in reactions with polar mechanisms, lewis acid catalysis often involves binding of the catalyst to lewis basic heteroatoms and withdrawing electron density, which in turn facilitates heterolytic bond cleavage (in the case of friedel-crafts reaction) or directly activates the substrate toward nucleophilic attack (in the case of carbonyl addition reactions). the dichotomy can have important consequences in some reactions, as in the case of lewis acid-promoted acetal substitution reactions, where the s1 and s2 mechanisms shown below may give different stereochemical outcomes. studying the product ratio in a bicyclic system, denmark and colleagues showed that both mechanisms could be operative depending on the denticity of the lewis acid and the identity of the r' group.