Process Intensification for Obtaining a Cannabidiol Intermediate by Photo-oxygenation of Limonene under Continuous-Flow Conditions

We have investigated the synthesis of p-mentha-2,8-dien-1-ol (1) as a cannabidiol precursor by photochemical oxidation of (R)-limonene (2) with singlet oxygen (¹O₂) using meso-tetraphenyl­porphyrin (TPP) as a photosensitizer. Different home-made engineered set-ups were used for reaction intensification. Under optimized conditions, the target compound was obtained at up to 580.8 μmol·min^–1 (66.7 g·day^–1) using a low concentration of TPP (0.1 mM) and requiring 20 min for total conversion. Low-temperature photoreactor approaches made it possible to raise the level of selectivity of the desired limonene derivative up to 66%, with 55% conversion.