Mechanistic proposals

We propose two possible reaction pathways to account for pyrrolinone formation during SES cyclization (Schemes A and B). Scheme A is based on chemistry much more common for furans than pyrroles. [4] Scheme B relies on the well known oxidizing power of sulfoxides [5], although we have found no literature precedence for the oxidation of pyrrole by sulfoxides. Hydrolysis of the initially formed trifluororacetate ester in this pathway to the observed pyrrolinones under the prolonged alkaline work-up conditions would not be surprising.
Scheme A

Scheme B

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