Previously a novel and efficient one-pot method has been developed for the synthesis of 1-substituted-3,4-dihydro-β-carbolines starting from tryptamine and a wide variety of carboxylic acids. The first reaction step was the acylation of tryptamine leading to the corresponding acyltryptamines, followed by a ring-closure step affording the tricyclic target molecules. In the present paper, possible mechanisms are proposed for the acylation of tryptamine with benzoic acid and for the closure of the β-carboline ring, including a detailed discussion of the effect of the T3P® reagent. In order to compare the innovative T3P®-promoted method with the traditional POCl3-promoted one, mechanistic computations have been performed for both methods. The computed mechanistic pathways explain why less than two equivalents of the T3P® reagent are sufficient for the two consecutive reaction steps and a parallelism is shown between the energetics of the T3P®-disruption and the biological energy storage of ATP. It can also be understood, why a higher temperature and a longer reaction time are required for the T3P®-promoted synthesis. Although the history of the Bischler-Napieralski reaction goes back more than 120 years, it is the first time that the reaction is studied by DFT quantum chemical calculations.