An Efficient Synthesis of a 1,3-Dioxolan-4-one-Based Impurity of Allisartan Isoproxil

Hypertension is closely associated with aberrant activation of the renin–angiotensin system. Allisartan isoproxil, a prodrug angiotensin II receptor blocker, exhibits improved pharmacokinetic stability and medication safety compared with agents that rely on CYP450-mediated metabolism. However, multiple process-related impurities are inevitably generated during the industrial synthesis of allisartan isoproxil. Among its impurity profile, Impurity V is particularly noteworthy because of its unique 1,3-dioxolan-4-one structure. As a typical process-related impurity, it cannot be readily accessed through simple chemical transformations. The aim of this study is to establish an efficient synthetic route to Impurity V for the first time. Starting from N-benzylimidazole, we designed and developed a novel 13-step synthetic route to the target product. A key intramolecular cyclization catalyzed by I2 under mild conditions enabled the efficient construction of the imidazo-fused 1,3-dioxolane lactone core in 57% yield while effectively suppressing the undesired decarboxylation side reaction. Furthermore, optimization of the C5 chlorination step using a dimethyl sulfoxide/NaHCO3/CHCl3 system significantly improved the yield from 36 to 81%. Impurity V was subsequently obtained through sequential multistep oxidation, alkylation, and deprotection reactions. This route enabled the 10-g scale preparation of Impurity V with an overall yield of 3.6%. This work provides a practical synthetic route to an unstable lactone system and delivers a reliable reference standard for analytical method development, thereby supporting quality control and regulatory submission of allisartan isoproxil.