Minimising conformational bias in fluoroprolines through vicinal difluorination

Abstract

Monofluorination at the proline 4-position results in conformational effects, which is exploited for a range of applications. However, this conformational distortion is a hindrance when the natural proline conformation is important. Here we introduce (3S,4R)-3,4-difluoroproline, in which the individual fluorine atoms instil opposite conformational effects, as a suitable probe for fluorine NMR studies.

Comparative effects of trifluoromethyl- and methyl-group substitutions in proline

“…92,93 Recently vicinal difluoroprolines were proposed for use in 19 F NMR labelling, as these derivatives have a reduced conformational bias in the side chain and the amide bond. 94 To a great dissatisfaction, studies of the polarity changes introduced in ProAs are quite underrepresented in the literature. Even for fluoroprolines, which are most well studied, there is a very little awareness of the polarity changes introduced by these residues in the polypeptide structures.…”

“…96 However, these would also have poor NMR properties due to a large coupling between the fluorine atoms. 94 Alternatively, Tressler and Zondlo adapted the use of perfluoro-tert-butytoxy 19 F probes 97,98 and proposed the use of O-perfluoro-tert-butyl-4-hydroxyprolines in NMR applications, 99 although these bulky lipophilic compounds also demonstrated a significant conformational bias in trans/cis amide equilibrium. Other proposed Pro substitutes are trifluoromethyl-4,5-methanoprolines, which were originally introduced as 19 F NMR labels for solid-state NMR studies in lipid membranes.…”

Synthesis of complex unnatural fluorine-containing amino acids

“…Proline is often a critical residue in terms of defining a peptide or protein’s secondary structures and it is ubiquitous within the sequences of cyclic peptides. Given this, there has been significant effort invested into accessing fluorinated proline derivatives [ 84 ], and multiple reports on the stereoselective synthesis of mono-fluoro prolines have been disclosed [ 85 , 86 ]. Several research groups, however, have focused on the synthesis of prolines substituted with multiple fluorines.…”

“…It is known that the introduction of a fluorine atom to either the 3 or 4 position of the proline ring can have a dramatic impact on the conformation adopted by the ring. In particular, their presence will encourage puckering of the ring and a change in cis/trans isomerisation [ 85 , 86 ]. Having a fluorine atom on both the 3 and 4 positions of the ring leads to a question as to what conformation the different stereoisomers will adopt.…”

Polarity effects in 4-fluoro- and 4-(trifluoromethyl)prolines

“…There is a rich portfolio of fluorine-containing proline analogues that have been developed to date (Figure 1B): fluorinated [6][7][8][9][10][11][12], trifluoromethylated [13][14][15][16][17][18][19][20][21][22][23], chimeric [16,19,[24][25][26][27][28][29], conformationally restricted [30][31][32][33] having variations in the ring size [34][35][36][37][38][39], non-α [40][41][42][43][44], and other analogues [45][46][47][48]. The fluorine-containing functional groups are usually chemically inert under most biologically relevant conditions.…”