Syntheses From Other Heterocycles

Such synthetic approaches can only be valid if the other heterocycles are readily available, or if their transformations lead to imidazoles difficult to make by other means. It is certainly important to be able to aromatize imidazolines since a number of ring-synthetic procedures lead to reduced imidazoles. 4-Aminoisoxazoles are sources of a-acylaminoenaminones which cyciize with bases to give 4-acylimidazoles. Oxazole-imidazole conversion has largely historical importance, but it is also implicated in some ring-synthetic procedures (e.g. the Bredereck method, see Chapter 5). Transformations of benzofuroxans into 2-substituted benzimidazole iV-oxides have some synthetic importance. Few, if any, ring contractions appear to have major application. 

Although some conversions of other heterocyclic compounds into benzimidazoles are of little more than academic interest, there may be synthetic value if the reactions occur under neutral conditions (the usual cyclizadons of o-phenylenediamines are in acidic media). 

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Primary Syntheses from Other Heterocyclic Systems... 

Syntheses from other heterocyclic systems via ring expansion are well developed (Section 14.08.8.1). Each of the approaches reported thus far for this type of ring construction appears rather promising, although ionic strategy is the most advanced. 

C. Syntheses from Other Heterocycles and Acyclic Precursors. 185... 

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