Combining organometallic reagents, the sulfur dioxide surrogate DABSO, and amines: A One-pot preparation of sulfonamides, amenable to array synthesis

Article abstract of DOI:10.1002/anie.201409283

We describe a method for the synthesis of sulfonamides through the combination of an organometallic reagent, a sulfur dioxide equivalent, and an aqueous solution of an amine under oxidative conditions (bleach). This simple reaction protocol avoids the need to employ sulfonyl chloride substrates, thus removing the limitation imposed by the commercial availability of these reagents. The resultant method allows access to new chemical space, and is also tolerant of the polar functional groups needed to impart favorable physiochemical properties required for medicinal chemistry and agrochemistry. The developed chemistry is employed in the synthesis of a targeted 70 compound array, prepared using automated methods. The array achieved a 93% success rate for compounds prepared. Calculated molecular weights, lipophilicities, and polar surface areas are presented, demonstrating the utility of the method for delivering sulfonamides with drug-like properties.

Full text of DOI:10.1002/anie.201409283

Angewandte
Chemie
DOI: 10.1002/anie.201409283
Sulfonamide Synthesis
Combining Organometallic Reagents, the Sulfur Dioxide Surrogate
DABSO, and Amines: A One-Pot Preparation of Sulfonamides,
Amenable to Array Synthesis**
Alex S. Deeming, Claire J. Russell, and Michael C. Willis*
Abstract: We describe a method for the synthesis of sulfon-
amides through the combination of an organometallic reagent,
a sulfur dioxide equivalent, and an aqueous solution of an
amine under oxidative conditions (bleach). This simple
reaction protocol avoids the need to employ sulfonyl chloride
substrates, thus removing the limitation imposed by the
commercial availability of these reagents. The resultant
method allows access to new chemical space, and is also
tolerant of the polar functional groups needed to impart
favorable physiochemical properties required for medicinal
chemistry and agrochemistry. The developed chemistry is
employed in the synthesis of a targeted 70 compound array,
prepared using automated methods. The array achieved a 93%
success rate for compounds prepared. Calculated molecular
weights, lipophilicities, and polar surface areas are presented,
demonstrating the utility of the method for delivering sulfon-
amides with drug-like properties.
T
he defining features of sulfonamides—heteroatom-rich
composition, balanced lipophilicity, good chemical and meta-
bolic stability, H-bond donor and acceptor ability, and three-
dimensional structure—combine to make them one of the
pre-eminent functional groups found in designed medicinal
and agrochemical agents. Accordingly, sulfonamides are
present in molecules active against a diverse range of
biological effects (Figure 1a).[1] Despite the prevalence of
sulfonamides, methods for their synthesis are largely confined
to the combination of amines with isolated, pre-activated
sulfonyl derivatives, most usually sulfonyl chlorides.[2] This is
a highly effective way to prepare sulfonamides, but it is not
without limitations. Importantly, the chemical space acces-
sible using this approach is defined by the availability of the
corresponding sulfonyl chlorides. These, in turn, are limited
by the largely classical methods used for their synthesis
(Figure 1b).[3] Alkyl, alkenyl, and heteroaryl sulfonyl chlor-
Figure 1. a) Selected bioactive sulfonamides. b) Classic sulfonamide
synthesis based on an initial electrophilic aromatic substitution
reaction to prepare a sulfonyl chloride. c) Present work: Sulfonamide
synthesis based on the combination of an in situ-generated sulfinate
intermediate and an electrophilically activated amine.
ides have a narrow commercial availability, reflecting both the
methods available for their synthesis and the stability of these
reagents. It follows that an effective way to prepare sulfon-
amides that occupy new areas of chemical space would be to
devise a synthetic route that avoids the use of sulfonyl
chlorides, and instead employed starting materials of greater
diversity. If the focus is on providing sulfonamides applicable
as bioactive molecules, then in addition to delivering novel
structures—new chemical space[4]—the method must also
allow the preparation of molecules with the physiochemical
profiles that have been shown to correlate with increased
success in achieving drug- and agrochemical-like proper-
ties.[5,6] An additional desirable feature for new methods
focused on delivering bioactive molecules is the compatibility
of the method for array, or library synthesis.[7,8] In the
following Communication we describe the development of
such a process, based on the combination of organometallic
reagents, a sulfur dioxide surrogate, and amines (Figure 1c).
To develop a sulfonamide synthesis that avoids the use of
sulfonyl chlorides, as either substrates or formal intermedi-
ates, we were attracted to sulfinates as suitable precursors.
[*] A. S. Deeming, Prof. M. C. Willis
Department of Chemistry, University of Oxford
Chemistry Research Laboratory
Mansfield Road, Oxford, OX1 3TA (UK)
E-mail: michael.willis@chem.ox.ac.uk
Homepage: http://mcwillis.chem.ox.ac.uk/MCW/Home.html
Dr. C. J. Russell
Syngenta, Jealott’s Hill International Research Centre
Bracknell, Berkshire, RG42 6EY (UK)
[**] This work was supported by the EPSRC and Syngenta. We thank
Russell Ellis and David Langton for assistance with the array
chemistry experiments.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201409283.
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Table 1: Organometallic reagent scope for the one-pot preparation of
sulfonamides (3).[a]
Although the commercial availability of sulfinates is poor,
they are readily generated in situ from the combination of an
organometallic reagent and sulfur dioxide gas,[9] or more
attractively, a sulfur dioxide surrogate.[10] Barrett and co-
workers reported a sulfonamide synthesis based on the
combination of Grignard reagents and sulfur dioxide gas,
followed by treatment with sulfuryl chloride and an amine.[11]
Our laboratory subsequently reported a related process, in
which we were able to replace the gaseous reagent with
a sulfur dioxide surrogate, the bis-SO2 adduct of 1,4-
diazabicyclo[2.2.2]octane (DABCO), DABSO.[12,13] Although
this was effective for simple substrates, it was hampered by
the need to employ sulfuryl chloride, a reagent that requires
frequent purification and has limited functional group
compatibility. To deliver a robust, user-friendly process that
would be tolerant against functional groups, led us to consider
alternative strategies based on the nucleophilic character of
the in situ-generated sulfinates. We had recently shown that
such sulfinates could be combined with a broad range of
carbon-based electrophiles to achieve an efficient sulfone
synthesis,[10] and were intrigued as to whether we could
replace the C-electrophiles with a range of N-based electro-
philes and therefore access sulfonamides (Figure 1c).
À
À
Entry R M
Yield [%] Entry R M
Yield [%]
1
2
3
82
52
65
9[b]
51
72
62
10[c]
11[c]
4
5
64
86
12[d]
13
62
89
6
84
14
78
7[b]
8
71
68
15[e]
16[f]
65
50
N-Chloroamines have gained popularity as an effective
source of electrophilic amine fragments, and have been used
in metal- and non-metal-catalyzed coupling reactions.[14]
Although some precedent exists for the combination of
isolated sodium sulfinates and N-chloroamines,[15] we were
interested to explore their reactivity with in situ-generated
sulfinates. Pleasingly, we found that magnesium sulfinates
formed in situ could be successfully coupled with N-chlor-
omorpholine, delivering the corresponding sulfonamides 1a,b
in high yields [Eq. (1), Scheme 1]. Although the inherent
[a] Reaction conditions: Organometallic reagent (1 equiv), DABSO
(0.6 equiv), THF À408C; then amine, H2O, and NaOCl at 08C followed by
stirring at rt; [b] RMgX generated from the corresponding iodide and
n
iPrMgCl; [c] RLi generated from the corresponding bromide and BuLi;
[d] RZnX generated from the corresponding iodide by zinc insertion;
[e] RLi generated through deprotonation with tBuLi; [f] Product isolated
following wash with 1m HCl(aq)
.
nesium bromide, morpholine, and an aqueous solution of
sodium hypochlorite (bleach). After some experimentation[16]
we identified reaction conditions that allowed the target
sulfonamide 2 to be isolated in good yield [Eq. (2), Scheme 1].
Our next task was to evaluate the range of organometallic
reagents that could be employed (Table 1). Pleasingly, we
found that Mg-, Li-, and Zn-based reagents, featuring a range
of alkyl, aryl, alkenyl, and heteroaryl substituents, could be
efficiently converted to the corresponding sulfonamides. For
the alkyl examples, Grignard reagents were the most efficient
compared to the corresponding lithium and zinc reagents
(entries 1–4). There was less variation in reactivity for the aryl
series (entries 7, 10, and 12). The examples shown in Table 1
feature commercial organometallic reagents as well as
organometallics generated by halogen–metal exchange with
both zero-valent metal and alkyl metals, and by deprotona-
tion.
We next explored the scope of amines that could be
employed (Table 2). A range of primary and secondary
amines were found to be compatible with this system
(entries 1–4). Pleasingly, aniline derivatives also delivered
sulfonamides in good yields (entries 5–9). Amino acids were
found to perform well with pure products readily obtained
after acidification (entries 10 and 11). The enantiopurity of
these substrates was unaffected during the process. Although
the formation of N-chloro derivatives of amides is known,[17]
these nucleophiles were not amenable to the present system,
Scheme 1. The combination of in situ-generated magnesium sulfinates
with N-chloroamines.
reactivity achieved in these reactions was encouraging, the
use of isolated N-chloroamines as electrophilic partners was
not ideal, due to the need for each chloroamine to be
individually prepared, and also the handling difficulties
associated with these molecules. Accordingly, we were
attracted to the possibility of generating both a metal
sulfinate and a N-chloroamine in situ. Such a process is
unprecedented, and would allow simple organometallic
reagents and simple amines to be the immediate precursors
that are combined (along with an SO2 surrogate) for the
proposed sulfonamide synthesis. To explore this possibility we
began by studying the combination of 3-methoxyphenylmag-
2
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Table 2: Variation of amines employed in the one-pot preparation of
sulfonamides 4.[a]
Entry Amine
1
Yield [%] Entry Amine
Yield [%]
73
79
68
76
7[c]
8[c]
9
2[b]
3
81
57
4
5
6
78
83
64
10[d]
11[b,d]
12[c]
83
73
51
[a] Reaction conditions: Grignard (1 equiv), DABSO (0.6 equiv), THF
À408C; then amine, solvent, and NaOCl at 08C followed by stirring at rt;
[b] ee confirmed by HPLC on a chiral stationary phase; [c] AcOH
(5 equiv) added to aid solubility; [d] product isolated following wash with
Figure 2. A 70-compound array synthesis of sulfonamides, together
with control examples. [a] Desilylated products isolated. THP=tetrahy-
dropyranyl, TMS=trimethylsilyl, TIPS=triisopropylsilyl.
1m HCl(aq)
.
resulting in no sulfonamide formation. We were able to
exploit this reactivity difference with the use of an amido-
substituted amine nucleophile; the sulfonamide obtained was
formed by exclusive reaction at the amine N-atom (entry 12).
Having examined the general reactivity and scope of the
developed sulfonamide synthesis, we next wanted to test the
methodology by application to an array synthesis format. In
particular, we sought to establish that the developed chemis-
try could be used to rapidly prepare a collection of molecules
featuring the physiochemical properties associated with drug-
likeness.[5,6] This meant selecting a group of functional-group-
rich, polar, and relatively small monomers; characteristics
often associated as being incompatible with new synthetic
methodology.[18] In addition, when selecting the organome-
tallic partners we focused on reagents for which the corre-
sponding sulfonyl chloride was not readily (commercially)
available. The reactions were performed as a plate experi-
ment, employing an XT-block reactor, with seven organome-
tallic reagents (M = MgX, Li, and ZnX) and ten amines
(Figure 2). Crude compounds were purified by mass-directed
preparative HPLC, allowing for isolated product yields to be
determined.
As can be seen from Figure 2, 65 out of a possible 70
organometallic/amine combinations delivered pure sulfon-
amide products (as determined by LC-MS, and in some cases
1H NMR spectroscopy). This equates to a 93% success rate
for the array. Of the five failed examples there was only
a single case in which no product was observed. The
remaining four “fails” contained impurities from inefficient
HPLC purification. To evaluate the effect of performing the
reactions in an array format, two substrate combinations were
individually prepared, employing the optimized conditions
used in Tables 1 and 2. Compounds G6 and D4 were obtained
in 39% and 54% yields, respectively, corresponding to
deviations of 6% and 10% by comparison with the results
from the array synthesis (Figure 2). Although these isolated
synthesis yields are respectable, it is the “array yield” of 93%
that should be used to measure the success of the experiment.
From a discovery chemistry perspective it is important to
assess the physiochemical properties of the 65 synthesized
molecules, and to determine their fit with the various
guidelines for medicinal and agrochemical applications.[5,6]
The array had been designed to deliver molecules with logP,
topological surface area and molecular weights to fit these
criteria; the calculated values are shown graphically in
Figure 3.[19] The majority of the products were found to
have logP values within the range of 0 and 3, PSA values
between 75 and 100 ꢀ2, and molecular weights centered on
325 Da; values desirable for drug-like space.[5,6] The calcu-
lated properties for three example sulfonamides prepared in
the array are also shown in Figure 3. This encouraging
physiochemical profile for the array translated into observed
in vivo activity in a range of agrochemical assays. In
particular, 67% of the molecules examined displayed at
least partial control in preliminary fungicide, herbicide, and
insecticide assays.[16]
In conclusion, we have developed a simple and efficient
one-pot synthesis of alkyl, alkenyl, and (hetero)aryl sulfon-
amides based on the union of metal sulfinates and N-
chloroamines, both generated in situ. The robustness of the
chemistry allowed the method to be challenged in an array
format; this was exemplified by delivering 65 sulfonamides,
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corresponding to a 93% success rate for the array (the
“array yield”). The synthesized array, containing 56 previ-
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[19] Calculations were performed using ACDꢁs logP calculator.
Received: September 19, 2014
Revised: November 5, 2014
Published online: && &&, &&&&
Keywords: amines · array synthesis · organometallics ·
.
physiochemical properties · sulfonamides
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4
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Sulfonamide Synthesis
A. S. Deeming, C. J. Russell,
M. C. Willis*
&&&& — &&&&
Combining Organometallic Reagents, the
Sulfur Dioxide Surrogate DABSO, and
Amines: A One-Pot Preparation of
Sulfonamides, Amenable to Array
Synthesis
Array array. A simple aqueous solution of
an amine and bleach, added to in situ-
generated metal sulfinates, allows the
effective preparation of a varied range of
sulfonamides. The sulfinate intermedi-
ates are formed by organometallic addi-
tion to the solid SO2 surrogate DABSO. A
70-compound array was prepared to
demonstrate the utility of the method,
with an emphasis on delivering mole-
cules with drug-like physiochemical
properties.
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