The exceptionally high selectivity towards EtO-4H-Qui when using the toluene sulfonic acid (TSS) is reasoned with the overall highest concentration of H + , leading to an accelerated formation of the ethylidene aniline and ethylvinyl ether at the same time, which subsequently react to form EtO-4H-Qui. Under the chosen reaction conditions, the subsequent reaction steps to form quinaldine are slower, which results in the very high selectivity towards EtO-4H-Qui for the experiment with triflate when compared to the other experiments.