The achiral cyclohexene derivative dimethyl cis-1,2,3,6-tetrahydrophthalate has been subjected to oxidation catalyzed by cytochrome P450 monooxygenase P450-BM3, leading to diastereoselective epoxidation rather than oxidative hydroxylation. This reaction occurs with 94% diastereoselectivity in favor of the anti-epoxide, in contrast to m-CPBA which delivers unselectively a 70:30 mixture of anti/syn diastereomers. The experimental results are nicely explained on a molecular level by docking experiments and molecular dynamics computations.