Biocatalytic Access to Natural Mosquito Repellent <i>p</i> ‐Menthane‐3,8‐diol via Direct Asymmetric Prins Cyclohydration

Vector-borne diseases pose a rising global health challenge, necessitating the development of safe and effective pest protection agents. Here, we report a highly selective biocatalytic direct Prins cyclohydration for the synthesis of (1R)-cis-p-menthane-3,8-diol (PMD), a natural insect repellent with high efficacy. By strategically engineering squalene-hopene cyclases (SHCs), we achieved >96% diastereomeric excess, surpassing previous synthetic methods. Structural and mechanistic analyses suggest direct Prins cyclohydration and a precisely positioned water molecule within the enzyme's active pocket adjacent to the final carbocation that drives hydration and catalytic efficiency. Fine-tuning the biocatalytic setup enabled preparative scale production, without losing much product selectivity. Moreover, we demonstrate access to the other naturally occurring PMD isomers from (R)- and (S)-citronellal, as well as a one-pot cascade starting from E/Z-citral. This study paves the way for highly selective access to stereodefined terpene-derived repellents and establishes engineered squalene-hopene cyclases as a tool for direct asymmetric Prins cyclohydration.