Imidazole-4-carbaldehydes, formation

Deacylations are known. C-Acyl groups in 1,3,4-thiadiazoles are cleaved by sodium ethoxide in ethanol (68AHC(9)165). Imidazole-2-carbaldehyde behaves similarly, yielding imidazole and ethyl formate this reaction involves an ylide intermediate. 3-Acylisoxazoles (405) are attacked by nucleophiles in a reaction which involves ring opening (79AHC(25)147). 

Acyl-substituted imidazoles have distinctive UV and IR spectra, and can exist as hydrates in solution, e.g. imidazole-2-carbaldehyde. The aldehyde group is sufficiently electron withdrawing to assist nucleophilic displacement of an adjacent halogen atom. The normal aldehyde derivatives such as oximes, acetals and hydrazones can be formed, and the kinetics of oxime formation with the 4-carbaldehyde have been studied. The tautomeric ratio (zwitterion uncharged aldehyde form) of 1-substituted imidazole- and benzimidazole-2-aldoximes rises in parallel with an increase in basicity of the parent molecule (73CHE1074). These compounds have been shown by NMR studies to adopt the syn configuration. With hydrazine hydrate, imidazole-4,5-dicarbaldehydes give imidazo[4,5-d]pyridazines (208 Scheme 110). 

---