Ionic liquid supported multistep divergent synthesis of benzimidazole linked pyrrolo-/pyrido-/isoindolo-benzimidazolones

Article abstract of DOI:10.1016/j.tetlet.2011.03.046

Diversity oriented parallel synthesis for bis-heterocyclic skeletal novel benzimidazole linked pyrrolo-/pyrido-benzimidazolones and benzimidazole linked isoindolo-benzimidazolones has been developed on ionic liquid support under microwave irradiation by u

Full text of DOI:10.1016/j.tetlet.2011.03.046

Tetrahedron Letters 52 (2011) 2818–2822
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ionic liquid supported multistep divergent synthesis of benzimidazole linked
pyrrolo-/pyrido-/isoindolo-benzimidazolones
⇑
Suman Thummanagoti, Gorakh S. Yellol, Chung-Ming Sun
Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300-10, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Diversity oriented parallel synthesis for bis-heterocyclic skeletal novel benzimidazole linked pyrrolo-/
pyrido-benzimidazolones and benzimidazole linked isoindolo-benzimidazolones has been developed
on ionic liquid support under microwave irradiation by utilizing the cascade cyclization. The key tandem
transformation comprises (i) amino-alkylation of immobilized o-phenylenediamine with ketoacids, (ii)
intramolecular cyclization through secondary amine on electrophilic imine carbon toward pentacyclic
aza-ring and (iii) second amido-cyclization to deliver cycloamide ring. The synergy arises by combined
use of microwave heating with ionic liquid support which is very effectively used to speed up multistep
synthesis of biological interesting heterocycles.
Received 9 January 2011
Revised 26 February 2011
Accepted 10 March 2011
Available online 30 March 2011
Keywords:
Ionic liquid support
Microwave synthesis
Cascade reaction
Bis-heterocycles
Benzimidazoles
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
activities associated with the pyrrolo[1,2-a]benzimidazole, pyr-
ido[1,2-a]benzimidazole, and imidazo[2,1-a]isoindole, there are
It is well known that heterocyclic compounds are the major
constituents of pharmaceutical ingredients and has embraced sig-
nificant space in medicinal chemistry, synthetic chemistry, and
natural products. Highly functionalized various ring size bis-het-
erocycles with different hetero atoms and substitution patterns
are of major interest in the drug discovery process.1 Bis-heterocy-
clic compounds offer better binding opportunities with enzyme ac-
tive site owing to its three dimensional special arrangement and
consequently depict many intrinsic biological properties.2 Particu-
larly, bis-heterocycles comprising pyridobenzimidazoles (I) are li-
gands for the BZD site on GABA-A receptors and are thus useful
for the treatment of disorders of the central nervous system includ-
ing convulsion such as epileptic seizures, anxiety, depression, mus-
cular spams, and attention deficit hyperactivity disorder (Fig. 1).3
Polycyclic bis-heterocycles containing imidazopyridine, imidazo-
pyridine, or imidazoisoindole moieties (II) constitute basic struc-
tural frameworks of potent inhibitors of respiratory syncytical
virus while some imidazoisoindoles (III) show antiviral activity.4
Benzimidazoles in combination with the pyrrolo–isoidolones (IV)
exhibit anticancer activity via the inhibition of the ATPase-type
catalytic activity of the Hsp90 chaperone protein.5 The geometric
assembly of bis-heterocycles characterized by the benzimidazole,
as a core connected to various fused heterocycles, is apparently
interesting to explore. Despite the broad range of biological
still very few reports for the construction of aforementioned three
classes linked with benzimidazole motif in linear fashion as bis-
heterocycles.6 Hence, interest remains strong to develop new ap-
proaches to produce novel bis-heterocyclic molecules and further
exploration of their biological applications.
Cl
N
O
N
S
F
N
CH3
N
O
N
HN
NH
N
N
O
O
BZD receptor antagonist (I)
RSV replication inhibitor (II)
O
N
N
H
O
N
N
N
N
N
H
n
H
Antiviral agent (III)
HSP90 Inhibitor (IV)
⇑
Corresponding author. Tel.: +886 3 5131511; fax: +886 3 5736007.
E-mail address: cmsun@mail.nctu.edu.tw (C.-M. Sun).
Figure 1. Biologically active bis-heterocyclic molecules.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.03.046

S. Thummanagoti et al. / Tetrahedron Letters 52 (2011) 2818–2822
2819
Integrating a variety of advanced technologies for the rapid syn-
thesis of numerous multi-functionalized heterocyclic molecules is
of high interest to provide rapid path for modern drug discovery.
Owing to the disadvantages like heterogeneous reaction conditions
of solid phase synthesis7 and low loading capacity of soluble poly-
mer supported synthesis,8 successful implementation of ionic li-
quid supported synthesis in small molecule synthesis retained
advantages over conventional solution phase chemistry.9 Depend-
ing on the choice of anions and cations, the solubility of the ionic
liquids can be tuned readily to control the phase trafficking in or-
ganic and aqueous phase. Ionic liquid supported synthesis has be-
come a very effective method for the production of combinatorial
libraries with high degree of chemical diversity.10 The numerous
advantages associated with the ionic liquid as a support has been
strongly appealing for the green synthesis.11 In addition to the
advantages like high loading capacity and recyclability of ionic li-
quid support, ionic liquids absorb microwave irradiation extremely
well due to the ionic conduction and thus reactions can be run in
non-polar solvents.12 Hence, application of microwave heating in
ionic liquid supported synthesis is a fast growing research area.
In continuation with our effort to develop multidisciplinary
synergetic approaches for rapid synthesis of medicinally interest-
ing compounds,13 here we report the efficient synthesis of bis-het-
erocyclic skeletal benzimidazole linked pyrrolo-/pyrido-/
isoindolone-benzimidazolones on ionic liquid support under
microwave conditions.
ingly, anilide conjugates 4 were obtained by the condensation of
acid 2 with IL conjugates 3 in presence of pyridine within 12 h in
refluxing dichloroethane. However, the application of microwave
irradiation at 100 °C reduced the reaction time to 10 min. By taking
the distinct solubility features of the ionic liquids, purification has
been carried out by precipitation of IL bound amide 4 in the low
polar solvent such as ether. The IL bound amide 4 was further cy-
clized to benzimidazole derivative 5 in the presence of trifluoro-
acetic acid and magnesium sulfate under microwave irradiation
in 5 min at 100 °C. However, it took 14 h to complete cyclization
under refluxing conditions. The progression of this transformation
was monitored directly on ionic liquid tag by the observation on
the chemical shift of aromatic region in proton NMR spectrum.
IL supported fluoronitrobenzoate 5 was further treated with
various primary amines in order to create second substitutional
diversity in the present skeleton by ipso-fluoro displacement reac-
tion. This reaction was performed in microwave conditions
(100 °C) for 5 min to offer ionic liquid attached conjugate 6, while
same reaction took 5 h in refluxing condition. Here it is noteworthy
to mention that primary amines did not cleave ionic liquid tag un-
der microwave harsh condition. Further nitro-group of IL conjugate
6 was reduced by treatment with zinc and ammonium formate for
5 min under microwave irradiations at 80 °C to furnish IL conju-
gate diamine 7. After completion of the reaction the IL supported
conjugates 7 was precipitated with ether and obtained in pure
form with good yields.
IL immobilized ortho-phenylenediamine 7 was used as a com-
mon scaffold to generate skeletal diversity in targeted bis-hetero-
cyclic molecules by one pot tandem transformation using various
ketoacids. The various 3-keto-acids were used to provide the pyr-
rolo fused benzimidazolone tricyclic skeleton while 4-keto-acids
were used to achieve skeletally different tricyclic framework of
pyrido fused benzimidazolones along with the substitutional
diversity depending on the groups present on the ketoacids.
Accordingly, IL immobilized ortho-phenylenediamine 7 was trea-
ted with various 3- and 4-keto-acids 8 in presence of TFA under
microwave irradiations. It is noteworthy to mention that the selec-
tive product generated in the coupling of ketoacid with o-phenyl-
enediamine is dependent on the quantity of TFA used in the
reaction mixture. With the stoichometric amount of TFA, diamine
groups of 7 cyclized with acid functionality of 8 afforded 2-alkyl
2. Results and discussion
The readily available 3-hydroxyethyl-(1-methylimidazolium)-
tetrafluoroborate 1 is selected as a suitable ionic liquid (IL) support
for multistep synthesis. The present strategy commenced with the
synthesis of IL immobilized ortho-phenylenediamine 3 from 4-flu-
oro-3-nitrobenzoic acid 2 with built-in structural diversity (R1)
through three step protocol. The ionic liquid 1 was attached to 4-
fluoro-3-nitrobenzoic acid 2 by esterification followed by ipso-flu-
oro substitution of primary amines and subsequent reduction of
nitro-group provided IL immobilized ortho-phenylenediamine 3
in overall good yields (Scheme 1). In an effort to attain the target
molecule, compound 3 was further N-acylated at the primary
amine functionality with 4-fluoro-3-nitrobenzoic acid 2. Accord-
O
O
IL
2, Pyridine
NH2
NO2
F
R1 NH2
O
OH
N
O
N
ClCH2CH2Cl
H3C
HO
MW, 100 oC, 10 min
BF4
NH
R1
1
2
3
F
O
O
NO2
H
N
IL
R2 NH2
CH2Cl2
MW, 100 oC, 5 min
IL
TFA, MgSO4
ClCH2CH2Cl
MW,100 oC, 5 min
N
O
NO2
F
O
N
NH
R1
R1
4
5
O
O
NO2
NH2
IL
IL
Zn, NH4COOH
MeOH
MW, 80 oC, 5 min
N
N
N
N
O
O
NH
R2
NH
R2
R1
R1
6
7
Scheme 1. Synthesis of IL supported diamine scaffold 7.

2820
S. Thummanagoti et al. / Tetrahedron Letters 52 (2011) 2818–2822
substituted bis-benzimidazoles 9a. The formation of compound 9a
was attributed to activation of acid functionality in presence of sto-
ichometric amount of TFA. The structure of compound 9a was later
confirmed as 9b by removal of ionic liquid support in sodium
methoxide solution. Alternatively, the intended cascade cyclization
of IL diamine conjugate 7 with ketoacids 8 was successfully
achieved in 20 mole % TFA in the presence of magnesium sulfate
and dichloroethane under focused microwave irradiation at
110 °C for 10 min (Scheme 2). The reaction mixture was purified
by precipitation, filtration, and washing to furnish IL immobilized
benzimidazole linked pyrrolo/pyrido-benzimidazolone derivatives
10 with good yields (72–85%). The same reaction was carried out
under conventional refluxing for 24 h to furnish the same product
which clearly depicts the superiority of microwave heating over
conventional heating reaction. The synthetic pathway leading to
pyrrolo/pyrido-benzimidazolones through one pot cascade reac-
tions includes (i) amino-alkylation of ionic liquid immobilized
benzimidazole linked diamine with ketoacids, (ii) intramolecular
cyclization through attack of secondary amine on electrophilic
imine carbon toward pentacyclic aza-ring formation and (iii) intra-
molecular cyclization to deliver second cyclic amine ring. A variety
of 4-ketoacids and 5-ketoacids were used in combination with the
N-alkyl substitution on diamine moiety for this tandem transfor-
mation to furnish pyrrolo-/pyrido-benzimidazoles with substitu-
tional diversity as depicted in Table 1. The formation of pyrrolo/
pyrido-benzimidazolone derivatives 10 was confirmed by proton
NMR spectrum and mass spectrum directly with IL support. The io-
nic liquid support was cleaved from 10 using sodium methoxide
methanol solution under microwave irradiation at 110 °C in
10 min to obtain benzimidazole linked pyrrolo/pyrido-benzimi-
dazolone derivatives 11 and 12. The ionic liquid was removed from
the reaction mixtures through precipitation by ether and filtration.
The characterization of recovered ionic liquid shows 96% purity
and 68% recovery after first cycle. The recovered ionic liquid was
recycled in the synthetic process. The separated desired products
were subjected to HPLC analysis for crude purity. Further column
chromatography purification furnished benzimidazole linked pyr-
rolo[1,2-a]benzimidazolones 11 and benzimidazole linked pyr-
ido[1,2-a]benzimidazolones 12 in good yields14 (Table 1). The
structure of final products was confirmed by spectroscopic analy-
sis. The appearance of the peaks due to alkyl protons present on
ketoacid moiety and the absence of peaks corresponding to amine
and acid protons were observed in the proton NMR spectrum of
compound 11 and 12. Additionally, the stereogenic quaternary car-
bon has appeared around 89.5 ppm in the 13C NMR spectrum along
with the amide carbonyl carbon absorbance peak at 175 ppm. The
formation of amide bond is further confirmed by the IR spectros-
copy which depicts amide frequency band around 1720 cmÀ1; par-
ticularly little higher due to the ketonic nature of tert-aminde
carbonyl functionality.
In order to expand the intended diversity profile of resulting
bis-heterocyclic molecules, the additional sulfur element was
incorporated in the skeleton. The IL supported benzimidazole
linked pyrido-benzimidazolone 10 was treated with Lawesson’s re-
agent in toluene under focused microwave conditions at 150 °C.
The thio derivative of IL supported pyrido-benzimidazolone 13
was obtained in good yield in 15 min while the same reaction took
18 h under the refluxing condition in toluene (Scheme 2). Further
removal of IL support affords the benzimidazole linked thiopyri-
do-benzimidazolone 14.
O
O
O
O
R3
NH2
R3
OH
N
IL
IL
n
n
N
N
N
O
O
8
N
O
NH
R2
R2
1 eq. TFA, MgSO4
ClCH2CH2Cl
MW,130 oC,10min
N
R1
R1
9a
9b
NaOMe, MeOH
7
MW, 80 oC, 10 min
8, 20 mol% TFA
MgSO4,ClCH2CH2Cl
MW, 130 oC, 10min
n = 1, R1 = 2-phenylethyl,
R2 = 2-cyclohexenylethyl, R3 = CH3
O
O
O
O
N
N
n
n
IL
NaOMe, MeOH
H3C
R3
R3
N
N
N
O
O
N
N
MW, 80 oC, 10 min
R2
R2
N
R1
10
R1
11: n =0; 12: n =1
Lawesson's reagent
ClCH2CH2Cl
MW,150 oC,15 min
S
S
O
O
N
N
n
n
IL
NaOMe, MeOH
H3C
R3
R3
N
N
N
O
O
N
N
MW, 80 oC, 10 min
R2
R2
N
R1
R1
14: n = 1, R1 = 2-phenylethyl,
13
R2 = 2-cyclohexenylethyl, R3=CH3
Scheme 2. Synthesis of pyrrolo-benzimidazolones and pyrido-benzimidazolones.

S. Thummanagoti et al. / Tetrahedron Letters 52 (2011) 2818–2822
2821
Table 1
Benzimidazole linked pyrrolo-/pyrido-benzimidazolones
O
O
N
n
R3
H3C
N
N
O
N
R2
R1
11 (n=0); 12 (n=1)
Entry
R1
R2
R3
MASSa
476
Yields (%)b
O
O
CH3
CH3
11a
80
82
11b
11c
11d
526
528
636
CH3
85
75
O
O
O
O
O
O
11e
11f
11g
11h
11i
F
570
570
582
580
580
78
75
73
72
75
F
F
Et
Et
Et
11j
592
490
76
80
O
CH3
CH3
12a
12b
12c
588
536
81
80
CH3
O
O
O
12d
12e
12f
566
566
578
78
75
77
a
Mass (LRMS) were detected as M+.
The overall yields were determined on the weight of purified samples obtained over two steps from 7 (%).
b
The scope of present cascade process was further extended by
simple precipitation and washing in ether. The same reaction re-
quired 24 h for complete cyclization in the refluxing condition.
The outcome of tandem transformation on aromatic ketobenzoic
acid is confirmed by analyzing the structure of compound 15 di-
rectly on ionic liquid support. The successive removal of ionic li-
quid support from 15 furnishes benzimidazole linked isoindolo-
benzimidazolones 16 with good yield (Scheme 3). The structure
of 18 was confirmed by observing the absence of peaks corre-
exploring ortho-ketobenzoic acids with selective ionic liquid conju-
gated diamines under the same reaction conditions to offer skele-
tally distinct bis-heterocyclic molecules. The IL immobilized
diamine 7 was treated with 2-acetylbenzoic acid with 20 mole %
TFA in dichloroethane under microwave irradiation at 110 °C
(Scheme 3). After 10 min, an IL supported isoindolo-benzimidazol-
ones 15 were obtained in good yields which were purified by

2822
S. Thummanagoti et al. / Tetrahedron Letters 52 (2011) 2818–2822
liquid as support for multistep organic synthesis possesses the
O
O
advantages like high loading soluble support, homogeneous reac-
tion phase, and recyclability. The powerful potential of multidisci-
plinary synthetic approach, integrating ionic liquid support, and
microwave synthesis with multistep synthesis has great potential
to produce biologically interesting molecules for drug discovery.
R3
O
N
IL
R3
R2
N
N
R1
COOH
O
7
N
20 mol% TFA, MgSO4
ClCH2CH2Cl
MW, 130 oC, 10 min
15
Acknowledgments
O
We thank the NSC (National Science Council) and the NCTU
(National Chiao Tung University) at Taiwan for financial support
of this work.
O
N
NaOMe, MeOH
H3C
R3
R2
N
N
R1
MW, 80 oC, 10 min
O
N
References and notes
16
1. (a) Jian, C.; Xian, H. J. Comb. Chem. 2010, 12, 1–4; (b) Dolle, R. E.; Le Bourdonnec,
B.; Goodman, A. J.; Morales, G. A.; Thomas, C. J.; Zhang, W. J. Comb. Chem. 2008,
10, 753–802.
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1179–1200; (b) Zambrowicz, B. P.; Sands, A. T. Nat. Rev. Drug Disc. 2003, 2, 38–
51.
Entry
16a
R1
R2
R3
Massa
Yieldsb
CH3
574
79
3. Ohta, S.; Naita, Y.; Yuasa, T.; Hatakeyama, S.; Kobayashi, M.; Kaibe, K.;
Kawasaki, I.; Yamashita, M. Chem. Pharm. Bull. 1991, 39, 2787.
4. Kuo-Long, Y.; Civiello, R. L.; Combrink, K. D.; Gulgeze, H. B.; Sin, N.; Wang, X.;
Nicholas, A. Venables, B. L. WO Patent 2001/95910 A1, 2001.
O
16b
16c
CH3
CH3
572
570
80
80
5. Smith, D. F. et al Pharmcol. Rev. 1998, 50, 493–513.
6. (a) Soural, M.; Bouillon, I.; Krchnak, V. J. Comb. Chem. 2008, 10, 923–933; (b)
Nefzi, A.; Giulianotti, M. A.; Houghten, R. A. J. Comb. Chem. 2001, 3, 68–70.
7. (a) Sears, P.; Wong, C. H. Science 2001, 291, 2344–2350; (b) Seeberger, P. H.;
Danishefsky, S. J. Acc. Chem. Res. 1998, 31, 685–695.
8. (a) Wentworth, P., Jr.; Janda, K. D. Chem. Commun. 1999, 1917–1918; (b)
Gravert, D. J.; Janda, K. D. Chem. Rev. 1997, 97, 489–509.
16d
CH3
622
82
9. (a) Ni, B.; Headley, A. D. Chem. Eur. J. 2010, 16, 4426–4436; (b) Miao, W.; Chan,
T. H. Org. Lett. 2003, 5, 5003–5005.
10. (a) Legeay, J. C.; Goujon, J. Y.; Vanden Eynde, J. J.; Toupet, L.; Bazureau, J. P. J.
Comb. Chem. 2006, 8, 829–833; (b) Yi, F. P.; Peng, Y. Q.; Song, G. H. Tetrahedron
Lett. 2005, 46, 3931–3933.
Scheme 3. Synthesis of benzimidazole linked isoindolo-benzimidazolones. aMass
(LRMS) were detected as M+. bThe overall yields were determined on the weight of
purified samples obtained over two steps from 7 (%).
11. (a) Miao, W. S.; Chan, T. H. Acc. Chem. Res. 2006, 39, 897–908; (b) Yao, Q.;
Zhang, Y. Angew. Chem. Int. Ed. 2003, 42, 3395–3398.
12. Roberts, B. A.; Strauss, C. R. Acc. Chem. Res. 2005, 38, 653–661.
13. (a) Yellol, G. S.; Chung, T. W.; Sun, C. M. Chem. Commun. 2010, 46, 9170–9172;
(b) Lai, J. J.; Salunke, D. B.; Sun, C. M. Org. Lett. 2010, 12, 2174–2177; (c) Maiti,
B.; Chanda, K.; Sun, C. M. Org. Lett. 2009, 11, 4826–4829.
sponding to amine and acid protons together with the emergence
of the new peaks due to aromatic protons of 2-acetyl benzoic acid
moiety in the proton NMR spectrum of compound 16 along with
amide frequency band around 1720 cmÀ1 in the IR spectra.
It is noteworthy to mention that the ionic liquid support is sta-
ble in microwave harsh conditions. Owing to high polar microwave
absorbance medium created by the ionic liquid support, time taken
to attain the desired reaction temperatures for all the reactions un-
der microwave exposure has substantially shortened and reactions
can run in low polar solvent media. The monitoring reaction pro-
gress is practicable by conventional spectroscopic methods with
ionic liquid support.
14. General procedure: (All microwave experiments performed under CEM Discover
Microwave system at the frequency of 2450 Hz and 0–300 W power in closed
vessel system.) To the solution of 7 in ethylenedichloride (9 mL), various
aliphatic 3- or 4-keto-acids 8 (1.5 equiv) were added, followed by addition of
dry MgSO4 and 20 mole % trifluoroacetic acid. The reaction mixture was
subjected to microwave irradiation for 10 min at 110 °C. The reaction mixture
was filtered and then precipitated by ether (60 mL). The precipitate was
filtered and washed with ether to get the IL immobilized benzimidazole linked
pyrrolo/pyrido-benzimidazoles 10. The solution of 0.1 M NaOMe in MeOH
(10 mL) was added to the solution of 10 in methanol. The reaction mixture was
subjected to microwave irradiation for 10 min at 110 °C. The cleaved ionic
liquid was precipitated by addition of ether solution and separated by
filtration. The filtrates were concentrated and subjected to HPLC analysis
(68–99%). The products were further purified by column chromatography over
silica gel using ethyl acetate/n-hexane (1:1) as eluent to obtain pure
benzimidazole linked pyrrolo/pyrido-benzimidazoles 11 and 12 in good
3. Conclusion
We have successfully developed rapid and efficient solution
phase approach to synthesize benzimidazole linked pyrrolo[1,2-
a]benzimidazolones, pyrido[1,2-a]benzimidazolones, and isoindo-
lo[1,2-a]benzimidazoles with three sets of diversity on ionic liquid
support under microwave conditions. A cascade reaction was
systematically applied to furnish skeletally diverse bis-heterocyclic
molecular libraries. This tandem transformation involves amino-
alkylation, intramolecular cyclization to form benzimidazole, and
successive cyclization to furnish fused cycloamide ring. Use of ionic
yields (72–85%). Compound 11a: 1H NMR (300 MHz, CDCl3)
d 8.63 (d,
J = 2.1 Hz, 1H), 8.17 (dd, J = 8.7, 2.1 Hz, 1H), 7.73 (d, J = 8.7 Hz, 1H), 7.63 (d,
J = 2.0 Hz, 1H), 7.56 (d, J = 8.7 Hz, 1H), 6.65 (d, J = 8.7 Hz, 1H), 4.33–4.28 (m,
2H), 3.97 (s, 3H), 3.62–3.52 (m, 4H), 3.39 (s, 3H), 2.84 (m, 1H), 2.63–2.52 (m,
2H), 2.43 (m, 1H), 2.26 (pent, J = 6.4 Hz, 1H), 1.55 (s, 3H), 0.91 (d, J = 6.4 Hz, 3H),
0.89 (d, J = 6.4 Hz, 3H); 13C NMR (75 MHz, CDCl3) d 175.0, 166.6, 154.1, 147.4,
136.5, 133.6, 130.9, 128.9, 126.8, 118.7, 114.3, 111.8, 111.4, 106.8, 89.8, 70.7,
67.1, 59.4, 53.3, 52.9, 43.4, 37.0, 33.4, 29.3, 23.5, 20.4, 20.22; MS (ESI): m/z 476
(M)+; HRMS (ESI): calcd for C27H32N4O4: m/z 476.2424; found 490.2588 (M)+;
IR (neat): 2360, 1712, 1658 cmÀ1
.