Synthesis of 4,6-Difluoro-Tryptophan as a Probe for Protein ¹⁹F NMR

A scalable procedure for the synthesis of 4,6-difluorotryptophan is reported based on a deaminative coupling of a 4,6-difluorogramine with 2-benzylthio-1,5-dihydro-4H-imidazolone as glycine equivalent. Thus prepared 4,6-difluorotryptophan was incorporated into the C-terminal domain of the HIV-1 capsid protein (CA-CTD), and F-19 spectra of the 4,6-difluoro Trp CA CTD were recorded and compared to the singly fluorinated counterparts.