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The kinetics of the intramolecular S→N acyl transfer in diastereoisomeric 3-amino- and 3methylamino-1,2,3-triphenylpropyl thiolacetates catalyzed by triethylamine could be conveniently studied by means of IR spectroscopy. The rates of migration depend strongly on the N-substitution as well as on the configuration. These rates clash with the steric effects expected in the cyclic tetrahedral intermediate, but are in full agreement with the requirements of Deslongchamps’ stereoelectronic hypothesis of two lone pairs on adjacent heteroatoms to be antiperiplanar to the cleaving bond. The formation or breaking of the endocyclic C-S bond demands an equatorial lone pair on nitrogen forcing an N-methyl group into the axial position thus changing the order of steric hindrance in the diastereoisomers.

Stereoelectronic effects in intramolecular S→N acyl migrations in diastereoisomeric 3-amino- and 3-methylamino-1,2,3-triphenylpropyl thiolacetates