Synthesis of optically pure N-Boc-protected (2R,3R)- and (2R,3S)-3-fluoroprolines

Article abstract of DOI:10.1016/S0040-4039(00)02030-X

Non-protein amino acids (2R,3R)- and (2R,3S)-3-fluoroprolines were synthesized as novel probes for studying the cis/trans isomerization of the amino acyl-proline peptide bond. The N-Boc-protected target compounds were obtained as optically pure material s

Full text of DOI:10.1016/S0040-4039(00)02030-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 651–653
Synthesis of optically pure N-Boc-protected (2R,3R)- and
(2R,3S)-3-fluoroprolines
Luc Demange, Je´roˆme Cluzeau, Andre´ Me´nez and Christophe Dugave*
De´partement d’Inge´nierie et d’Etudes des Prote´ines (DIEP), CEA/Saclay, 91191 Gif-sur-Yvette, France
Received 8 October 2000; accepted 10 November 2000
Abstract—Non-protein amino acids (2R,3R)- and (2R,3S)-3-fluoroprolines were synthesized as novel probes for studying the
cis/trans isomerization of the amino acylꢀproline peptide bond. The N-Boc-protected target compounds were obtained as optically
pure material starting from (2S,3S)-3-hydroxyproline. © 2001 Elsevier Science Ltd. All rights reserved.
The amino acylꢀproline cis/trans isomerization is a
rate-limiting step of protein folding1 and modulates the
biological activity of peptides.2 Therefore, proline plays
a particular role in protein structure and peptide con-
formation.3 As a consequence, many proline surrogates
have been designed for the study and control of confor-
mational transitions in peptides and proteins.4 We
focused our interest on fluorinated prolines which com-
bine the strong electroattractive effect of fluorine with a
low steric hindrance.5 We describe herein the first syn-
thesis of N-Boc-protected (2R,3R)- and (2R,3S)-3-
fluoroprolines 1 and 2 (Fig. 1).
The synthesis of optically pure 3-fluoroprolines 1 and 2
was not obvious due to the marked tendency of b-acti-
vated amino acids to epimerize or to undergo an a,b-
elimination. In a preliminary attempt, the (2S,3S)-3-hy-
droxyproline 36 was protected as a Boc-methyl ester
derivative 4a and was fluorinated with DAST (Scheme
1).7 However, saponification of the methyl ester 5a with
lithium hydroxide led exclusively to the corresponding
a,b-dehydroproline 6a. In order to ensure a complete
orthogonality of protection in later steps of the synthe-
sis, compatible with the moderately stable fluorine at
Cb, the methyl ester was replaced with a benzyl ester.
Fluorination of compound 4b with DAST yielded the
corresponding 3-fluoroproline 5b in moderate yield.
Finally, the title compound 18 was obtained in 26%
overall yield by prolonged hydrogenolysis of the benzyl
ester.
F
OH
CO2H
F
CO2H
CO2H
N
N
H
N
Boc
Boc
1
2
3
Figure 1.
OH
CO2R
F
a,b
c
d
+
3
1
CO2R
CO2R
N
N
N
Boc
Boc
Boc
4a 100%
4b 87%
5a 40%
5b 41%
6a 55%
0%
R = Me
R = Bn
72%
-
Scheme 1. R=Me; (a) SOCl2 in methanol; (b) Boc2O, NEt3 in DCM; (c) DAST in dry DCM; (d) LiOH in acetonitrile/water;
R=Bn; (a) Boc2O, NaOH in THF; (b) Cs2CO3 in water then BnBr in DMF; (c) DAST in dry DCM; (d) H2, Pd/C in methanol.
* Corresponding author. Fax: (33) 169 08 90 71; e-mail: christophe.dugave@cea.fr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02030-X

652
L. Demange et al. / Tetrahedron Letters 42 (2001) 651–653
OH
CO2Bn
OH
CO2Bn
F
b
d
e
2
CO2Bn
N
R
N
R
N
71%
38%
96%
Boc
4b (R = Boc)
(R = Tr) 70%
8 (R = Tr)
10
a
c
7
9 (R = Boc) 82%
Scheme 2. (a) TFA in DCM; (b) PPh3, DEAD, PhCO2H in toluene then KOH in methanol; (c) HCO2H in 1,2-dichloroethane
then Boc2O, NEt3 in DCM; (d) DAST in dry DCM; (e) H2, Pd/C in methanol (R=Bn).
Inversion of configuration of the alcohol was carried
out starting from compound 4a or 4b (Scheme 2) by a
three-step procedure, as previously described for 4-
hydroxyproline.5 In the methyl ester series, fluorination
with DAST gave the C3 epimer of 5a. Unexpectedly, its
saponification with lithium hydroxide gave a mixture of
6a (61%) and 2 (6%). This suggested that (2R,3S)-N-
Boc-3-fluoroproline methyl ester exists in a twisted
conformation which enables a pseudo anti-elimination
of fluorine.
and 19F NMR spectra of each diastereomer clearly
showed significative differences with no trace of the
other isomer (Fig. 2). As already observed with Boc-4-
fluoroprolines, a significative doubling of several 1H,
13C and 19F NMR signals reflects the well-known ure-
thane cis/trans equilibrium.10,11
Therefore, (2R,3R)- and (2R,3S)-3-fluoroprolines 1 and
2 were obtained by a straightforward synthetic route in
26% (four steps) and 13% (seven steps), respectively. A
diastereomeric purity higher than 99% was assessed by
NMR.
Starting from compound 4b, (2S,3R)-N-Boc-3-hydroxy-
proline benzyl ester 9 was isolated in 45% overall yield.
NMR analysis showed that the Mitsunobu reaction
took place with a complete inversion of configuration.9
Fluorination and hydrogenolysis led to compound 210
in 36% yield (two steps).
Acknowledgements
The optical purity of compounds 5b, 10, 1 and 2 was
investigated since the generation of a proline b-cation
equivalent might cause a partial or complete racemiza-
This work was supported by SIDACTION (grant c
99014) and the Agence Nationale de Recherches sur le
Sida (ANRS) (grant c 70000016-01) and the Atomic
Energy Commission (CEA).
1
tion of both Ca and Cb asymmetric centers. H, 13C
1
Figure 2. Comparison of H NMR spectra of compounds 5b (top) and 10 (below) at 250 MHz showed that no epimerization
occurred either at Ca or Cb during the inversion and fluorination steps.

L. Demange et al. / Tetrahedron Letters 42 (2001) 651–653
653
References
Boc-3-chloroprolines (up to 60% from 4b and 22% from
9; DCI/NH3: m/z=340+342 (MH+)) as the major by-
product.
1. Stein, R. Adv. Protein Chem. 1993, 44, 1–23.
2. (a) Budisa, N.; Minks, C.; Medrano, F. J.; Lutz, J.;
Huber, R.; Moroder, L. Proc. Natl Acad. Sci. USA 1998,
95, 455–459; (b) Keller, M.; Sager, C.; Schutkowski, M.;
Fischer, G. S.; Mutter, M. J. Am. Chem. Soc. 1998, 120,
2714–2720; (c) An, S. S. A.; Lester, C. C.; Peng, J.-L.; Li,
Y.-J.; Rothwarf, D. M.; Welker, E.; Thannhauser, T. W.;
Zhang, L. S.; Tam, J. P.; Shegara, H. A. J. Am. Chem.
Soc. 1999, 121, 11558–11566; (d) Be´lec, L.; Slaninova, J.;
Lubell, W. D. J. Med. Chem. 2000, 43, 1448–1455.
3. Weiss, M. S.; Jabs, A.; Hilgenfeld, R. Nat. Struct. Biol.
1998, 5, 676.
4. (a) Avent, A. G.; Bowler, A. N.; Doyle, P. M.;
Marchand, C. M.; Young, D. W. Tetrahedron Lett. 1992,
33, 1509–1512; (b) Panasik Jr., N.; Eberhardt, E. S.;
Edison, A. S.; Powell, D. R.; Raines, R. T. Int. J. Peptide
Protein Res. 1994, 44, 262–269; (c) Eberhardt, E. S.;
Panasik Jr., N.; Raines, R. T. J. Am. Chem. Soc. 1996,
118, 12261–12266.
1
8. 1: [h]2D1=−86 (c=1, CHCl3); H (CDCl3): l (ppm) 7.8 (s,
1H), 5.41 (d, 1H, 2JH-F=55 Hz), 4.53–4.45 (m, 1H),
3.82–3.55 (m, 2H), 2.34–1.95 (m, 2H), 1.46 (minor) and
1.42 (major); 13C (CDCl3): l (ppm) 172.5 (major) and
171.6 (minor), 154.4 (minor) and 153.7 (major),
1
1
93.1(major, d, JC-F=185 Hz) and 92.2 (minor, d, JC-F
=
182 Hz), 81.0 (major) and 80.8 (minor), 64.1 (d, 2JC-F=22
Hz), 44.6 (minor) and 44.1 (major), 32.0 (minor, d,
2JC-F=23 Hz) and 31.4 (major, d, 2JC-F=21 Hz), 29.2
(minor) and 28.1 (major); 19F NMR (CDCl3): l (ppm)=
−182.6 (major) and −183.0 (minor); MS (DCI, NH3):
m/z 251 (MNH+4, 26%), 234 (MH+, 85%), 195 (MNH+4−
C4H8, 30%), 178 (MH+−C4H8, 100%). Anal.
(C10H16FNO4) calcd: C, 51.50; H, 6.91; N, 6.0. Found: C,
52.15; H, 7.14; N, 5.88.
9. Comparison of 1H NMR spectra of the corresponding
3-hydroxyprolines showed significative differences with
no traces of the other isomer, see: Jurczac, J.; Prokopow-
icz, P.; Golebiowski, A. Tetrahedron Lett. 1993, 34, 7107–
7110.
5. Demange, L.; Me´nez, A.; Dugave, C. Tetrahedron Lett.
1998, 39, 5775–5778.
6. (a) Sibi, M. P.; Christensen, J. W. Tetrahedron Lett. 1995,
36, 6213–6216; (b) Dell’Uomo, N.; Di Giovanni, M. C.;
Misiti, D.; Zappia, G.; Delle Monache, G. Tetrahedron:
Asymmetry 1996, 7, 181–188; (c) Mulzer, J.; Meier, A.;
Buschmann, J.; Luger, P. J. Org. Chem. 1996, 61, 566–
572.
7. General procedure of fluorination using DAST: 5 equiv. of
DAST were added dropwise to a solution of Boc-3-
fluoroproline benzyl ester (1 mmol) in dry dichloro-
methane under argon at −78°C. The mixture was stirred
for 5 hours at room temperature. The reaction was
quenched with 1 M sodium bicarbonate and the aqueous
layer was extracted with dichloromethane. The combined
organic layers were washed with 10% citric acid and brine
and were dried over sodium sulfate. Purification by silica
gel flash chromatography (eluent: ethyl acetate:hexane
20:80) gave the product along with the corresponding
10. 2: [h]2D1=−69 (c=0.75, CHCl3); 1H NMR (CDCl3): l
(ppm) 8.15 (bs, 1H), 5.43 (major, d, 1H, 2JH-F=52 Hz)
and 5.24 (minor, d, 1H, 2JH-F=48 Hz), 4.59 (major, d,
1H, 3JH-F=22 Hz) and 4.50 (minor, d, 1H, 3JH-F=23
Hz), 3.80–3.48 (m, 2H), 2.24–1.83 (m, 2H), 1.50 (major,
s) and 1.43 (minor, s) (9H); 13C NMR (CDCl3): l (ppm)
174.3 (minor) and 171.2 (major), 156.7 (major) and 153.6
1
(minor), 95.0 (minor, d, JC-F=186 Hz) and 93.7 (major,
d, 1JC-F=182 Hz), 82.5 (major) and 81.0 (minor), 59.9
(minor) and 58.4 (major), 44.8 (major) and 44.6 (minor),
34.2 (major) and 33.7 (minor), 28.2; 19F NMR (CDCl3):
l (ppm) −171.1 (major), −176.7 (minor); MS (DCI,
NH3): m/z 251 (MNH4+, 53%), 234 (MH+, 100%), 216
(MH+−H2O, 35%), 195 (MNH4+−C4H8, 41%), 178 (MH+−
C4H8, 85%). Anal. (C10H16FNO4) calcd: C, 51.50; H,
6.91; N, 6.0. Found: C, 51.38; H, 7.01; N, 6.10.
11. Cox, C.; Lectka, T. J. Org. Chem. 1998, 63, 2426–2427.
.