The reaction progresses in an enforced concerted manner (Figure ), as in the case of 2a, with rate-limiting proton transfer. The simulation also showed a higher transition state energy in case of the pro-R decarboxylation of 3a, with a transition state energy of 34.9 kcal mol -1 for the rate-limiting proton transfer step (Table ). This is in line with the fact that (R)-3b could barely be detected after enzymatic decarboxylation.