A theoretical study of the intramolecular cyclization of a series of propanamides to yield beta-lactams was performed at the B3LYP/6-31+G(d,p) and MP2/6-31+G(d,p)//B3LYP/6-31+G(d,p) levels. The effect of the Cl- and OH- leaving groups and of several substituents on N1 and on C4 was investigated. As expected, OH- is a much worse leaving group than Cl-, rendering an energy barrier about 2 times larger. An SO3- substituent on N1 diminishes the energy barrier by destabilizing the intermediate prior to the rate-determining TS, whereas OH and OCH3 substituents do not produce an appreciable effect. According to the MP2 method, one and two methyl substituents on C4 increase the energy barrier of the process. The simultaneous presence of a sulfonate group on N1 and a methyl group on C4 render the corresponding combined effect, while combination of an OH on N1 and two methyl groups on C4 is not simply additive.