A theoretical study to elucidate the mechanistic aspects involved in the tosylation-cyclization reaction of diastereomeric phytosphingosines 1a-1d to jaspines 4a-4d is presented. The stereochemistry of the starting stereoisomers is crucial for the development of weak interactions, both in the reactants and in the transition states. The analysis of the energy barriers of each elementary reaction is consistent with the observed reluctance of tosylate 2d to undergo cyclization. In addition, the initial tosylation can be identified as the limiting step in cyclizations from 1a and 1b.