N-Hydroxy sulfonimidamides as new nitroxyl (HNO) donors

Article abstract of DOI:10.1016/j.bmcl.2005.02.082

Chlorination and condensation of simple sulfinamides with O-benzyl and O-tert-butyl dimethyl siloxy hydroxylamine gives O-protected N-hydroxy sulfonimidamides. Deprotection of these compounds produces the corresponding sulfinamide and nitrous oxide, which

Full text of DOI:10.1016/j.bmcl.2005.02.082

Bioorganic & Medicinal Chemistry Letters 15 (2005) 2331–2334
N-Hydroxy sulfonimidamides as new nitroxyl (HNO) donors
Richard L. Pennington,a Xin Shab and S. Bruce Kingb,*
aDepartment of Chemistry, College of St. Mary, 1901 S. 72nd Street, Omaha, NE 68124-2377, USA
bDepartment of Chemistry, Salem Hall, Wake Forest University, Winston-Salem, NC 27109, USA
Received 26 January 2005; revised 24 February 2005; accepted 28 February 2005
Available online 25 March 2005
Abstract—Chlorination and condensation of simple sulfinamides with O-benzyl and O-tert-butyl dimethyl siloxy hydroxylamine
gives O-protected N-hydroxy sulfonimidamides. Deprotection of these compounds produces the corresponding sulfinamide and
nitrousoxide, which providesevidence for the intermediacy of nitroxyl (HNO) and identifiesthese compoundsasnew potential
HNO donors.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
work revealsa unique redox role for the H 4B co-factor.3
A number of potential mechanisms featuring activated
iron heme-based oxidants and both oxygen and nitrogen
centered radicals exist and have been summarized.4 These
mechanisms generally include a peroxy-iron species that
reactswith either NG-hydroxy-L-arginine or an NG-hy-
droxy-L-arginine-derived radical to give a tetrahedral
intermediate (1) that decomposes to L-citrulline and
NO.4 Given the presence of tetrahedral intermediates in
these proposals, tetrahedral transition state analogs of
NG-L-hydroxyarginine, such as the L-arginine derived
N-hydroxy sulfonimidamide (2) may be useful mechanis-
tic probesfor the NOS catalyzed converison of NG-L-
hydroxyarginine to NO and potential NOS inhibitors.
Also, the recent identification of nonamino acid derived
N-hydroxyguanidinesascompetent NO producing sub-
strates of NOS indicate that simple N-hydroxy sulfonimi-
damides, such as 3, may also act as transition state
analogs.5 We report synthetic approaches toward N-hy-
droxy sulfonimidamides and their unexpected rapid
decomposition to the corresponding sulfinamide and ni-
troxyl (HNO), the one-electron reduced form of NO.
These results identify N-hydroxy sulfonimidamides as a
The nitric oxide synthases (NOS) catalyze the oxidation
of the terminal guanidino group of L-arginine to nitric
oxide (NO), a neutral molecule that playsimportant
roles in blood pressure control, neurotransmission,
and the immune response (Scheme 1).1 The first step
in the conversion of L-arginine to NO, N-hydroxylation
of L-arginine to NG-L-hydroxyarginine, requirestwo
NADPH-derived electronsto activate molecular oxygen
and findsprecedence in the cytochrome P450 catalyzed
N-hydroxylation of amidines.2 Thisconverison follows
a classic P450 type mechanism including formation of
peroxo and perferryl iron heme complexes, hydrogen
atom abstraction, and oxygen rebound.1
The second step, oxidation of NG-L-hydroxyarginine to
NO and L-citrulline isnot aswell understood and recent
H2N
HN
H2N
HN
NOS
O2, NADPH
H2N
H2N
HN
NH
NOH
O
NOS
O2, NADPH
H2N
N
O
+
nitric
H2N
oxide
O
O
O
Fe
HO
HO
HO
H2N
HN
R
O
N
O
O
O
R
O
S
L-NG-Hydroxyarginine
S
L-Arginine
L-Citrulline
HN OH
HN
HN OH
HN
Scheme 1. NOS catalyzed oxidation of L-arginine to NO.
H2N
HO
H2N
O
O
Keywords: Nitroxyl; Nitric oxide; PilotyÕsacid; Nitric oxide synthase.
Corresponding author. Tel.: +1 336 758 5774; fax: +1 336 758
4656; e-mail: kingsb@wfu.edu
HO
*
1, Proposed Tetrahderal
3, Simple N-hydroxy
2, Proposed N-Hydroxy
Intermediate
sulfonimidamide
Sulfonimidamide
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.02.082

2332
R. L. Pennington et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2331–2334
H
N
H
N
H
N
R1
c then
d or e
H
N
R
N
S
O
R
a or b
S
O
S
O
R
S
O
a or b
N
O
S
N
O
R2
H
N
O
O
H
tBDMS
H
H
6, R1 = -n-Bu, R2 = -Bn
4, R = -n-Bu
5, R = -Ph
7, R1 = -Ph, R2 = -Bn
11, R = -n-Bu
12, R = -Ph
8, R = -n-Bu
9, R = -Ph
11, R1 = -n-Bu, R2 = -tBDMS
12, R1 = -Ph, R2 = -tBDMS
H
N
R
HNO
H
S
O
N
R1
+
HNO
OH
N2O + H2O
H2N2O2
H
S
O
f, g or h
N
R
N
O
S
O
R2
4 and 5
H
N
OH
4, R = -n-Bu
5, R = -Ph
H
6, R1 = -n-Bu, R2 = -Bn
7, R1 = -Ph, R2 = -Bn
8, R = -n-Bu
9, R = -Ph
O
S
O
S
O
11, R1 = -n-Bu, R2 = -tBDMS
12, R1 = -Ph, R2 = -tBDMS
OH
N
NOT FORMED
HNO
+
H
H
N
R
S
Piloty's Acid
NH2
O
Scheme 3. Reagents: (a) TBAF; (b) CH3CO2H–H2O–THF.
10, R = -n-Bu
Scheme 2. Reagents: (a) n-BuLi; (b) PhMgBr; (c) t-BuOCl; (d)
BnONH2; (e) t-BDMSONH2; (f) Pd/C, H2; (g) TBAF; (h)
CH3CO2H–H2O–THF.
tion of hydroxylamine failsto give the desired N-hydr-
oxy sulfonimidamide 8.
Gaschromatographic analysisof the headspace of either
the TBAF or acidic deprotection of 11 and 12 revealsthe
formation of nitrousoxide (N 2O, 75% and 65%, respec-
tively, after 2 h, Scheme 3).11 The identification of N2O
during these reactions provides strong evidence for the
intermediacy of nitroxyl (HNO), which rapidly dimerizes
and dehydratesto give N 2O.12 The addition of an aque-
ous solution of TBAF to a solution of 11 and 12 in 1:1
CH3OH–H2O also produces N2O in 51% and 55% yield,
respectively, demonstrating the aqueous decomposition
of 11 and 12 to HNO. Addition of glutathione (2 equiv)
to these reaction mixtures completely suppresses N2O
formation providing further evidence of the intermediacy
of HNO. These results suggest that removal of the pro-
tecting group of 11 and 12 initially formsthe N-hydroxy
sulfonimidamides 8 and 9 that immediately decompose
to HNO and the sulfinamides 4 and 5 (Scheme 3). In ret-
rospect, the decomposition of 8 and 9 to HNO may have
been anticipated given the structural similarity of 8 and 9
to PilotyÕsacid, a compound known to decompoes to
HNO and benzene sulfinic acid under basic conditions
(Scheme 3).13
new group of potential donorsof HNO, which hasre-
ceived considerable recent interest given its different bio-
logical profile from NO. The chemistry and biology of
6
HNO hasrecently been extensively reviewed.
2. Results and discussion
Scheme 2 depictsthe initial attempt at the preparation of
model N-hydroxy sulfonimidamides. Treatment of N-
thionylaniline with either n-butyl lithium or phenyl mag-
nesium bromide yields the sulfinamides (4 and 5) in 82%
and 73% yield, respectively (Scheme 2).7,8 Exposure of 4
and 5 to t-butyl hypochlorite followed by condensation
with O-benzyl hydroxylamine givesthe correpsonding
benzyl protected N-hydroxy sulfonimidamides (6 and
7) in 59% and 67% yield, respectively (Scheme 2).9,10 Cata-
lytic hydrogenation (10% Pd/C catalyst, room tempera-
ture, 1 atm of H2, CH3OH) of 6 and 7 doesnot form
the expected N-hydroxy sulfonimidamides (8 and 9)
but surprisingly produces the corresponding sulfin-
amides( 4 and 5) asjudged by TLC and 1H NMR spec-
troscopy as compared to authentic samples (Scheme 2).
The preparation of 10 by treatment of 4 with t-butyl
These synthetic studies show the general instability of N-
hydroxy sulfonimidamides, which severely limits their
use as transition state analogs for NOS. However, these
experimentsreveal that thisunique functional group acts
asan HNO donor. Thus, O-protected N-hydroxy sulfon-
imidamides(such as 6, 7, 11, and 12) represent prodrugs
capable of HNO release upon deprotection (dealkyla-
tion) similar to N,O-diacylated or alkylated N-hydroxy-
arylsulfonamides previously reported by Nagasawa and
co-workers.14 While AngeliÕs salt (sodium trioxodini-
trate, Na2N2O3) and PilotyÕsacid remain the most widely
used HNO donors, the intense biological, chemical, and
theoretical interest in HNO highlights the need for new
HNO donorsasbiochemical and pharmacological
probesand possible therapeutic agents. 6,15 For example,
HNO and NO donors demonstrate discrete effects in
both normal and failing heart modelswith HNO donors
specifically increasing levels of calcitonin gene related
peptide and NO donorsincreaisng cyclic guanylate
monophosphate levels.16 These results have led to the
suggestion of HNO donors as a new and unique
1
hypochlorite followed by ammonia and H NMR and
TLC characterization eliminates 10, which could form
by N–O bond reduction, asthe reaction product.
Scheme 2 also summarizes the alternative use of the t-
butyl dimethyl silyloxy (t-BDMS) protecting group in
the preparation of N-hydroxy sulfonimidamides. Expo-
sure of 4 and 5 to t-butyl hypochlorite followed by
condensation with O–t-BDMS hydroxylamine yields
the O–t-BDMS protected N-hydroxy sulfonimidamides
(11 and 12) in 48% and 41% yield, respectively (Scheme
2). Treatment of 11 and 12 with tetra-n-butyl ammo-
nium fluoride (TBAF) again givesthe uslfinamides
4
and 5 with no indication of the N-hydroxy sulfonimid-
amides( 8 and 9, Scheme 2). Exposure of 11 and 12 to
acidic conditions(3:1:1, CH 3CO2H–H2O–THF, room
temperature) only yields 4 and 5 with no evidence of 8
and 9 (Scheme 2). Finally, treatment of the sulfinamide
4 with t-butyl hypochlorite followed by the direct addi-

R. L. Pennington et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2331–2334
2333
treatment for heart failure.15 These results describe a new
functional group (N-hydroxy sulfonimamides) as an
alternative HNO donor that may be useful in distin-
guishing the effects of NO from HNO in some systems.
combined, dried (MgSO4), concentrated, and purified by
flash chromatography to give the protected N-hydroxy-
sulfonimidamides (6, 7, 11, 12) in 41–67% yield.
Compound 6: Rf 0.69 (50:50 pentane–ether); mp 68–
69 ꢁC; 1H NMR (500 MHz, CDCl3): d 7.51–7.20 (m,
9H, ArH), 7.02–7.00 (t, 1H, J = 7.20 Hz, ArH), 6.86
(br s, 1H, NH), 4.87–4.80 (m, 2H, CH2OPh), 3.37–
3.32 (m, 2H, a-CH2), 1.91–1.86 (m, 2H, b-CH2), 1.52–
1.47 (sext., 2H, J = 7.51 Hz, v-CH2), 0.99–0.96 (t, 3H,
J = 7.40 Hz, CH3); 13C NMR (75.5 MHz, CDCl3): d
145.5, 129.7, 129.3, 129.0, 128.8, 124.1, 122.8, 79.2,
51.4, 23.3, 21.8, 13.9; LRMS (ESI) m/z 319 (MH+).
3. N-Phenylbutane sulfinamide (4)
N-Butyllithium (5.74 mL of a 2.5 M solution in hexanes,
1.44 · 10À2 mol wasadded to a stirred solution of N-thion-
ylaniline (0.81 mL, 7.2 · 10À3 mol) in THF (5 mL) at
À78 ꢁC (acetone–dry ice bath). The reaction mixture
wasmonitored by TLC and after 30 min. wascarefully
worked up by the addition of water, diluted with ether
and extracted with 1 M HCl, NaHCO3 (satd) and water.
The organic layerswere combined, dried (MgSO 4), con-
centrated, and purified by flash chromatography (silica
gel, 50:50 pentane–ether, Rf 0.23) to give 1.20 g
(84.5%) of 4 asa white powder: mp 73–74 ꢁC; 1H
NMR (300 MHz, CDCl3): d 7.32–7.26 (m, 2H, ArH),
7.08–7.04 (m, 3H, ArH), 5.85 (br s, 1H, NH), 2.97–
2.91 (m, 2H, a-CH2), 1.81–1.71 (p, 2H, J = 7.59 Hz, b-
CH2), 1.56–1.46 (m, 2H, v-CH2), 1.00–0.95 (t, 3H,
J = 7.31 Hz, CH3); 13C NMR (75.5 MHz, CDCl3): d
141.0, 129.5, 123.4, 118.8, 56.0, 25.0, 21.8, 13.7; LRMS
(ESI) m/z 198 (MH+).
Compound 7: 0.75 (50:50 pentane–ether); mp 100–
103 ꢁC; 1H NMR (300 MHz, CDCl3): d 8.17–8.15
(m, 2H, ArH), 7.64–7.53 (m, 3H, ArH), 7.35–7.29 (m,
7H, ArH), 7.25–7.20 (m, 2H, ArH), 7.08–7.05 (m, 1H,
ArH), 6.91 (br s, 1H, NH), 4.79–4.78 (m, 2H, CH2OPh),
13C NMR (75.5 MHz, CDCl3): d 143.4, 137.6, 136.0,
133.6, 129.5, 129.3, 128.9, 128.8, 128.8, 124.4, 123.1,
79.0; LRMS (ESI) m/z 339 (MH+).
Compound 11: Rf 0.53 (50:50 pentane–ether); mp 78–
81 ꢁC; 1H NMR (300 MHz, CDCl3): d 7.23–7.20 (t,
2H, J = 7.75 Hz, ArH), 7.15–7.13 (d, 2H, J = 7.80 Hz,
ArH), 6.96 (t, 1H, J = 7.79 Hz), 6.43 (br s, 1H, NH),
3.36–3.34 (m, 2H, a-CH2), 1.94–1.91 (p, 2H,
J = 7.64 Hz, b-CH2), 1.54–1.53 (m, 2H, v-CH2), 1.02–
0.99 (t, 3H, J = 7.57 Hz, CH3), 0.93 (s, 9H, SiC(CH3)3),
0.17 (s, 3H, SiCH3), 0.08 (s, 3H, SiCH3); 13C NMR
(75.5 MHz, CDCl3): d 143.5, 129.0, 123.8, 122.3, 50.0,
26.1, 25.2, 21.7, 18.2, 13.7; LRMS (ESI) m/z 343 (MH+).
4. N-Phenylbenzenesulfinamide (5)
Phenylmagnesium bromide (3.6 mL of a 3 M solution in
THF, 1.08 · 10À2 mol) was added to a stirred solution
of N-thionylaniline (0.81 mL, 7.18 · 10À3 mol) in THF
(5 mL) at 0 ꢁC. The reaction mixture wastsirred at
0 ꢁC for 1 h, and then at room temperature for 2 h, with
monitoring by TLC until completion. The reaction mix-
ture wasworked up by the addition of water, dilution
with ether, and extraction with 1 M HCl, NaHCO3
(satd) and water. The organic layers were combined,
dried (MgSO4), concentrated and he resultant oil was
purified by flash chromatography (silica gel, 50:50 pen-
tane–ether Rf 0.31), to yield 1.13 g (72.8%) of 5 asa
white solid: mp 108–111 ꢁC; 1H NMR (300 MHz,
CDCl3): d 7.83–7.81 (m, 2H, ArH), 7.56–7.55 (m, 3H,
ArH), 7.32–7.29 (m, 2H, ArH), 7.12–7.07 (m, 3H,
ArH), 6.06 (br s, 1H, NH); 13C NMR (75.5 MHz,
CDCl3): d 131.8, 129.9, 129.5, 125.8, 124.1, 119.4.
LRMS (ESI) m/z 239 (M+Na+).
Compound 12: Rf 0.75 (50:50 pentane–ether); mp 91–
93 ꢁC; 1H NMR (300 MHz, CDCl3): d 8.17–8.15 (m,
2H, ArH), 7.67–7.65 (m, 3H, ArH), 7.30–7.22 (m, 4H,
ArH), 7.06–7.00 (m, 1H, ArH), 6.56 (br s, 1H, NH),
0.89 (s, 9H, SiC(CH3)3), 0.07 (s, 3H, SiCH3), 0.01 (s,
3H, SiCH3); 13C NMR (75.5 MHz, CDCl3): d 143.0,
136.7, 133.4, 129.1, 129.1, 128.9, 124.1, 122.6, 26.0,
18.0; LRMS (ESI) m/z 363 (MH+).
6. N-Phenylbutane sulfonimamide (10)
Prepared in a similar fashion to 6, 7, 11, and 12 with
ammonia being added to give 10: Rf 0.75 (ether); mp
1
125–128 ꢁC; H NMR (300 MHz, CDCl3): d 7.33–7.25
(m, 2H, Ar), 7.18–7.15 (m, 2H, Ar), 7.07–7.02 (t, 1H,
J = 7.25 Hz, Ar), 4.42 (br s, 1H, NH), 3.28–3.23 (m,
2H, a-CH2), 1.97–1.88 (m, 2H, b-CH2), 1.50–1.42 (m,
2H, J = 7.36 Hz, v-CH2), 0.99–0.94 (t, 3H, J =
7.33 Hz, CH3); 13C NMR (75.5 MHz, CDCl3): d 129.6,
123.3, 122.9, 56.0, 26.3, 21.7, 14.0; LRMS (ESI) m/z
213 (MH+), 235 (M+Na+).
5. Protected N-hydroxy sulfonimidamides (6, 7, 11, 12)
t-Butyl hypochlorite (1.2 equiv) in CCl4 (1 mL) was
added dropwise to a solution of the sulfinamide
(1 equiv) in CCl4 (2 mL) and stirred at 0 ꢁC in an alumi-
num foil-covered flask. After stirring for 1.5 h, the mix-
ture was concentrated and the resultant residue was
taken up in CCl4 (2 mL) at room temperature. A solu-
tion of benzyl hydroxylamine or t-butyldimethylsilyl
hydroxylamine (2.4 equiv) in CCl4 wasadded to this
solution dropwise. After stirring overnight, the reaction
mixture wasdiluted with ether and extracted with 1 M
HCl, satd NaHCO3, and water. The organic layerswere
7. Attempted deprotection of 11 and nitrous oxide
detection
A
solution of tetra-n-butyl ammonium fluoride
(4.38 mL, 1 M, 4.37 · 10À3 mol) in THF wasadded

2334
R. L. Pennington et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2331–2334
dropwise to a solution of 11 (500 mg, 2.18 · 10À3 mol)
in THF (10 mL) at room temperature. After stirring
for 15 min, the reaction wasjudged complete by TLC.
The reaction mixture wasdiluted with ether and ex-
tracted with water, and the ether layer was subsequently
dried (MgSO4) and concentrated to give a residue that
was purified by flash chromatography (silica gel; 50:50
pentane–ether) to yield 255 mg (52%) of 4 asjudged
by TLC and NMR spectroscopy. An aliquot (250 lL)
of the reaction headspace from a sealed reaction flask
wasinjected onto a 6890 Hewlett–Packard gaschro-
matograph equipped with a thermal conductivity detec-
tor and a 6 ft X 1/8 in. Porapak Q column at an
operating oven temperature of 50 ꢁC (injector and detec-
tor 150 ꢁC) with a flow rate of 16.67 mL/min (He, carrier
gas). The retention time of nitrous oxide was 2.61 min
and identical to a known sample. Yields were deter-
mined using a standard curve generated from known
amountsof nitrousoxide.
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Acknowledgements
This work was supported by grants (0140020N, Estab-
lished Investigator Award) from the American Heart
Association and the National Institutes of Health
(HL62198).
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