Reactive intermediate generation infrared spectroscopy

Sordo et al. [144] explained the stereoselectivity on the basis of torquoelectronic effects. Low-temperature infrared spectroscopy was also used to identify the reactive intermediates [145]. Two mechanisms were proposed to explain the product distribution in the (3-lactam formation reaction. The ketene mechanism was observed in a low temperature infrared spectroscopy study [145], while the acylation of imine mechanism was believed to be involved in some [122]. Both mechanisms were supported by evidences. It had been hypothesized that cycloaddition of the imine occurs from the least hindered side of the ketene, and this process generates zwitterionic intermediates conrotatory cyclization of these intermediates then produce cis- and //Y/ .v-[S-lactanis. Acylation of the imine by the acid chloride to form /V-acyliminium chloride also produced zwitterionic intermediates (Scheme 10). 

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