5-Iodo-3-Ethoxypyrazoles: An Entry Point to New Chemical Entities

Our program, which has focused on the preparation of new pyrazole derivatives, has led us to report here an original and simplified preparation of ethyl 3‐ethoxy‐1H‐pyrazole‐4‐carboxylate. This is based on the reaction of hydrazine monohydrochloride and diethyl 2‐(ethoxymethylene)malonate. Further transformations of this key compound allowed the preparation of the two possible iodinated isomers, namely, 3‐ethoxy‐4‐iodo‐ and 3‐ethoxy‐5‐iodo‐1H‐pyrazole. These compounds have opened the way to a quick access to many original pyrazole series. As an illustration, we report here on the selectivity of N‐arylation, by using the Lam and Cham method, the C4‐ and C5‐arylation of some of these 3‐ethoxypyrazole derivatives by using the Suzuki–Miyaura reaction, and C5‐benzylation reactions by means of the Negishi reaction. This was followed by hydrolysis of the ethoxy group, which led to the corresponding pyrazol‐3‐one derivatives. As a conclusion of this work, we conducted an investigation into the regiochemistry of the condensation between diethyl 2‐(ethoxymethylene)malonate and the hydrochloride salts of methyl, benzyl, or phenyl hydrazine. Open up your chemical mind! After achieving a simple preparation of various iodinated 3‐alkoxypyrazoles, their potential use as building blocks to gain access to new chemical entities was illustrated by the use of Lam and Cham N‐arylation, Suzuki–Miyaura C4‐ and C5‐arylation, and Negishi C5‐benzylation (see scheme).