At 160–180 °C, in the presence of alkali metal exchanged faujasites (MX or MY; M = Li, Na, K), the reaction of dimethyl carbonate with indolyl-3-acetic, -3-propionic, and -3-butyric acids proceeds towards the formation of the corresponding methyl esters or carbamate esters which can be isolated in 93–99% yields. The methylation of the indolyl-NH group is never observed. This high chemoselectivity is driven by the nature of the catalyst and the reaction temperature. In particular, among the six different zeolites used, the more basic MX faujasites show better performances in terms of both activity and selectivity than MY solids. A similar trend also holds for the reaction of dimethyl carbonate with indolyl-carboxylic acids, where MX compounds are still efficient catalysts for the formation of methyl esters. In this case, however, the overall reactivity/selectivity also reflects the relative positions of the NH and CO2H groups which may account for significant decarboxylation reactions observed for indolyl acids substituted at positions 2 and 3. This process is totally absent for indolyl-6-carboxylic acid.
