SYNLETT
Accounts and Rapid Communications in Synthetic Organic Chemistry
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Synthesis of cis-5-Trifluoromethylproline from L-Glutamic Acid letter
Stéphanie Ortial,a,b Rajesh Dave,1 Zohra Benfodda,a,b David Bénimélis,a,b Patrick Meffre*a,b Synthesis of cis-5-Trifluoromethylproline
a
Institut des Biomolécules Max Mousseron (IBMM), UMR CNRS 5247, Université de Montpellier I & II, Place Eugène Bataillon, 34095 Montpellier Cedex 5, France b UNIVERSITE DE NIMES, Laboratoire de Chimie BioOrganique (LCBO), Place G. Péri, 30021 Nîmes Cedex 1, France Fax +33(4)66364587; E-mail:
[email protected] Received: 03.11.2013; Accepted after revision: 09.12.2013
Abstract: The diastereoselective synthesis of cis-5-trifluoromethylproline (5-Tfm-Pro) from L-glutamic acid is described. 5-Tfm-Pro could be obtained through a seven-step linear sequence. Trifluoromethylation of the glutamic-derived ester or aldehyde and subsequent reduction of the cyclic imine are the key steps in the synthesis. Key words: amino acids, cis-5-trifluoromethylproline, fluorine, diastereoselective synthesis, imine, reduction
The conformation of peptides is known to influence their biological properties. The increased rigidity of peptidomimetics containing constrained amino acids has been correlated with an enhanced activity toward biological targets.2 It has been demonstrated that the introduction of cyclic amino acids such as proline into peptides helps controlling the cis/trans conformations of the amide bond, leading to a modification of the peptide global conformation.3 On the other hand, there is a growing interest in fluorinated amino acids because of their various biomedical applications.4 The presence of fluorine atoms in molecular and supramolecular structures confers onto them different physical and biological properties compared to the natural parents, such as greater hydrophobicity, thermal stability and increased protein–ligand interactions.5 Numerous trifluoromethylprolines have already been synthesized. 2-Trifluoromethylprolines were obtained in their enantiopure forms from chiral oxazolidines6 or chiral allylmorpholinones7 using ethyl trifluoropyruvate as a trifluoromethylated partner and coupled to other amino acids to give peptides.8,9 Gulevich et al. also published the preparation of dipeptides containing a 2-trifluoromethylproline residue.10 Surprisingly, no synthesis of simple 3-trifluoromethylproline has been reported. Fully substituted derivatives have been prepared via Cu(I)-catalyzed 1,3-dipolar cycloaddition of azomethine ylides with trifluorocrotonates or trifluoromethyl-substituted nitroalkenes.11-13 N-Protected-4 trifluoromethylproline was synthesized from Garner’s aldehyde14 and in a more efficient way from L-hydroxyproline.15 In this work, the trifluoromethylation of the corresponding ketone with Ruppert’s reagent and tetra-n-butylammonium fluoride (TBAF) was used to introduce the trifluoromethyl group. Similarly, Del Valle et al. reported the synthesis of NSYNLETT 2014, 25, 0569–0573 Advanced online publication: 14.01.201409 36-521 41437-2 096 DOI: 10.1055/s-0033-1340553; Art ID: ST-2013-D1028-L © Georg Thieme Verlag Stuttgart · New York
Boc-4-trifluoromethylproline from 4-hydroxyproline methyl ester involving a stereoselective substrate-directed reduction using Crabtree’s catalyst.16 5-Trifluoromethylprolines have attracted less attention. In 2010, Brigaud et al. reported the synthesis of 5-trifluoromethyl-1,3-oxazolidines as pseudoprolines17 from the condensation reaction of serine derivatives and trifluoroacetaldehyde ethyl hemiacetal and their incorporation into peptides.18 More recently, 5-trifluoromethylated polyhydroxypyrrolidines were synthesized from the respective nitrone using Ruppert’s reagent and TBAF as tetrasubstituted proline derivatives.19 The first diastereopure 5-trifluoromethylproline (Figure 1) has been prepared 20 in 17% overall yield from a trifluoromethyl enone and ethyl isocyanoacetate. In this strategy, 5-Tfm-Pro was obtained after reduction of the corresponding pyrrole, using H2, Pd/C, therefore leading to the mixture of both cis enantiomers.
HO2C
N H
CF3
Figure 1 rac-cis-5-Trifluoromethyl proline (cis-5-Tfm-Pro)
The synthesis of the enantiopure or enantioenriched cis-5trifluoromethyl-L-proline remains challenging. We report herein the synthesis of the cis-5-Tfm-Pro from naturally occurring L-glutamic acid. Our strategy is based on the chemoselective trifluoromethylation and selective reduction of a cyclic trifluoromethyl imine (Scheme 1). O HO2C
N H
CF3
HO2C
N
CF3
O
HO
OH NH2
rac-cis-5-Tfm-Pro
L-glutamic
acid
Scheme 1 Retrosynthesis
The synthesis of cyclic imine 6 is described in Scheme 2. Boc-Glu(OMe)-OMe 1 is commercially available or can be prepared quantitatively from L-glutamic acid in a onepot two-step process by esterification of the carboxylic acid followed by protection of the amino group with ditert-butyl dicarbonate (Boc2O). Enhanced selectivity for the less sterically hindered methyl ester toward various transformations could be achieved by the addition of a second tert-butyloxycarbonyl group onto the amino function.
Synthesis 2000, No. X, x–xx
ISSN 0039-7881
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is a copy of the author's personal reprint l
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S. Ortial et al. O
O
O
O
O
a HO
OH
O
O
MeO
NH2
OMe
MeO
OMe
O
NBoc2 3 e O
O
g or h N 6
MeO
NBoc2 2 c
NHBoc 1
HO2C
CF3
O
d
b
HO H
f MeO
CF3 NBoc2 5
MeO
CF3 NBoc2 4
Scheme 2 Reagents and conditions: (a) TMSCl, MeOH, r.t., 48 h then Boc2O, TEA, r.t., 24 h; (b) Boc2O, DMAP, MeCN, r.t., 16 h, 92% (2 steps); (c) CF3SiMe3, cat. TBAF, toluene, –78 °C to r.t., 16 h then TBAF, 0 °C to r.t., 15 min, 65%; (d) DIBAL-H, Et2O, −78 °C, 15 min, 84%; (e) CF3SiMe3, TBAF, neat, 0 °C to r.t., 4 h, 95%; (f) Dess–Martin periodinane, CH2Cl2, r.t., 4 h, 79%; (g) 4 M HCl, THF, r.t., 30 min then 1 M NaOH, r.t., 2 h, then Dowex 50WX8, 69%; (h) 6 M HCl, 60 °C, 24 h then Dowex 50WX8, 59%.
Thus, the reaction of secondary amine 1 with Boc2O in acetonitrile using N,N-dimethylaminopyridine (DMAP)21 gave the fully protected intermediate 2 in 92% yield from L-glutamic acid after purification by flash chromatography. Introduction of the trifluoromethyl group was less simple. Our initial strategy was the direct trifluoromethylation of the ester with Ruppert’s reagent (CF3SiMe3) and a catalytic amount of TBAF as described by Olah.22 Although the selective trifluoromethylation of the γ-methyl ester group could be achieved in 65% yield (20% of starting material was recovered) on a 2-gram scale, this reaction was difficult to reproduce in our hands.23 Therefore, we decided to use a more reliable strategy to prepare the trifluoromethyl ketone 5, involving the nucleophilic trifluoromethylation of the aldehyde 3. The latter was synthesized through the selective reduction of 2 with diisobutylaluminium hydride (DIBAL-H) at −78 °C in diethyl ether in good yield (84%).24 Trifluoromethylation of the resulting aldehyde was conducted with an excess of Ruppert’s reagent in the presence of a stoichiometric amount of TBAF at 0 °C on the neat substrate following a recently published method.25 After five hours at room temperature, the carbinol 4 was obtained in excellent yield (95%) and used without any further purification. Dess– Martin oxidation of 4 led to the corresponding trifluoromethyl ketone 5 in 79% yield. This three-step sequence was reproduced on various scales (0.5 g to 3 g) and remains competitive with the direct trifluoromethylation of ester 2 with a similar overall yield (63%). The key cyclic imine 6 was obtained after cleavage of the tert-butoxycarbonyl groups and saponification of the methyl ester. Under classical conditions, saponification of methyl ester containing amino acids can lead to epimerization.26 Therefore, we chose to deprotect the tert-butoxycarbonyl groups using 4 M HCl in THF before removing the methyl ester under basic aqueous conditions (Scheme 2, path g). Similarly, the fully deprotected amino acid could be prepared using strongly acidic conditions and heating (6 M HCl, 60 °C, Scheme 2, path h). The neutral amino group was then regenerated using an acidic resin, spontaneously giving the cyclic imine 6 in 69% and 59% yields, respectively after removal of the water in vacuo.27 The cyclic im-
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ine 6 must be stored in a dry environment as a solid to avoid any isomerization or hydration (Scheme 3). In anhydrous organic solvents, 6 slowly isomerized irreversibly leading to the loss of chirality at C-2. This observation was not completely unexpected since Soloshonok had described the [1,3]-proton shift reaction of fluorinated imines under thermal or basic conditions.28 In our case, the isomerization occurred at room temperature under neutral conditions due to the conjugation of 6′. Water adds non-selectively to the double bond of 6 giving a mixture of both diastereomeric acetals and the opened form. Methanol and ethanol also add slowly to the double bond of the imine. Considering the lack of stability and the high polarity of 6, finding a suitable solvent for its reduction to 5-trifluoromethylproline was problematic. We turned our attention to N,N-dimethylformamide since it dissolves the imine 6 and does not react with the double bond. Three methods were assessed to reduce 6 selectively and the results are depicted in Table 1. Firstly, classical hydrogenation using palladium on carbon under mild conditions afforded the amine 7 in quantitative yield after 16 hours.29 The relative configuration of 7 could not be definitively proven by NOE experiments. The correlation between the protons on C-2 and C-5 is known to be weak and in this case, those protons are too close in the 1H NMR spectrum of 7 (ca. 0.1 ppm) to give an interpretable result in a NOE experiment, appearing as a single multiplet when 7 is the hydrochloride salt. Kondratov et al. also reported such an issue (see ref. 20 and references therein). Therefore, the cis configuration of 7 was deduced by analogy with hydrogenation results of similar compounds. This consideration was also supported by the fact that hydrogen addition on the less hindered face of the imine is favored.30 We were also interested in isolating the trans5-Tfm-Pro as the major isomer. Thus, cyclic imine 6 was reacted with sodium triacetoxyborohydride in methanol31 at room temperature to produce a complex mixture of amine 7 and acetals because of the addition of methanol onto the double bond. Only 50% conversion was observed
© Georg Thieme Verlag Stuttgart · New York
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Synthesis of cis-5-Trifluoromethylproline
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Scheme 3 Cyclic imine 6 behavior in water and organic solvents. 19F NMR spectrum A (DMSO-d6): (a) t = 0, no 6′, (b) t = 7 h, 6% 6′, (c) t = 48 h, 35% of 6′, (d) t = 8 d, 72% of 6′; 19F NMR spectrum B: (a) in DMSO-d6, (b) in DMSO-d6 + D2O, (c) in D2O.
after 20 hours and the cis/trans ratio was not favorable (Table 1). Table 1 Synthesis of 5-Tfm-Pro through Reduction of the Imine 6 HO2C
N H (±)-7
CF3
reduction HO2C
N 6
or
CF3 O
N
CF3
CHO 8
Reagents
Conversion (%)
Product (cis/trans)a
H2, Pd/C, DMF
100
7 (87:13)
50
7 (67:33)
NaBH(OAc)3, MeOH (R)-TRIP, ethidine, DMF a
100
8
intermediate. 1H NMR and 13C NMR spectroscopy showed the disappearance of the proton at C-2 and mass spectrometry using negative electrospray ionization, indicated a major peak at m/z = 180.00 ([M − H]−, 80%). To assess the enantiopurity of 7, the more convenient to handle methyl ester was prepared. Reaction with trimethylsilyl diazomethane in methanol afforded the desired compound, and only the major isomer 9 was isolated after chromatography (50% yield, Scheme 4). Unfortunately, chiral chromatography showed that a racemic mixture of 9 had been obtained. If partial isomerization can occur during the reduction step (reaction time is 16 h), this cannot explain the extensive racemization observed. Lubell et al. reported such racemization during the deprotection and cyclization of their iminium intermediate under acidic conditions in the synthesis of 5-tert-butylproline.30c Further investigations must be conducted to ascertain if the deprotection of trifluoromethylated ketone 5 using 4 or 6 M HCl led to the racemization of the steoreogenic center.
19
The cis/trans ratio was assessed using F NMR. TMSCHN2, MeOH HO2C
Finally, the chiral phosphoric acid–Hantzsch ester system was also examined for catalytic transfer hydrogenation of imine 6.32 At room temperature, in the presence of (R)-TRIP (CAS# 791616-63-2) and ethidine (CAS# 1143-23-1) no reaction occurred, so the mixture was heated at 50 °C for 48 hours, to ensure complete conversion of the starting material. At such a temperature, the cyclic imine 6 isomerizes (Scheme 3) to give the more stable α,β-unsaturated compound. Further heating led to the proposed formation of 2-oxo-5-(trifluoromethyl)pyrrolidine1-carbaldehyde (8) through addition of DMF onto the C-2 position followed by the decarbonylation of the cyclic © Georg Thieme Verlag Stuttgart · New York
N H (±)-7
CF3
r.t., 2 h
MeO2C
N H (±)-9
CF3
Scheme 4 Synthesis of cis-5-Tfm-Pro methyl ester
In summary, we report herein the diastereoselective synthesis of cis-5-Tfm-Pro from L-glutamic acid in 40% overall yield. Developing a related strategy avoiding strongly acidic or basic conditions during the synthesis of 5-Tfm-Pro is currently being investigated to afford the enantiopure compound. Finally, incorporation of cis-5-TfmPro into peptides to study its influence on the conformation of the peptide will be examined. Synlett 2014, 25, 569–573
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Acknowledgment We thank Marion Jean for chiral HPLC separations (ISM2-UMR 7313, Aix-Marseille Université).
Supporting Information for this article is available online at http://www.thieme-connect.com/ejournals/toc/synlett. Included are detailed experimental procedures and 1H NMR, 13C NMR, 19F NMR and HRMS data.SmoInurfgiptSa
References and Notes (1) Present address: Calyx Chemicals & Pharmaceuticals Ltd, A-37/38, MIDC, Dombivli East, Dist. Thane 421 203, Maharashtra, India. (2) (a) Kowalczyk, V.; Prahl, A.; Derdowska, I.; Dawidowska, O.; Slaninovà, J.; Lammek, B. J. Med. Chem. 2004, 47, 6020. (b) Brackmann, F.; De Meijere, A. Chem. Rev. 2007, 107, 4493. (c) Fülöp, F.; Martinek, T. A.; Toth, G. K. Chem. Soc. Rev. 2006, 35, 323. (3) (a) Yaron, A.; Naider, F.; Scharpe, S. Crit. Rev. Biochem. Mol. Bio. 1993, 28, 31. (b) Vanhoof, G.; Goossens, F.; De Meester, I.; Hendriks, D.; Scharpé, S. FASEB J. 1995, 9, 736. (c) Cai, M.; Cai, C.; Mayorov, A. V.; Xiong, C.; Cabello, C. M.; Soloshonok, V. A.; Swift, J. R.; Trivedi, D.; Hruby, V. J. J. Peptide Res. 2004, 63, 116. (4) (a) Salwiczek, M.; Nyakatura, E. K.; Gerling, U. I. M.; Ye, S.; Koksch, B. Chem. Soc. Rev. 2012, 41, 2135; and references therein. (b) Qiu, X.-L.; Qing, F.-L. Eur. J. Org. Chem. 2011, 3261; and references therein. (c) Yoder, N. C.; Kumar, K. Chem. Soc. Rev. 2002, 31, 335. (d) Aceña, J. L.; Sorochinsky, A. E.; Soloshonok, V. A. Synthesis 2012, 44, 1591. (e) Aceña, J. L.; Sorochinsky, A. E.; Moriwaki, H.; Sato, T.; Soloshonok, V. A. J. Fluorine Chem. 2013, 155, 21. (f) Sorochinsky, A. E.; Soloshonok, V. A. J. Fluorine Chem. 2010, 131, 127. (5) (a) Soloshonok, V. A. Fluorine-Containing Synthons, Oxford University Press, 2005. (b) Ojima, I. Fluorine in Medicinal Chemistry and Chemical Biology; Blackwell Publishing, Ltd: Chichester, 2009. (c) Mikami, K.; Fustero, S.; Sanchez-Rosello, M.; Aceña, J. L.; Soloshonok, V.; Sorochinsky, A. Synthesis 2011, 3045. (6) Chaume, G.; Van Severen, M.-C.; Marinkovic, S.; Brigaud, T. Org. Lett. 2006, 8, 6123. (7) Caupène, C.; Chaume, G.; Ricard, L.; Brigaud, T. Org. Lett. 2009, 11, 209. (8) Chaume, G.; Lensen, N.; Caupène, C.; Brigaud, T. Eur. J. Org. Chem. 2009, 5717. (9) Jlalia, I.; Lensen, N.; Chaume, G.; Dzhambazova, E.; Astasidi, L.; Hadjiolova, R.; Bocheva, A.; Brigaud, T. Eur. J. Med. Chem. 2013, 62, 122. (10) Gulevich, A. V.; Shevchenko, N. E.; Balenkova, E. S.; Röschenthaler, G.-V.; Nenajdenko, V. G. Synlett 2009, 403. (11) Li, Q.; Ding, C.-H.; Li, X.-H.; Weissensteiner, W.; Hou, X.L. Synthesis 2012, 44, 265. (12) Li, G.-H.; Tong, M.-C.; Li, J.; Tao, H.-Y.; Wang, C.-J. Chem. Commun. 2011, 47, 11110. (13) Li, Q.-H.; Xue, Z.-Y.; Tao, H.-Y.; Wang, C.-J. Tetrahedron Lett. 2012, 53, 3650. (14) Qiu, X.-L.; Qing, F.-L. J. Chem. Soc., Perkin Trans. 1 2002, 2052. (15) Qiu, X.-L.; Qing, F.-L. J. Org. Chem. 2002, 67, 7162. (16) Del Valle, J. R.; Goodman, M. Angew. Chem. Int. Ed. 2002, 41, 1600. (17) Chaume, G.; Barbeau, O.; Lesot, P.; Brigaud, T. J. Org. Chem. 2010, 75, 4135.
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LETTER (18) (a) Chaume, G.; Feytens, D.; Chassaing, G.; Lavielle, S.; Brigaud, T.; Miclet, E. New J. Chem. 2013, 37, 1336. (b) Chaume, G.; Simon, J.; Caupène, C.; Lensen, N.; Miclet, E.; Brigaud, T. J. Org. Chem. 2013, 78, 10144. (19) Khangarot, R. K.; Kaliappan, K. P. Eur. J. Org. Chem. 2013, 13, 2692. (20) Kondratov, I. S.; Dolovanyuk, V. G.; Tolmachova, N. A.; Gerus, I. I.; Bergander, K.; Fröhlich, R.; Haufe, G. Org. Biomol. Chem. 2012, 10, 8778. (21) Padrón, J. M.; Kokotos, G.; Martín, T.; Markidis, T.; Gibbons, W. A.; Martín, V. S. Tetrahedron: Asymmetry 1998, 9, 3381. (22) Wiedemann, J.; Heiner, T.; Mloston, G.; Prakash, G. K. S.; Olah, G. A. Angew. Chem. Int. Ed. 1998, 37, 820. (23) The level of drying of all reagents and solvents required for this reaction might be the cause of the capricious nature of this transformation as discussed by the authors (Note: commercially available 1 M solution of TBAF in THF usually contains 5% of H2O). (24) Adamczyk, M.; Johnson, D. D.; Reddy, R. E. Tetrahedron: Asymmetry 1999, 10, 775. (25) Ilies, M.; Dowling, D. P.; Lombardi, P. M.; Christianson, D. W. Bioorg. Med. Chem. Lett. 2011, 21, 5854. (26) Bodanszky, M.; Bodanszky, A. In The Practice of Peptide Synthesis; Springer Verlag: Berlin, 1984, 177. (27) Experimental Procedure for 6: Using 4 M HCl–1 M NaOH: Compound 5 (1.2 mmol, 1 equiv) was dissolved in THF (10 mL) and a THF–12 M HCl mixture (11 mL/9 mL) was added dropwise and the solution was stirred at r.t. for 30 min. After concentration in vacuo, MeOH (2.4 mL) and 1 M NaOH (3.36 mL) were added to the crude product. The solution was stirred at r.t. for 2 h and concentrated in vacuo. Purification over Dowex 50WX8, eluting with H2O then 5% aq NH3 followed by purification over RP 18 silica gel, eluting with H2O afforded the imine 6 as an off-white solid after complete removal of the H2O (0.15 g, 0.83 mmol, 69%). Using 6 M HCl: Compound 5 (0.84 mmol, 1 equiv) was dissolved in 6 M HCl (3.5 mL). The solution was heated at 60 °C until reaction was complete (24 h) and the brown mixture then washed with Et2O (2 ×) and concentrated in vacuo. Purification following the procedure previously described afforded 6 in 59% yield (0.09 g, 0.50 mmol). 1H NMR (300 MHz, DMSO-d6): δ = 4.62 (m, 1 H), 2.76 (m, 2 H), 2.13 (m, 2 H). 13C NMR (75.45 MHz, DMSO-d6): δ = 173.3 (C), 164.3 (q, JCCF = 34.8 Hz, C), 120.5 (q, JCF = 273 Hz, C), 77.9 (CH), 33.5, 27.0 (CH2). 19F NMR (282.4 MHz, DMSO-d6): δ = −68.8 (d, JHF = 2.8 Hz). [α]D20 6.1 (c = 1.0, DMSO). HRMS (ESI−): m/z [M − H]− calcd for C6H5NO2F3: 180.0272; found: 180.0273. (28) (a) Yasumoto, M.; Ueki, H.; Soloshonok, V. A. J. Fluorine Chem. 2007, 128, 736. (b) Soloshonok, V. A.; Yasumoto, M. J. Fluorine Chem. 2006, 127, 889. (c) Ono, T.; Kukhar, V. P.; Soloshonok, V. A. J. Org. Chem. 1996, 61, 6563. (29) Experimental Procedure for 7: The cyclic imine 6 (0.27 mmol, 1 equiv) was dissolved in anhyd DMF (2 mL) and Pd/C was (10−15 wt%) added. The solution was degassed at −78 °C for 5 min and the flask was filled with H2. After 16 h at r.t., the reaction was complete. The solution was filtered over a pad of Celite® and concentrated under high vacuum. Purification over RP 18 silica gel eluting with H2O containing a few drops of 12 M HCl afforded 5-Tfm-Pro as its hydrochloride salt (0.061 g, 0.27 mmol, 100%). 1H NMR (300 MHz, D2O): δ = 4.41 (m, 2 H), 2.05−2.42 (m, 4 H). 13C NMR (75.45 MHz, D2O): δ = 171.2 (C), 123.0 (q, JCF = 278 Hz, C), 61.5 (CH), 59.5 (q, JCCF = 33.7 Hz, C), 27.1, 23.7 (CH2). 19F NMR (282.4 MHz, D2O): δ = −72.3 (d, JHF = 7.06
© Georg Thieme Verlag Stuttgart · New York
LETTER Hz, cis isomer), −73.0 (d, JHF = 7.34 Hz, trans isomer). MS (ESI−): m/z = 181.94 ([M − H]−). HRMS (ESI−): m/z [M − H]− calcd for C6H7NO2F3: 182.0429; found: 182.0430. (30) (a) Brun, M.-P.; Martin, A.-S.; Garbay, C.; Bischoff, L. Tetrahedron Lett. 2003, 44, 7011. (b) Mota, A. J.; Langlois, N. Tetrahedron Lett. 2003, 44, 1141. (c) Beausoleil, E.; L’Archevêque, B.; Bélec, L.; Atfani, M.; Lubell, W. D. J. Org. Chem. 1996, 61, 9447.
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(31) Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff, C. A.; Shah, R. D. J. Org. Chem. 1996, 61, 3849. (32) (a) Connon, S. J. Angew. Chem. Int. Ed. 2006, 45, 3909. (b) Guo, Q.-S.; Du, D.-M.; Xu, J. Angew. Chem. Int. Ed. 2008, 47, 759. (c) Rueping, M.; Tato, F.; Schoepke, F. R. Chem. Eur. J. 2010, 16, 2688.
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