Two-step continuous flow synthesis of α-terpineol

Abstract

α-Terpineol is a naturally occurring monoterpene present in essential oils, of high value on the market as it is widely used as flavoring in the cosmetics and food industry. This study aims to produce α-terpineol by two different synthetic strategies, using both batch and continuous flow systems, focusing on the optimization of the process, improving the reaction conversion and selectivity. The first strategy adopted was a one-stage hydration reaction of (+)-α-pinene by an aqueous solution of chloroacetic acid (molar ratio 1:1 between pinene and the acid) in continuous flow conditions. This reaction was carried out at 80 °C with a residence time of 15 min, obtaining good conversion (72%) and selectivity (76%), and productivity of 0.53 kg.day−1. The second strategy accomplished was a two-step cascade reaction with (+)-limonene as starting material, where the first step is a chemospecific double bond addition using trifluoroacetic acid, and the second step is the basic hydrolysis of the ester promoted by a solution of sodium hydroxide (2.25 M) in methanol (1:1). This reaction was adapted to a continuous flow condition, where all steps involved a residence time of 40 min, at 25 °C, with no quenching between steps required, with 97% conversion, 81% selectivity and up to 0.14 kg.day−1.