Indoxazenes

All these special cases involve benzologs with an ortho quinonoid structure. But even this pattern is not universal, a deviation being 2-methylindazole (40), which quaternizes 2.2 times more slowly than its parent compound 1-methylpyrazole (37).122 Indeed, except for the molecules just considered and 1,2-benzisothiazole,122 benzo-fusion is rate retarding. Thus, 1,2-benzisoxazole (indoxazene, 39), reacts 3.2 times, and 1-methylindazole (39) 7.1 times, more slowly than their parent compounds.122 Fusing a benzene ring onto an azole where the heteroatoms are situated 1,3 leads to decreases in rate constants by factors of 5.0, 6.3, and 6.8, respectively, when X of 38 is NMe, S, and O.122 These factors are not much smaller than that obtained from a comparison of pyridine and quinoline reactivities.61,78,79... 

The most generally applicable methods of synthesis of indoxazenes fall into two main classes ... 

Unsubstituted indoxazenes cannot be prepared in this way instead of the expected product one obtains the corresponding isomeric salicylonitrile (9) or its hydrolysis products.20 With o-bromophenyl-glyoxylic acid oxime (10) in alkaline solution the reaction proceeds similarly, with decarboxylation, ring opening, and hydrolysis to salicylic acid.21... 

The configuration of the oxime is important to the course of the reaction. Thus, while the j8-oxime (11) gives the indoxazene or its degradation products, the a-oxime (12) undergoes a Beckmann rearrangement, finally yielding benzoxazole (13). This difference in reaction pathway between stereoisomeric oximes was used in the determination of their configurations.7,8 22,23... 

Again, the configuration of the oxime is of significance. The acetate (20, R = Ph) provides the indoxazene smoothly on pyrolysis, while its isomer (21) gives only a small yield of the same product.42 A Beckmann rearrangement diverts the pyrolysis of some of the oxime benzoate (22) to the formation of the benzoxazole (23).43... 

Salicylamidoxime cannot be dehydrated directly to an indoxazene for example, with phosphorus pentoxide it rearranges to give 2-aminobenzoxazole.43 However, itsoxime-O-ethoxycarbonyl derivative (24) provides 3-aminoindoxazene (25) on heating.44... 

The benzoxazolinone (26) is formed via a rearrangement, on heating the acetylhydroxamic acid (27)—preferably with alkali.47 The compound which the same authors described as indoxazen-3-one was later46 shown to be salicylamide. 

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