Nonenzymatic reactions, though critical in natural product biosynthesis, are significantly challenging to control. Adding 3% NaI to the culture medium of Penicillium janczewskii significantly increased pseurotin D (1) production and decreased pseurotin A (2) production. Previously, 1 and 2 were suggested to be produced via a nonenzymatic reaction, where the epoxide at C-10 undergoes SN2 (2) or SN2' (1) reactions. We confirmed that 1 was isolated as a 1:1 mixture of C-13 epimers by spectral elucidation via CP3 analysis aided by selective excitation NMR methods, which supported that 1 was produced through a nonenzymatic SN2' reaction. We propose that NaI increased the ratio of 1 by causing steric hindrance at the C-11 position of the transient intermediate, which makes C-13 more preferred in the SN2/SN2' competition.