Insight into the Mechanism of the Pechmann Condensation Reaction Using NMR

The mechanism of the Pechmann condensation is still controversial despite the technological and biochemical importance of coumarins. Here, we present NMR evidence for a mechanism featuring the sequence of initial electrophilic aromatic substitution followed by transesterification and a final dehydration. This mechanism has been convincingly defined and supported by the temporal evolution of two key intermediates which could be purified and identified.