Ring cleavage via C-deprotonation

Isoxazoles unsubstituted in the 3-position react with hydroxide or ethoxide ions to give (3-keto nitriles (321) — (322). This reaction involves nucleophilic attack at the 3-CH group. 1,2-Benzisox-azoles unsubstituted in the 3-position similarly readily give salicylyl nitriles (67AHC(8)277), and 5-phenyl-l,3,4-oxadiazole (323) is rapidly converted in alkaline solution into benzoylcyanamide (324) (6lci(L)292). A similar cleavage is known for 3-unsubstituted pyrazoles and indazoles the latter yield o-cyanoanilines. 

3-Unsubstituted isoxazolium salts (325) lose the 3-proton under very mild conditions, e.g. at pH 7 in aqueous solution, to give intermediate acylketenimines (326) which convert carboxylic acids into efficient acylating agents (327) (79AHC(25)147). 

For ring-opening reactions of C-metallated azoles, see Section 3.4.3.10. 

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Electrochemically generated cation radical 252 derived from the cyclic alkylphenyl selenide 251 can react via two parallel paths deprotonization with the formation of compounds containing double bonds in the selenium-bearing rings and homolytic cleavage of the C(sp3)-selenium bond, followed by dimerization and formation of a diselenide 253 with an extended ring (Scheme 38) <1987ZOB609>. 

Benzo[c]azepine-l-ones have been fonned by palladium-catalysed, silver-promoted intramolecular cyclization of cyclopropyl benzamides in a reaction that proceeds via a cyclopropyl C(s/ )-H cleavage step followed by ring opening, deprotonation, and reductive elimination (Scheme 143). ° ... 

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