Seven-Step Continuous Flow Synthesis of Linezolid Without Intermediate Purification

Article abstract of DOI:10.1002/anie.201901814

Herein, the blockbuster antibacterial drug linezolid is synthesized from simple starting blocks by a convergent continuous flow sequence involving seven (7) chemical transformations. This is the highest total number of distinct reaction steps ever performed in continuous flow without conducting solvent exchanges or intermediate purification. Linezolid was obtained in 73 % isolated yield in a total residence time of 27 minutes, corresponding to a throughput of 816 mg h^?1.

Full text of DOI:10.1002/anie.201901814

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
www.angewandte.org
Accepted Article
Title: Seven-Step Continuous Flow Synthesis of Linezolid Without
Intermediate Purification
Authors: M. Grace Russell and Timothy F Jamison
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901814
Angew. Chem. 10.1002/ange.201901814
Link to VoR: http://dx.doi.org/10.1002/anie.201901814
http://dx.doi.org/10.1002/ange.201901814

10.1002/anie.201901814
Angewandte Chemie International Edition
COMMUNICATION
Seven-Step Continuous Flow Synthesis of Linezolid Without
Intermediate Purification
M. Grace Russell and Timothy F. Jamison*[a]
Abstract: Herein the blockbuster antibacterial drug linezolid is
synthesized from simple starting blocks via a convergent continuous
flow sequence involving seven (7) chemical transformations. This is
the highest total number of distinct reaction steps ever performed in
continuous flow without conducting solvent exchanges or
intermediate purification. Linezolid was obtained in 73% isolated yield
in a total residence time of 27 minutes, corresponding to a throughput
of 816 mg h-1.
regioselective ring-opening reaction with aniline 3. Following late-
stage oxazolidinone formation of alcohol 2, linezolid (1) would be
obtained. Although 4 has previously been proposed as an
intermediate in the synthesis of linezolid,[14c] its synthetic utility
was limited due to competitive intramolecular cyclization leading
to oxazoline formation. We postulated that the judicious choice of
reagents in tandem with the precise control of reaction conditions
enabled by a continuous flow strategy such as temperature,
stoichiometry, and reaction time, would allow us to successfully
engage 4. In this manner, an efficient synthesis of 1 could be
achieved.
The adoption of continuous flow technology[1] for the
synthesis of natural products,[2] active pharmaceutical ingredients
(APIs),[3] functional materials[4] and other molecules of interest[5]
has been largely motivated by its key advantages[6] relative to
batch methodologies. Among these advantages are the ability to
access underutilized chemical transformations,[7] improve the
selectivity of target molecules through greater control of reaction
conditions,[8] and achieve faster scale-up and greater synthetic
efficiency.[9] While a continuous flow strategy offers many benefits,
it is interesting to note that a majority of reported syntheses do not
exceed two steps and those that do, typically require off-line
intermediate purification. To illustrate, we performed a literature
survey by searching the key words ‘continuous flow’ in SciFinder®.
During the period of January 2018 to October 2018 we located
190 examples of fully-continuous flow syntheses and 96% of
these were one or two steps. Often, the salient challenges in
demonstrating longer continuous reaction sequences (≥ 3 steps)
are the proper management of solvent compatibilities as well as
byproduct and side-product formation. Such challenges have
been overcome in several elegant four-[10] and five-step[11]
continuous flow syntheses, and in the six-step continuous flow
synthesis of ciprofloxacin we recently reported.[12] Here we further
demonstrate the capabilities of flow through a seven step
continuous flow synthesis of linezolid (1) (Figure 1). To the best
of our knowledge, this is the highest total number of chemical
transformations performed in any continuous flow synthesis
reported to-date that is void of solvent exchanges or intermediate
purification.
We began our investigation into the synthesis of linezolid (1)
with the preparation of amide 7 via a Ritter-type reaction between
(+)-epichlorohydrin (8) and acetonitrile (Figure 2). Inspired by
previous reports,[15] we employed BF3·OEt2 as a stoichiometric
Lewis acid and allowed the reaction to proceed for a five minute
residence time at room temperature. Upon performing an
aqueous NaHCO3 quench and purification under batch conditions,
amide 7 was obtained in an average of 62% yield. Although
competitive oxazoline 9 formation was largely suppressed under
these conditions, appreciable amounts of 10 were obtained (ca.
15%), suggesting the participation of diethyl ether from the Lewis
acid as a nucleophile.[16] As an effort to eliminate this side-product
formation, we investigated the use of different boron trifluoride
etherate complexes with increased steric effects. We were
.
pleased to find that using BF3 OBu2 (1.1 equiv.) significantly
improved the isolated yield of 7 to 90% under otherwise identical
conditions (Figure 2).
Having realized reaction conditions to access 7, we
attempted to integrate the aqueous quench into the continuous
flow sequence via the use of a T-mixer, but observed rapid
clogging of the reactor tubing due to the precipitation of the boric
acid. As shown in Figure 3, this was remedied by quenching the
Lewis acid with 2-propanol at -35 °C for 1.2 minutes to generate
the soluble boronic ester by-product (Reactor II, Zone 1). Under
these conditions, intermediate nitrillium ion 11 also reacted with
2-propanol to provide imidate 12 as confirmed by 1H-NMR studies.
Epoxide 13 was then formed by treating the stream of 12 with
lithium tert-butoxide in a 1:1 mixture of THF:1,2-dichloroethane at
-35 °C for a residence time of 4 minutes (Reactor III, Zone 1).[14c]
The 1,2-dichloroethane solvent solubilized the lithium chloride
salts generated during the course of the reaction and was
compatible with the use of the cold reaction temperature (melting
point -35 °C). Overall, epoxide 13 featuring a masked amine, was
formed within a residence time of 10.2 minutes starting from (+)-
epichlorohydrin (8). It is important to note that variations in the
reaction conditions within Reactors II and III, namely the
equivalents of 2-propanol and lithium tert-butoxide respectively,
as well as variations in the cold bath temperatures and residence
times, led to significant decreases in the yield of amino alcohol 2
(Reactor VI, Zone 3) as monitored by HPLC versus an internal
standard. Additionally, by masking the reactive amide group of 12
as an imidate, side-product formation was minimized in the
epoxide-forming reaction performed in Reactor III.[14c]
Linezolid (1) (trade name Zyvox®, Pfizer) is an antibiotic that
is on the World Health Organization’s list of essential medicines
(Figure 1).[13] This API is primarily used as a last line of defense
treatment against multi-drug resistant gram-positive bacteria.
While the reported synthetic routes to linezolid are lengthy (> 7
steps) or rely on protecting group manipulations,[14] we envisioned
a convergent, protecting group-free synthesis that is void of
purification steps between any of the transformations. As shown
in Figure 1, our approach was to incorporate epoxide 4 via a
[a]
M. G. Russell, Prof. Dr. T. F. Jamison
Department of Chemistry
Massachusetts Institute of Technology
77 Massachusetts Ave., Cambridge, MA 02139 (USA)
E-mail: tfj@mit.edu
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201901814
Angewandte Chemie International Edition
COMMUNICATION
Figure 1. Synthesis planning for linezolid (1). In the synthetic strategy proposed for 1, epoxide 4 was to be accessed via a Ritter-type reaction of (+)-epichlorohydrin
(8) followed by ring closure of amide 7. Synthesis and coupling of aniline 3 with 4 followed by oxazolidinone formation would generate the target API.
Figure 2. Optimization of the continuous flow synthesis of amide 7 from (+)-epichlorohydrin (8). The yield of amide 7 significantly improved in the presence of
.
.
excess BF3 OBu2 relative to BF3 OEt2. tR, residence time.
With access to epoxide 13, focus was next directed to the
preparation of aniline 3 via a nucleophilic aromatic substitution
(SNAr) reaction (Reactor IV)[17] followed by hydrogenation
(Reactor V) as shown in Zone 2 of Figure 3. We found that the
resulting scrubbed reaction stream containing 3 was immediately
reintroduced to the system with an additional pump and reacted
with epoxide 13 (Figure 3, Zone 3).[19] Compound 2 was prepared
in 12 minutes at 40 °C in Reactor VI without any additional
activating agent. We hypothesize the imidate is hydrolyzed to the
corresponding amide in this step due to water produced from the
hydrogenation process in Zone 2. Attempts to increase
temperature and decrease the residence time resulted in
diminished conversion. It should also be noted that amino alcohol
2 and aniline 3 could not be isolated from the crude reaction
mixtures due to rapid oxidation. By utilizing continuous flow
technology and designing the synthesis in manner that enabled it
to be fully continuous, deleterious oxidation was minimized
through the rapid consumption of these intermediates.
SNAr
reaction
between
morpholine
5
and
3,4-
dinitrofluoronitrobenzene (6) to form 14 required 10 minutes at
150 °C to proceed to completion in a mixed solvent system
comprised of 1,4-dioxane and N,N-dimethylformamide. Next, the
resulting crude reaction mixture was reduced by introducing
hydrogen gas via a mass flow controller (hydrogen gas: liquid flow
rate, 15:1) and passing through a compact stainless steel packed
bed of palladium(0) at 100 °C and 100 psi of back-pressure.[18]
After a residence time of only 0.33 minutes, intermediate 14
underwent complete conversion to aniline
3
and no
dehalogenation was observed. The 1,4-dioxane-N,N-
The final oxazolidinone ring forming reaction to generate
linezolid (1) was performed in Reactor VII by treating the stream
of 2 to a solution of N,N-carbonyldiimidazole in dioxane at 150 °C
for five minutes of residence time (Figure 3, Zone 3). Other
coupling agents such as phosgene derivatives did not provide 1
presumably due to reaction with nucleophilic components in the
crude mixture such as 2-propanol. After the addition of HCl and
dimethylformamide solvent system chosen for this telescoped
sequence enhanced the rate of the SNAr, solubilized all starting
materials and byproducts, and ensured good compatibility with
the palladium packed bed.
Following hydrogenation, the excess hydrogen gas was
removed through the use of gravity in a sealed container and the
This article is protected by copyright. All rights reserved.

10.1002/anie.201901814
Angewandte Chemie International Edition
COMMUNICATION
in-line separation, the product was collected and purified off-line.
Overall, linezolid (1) was synthesized in 73% isolated yield,
corresponding to a throughput of 816 mg h-1. HPLC analysis
confirmed that the enantio-enrichment of the product was
conserved throughout the entire sequence.
In summary, we have developed an efficient synthesis of
linezolid (1). The total residence time for the seven-step sequence
is 27 minutes, which is significantly shorter than reported batch
procedures (> 60 hours).[14c] The E-factor calculated for this
process is 25 (an average of 3.57 per chemical transformation).
For comparison, industrial pharmaceutical production processes
typically attain E-factors between 25-100.[20] Notably, to the best
of our knowledge, we achieve the highest number of chemical
transformations in a fully-continuous manner without solvent
exchanges, or interruptions for intermediate work-ups or
purifications. These achievements were possible through
thoughtful planning of the reaction sequence in a manner that
benefitted from the attributes of flow as well as by choosing
reagents and solvents that had downstream compatibility and led
to minimal byproduct and side-product formation.
Figure 3. Seven-step, continuous flow synthesis of linezolid (1). The continuous flow process is visualized as three main zones. Zone 1 generates epoxide 13 via
three chemical transformations starting from 8. Aniline 3 is simultaneously generated in Zone 2 from 5 and 6 via two chemical reactions. Finally, the synthesis
converges in Zone 3 with the coupling of 3 and 13, followed by oxazolidinone formation. DCE, 1,2-dichloroethane; DMF, N,N-dimethylformamide; THF,
tetrahydrofuran; tR, residence time; I.D., inner diameter; O.D., outer diameter.
This work was supported by the Defense Advanced Research
Acknowledgements
Project Agency (DARPA) (N66001-16-C-4005). We are grateful
This article is protected by copyright. All rights reserved.

10.1002/anie.201901814
Angewandte Chemie International Edition
COMMUNICATION
[13] World Health Organisation. WHO Model List of Essential Medicines. 20th
List (2017).
to Dr. Rachel L. Beingessner and Dr. Anne-Catherine Bédard for
their advice and support.
[14] a) S. J. Brickner, et al. J. Med. Chem. 1996, 39, 673–9; b) S.D. Ramgren,
A. L. Silberstein, Y. Yang, N. K. Garg, Angew. Chem. Int. Ed. 2011, 50,
2171 –2173; Angew. Chem. 2011, 123, 2219–2221; c) W. R. Perrault, et
al. Org. Process Res. Dev. 2003, 7, 533–546; d) W. R. Perrault, et al.
Scalable Green Chemistry - Case Studies from the Pharmaceutical
Industry. Pan Stanford Publishing, 2013, pp. 157–166.
Keywords: Antibiotics • Continuous Flow • Reactive
intermediates • Total synthesis • Waste prevention
[1]
[2]
[3]
M. B. Plutschack, B. Pieber, K. Gilmore, P. H. Seeberger, Chem. Rev.
2017, 117, 11796-11893.
[15] a) J. M. Concellón, J. R. Suárez, V. Del Solar, J. Org. Chem. 2005, 70,
7447–7450; b) J. C. Brandt, S. C. Elmore, R. I. Robinson T. Wirth, Synlett.
2010, 20, 3099-3103; c) L. Audiger, K. Watts, S. C. Elmore, R. I.
Robinson, T. Wirth, ChemSusChem. 2012, 5, 257-260; d) C. Battilocchio,
I. R. Baxendale, M. Biava, M. O. Kitching, S. V. Ley, Org. Process Res.
Dev. 2012, 16, 798-810.
J. C. Pastre, D. L. Browne, S. V. Ley, Chem. Soc. Rev. 2013, 42, 8801-
9198.
a) M. Baumann, I. R. Baxendale, Beilstein J. Org. Chem. 2015, 11, 1194-
1219; b) R. O. M. A. De Souza, P. Watts, J. Flow. Chem. 2017, 7, 146-
150; c) R. Porta, M. Benaglia, A. Puglisi, Org. Process Res. Dev. 2016,
20, 2–25.
[16] J. L. Boyer, J. E. Krum, M. C. Myers, A. N. Fazal, C. T. Wigal, J. Org.
Chem. 2000, 65, 4712-4714.
[4]
[5]
R. M. Myers, D. E. Fitzpatrick, R. M. Turner, S. V. Ley, Chem. Eur. J.
2014, 20, 12348-12366.
[17] a) E. Comer, M. G. Organ, J. Am. Chem. Soc. 2005, 127, 8160-8169; b)
A.-C. Bédard et al. Science, 2018, 361, 1220-1225; c) K. D. Nagy, B.
Shen, T. F. Jamison, K. F. Jensen, Org. Process Res. Dev. 2012, 16,
a) M. Movsisyan, E. I. Delbeke, J. K. Berton, C. Battilocchio, S. V. Ley,
C. V. Stevens, Chem. Soc. Rev. 2016, 45, 4892-4928; b) J. Wegner, S.
Ceylan, A. Kirschning, Adv. Synth. Catal. 2012, 354, 17-57.
J. A. M. Lummiss, P.D. Morse, R. L. Beingessner, T. F. Jamison, Chem.
Rec. 2017, 17, 667-680.
976-981;
d)
A.
G.
O'Brien, et al., Angew.
Chem.
Int.
Ed. 2012, 51, 7028– 7030; Angew. Chem. 2012, 124, 7134– 7137; e) C.
A. Hone, N. Holmes, G. A. Akien, R. A. Bourne, F. L. Muller, React. Chem.
Eng., 2017, 2, 103-108.
[6]
[7]
D. Cambié, C. Bottecchia, N. J. W. Straathof, V. Hessel, T. Noël, Chem
Rev. 2016, 116, 10276-10341.
[18] a) A. K. Rathi et al. Catal. Sci. Technol. 2016, 6, 152-160; b) J. D. Lewis,
S. Van de Vyver, A. J. Crisci, W. R. Gunther, V. K. Michaelis, R. G. Griffin
and Y. Román-Leshkov, ChemSusChem, 2014, 7, 2255–2265.
[19] a) M. W. Bedore, N. Zaborenko, K. F. Jensen, T. J. Jamison, Org.
Process Res. Dev. 2010, 14, 432– 440; b) N. Zaborenko, M. W. Bedore,
T. F. Jamison, K. F. Jensen, Org. Process Res. Dev. 2011, 15, 131– 139.
[20] R. A. Sheldon, Green Chemistry. 2007, 9, 1261-1384.
[8]
[9]
V. Hessel, Chem. Eng. Technol. 2009, 32, 1655–1681.
P. D. Morse, R. L. Beingessner, T. F. Jamison, Isr. J. Chem. 2017, 57,
218-227.
[10] a) S. Ogasawara, Y. Hayashi, Synthesis, 2017, 49, 424-428; b) P. R. D.
Murray, D. L. Browne, J. C. Pastre, C. Butters, D. Guthrie, S. V. Ley, Org.
Process Res. Dev. 2013, 17, 1192 –1208; c) C. E. M. Salvador, B. Pieber,
P. M. Neu, A. Torvisco, C. Kleber, Z. Andrade, C. O. Kappe , J. Org.
Chem., 2015, 80, 4590 -4602.
[11] D. Ghislieri, K. Gilmore, P. H. Seeberger, Angew. Chem. Int. Ed. 2015,
54, 678 –682; Angew. Chem. 2015, 127, 688 –692.
[12] H. Lin, C. Dai, T. F. Jamison, K. F. Jensen, Angew. Chem. Int. Ed. 2017,
56, 8870-8873; Angew. Chem. 2017, 129, 8996-8999.
This article is protected by copyright. All rights reserved.

10.1002/anie.201901814
Angewandte Chemie International Edition
COMMUNICATION
COMMUNICATION
Herein the blockbuster antibacterial
drug linezolid is synthesized in 73%
yield from simple starting blocks via a
convergent continuous flow sequence
involving seven (7) chemical
M. Grace Russell and Timothy F.
Jamison*
Page No. – Page No.
Seven-Step Continuous Flow
Synthesis of Linezolid Without
Intermediate Purification
transformations.
This article is protected by copyright. All rights reserved.