Synthesis of N-phthalimido β-aminoethanesulfonyl chlorides: The use of thionyl chloride for a simple and efficient synthesis of new peptidosulfonamide building blocks

Article abstract of DOI:10.1016/j.tetlet.2005.04.011

N-Phthalimido β-aminoethanesulfonyl chlorides, new building blocks for the synthesis of peptidosulfonamide peptidomimetics, were prepared in a straightforward manner from amino acids. In the crucial synthetic step, sulfonic acids or their sodium salts were converted into the corresponding sulfonyl chlorides using an excess of refluxing thionyl chloride or thionyl chloride/DMF. This simple and effective chlorinating method is also applicable to β-aminoethane sulfonic acids and their sodium salts with other N-protecting groups.

Full text of DOI:10.1016/j.tetlet.2005.04.011

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 4069–4072
Synthesis of N-phthalimido b-aminoethanesulfonyl chlorides:
the use of thionyl chloride for a simple and efficient synthesis
of new peptidosulfonamide building blocks
Jan Humljana,b and Stanislav Gobecb,*
a
ˇ
Lek d. d.,Verovskova 45, 1000 Ljubljana, Slovenia
ˇ ˇ
Faculty of Pharmacy,University of Ljubljana, Askerceva 7, 1000 Ljubljana, Slovenia
b
Received 21 February 2005; revised 25 March2005; accepted 5 April 2005
Available online 19 April 2005
Abstract—N-Phthalimido b-aminoethanesulfonyl chlorides, new building blocks for the synthesis of peptidosulfonamide peptido-
mimetics, were prepared in a straightforward manner from amino acids. In the crucial synthetic step, sulfonic acids or their sodium
salts were converted into the corresponding sulfonyl chlorides using an excess of refluxing thionyl chloride or thionyl chloride/DMF.
This simple and effective chlorinating method is also applicable to b-aminoethane sulfonic acids and their sodium salts with other
N-protecting groups.
ꢀ 2005 Elsevier Ltd. All rights reserved.
Bioisosteric replacement of a peptide bond withvarious
surrogates is a widespread strategy for improving bio-
logical activity, physicochemical properties, and stability
of peptides.1–3 In the past, a wide variety of modified
peptides have been developed (e.g., azapeptides,
depsipeptides, retro and retro-inverso peptides, phos-
pha- peptides, ureidopeptides, etc.).4 Among these, pept-
idosulfonamides have been recognized as building
blocks, not only for preparing peptidomimetics, but also
for developing enzyme inhibitors, catalysts, foldamers,
tweezers, etc.5–22 Due to the intrinsic chemical instability
of a-peptidosulfonamides, most of the studies of pep-
tides containing the SO2NH junction are limited to
b-peptidosulfonamides (Fig. 1).23 These possess a geome-
try similar to the tetrahedral intermediate formed during
peptide bond cleavage.24 Their polarity and hydrogen-
bond donation capability are greater than those of the
corresponding natural peptides.13 Additionally, the sta-
bility of peptidosulfonamide peptidomimetics toward
degradation by proteases is significantly increased.25
R
O
α
O
O
O
S
S
N
H
N
N
H
N
β
H
H
R
Figure 1. a- and b-substituted b-aminoethane sulfonamide mimics
of natural amino acids.
having the side chain on the a-carbon withrespect to
the sulfonyl moiety and one with the side chain on the
b-carbon atom (Fig. 1). Most of the research in this field
has been devoted to b-substituted b-sulfonopeptides.
Originally, peptidosulfonamides were synthesized by
coupling b-aminoethanesulfinyl chlorides with C-pro-
tected amino acids, followed by oxidation of the result-
ing sulfinamides to sulfonamides.8,9,24 The oxidation
step did not always result in satisfactory yields.9 In the
most general approach N-protected b-sulfonyl chlorides
are used to prepare sulfonopeptides. They are usually
obtained from N-protected b-sulfonic acids or their so-
dium salts, using chlorinating agents such as PCl5,26,27
phosgene,17,18,20 or triphosgene.9,11,13 The N-protected
b-sulfonic acids are conveniently prepared from readily
available N-protected a-amino acid esters, which are
reduced to alcohols, mesylated, then converted into
the corresponding thioacetates, and finally oxidized to
the target sulfonic acids.18 In these procedures Cbz,
Fmoc, and Boc N-protecting groups have been used.
In principle, two types of b-aminosulfonic acid mimics
of naturally occurring a-amino acids are possible: one
Keywords: Peptidomimetics; Peptidosulfonamides; b-Aminosulfonyl
chlorides.
*
Corresponding author. Tel.: +386 1 476 9500; fax: +386 1 425
8031; e-mail: gobecs@ffa.uni-lj.si
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.04.011

4070
J. Humljan, S. Gobec / Tetrahedron Letters 46 (2005) 4069–4072
O
R
1) NaBH4 - I2, THF,
0 oC to reflux
2) MeOH
MsCl, Et3N,
Phthalic acid -
R
R
anhydride, 140 oC
OH
CH2Cl2, 0 oC to r.t.
OH
N
H2N
H2N
COOH
1a-c
3a-c
2a-c
O
O
O
R
O
R
R
Cs2CO3, HSAc,
DMF, 50 oC
1) H2O2, HOAc, r.t.
2) Pd/C
OMs
SAc
SO3H
N
N
N
5a-c
6a-c
4a-c
O
O
O
O
R
O
R
SOCl2, DMF,
reflux
SOCl2, reflux
SO2Cl
SO3-Na+
N
N
8a-d
7
O
O
Scheme 1. Synthesis of N-phthalimido b-aminoethanesulfonyl chlorides 8a–d.
As a part of our researchon peptidosulfonamide en-
zyme inhibitors we wanted to introduce a new protec-
tion strategy using the N-phthalimido protecting group
(Scheme 1). We started the synthesis with free amino
acids 1 (Ala, Val, Phe), which were reduced to amino-
alcohols 2 using the NaBH4–I2 system28 and phthalo-
ylated with phthalic anhydride to give N-phthalimido
protected aminoalcohols 3 in high yield. In the next
three steps, we slightly modified the procedure of Brou-
wer et al.18 The protected aminoalcohols 3 were
mesylated withmethanesulfonyl chloride and Et 3N in
dichloromethane. In the next step, mesylates 4 were
added to a mixture of thioacetic acid and Cs2CO3 in
DMF and stirred at 50 ꢁC for 24 h. If the reaction was
carried at room temperature, as described originally,18
the yields were low and irreproducible. Furthermore, a
slight excess of Cs2CO3 was needed for clean conversion.
Although we used more vigorous conditions we did not
notice any cleavage of the phthalimido group as was re-
ported in the case of the Fmoc N-protecting group.18
Thioacetates 5 were then oxidized to the corresponding
sulfonic acids 6 using aqueous hydrogen peroxide and
acetic acid and, after 24 hat rt, the excess of peroxide
was destroyed by adding 10% Pd/C. The resulting crude
sulfonic acids 6 were finally refluxed in excess thionyl
chloride to give sulfonyl chlorides 8a–c in high yields
on a 7–35 mmol scale (Table 1).29 We did not observe
any racemization of compounds 2–6 and 8 during this
procedure. The optical purity of all products was con-
firmed by optical rotation. In addition, sulfonyl chlo-
rides 8a–c were coupled withenantiomerically pure
amino acids (e.g., L-Ala(OBzl)) and the products were
examined by 1H NMR (data not shown). No multiplica-
sodium salts in thionyl chloride. Nevertheless, the corre-
sponding sulfonyl chloride 8d could be obtained by a
slight modification of the procedure in which a small
amount of dry DMF was added to the reaction mixture
to achieve clean and rapid chlorination of sodium salt 7
(Scheme 1).30
The crucial step in the synthesis of peptidosulfonamides
is the conversion of sulfonic acids to the corresponding
sulfonyl chlorides. b-Substituted b-aminoethane sulfon-
yl chlorides are usually obtained from sulfonic acids
or their salts using triphosgene9,11,13 or phosgene17,18,20
as chlorinating agents. Both agents give sulfonyl chlo-
rides in high yields but also suffer from some drawbacks.
1
Triphosgene is not detectable by H NMR and TLC
which makes purification of sulfonyl chlorides very dif-
ficult. In addition, it was reported that residual triphos-
gene caused serious problems in the subsequent coupling
reactions, thus lowering overall yields.18 These problems
were overcome by replacing triphosgene with phosgene
which can be easily removed by evaporation in vacuum.
However, phosgene is toxic on acute inhalation expo-
sure and severe respiratory effects have been reported
in humans.32 To circumvent these problems we sought
Table 1. Yields of compounds 3–5 and 8
Starting amino acids 1
R
Yield (%)
3
4
5
8
L-Ala (a)
L-Val (b)
CH3
CH(CH3)2
86
75
96
90
82
71
85a
67a
(<20)b
a
D-Phe (c)
Tau (d)c
CH2Ph96
H
84
80
62
1
(<20)b
<20
(89)d
tion of H NMR signals, due to possible formation of
diastereomers, was observed which provided further
evidence that the procedure described in this paper is
racemization free.
—
—
—
a Overall yields from the conversion of thioacetates 5a–c into sulfonyl
chlorides 8a–c (two steps).
b Yields from the SOCl2 procedure, applying the corresponding
sodium salts of 6.
The reaction of sulfonic acid sodium salt 7 withan
excess of refluxing thionyl chloride was also examined
(Table 1). The reaction proved to be sluggish, most
probably because of the low solubility of sulfonic acid
c The sulfonic acid sodium salt 7 was prepared as described.31
d Yield from the SOCl2 /DMF procedure.

J. Humljan, S. Gobec / Tetrahedron Letters 46 (2005) 4069–4072
4071
Table 2. Synthesis of Fmoc- and Cbz-protected b-aminoethane sulfonyl chlorides from the corresponding sulfonic acids or their salts
Starting materiala
Productb
Chlorinating agent
Yield (%)
SO3H
Fmoc
N
SO2Cl
SO2Cl
Fmoc
SOCl2
70
85
N
H
c
H
13
14
9c
SO3-Na+
Fmoc
Fmoc
N
N
H
SOCl2
H
10
Ph
Ph
Cbz
SO2Cl
SOCl2/DMF
58
75
Cbz
Cbz
SO3-Na+
N
N
H
H
c
c
11
15
Cbz
SO2Cl
SO3-Na+
N
H
N
H
SOCl2/DMF
12
16
a The sulfonic acids or their sodium salts were prepared as described.18
b The compounds were previously prepared by the procedure using phosgene.18
c (S)-enantiomers.
a different chlorinating agent which would be suitable
for activating the sulfonic acid group.
Supplementary data
Supplementary data associated withthis article can be
found, in the online version, at doi:10.1016/j.tetlet.
2005.04.011.
Thionyl chloride has been reported to chlorinate N-pro-
tected taurine, however the reaction was carried out in
dichloromethane solution, starting from the tetrabutyl
ammonium salt.12 Using excess refluxing SOCl2 or
SOCl2/DMF we achieved smooth and convenient con-
version of b-substituted b-aminoethane sulfonic acids
6a–c or sulfonic acid sodium salt 7 into the correspond-
ing sulfonyl chlorides 8a–d. Since the broad applicability
of thionyl chloride as a chlorinating reagent for the
synthesis of peptidosulfonamide building blocks has
not been reported, we investigated the generality of this
interesting reaction by using a series of sulfonic acids or
their sodium salts 9–12 which were N-protected with
either Fmoc- or Cbz-protecting groups. The sulfonic
acids or their sodium salts were refluxed in an excess
of either SOCl2 or SOCl2/DMF for 4 hto give sulfonyl
chlorides 13–16 in high yields on the 7–15 mmol scale
(Table 2).
References and notes
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Liskamp, R. M. J. Bioorg. Med. Chem. Lett. 1996, 6,
3035–3040.
To conclude, the synthesis of N-phthalimido b-amino-
ethanesulfonyl chlorides is presented, which are new
building blocks for the synthesis of peptidosulfon-
amides, an emerging group of peptidomimetics. In the
crucial step, sulfonic acids or their sodium salts were
converted into the corresponding sulfonyl chlorides
using an excess of either SOCl2 or SOCl2/DMF. This
simple and effective chlorinating method is also applica-
ble to b-aminoethane sulfonic acids with other N-pro-
tecting groups.
10. de Bont, D. B. A.; Moree, W. J.; Liskamp, R. M. J.
Bioorg. Med. Chem. 1996, 4, 667–672.
11. Gude, M.; Piarulli, U.; Potenza, D.; Salom, B.; Gennari,
C. Tetrahedron Lett. 1996, 37, 8589–8592.
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N. A.; Gallant, D. L.; Briskin, M. J. Bioorg. Med. Chem.
Lett. 1997, 7, 711–714.
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Eur. J. 1998, 4, 1924–1931.
´
14. Bischoff, L.; David, C.; Roques, B. P.; Fournie-Zaluski,
Acknowledgements
This work was supported financially by the Ministry of
Education, Science and Sport of the Republic of Slove-
nia and Lek Pharmaceuticals d.d. The authors thank
Professor Dr. Roger Pain for critical reading of the
manuscript.
M.-C. J. Org. Chem. 1999, 64, 1420–1423.
15. Calcagani, A.; Ciattini, P. G.; Di Stefano, A.; Dupre, S.;
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Luisi, G.; Pinnen, F.; Rossi, D.; Spirito, A. Farmaco 1999,
54, 673–677.

4072
J. Humljan, S. Gobec / Tetrahedron Letters 46 (2005) 4069–4072
16. Lowik, D. W. P. M.; Liskamp, R. M. J. Eur. J. Org. Chem.
2000, 1219–1228.
17. van Ameijde, J.; Liskamp, R. M. J. Tetrahedron Lett.
2000, 41, 1103–1106.
18. Brouwer, A. J.; Monnee, M. C. F.; Liskamp, R. M. J.
Synthesis 2000, 1579–1584.
19. Monee, M. C. F.; Marijne, M. F.; Brouwer, A. J.;
Liskamp, R. M. J. Tetrahedron Lett. 2000, 41, 7991–
7995.
20. de Jong, R.; Rijkers, D. T. S.; Liskamp, R. M. J. Helv.
Chim. Acta 2002, 85, 4230–4243.
21. Giordano, C.; Lucente, G.; Nalli, M.; Zecchini, G. P.;
Paradisi, M. P.; Varani, K.; Spisani, S. Farmaco 2003, 58,
1121–1130.
S-[(2S)-2-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl) propyl]
ethanethioate (5a): Thioacetate (3.6 ml, 51 mmol) was
added to a suspension of Cs2CO3 (15.25 g, 47 mmol) in
DMF (70 ml). The mesylate 4a (12.05 g, 43 mmol) was
added in one portion to the resulting solution and stirring
was continued at 50 ꢁC for 24 h, prior to which the
reaction flask was covered withaluminum foil. Teh
mixture was poured into distilled H2O (250 ml), and
the aqueous phase extracted with EtOAc (3 · 150 ml). The
combined organic layers were washed with H2O (150 ml),
NaHCO3 (5% w/w, 150 ml), and brine (150 ml). The
organic phase was dried over Na2SO4, filtered, and
evaporated. The resulting residue was purified by column
chromatography (EtOAc/hexane = 1/1) to afford 5a as a
white solid (9.20 g, 82%): Rf = 0.40 (EtOAc/Hex = 1/1);
22. Cama, E.; Shin, H.; Christianson, D. W. J. Am. Chem.
Soc. 2003, 125, 13052–13057.
23
mp 54–57 ꢁC; ½aꢀD +170.1 (c 0.332, MeOH); 1H NMR
23. Paik, S.; White, E. H. Tetrahedron 1996, 52, 5303–5318.
24. (a) Moree, W. J.; van der Marel, G. A.; Liskamp, R. M. J.
Tetrahedron Lett. 1991, 32, 409–412; (b) Moree, W. J.; van
Gent, L. C.; van der Marel, G. A.; Liskamp, R. M. J.
Tetrahedron 1993, 49, 1133–1150.
25. de Bont, D. B. A.; Sliedregt-Bol, K. M.; Hofmeyer, L. J.
F.; Liskamp, R. M. J. Bioorg. Med. Chem. 1999, 7, 1043–
1047.
(300 MHz, CDCl3) d = 1.58 (d, 3H, J = 6.9 Hz, CH3), 2.30
(s, 3H, CH3), 3.40 (dd, 1H, J = 13.9, 5.5 Hz, CH2), 3.52
(dd, 1H, J = 13.9, 9.7 Hz, CH2), 4.42–4.58 (m, 1H, CH),
7.68–7.78 (m, 2H, Pht-H), 7.80–7.90 (m, 2H, Pht-H); IR
(KBr, cmꢁ1) 3453, 2976, 1698, 1466, 1356, 1106, 944, 884,
714, 630; FABMS: m/z = 264 (M+H)+; Anal. Calcd for
C13H13NO3S: C, 59.30; H, 4.98; N, 5.32. Found: C, 59.29;
H, 4.89; N, 5.23.
26. Frankel, M.; Moses, P. Tetrahedron 1960, 9, 289–294.
27. Kircheldorf, R. H.; Leppert, E. Synthesis 1976, 43–45.
28. McKennon, M. J.; Meyers, A. I.; Drauz, K.; Schwarm, M.
J. Org. Chem. 1993, 58, 3568–3571.
(2S)-2-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)-1-propane-
sulfonyl chloride (8a): A mixture of H2O2 (30% w/w in
H2O, 30 ml) and HOAc (60 ml) was added to a solution of
thioacetate 5a (9.05 g, 34.4 mmol) in HOAc (30 ml). After
stirring for 24 hat rt, 10% Pd/C was added to destroy the
excess peroxide. Filtration, concentration, and coevapo-
ration withtoluene (2 · 20 ml) and ether (2 · 20 ml) under
reduced pressure afforded the crude sulfonic acid 6a. Th e
sulfonic acid 6a was dried at 40 ꢁC for 48 hin vacuo over
P2O5 and NaOH and afterwards refluxed in SOCl2 (20 ml)
for 7 h. Excess SOCl2 was removed by evaporation,
followed by coevaporation withtoluene and ether under
reduced pressure. The resulting residue was purified
through a silica plug (CH2Cl2) to afford a white solid 8a
(8.41 g, 85%). Analytical samples of the product were
precipitated from CH2Cl2/hexane: Rf = 0.65 (CH2Cl2/ace-
tone = 18/1); mp 83–85; ½aꢀD +78.1 (c 0.310, MeOH); H
NMR (300 MHz, CDCl3) d = 1.66 (d, 3H, J = 7.2 Hz),
3.97 (dd, 1H, J = 14.5, 3.6 Hz, CH2), 4.77 (dd, 1H,
J = 14.3, 9.8 Hz, CH2), 5.13–5.28 (m, 1H, CH), 7.72–
7.81 (m, 2H, Pht-H), 7.84–7.93 (m, 2H, Pht-H); IR (KBr,
cmꢁ1) 3467, 1776, 1711, 1374, 1169, 1062, 860, 724, 605,
525; EI MS: 287, 289 (M+); Anal. Calcd for
C11H10ClNO4S: C, 45.92; H, 3.50; N, 4.87. Found: C,
46.18; H, 3.52; N, 4.68.
29. The structures of all compounds were confirmed by 1H
NMR, IR, and mass spectroscopy. For all new com-
pounds satisfactory elemental analyses data were obtained.
A typical procedure for preparing compounds 8a–c is
as follows:
(S)-2-Phthalimidopropanol (3a): Phthalic anhydride
(20.00 g, 135 mmol) and (S)-alaninol (9.66 g, 129 mmol)
were fused at 140 ꢁC for 7 h. The reaction mixture was
cooled to rt and the resulting solid dissolved in ethyl
acetate (200 ml). The solution was washed successively
withsaturated aqueous NaHCO (60 ml), H2O (60 ml),
3
citric acid (10% w/w, 60 ml) and brine (60 ml). Drying
(Na2SO4), followed by concentration in vacuo, afforded
compound 3a (22.70 g, 86%) as a white solid: Rf = 0.48
23
1
23
(CHCl3/MeOH = 9/1); mp 79–82 ꢁC (lit.33 mp 77 ꢁC); ½aꢀD
+32.7 (c 0.312, MeOH); 1H NMR (300 MHz, CDCl3):
d = 1.47 (d, 3H, J = 7.1 Hz, CH3), 2.70 (br s, 1H, OH),
3.91 (dd, 1H, J = 11.8, 3.8 Hz, CH2), 4.05 (dd, 1H,
J = 11.7, 7.5 Hz, CH2), 4.45–4.63 (m, 1H, CH), 7.70–
7.78 (m, 2H, Pht-H), 7.82–7.90 (m, 2H, Pht-H); FAB MS:
m/z = 206 (M+H)+.
(2S)-2-(1,3-dioxo-1,3-dihydro-2H-isoindol-2-yl)propyl
methanesulfonate (4a): To a solution of alcohol 3a (9.76 g,
48 mmol) in CH2Cl2 (150 ml) was added Et3N (8.0 ml,
57 mmol). After cooling to 0 ꢁC, methanesulfonyl chloride
(4.5 ml, 57 mmol) was added dropwise. Stirring was
continued overnight at rt, followed by addition of CH2Cl2
(100 ml). The mixture was washed with NaHCO3 (5%
w/w, 2 · 100 ml), H2O (2 · 100 ml), and brine (80 ml). The
organic phase was dried over Na2SO4, filtered and
evaporated. The mesylate 4a was crystallized from ethyl
acetate/hexane. Crystallization afforded white crystals
(12.90 g, 96%): Rf = 0.64 (CHCl3/MeOH = 9/1); mp 71–
74 ꢁC; ½aꢀD +34.0 (c 0.315, MeOH); H NMR (300 MHz,
CDCl3) d = 1.53 (d, 3H, J = 6.8 Hz, CH3), 2.99 (s, 3H,
CH3), 4.45 (dd, 1H, J = 9.8, 4.4 Hz, CH2), 4.68–4.90 (m,
2H, CH2 + CH), 7.71–7.80 (m, 2H, Pht-H), 7.82–7.91 (m,
2H, Pht-H); IR (KBr, cmꢁ1) 3012, 1771, 1709, 1467, 1354,
1170,1042, 992, 821, 719, 517; FAB MS: m/z = 284
(M+H)+; Anal. Calcd for C12H13NO5S: C, 50.87; H,
4.63; N, 4.94. Found: C, 51.16; H, 4.70; N, 4.96.
30. A typical procedure: To a cooled (ice bath) mixture of
sulfonic acid sodium salt 7 (5.00 g, 17.9 mmol) and excess
thionyl chloride (10 ml) was added DMF (1 ml) dropwise.
The mixture was heated at reflux for 5 h. The chlorinating
species was removed by evaporation, followed by coeva-
poration withtoluene and ether under reduced pressure.
The residue was dissolved in EtOAc (100 ml) and washed
withH 2O (60 ml), saturated aqueous NaHCO3 (60 ml)
and brine (50 ml). The organic phase was dried over
Na2SO4, filtered, evaporated, and the residue purified
through a silica plug (CH2Cl2) to afford sulfonyl chloride
8d as a white solid (4.90 g, 89%); mp 160–162 ꢁC (lit.31 mp
159–162 ꢁC).
23
1
31. Winterbottom, R.; Clapp, J. W.; Miller, W. H.; English, J.
P.; Roblin, R. O. J. Am. Chem. Soc. 1947, 69, 1393–1401.
32. e-EROS Encyclopedia of Reagents for Organic Synthesis.
John Wiley
& Sons, Ltd (http://www3.interscience.
wiley.com/cgi-bin/mrwhome/104554785).
33. Casara, P.; Danzin, C.; Metcalf, B.; Jung, M. J. Chem.
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