The sulphonation of indole, 1-methyl-, 2-methyl-, and 1,2-dimethylindole by pyridinium-1-sulphonate in refluxing pyridine occurs smoothly at C3; analogous sulphonation of 3-methyl- and 1,3-dimethylindole occurs at C2; 2,3-dimethylindole does not react under these conditions. © 1973.
Smith, G. F., & Taylor, D. A. (1973). The sulphonation of indole and some of its simple alkyl derivatives. Tetrahedron, 29(4), 669–672. https://doi.org/10.1016/0040-4020(73)85012-4