Multigram-scale synthesis of enantiopure 3,3-difluoroproline

Article abstract of DOI:10.1016/j.tetlet.2016.11.012

An efficient route for the synthesis of enantiopure 3,3-difluoroproline on multigram-scale is described herein. The deoxofluorination can be achieved with DAST on the corresponding racemic pyrrolidinone in good yield. Resolution of the racemate by crystallization with D- and L-tyrosine hydrazide provides both enantiomers of 3,3-difluoroproline in high yield and ee%.

Full text of DOI:10.1016/j.tetlet.2016.11.012

Accepted Manuscript
Multigram-scale Synthesis of Enantiopure 3,3-Difluoroproline
Christelle Doebelin, Yuanjun He, Theodore M. Kamenecka
PII:
S0040-4039(16)31468-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.11.012
TETL 48298
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
20 October 2016
31 October 2016
2 November 2016
Please cite this article as: Doebelin, C., He, Y., Kamenecka, T.M., Multigram-scale Synthesis of Enantiopure 3,3-
Difluoroproline, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.11.012
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Tetrahedron Letters
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Multigram-scale Synthesis of Enantiopure 3,3-Difluoroproline
Christelle Doebelin, Yuanjun He, and Theodore M. Kamenecka*
Department of Molecular Therapeutics, The Scripps Research Institute, Scripps Florida, 130 Scripps Way #A2A, Jupiter, FL 33458, USA
ARTICLE INFO
ABSTRACT
Article history:
Received
An efficient route for the synthesis of enantiopure 3,3-difluoroproline on multigram-
scale is described herein. The deoxofluorination can be achieved with DAST on the
corresponding racemic pyrrolidinone in good yield. Resolution of the racemate by
crystallization with D- and L-tyrosine hydrazide provides both enantiomers of 3,3-
difluoroproline in high yield and ee%.
Received in revised form
Accepted
Available online
Keywords:
3,3-dilfuoroproline
Chiral resolution
Gram-scale synthesis
2009 Elsevier Ltd. All rights reserved.
For 60 years, the replacement of hydrogen by fluorine has
been used in medicinal chemistry to modulate properties of
druglike compounds.1–5 A fluorine atom is regarded as a
bioisostere of hydrogen. The substitution of a hydrogen atom by
fluorine can block undesirable metabolism at a specific site, can
increase lypophilicity and/or binding affinity and can modulate
ADME parameters. Countless drugs and clinical candidates take
advantage of fluorine atoms for these reasons. More recently,
fluorinated amino acids have been incorporated in proteins to
study stability, configuration, protein folding, and protein-protein
interactions.6–9 Optically pure 4,4-difluoroproline was
synthesized over 30 years ago. It has shown is usefulness
especially in medicinal chemistry to significantly improve
potency, selectivity and/or solubility.10–13 We were in need of
enantiomerically pure 3,3-difluoroproline in gram quantities, and
were surprised by the lack of suitable methods to synthesize it.
Hence, we devised a synthetic route that could deliver optically
pure material in just a few steps with minimal purifications.
Herein we describe our approach.
This process was then optimized to allow the synthesis of
chiral β,β-difluoroproline derivatives (Scheme 2).17 The number
of steps in these routes and the use of specific reagents (ie.
allyltributyltin/AIBN, ozone) make these synthetic pathways
time-consuming and difficult to scale up. We envisioned an
alternate strategy to 3,3-difluoroproline starting from
a
commercially available β-ketoproline derivative that was both
fast and scalable. For larger scale synthesis (> 30 g), we
synthesized the β-ketoproline and adjusted the protecting groups
to suit our needs.
Scheme 1.
Synthesis of DL-3,3-difluoroproline derivative from masked 3-
hydroxyprolinol 3.a
Two major synthetics pathways have been described for the
synthesis of 3,3-difluoroproline derivatives. In 1993, Hart and
Coward, described the synthesis of racemic 3,3-difluoroproline
from the masked 3-hydroxyprolinol 3 (Scheme 1).14 Compound
3 can be obtained in five steps following the procedure describe
by Tamaru et al.15 In 1995, Shi et al. reported a synthetic pathway
providing racemic β,β-difluoroproline starting from β,β-difluoro-
aReagents: a) (CF3CO)2O, DMSO, Et3N, CH2Cl2, 73%; b) DAST, CH2Cl2, -
78°C to rt, 64%; c) 6N HCl, reflux; d) (Boc)2O, NaHCO3, CHCl3, H2O, 92%
from 5; e) RuO2.xH2O, 10%NaIO4, EtOAc; f) CH2N2, Et2O, 93% from 6.
α-keto-esters.16
———
*
Corresponding author.
E-mail address: kameneck@scripps.edu

2
Tetrahedron
We initiated our synthetic investigations from commercially
Scheme 3.
Difluorination of N-Boc-3-oxopyrrolidine-2-carboxylate. a
available ethyl N-Boc-3-oxopyrrolidine-2-carboxylate 19. The
key step is the deoxofluorination of the β-oxo group.18 Literature
data as well as our own in-house studies confirm that the use of
nucleophilic fluorination agents like DAST and its relatives on
acyclic 3-keto esters fails to provide the desired 3,3-difluoro
ester.
aReagents: a) DAST, neat, 0°C-rt, 18h, 64%; b) HCl 6N, 60°C, 5h; c)
CbzCl, NaHCO3, H2O, THF, rt, 24h, 81% over two steps.
Scheme 2.
Initial synthesis of enantiopure 3,3-difluoroproline derivative.a
This synthetic pathway was useful to prepare several grams of
3,3-difluoroproline, but for larger batches, we used Moyer et
al.’s26 synthesis of cyclic α-amino-β-keto ester 27 (Scheme 4),
but modified the protecting groups to simplify the chemistry. The
tert-butyl ester could be cleaved at room temperature, leaving the
Cbz-group needed for the resolution step. The key step of the
synthetic pathway is the cyclization of α-diazo-β-keto ester in
presence of Rh2(OAc)4.
Scheme 4.
Preparation and fluorination of N-Cbz-3-oxopyrrolidine-2-
carboxylate. a
aReagents: a) Mg, NaI, Me3SiCl, DMF, 50%; b) NBS, CH2Cl2, rt, 97%; c) H2,
Pd(OCOCF3)2, (R)-BINAP, rt; d) allyltributyltin, AIBN, toluene, 90°C, 87%
from 10; e) CHTD, BnOH, toluene, reflux, 89%; f) i CAN, CH3CN, 0°C, ii
Na2S2O3, CBzCl, NaHCO3, 0°C, 39%; g) i O3/O2, CH2Cl2, -78°C, ii Me2S, -
78°C to rt, 77%; h) PPh3.Cl2, DMF, 0°C to rt; i) H2 (1 atm), RhCl(PPh3)3,
benzene, rt, 87%, > 99%ee.
These reactions lead to the formation of difluoro olefin esters
and the occurrence of several side products.19 To our knowledge,
the fluorination of an α-amino β-keto esters had only been
described in the patent literature.20–22 With this in mind, we tested
three different fluorination agents (Deoxo-Fluor®23, DAST24 and
XtalFluor-E®25) with ketone 19 in CH2Cl2 (Scheme 3). In all
reactions conditions tested, desired product 20 was detected,
however despite using six equivalents of fluorination agent, the
reactions failed to go to conversion. The reaction was ultimately
improved by using 3 equivalents of DAST without solvent.
Under these reaction conditions, 80% conversion of ketone 19
was observed by 1H-NMR. The 3,3-difluoro ester 20 was isolated
in 64% yield on multigram scale (5-6 g) starting with ketone 19.
Surprisingly, the reaction was quite clean, affording none of the
by-products observed when using acyclic β-ketoesters. The
hydrolysis of the Boc protecting group and the ester was
achieved in a single step using aqueous 6N HCl at 60°C for 5h.
We observed significant degradation of desired compound 21 at
higher temperature (> 70°C) likely through competing β-
elimination pathways. After removing excess HCl, the obtained
amino acid was directly protected with benzyl chloroformate for
easier isolation producing racemic Cbz-3,3-difluoroproline
derivate 22 in 81% yield over three steps from 20.
aReagents: a) (i) CDI, THF, rt, 18h; (ii) iPrMgBr, 0°C-rt, 5h, 95%; b) m-
carboxybenzenesulfonyl azide, CH3CN, Et3N, rt, 2h, 95%; c) Rh2(OAc)4,
toluene, 90°C, 1h, 91%; d) DAST, DCM, rt, 18h, 71%; e) TFA, DCM, rt, 5h,
qt.
β-keto ester 25 was obtained by a Masamune-Claisen
condensation between the two commercially available carboxylic
acids 23 and 24 in presence of 1,1’-carbonyldiimidazole (CDI)
and isopropylmagnesium bromide. The diazo group was
introduced by diazo transfer from the 3-carboxybenzenesulfonyl
azide27
in
95%
yield.
The
resulting
m-
carboxybenzenesulfonamide can be easily removed from desired
compound 26 by an alkaline wash. Cyclization of the α-diazo β-
keto ester 26 in the presence of Rh2(OAc)4 gave 27 in 91% yield.
Deoxofluorination was performed in the presence of DAST to
afford the β,β-difluoroproline derivate 28 in 71% yield. Prior to
the fluorination step, no silica gel chromatography was needed
which was advantageous on multi-gram scale (> 30 g).
Moreover, the presence of the Cbz protecting group from the
beginning of the synthesis facilitated reaction monitoring by UV
and was desired for the resolution step. Another advantage of this
synthetic pathway was the presence of the t-butyl ester which
was cleaved under milder conditions than the ethyl ester. The use
of TFA/DCM at rt, during the hydrolysis instead of 6N HCl at
60°C reduced the risk of formation of undesired side products.

5. Gillis, E. P.; Eastman, K. J.; Hill, M. D.; Donnelly, D. J.; Meanwell, N.
A. J. Med. Chem. 2015, 58 (21), 8315–8359.
6. Hodgest, J. A.; Raines, R. T. J. Am. Chem. Soc. 2003, 125 (31), 9262–
9263.
Scheme 5.
Chiral resolution a
7. Jäckel, C.; Salwiczek, M.; Koksch, B. Angew. Chemie - Int. Ed. 2006, 45
(25), 4198–4203.
8. Cornilescu, G.; Hadley, E. B.; Woll, M. G.; Markley, J. L.; Gellman, S.
H.; Cornilescu, C. C. Protein Sci. 2007, 16 (9), 14–19.
9. Marsh, E. N. G. Acc. Chem. Res. 2014, 47 (10), 2878–2886.
10. Qiu, Z.; Lin, X.; Zhou, M.; Liu, Y.; Zhu, W.; Chen, W.; Zhang, W.; Guo,
L.; Liu, H.; Wu, G.; Huang, M.; Jiang, M.; Xu, Z.; Zhou, Z.; Qin, N.;
Ren, S.; Qiu, H.; Zhong, S.; Zhang, Y.; Zhang, Y.; Wu, X.; Shi, L.; Shen,
F.; Mao, Y.; Zhou, X.; Yang, W.; Zhen Wu, J.; Yang, G.; Mayweg, A. V;
Shen, H. C.; Tang, G. J. Med. Chem. 2016, 59 (16), 7651–7666.
11. Jansen, K.; Heirbaut, L.; Verkerk, R.; Cheng, J. D.; Joossens, J.; Cos, P.;
Maes, L.; Lambeir, A.; Meester, I. De; Augustyns, K.; Van Der Veken, P.
J. Med. Chem. 2014, 57 (7), 3053–3074.
12. Gardelli, C.; Nizi, E.; Muraglia, E.; Crescenzi, B.; Ferrara, M.; Orvieto,
F.; Pace, P.; Pescatore, G.; Poma, M.; Ferreira, M. D. R. R.; Scarpelli, R.;
Homnick, C. F.; Ikemoto, N.; Alfieri, A.; Verdirame, M.; Bonelli, F.;
Paz, O. G.; Taliani, M.; Monteagudo, E.; Pesci, S.; Laufer, R.; Felock, P.;
Stillmock, K. A.; Hazuda, D.; Rowley, M.; Summa, V. J. Med. Chem.
2007, 50 (20), 4953–4975.
13. De Bruin, G.; Huber, E. M.; Xin, B. T.; Van Rooden, E. J.; Al-Ayed, K.;
Kim, K. B.; Kisselev, A. F.; Driessen, C.; Van Der Stelt, M.; Van Der
Marel, G. A.; Groll, M.; Overkleeft, H. S. J. Med. Chem. 2014, 57 (14),
6197–6209.
14. Hart, B. P.; Coward, J. K. Tetrahedron Lett. 1993, 34 (31), 4917–4920.
15. Tamaru, Y.; Kawamura, S.; Bando, T.; Tanaka, K.; Yoshida, Z. J. Org.
Chem. 1988, 53 (2), 5491–5501.
16. Shi, G.-Q.; Cai, W.-L. J. Org. Chem. 1995, 60 (20), 6289–6295.
17. Suzuki, A.; Mae, M.; Amii, H.; Uneyama, K. J. Org. Chem. 2004, 69
(15), 5132–5134.
18. Kirk, K. L. Org. Process Res. Dev. 2008, 12 (2), 305–321.
19. Bildstein, S.; Ducep, J.-B.; Jacobi, D.; Zimmermann, P. Tetrahedron Lett.
1995, 36 (28), 5007–5010.
20. Li, W.; Oliver, E.; Rojas, C.; Kalish, V.; Belyakov, S. WO 2004/071454.
21. Burnett, D.; Bursavich, M. G.; Mcriner, A. WO 2015/109109.
22. Kamenecka, T. M.; He, Y. WO 2016/025669.
23. Lal, G. S.; Pez, G. P.; Pesaresi, R. J.; Prozonic, F. M.; Cheng, H. J. Org.
Chem. 1999, 64 (19), 7048–7054.
24. Hudlicky, M. Org. React. 1988, 35, 513–637.
aReagents: a) (i) D-tyrosine hydrazide, iPrOH, MeOH, 90°C-rt, 18h; (ii)
HCl 1N, 80%, 99%ee; b) L-tyrosine hydrazide, iPrOH, MeOH, 90°C-rt, 18h;
(ii) HCl 1N, 80%, 99%ee; c) Pd/C, H2, EtOAc, rt, 18h, 99%.
We next tackled the resolution of our racemic material.
Tyrosine hydrazide (Tyr-NHNH2) has been shown to efficiently
resolve several amino acid derivatives.28–30 Vogler and Lanz.28
reported that L-tyrosine hydrazide could resolve DL-amino acids
to provide the D-antipodes with high efficiency and enantiomeric
excess. Fortunately, in this case, the 3,3-bis-fluorine substitution
did not interfere with the resolution, and using D-tyrosine
hydrazide, we isolated the salt of acid 29 after filtration, with
99%ee and 80% yield after the first crystallization (Scheme 5).
D-tyrosine hydrazide was easily prepared from D-tyrosine
($0.50/gram) in two steps (93% yield on 3 kg scale) following
the same method as described for the L-tyrosine hydrazide after
recrystallization.31 An acidic wash removed the hydrazide, which
could be recovered and recycled.28 A second crystallization of the
mother liquor with L-tyrosine hydrazide afforded the opposite
enantiomer, (S)-3,3-difluoro-Cbz-proline 30 with 99%ee and
80% yield following acidic work-up. The same yield and
enantiomeric excess were observed during the resolution of acid
22 on milligram scale (20 mg) as well as on 70 g scale.
Enantiomeric excess was determined using chiral HPLC (see
supporting information). The deprotection of the Cbz group by
hydrogenation (99% yield) afforded enantiopure 3,3-
difluoroproline in 5 steps from commercially available starting
materials.
25. L’heureux, A.; Beaulieu, F.; Bennett, C.; Bill, D. R.; Clayton, S.;
Laflamme, F.; Mirmehrabi, M.; Tadayon, S.; Tovell, D.; Couturier, M. J.
Org. Chem. 2010, 75 (10), 3401–3411.
26. Moyer, M. P.; Feldman, P. L.; Rapoport, H. J. Org. Chem 1985, 50 (12),
5223–5230.
27. Moreau, R. J.; Sorensen, E. J. Tetrahedron 2007, 63 (28), 6446–6453.
28. Vogler, K.; Lanz, P. Helv. Chim. Acta 1966, 49 (4), 1348–1354.
29. Rodebaugh, R. M.; Cromwell, N. H. J. Heterocycl. Chem 1969, 6 (6),
993–994.
30. Jaen, J. C. e-Eros Encycl. Reagents Org. Synth. John Wiley Sons, LTd,
Ed. Chichester, UK 2011.
In summary, the synthetic pathway described herein gives
access to optically pure (S)- and (R)-3,3-difluoroproline with
good yields at each step. Purification by flash chromatography
was only necessary after the key fluorination step, allowing for
the easy production of gram quantities of the desired compound.
3,3-difluoroproline could be used as a scaffold in medicinal
chemistry or in non-natural peptide synthesis.
31. Kudelko, A.; Zielinski, W.; Ejsmont, K. Tetrahedron 2011, 67 (40),
7838–7845.
Acknowledgments
Supported by the National Institute on Drug Abuse
(DA033622 to T.M.K.).
Supplementary data
Supplementary
data
(experimental
protocols
and
spectroscopic data) associated with this article can be found in
the online version.
References
1. Swallow, S. Prog. Med. Chem. 2015, 54 (2), 65–133.
2. Purser, S.; Moore, P. R.; Swallow, S.; Gouverneur, V. Chem. Soc. Rev.
2008, 37 (2), 267–432.
3. Hagmann, W. K. J. Med. Chem. 2008, 51 (15), 4359–4369.
4. Müller, K.; Faeh, C.; Diederich, F. Science 2007, 317 (317), 1881–1886.

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Multigram-scale Synthesis of Enantiopure
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3,3-Difluoroproline
Christelle Doebelin, Yuanjun He, Theodore M. Kamenecka

*Highlights



Enantioselective synthesis of 3,3-difluoroproline
Short scalable synthesis
Resolution provided both enantiomer in high ee and yield