A new synthesis of difluoromethanesulfonamides - A novel pharmacophore for carbonic anhydrase inhibition

Article abstract of DOI:10.1039/b416642f

Preparation of the key intermediate carboxydifluoromethanesulfonamide provides direct synthetic access to a wide range of novel difluoromethanesulfonamides, including the acetazolamide analogue (2-ethanoylamino-1,3,4-thiadiazol-5-yl)-difluoromethanesulfonamide. Their water solubility and stability, ether partition coefficient, pK, and submicromolar dissociation constants for human carbonic anhydrase isozyme II (HCA II) make them promising candidates for topical glaucoma therapy.

Full text of DOI:10.1039/b416642f

View Article Online / Journal Homepage / Table of Contents for this issue
C O M M U N I C A T I O N
A new synthesis of difluoromethanesulfonamides–a novel
pharmacophore for carbonic anhydrase inhibition†
b
Nicholas A. Boyle,‡a W. Richard Chegwidden§ and G. Michael Blackburn*a
a Krebs Institute, Chemistry Department, Sheffield University, Sheffield, UK S3 7HF.
E-mail: g.m.blackburn@shef.ac.uk; Fax: +44 114 222 9346; Tel: +44 114 222 9462
b Sheffield Hallam University, UK S1 1WB
Received 2nd November 2004, Accepted 25th November 2004
First published as an Advance Article on the web 15th December 2004
Preparation of the key intermediate carboxydifluo-
romethanesulfonamide provides direct synthetic access to a
wide range of novel difluoromethanesulfonamides, includ-
ing the acetazolamide analogue (2-ethanoylamino-1,3,4-
thiadiazol-5-yl)-difluoromethanesulfonamide. Their water
solubility and stability, ether partition coefficient, pKa and
submicromolar dissociation constants for human carbonic
anhydrase isozyme II (HCA II) make them promising
candidates for topical glaucoma therapy.
to the difluoromethanesulfonamides 3. The sulfonylation was
accomplished in good yield for a range of amine-substituted
thiadiazoles 2a–e using aqueous sodium bisulfite (Scheme 1).
Unfortunately, all efforts to convert these sulfonic acids or their
salts into the respective sulfonamides using electrophilic agents
(SOCl2, POCl3, PCl5, Tf2O, etc.) followed by liquid ammonia
treatment failed. Reports of the transformation of simple
difluoromethanesulfonic acids into sulfonamides via sulfonyl
chlorides are limited.6
To date, virtually all sulfonamide compounds evaluated as car-
bonic anhydrase inhibitors, CAIs, share the common structural
feature of a primary sulfonamide group (SO2NH2) attached
to a substituted or unsubstituted aromatic or heteroaromatic
moiety (ArSO2NH2). Acetazolamide has long been the CAI of
choice for glaucoma therapy despite its poor water solubility.
Consequently, there has been a systematic search for CAIs with
improved water solubility as agents for the topical treatment
of glaucoma.1 The inherent problem is the three-way balance
between the acidity of the sulfonamide function, the water
solubility and the lipophilicity2 needed to provide the requisite
bioavailability. Thus, while trifluoromethanesulfonamide has
been found to be one of the most potent inhibitors of carbonic
anhydrase isozyme II (CA II) and is very water soluble, toxicity
and limited bioavailability have denied its clinical development.3
Moreover, its use as a lead compound has been held back by the
problems of synthesis of a-fluoro-sulfonamides.4
Scheme 1 Reagents and conditions: i) Na2SO3 aq., dioxane, rt, 5 h; ii)
POCl3 etc.; iii) NH3, −78 ◦C.
We therefore chose to approach the desired synthesis by
constructing the heterocyclic ring onto a preformed difluo-
romethanesulfonamide entity. Retrosynthetic analysis identified
carboxydifluoromethanesulfonamide 9 as a suitable intermedi-
ate and we accomplished its synthesis in six steps and 46%
overall yield from inexpensive chlorodifluoroethanoic acid 4
(Scheme 2).7 The carboxyl group of 9 shows normal reactivity,
being converted into its methyl ester in 90% yield on heating
with methanolic HCl.
The inhibitory potency of CF3SO2NH2 has been attributed in
part to its very strongly acidic sulfonamide, pKa 6.3. Given that
CH3SO2NH2, pKa 10.3, is a poor CA inhibitor, the difference
in activity must be a direct result of the electronegativity of
the CF3 group and thereby satisfies one of the established
prime requirements5 for an effective CAI, namely a sulfonamide
function of low pKa. We therefore sought to link a proven
heterocyclic pharmacophore, Het, with the difluoromethylene-
sulfonamide group, -CF2SO2NH2, to generate a novel range of
CA II inhibitors of general structure HetCF2SO2NH2. In this
work, we have employed the 1,3,4-thiadiazole to provide the
Het function for a number of viable CAIs.1
We have recently achieved the first successful syntheses
of aryldifluoromethanesulfonamides by direct fluorination of
arylmethanesulfonamides.4 We next sought to develop a method
that would be more general for a range of functionalised
heterocyclic methanesulfonamides, especially in the amino-
1,3,4-thiadiazole series. Our initial approach involved the
synthesis of 5-chlorodifluoromethyl-2-amino-1,3,4-thiadiazole
1a and its derivatives and their sulfonylation to 2 leading
Scheme 2 Reagents and conditions: i) PhCH2SNa, dioxane; ii) POCl3
etc.; iii) PhNHMe, −78 ◦C; iv) Cl2–AcOH–H2O, 0 ◦C, 1 h; v) NH3 (liq.),
2 h; vi) HCl conc., reflux, 16 h.
More significantly, 9 was condensed with thiosemicarbazide
in the presence of phosphoryl chloride to give (2-amino-1,3,4-
thiadiazolyl)-difluoromethanesulfonamide† 3 in 33% yield. The
amino function of 3 was readily converted into the acetamide
10 (the difluoromethylene homologue of acetazolamide) and
the trifluoroacetamide 11 by treatment with the corresponding
carboxylic anhydride (Scheme 3).
In other studies, attention has been focused on achieving water
solubility by the incorporation of charged functions in the acyl
side chain of acetazolamide, including aminoacyl functions8
and carboxylic acids.9 We therefore prepared a further six
derivatives of 3 incorporating glycyl 12, b-alanyl 13, succinyl
14 and 3-chloropropionyl 15 functions to provide additional
† Electronic supplementary information (ESI) available: characterisa-
tion of all compounds by HRMS or combustion analysis and by 1H, 13C,
and 19F NMR. See http://www.rsc.org/suppdata/ob/b4/b416642f/
‡ Present address: Biota Research Laboratories (U.S.), Carlsbad Re-
search Center, 2232 Rutherford Road, Carlsbad, CA 92008, USA.
§ Present address: Lake Erie College of Osteopathic Medicine, 1858 West
Grandview Blvd., Erie, PA 16509, USA.
2 2 2
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 2 2 – 2 2 4
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
©

View Article Online
species (Table 1). They show a 300-fold range but with several
values close to unity, especially for the most powerful CA
inhibitors.
Dissociation constants of inhibitors for human CA II
were measured by a fluorimetric competition method12 accu-
rate to 0.5%. Kd values for these novel a,a-difluoromethane-
sulfonamides range from 15 to 210 nM (Table 1), which bracket
the value of 50 nM for difluoromethanesulfonamide.3 No other
alkanesulfonamides remotely approach such affinity for CA II.1
For comparison, the desfluoro-isostere of 10 has Kd 2.5 lM,
that is a 12-fold increase resulting from the loss of the two
fluorines adjacent to the sulfonamide.
Scheme 3 Reagents and conditions: i) NH2CSNHNH2, POCl3, 70 ◦C,
3 h; ii) (RCO)2O, rt, 12 h.
thiadiazolyl-a,a-difluoromethanesulfonamides (Fig. 1) along
with carboxamidodifluoromethanesulfonamide 16.
Taken together, the results described here show that the
difluoromethanesulfonamide group delivers high affinity ligands
for CA II and it can be considered a new sulfonamide pharma-
cophore of general potential. Its major significance is that it
effectively uncouples the desired sulfonamide acidity and drug
lipophilicity for CAI activity that hitherto have been achieved
only with arenesulfonamides and heteroarenesulfonamides.1,3
Moreover, it can readily be incorporated into other glaucoma
agents, such as ethoxzolamide and methazolamide,1 and it may
well have potential for incorporation into analogues of other
sulfonamide therapeutic agents.
Maren has established a near linear inverse correlation
between sulfonamide pKa and logKi for CA II over five decades.5
Most importantly, the identification of other binding affinities
in the non-sulfonamide moiety has enabled the successful de-
velopment of anti-glaucoma drugs with over 100-fold improved
affinity for CA II over that predicted from their sulfonamide
pKa.5 Of the compounds described here, 3, 14, and 15 identify
opportunities for such further progress in the use of a,a-
difluoroalkanesulfonamides as CAI agents.
Fig. 1 N-2-Substituted thiadiazolyldifluoromethanesulfonamides.
Sulfonamide pKa values were determined by potentiometric
titration for trifluoromethanesulfonamide and compounds 8–16
(25 ◦C, 0.1 M salt) and overlapping pKas were deconvoluted
using KaleidagraphTM (Table 1). For the thiadiazoles 10 and
14, pKas were also determined spectroscopically from UV scans
using various buffers in the range pH 5–10, especially to discrim-
inate between the ionisations of sulfonamide and carboxamide
functions. Clearly, insertion of the CF2 group next to the
sulfonamide markedly enhances its acidity (Table 1). A DpKa
increment of +2.2 for acetazolamide (relative to 10) was matched
by similar increments for other thiadiazolesulfonamides of
+2.2 for glyazolamide10 (relative to 12), +1.9 for betazolamide
(relative to 13) and +2.3 for chlorpropazolamide (relative to
15). These are well in line with the incremental effects of a-
fluorination on sulfonamide pKa values for CH3SO2NH2 (10.8),
CFH2SO2NH2 (9.32),11 CF2HSO2NH2 (8.06)11 and CF3SO2NH2
(6.3).
Finally, it is noteworthy that the observed affinities of these
difluoromethanesulfonamides for CA II relate well to those of a
family of 2-(aryloxy)-ethyl sulfamates, ArOCH2CH2OSO2NH2,
the best of which have Ki values in the 5 to 50 nM range.13
This comparison suggests that in the sulfonamide series, as for
the phosphonic acids, the CF2 group can be considered as an
isopolar and isosteric surrogate for oxygen.14
Water solubilities were determined spectroscopically in phos-
phate buffered saline, PBS, at pH 7.2 and 25 ◦C for the
thiadiazoles and gravimetrically for other sulfonamides. The
data (Table 1) shows that the marked hydrophobic character
of fluoromethylene groups does not have an adverse effect
on water solubility of the a,a-difluoromethanesulfonamides.
Indeed, many of these compounds have solubilities suitable for
their application as topical agents in aqueous solution.
Ether partition coefficients were measured for equilibria
between diethyl ether and PBS at 25 ◦C and pH 7.2 and corrected
for ionisation of the sulfonamide, with the zwitterionic forms of
12 and 13 and the carboxylate of 9 and 14 taken as uncharged
Acknowledgements
We thank the University of Sheffield and Sheffield Hallam
University for a Studentship (to NAB).
Notes and references
1 U. F. Mansoor, X.-R. Zhang and G. M. Blackburn, in The Carbonic
Anhydrases: New Horizons, ed. W. R. Chegwidden, N. D. Carter and
Y. H. Edwards, Birkhauser Verlag, Basel, 2000, 437–459.
2 A. Jain, G. M. Whitesides, R. S. Alexander and D. W. Christianson,
J. Med. Chem., 1994, 37, 2100–5.
Table 1 Some properties of fluoromethanesulfonamides
Sulfonamide
pKa
Water sola/wv%
Pether (pH 7.2)
Kd (CAI)/nM
Acetazolamide
Dorzolamide
CF3SO2NH2
3
8
8.9
8.4
6.3
6.8
7.2
8.3
6.7
6.5
6.8
6.9
6.7
6.8
7.0
0.17
2.17
> 10
2.83
0.07
> 10
0.21
0.66
> 10
1.30
> 10
0.36
> 5
0.10
2.0b
9.3
8.0c
<2
0.003
0.72
5.39
0.03
1.08
1.37
0.075
1.05
0.33
0.98
0.3
15.1
186
n.d.
208
150
122
117
45.4
31.7
40
9 HOCOCF2SO2NH2
10
11
12
13
14
15
16 H2NCOCF2SO2NH2
a Determined at pH 7.2. b Pchloroform reported1 . c Kd value reported.1
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 2 2 – 2 2 4
2 2 3

View Article Online
3 T. H. Maren and C. W. Conroy, J. Biol. Chem., 1993, 268, 26233–
9.
4 G. M. Blackburn and H. Tu¨rkmen, Org. Biomol. Chem., 2005, DOI:
10.1039/b417327a; B. Hill, Y. Liu and S. D. Taylor, Org. Lett., 2004,
6, 4285–4288.
8 G. D. S. Jayaweera, S. A. MacNeil, S. F. Trager and G. M. Blackburn,
Bioorg. Med. Chem. Lett., 1991, 1, 407–10.
9 S. Antonaroli, A. Bianco, M. Brufani, L. Cellai, G. Lo Baido, E.
Potier, L. Bonomi, S. Perfetti, A. I. Fiaschi and G. Segro, J. Med.
Chem., 1992, 35, 2697–2703.
5 T. H. Maren, Drug Dev. Res., 1987, 10, 255–276.
6 (a) Difluoromethanesulfonic acid has been converted into difluoro-
methanesulfonanilide with PCl5 followed by treatment’ with aniline;
(b) W. V. Farrar, J. Chem. Soc., 1960, 3058–62.
7 (a) Since the completion of this work, 2-(aminosulfonyl)-2,2-difluoro-
ethanamide has been advertised commercially but no synthetic
details are provided (Ambinter Screening Library. Ambinter SARL,
50 avenus de Versailles, F-75016 Paris, France); (b) see also G. A.
Sokol’skii, M. A. Dmitriev and I. L. Knunyants, IAN SSSR Otd.
Khim. Nauk, 1961, 621–2.
10 Glyazolamide, betazolamide, and chlorpropazolamide have been
prepared and analysed; X.-R. Zhang, PhD Thesis, Sheffield Uni-
versity, 1995.
11 R. D. Trepka, J. W. Belisle and J. K. Harrington, J. Org. Chem., 1974,
39, 1094–8.
12 S. K. Nair, J. F. Krebs, D. W. Christianson and C. A. Fierke,
Biochemistry, 1995, 34, 3981–4.
13 T. H. Maren, A. Bar-Ilan, C. W. Conroy and W. F. Brechue, Exp. Eye
Res., 1990, 50, 27–36.
14 G. M. Blackburn, Chem. Ind. (London, UK.), 1981, 134.
2 2 4
O r g . B i o m o l . C h e m . , 2 0 0 5 , 3 , 2 2 2 – 2 2 4