Activation of hydrocinnamic acids with pentafluorophenol versus pentafluorothiophenol: Reactivity towards hexylamine

In this work we describe and compare the synthesis of four new hexylamides of hydrocinnamic acids, namely hexylamide of hydrocinnamic, 3,4-dimethoxyhydrocinnamic, 4-hydroxy-3-methoxyhydrocinnamic and 3,4-dihydroxyhydrocinnamic acids via pentafluorophenyl esters (PFPEs) versus pentafluorophenyl thioesters (PFPTs) intermediates. It was found that the PFPE are the best intermediates for this kind of synthesis giving reactions with less by products, easier work-up, higher overall yields and with the best reactivity towards hexylamine. The X-ray structures of two PFPE are also reported.