Old is Gold? Nefopam Hydrochloride, a Non-opioid and Non-steroidal Analgesic Drug and Its Practical One-Pot Synthesis in a Single Solvent for Large-Scale Production

Abstract

Nefopam hydrochloride is extensively used in most of the European countries until today as an analgesic because of its non-opiate (non-narcotic) and non-steroidal action with fewer side effects compared with opioid and other analgesics, which cause more troublesome side effects. A multikilogram synthesis of nefopam hydrochloride has been achieved in one pot using a single solvent (toluene). A ≥99.9% purity of the active pharmaceutical ingredient (API) was achieved in excellent overall yield (≥79%). The one-pot, five-step synthetic process involves formation of an acid chloride (3) from benzoylbenzoic acid (2) followed by amidation (4), reduction (5), cyclization (6), and formation of the hydrochloride salt (1). The major advantages include (i) use of a single solvent, (ii) >90% conversion in each step, (iii) a cost-effective and operationally friendly process, (iv) averting the formation of genotoxic impurities, and (v) improved overall yield (≥79%) provided by the one-pot operation. For the first time, we report the characterization data of API 1, intermediates 3, 4, and 5, and also a possible impurity (5a).