We note that neither simple pKa trends nor steric effects can readily explain why 3j emerged as the optimal catalyst with minimal decay. Control experiments indicated that esterification is unlikely to explain the catalyst decay observed with less optimal catalysts (no Me ester derived from 3b could be detected under the reaction conditions; see the SI). Nevertheless, we noted that replacing 2,2dimethoxypropane 2a with 2-methoxypropene 2d accelerated the consumption of allyl alcohol (2a t1/2 ≈ 2 h, 2d t1/2 ≈ 0.5 h) and raised the conversion to product 6a to 85 %, providing a set of optimized conditions for further elaboration. a Examples of same excess experiments for two benzoic acids, 3b and 3j. Typical initial conditions: for catalyst 3b, experiment 1 (red dots) 1a (0.39 M), 2a (1.17 M), 3b (0.039 M); experiment 2 (blue dots) 1a (0.20 M), 2a (0.98 M), 3b (0.039 M). For details, see the Supporting Information.