Symmetry-based design for the chemoenzymatic synthesis of oseltamivir (Tamiflu) from ethyl benzoate

Full text of DOI:10.1002/anie.200901345

Angewandte
Chemie
DOI: 10.1002/anie.200901345
Tamiflu Synthesis
Symmetry-Based Design for the Chemoenzymatic Synthesis of
Oseltamivir (Tamiflu) from Ethyl Benzoate**
Bradford Sullivan, Ignacio Carrera, Melissa Drouin, and Tomas Hudlicky*
The possibility of a major influenza pandemic (especially the
avian H5N1 influenza) continues to be a serious health
concern. The development of effective antiviral medicines is
hampered by the exceptionally high mutation rates of the
influenza virus and, for the research effort to be successful,
any new drugs must target the molecular mechanisms specific
to the proliferation of the virus. The mechanism of infection
involves the protein neuraminidase (NA), which is essential
to viral replication. It is responsible for the glycosidic cleavage
of sialic acid from a glycoprotein of a host cell in a process that
2, derived from ethyl benzoate, already contains this func-
tionality and offers several other advantages in the flexibility
of the approach.
[6]
Oseltamivir contains a latent symmetry axis through C1
and C4, a feature that can be exploited in the design of
flexible synthetic routes starting from either of the two
diastereomeric aziridines 3 or 4, respectively. The concept of
latent symmetry has been applied to the enantiodivergent
[
6a]
[7]
[8]
syntheses of pinitol, pancratistatin, carbohydrates, and
[
9]
trihydroxyheliotridanes. Its application in the synthesis of
oseltamivir is shown in Scheme 1.
[
1]
liberates the virion from the infected cell. Of the compounds
that have been found to be effective as inhibitors of NA, by
mimicking the oxonium intermediate of sialic acid glycolysis,
oseltamivir (1), as its phosphate, or Tamiflu appears to be
superior. It is orally active and serves as a prodrug, the active
form of which is the corresponding carboxylic acid. It also has
The two representations of oseltamivir shown are identi-
cal structures; however, if the substituents at C3, C4, and C5
are not specifically defined then the configurations of these
three carbon atoms represent an “enantiomeric switch” which
would be controlled by the translocation of the double bond.
We can exploit this latent symmetry by designing an approach
in which the order and the site of introduction of either the
nitrogen or oxygen atom can be interchanged. In approach A
[2]
a superior bioavailability and is active at nanomolar levels.
Although oseltamivir is not a particularly complex
molecule, its practical synthesis on a scale large enough to
guard against an influenza pandemic presents a formidable
challenge. To date many ingenious syntheses have been
[
3]
developed and published, several of which have the
potential for optimization on a scale large enough to meet
[
4]
the requirements of a commercial synthesis. Herein we
report a flexible symmetry-based synthesis of oseltamivir,
starting from a cis-dihydrodiol which is derived by enzymatic
dihydroxylation of ethyl benzoate.
Three chemoenzymatic approaches to oseltamivir, start-
ing from the cis-dihydrodiol derived from bromobenzene,
[
5]
[3p]
were reported in 2008: those of Hudlicky, Fang,
Banwell.
and
[
3o]
Although the cis diol derived from bromoben-
zene is a commercially available material, any synthesis
originating from this compound will require a palladium-
catalyzed carbonylation step to introduce the ester function-
ality, usually at an advanced stage of the synthesis. The cis-diol
[
*] B. Sullivan, M. Drouin, Dr. T. Hudlicky
Department of Chemistry and Centre for Biotechnology
Brock University, 500 Glenridge Avenue
St. Catharines, Ontario, L2S 3A1 (Canada)
Fax: (+1)905-984-4841
E-mail: thudlicky@brocku.ca
I. Carrera
Facultad de Quꢀmica, Universidad de la Repfflblica
Av. Gral. Flores 2124, Montevideo 11800 (Uruguay)
[
**] The authors are grateful to the following agencies for financial
support of this work: National Sciences and Engineering Research
Council of Canada (Idea to Innovation and Discovery Grants),
Organization for the Prohibition of Chemical Weapons for a
research fellowship to I.C., TDC Research, Inc., Brock University,
and the Ontario Partnership for Innovation and Commercialization.
Scheme 1. Symmetry-based design for oseltamivir from cis-dihydrodiol
via diastereomeric vinylaziridines.
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the ether functionality is introduced to the activated allylic
outlined in Scheme 2, begins with the whole-cell fermentation
position in 3, with subsequent inversion at C5 with a nitrogen
nucleophile, and removal of the C6 hydroxy group by
reduction. In this approach the acrylate double bond remains
in its original position. Conversely, the nitrogen functionality
is introduced in aziridine 4, a subsequent “symmetry switch”
of the double bond with concomitant elimination of the C6
hydroxy group, and final alkylation of the C5 hydroxy group.
Approach A has already been tested in our first-generation
attempt from the diol derived from bromobenzene.
Approach B relies on the synthesis of the N-acyl anti-aziridine
of ethyl benzoate with a recombinant strain Escherichia coli
[
10]
JM109(pDTG601A) to afford diol 2 in yields of approx-
À1
[11]
imately 1 gL
(15 l fermentor scale).
The diol was
protected as its acetonide 6 and then reacted without
purification with N-hydroxy acetamide in the presence of
sodium periodate to afford the bicyclic oxazine 7 via an
inverse electron-demand Diels–Alder cycloaddition with the
[
12]
acyl nitroso dienophile. Reduction of the NÀO bond with
[Mo(CO) ] furnished the allylic alcohol 8, which upon
6
exposure to methanesulfonyl chloride led directly to oxazo-
line 9.
4, which upon ring-opening would provide the allylic nitrogen
substituent and necessitate the translocation of the double
bond by hydrogenation and final base-catalyzed elimination
of the C6 hydroxy group. We have tested this approach by
preparing 4 (R = Ts), and we have converted this material
into an advanced oseltamivir intermediate wherein we
encountered problems with the removal of the N-tosyl
group. As the synthesis of 4 (R = Ac) was not trivial we
chose an alternate and more convenient route to 5 (the
putative product of opening of N-acetyl 4 with a nitrogen
nucleophile) to validate the symmetry-based synthetic route
shown in approach B. This strategy has an additional advant-
age in that the C6 hydroxy group need not be removed by
reduction, but rather by elimination upon the translocation of
the olefin and the creation of the allylic alcohol.
Treatment of the oxazoline with calcium carbonate in
refluxing aqueous ethanol furnished acetamide 10, which was
hydrogenated, without purification, to the saturated ester 11.
Conversion of the alcohol into mesylate 12, displacement with
azide to give 13, and treatment with DBU completed the
formal synthesis of oseltamivir via Fangꢀs intermediate 14,
which is attained in ten steps (seven operations) from ethyl
benzoate. The base-induced collapse of the acetonide gen-
erates the required allylic alcohol and provides for the
“symmetry switch” of the double bond.
In summary, a short formal synthesis of oseltamivir from
ethyl benzoate has been accomplished. It is comparable to
other published routes in terms of brevity and practicality, but
its symmetry-based design provides flexibility and more
options for choosing the order of the introduction of the
nitrogen and oxygen substituents onto the periphery of the
cyclic diene diol. Ethyl benzoate, a commodity chemical,
contains all of the carbon atoms of the product except for the
3-pentyl ether and is therefore a very convenient starting
The equivalent of the trans-substituted diamine derivative
of type 5 became available in a short sequence through an
inverse electron-demand hetero-Diels–Alder reaction of 2
with an acyl nitroso compound, subsequent reduction of the
oxazine, and additional functionalization. The synthesis,
Scheme 2. Synthesis. DMP=dimethoxypropane, Ts=4-tolylsulfonyl, Ms=methanelsulfonyl, DMAP=4-dimethylamino pyridine, DBU=1,8-
diazabicyclo[5.4.0]undec-7-ene.
4
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material for a potential large-scale synthesis of oseltamivir.
Our next effort will address some additional improvements in
the brevity, such as a direct introduction of Boc-protected
amine at the allylic position of 10 and conversion of this
material into 15 to intercept Coreyꢀs intermediate 16 or to
produce oseltamivir by alkylation of the allylic alcohol. A side
benefit of our investigation is also the potential for a new
route to protected derivatives of 1,2-amino alcohols from
conjugated dienes. We will report on additional ameliorations
of this route in due course.
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Published online: May 7, 2009
Keywords: cyclitols · dihydroxylation · Diels–Alder reaction ·
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