Background levels of etheno adducts have been attributed to the reaction of DNA with 2,3epoxyaldehydes, a proposed product of lipid peroxidation. We have examined the reaction of (2R,3S)epoxyhexanal with dGuo to give 7-(1S-hydroxybutyl)-1,N 2 -etheno-dGuo. We observed that the stereochemistry of the side chain scrambled over time. This process provided insight into the mechanism for the formation of 1,N 2 -etheno-dGuo from 4,5-epoxy-2-decenal [Lee, S. H., et al. ( 2002) Chem. Res. Toxicol. 15,[300][301][302][303][304]. The mechanistic proposal predicts that 2-octenal is a by-product of the reaction. The reaction of 4,5-epoxy-2-decenal was reinvestigated, and the 2-octenal adduct of dGuo was identified as a product of this reaction in support of the mechanistic proposal. Also observed are products that appear to be derived from 2,3-epoxyoctanal, which can be formed through Schiff base formation of 4,5-epoxy-2-decenal with the dGuo followed by hydration of the double bond and retro-aldol reaction.