To further investigate this, the reactions were followed by NMR. As the peak trends were nearly identical for the two acceptors, only reactions with the (S)-acceptor were investigated by NMR. Two reactions were carried out in NMR tubes, one with the 1-α donor and one with the 1-β donor (Figure ). 1 to 2 spectra were recorded each hour and the stacked spectra are shown in Figure . For the reaction with 1-α, a relatively simple reaction picture is observed (3B left), where the signal from the α-TCA (1-α) decreases, while the signal from the β-product (6β) increases. Further, a small formation of hemiacetal side-product is observed. A more complicated picture is observed for the reaction with the β-TCA as seen in the stacked spectra to the right in Figure . As expected, the signal from the β-TCA decreases over time, and formation of the α-product (6-α) is observed. In addition, a significant amount of hemiacetal and β-product is formed during the reaction. It is also seen that α-TCA is formed during the reaction, which would inevitably react to form the β-product. Isomerization of the β-TCA has previously been observed for this type of reaction. Interestingly, a signal, that initially increases and afterward decreases, is observed to the left of the signal from the α-product (Figure right). As this behavior is similar to the behavior observed by IR, it is likely that it is the same intermediate giving rise to both peaks. However, it should be noted that the findings from the NMR experiments cannot be completely transferred to the observations in the IR experiments, as the concentration is lower, 0.044 M in the NMR experiments vs 0.2 M in the IR experiments, and also the solvent has been changed from CH 2 Cl 2 to CDCl 3 . In Figure , the integral of the peaks from the observed species is plotted as a function of time. From these plots the difference in product distribution is very pronounced, indicating that one major pathway is dominating for the reaction with 1-α, whereas for the reaction with 1-β there are multiple competing pathways.