The use of propyl formate or ethyl propionate instead of butyl formate afforded the propyl or ethyl monoesters of hexanedioic acid. The reaction selectivity was high, the diesters being obtained in only 3-8% yield. The amount of catalyst was crucial: to realize high selectivities, 1 g/mmol with respect to the dicarboxylic acid was employed, which is about 10 times as much as that used in the selective acyla- tion163 and tetrahydropyranylation164 of diols. The authors showed that a certain amount of water in the resin is essential for the high selectivity. Taking into account that the diacid reacts much more rapidly than the monoester, they presumed that the selec- tivity arises from a partition equilibrium between the aqueous layer and the aprotic ester-hydrocarbon layer which is different for the diacid and monoester.