Instead of optimizing conversion at a certain endpoint, we decided to optimize for catalyst stability by same excess experiment with each catalyst. After initial preliminary tests with other acids, we focused on differently substituted benzoic acids (3a-3j, see Scheme 3), with 1-octen-3-ol (1a) and 2,2-dimethoxypropane (2a) as the substrates (Scheme 3). For simplicity, catalyst decay was evaluated by qualitatively plotting the consumption of 1a (monitored by GC, see the SI for details) under two different starting conditions (same excess). The catalyst stability studies revealed that 4-bromobenzoic acid (3i) and 4-chlorobenzoic acid (3j) were generally the best catalysts, demonstrating only slight (3i) or no observable catalyst decay (3j) (for details, see the Supporting Information (SI)). Catalyst 3j also led to the highest conversion to product (80 %).