Imidazo thiazoles, nitrosation

Very little is known about the reactivity of imidazo[5,l-h]thiazoles. It seems that for 3,5-di, and 2,3,5-trisubstituted compounds, nitrosation occurs at the 7-position (64LA144). 

Under alkaline conditions, alkyl nitrites nitrosate imidazoles that possess a free NH group, at the 4-position. Nitrosation of 3,5-dimethylpyrazoles gives the 4-diazonium salt by further reaction of the nitroso compound with more NO+. 4-Nitrosopyrazoles 188 can be used as spin traps. 5-Pyrazolinones are nitrosated readily at the 4-position. Imidazo[l,2-tf] benzimidazoles are nitrosated with NaN02 in acetic acid giving 3-nitrosoderivatives 189. Nitrosation of 2-(dimethyla-mino)thiazoles under acid conditions gives 5-nitroso-derivatives 190. 3-Alkyl-5-acetamidoisothiazoles undergo 4-nitrosation. 

Combinations of N- and 5-alkylation in 2-imidazoline-2-thiols can lead to 5,6-dihydro-4//-imidazo[2,l-6]thiazoles when the heterocycles are treated with ketones in the presence of a halo-genating agent. This is a variant of the Hantzsch thiazole synthesis <92SC1293>. A further example of A-acylation in combination with nucleophilic substitution is the conversion of 2-chloro-2-imi-dazoline into (131) when it is treated with pyridine and an aryl isocyanate <87JCS(P1)1033>. 2-Imidazolines like clonidine are also known to A-nitrosate <93JCS(P2)59l>. Intramolecular alkylation is exemplified in the base-induced rearrangement of 2,5-diaryl-4-chloromethyl-2-imidazolines (132) into pyrimidines (Scheme 64) <93JOC6354>. 

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