Synthesis of β-aminoethanesulfonyl fluorides or 2-substituted taurine sulfonyl fluorides as potential protease inhibitors

Article abstract of DOI:10.1016/j.tetlet.2009.02.130

Substituted taurine sulfonyl fluorides derived from taurine or protected amino acids are conveniently synthesized from β-aminoethanesulfonyl chlorides using KF/18-crown-6 or from β-aminoethanesulfonates using DAST.

Full text of DOI:10.1016/j.tetlet.2009.02.130

Tetrahedron Letters 50 (2009) 3391–3393
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of b-aminoethanesulfonyl fluorides or 2-substituted taurine
sulfonyl fluorides as potential protease inhibitors
*
Arwin J. Brouwer, Tarik Ceylan, Tima van der Linden, Rob M. J. Liskamp
Department of Medicinal Chemistry and Chemical Biology, Utrecht Institute for Pharmaceutical Sciences, Faculty of Science, Utrecht University, PO Box 80082,
NL-3508 TB Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
Substituted taurine sulfonyl fluorides derived from taurine or protected amino acids are conveniently
synthesized from b-aminoethanesulfonyl chlorides using KF/18-crown-6 or from b-aminoethanesulfo-
nates using DAST.
Received 2 January 2009
Revised 8 February 2009
Accepted 18 February 2009
Available online 21 February 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Probably one of most well-known and commonly used protease
inhibitors is PMSF (phenyl methyl sulfonyl fluoride). This com-
pound or the less known AEBSF (4-(2 aminoethyl)-benzenesulfonyl
fluoride hydrochloride, Fig. 1) is often present in protease inhibitor
‘cocktails’1 or is added together with, for example, leupeptin, dur-
ing isolation or handling procedures of proteins to prevent their
premature proteolytic degradation.2 PMSF is capable of inhibition
of a broad spectrum of serine proteases, for example, chymotryp-
sin, trypsin and thrombin. It is cheap and relatively easy to handle.
Its moderate reactivity prevents rapid (chemical) degradation by
hydrolysis or fast reaction with a variety of nucleophiles. In this re-
spect, the sulfonyl fluoride, which is the reactive moiety, is suitable
for incorporation into potential protease inhibitors. Despite this,
and to our knowledge, the synthesis of other functionalized sulfo-
nyl fluorides has not been described in the literature together with
applications as protease inhibitors or in other areas. This may be
due to a relative lack of synthetic procedures for the mild prepara-
tion of aliphatic sulfonyl fluorides.
It was decided to start with the most simple sulfonyl fluoride
derived from taurine. To this end, the amino group of taurine (1)
was Cbz-protected using Cbz-chloride and NaOH (Scheme 1) lead-
ing to sodium sulfonate 2, which was treated with a solution of
phosgene in toluene to afford sulfonyl chloride 3a. In an initial at-
tempt to convert the sulfonyl chloride into a sulfonyl fluoride, TBAF
in THF was used,4 which afforded sulfonyl fluoride 4a in only 31%
yield. Although the TBAF solution was dried over molecular sieves
before use, a significant amount of the sulfonyl chloride was
hydrolyzed. Also, it was difficult to monitor the reaction on TLC
since both the sulfonyl chloride and sulfonyl fluoride ran and
stained similarly. Furthermore, repetition of this reaction showed
that the yield was not reproducible (0–73%), probably because of
the varying quantities of residual water in the TBAF solution.
Therefore, it was decided to try a procedure using dry potassium
fluoride and 18-crown-6 in acetonitrile,5 which afforded sulfonyl
fluoride 4a in a good yield of 70%. The progress of the reaction
was monitored from the shift of the SO2CH2 signals in the 1H
NMR spectrum. After developing this successful procedure, we
were interested in whether it could also be applied to the crude
Recently, our successful efforts towards an efficient synthesis of
amino acid-derived sulfonyl chlorides3 provided us with a possible
attractive entry for obtaining functionalized, amino acid-based sul-
fonyl fluorides for possible future use as selective protease inhibi-
tors and for proteomics applications. Our results are reported
herein.
Figure 1. The structures of PMSF and AEBSF.
* Corresponding author. Tel.: +31 30 2537396/+31 30 253 7307; fax: +31 30
2536655.
E-mail address: R.M.J.Liskamp@uu.nl (R.M.J. Liskamp).
Scheme 1. Synthesis of taurine-derived sulfonyl fluoride 4a.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.02.130

3392
A. J. Brouwer et al. / Tetrahedron Letters 50 (2009) 3391–3393
sulfonyl chloride in a one-pot reaction. Thus, sodium sulfonate 2
was converted to sulfonyl chloride 3a and subsequently treated
with potassium fluoride and 18-crown-6 after evaporation of ex-
cess phosgene and solvents. Sulfonyl fluoride 4a was obtained in
60% yield, which was higher than that of the procedure with puri-
fication of the intermediate sulfonyl chloride 3a (53%). The higher
yield combined with omitting one purification step made this the
procedure of choice. Although having found a good two-step,
one-pot procedure for the preparation of the amino acid-based sul-
fonyl fluoride, we were left wondering whether diethylamino sul-
fur trifluoride (DAST)6 could also be used to transform sodium
sulfonate 2 directly into sulfonyl fluoride 4a. This was indeed pos-
sible and 4a was obtained, albeit in a lower yield (42%). Therefore,
it was decided to use the former two-step, one-pot procedure with
KF via the crude sulfonyl chloride.
After preparation of taurine-derived sulfonyl fluoride 4a, the
next step was the preparation of sulfonyl fluorides derived from
a variety of side-chain-containing amino acids. Thus, Cbz-pro-
tected amino acids 5b–e (Ala, Val, Leu and Phe) were first reduced
to the corresponding alcohols 6b–e (Scheme 2) using sodium boro-
hydride treatment of the in situ prepared mixed anhydride.7
Mesylates 7b–f were obtained in reasonable to high yields (Ta-
ble 1) by reaction of alcohols 6b–f with methanesulfonyl chloride
in the presence of base (Et3N). Next, thioacetates 8b–f were pre-
pared in good to high yields (65–91%) by reaction with in situ pre-
pared cesium thioacetate. Oxidation of the thioacetates using
aqueous hydrogen peroxide and subsequent addition of sodium
acetate afforded sodium sulfonates 9b–f. These sodium sulfonates
were treated with a phosgene solution, and after evaporation, the
crude sulfonyl chlorides 3b–f were reacted directly with potassium
fluoride in the presence of 18-crown-6 in acetonitrile. Unfortu-
nately, the yields (Table 1) were much lower compared to tau-
rine-derived sulfonyl fluoride 4a (Scheme 1). Due to the more
complex 1H NMR spectra combined with some impurities, it was te-
dious to monitor these reactions. Therefore, it was decided to per-
form the substitution reaction with potassium fluoride and 18-
crown-6 after purification of the sulfonyl chlorides. The reactions
were now monitored more easily and sulfonyl fluorides 4b, 4d
and 4e were isolated in acceptable to good yields (45–67%). Using
the TBAF procedure, a sulfonyl fluoride containing an Fmoc-protect-
ing group was prepared (4f); this is useful since most of the earlier
prepared amino acid-based sulfonyl chlorides were Fmoc-pro-
tected.3 However, the yield was only moderate (30%), probably
due to removal of the Fmoc group by the basic fluoride ion.
For comparison with taurine-derived sulfonyl fluoride 4a, so-
dium sulfonates 9b–e were reacted with DAST in dichloromethane.
Surprisingly, the yields of sulfonyl fluorides 4b–e (40–65%, two
steps from the thioacetates (8b–e)) obtained (Table 1) were similar
or even higher than the yield of 4a (42%). In addition, the yields
with DAST were higher than the yields obtained via the KF/crown
ether procedure (20–58%, three steps from the thioacetates (8b–
e)), thereby providing a good alternative procedure. Furthermore,
by using DAST, the acidic phosgene reaction is avoided, and there-
fore acid labile protecting groups for functionalized amino acids
can be used.
1) NMM, DME
O
iBuOC(O)Cl
H
H
PN
PN
OH
OH
2) NaBH4,H2O
-15 ºC
R
R
5b-f
6b-f
MsCl, Et3N
CH2Cl2, 0 ºC
Cs2CO3, HSAc
DMF, r.t.
H
H
PN
PN
OMs
SAc
Cl
R
R
7b-f
8b-f
COCl2/toluene
CH2Cl2, DMF, r.t.
H
H
1) H2O2, HOAc
2) NaOAc, Pd/C
PN
ONa
PN
S
S
O
O
O
O
R
R
9b-f
3b-f
CH2Cl2, r.t
DAST
KF, CH3CN
18-crown-6
4
b
c
d
e
f
R
CH3
CH(CH3)2
CH2CH(CH3)2
CH2Ph
Amino Acid
Ala
P
H
Cbz
Cbz
Cbz
Cbz
Fmoc
PN
F
S
O
Val
Leu
Phe
Val
O
R
4b-f
CH(CH3)2
Scheme 2. Synthesis of amino acid-based sulfonyl fluorides 4b–f starting from
N-protected amino acids.
Table 1
Yields (%) for the synthesis of sulfonyl fluorides 4b–f from Cbz- or Fmoc-protected
amino acids
Amino acid
7
8
3a
4
4a,d
Cbz-Ala-OH (b)
Cbz-Val-OH (c)
Cbz-Leu-OH (d)
Cbz-Phe-OH (e)
Fmoc-Val-OH (f)
82
50
—
91
61
51
44
87e
71
65c
nd
65
40
nd
62
nd
65
58a
86e
86
45c
67c
30b
80e
90
For future biological evaluation of the prepared amino acid-
based sulfonyl fluorides 4a–f, it was decided to check the reactivity
towards different nucleophiles. To this end, sulfonyl fluoride 4a
was subjected to reactions with an alkoxide, a primary amine
and a secondary amine (Scheme 3).
a
Yield over two steps.
Yield obtained using the TBAF procedure.
Yield obtained using the KF/18-crown-6 ether procedure.
Yield obtained using the DAST procedure.
Ref. 4.
b
c
d
e
Scheme 3. Reactions of sulfonyl fluoride 4a with different nucleophiles.

A. J. Brouwer et al. / Tetrahedron Letters 50 (2009) 3391–3393
3393
phenylalanine-derived sulfonyl chloride 3e using NMM as the base
to give sulfonyl fluoride 12 in high yield (73%, two steps). The abil-
ity of the sulfonyl fluoride to resist these harsh deprotection condi-
tions points to its relative stability as a ‘reactive’ group.
In conclusion, we have developed a successful synthesis of
substituted b-aminoethanesulfonyl fluorides, which can be pre-
pared, starting in principle, from any Cbz- or Fmoc-protected ami-
no acid. Different procedures were used for fluorination of which
DAST and KF/18-crown-6 were the best. By using DAST for intro-
duction of the fluorine atom, strong acidic conditions were
avoided, which allows the use of acid labile protecting groups pres-
ent in functionalized amino acids. Under present investigation are
the protease inhibitory properties of these new functionalized sul-
fonyl fluorides.
Scheme 4. Modification of the N-terminus of sulfonyl fluoride 4a.
References and notes
When sulfonyl fluoride 4a was treated with sodium ethoxide in
ethanol, full conversion was observed after stirring for only 5 min
at room temperature. Treatment with benzylamine in dichloro-
methane gave only 30% conversion after 18 h of stirring at room
temperature. A higher conversion was expected after reaction with
the more nucleophilic piperidine, and after 18 h at room tempera-
ture, all the sulfonyl fluoride had been consumed. These test-reac-
tions with sulfonyl fluoride 4a clearly showed the potential of the
amino acid-based sulfonyl fluorides 4a–f as possible serine and
cysteine protease inhibitors. Since they react slowly with primary
and secondary amines compared to alkoxides they will probably
also react slowly with nitrogen group-containing residues present
in enzymes, for example, lysine, histidine and tryptophan.
It is expected that selective protease inhibitors can be prepared
by further functionalization of the sulfonyl fluorides at the N-ter-
minus. Therefore, the Cbz-group was cleaved from taurine sulfonyl
fluoride 4a using HBr in acetic acid (Scheme 4).
After treatment with HBr, an ion exchange resin was used to ob-
tain the non-hygroscopic hydrochloride salt 10. Taurine-derived
hydrochloride salt 10 was coupled to Boc-Phe-OH using BOP and
DiPEA as a base and sulfonyl fluoride 11 was obtained in high yield
(80%, two steps). Hydrochloride salt 10 was also reacted with
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