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An Unprecedented Retro-Mumm Rearrangement Revealed by ESI-MS/MS, IRMPD Spectroscopy, and DFT Calculations.

Claudio IacobucciSamantha RealeMassimiliano AschiJos OomensGiel BerdenFrancesco De Angelis
Published in: Chemistry (Weinheim an der Bergstrasse, Germany) (2018)
Brønsted acids and protic solvents mediate acyl transfer, known as the Mumm rearrangement, from imidates to the corresponding acylamides. This represents a key step in several reactions, for example, the Ugi four-component reaction (U-4CR) and Passerini three-component reaction (P-3CR). Herein, an unprecedented break of the non-reversibility of the Mumm rearrangement is reported. A combination of electrospray tandem mass spectrometry (ESI-MS/MS), infrared multiphoton dissociation (IRMPD) ion spectroscopy and theoretical calculations demonstrates the occurrence of the retro-Mumm rearrangement of protonated isopeptides in the gas phase. In the gas phase, the extra proton acquired during ESI promotes the backward isomerisation reaction in a catalytic fashion.
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