Synthesis of a Cannabidiol Precursor: Experimental Challenges and DFT Insights into β-Elimination Barriers

The synthesis of cannabidiol (CBD) from limonene derivatives involves a key β-elimination step that remains challenging to reproduce efficiently. In this work, we revisited a known racemic synthetic route to CBD and investigated the mechanistic origin of the low yield associated with the β-hydrogen elimination step. Alternative synthetic approaches were tested experimentally by comparing the traditional selenoxide-mediated pathway with a direct elimination attempt from bromohydrin intermediates. Despite optimization of reaction and workup conditions, β-elimination consistently failed, regenerating epoxide 1 instead of olefin 3. Density functional theory (DFT) calculations revealed that conformational constraints and electronic effects disfavor the reactive rotamer required for β-hydrogen elimination, explaining the experimentally observed lack of reactivity. The results clarify why the selenoxide pathway remains the only viable route to p-mentha-2,8-dien-1-ol (3) and provide mechanistic insight that may guide the development of future selenium-free synthetic methods.