Regioselective Synthesis of Imidazo[1,5‐b]pyridazines by Cascade Cyclizations of 1,2‐Diamino‐4H‐phenylimidazole with 1,3‐Diketones, Acetoacetic Ester and Their Derivatives

In this study we investigated the cascade heterocyclisation reactions of 1,2‐diamino‐4H‐phenylimidazole with dimethylaminomethylidene (ethoxymethylidene) by the derivatives of acetoacetic ester, acetyl acetone, and aryl(hetaryl)butane‐1,3‐diones. As a result of the studied processes 7‐amino‐2‐R‐3‐R1‐5‐phenylimidazo[1,5‐b]pyridazines were obtained, whose structures were proved by means of 1H NMR, 13C NMR, XRS, and NOESY spectroscopy. Heterocyclisation conditions were optimised to obtain target products with high yields. A probable mechanism for regioselective processes was suggested. It was found that three‐component processes with initial diamine, 1,3‐dicarbonyl compounds and triethyl orthoformate or dimethylformamide dimethylacetal result in similar products. The yield of these reactions was slightly reduced in comparison to the two‐component processes. In this study the cascade heterocyclisation reactions of 1,2‐diamino‐4H‐phenylimidazole with dimethylaminomethylidene (ethoxymethylidene) by the derivatives of acetoacetic ester, acetyl acetone, and aryl(hetaryl)butane‐1,3‐diones were investigated. As a result of the studied processes 7‐amino‐2‐R‐3‐R1‐5‐phenylimidazo[1,5‐b]pyridazines were obtained. Heterocyclisation conditions were optimised to obtain target products with high yields. A mechanism explaining the regioselectivity of these processes was proposed. It was found that three‐component processes with initial diamine, 1,3‐dicarbonyl compounds and triethyl orthoformate or dimethylformamide dimethylacetal resulted in similar products.