In Situ Preparation and Consumption of O -Mesitylsulfonylhydroxylamine (MSH) in Continuous Flow for the Amination of Pyridines

Article abstract of DOI:10.1055/s-0036-1588799

The paper demonstrates a safe method in which highly unstable O -mesitylsulfonylhydroxylamine (MSH) can be prepared and consumed in continuous flow. MSH was prepared in situ and used for the flow amination of a range of pyridines, which were subsequently transformed into useful pyrazolopyridine building blocks.

Full text of DOI:10.1055/s-0036-1588799

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
© Georg Thieme Verlag Stuttgart · New York
2017, 28, A–E
letter
A
en
C. E. Brocklehurst et al.
Letter
Synlett
In Situ Preparation and Consumption of O-Mesitylsulfonylhydrox-
ylamine (MSH) in Continuous Flow for the Amination of Pyridines
Cara E. Brocklehurst*
Guido Koch
Stephanie Rothe-Pöllet
Luigi La Vecchia
Synthesis and Technologies, Global Discovery Chemistry,
Novartis Institutes for Biomedical Research,
Klybeckstrasse 141, 4057 Basel, Switzerland
cara.brocklehurst@novartis.com
Received: 08.02.2017
Accepted after revision: 28.03.2017
Published online: 02.05.2017
solid), using flow chemistry, would give us a safe method to
prepare pyrazolopyridine building blocks for our medicinal
chemistry programs.
DOI: 10.1055/s-0036-1588799; Art ID: st-2017-b0095-l
MSH (1) has been widely used for the N- or S-amination
of heteroaromatic systems,4 especially the amination of
pyridines 2 followed by cyclization with acetylenes yielding
3-functionalized pyrazolo[1,5-a]pyridines 5 (Scheme 1).5
The corresponding 2-alkyl pyrazolo[1,5-a]pyridinecarbox-
ylates can be prepared by a two-step Knoevenagel/Hemets-
berger–Knittel sequence,6 albeit with associated hazards in
handling azido-3-(pyridin-2-yl)acrylates, some of which
have been overcome by adaptation to flow.7
Abstract The paper demonstrates a safe method in which highly un-
stable O-mesitylsulfonylhydroxylamine (MSH) can be prepared and
consumed in continuous flow. MSH was prepared in situ and used for
the flow amination of a range of pyridines, which were subsequently
transformed into useful pyrazolopyridine building blocks.
Key words safety, pyrazolopyridines, flow, O-mesitylsulfonylhydroxyl-
amine, pyridine
O-Mesitylsulfonylhydroxylamine (MSH, 1) is a highly
unstable reagent, with an onset of decomposition only
slightly above room temperature. The stability of 1 is de-
pendent upon its water content; however it is strongly rec-
ommended that even damp MSH (containing 40% water) is
prepared immediately prior to use and not stored.1
Our group was searching for a method in which MSH
could be safely used, ultimately on a multi-hundred gram
scale, for the amination of pyridines and their subsequent
1,3-dipolar cycloaddition to give pyrazolpyridine building
blocks. Pyrazolo[1,5-a]pyridines are useful building blocks
for medicinal chemistry and have been widely used in a
number of medicinal chemistry programs.2 Given the low
thermal stability of MSH, its formation and consumption
under continuous flow became an attractive target.
R1
R1
O
O
S
NH2
S
N+
O
O–
N
O
O
NH2
1 (MSH)
2
3
R2O2C
base
4
R1
CO2R2
N
N
5
Scheme 1 General scheme for the amination of pyridines and the sub-
sequent cycloaddition with acetylenes 4 yielding pyrazolo[1,5-a]pyri-
dines 5
One of the many advantages of flow chemistry is the
safe handling of hazardous reagents by minimizing their
quantity at a specific time point during a chemical transfor-
mation. High-energy reagents can be prepared in situ and
used immediately in a subsequent transformation.3 We en-
visaged that the formation of only a small quantity of MSH
in solution at any one time (and avoiding its isolation as a
The advantage of using MSH over other aminating re-
agents, such as O-2,4-dinitrophenylhydroxylamine (DPH),8
is that MSH reacts rapidly and cleanly in the amination of
many pyridines. There are two generally accepted methods
for the preparation of MSH (outlined in Scheme 2): (i)
deprotection of the Boc-hydroxylamine derivative 6 with
TFA (Carpino’s method),9 or (ii) deprotection of the acet-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

B
C. E. Brocklehurst et al.
Letter
Synlett
amide 7 using perchloric acid (Zinner’s method).10 Both Boc
derivative 6 and acetimidate 7 are commercially available.
The Boc-deprotection is performed in neat TFA and requires
1.5 hours at 0 °C. In contrast, the acetimidate deprotection
with perchloric acid only requires one equivalent of acid
and is complete within just 15 minutes at room tempera-
ture. Both reactions are described in the literature and re-
quire quenching into ice cold water, followed by filtration
and drying under vacuum.11 We wanted to avoid the isola-
tion of MSH as a solid and sought to implement both, the
formation of MSH and its consumption, in a flow process.
O
15 min
30 °C
20 mL coil
(MSH
S
N
OEt
O
O
7 (1 M in MeCN)
deprotection)
Br
70% HClO4
(1 equiv,
11.6 M neat)
N
2a (1 equiv,
2M in MeCN)
1 min
30 °C
2 mL coil
(mixing)
1 M NaOH
(1 equiv,
diluted
with DMF,
0.667 M)
2.6 min
30 °C
10 mL coil
(amination)
O
S
NHBoc
neat TFA
0 °C, 1.5 h
O
O
6
O
S
NH2
Br
O
O
O
S
HClO4 (1 equiv)
MeCN,
rt, 15 min
N+
O
1 (MSH)
O–
O
S
N
OEt
NH2
O
3a
O
(quant.)
7
Scheme 3 Optimal set-up for the amination of 3-bromopyridine (2a)
Scheme 2 Preparation of MSH using Carpino’s (upper arrow) and Zin-
ner’s (lower arrow) methods
in flow13
The method by which the MSH deprotection and con-
sumption was adapted to flow, using 3-bromopyridine (2a)
as a model reaction, is detailed in Scheme 3. After multiple
batch trials, in which the concentration of reagents and sol-
vents were sequentially changed to check that every com-
pound involved remained in solution, we arrived at the op-
timized set-up shown (Scheme 3).12
chloric acid. The power trace for the third reactor (in which
the amination occurred) showed that less power was re-
quired to heat to 30 °C, indicating an exothermic reaction.
We have been able to show that 1,3-dipolar cycloaddi-
tion reactions between the 1-amino-3-bromopyridinium
salt 3a and methyl propiolate (4a) can also be adapted to
flow (Scheme 4). In the first Y-piece the aminated pyridini-
um salt 3a [prepared by mixing 3-bromopyridine (2a) and
damp solid MSH (1)] was mixed with triethylamine, and
the deprotonation occurred in the first reactor. In the sec-
ond Y-piece, methyl propiolate (4a) was added, and the cy-
clization step reached completion in the second reactor coil
at 90 °C within just 2 minutes.14 Pyrazolopyridines 5a and
5b were isolated in 42% and 14% yield, respectively (com-
pared to 17% yield of 5a and 27% yield of 5b using a batch
method).
Differential scanning calorimetry (DSC) of the aminated
salts 3 showed that the latter undergo decomposition at a
considerably higher temperature than MSH [decomposition
above 200 °C vs. an exothermic decomposition with an on-
set at 50 °C for solid MSH (1)]. In order to reduce the com-
plexity, and also to reduce the number of pumps required,
we did not regard the last deprotonation/cyclization steps
as essential to be adapted to flow, because by this point the
safety risk of handling explosive MSH has been circumvent-
ed. The output solution of aminated pyridine salt 3a (from
Scheme 3) was therefore treated directly with triethyl-
A solvent screen revealed that MSH shows reasonable
stability over several hours in acetonitrile. Acetimidate 7
(used as a 1 M solution in acetonitrile) was mixed with one
equivalent of 70% perchloric acid in the first Y-piece. The
MSH formation subsequently occurred in the first reaction
coil within 15 minutes at 30 °C. We observed that the thus-
formed MSH solution, which still contained one equivalent
of perchloric acid, did not react with 3-bromopyridine (2a).
For this reason, the second coil shown acted as a mixing
coil. One equivalent of sodium hydroxide solution was pro-
vided by the fourth pump, and in the third reactor, the per-
chloric acid was quenched and the neutralized MSH reacted
rapidly with the pyridine to form the aminated salt 3a. DMF
was added to the incoming sodium hydroxide solution pro-
vided by the fourth pump, not only to ensure that the ami-
nated salt 3a remained in solution, but also to facilitate any
subsequent cyclization steps. It should be noted that the
rate-limiting step in the whole sequence remained the
deprotection step, with all other subsequent steps being in-
ferred by the initial flow rates of the acetimidate 7 and per-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

C
C. E. Brocklehurst et al.
Letter
Synlett
yields are modest, they are not far away from those ob-
served in the literature (20–40% yield).2a The productivity
of pyrazolopyridine 5e (Table 1, isolated by direct filtration
of the reaction mixture quenched into water) was 4.2
grams per hour using the flow methodology, which demon-
strates that useful quantities of difficult-to-obtain scaffolds
can be prepared in a safe and efficient manner, on a gram
scale using a lab flow machine.
Pyrazolopyridines 5a–d (Scheme 4 and Table 1, entries
1 and 2) are useful heterocyclic intermediates in which the
bromo-handle provides the possibility of diversification
via, for example, palladium-catalyzed coupling reactions.2b
The amino-substituted pyrazolopyridines 5f and 5g (Table
1, entries 4 and 5) provide a branching point for amide cou-
plings, reductive aminations or conversion into halides via
Sandmeyer chemistry.
Br
O
N+
NH2
S
O–
O
31 s
30 °C
2 mL coil
(deprotonation)
3a
(0.2 M in THF/H2O 1:1)
MeO2C
Et3N (1.5 equiv,
0.8 M in THF)
4a
(1.5 equiv,
0.8 M
in THF)
2 min
90 °C
10 mL coil
(cyclization)
Br
Br
CO2Me
CO2Me
N
N
N
Our flow methodology to prepare MSH in situ has also
been used to aminate methyl 2-(pyridin-2-yl)acetate (2d)
and methyl 2-(5-bromopyridin-2-yl)acetate (2e) under
continuous flow conditions. Both undergo an intramolecu-
N
5a
5b
(14% after
chromatography)
(42% after
chromatography)
Scheme 4 Flow chemistry deprotonation and cyclization steps to yield
pyrazolopyridines 5a and 5b
O
15 min
30 °C
20 mL coil
(MSH
amine and ethyl propiolate (4b) in the outlet flask to yield
pyrazolopyridines 5c and 5d in 29% combined yield (Table
1, entries 1 and 2).
S
N
OEt
O
O
7 (2 equiv,
deprotection)
1 M in MeCN)
R
70% HClO4
(2 equiv,
11.6 M neat)
Table 1 Yields of Pyrazolopyridines via Flow Amination and Batch Cy-
clization
N
CO2Me
2d R = H
2e R = Br
(1 equiv, 2 M in MeCN)
2.6 min
30 °C
10 mL coil
(mixing)
R
2a R = m-Br
2b R = p-CO2Me
2c R = m-NHBoc
N
1 M NaOH
(2 equiv,
diluted
with DMF,
0.667 M)
MSH prepared
and consumed
in flow
O
8.6 min
30 °C to rt
30 mL coil
(amination)
S
NH2
O
O
R2O2C
4a R2 = Me
4b R2 = Et
(1.5 equiv)
R1
R1
R
O
O
S
CO2R2
S
N+
NH2
N
O–
Et3N (2 equiv), rt, 15 min
N+
O
O–
O
N
3
NH2
CO2Me
5
outlet solution
3d R = H
3e R = Br
Entry
Product
R1
R2
Yield (%)
Et3N (2 equiv), rt, 15 min
R
1
2
3
4
5
5c
5d
5e
5f
6-Br
Et
11
18
24
2
4-Br
Et
3-CO2Me
6-NHBoc
4-NHBoc
Me
Et
N
N
OH
5g
Et
23
8a R = H (21% yield after chromatography)
8b R = Br (27% yield after crystallization)
Scheme 5 Amination of methyl 2-(pyridin-2-yl)acetate (2d) and meth-
yl 2-(5-bromopyridin-2-yl)acetate (2e) with subsequent intramolecular
ring closure
Three substituted pyridines (2a, 2b and 2c) were treated
under our optimized conditions shown in Scheme 3 to yield
pyrazolopyridines 5c–g shown in Table 1. Although the
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

D
C. E. Brocklehurst et al.
Letter
Synlett
lar cyclization to give pyrazolopyridin-2-ol (8a) and 6-bro-
mopyrazolopyridin-2-ol (8b) in 21% and 27% yield, respec-
tively, after purification. In contrast to previous examples
for pyrazolopyridine formation by intermolecular cycliza-
tion, an excess of MSH (1) was shown to give higher yields
of product in the intramolecular case. For this reason, two
equivalents of acetimidate 7, perchloric acid and sodium
hydroxide solution were used for the transformations
shown in Scheme 5. We also observed that the amination of
pyridines 2d and 2e was much slower than that of pyridines
2a–c, and a longer reaction coil (30 mL) was required in the
third reactor. Without the longer third coil, decomposition
of MSH occurred in the back-pressure regulator before it
had a chance to aminate the pyridine.
Fluorinated heterocyclic cores are privileged structures
in medicinal chemistry;15 however, little is known about
the preparation of 2-fluoropyrazolopyridines in the litera-
ture. We have therefore demonstrated that pyrazolopyri-
dine-3-carboxylate 5c prepared in flow can be fluorinated
using Selectfluor at 100 °C, followed by saponification and
(2) (a) Kendall, J. D.; O’Connor, P. D.; Marshall, A. J.; Frederick, R.;
Marshall, E. S.; Lill, C. L.; Lee, W. J.; Kolekar, S.; Chao, M.; Malik,
A.; Yu, S. Q.; Chaussade, C.; Buchanan, C.; Rewcastle, G. W.;
Baguley, B. C.; Flanagan, J. U.; Jamieson, S. M. F.; Denny, W. A.;
Shepherd, P. R. Bioorg. Med. Chem. 2012, 20, 69. (b) Takahashi,
Y.; Hibi, S.; Hoshino, Y.; Kikuchi, K.; Shin, K.; Murata-Tai, K.;
Fujisawa, M.; Ino, M.; Shibata, H.; Yonaga, M. J. Med. Chem. 2012,
55, 5255. (c) Kendall, J. D.; Marshall, A. J.; Giddens, A. C.; Tsang,
K. Y.; Boyd, M.; Frederick, R.; Lill, C. L.; Lee, W. J.; Kolekar, S.;
Chao, M.; Malik, A.; Yu, S.; Chaussade, C.; Buchanan, C. M.;
Rewcastle, G. W.; Baguley, B. C.; Flanagan, J. U.; Denny, W. A.;
Shepherd, P. R. MedChemComm 2014, 5, 41. (d) Moller, D.;
Salama, I.; Kling, R. C.; Hubner, H.; Gmeiner, P. Bioorg. Med.
Chem. 2015, 23, 6195. (e) Tang, J.; Wang, B. X.; Wu, T.; Wan, J. T.;
Tu, Z. C.; Njire, M.; Wan, B. J.; Franzblauc, S. G.; Zhang, T. Y.; Lu,
X. Y.; Ding, K. ACS Med. Chem. Lett. 2015, 6, 814. (f) Degorce, S.
L.; Boyd, S.; Curwen, J. O.; Ducray, R.; Halsall, C. T.; Jones, C. D.;
Lach, F.; Lenz, E. M.; Pass, M.; Pass, S.; Trigwell, C. J. Med. Chem.
2016, 59, 4859.
(3) (a) Ley, S. V. Chem. Rec. 2012, 12, 378. (b) Movsisyan, M.;
Delbeke, E. I. P.; Berton, J.; Battilocchio, C.; Ley, S. V.; Stevens, C.
V. Chem. Soc. Rev. 2016, 45, 4892. (c) Porta, R.; Benaglia, M.;
Puglisi, A. Org. Process Res. Dev. 2016, 20, 2.
decarboxylation to yield 2-fluoropyrazolopyridine
(Scheme 6) in high yield.
9
(4) Tamura, Y.; Minamikawa, J.; Ikeda, M. Synthesis 1977, 1.
(5) Huisgen, R.; Grashey, R.; Krischke, R. Tetrahedron Lett. 1962,
387.
(6) Roy, P. J.; Dufresne, C.; Lachance, N.; Leclerc, J. P.; Boisvert, M.;
Wang, Z. Y.; Leblanc, Y. Synthesis 2005, 2751.
(7) O’Brien, A. G.; Levesque, F.; Seeberger, P. H. Chem. Commun.
2011, 47, 2688.
(8) Legault, C.; Charette, A. B. J. Org. Chem. 2003, 68, 7119.
(9) Carpino, L. A. J. Am. Chem. Soc. 1960, 82, 3133.
Br
Br
(i) Selectfluor, DMF,
100 °C, 20 min
N
CO2Et
N
(ii) LiOH, H2O, THF,
0 °C, 5 min
N
N
F
5c
9 (91% yield)
(10) Tamura, Y.; Minamika, J.; Sumoto, K.; Fujii, S.; Ikeda, M. J. Org.
Chem. 1973, 38, 1239.
(11) When MSH (1) was isolated as a damp solid and analyzed by ion
chromatography, no anions were observed, indicating the for-
mation of free MSH and not a salt.
Scheme 6 Fluorination and decarboxylation of pyrazolopyridine-3-car-
boxylate 5c
In conclusion, we demonstrated a safe method in which
MSH (1) can be prepared and consumed in continuous flow
and showed how this method can be applied to the synthe-
sis of pyrazolopyridine building blocks.
(12) It should be noted that, when damp solid MSH was dissolved in
acetonitrile and pumped through the Vapourtec Knauer pump
heads, decomposition occurred presumably due to the mechan-
ical action of the pistons. Both the batch and the flow deprotec-
tion of 7 required 15 minutes. Warming of the first reaction coil
above 30 °C resulted in the decomposition of MSH. Combining
the inlet solutions of pyridine 2a and sodium hydroxide was not
tolerated and also resulted in decomposition.
Acknowledgment
(13) 1-Aminopyridin-1-ium
2,4,6-trimethylbenzenesulfonate
The authors would like to thank Christoph Heuberger for his advice
concerning safety, Hansjoerg Lehmann for collaborating on flow
chemistry, as well as Guillaume Ngo and Corinne Marx for high reso-
lution mass spectrometry.
Salts 3, General Flow Procedure
All reactions were performed using a commercially available
Vapourtec R-series set-up equipped with four pumps. (E)-Ethyl
N-(mesitylsulfonyl)oxyacetimidate (7) was dissolved in MeCN
(1 M) and filtered. Perchloric acid (neat, 11.6 M) was mixed
with the first inlet via a Y-piece with flow rates of 1.228 mL/min
and 0.106 mL/min, respectively. Pyridine 2 was dissolved in
MeCN (2M), filtered and introduced into a second Y-piece at a
flow rate of 0.614 mL/min. Sodium hydroxide (1 M, aq.) was
diluted with DMF to a concentration of 0.667 M and introduced
in a third Y-piece at a flow rate of 1.840 mL/min. The stoichio-
metric ratio of all four inlets was 1:1:1:1. The system solvent
was MeCN for the first three inlets and H2O/DMF (2:1) for the
fourth inlet. The PFA (polyfluoroalkoxy alkane polymer) reactor
coils, with volumes of 20 mL, 2 mL and 10 mL, respectively,
were all set to a temperature of 30 °C. The reaction mixture
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0036-1588799.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
References and Notes
(1) Mendiola, J.; Rincon, J. A.; Mateos, C.; Soriano, J. F.; de Frutos, O.;
Niemeier, J. K.; Davis, E. M. Org. Process Res. Dev. 2009, 13, 263.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E

E
C. E. Brocklehurst et al.
Letter
Synlett
from the first two inlet streams had a residency time of 15 min
in the first reactor, of 1.02 min in the second and of 2.64 min in
the third.
121.4, 128.6, 129.9, 135.8, 136.4, 138.6, 141.4, 142.5, 166.0.
+
HRMS (FAB): m/z calcd for C5H6BrN2
: 172.97144; found:
172.97105; m/z calcd for C9H11O3S–: 199.04289; found:
199.04277. DSC showed small exotherm with 61 J/g onset
249 °C and larger exotherm with 573 J/g onset 299 °C.
(14) It should also be noted that combining any of the three inlet
solutions, in order to simplify the set-up, led to decomposition
and poor yield.
1-Amino-3-bromopyridin-1-ium
sulfonate (3a)
2,4,6-Trimethylbenzene-
The reaction was performed by adapting the general flow pro-
cedure to the reaction of 3-bromopyridine (2a) with MSH. The
outlet solution (25 mL, collected over 3.6 min) was concen-
trated in vacuo to give an orange solid (3.6 min collection time,
>99%). 1H NMR (400 MHz, DMSO-d6): δ = 2.18 (s, 9 H, 3 × CH3),
6.77 (s, 2 H, NH2), 7.93 (dd, J = 4, 8 Hz, 1 H, ArH), 7.95 (s, 2 H,
ArH), 8.49 (d, J = 8 Hz, 1 H, ArH), 8.81 (d, J = 8 Hz, 1 H, ArH), 9.17
(s, 1 H, ArH). 13C NMR (101 MHz, d6-DMSO): δ = 20.3, 22.7,
(15) (a) Chen, Z.; Cogan, D.; Guo, X.; Marshall, D. R.; Meyers, K. M.;
Zhang, Y. WO2016089800A1, 2016. (b) Dominguez, C.; Munoz-
San Juan, I.; Maillard, M.; Raphy, G.; Haughan, A. F.; Luckhurst,
C. A.; Jarvis, R. E.; Burli, R. W.; Wishart, G.; Hughes, S. J.; Allen, D.
R.; Penrose, S. D.; Breccia, P. WO2014159224A1, 2014.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–E