Synthesis and biological evaluation of nitric oxide releasing derivatives of 6-amino-3-n-butylphthalide as potential antiplatelet agents

Article abstract of DOI:10.1016/j.bmcl.2013.02.035

A series of novel nitric oxide releasing derivatives of 6-amino-3-n-butylphthalide were designed, synthesized and evaluated as potential antiplatelet agents. Compound 10b significantly inhibited the adenosine diphosphate (ADP)-induced platelet aggregation

Full text of DOI:10.1016/j.bmcl.2013.02.035

Bioorganic & Medicinal Chemistry Letters 23 (2013) 1985–1988
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis and biological evaluation of nitric oxide releasing deriva-
tives of 6-amino-3-n-butylphthalide as potential antiplatelet agents
Xiaoli Wang a,b, , Linna Wang a,c, , Zhangjian Huang a,b, , Xiao Sheng a,b, Tingting Li a,c, Hui Ji a,c,
,
⇑
Jinyi Xu a,b, , Yihua Zhang a,b,
⇑
⇑
a State Key Laboratory of Natural Medicines, China Pharmaceutical University, Nanjing 210009, PR China
b Center of Drug Discovery, China Pharmaceutical University, Nanjing 210009, PR China
c Department of Pharmacology, China Pharmaceutical University, Nanjing 210009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel nitric oxide releasing derivatives of 6-amino-3-n-butylphthalide were designed, synthe-
sized and evaluated as potential antiplatelet agents. Compound 10b significantly inhibited the adenosine
diphosphate (ADP)-induced platelet aggregation in vitro, superior to 6-amino-3-n-butylphthalide,
3-n-butylphthalide (NBP) and ticlopidine. Meanwhile 10b released moderate levels of NO, which could
be beneficial for improving cardiovascular and cerebral circulation. Furthermore, 10b had an enhanced
aqueous solubility relative to NBP. These findings may provide new insights into the development of
novel antiplatelet agents for the treatment of thrombosis-related ischemic stroke.
Received 17 December 2012
Revised 29 January 2013
Accepted 6 February 2013
Available online 15 February 2013
Keywords:
3-n-Butylphthalide (NBP)
6-Amino-3-n-butylphthalide
NO-releasing derivatives
Antiplatelet agents
Ischemic stroke
Ó 2013 Elsevier Ltd. All rights reserved.
Stroke is a major cause of adult disability and death worldwide1
which can be subdivided into two categories, ischemic and hemor-
rhagic. Ischemic strokes are more prevalent than hemorrhagic
ones, making up approximately 87% of all cases, and have been
the target of most drug trials.2 Previous studies have demonstrated
that platelet-mediated thrombosis is a major cause of ischemic
stroke, and the activation of platelets and the resultant aggregation
play a central role in the pathogenesis of ischemic stroke.3 Thus,
platelet adhesion and aggregation have been identified as impor-
tant targets for the development of antithrombotic drugs against
ischemic stroke. Because of the multifactorial nature of ischemic
stroke, the traditional approach of single-target drugs could gener-
ally only offer limited and transient benefits. Hence, development
of new drugs with multiple actions could be of great significance
for the treatment of ischemic stroke.
multiple actions,4–6 such as inhibiting platelet aggregation and
thrombosis, improving microcirculation, and decreasing brain in-
farct volume. Despite these benefits, the clinic application of NBP
has been limited due to its poor aqueous solubility, and NBP should
be administered by combination with other antiplatelet drugs to
improve therapeutic effects. Thus, more potent drugs than NBP
for the treatment of ischemic stroke are needed.
In recent years, it has been reported that the introduction of a
proper substituent into position 6 of the phenyl ring of NBP could
impact its activity, and 6-amino-3-n-butylphthalide, that is, 6-ami-
no-NBP (compound 1, Fig. 1) showed stronger potency than NBP,
and could significantly inhibit the platelet aggregation, reduce
the area of cerebral infarct, improve mitochondrial function and
decrease oxidative damage as well as neuronal apoptosis.7
Nitric oxide (NO), a free radical gas, is a multifunctional mes-
senger molecule with diverse physiological roles, such as dilation
of blood vessels, immune responses, and potentiation of synaptic
transmission.8,9 Indeed, increasing NO production either from
endothelial NOS (eNOS) or NO-donors mimicking eNOS-derived
NO has already been applied as a therapeutic approach for throm-
botic complications, such as myocardial infarction and ischemic
stroke.10 Recently, we reported several NO-based ring-opening
derivatives of NBP (2a–l, (S)- and (R)-ZJM-289, Fig. 1) as potent
anti-ischemic stroke agents which showed excellent antiplatelet
and antithrombotic activities.11,12
The racemic 3-n-butylphthalide (NBP) (Fig. 1), a component ex-
tracted from seeds of Apium graveolens Linn, was approved by the
State Food and Drug Administration (SFDA) of China as a new drug
for the treatment of ischemic stroke in 2002. It has already been re-
ported that NBP possesses beneficial effects on stroke through
⇑
Corresponding authors. Tel./fax: +86 25 83271015 (Y.Z.); tel.: +86 25 86021369;
fax: +86 25 86635503 (H.J.); tel.: +86 25 83271445; fax: +86 25 83302827 (J.X.).
E-mail addresses: huijicpu@163.com (H. Ji), jinyixu@china.com (J. Xu),
zyhtgd@163.com (Y. Zhang).
These authors contributed equally to this work.
0960-894X/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.bmcl.2013.02.035

1986
X. Wang et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1985–1988
O
O
H2N
O
O
1
NBP
HCl
O
O
R2
HCl
N
O
O
O
O
O
O
O
O(CH2)nONO2
O(CH2)4ONO2
R1
O
O
2a-i
n = 3~5
R1 = H, OMe
(S)-ZJM-289
(R)-ZJM-289
R2
=
N
O
N
Figure 1. Structrues of NBP, 1, 2a–i, (S)- and (R)-ZJM-289.
O
O
O
O
linker
R
N
H
H2N
N
H
NO-donor
O
O
O
1
10a-l
Figure 2. Strategy for the design of NO releasing derivatives of 6-amino-3-n-butylphthalide.
In the present study, to obtain new agents with stronger po-
tency, we employed 1 as the lead compound to design and synthe-
size a novel class of NO-releasing derivatives (10a–l, Fig. 2). Their
antiplatelet activity in vitro, NO-releasing ability as well as aque-
ous solubility were evaluated.
The synthesis of target compounds 10a–l was depicted in
Scheme 1. The bromo-substituted alkylcarboxylic acids 3a–c were
converted to the corresponding nitrates 4a–c using AgNO3 in
CH3CN. Treatment of hydroxybenzaldehyde 5a–c with Br(CH2)mBr
yielded bromids 6a–i, which were treated with AgNO3 leading to
nitrats 7a–i. Subsequent oxidation reactions of 7a–i using potas-
sium permanganate yielded acids 8a–i. On the other hand, NBP
was nitrated using potassium nitrate in H2SO4 to give nitro com-
pound 9, which was reduced with Fe and NH4Cl in THF/H2O to give
amino compound 1. Finally, the target compounds 10a–l were ob-
tained by acylation of 1 with 4a–c or 8a–i, respectively. All of new
compounds were purified by column chromatography and charac-
terized by IR, ESI-MS, 1H NMR, 13C NMR, and HRMS.13,14
The in vitro inhibitory effects of the target compounds 10a–l on
adenosine diphosphate (ADP)-induced platelet aggregation in rab-
bit platelet rich plasma (PRP) were assayed using Born’s turbidi-
metric method.15 As shown in Table 1, 1 (inhibition rate 18.43%
a
HOOC mONO2
m = 2-4
OHC
HOOC mBr
3a-c
4a-c
b
c
d
n = 2-4
HOOC
OHC
OHC
n Br
OH
O
n ONO2
O n ONO2
O
8a-i
7a-i
6a-i
5a-c
g
f
e
O
O
O
O
O
O
O
O
H2N
O2N
R
N
H
O
1
9
NBP
10a-l
10a
10b
10c
10d
10e
10g
10h
10j
: R = CH2ONO2;
: R = (CH2)2ONO2;
: R = (CH2)3ONO2;
: R = o-ArO(CH2)2ONO2;
: R = p-ArO(CH2)2ONO2;
: R = m-ArO(CH2)2ONO2;
: R = o-ArO(CH2)3ONO2;
: R = p-ArO(CH2)3ONO2;
: R = m-ArO(CH2)3ONO2;
: R = o-ArO(CH2)4ONO2;
: R = p-ArO(CH2)4ONO2;
: R = m-ArO(CH2)4ONO2
10k
10f
10i
10l
Scheme 1. Synthesis of compounds 10a–l. Reagents and conditions: (a) AgNO3, CH3CN, 70 °C, 2 h; (b) Br(CH2)mBr, K2CO3, acetone, reflux, 2–4 h; (c) AgNO3, CH3CN, 70 °C, 3 h;
(d) K2MnO4, acetone, reflux, 10 h; (e) KNO3, H2SO4, rt, 12 h; (f) Fe, NH4Cl, THF/H2O, 65 °C, 2 h; (g) 4a–c or 8a–i, EDCI, DMAP, CH2Cl2, rt, 6–8 h.

X. Wang et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1985–1988
1987
Table 1
ꢀ
The effects on platelet aggregation induced by ADP in vitro (n = 6, x s)
Compd
(0.1 mM)
Inhibition rate (%) Compd
(0.1 mM)
Inhibition rate (%)
Ticlid
NBP
1
10a
10b
10c
10d
10e
26.45 6.38
15.72 3.99
18.43 4.22
22.16 4.36
30.42 5.49*
28.56 4.54
15.20 4.41
20.04 5.14
10f
10g
10h
10i
10j
10k
10l
26.58 7.14
16.71 6.76
22.78 4.38
28.76 8.64
13.57 3.64
18.25 5.05
21.55 4.72
PRP were preincubated with tested compounds (0.1 mm) at 37 °C for 5 min fol-
lowed by the addition of ADP (10 M). Data are expressed as mean SD of each
l
group (n = 6) and analyzed by one-way analysis of variance (ANOVA) followed by
post hoc Tukey test.
*
P <0.05 versus NBP.
Figure 4. NO-releasing assay of selected compounds in vitro. The levels of NO
produced by selected compounds were determined by Griess assay. Individual
compounds were incubated for the indicated periods and reacted with Griess
reagent, followed by measuring at 540 nm. The levels of NO produced by individual
compounds were calculated, according to the standard curve of nitrite. NO data are
at 0.1 mM) showed slight stronger inhibitory effect than NBP
(15.72%) at the same concentration, which was consistent with
the previous report.7 Furthermore, 10b (30.42%), 10c (28.56%),
10f (26.58%) and 10i (28.76%) displayed significantly inhibitory ef-
fects, which were superior to 1 (18.43%) and two clinically used
anti-ischemic stroke drugs, NBP (15.72%) and Ticlid (26.45%).16
Notably, 10b was almost twofold more potent than NBP and 1.
Since 10b is composed of two moieties, that is, 6-amino-NBP (1)
and NO donor 4b, we further investigated their individual contri-
bution to the overall activity against ADP-induced platelet aggre-
gation in vitro. We found that although 1 and 4b displayed a
certain degree of antiplatelet activity (18.43% and 8.32%, respec-
tively), both of them were less effective than 10b (30.42%, Fig. 3),
suggesting that the two moieties may have synergistic effects on
inhibiting the ADP-induced platelet aggregation in vitro. In addi-
tion, the antiplatelet effect of 10b was dramatically attenuated
by treatment with hemoglobin (He), an endogenous NO scavenger,
resulting in reduction of inhibition from 30.42% to 23.05% (Fig. 2).
These results strongly demonstrate that NO has an important im-
pact on the antiplatelet effects of 10b.
expressed as mean SD (nitrite) in
variations were less than 10%.
lmol/L at each time point and intra-group
NO-releasing ability of these compounds was positively correlated
to their platelet aggregation inhibitory activity (R2 = 0.98, P <0.01,
determined by logistic regression analysis).18
As the poor aqueous solubility of NBP affects its therapeutic effi-
cacy leading to limited clinical application, we next investigated
whether an increase in aqueous solubility of target compounds
could be associated with their enhanced activity. Aqueous solubil-
ity of compounds 10b, 10g–i and NBP with different levels of anti-
platelet aggregation activity were tested as described previously.19
It was observed that the aqueous solubility of 10b and 10i with
strong antiplatelet aggregation activity were 1.18 and 1.06 mM,
respectively, which were higher than that of NBP (0.54 mM)
whereas the aqueous solubility of 10g and 10h with weak anti-
platelet aggregation activity were 0.58 and 0.75 mM, which were
similar to that of NBP (Table 2).20 In addition, the logP values of
10b (2.94) and NBP (3.19) were calculated using the ChemDraw Ul-
tra program, version 12.0, CambridgeSoft company.21 These results
suggest that the appropriate lipid-water partition coefficient of
these target compounds might be contributed to their antiplatelet
aggregation activity.
To further examine the relationship between NO and antiplate-
let aggregation activity, the levels of NO produced by 10a–i were
determined by Griess assay.17 As shown in Figure 4, the active
compounds 10b, 10i, 10c and 10f released moderate amount of
NO (12.88, 7.27, 7.10 and 6.82 lmol/L, respectively). In contrast,
the less active compounds 10h, 10a, 10e, 10g and 10d under the
Analysis of structure and antiplatelet aggregation activity rela-
tionship revealed that the target compounds 10a–l with different
linkers displayed variable antiplatelet aggregation activity. In gen-
eral, the compounds with straight chain alkanes exhibited higher
antiplatelet potency than those with aromatic alkanes. For straight
chain alkanes, the compounds with two-carbon alkanes showed
best inhibitory activity. For aromatic alkanes, the different position
of the substituents showed different activity (para > meta > ortho).
Evidentially, 10b with a two-carbon linker displayed the strongest
antiplatelet aggregation activity. These results suggest that varied
linkers have different abilities to modulate the structure, stability,
metabolism and penetrability of these compounds, which affect
same conditions released lower levels of NO (3.88, 3.67, 2.21,
2.18 and 1.82 lmol/L, respectively). The resluts indicated that the
Table 2
The solubility of NBP, 1, 10b and 10g–ia
Compd
Solubility (mM)
Compd
Solubility (mM)
Figure 3. Inhibition of ADP-induced rabbit platelet aggregation by selected
compounds in vitro. PRP were pre-incubated with tested compounds (0.1 mM) at
37 °C for 5 min followed by addition of ADP (10 lM) Hemoglobin (20 lM) was
NBP
10b
10g
0.54
1.18
0.58
10h
10i
0.75
1.06
added and incubated with the drug-platelet suspension in the indicated group. Data
are expressed as mean SD (n = 6) and analyzed by one-way analysis of variance
(ANOVA) followed by post hoc Tukey test. ⁄⁄⁄P <0.001 versus 10b.
a
Data are expressed as mean concentration of individual compounds saturating
in 5 ml saline at 25 °C.

1988
X. Wang et al. / Bioorg. Med. Chem. Lett. 23 (2013) 1985–1988
(t, 3H, CH3, J = 6.9 Hz), 1.26–1.50 (m, 4H, 2 ꢁ CH2CH3), 1.71–2.07 (m, 2H,
CHCH2), 2.92 (t, 2H, NHCOCH2, J = 6.1 Hz), 4.87 (t, 2H, ONO2CH2, J = 6.7 Hz),
5.47–5.51 (m, 1H, COOCHCH2), 7.41 (d, 1H, ArH, J = 8.3 Hz), 7.93 (s, 1H, ArH),
8.26 (dd, 1H, J = 1.4, 8.3, ArH), 8.70 (s, 1H, NH). 13C NMR (75 MHz, CDCl3): d
171.08, 167.45, 145.66, 139.16, 126.57, 126.40, 122.52, 115.96, 82.13, 68.48,
the NO production, leading to various bioactivities of the target
compounds.
In summary, we designed and synthesized a series of NO-
releasing derivatives of 6-amino-NBP and found that some com-
pounds had strong inhibitory effects on ADP-induced platelet
aggregation in vitro, among which 10b was the most potent, supe-
rior to NBP, 6-amino-NBP and Ticlid. Further investigation demon-
strated that 10b could produce moderate levels of NO in vitro,
which could be beneficial for improving cardiovascular and cere-
bral circulation. Importantly, since 10b had an enhanced aqueous
solubility and appropriate lipid–water partition coefficient relative
to NBP, it may more favorably penetrate the blood–brain barrier
(BBB) to exert its action. Altogether, the multifunctional effects of
10b qualified it as a potential antiplatelet agent for the treatment
of thrombosis-related ischemic stroke.
34.48, 29.66, 26.76, 22.36, 13.79. HRMS (ESI): m/z Calcd for
[M+Na]+ 345.1063; found 345.1067. Analytical data for 10i: Mp: 103–106 °C.
MS (ESI): m/z 451.1 [M+Na]+. IR (cmꢀ1, KBr):
max 763, 1280, 1400, 1501, 1752,
C15H18N2O6,
m
2924. 1H NMR (300 Hz, CDCl3): d 0.902 (t, 3H, CH3, J = 6.8 Hz), 1.38–1.50 (m,
4H, 2 ꢁ CH2CH3), 1.70–2.06 (m, 2H, CHCH2), 2.17–2.27 (m, 2H, ONO2CH2CH2),
4.16 (t, 2H, ONO2CH2, J = 6.8 Hz), 4.68 (t, 2H, OCH2, J = 6.3 Hz), 5.45–5.49 (m,
1H, COOCHCH2), 7.06–7.60 (m, 5H, ArH), 8.07 (s, 1H, ArH), 8.32 (dd, 1H, J = 1.9,
8.3 Hz, ArH), 8.69 (s, 1H, NH). 13C NMR (75 MHz, CDCl3): d 170.70, 165.69,
158.81, 145.62, 139.42, 135.68, 129.88, 126.76, 123.34, 119.72, 118.95, 116.63,
112.95, 81.75, 69.87, 63.88, 34.39, 29.65, 26.94, 26.78, 22.38, 13.80. HRMS
(ESI): m/z Calcd for C22H24N2O7, [M+Na]+ 451.1481; found 451.1486.
15. Jones, M.; Inkielewicz, I.; Medina, C.; Santos-Martinez, M. J.; Radomski, A.;
Radomski, M. W.; Lally, M. N.; Moriarty, L. M.; Gaynor, J.; Carolan, C. G.; Khan,
D.; O’Byrne, P.; Harmon, S.; Holland, V.; Clancy, J. M.; Gilmer, J. F. J. Med. Chem.
2009, 52, 6588.
16. Antiplatelet aggregation effect in vitro: Blood samples were withdrawn from
rabbit carotid artery and mixed with 3.8% sodium citrate solution (9:1, v/v),
followed by centrifuging at 500 rpm for 10 min at room temperature. After the
resulting platelet-rich plasma (PRP) supernatant was collected, the residue was
centrifuged at 3000 rpm for another 10 min at room temperature to obtain
platelet-poor plasma (PPP). The PRP was adjusted with PPP in order to obtain
platelet counts of 400–450 ꢁ 109 Pl/L. Platelet aggregation was determined by
Born’s turbidimetric method using a four-channel aggregometer (LG-PABER-I
Platelet-Aggregometer, Beijing, China) within 3 h after blood collection. Briefly,
Acknowledgments
This study was financially supported by Grants from the Major
National Science and Technology Program of China for Innovative
Drug during the Eleventh Five-Year Plan Period (No.
2009ZX09103-095) and the Project Program of State Key Labora-
tory of Natural Medicines, China Pharmaceutical University
(No.ZJ11176).
PRP (240
concentrations of individual compounds (30
the addition of 10
M of Adenosine 50-diphosphate sodium salt (ADP, Sigma–
l
l) was pre-incubated with vehicle, positive control or different
l
L) for 5 min at 37 °C, followed by
l
Aldrich, USA) to induce the platelet aggregation. The maximum aggregation
rate (MAR) was recorded within 5 min at 37 °C. The inhibition rate of the tested
compounds on platelet aggregation was calculated with the following formula:
Inhibition rate (%) = (100% ꢀ MAR of tested compound/MAR of vehicle).
17. Velàzquez, C.; Vo, D.; Knaus, E. E. Drug Dev. Res. 2003, 60, 204.
18. Nitrate/nitrite measurement in vitro: Brifly, 0.1 mM of each compound in
phosphate buffer solution (PBS) was incubated at room temperature for 15–
300 min and were sampled every 15 min for 120 min and then every 30 min
References and notes
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3. Mackman, N. Nature 2008, 451, 914.
4. Zhu, X. Z.; Li, X. Y.; Liu, J. Eur. J. Pharmacol. 2004, 500, 221.
5. Zhang, Y.; Wang, L.; Li, J.; Wang, X. L. J. Pharmacol. Exp. Ther. 2006, 317, 973.
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260, 106.
7. Guo, Z. R.; Chu, F. M.; Zhang, J. T. [P] CN1330069A, 2002.
8. Furchgott, R. F.; Zawadzki, J. V. Nature 1980, 288, 373.
9. Moncada, S.; Higgs, A. N. Eng. J. Med. 2002, 1993, 329.
for the remaining time. The collected samples (50
ll) were mixed with 50 ll of
sulfanilamide solution and 50 of N-[-1-naphthyl-ethylenediamine
lL
dihydrochloride], then incubated 5–10 min at room temperature, protected
from light. Nitrite concentration was determined by microtiter plate reader at
540 nm from
(Beyotime Biotechnology). NO data are expressed as mean SD (nitrite) in
mol Lꢀ1
a ) derived from NaNO2
standard curve (0–100 mmol Lꢀ1
10. (a) Megsonand, I. L.; Webb, D. J. Expert Opin. Invest. Drugs 2002, 11, 587; (b)
Martelli, A.; Rapposelli, S.; Calderone, V. Curr. Med. Chem. 2006, 13, 609.
11. Wang, X. L.; Li, Y.; Zhao, Q.; Min, Z. L.; Zhang, C.; Lai, Y. S.; Ji, H.; Peng, S. X.;
Zhang, Y. H. Org. Biomol. Chem. 2011, 9, 5670.
l
.
19. Casini, A.; Scozzafava, A.; Mincione, F.; Menabuoni, L.; Ilies, M. A.; Supuran, C. T.
J. Med. Chem. 2000, 43, 4884.
20. Aqueous solubility assays: Individual compounds at ca. 1 mg were dissolved in
10 ml of methanol and the maximum UV absorption of each compound was
determined in a UV755B spectrophotometer, eventually diluting the solution
(with MeOH) as necessary. A saturated solution of each compound was then
prepared by stirring magnetically a small volume of normal saline in the
presence of excess compound for 5 h. The saturated solution was filtered with
a Millipore 0.45-mm filter to remove solid compound and measured by UV-
spectrometry at the wavelength determined. Total solubility was determined
12. Wang, X. L.; Zhao, Q.; Wang, X. L.; Li, T. T.; Lai, Y. S.; Peng, S. X.; Ji, H.; Xu, J. Y.;
Zhang, Y. H. Org. Biomol. Chem. 2012, 10, 9030.
13. General procedure for the synthesis of the target compounds 10a–l: Compound
1 and one molar amount of EDCI were dissolved in 20 mL of dry CH2Cl2 and the
mixture was stirred at room temperature for 0.5 h. Then a solution of one equiv
molar amount of 4a–c or 8a–i in 5 mL of dry CH2Cl2 was added, followed by a
catalytic amount of DMAP. The solution was left stirring at room temperature
for 6–8 h. Then the resulting mixture was washed sequentially with 1 M HCl,
water, and brine. The solution was then dried, filtered, and evaporated to
dryness. The residue was purified by column chromatography (petroleum
ether/EtOAc 5:1 v/v) to obtain the target compounds 10a–l as white solids.
by the relationship: C0 = A0CAꢀ1
, where C = the concentration of standard
solution (mg mLꢀ1), A = absorbance of standard solution, A0 = absorbance of
saturated solution, and C0 = concentration of saturated solution (mg mLꢀ1).
21. Huang, Z. J.; Velázquez, C. A.; Abdellatif, K. R.; Chowdhury, M. A.; Reisz, J. A.;
Dumond, J. F.; King, S. B.; Knaus, E. E. J. Med. Chem. 2011, 54, 1356.
14. Analytical data for 10b: Mp: 116–120 °C. MS (ESI): m/z 345.1 [M+Na]+. IR (cmꢀ1
KBr): mmax 763, 1284, 1400, 1618, 1752, 2925. 1H NMR (300 Hz, CDCl3): d 0.90
,