The epoxidation of β-methylstyrene catalyzed by the Shi fructose-derived ketone is studied using experimental kinetic isotope effects and DFT calculations. The observation of a large β olefinic C-13 isotope effect and small a carbon isotope effect is indicative of an asynchronous transition state with more advanced formation of the C-O bond to the β olefinic carbon. By varying the catalyst conformation and alkene orientation, diverse transition structure geometries were located calculationally, and the lowest-energy structure leads to an accurate prediction of the isotope effects. Given this support for the accuracy of the calculations employed, the nature of enantioselectivity in this and related epoxidations is considered. The lowest-energy transition structures are generally those in which the differential formation of the incipient C-O bonds, the "asynchronicity," resembles that of an unhindered model, and the imposition of greater or lass asynchronicity leads to higher barriers. In reactions of cis-disubstituted and terminal alkenes using Shi's oxazolidinone catalyst, the asynchronicity of the epoxidation transition state leads to increased steric interaction with the oxazolidinone when a T-conjugating substituent is distal to the oxazolidinone but decreased steric interaction when the π-conjugating substituent is proximal to the oxazolidinone. Overall, the asynchronicity of the transition state must be considered carefully to understand the enantioselectivity.