Sulfation made simple: A strategy for synthesising sulfated molecules

Article abstract of DOI:10.1039/c9cc01057b

The study of organosulfates is a burgeoning area in biology, yet there are significant challenges with their synthesis. We report the development of a tributylsulfoammonium betaine as a high yielding route to organosulfates. The optimised reaction conditions were interrogated with a diverse range of alcohols, including natural products and amino acids.

Full text of DOI:10.1039/c9cc01057b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc
first methanofullerene derivatives of Sc
3
N@C2n (2n
=
68 and 80). Synthesis of the
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Sulfation made simple: A strategy for synthesising sulfated
molecules
Received 00th January 20xx,
Accepted 00th January 20xx
a
b
a
Daniel M. Gill, Louise Male and Alan M. Jones *
DOI: 10.1039/x0xx00000x
www.rsc.org/
The study of organosulfates is a burgeoning area in biology,
yet there are significant challenges with their synthesis. We
report the development of a tributylsulfoammonium betaine
as a high yielding route to organosulfates. The optimised
reaction conditions were interrogated with a range of diverse
alcohols, including natural products and amino acids.
Organosulfates play a variety of important roles in biology
from xenobiotic metabolism to the downstream signalling of
steroidal sulfates in disease states. Sulfate groups on
®
Figure 1 Examples of drug molecules containing organosulfates: heparin, sotradecol
and avibactam ; a chemical tool compound (C3) and a metabolite of paracetamol.
1
®
glycosaminoglycans (GAGs) such as heparin, heparan sulfate
and chondroitin sulfate, facilitate molecular interactions and
The preparation of organosulfate esters include a microwave
3 3
assisted approach to the sulfation of alcohols, using Me N-SO
2
protein ligand binding at the cellular surface, an area of
15,16
3
and PySO , Chart 1 (i)).
The addition of catalytic diaryl
3
interest in drug discovery.
borinic acid can be used with Me
3
NSO
3
to sulfate
4
®
Heparin (an anticoagulant) , avibactam (a β-lactamase
17
carbohydrates. The limitations with these reagents include
the need for stoichiometric excess of reagent per alcohol
group (up to 10 eq. per hydroxyl group) and difficulties with
5
6
inhibitor) , sotradecol (a treatment for varicose veins) , the
sulfate metabolite of paracetamol (an analgesic), and the
glycomimetic C3 (used to study atherosclerosis) , contain
7
8
organosulfate motifs (Figure 1). There are many other natural
9
sources of bioactive sulfated compounds.
The incorporation of polar hydrophilic organosulfate groups
onto drug-like molecules is timely to facilitate research
1
0
investigating GAGs as potential new therapies. However, the
insertion and isolation of sulfate groups into target molecules
1
1
remains a challenging aspect of their synthesis, prompting
1
2
recent advances into sulfate revealing pro-drugs.
The presence of one or more sulfate group makes chemical
synthesis and purification of (per)sulfated compounds
challenging, primarily due to their poor solubility in organic
solvents.13 Therefore the insertion of organosulfate groups is
typically the final step in a synthetic method, limiting further
1
4
chemical modifications. A variety of methods to prepare
organosulfates are shown in Chart 1.
a.
School of Pharmacy, University of Birmingham, Edgbaston, B15 2TT
School of Chemistry, University of Birmingham, Edgbaston, B15 2TT
b.
*e-mail: a.m.jones.2@bham.ac.uk; +44(0)1214147288
Electronic Supplementary Information (ESI) available: Preparative routes,
1
13
compound characterisation, copies of H and C spectra and the cif file for the X-
Chart 1. Previous strategies to the synthesis of sulfate esters: This work: Sulfate ester
formation using tributylsulfoammonium betaine gives direct access to an organic
soluble organosulfate intermediate that can be readily ion exchanged.
ray crystal structure of 1.See DOI: 10.1039/x0xx00000x
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the preparation for nucleoside phophosulfates22 highlighted
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DOI: 10.1039/C9CC01057B
that the solubility of organosulfate ester can be modulated by
increasing the lipophilicity of the corresponding cation.
During a medicinal chemistry programme we encountered
difficulties using the established sulfation methods to per-
sulfate compounds due to the poor solubility of the
organosulfate intermediates and their resulting purification.
We sought to develop an all in one reagent, to improve the
solubility of the intermediate organosulfate ester. Combining
3 3
Scheme 1. (a) Synthesis of Bu NSO (1); (b) Alternative views of the crystal structure of
1
obtained from small molecule single crystal X-ray diffraction.
tributylamine (pKa, 10.89) with SO
3
, we envisioned this would
purification. Poor solubility in organic solvents makes aqueous
purification protocols and ion-exchange chromatography a
standard procedure, limiting the practicality. An alkyl
chlorosulfate ester with subsequent de-protection has proven
a reliable method, but limitations include the need to use a
strong base and a deprotection step leads to side products
create a complex (Bu N-SO , 1) for the persulfation of alcohols,
3
3
with increased lipophilicity of the intermediate sulfate ester,
improving the overall solubility of the organosulfates in
organic solvents. We rationalised that 1 would retain similar
3 3 3
activity to SO complexes with Et N and Me N, due to their
Chart 1 (ii)).18 A DCC/H
demonstrated but is not amenable to acid sensitive substrates
similar Lewis base strengths, pKa = 11.01 and 10.63,
respectively. Overall, 1 may permit sequential chemical steps
in organic solvents, with a simple purification method to the
corresponding sodium salt, streamlining the synthesis of
organosulfates.
(
2 4
SO -sulfate coupling has been
Chart 1 (iii)).19 A sulfitylation-oxidation protocol (Chart 1 (iv))
(
involves the synthesis of a protected sulfite ester, oxidation to
the protected sulfate ester and cleavage to the sodium sulfate
2
0
The complexation of sulfur trioxide to nitrogen or oxygen
salt. However, the use of multiple steps and purification
2
1
containing molecules (such as pyridine, NMe
3 3
, NEt , DMF, THF
sequences is limiting. A process route to Avibactam (Figure 1
and Chart 1 (v) involved sulfation of the hydroxylamine
1
1,15
and dioxane) is well known,
the use of an organic
solubilising partner, tributylamine, is however not. To the best
of our knowledge, the only literature report of the synthesis
and physical study of 1 was by Moede in 1949. It was not
until 1976 that Parshikov and co-workers studied 1 as a
sulfating agent on simple aliphatic alcohols (without
3 3
intermediate using Me NSO , followed by cation exchange
with tetrabutylammonium acetate, gave the organosulfate as
its tetrabutylammonium salt. The sodium salt was obtained by
precipitation in 77% yield over 2 steps on a multi-kg scale.
2
3
2
4
spectroscopic characterisation) and found that 1 reacts via an
2
S
N
mechanism driven by the hydrogen-bonding propensity of
the alcohol under study. However, to the best of our
knowledge no further use or development of this reagent has
been reported.
We synthesised
1 by reaction of tributylamine with
chlorosulfonic acid, affording a 90% yield on a 60 g scale
2
5
(
Scheme 1 (a)). For the first time both NMR spectral data and
the crystal structure of 1, obtained from small molecule single
2
6
crystal X-ray diffraction, was determined (Scheme 1 (b)).
Bu NSO (1) adopts a Gauche conformation within an
3
3
asymmetric unit cell caused by hydrogen bonding between the
methylene hydrogen atoms α to the nitrogen and the oxygens
of SO
3
. The measured N-S bond length in 1 is 1.886(3) Å, a
comparable bond length to a single N-S bond (typically: 1.73-
2
7
1
.83 Å versus 2.06 Å for a donor-acceptor system), suggesting
that 1 exists as a betaine in the solid state which may have
3
implications for the other unsolved amine-SO complexes and
their associated mechanisms.
Benzyl alcohol (2a) was selected for the optimisation study
(
Chart 2) due to a distinct down-field shift (+ 0.35 ppm) of the
1
benzylic signal after sulfation (by H-NMR spectroscopy).
We examined the sulfation of 2a with varying equivalents of 1
(entries 1 to 4). It was found that 2.0 equivalents of 1 was
optimal for high conversions (>99%) and isolated yields (95%,
entry 3). Less than 2.0 equivalents of 1 (entries 1 and 2) gave
incomplete conversion to 3a. We next surveyed the effect of
temperature (entries 5-8 vs entry 3). 90 °C and 2.0 equivalents
of 1 gave complete conversion to 3a within 2h. Notably,
Chart 2. Optimisation of a model system. a not isolated; isolated as the
b
c
d
pyridinium salt; additive used during work-up; microwave irradiation.
Similarly, the use of a sulfate bis(tributylammonium) salt for
2
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steric factors. Differences occurred when converting the
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intermediate into a crystalline sodium salt; these are most
likely due to the variability of precipitation of the sodium salt.
Therefore differences of electron-withdrawing and electron-
donating factors are difficult to draw with certainty. Steric bulk
adjacent to the reacting centre was accommodated with ease
(4h) and the reaction could also be applied to phenolic
alcohols (4i).
Driving an organosulfate ester reaction to completion, by
sulfating all possible hydroxyl sites in a structure remains a
synthetic challenge. This is influenced by anionic crowding,
1
6
making per-sulfation progressively more difficult. Therefore
we applied the conditions to examples of compounds requiring
two or more sulfation events, including natural products, and
compounds containing different hydroxyl moieties within the
same structure to probe the selectivity profile (Chart 4).
Cyclohexane-1,2-diol afforded the disulfate in high conversion
and isolated yield, likewise with glycerol, the tri-sulfated
analogue was prepared with ease (5 and 6, respectively). The
Chart 3. .Application of methodology to benzylic and phenolic alcohols.
1
(Parentheses indicates reaction conversion as measured by H NMR
spectroscopy).
3
reaction of 1 with menthol (7) delivered the NHBu salt with
ease.
reaction completion was achieved, even at 30 °C (with
increased reaction time) demonstrating that 1 could be a
suitable reagent for temperature sensitive substrates such as
proteins.13
To probe the alcohol selectivity profile, a primary alcohol was
functionalised in preference to a phenol in 8. Therefore,β-
estradiol afforded 17-β-estradiol sulfate over the more
common metabolite 3-β-esteradiol in 60% isolated yield (9a)
using 1.5 eq. 1. Using 5.0 eq. of 1 both the 17- and 3-positions
were sulfated in 84% isolated yield (9b).
2
8
We compared the use of two commercially available amine-
SO
3
complexes, namely pyridine and trimethylamine (entries 9
was more reactive than 1, giving
and 12, respectively). Py-SO
3
We then applied the methodology to the original medicinal
chemistry challenge, the sodium salt of glycomimetic C3 (10).
complete conversion to 3a in 0.5 h but only a 17% isolated
yield of the pyridinium species. As a control experiment, the
8
High conversion and an isolated yield of 76% on a 500 mg scale
was achieved. The methodology was applied to the amino
acid, serine, resulting in excellent yields (95% under normal
conditions) and 90% at a non-denaturing temperature (11)
both using 4.0 eq. of 1. Importantly, no loss in enantiomeric
ratio was observed upon sulfation (>99:1).
In summary, we have reported the first scalable preparation
and reaction scoping study of 1 as a mild, bench-stable, and
chromatography-free method to access organic sulfate esters
as their ammonium or sodium salts. The reaction holds
promise with the ability to install up to three sulfate groups on
sequential exchange of the pyridinium salt with different Bu
cations afforded the isolation of 4a in 47% (Bu NI) and 76%
Bu NOAc) yield, respectively (entries 10 and 11). The use of
Me N-SO gave poor results (0% isolated yield of 4a from a
9% conversion to 3a). The addition of Bu NI to the reaction
4
N
4
(
4
3
3
2
4
mixture significantly improved the reaction, affording an 88%
conversion to 3a, potentially via the in situ formation of 1
(entry 13). Unfortunately, a more complex reaction mixture
was detrimental to the isolation of 4a. These results
demonstrate the isolation benefits associated with the
protocol developed with 1.
We observed that 1 can achieve high conversions of 2a to
3
a/4a without microwave irradiation (entries 1-7 vs 14-15)
unlike other reported sulfating agents. The addition of a
hetero or homogenous base (entries 16-19) was investigated
and in all cases this led to a decrease in conversion to 3a. Most
notably, the addition of tributylamine (entry 19) initially
increased the rate conversion to 3a, but reduced the overall
2
4
conversion contradicting the original report. We rationalised
that Et
reaction, forming Et
been observed in the reaction of Et
3
N (with a higher pKa value than Bu
3
N) competes in the
exchange has also
3
NSO in situ. This SO
3
3
2
1
3
N with PySO
3
.
We
applied the optimal conditions to the synthesis of a range of
sulfate esters (Chart 3).
In all examples (Chart 3) we observed a near-quantitative
conversion to the corresponding sulfate ester as the
tributylammonium salt, independent of both electronic and
Chart 4. Reaction scope on diverse alcohols (2.0 eq. of 1 per OH group except where
indicated). a isolated as NBu H salt; b 1.5 eq of 1; 5.0 eq. of 1; reaction performed at
c
d
3
3
8 °C using 4.0 eq. of 1 in DMF . (Parentheses indicate reaction conversion as measured
1
by H NMR spectroscopy).
complex scaffolds including examples where sterics would
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5 The original report of 1$23 used the reaction of NBu
2
2
3
with
limit other methods. Further work to elucidate the scope of
the reaction on human sulfate metabolites is ongoing.
The authors thank Dr Chi Tsang and the Centre for Chemical
and Materials Analysis in the School of Chemistry, University of
Birmingham for analytical support. D.M.G. thanks the
University of Birmingham for a PhD scholarship. The authors
thank Dr J. W. Mueller, Dr K. A. Roper, and Ms. Anna M.
Benedetti for helpful discussions.
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3 4
DOI: 10.1039/C9CC01057B
liquid SO in CCl but no isolated yield or characterisation
data (apart from the melting point) is available.
6 CCDC 1894165 (1) contains the supplementary crystallographic
3
data for this paper. Crystal Data for 1: C12H27NO S (M =265.40
g/mol): trigonal, space group R3c (no. 161), a = 14.3352(2) Å,
3
c = 12.2455(2) Å, V = 2179.28(8) Å , Z = 6, T = 100.01(10) K,
-
1
3
μ(CuKα) = 1.969 mm , Dcalc = 1.213 g/cm , 8776 reflections
measured (16.14° ≤ 2Θ ≤ 147.692°), 977 unique (Rint
=
0
.0273, Rsigma = 0.0115) which were used in all calculations.
1 2
The final R was 0.0209 (I > 2σ(I)) and wR was 0.0570 (all
data).
Conflicts of interest
There are no conflicts to declare.
27 (a) F. A. Kanda, A. J. King, J. Am. Chem. Soc. 1951, 73, 2315-
2
1
319; (b) G. J. Kubas, A. C. Larson, R. R. Ryan, J. Org. Chem.
979, 44, 3867-3871.
2
2
8 The only other report to the preparation of 12a involved a
protection-deprotection cascade: H. Fex, K. E. Lundvall, A.
Olsson, Acta. Chem. Scand. 1968, 22, 254-264.
9 We thank reviewer 2 for this suggestion and the insight it
provided. Chiral HPLC traces are provided in the ESI.
Notes and references
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