A multistep continuous-flow system for rapid on-demand synthesis of receptor ligands

Article abstract of DOI:10.1021/ol902101c

A multistep continuous-flow system for synthesis of receptor ligands by assembly of three variable building blocks in a single unbroken flow is described. The sequence consists of three reactions and two scavenger steps, where a Cbz-protected diamine is reacted with an isocyanate, deprotected, and reacted further with an alkylating agent.

Full text of DOI:10.1021/ol902101c

ORGANIC
LETTERS
2009
Vol. 11, No. 22
5134-5137
A Multistep Continuous-Flow System for
Rapid On-Demand Synthesis of
Receptor Ligands
Trine P. Petersen,†,‡ Andreas Ritze´n,‡ and Trond Ulven*,†
Department of Physics and Chemistry, UniVersity of Southern Denmark,
CampusVej 55, DK-5230 Odense M, Denmark, and Medicinal Chemistry Research DK,
H. Lundbeck A/S, OttiliaVej 9, DK-2500 Valby, Denmark
ulVen@ifk.sdu.dk
Received September 10, 2009
ABSTRACT
A multistep continuous-flow system for synthesis of receptor ligands by assembly of three variable building blocks in a single unbroken flow
is described. The sequence consists of three reactions and two scavenger steps, where a Cbz-protected diamine is reacted with an isocyanate,
deprotected, and reacted further with an alkylating agent.
Continuous-flow reactions, well established in the process
industry, have recently also received increased attention as
a tool for small-scale organic synthesis.1 Micro- and meso-
flow reactors in synthesis offer numerous potential advan-
tages over traditional batch synthesis. For example, (1)
mixing of reagents in a microflow system can be dramatically
more efficient than in a reaction flask, (2) a microflow system
offers precise heating and cooling of the reaction flow over
a very wide temperature and pressure span, (3) exothermic
and run-away reactions usually pose a negligible risk in a
flow system due to the small reactor volume, (4) reactions
and processes can be coupled in series and in parallel, (5)
flow systems are ideal for immobilized catalysts, (6) flow
chemistry is ideal for automation, and (7) once a flow system
is optimized for a reaction, the reaction is easily rerun with
the same or analogous reagents and can be scaled up within
reasonable limits by simply running the flow reaction for a
longer period of time. A host of diverse reactions have been
performed in single-step continuous-flow systems. In the
more advanced systems, of which there are relatively few
examples, several reactors or processes are coupled in series
to enable multistep synthesis in a single continuous flow.
† University of Southern Denmark.
‡ H. Lundbeck A/S.
(1) Reviews on flow chemistry: (a) Mason, B. P.; Price, K. E.;
Steinbacher, J. L.; Bogdan, A. R.; McQuade, D. T. Chem. ReV. 2007, 107,
2300–2318. (b) Yoshida, J.; Nagaki, A.; Yamada, T. Chem.sEur. J. 2008,
14, 7450–7459. (c) Kirschning, A. Beilstein J. Org. Chem. 2009, 5, 15.
and accompanying reviews. (d) Wiles, C.; Watts, P. Eur. J. Org. Chem.
2008, 1655–1671. (e) Ahmed-Omer, B.; Brandt, J. C.; Wirth, T. Org.
Biomol. Chem. 2007, 5, 733–740. (f) Kirschning, A.; Solodenko, W.;
Mennecke, K. Chem.sEur. J. 2006, 12, 5972–5990. (g) Jas, G.; Kirschning,
A. Chem.sEur. J. 2003, 9, 5708–5723. (h) Jahnisch, K.; Hessel, V.; Lowe,
H.; Baerns, M. Angew. Chem., Int. Ed. 2004, 43, 406–446. (i) Belder, D.
Angew. Chem., Int. Ed. 2009, 48, 3736–3737. (j) Geyer, K.; Codee, J. D.;
Seeberger, P. H. Chem.sEur. J. 2006, 12, 8434–8442. (k) Baxendale, I. R.;
Hayward, J. J.; Ley, S. V. Comb. Chem. High Throughput Screening 2007,
10, 802–836. (l) Fukuyama, T.; Rahman, M. T.; Sato, M.; Ryu, I. Synlett
2008, 2008, 151–163. (m) Benito-Lo´pez, F.; Egberink, R. J. M.; Reinhoudt,
D. N.; Verboom, W. Tetrahedron 2008, 64, 10023–10040. (n) Glasnov,
T. N.; Kappe, C. O. Macromol. Rapid Commun. 2007, 28, 395–410.
10.1021/ol902101c CCC: $40.75
Published on Web 10/23/2009
 2009 American Chemical Society

Most described systems of this type are designed to carry
out a single reaction that requires several operations or
perform several simple transformations on a single starting
material.2 Only a small handful of multistep systems have
been described which can assemble several building blocks
to more complex molecules.3,4 One outstanding example is
the synthesis of oxomaritidine from two simple starting
materials in six flow steps, where only one intermediate
product handling operation by the user was required in
connection with change of solvent.3a,5 Another recent
example describes the conversion of an acid chloride to an
isocyanate by a Curtius reaction, and subsequent reaction
with alcohols to produce carbamates in a single flow.3d This
system is relevant to our system, since implementation of a
Curtius step would allow acid chlorides rather than isocy-
anates as starting materials, and thereby expand the number
of easily accessible structures. A third notable flow system
performs two amide couplings, an asymmetric chlorination
and a nucleophilic substitution in a single continuous flow
carried out in a system of gravity columns.4a The scarcety
of reports of multistep continuous-flow syntheses is probably
related to the practical challenges that appear when such
systems are assembled, which include the frequent need for
different solvents in different reactions, accumulation of
byproduct and impurities which affect downstream reactions,
timing of reactant flows, build-up of high back-pressure over
several packed columns in series, precipitation and clogging
of tubes and columns, and chromatographic separation of
dissolved reagents in packed columns. Another reason may
be the high-tech flavor that the field has, with discussions
often focusing on lab chips and microstructured devices,
giving the impression that considerable investment is neces-
sary to enter the field.
Multistep continuous-flow systems appear to be particu-
larly suitable for medicinal chemistry, where a large number
of analogues in the optimization process can be synthesized
by the same reaction sequence consisting of a few relatively
simple steps, like amide couplings, reductive aminations,
catalytic hydrogenations, and metal-catalyzed coupling reac-
tions.6 Previous flow systems which assemble three building
blocks have been described, but these have only been
exemplified by the synthesis of a single compound, and the
building blocks are in all cases assembled by amide bond
formations.4 Here, we describe the construction of a continu-
ous-flow sequence designed for rapid synthesis of new
ligands in medicinal chemistry by combination of three
building blocks, and we demonstrate its potential by synthesis
of a library of diverse drug-like test compounds. Targeting
the chemokine receptor CCR8, which is of interest for
treatment of various inflammatory and allergic conditions,7
our goal was to construct a system for on-demand production
of compounds with pharmacophores corresponding to known
CCR8 ligands (Figure 1). Three reactions were selected to
Figure 1.
Inspirational chemokine receptor CCR8 ligands.8
accomplish this: the reaction of an amine with an isocyanate,
a Cbz-deprotection, and alkylation of a secondary amine.
A stock solution stream of a Cbz-protected diamine is
mixed with an isocyanate solution stream in a T-piece, and
(2) For example: (a) Lee, C. C.; Sui, G.; Elizarov, A.; Shu, C. J.; Shin,
Y. S.; Dooley, A. N.; Huang, J.; Daridon, A.; Wyatt, P.; Stout, D.; Kolb,
H. C.; Witte, O. N.; Satyamurthy, N.; Heath, J. R.; Phelps, M. E.; Quake,
S. R.; Tseng, H. R. Science 2005, 310, 1793–1796. (b) Baumann, M.;
Baxendale, I. R.; Ley, S. V. Synlett 2008, 2111–2114. (c) Gustafsson, T.;
Gilmour, R.; Seeberger, P. H. Chem. Commun. 2008, 3022–3024. (d) van
der Linden, J. J. M.; Hilberink, P. W.; Kronenburg, C. M. P.; Kemperman,
G. J. Org. Process Res. DeV. 2008, 12, 911–920. (e) Fukuyama, T.;
Kobayashi, M.; Rahman, M. T.; Kamata, N.; Ryu, I. Org. Lett. 2008, 10,
533–536. (f) Nagaki, A.; Kim, H.; Yoshida, J. Angew. Chem., Int. Ed. 2008,
47, 7833–7836. (g) Usutani, H.; Tomida, Y.; Nagaki, A.; Okamoto, H.;
Nokami, T.; Yoshida, J. J. Am. Chem. Soc. 2007, 129, 3046–3047.
(3) Multistep flow systems combining two building blocks: (a) Baxen-
dale, I. R.; Deeley, J.; Griffiths-Jones, C. M.; Ley, S. V.; Saaby, S.; Tranmer,
G. K. Chem. Commun. 2006, 2566–2568. (b) Baxendale, I. R.; Griffiths-
Jones, C. M.; Ley, S. V.; Tranmer, G. K. Synlett 2006, 427–430. (c)
Baxendale, I. R.; Ley, S. V.; Mansfield, A. C.; Smith, C. D. Angew. Chem.,
Int. Ed. 2009, 48, 4017–4021. (d) Sahoo, H. R.; Kralj, J. G.; Jensen, K. F.
Angew. Chem., Int. Ed. 2007, 46, 5704–5708. (e) Baumann, M.; Baxendale,
I. R.; Ley, S. V.; Nikbin, N.; Smith, C. D.; Tierney, J. P. Org. Biomol.
Chem. 2008, 6, 1577–1586. (f) Baumann, M.; Baxendale, I. R.; Ley, S. V.;
Nikbin, N.; Smith, C. D. Org. Biomol. Chem. 2008, 6, 1587–1593. (g) Grant,
D.; Dahl, R.; Cosford, N. D. J. Org. Chem. 2008, 73, 7219–7223. (h)
Baxendale, I. R.; Ley, S. V.; Smith, C. D.; Tamborini, L.; Voica, A. F.
J. Comb. Chem. 2008, 10, 851–857. (i) Gustafsson, T.; Ponten, F.; Seeberger,
P. H. Chem. Commun. 2008, 1100–1102. (j) Griffiths-Jones, C. M.; Hopkin,
M. D.; Jonsson, D.; Ley, S. V.; Tapolczay, D. J.; Vickerstaffe, E.; Ladlow,
M. J. Comb. Chem. 2007, 9, 422–430. (k) Hafez, A. M.; Taggi, A. E.;
(6) For examples of flow systems with medicinal chemistry applications,
see: (a) Hamper, B. C.; Tesfu, E. Synlett 2007, 2257–2261. (b) Bogdan,
A. R.; Sach, N. W. AdV. Synth. Catal. 2009, 351, 849–854.
(7) (a) Pease, J. E.; Horuk, R. Expert Opin. Ther. Pat. 2009, 19, 199–
221. (b) Mitsuyama, E.; Kunori, Y.; Kamimura, T.; Kaminuma, O. Mini-
ReV. Med. Chem. 2006, 6, 463–466.
(8) (a) Haskell, C. A.; Horuk, R.; Liang, M.; Rosser, M.; Dunning, L.;
Islam, I.; Kremer, L.; Gutierrez, J.; Marquez, G.; Martinez, A. C.; Biscone,
M. J.; Doms, R. W.; Ribeiro, S. Mol. Pharmacol. 2006, 69, 309–316. (b)
Jensen, P. C.; Nygaard, R.; Thiele, S.; Elder, A.; Zhu, G.; Kolbeck, R.;
Ghosh, S.; Schwartz, T. W.; Rosenkilde, M. M. Mol. Pharmacol. 2007,
72, 327–340. (c) Ghosh, S.; Patane, M. A.; Carson, K. G.; Chi, I. C. S.;
Ye, Q.; Elder, A. M.; Jenkins, T. J. Patent WO2003037271 , 2003.
(9) (a) Jones, R. V.; Godorhazy, L.; Varga, N.; Szalay, D.; Urge, L.;
Darvas, F. J. Comb. Chem. 2006, 8, 110–116. (b) Franckevicius, V.;
Knudsen, K. R.; Ladlow, M.; Longbottom, D. A.; Ley, S. V. Synlett 2006,
889–892.
(10) (a) Brinkman, J. A.; Cheung, A. W.-H.; Firooznia, F.; Guertin,
K. R.; Marcopulos, N.; Qi, L.; Racha, J. K.; Sarabu, R.; Tan, J.; Tilley,
J. W. Patent WO2007134958, 2007. (b) Chen, M.; Claret, E.; Cleve, A.;
Davey, D.; Guilford, W.; Khim, S.-K.; Kirkland, T.; Kochanny, M. J.; Liang,
A.; Light, D.; Parkinson, J.; Vogel, D.; Wei, G. P.; Ye, B.; Ye, H. US
patent US2007155727, 2007. (c) Jikyo, T.; Shudo, K. Patent WO2006028161
, 2006. (d) Campbell, M.; Mason, A. M.; Pinto, I. L.; Pollard, D. R.; Smith,
I. E. D. Patent WO2006085108, 2006. (e) Tassoni, E.; Giannessi, F.; Dell’
Uomo, N.; Gallo, G.; Conti, R.; Tinti, M. O. Patent WO2007096251, 2007.
(f) Ahn, K.; Johnson, D. S.; Fitzgerald, L. R.; Liimatta, M.; Arendse, A.;
Stevenson, T.; Lund, E. T.; Nugent, R. A.; Nomanbhoy, T. K.; Alexander,
J. P.; Cravatt, B. F. Biochemistry 2007, 46, 13019–13030.
Dudding, T.; Lectka, T. J. Am. Chem. Soc. 2001, 123, 10853–10859
.
(4) Multistep flow systems combining three building blocks: (a) France,
S.; Bernstein, D.; Weatherwax, A.; Lectka, T. Org. Lett. 2005, 7, 3009–
3012. (b) Watts, P.; Wiles, C.; Haswell, S. J.; Pombo-Villar, E. Tetrahedron
2002, 58, 5427–5439. (c) Baxendale, I. R.; Ley, S. V.; Smith, C. D.;
Tranmer, G. K. Chem. Commun. 2006, 4835–4837
.
(5) Kundig, P. Science 2006, 314, 430–431
Org. Lett., Vol. 11, No. 22, 2009
.
5135

Table 1. Three-Step Continuous-Flow Synthesizer
a Method A: Stock solutions of Cbz-diamine and alkylating agent in ethanol. Method B: All stock solutions in DMF. b Yields are after purification by
semiautomatic flash chromatography (Combiflash Companion).
5136
Org. Lett., Vol. 11, No. 22, 2009

the mixture stream is heated to 75 °C (Table 1). Excess
isocyanate is scavenged in a column with trisamine on
macroporous polystyrene (MP-trisamine). The flow continues
into an H-Cube hydrogenator, where the Cbz-group is
removed by hydrogenation over palladium.9 The byproducts
from this reaction, toluene and carbon dioxide, cause no
problems in the downstream processes. In the final step the
secondary amine is alkylated by a benzyl bromide or another
appropriate alkylating agent in the presence of polystyrene-
bound N-methylmorpholine. Excess alkylating agent is
removed by leading the flow through a final column with
MP-trisamine at 75 °C. The outlet flow is concentrated, and
the residue is purified semiautomatically in a Combiflash
system. To the best of our knowledge, this represents the
first examples of tertiary amine synthesis in flow by
N-alkylation with alkyl halides, as well as the first synthesis
of ureas from isocyanates in flow, although examples related
to the latter reaction exist.3d,e
The H-Cube hydrogenation is known to work well with
ethanol, and the system was initially fed with stock solutions
of Cbz-diamines and benzyl bromides in this solvent. The
flow system was able to produce compounds of good quality
in useful amounts, but yields were generally low (Table 1,
entries 1-6), partly ascribed to the competing reaction
between isocyanates and ethanol. The solvent was therefore
changed to DMF in all stock solutions (Table 1, entries
6-15). Fortunately, this worked well with the H-Cube, and
the yields generally improved to on average 50-60% over
the complete sequence and up to 96% in one example (entry
12). The solvent change also brought other advantages, since
DMF is a powerful solvent and eluent for most organic
compounds, minimizing problems related to precipitation and
clogging of the system and retention on packed columns.
The residence time in the optimized system is approximately
45 min. With synthesis proceeding for 10-15 min followed
by flushing for 45 min, the system can deliver approximately
one crude product per hour.
isocyanates and alkylating agents demonstrate that the system
is potentially useful for synthesis of highly diverse com-
pounds. The system also has potential applications beyond
targeting the CCR8 receptor. For example, ligands for a large
number of diverse receptors, enzymes, and ion channels have
the N-benzyl-(homo)piperazine-urea scaffold, including ad-
enosine A2B antagonists,10a leukotriene A4 hydrolase
inhibitors,10b serotonin 5-HT3 antagonists,10c nicotinic acid
HM74A agonists,10d carnitine palmitoyl transferase
inhibitors,10e and fatty acid amide hydrolase inhibitors,10f
suggesting that our multistep system can be useful for
synthesis and optimization of ligands for vastly diverse
targets.
Apart from the H-Cube, our flow system consists entirely
of inexpensive and readily available components. It is
conveniently assembled and modified, and the modules can
be easily rearranged. The implemented reactions were
carefully selected, with special attention to any stoichiometric
byproducts. Addition reactions are generally preferable, and
reactions which produce inert byproducts are also usually
good choices. Scavenging of stoichiometric amounts of
byproduct is possible, but less economic and practical.
In summary, we have demonstrated the value of multistep
flow synthesis in medicinal chemistry with a three-step flow
system which conveniently assemble three varied building
blocks in a single continuous flow. This represents the first
demonstration of robustness and usefulness of such a system
by the synthesis of a diverse library of test compounds. An
important aim of this report is to demonstrate that the general
medicinal chemistry laboratory with only minor investments
can construct simple systems for flow synthesis, which can
be very effective and flexible tools for rapid synthesis of
desired lead analogues in the drug discovery and optimization
process.
Acknowledgment. The Danish Natural Sciences Research
Council and the Danish Medical Sciences Research Council
are thanked for financial support.
Once the three-step flow sequence was assembled and
optimized, it proved to be a powerful and robust tool for
rapid production of diverse compounds. Sequential produc-
tion with a minimum of effort compared to batch or parallel
synthesis contributes to minimizing the turnaround time of
the critical design-synthesis-screening cycle. Although the
examples in Table 1 are mostly limited to compounds that
fit the targeted pharmacophore, the diverse selection of
Supporting Information Available: Experimental pro-
cedures, description of flow equipment, and 1H and 13C NMR
of target compounds. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL902101C
Org. Lett., Vol. 11, No. 22, 2009
5137