Biomimetic synthesis of Tramadol

Article abstract of DOI:10.1039/c5cc05948h

Tramadol has recently been isolated from the roots and bark of Nauclea latifolia. A plausible biosynthetic pathway has been proposed and the product-precursor relationship has been probed by ¹³C position-specific isotope analysis. By further exploring this pathway, we demonstrate that a key step of the proposed pathway can be achieved using mild conditions that mimic in vivo catalysis.

Full text of DOI:10.1039/c5cc05948h

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Chem. Commun., 2015, DOI: 10.1039/C5CC05948H.
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Biomimetic synthesis of Tramadol
Florine Lecerf-Schmidt,a,b,† Romain Haudecoeur,a,b,† Basile Peres,a,b Marcos Marçal Ferreira
Queiroz,c Laurence Marcourt,c Soura Challal,c Emerson Ferreira Queiroz,c Germain Sotoing Taiwe,d
Thierry Lomberget,e Marc Le Borgne,e Jean-Luc Wolfender,c Michel De Waard,f Richard J. Robins,g
and Ahcène Boumendjel*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
In 2013, we reported that the root bark extract of Nauclea latifolia samples of N. latifolia, as reported in the original work of
Sm. (Rubiaceae) collected in a biosphere reserve in the north of Boumendjel et al.1, cannot be explained by anthropogenic
Cameroun contains large quantities (0.4% w/w) of (±)-(1R,2R)-2- contamination. In addition, the samples were collected in the
[(dimethylamino)methyl]-1-(3-methoxy-phenyl)- Tramadol has preserved Benoué biosphere reserve, in which human
recently been isolated from the roots and bark of Nauclea activities, as well as cattle pasturing, are prohibited.
latifolia. A plausible biosynthetic pathway has been proposed and
the product-precursor relationship probed by 13C position-specific
isotope analysis. Further exploring this pathway, we demonstrate
that a key step of the proposed pathway can be achieved in mild
conditions that mimic in vivo catalysis.
H2
N
NH2
NH2
CO2
CO2
H
L-Lysine
H
oxidative
deamination
L-Phenylalanine
O
NH2
cyclohexanol.1 The compound was isolated by a blind bioassay-
guided search and the structure rigorously established by
spectroscopic and X-ray diffraction analyses. It proved to be
structurally identical to the commercialized analgesic, Trama-
dol, previously only known as a synthetic pharmaceutical used
worldwide since 19772 as a painkiller by acting as a weak μ-
opioid receptor agonist.3 This discovery was highly publicized
and covered by the major worldwide press.4 Recently, Kusari
et al.5 reported the isolation of trace amounts of Tramadol
(<0.00002% w/w) and metabolites thereof from N. latifolia and
soils collected in the north of Cameroun and suggested a
possible anthropogenic contamination through cattle and
human overconsumption of Tramadol. However, this could
equally plausibly be due to simian dispersal following feeding
on N. latifolia, and the high concentration found in different
NH2
CO2H
L-2-amino-
adipic-6-semialdehyde
HO
CO2H
L-3'OH-
decarboxylation
Phenylalanine
O
NH2
MeO
N-methylation
Me
O
N
O
Me
1
2
aldolization
MeO
MeO
dehydration
reduction
O
O
Me
Me
N
N
Me
Me
HO
3
4
oxydation
MeO
MeO
MeO
O
O
HO
1
cyclisation
Me
Me
Me
2
N
N
N
iminium
reduction
Me
Me
Me
5a
5b
Tramadol
Scheme 1
Key intermediates and steps in the proposed biosynthesis of Tramadol,
partially corroborated by a recent position-specific isotope analysis.8
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J. Name., 2013, 00, 1-3 | 1
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In order to provide evidence to support the probable
natural origin of tramadol from N. latifolia, we have adopted
two approaches to provide a plausible biosynthetic route. As it
contains a basic amine, natural Tramadol can be classified as
an alkaloid, for which the biosynthesis frequently involves one
or more amino acid precursors, notably L-lysine, L-arginine, L-
tyrosine, L-phenylalanine or L-tryptophan.6 In the case of
Tramadol, as outlined in Scheme 1 the proposed pathway
invokes the condensation of 3’-methoxyacetophenone
N,N-dimethyl-5-aminopentanal to afford intermediate
which, upon reduction, provides amino ketone . The latter
1 with
2
3
4
would undergo an oxidation step to afford iminium 5a in
equilibrium with enamine 5b. Finally, the key step is the
cyclisation of enamine 5b followed by the reduction of the
resulting iminium to afford Tramadol.
Scheme 2
Preparation of dicarbonyl 8, precursor of the key intermediates 5a/5b.
exchange allows the nucleophilic addition, followed by a
subsequent dehydration step. The resulting compound was
In our first approach, the logical involvement of L-
6
phenylalanine, L-lysine,
1 and 2 in the putative biosynthetic
then submitted to a classical OsO4/NaIO4 treatment, leading to
the oxidative cleavage of the alkene group to afford compound
pathway of Tramadol has been detailed and related to known
biosynthetic compounds,7 a relationship partially corroborated
by the analysis of the position-specific isotope.8 In addition, L-
lysine is known to initiate, through decarboxylation and
oxidative deamination steps, the biosynthesis of 5-
aminopentanal which leads to indolo[2,3-a]quinolizidine
derivatives,9 a class of natural compounds widely present in N.
8, with an overall yield of 73%.
Having the dicarbonyl derivative
8 in hand, we proceeded
with its conversion to iminium 5a by treatment with dimethy-
lamine (Scheme 3). Despite several attempts, the isolation of
iminium 5a or enamine 5b in acceptable yields was
problematic and irreproducible. As an alternative, we
latifolia
(e.g.
angustine,
naucleamides11
nauclefine,
and
naucletine,10
strictosamide,
latifoliamides.12
attempted a one-pot conversion of
8
to Tramadol without
with
isolation of 5a and 5b. Upon treatment of
8
Furthermore, naturally occurring alkaloids sharing certain
characteristics of Tramadol, such as the presence of an
uncommon 3-methoxyphenyl substituent, have been
reported.13
dimethylamine followed by an in situ reduction with NaBH3CN
(or NaBH4), the formation of Tramadol as a mixture of two
diastereoisomers (
9 and 10) in 10% yield and of at least three
new compounds (11
,
12 and 13) was evidenced by UHPLC-TOF-
Two key steps of the proposed biosynthetic pathway can
be identified: (i) the aldolization to form the full open carbon
MS and 1H NMR analyses. The formation of the α,β-
unsaturated ketone 11 could readily originate from an
structure
3 and (ii) the cyclisation/reduction sequence by
intramolecular aldolization/crotonization of
8, promoted by
which enamine 5b is converted to Tramadol. In our second
approach, reported in this communication, we present
evidence that (ii) can occur under mild biomimetic conditions.
It should be noted that the cyclisation reaction could be
enzymatically-catalyzed or not, as found in other alkaloid
pathways.14
the basic dimethylamine. The aminoketone 12 could be
formed via an intramolecular Mannich reaction involving the
enolization ability of the ketone function of intermediate 5a
Finally, the aminol 13 is probably formed from the double
reduction of 5b
.
.
Herein, we focused our efforts on the synthesis of the
The formation of Tramadol proceeds via the iminium 5a
and subsequent isomerization to enamine 5b. An
intramolecular addition of the enamine onto the carbonyl
according to a 6-enolexo Hajos-Parrish-Eder-Sauer-Wiechert15
mechanism will afford an iminium intermediate, which upon
iminium 5a and its conversion via enamine 5b to Tramadol.
Iminium 5a could be obtained from dicarbonyl 8. This inter-
mediate was prepared through a series of four straightforward
steps (Scheme 2). Starting from 3-bromomethoxybenzene and
cycloheptanone in presence of n-BuLi, a lithium-halogen
reduction provides Tramadol 9 and its diastereoisomer 10.
Tramadol was then purified by semi-preparative HPLC,
2 | J. Name., 2012, 00, 1-3
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(g) W.-M. Chou, T. M. Kutchan, Plant J., 1998, 15, 289; (h) D.
K. Liscombe, G. V. Louie, J. P. Noel, Nat. Prod. Rep., 2012, 29
scaling-up analytical HPLC conditions using a gradient transfer
method.16 NMR analysis revealed a mixture of (±)-(1R,2R)-2-
,
1238; (i) C. Lapadatescu, C. Giniès, J. L. Quéré, P. Bonnarme,
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[(dimethylamino)methyl]-1-(3-methoxyphenyl)cyclohexanol
9
and (±)-(1R,2S)-2-[(dimethylamino)methyl]-1-(3-methoxy-phe-
nyl)-cyclohexanol 10 in a 7/3 ratio. The spectroscopic data of
an authentic synthetic sample of Tramadol were identical in all
8
9
K. M. Romek, P. Nun, G. S. Remaud, V. Silvestre, G. Sotoing
Taïwe, F. Lecerf-Schmidt, A. Boumendjel, M. De Waard, R. J.
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aspects with those of Tramadol
route. The observed diastereoselectivity in favor of the (±)-
(1R,2R)-isomer ( ) may be due to steric hindrance caused by
9 obtained by the biomimetic
9
the methoxyphenyl moiety, differentiating the face by which
the approaching nucleophile can access the diastereotopic
faces of the carbonyl group.
10 F. Hotellier, P. Delaveau, Phytochemistry, 1975, 14, 1407.
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Thus, under mild chemical conditions, we have demons-
trated that a key intermediate of Tramadol biosynthesis 5b can
be formed in situ and can be converted to Tramadol without
enzymatic catalysis. The involvement of a non-enzymatic ring
closure readily explains the occurrence of a natural racemate,
formed as a result of either re- or si- attack of the electron on
the C1 position. The finding that the formation of the (±)-
(1R,2S)-isomers is less favored and that these do not occur in
the N. latifolia extract, adds weight to the natural origin of this
compound.
12 A. A. Agomuoh, A. Ata, C. C. Udenigwe, R. E. Aluko, I. Irenus,
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As with the recent report regarding the position specific
isotope analysis studies8 this work gives further guidelines as
to how to conduct investigations of the biosynthesis of this
unusual compound by using labeling techniques.
M.L.B. and T.L. thank ISPB of Lyon for financial support.
A.B. and M.D.W. thank the University Grenoble Alpes for
financial support. A.B., B.P., F.L.-S. and R.H. are grateful to ANR
(Agence Nationale pour la Recherche) for financial support
(Labex Arcane (ANR-11-LABX-0003-01)). R.J.R. thanks the CNRS
for financial support. Authors thank Pr. J. Lebreton, Pr. J. Ver-
cauteren and Dr. G. Massiot for helpful discussions.
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This journal is © The Royal Society of Chemistry 20xx
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