oxazoles 853. This ring-contraction reaction can also be promoted by iron(ii) catalysts. Thus, 5-alkoxy- and 5-aminoisoxazoles isomerize to 27/-azirine-2-carboxylic esters and 2/7-azirine-2-carboxamides, respectively, in nearly quantitative yield by reaction with catalytic FeCb (Scheme 212) <1997T10911>. 

Photoisomerization of five-membered heterocycles commonly takes place via ring-contracted intermediates. The 2H-azirine (33)... 

Photolysis and thermolysis of isoxazoles and oxazoles causes ring-contraction to 2-acyl-2H-azirines, as shown in Scheme 92 . The azirines may be observed in some cases, or may be transformed without observation to other heterocycles. Equation (74) shows an example where the azirine was isolated in good yield <88TL6067>. An isolable azirine was also formed in the photolysis of a 4-acylisoxazole <90JOC40ll>. 

Alkoxy-isoxazoles undergo ring contraction with iron(II) chloride, producing azirine esters. ... 

In a similar way, irradiation converts many simpler pyrazoles into imidazoles, phenylisothiazoles and methylisothiazoles partially into the corresponding thiazoles, and 3,5-diarylisoxazoles converted into 2,5-disubstituted oxazoles. " 3-Alkoxyisoxazoles undergo an extraordinary ring contraction with iron(II) chloride, producing azirine esters... 

---