Stereo Control of -Lactam Formation

Formation of /1-lactams by the reaction of an acid chloride, a SchifFbase and a tertiary amine (Eq. 62) appears to involve multiple pathways, some of which are very fast at higher temperatures. When conducted in open vessels in unmodified micro-wave ovens, high level irradiation leads to preferential formation of the trans -lactams (55%) whereas, at low power, the cis isomer was obtained as the only product (84%). The failure of the cis isomer to isomerize to the trans compounds is an example of induced selectivity. 

This effect has recently been explained by considering that under the action of microwave irradiation the route involving direct reaction between the acyl chloride and the imine competes efficiently with the ketone-imine reaction pathway, a situation highlighted by theoretical calculations [113]. In the other words and according to our assumptions, it can be stated that the transition state leading to the trans isomer therefore seems to be more polar than that leading to the cis compound. 

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