Furthermore, comparing chemical shift changes at multiple host protons shows that the nature of binding of (S)-1 and (R)-1 is distinct qualitatively. While binding of (S)-1 leads to marked changes in all four protons plotted in Figs and, binding of (R)-1 affects H5 (deeper in the cyclodextrin cavity) rather less, suggesting a distinct binding conformation for (R)-1 resulting from a poorer 'fit' in the host. Since our reaction was intended to be performed at -20 °C (-253 K), a series of 1 H spectra were acquired at this temperature (see SI, Fig. ). Unusually, while all room-temperature spectra manifest fast guest exchange regimes, these spectra suggest slow guest exchange for the binding of (S)-1, but fast exchange of (R)-1: promising for selective (S)-1 protection, while allowing (R)-1 to react in these conditions.