Dedicated to Duilio Arigoni on the occasion of his 80th birthdayThe new electrophilic trifluoromethylating 1-(trifluoromethyl)-benziodoxole reagents A and B (Scheme 1) have been used to selectively attach CF 3 groups to the S-atom of cysteine side chains of aand b-peptides (up to 13-residues-long; products 7 -14). Other functional groups in the substrates (amino, amido, carbamate, carboxylate, hydroxy, phenyl) are not attacked by these soft reagents. Depending on the conditions, the indole ring of a Trp residue may also be trifluoromethylated (in the 2-position). The products are purified by chromatography, and identified by 1 H-,
13C-, and 19 F-NMR spectroscopy, by CD spectroscopy, and by high-resolution mass spectrometry. The CF 3 groups, thus introduced, may be replaced by H (Na/NH 3 ), an overall Cys/Ala conversion. The importance of trifluoromethylations in medicinal chemistry and possible applications of the method (spin-labelling, imaging, PET) are discussed. [7]. In continuation of this tradition, we have now investigated the conversion of CH 2 ÀSÀH groups of cysteine side chains in peptides to CH 2 ÀSÀCF 3 groups, by using a method [8] [9] that has been recently introduced by another group of co-authors of the present article 5 ). The method uses the hypervalent iodine reagents A
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