The absolute configuration of (+)- and (-)-erythro-mefloquine

Article abstract of DOI:10.1002/anie.201300258

The controversy over the absolute configuration of (+)-erythro-mefloquine, the less psychosis-causing enantiomer of the anti-malarial drug Lariam, has been resolved by Mosher ester crystallization. The configuration determined previously by physical metho

Full text of DOI:10.1002/anie.201300258

Angewandte
Chemie
DOI: 10.1002/anie.201300258
Crystal Structure Analysis
The Absolute Configuration of (+)- and (ꢀ)-erythro-Mefloquine**
Michael Mꢀller, Claudia M. Orben, Nina Schꢀtzenmeister, Manuel Schmidt, Andrei Leonov,
Uwe M. Reinscheid,* Birger Dittrich,* and Christian Griesinger*
The antimalarial agent rac-erythro-mefloquine hydrochloride,
established in the 1970s and marketed as Lariam by Roche, is
one of the standard medications for malaria chemoprophy-
laxis and treatment.[1] One major drawback of this drug is the
occurrence of dose-related neuropsychiatric adverse
effects.[1,2] Indeed, the enantiomer (ꢀ)-erythro-mefloquine is
known to block adenosine receptors in CNS and therefore
believed to be responsible for the neuropsychiatric symptoms,
while (+)-erythro-mefloquine does not.[3] Application of
(+)-erythro-mefloquine rather than the racemate could
therefore result in a better risk to benefit evaluation.
mentioned investigation.[5]
A first total synthesis of
(+)-erythro-mefloquine was based on a l-proline-catalyzed
asymmetric direct aldol reaction[6] and supported the CD
assignment of Carrol.[4] This version of the aldol reaction is
accepted to be reliable but it was not used before for
a synthesis similar to that of (+)-erythro-mefloquine. In our
endeavor to determine the absolute configuration of
(+)-erythro-mefloquine, we applied residual dipolar coupling
(RDC)-enhanced NMR spectroscopy in combination with
optical rotatory dispersion (ORD) and circular dichroism
(CD) spectroscopy,[7] and confirmed the result of the crystal
structure.[5] The approach had been successfully applied in the
past for several other flexible natural products[8] and is
a standard technique for rigid natural products. However,
another total synthesis[9] by the Coltart group confirmed the
absolute configuration determined by CD and the previous
total synthesis,[4,6] contradicting the X-ray[5] and NMR[7]
results. They utilized an asymmetric Darzens reaction, yield-
ing an enantiomerically pure intermediate with an absolute
configuration confirmed by crystal-structure analysis. After
eight further steps, (+)-erythro-mefloquine·HCl was obtained
and the absolute configuration was determined as (11R,12S).
A third total synthesis using an enantioselective transferhy-
drogenation to reduce the ketone presented (+)-erythro-
mefloquine with (11R,12S) as the absolute configuration,
relying on the assignment of the previous syntheses without
checking the optical rotation of the product.[10]
Owing to the different pharmaceutical activity of the
erythro-mefloquine enantiomers, a number of researchers
have attempted to determine the absolute configuration of
(+)-erythro-mefloquine 1 (Scheme 1) in the past. However,
Scheme 1. The antimalarial agent (+)-erythro-mefloquine HCl 1 and its
free base 2.
the results were ambiguous and caused controversy. The first
paper by Carroll in 1974 claimed the absolute configuration of
(+)-erythro-mefloquine as (11R,12S) based on circular
dichroism (CD) and empirical rules.[4] A crystal-structure
analysis of (ꢀ)-erythro-mefloquine HCl using the anomalous
signal from single-crystal X-ray diffraction determined the
absolute configuration as (11R,12S), contradicting the afore-
As it is puzzling for a simple albeit important antimalarial
agent as mefloquine to have the assignment of (+)-(11R,12S)
four times and of (ꢀ)-(11R,12S) twice, we will show in the
present study that the absolute configuration of (+)-erythro-
mefloquine is indeed (11S,12R) and of (ꢀ)-erythro-meflo-
quine (11R,12S). Our analysis is based on crystal-structure
determinations of the Mosher amides of both enantiomers of
mefloquine together with HPLC separations of the diaste-
reomeric Mosher esters to verify the correct starting materi-
als. We indeed obtained the two enantiomeric Mosher amides
and determined both crystal structures to ensure the reli-
ability of our results.
[*] Dr. M. Mꢀller, Dr. N. Schꢀtzenmeister, Dipl.-Chem. M. Schmidt,
Dr. A. Leonov, Prof. Dr. U. M. Reinscheid, Prof. Dr. C. Griesinger
Max-Planck-Institut fꢀr Biophysikalische Chemie
Department of NMR-Based Structural Biology
Am Fassberg 11, 37077 Gçttingen (Germany)
E-mail: cigr@nmr.mpibpc.mpg.de
(+)-Erythro-mefloquine hydrochloride 1 as well as rac-
erythro-mefloquine hydrochloride (rac-1) are commercially
available. For the (+)-erythro-enantiomer we obtained very
similar [a]D values as reported previously.[11] For comparison
with reported HPLC data, the hydrochloride was converted
into the free base.[12] The synthesis of the Mosher amide
derivatives was achieved with (+)-erythro-mefloquine as well
as the racemate using five equivalents of Hꢀnigꢁs base
(iPr2NEt) and (S)-MTPA-Cl as well as (R)-MTPA-Cl, respec-
tively.[13] With this procedure no difference in reactivity
between the hydrochloride and the free base was observed.
M. Sc. C. M. Orben, Dr. B. Dittrich
Institut fꢀr Anorganische Chemie
Georg-August-Universitꢁt Gçttingen
Tammannstrasse 4, 37077 Gçttingen (Germany)
E-mail: bdittri@gwdg.de
[**] Support by the MPG (to C.G.) and the DFG (FOR 934 to C.G. and
U.M.R.) is acknowledged. We thank Prof. L. F. Tietze for continuous
support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300258.
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(+)-(11S,12R)-Erythro-mefloquine-(R)-Mosher
amide
(3) was obtained in 74% yield by amidation of (+)-erythro
mefloquine 1 with (S)-MTPA-Cl in reaction I. Reaction II
was the amidation of the racemic free base rac-2 yielding 3 as
well as (+)-(11R,12S)-erythro-mefloquine-(R)-Mosher amide
(4), which could be separated by column chromatography
(Scheme 2) and stereochemically assigned by comparison of
the Rf-values with 3 obtained from reaction I.
Scheme 2. Synthesis of (+)-(11S,12R)-erythro-mefloquine-(R)-Mosher
amide (3) and (+)-(11R,12S)-erythro-mefloquine-(R)-Mosher amide (4).
Figure 1. Crystal structures of the enantiomeric pair: (+)-(11S,12R)-
erythro-mefloquine-(R)-Mosher amide (3; top) and (ꢀ)-(11R,12S)-
erythro-mefloquine-(S)-Mosher amide (ent-3; bottom). Ellipsoids set at
50% probability.
In reaction III we synthesized (ꢀ)-(11S,12R)-erythro-
mefloquine-(S)-Mosher amide (ent-4) in 98% yield, and in
reaction IV we amidated the racemic free base rac-2 yielding
(ꢀ)-(11R,12S)-erythro-mefloquine-(S)-Mosher amide (ent-3)
in 47% and ent-4 in 40% yield (Scheme 3). The identification
of ent-3 as enantiomer to 3 was verified through equal
retention times by TLC, whereas the retention time of ent-4
was indeed different but identical to the product of reactio-
n III.
atom model) refinement of the crystal structure was already
sufficient to allow an assignment of the absolute configuration
based on the anomalous signal of the fluorine atoms. 56% of
all Friedel pairs in ent-3 were measured.
To further increase the confidence of our absolute
structure determination, an invariom refinement,[15] which
better describes the bonding electrons accounting for the
asphericity of the electron density distribution, was per-
formed. A locally modified version of the least-squares
refinement program XD was used for that purpose.[16]
This procedure was used because invariom refinement
reduces the standard uncertainty of all refined parameters in
proportion to the improvement of the figures of merit. This
also holds for the Flack parameter x and its standard
deviation[17] u that measure the reliability of the determina-
tion of the absolute configuration. Indeed, after aspherical-
atom modeling with scattering factors from the invariom
database the Flack parameter for ent-3 changed from x =
ꢀ0.02 ꢁ 0.06 (IAM) to x = ꢀ0.03 ꢁ 0.05 (invariom). Accord-
ing to Flack and Bernardinelli,[18] u should be below 0.05 and
x should be close to zero within 2u to obtain the absolute
configuration with confidence. This is the case for ent-3. The
enantiomeric compound 3 has also been measured and its
configuration and conformation in the crystal is enantiomeric
(Figure 1, top). The Flack parameter for 3 is x = 0.00 ꢁ 0.07
(IAM) and also verifies the absolute configuration.[19] It
should be mentioned that the known absolute configuration
of the Mosher acid chloride was reproduced in both crystal
structures.
The structure, including the absolute configuration of ent-
3, was determined by single-crystal X-ray diffraction[14]
(Figure 1, bottom). The conventional IAM (independent
Scheme 3. Synthesis of (ꢀ)-(11S,12R)-erythro-mefloquine-(S)-Mosher
amide (ent-4) and (ꢀ)-(11R,12S)-erythro-mefloquine-(S)-Mosher amide
(ent-3).
2
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[8] a) U. M. Reinscheid, M. Kçck, C. Cychon, V. Schmidts, C. M.
In general, total synthesis is a reliable applied practice for
the determination of the absolute configuration of organic
molecules. However, in case of mefloquine the absolute
configuration of this rather simple molecule could not be
determined correctly by total synthesis. To be safe with this
statement it is necessary to quantify the reliability of the
described results. For crystal structure analyses there are two
major pitfalls that can cause a wrong determination. On the
one hand it is possible that a crystal of the minor diastereomer
was used for X-ray analysis. In our case this is extremely
improbable becuase we carried out independent experiments
on both enantiomers. (+)-(11S,12R)-Erythro-mefloquine-
(R)-Mosher amide (3) was obtained in reaction I with d.r. >
99:1 (the diastereomer was not detectable on HPLC) and ent-
3 was obtained with d.r. = 98:2 in reaction IV. Furthermore,
for the respective diastereomers, 4 and ent-4 no single crystal
could be obtained. As it is always difficult to obtain a pure
crystal of the minor component from a mixture; the chance
that in both cases the “wrong” crystal was used for X-ray
analysis is much lower than the calculable 0.2% (2/98·1/99).
On the other hand, the crystal structure analysis itself may be
erroneous. However, prior to and after Invariom refinement
the Flack parameter and its standard deviation as well as the
excellent figures of merit clearly indicate the high reliability
of our X-ray analyses. Therefore, this second possible pitfall
can also be excluded beyond reasonable doubt.
Thiele, C. Griesinger, Eur. J. Org. Chem. 2010, 6900 – 6903; b) H.
Sun, E. J. dꢁAuvergne, U. M. Reinscheid, L. Carlos Dias, C.
Kleber, Z. Andrade, R. Oliveira Rocha, C. Griesinger, Chem.
Eur. J. 2011, 17, 1811 – 1817; c) H. M. Ge, H. Sun, N. Jiang, Y. H.
Qin, H. Dou, T. Yan, Y. Y. Hou, C. Griesinger, R. X. Tan, Chem.
Eur. J. 2012, 18, 5213 – 5221; d) H. Sun, U. M. Reinscheid, E. L.
Whitson, E. J. dꢁAuvergne, C. M. Ireland, A. Navarro-Vꢃzquez,
C. Griesinger, J. Am. Chem. Soc. 2011, 133, 14629 – 14636;
e) K. M. Specht, J. Nam, D. M. Ho, N. Berova, R. K. Kondru,
D. N. Beratan, P. Wipf, R. A. Pascal, D. Kahne, J. Am. Chem.
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3121.
[10] W. P. Hems, W. P. Jackson, P. Nightingale, R. Bryant, Org.
Process Res. Dev. 2012, 16, 461 – 463.
[11] for commercially available (+)-erythro-mefloquine HCl 1 from
23
SynphaBase AG: ½aꢂD ¼ + 32.4 (c = 0.5, MeOH); > 99.5% ee.
22
20
Literature: ½aꢂD ¼ + 33.9 (c = 0.28, MeOH);[4] ½aꢂD ¼ + 36 (c =
0.6, MeOH).[6]
[12] Y. Qiu, S. Kitamura, J. K. Guillory, Pharm. Res. 1992, 9, 1640 –
1643.
[13] (S)-MTPA-Cl and (R)-MTPA-Cl were obtained from sigma
aldrich with ꢃ 99% ee.
[14] Refined formula C27H23F9N2O3, formula weight 594.47 gmolꢀ1
;
Crystal size 0.03 ꢄ 0.10 ꢄ 0.11 mm3, orthorhombic, C2221, Z = 8,
a = 8.6087(3), b = 28.4285(11), c = 21.1151(8) ꢅ, V=
5167.6(3) ꢅ3, 1 = 1.528, m = 1.267 mm, T= 100 K, F(000) = 2432
e, l = 1.54178 ꢅ (CuKa), 2qmax = 138.388, w- and f-scans with
Bruker Smart 6000 detector. 51480 measured reflections, of
which 4849 were symmetry independent and 4739 (with FO >
3s(FO)) used in the refinement against F, riding hydrogen atoms
and constrained fluorine distances for the disordered trifluor-
omethyl group, 364 parameters refined, Rint = 3.5%, R1 = 2.33%,
Rw = 2.43%, Flack parameter x = ꢀ0.02(6), after transfer of the
invariom multipole populations (not refined): R1 = 1.97%, Flack
parameter x = ꢀ0.03(5). CCDC 903889 contains the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Optical
rotation of the crystals could not be measured.
In conclusion, we have determined the absolute config-
uration of (+)-erythro-mefloquine hydrochloride
1 as
(11S,12R) and confirmed one of the previous assignments[5]
and our previous NMR analysis.[7] The three assignments
from syntheses[6,9,10] could not be confirmed. For example, in
one case,[9] a possible epimerization of the product of the
Darzens reaction was not investigated. Likewise in another,[9]
only the absolute configuration of an intermediate was
determined. We are confident that the absolute configuration
of 1 has now been unambiguously determined, closing a topic
that was controversial for 40 years. This allows to work out
enantioselective syntheses of (+)-erythro-mefloquine that
might provide improved anti-malaria medication.
[15] B. Dittrich, C. B. Hꢀbschle, M. Messerschmidt, R. Kalinowski,
D. Girnt, P. Luger, Acta Crystallogr. Sect. A 2005, 61, 314 – 320.
[16] A. Volkov, P. Macchi, l. J. Farrugia, C. Gatti, P. Mallinson, T.
Richter, T. Koritsꢃnszky, 2006, XD2006—A computer program
Package for Multipole refinement, Topological Analysis of
Charge densities and Evaluation of Intermolecular Energies
from Experimental or Theoretical Structure Factors.
Received: January 11, 2013
Published online: && &&, &&&&
Keywords: absolute configuration · structure elucidation ·
.
[17] B. Dittrich, M. Strumpel, M. Schꢆfer, M. A. Spackman, T.
Koritsꢃnszky, Acta Crystallogr. Sect. A 2006, 62, 217 – 223.
[18] H. D. Flack, G. Bernardinelli, J. Appl. Crystallogr. 2000, 33,
1143 – 1148.
malaria · mefloquine
[1] P. Schlagenhauf, M. Adamcova, L. Regep, M. T. Schaerer, H.-G.
Rhein, Malar. J. 2010, 9, 357.
[19] Refined formula C27H23F9N2O3, formula weight 594.47 gmolꢀ1
;
Crystal size 0.05 ꢄ 0.18 ꢄ 0.23 mm3, orthorhombic, C2221, Z = 8,
a = 8.624(2), b = 28.410(6), c = 21.114(4) ꢅ, V= 5167(2) ꢅ3, 1 =
1.528, m = 1.267 mm, T= 100 K, F(000) = 2432 e, l = 1.54178 ꢅ
(CuKa), 2qmax = 138.988, w- and f-scans with Bruker Smart 6000
detector. 83378 measured reflections, of which 4848 were
symmetry independent (4731 with FO2 > 2s(FO2)) and used in
the refinement against F2, riding hydrogen atoms and con-
strained fluorine distances for the disordered trifluoromethyl
group, 429 parameters refined, Rint = 4.0%, R1 = 2.37%, Rw =
6.01%, Flack parameter x = ꢀ0.00(7), Experimental intensities
are available on request.
[2] W. R. Taylor, N. J. White, Drug. Saf. 2004, 27, 25 – 61.
[3] a) C. Mulliꢂ, A. Jonet, C. Desgrouas, N. Taudon, P. Sonnet,
Malar. J. 2012, 11, 65; b) J. Shepherd, International patent
WO98/39003, 1998.
[4] F. I. Carroll, J. T. Blackwell, J. Med. Chem. 1974, 17, 210 – 219.
[5] J. M. Karle, I. L. Karle, Antimicrob. Agents Chemother. 2002, 46,
1529 – 1534.
[6] Z.-X. Xie, L.-Z. Zhang, X.-J. Ren, S.-Y. Tang, Y. Li, Chin. J.
Chem. 2008, 26, 1272 – 1276.
[7] M. Schmidt, H. Sun, P. Rogne, G. K. E. Scriba, C. Griesinger,
L. T. Kuhn, U. M. Reinscheid, J. Am. Chem. Soc. 2012, 134,
3080 – 3083.
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Angewandte
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Communications
Crystal Structure Analysis
The controversy over the absolute con-
figuration of (+)-erythro-mefloquine, the
less psychosis-causing enantiomer of the
anti-malarial drug Lariam, has been
resolved by Mosher ester crystallization.
The configuration determined previously
by physical methods (see scheme) is
correct, while the configuration deter-
mined by three enantioselective synthe-
ses is wrong.
M. Mꢁller, C. M. Orben,
N. Schꢁtzenmeister, M. Schmidt,
A. Leonov, U. M. Reinscheid,*
B. Dittrich,*
C. Griesinger*
&&&& — &&&&
The Absolute Configuration of (+)- and
(ꢀ)-erythro-Mefloquine
Kristallstrukturanalyse
Die Kontroverse ꢀber die absolute Kon-
figuration von (+)-erythro-Mefloquin,
dem weniger Psychosen auslçsenden
Enantiomer des Malariawirkstoffs Lariam
wurde durch Kristallisation der Mosher-
Ester beigelegt. Die frꢀher mit physikali-
schen Methoden bestimmte Konfigura-
tion (siehe Schema) ist richtig, wꢁhrend
die mit drei enantioselektiven Synthesen
bestimmte Konfiguration falsch ist.
M. Mꢁller, C. M. Orben,
N. Schꢁtzenmeister, M. Schmidt,
A. Leonov, U. M. Reinscheid,*
B. Dittrich,*
C. Griesinger*
&&&& — &&&&
The Absolute Configuration of (+)- and
(ꢀ)-erythro-Mefloquine
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Crystal Structure Analysis
The controversy over the absolute con-
figuration of (+)-erythro-mefloquine, the
less psychosis-causing enantiomer of the
anti-malarial drug Lariam, has been
resolved by Mosher ester crystallization.
The configuration determined previously
by physical methods (see scheme) is
correct, while the configuration deter-
mined by three enantioselective synthe-
ses is wrong.
M. Mꢁller, C. M. Orben,
N. Schꢁtzenmeister, M. Schmidt,
A. Leonov, U. M. Reinscheid,*
B. Dittrich,*
C. Griesinger*
&&&& — &&&&
The Absolute Configuration of (+)- and
(ꢀ)-erythro-Mefloquine
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!