The stereochemistry of the reaction between cyclopentyne and ethene has been modeled using statistical methods, based on RRKM theory and a master equation analysis, and by molecular dynamics. We show that the stereochemical retention observed experimentally is not compatible with statistical models that invoke a diradical mechanism but that it can be rationalized through analysis of short time diradical. dynamics. Within the first similar to 400 fs, reaction occurs from the initial diradical adduct to form a carbene, which may subsequently isomerize to give the final product. The carbene route has a significantly higher barrier than other channels; however, at short times the reaction energy is efficiently coupled into the reaction coordinate for carbene formation. Loss of the initial ethene stereochemistry by rotation about the former C=C bond occurs on a time scale of similar to 300 fs, so that stereochemistry is retained in the carbene on short time scales. The bond rotation required to pass directly through the low energy transition state leading from the diradical, to the [2+2] cycloaddition product is slow because of the attached heavy groups, occurring on a 1-2 ps time scale. Therefore, this low energy channel only becomes active on longer time scales, when memory of the initial ethene stereochemistry has been lost. Short time retention of stereochemistry via the carbene is thus related to the time scales for randomization of both the energy and the geometry. it is argued that these effects may combine to amplify the stereochemical retention for reaction of substituted ethenes in solution.