A novel multicomponent Zr-catalyzed synthesis of functionalized pyrano[3,2-b]pyrrole derivatives

Article abstract of DOI:10.1007/s11164-012-0965-3

A novel, efficient, and rapid procedure, one-pot condensation of 3-hydroxypyrrole, malononitrile, and aromatic aldehydes, with 10 mol % bis[N-(3,5-dicumylsalicylidene)anthracylaminato]zirconium(IV) dichloride as catalyst, in the presence of ultrasonic irradiation, has been developed for synthesis of 5-amino-7-aryl-6-cyano-4H-pyrano[3,2-b]pyrrole derivatives.

Full text of DOI:10.1007/s11164-012-0965-3

Res Chem Intermed
DOI 10.1007/s11164-012-0965-3
A novel multicomponent Zr-catalyzed synthesis
of functionalized pyrano[3,2-b]pyrrole derivatives
•
Saman Damavandi Reza Sandaroos
•
•
Gholam Hossein Zohuri Saied Ahmadjo
Received: 18 June 2012 / Accepted: 3 December 2012
Ó Springer Science+Business Media Dordrecht 2012
Abstract A novel, efficient, and rapid procedure, one-pot condensation of 3-hy-
droxypyrrole, malononitrile, and aromatic aldehydes, with 10 mol % bis[N-(3,5-
dicumylsalicylidene)anthracylaminato]zirconium(IV) dichloride as catalyst, in the
presence of ultrasonic irradiation, has been developed for synthesis of 5-amino-7-
aryl-6-cyano-4H-pyrano[3,2-b]pyrrole derivatives.
Keywords Pyrano[3,2-b]pyrrole Á One-pot Á Organometal
Introduction
Multicomponent reactions (MCRs) are very useful for rapid and efficient generation
of complex and diverse molecules. The exploratory power of MCRs is very high
and, because they are highly flexible and efficient, they can be regarded as among
the most versatile classes of tandem reaction [1]. In recent years, MCRs have been
important in synthetic organic chemistry because of their valued features, for
example convergence, productivity, facile execution, and generally high yield of
S. Damavandi (&)
Young Researchers Club, Sarvestan Branch, Islamic Azad University, Fras, Iran
e-mail: saman_damavandi@yahoo.com
R. Sandaroos (&)
Department of Chemistry, Faculty of Science, University of Birjand, Birjand, Iran
e-mail: r_sandaroos@birjand.ac.ir; r_sandaroos@yahoo.com
G. H. Zohuri
Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran
S. Ahmadjo
Department of Catalyst, Iran Polymer and Petrochemical Institute, Tehran, Iran
123

S. Damavandi et al.
products [2–4]. Recently, we have reported several MCRs for synthesis of
indenoquinolinones [5], pyranoindoles [6], and phenanthroimidazoles [7].
It is well known that pyrans are important core units in several natural products
and photochromic materials [8]. Much research has been devoted to the synthesis of
a variety of pyran derivatives, for example benzopyrans, naphthopyrans, and
4-substituted pyrans, to obtain more biologically potent heterocyclic systems
[9, 10]. Compounds with pyran ring systems have many pharmacological properties
and are important in biochemical processes [11, 12]. Recently, several methods
have been reported for the synthesis of pyran derivatives via three-component
condensation reactions [12–14].
The 4H-pyran derivatives are of the immense interest in the synthesis of drugs for
example their varied pharmacological and biological activity, for example
antimicrobial [14], mutagenicity [15], anti-proliferative [16], sex pheromone [17],
antitumor, and central nervous system activity [18]. Accordingly, the synthesis of
such compounds is an interesting challenge.
Use oforganometallic compounds enableextensive exploration for new methods and
techniques in organic synthesis. Our interest in organometallic compounds [19]
prompted us to investigate use of these as catalysts. To the best of our knowledge, one-
pot Zr-catalyzed synthesis of 5-amino-7-aryl-6-cyano-4H-pyrano[3,2-b]pyrrole deriv-
atives under ultrasonic irradiation conditions has not yet been reported. In continuation
of our recent MCR synthesis [20–24], we report herein the details of this study.
Experimental
General procedure for synthesis of pyranopyrrole derivatives
A mixture of aldehyde (1 mmol), 3-hydroxypyrrole (1 mmol), malononitrile
(1.1 mmol), and catalyst (0.1 mmol) in CH3CN (4 mmol) was stirred at 50 °C under
ultrasonication in an ultrasonic cleaner with a frequency of 40 kHz and a nominal power
100 W for the appropriate time (TLC). On completion of the reaction, the mixture was
extracted withAcOEt(2 9 10 ml) and washedwithaq. NaHCO3 solution(2 9 10 ml).
TheorganicphasewasthendriedoverMgSO4 andconcentratedunder reducedpressure.
1
The crude product was purified by column chromatography on silica gel. H NMR,
13C NMR, and DEPT 13C NMR spectra were recorded on a Bruker 400-MHz
spectrometer with chloroform as solvent and TMS as internal standard. IR spectra were
recorded using KBr disks on a FT-IR Bruker Tensor spectrometer. The procedure for
synthesis of the catalyst has been reported elsewhere [19].
5-Amino-1,7-dihydro-7-(4-hydroxyphenyl)pyrano[3,2-b]pyrrole-6-carbonitrile
(5a)
AnalCalcdforC14H11N3O2 (253.26):C, 66.40;H, 4.38;N, 16.59 %. FoundC, 66.38;H,
4.37; N, 16.55 %. IR (KBr, mmax, cm-1): 3,408 and 3,276 (asym. and sym. str. of –NH2),
3,402 (NH), 2,160 (–CN str.), 1,255 (asym. str. of cyclic ArC–O–C ether). 1H NMR
(400 MHz, CDCl3) dH (ppm): 5.48 (s, 1H, pyran H4), 6.15 (d, 1H, pyrrole H3), 6.59
123

Synthesis of functionalized pyrano[3,2-b]pyrrole derivatives
(d, 2H, Ar–H), 6.65 (d, 1H, pyrrole H2), 6.88 (s, 2H, D2O exch. NH2), 6.95 (d, 2H, Ar–
H), 7.44 (s, 1H, pyrrole NH). 13C NMR (250 MHz, CDCl3) dC (ppm): 27.9 (pyran C4),
58.5 (pyran C3), 104.1 (pyran C6), 104.8 (pyrrole C3), 120.3 (pyrrole, C1), 124.2 (CN),
125.1 (pyran C5), 119.1, 128.3, 130.9, 153.8 (Ar–C), 170.49 (pyran C2).
5-Amino-1,7-dihydro-7-(4-methoxyphenyl)pyrano[3,2-b]pyrrole-6-carbonitrile
(5b)
Anal Calcd for C15H13N3O2 (267.28): C, 67.40; H, 4.90; N, 15.72 %. Found C,
66.22; H, 4.78; N, 15.59 %. IR (KBr, mmax, cm-1): 3,411 and 3,279 (asym. and sym.
str. of –NH2), 3,400 (NH), 2,160 (–CN str.), 1,251 (asym. str. of cyclic ArC–O–C
1
ether). H NMR (400 MHz, CDCl3) dH (ppm): 3.37 (s, 3H, OCH3), 5.43 (s, 1H,
pyran H4), 6.11 (d, 1H, pyrrole H3), 6.63 (d, 2H, Ar–H), 6.68 (d, 1H, pyrrole H2),
6.89 (s, 2H, D2O exch. NH2), 7.05 (d, 2H, Ar–H), 7.54 (s, 1H, pyrrole NH). 13C
NMR (250 MHz, CDCl3) dC (ppm): 28.6 (pyran C4), 55.23 (O–CH3), 59.5 (pyran
C3), 101.5 (pyran C6), 104.9 (pyrrole C3), 119.3 (pyrrole, C1), 122.0 (CN), 126.1
(pyran C5), 119.2, 127.4, 130.3, 154.6 (Ar–C), 171.4 (pyran C2).
5-Amino-1,7-dihydro-7-phenylpyrano[3,2-b]pyrrole-6-carbonitrile (5c)
Anal Calcd for C14H11N3O (237.26): C, 70.87; H, 4.67; N, 17.71 %. Found C, 68.59; H,
4.62; N, 16.97 %. IR (KBr, mmax, cm-1): 3,405 and 3,283 (asym. and sym. str. of –NH2),
3,405 (NH), 2,157 (–CN str.), 1,247 (asym. str. of cyclic ArC–O–C ether). 1H NMR
(400 MHz, CDCl3) dH (ppm): 5.51 (s, 1H, pyran H4), 6.25 (d, 1H, pyrrole H3), 6.73
(d, 1H, pyrrole H2), 6.75 (s, 2H, D2O exch. NH2), 7.09–7.15 (m, 5H, Ar–H), 7.50 (s, 1H,
pyrrole NH). 13C NMR (250 MHz, CDCl3) dC (ppm): 30.6 (pyran C4), 55.8 (pyran C3),
101.6 (pyran C6), 106.4 (pyrrole C3), 117.9 (pyrrole, C1), 123.5 (CN), 129.7 (pyran C5),
122.8, 129.1, 127.9, 130.8 (Ar–C), 173.7 (pyran C2).
5-Amino-1,7-dihydro-7-p-tolylpyrano[3,2-b]pyrrole-6-carbonitrile (5d)
Anal Calcd for C15H13N3O (251.28): C, 71.70; H, 5.21; N, 16.72 %. Found C,
70.93; H, 5.08; N, 16.14 %. IR (KBr, mmax, cm-1): 3,422 and 3,272 (asym. and sym.
str. of –NH2), 3,419 (NH), 2,189 (–CN str.), 1,252 (asym. str. of cyclic ArC–O–C
1
ether). H NMR (400 MHz, CDCl3) dH (ppm): 2.31 (s, 3H, Ar–CH3), 5.60 (s, 1H,
pyran H4), 6.18 (d, 1H, pyrrole H3), 6.80 (s, 2H, D2O exch. NH2), 6.89 (d, 1H,
pyrrole H2), 6.98–7.07 (m, 4H, Ar–H), 7.38 (s, 1H, pyrrole NH). 13C NMR
(250 MHz, CDCl3) dC (ppm): 24.7 (–CH3), 29.21 (pyran C4), 54.6 (pyran C3),
100.1 (pyran C6), 107.3 (pyrrole C3), 115.7 (pyrrole C1), 126.5 (CN), 131.4 (pyran
C5), 128.3, 129.1, 133.6, 137.9 (Ar–C), 169.1 (pyran C2).
5-Amino-7-(4-bromophenyl)-1,7-dihydropyrano[3,2-b]pyrrole-6-carbonitrile
(5e)
Anal Calcd for C14H10BrN3O (316.15): C, 53.19; H, 3.19; N, 13.29 %. Found C,
52.79; H, 3.14; N, 12.64 %. IR (KBr, m, cm-1): 3,425 and 3,269 (asym. and sym. str.
123

S. Damavandi et al.
of –NH2), 3,434 (NH), 2,201 (–CN str.), 1,242 (asym. str. of cyclic ArC–O–C
ether). 1H NMR (400 MHz, CDCl3) dH (ppm): 5.40 (s, 1H, pyran H4), 6.11 (d, 1H,
pyrrole H3), 6.49 (s, 2H, D2O exch. NH2), 7.06 (d, 2H, Ar–H), 6.70 (d, 1H, pyrrole
H2), 7.25 (d, 2H, Ar–H), 7.39 (s, 1H, pyrrole NH). 13C NMR (250 MHz, CDCl3) dC
(ppm): 31.4 (pyran C4), 64.9 (pyran C3), 100.5 (pyran C6), 109.5 (pyrrole C3), 117.3
(pyrrole, C1), 127.2 (CN), 130.3 (pyran C5), 123.6, 129.4, 130.9, 138.4 (Ar–C),
179.7 (pyran C2).
5-amino-7-(4-fluorophenyl)-1,7-dihydropyrano[3,2-b]pyrrole-6-carbonitrile (5f)
Anal Calcd for C14H10FN3O (255.25): C, 65.88; H, 3.95; N, 16.46 %. Found C,
65.81; H, 3.90; N, 16.43 %. IR (KBr, m, cm-1): 3,422 and 3,267 (asym. and sym. str.
of –NH2), 3,439 (NH), 2,211 (–CN str.), 1,240 (asym. str. of cyclic ArC–O–C
ether). 1H NMR (400 MHz, CDCl3) dH (ppm): 5.36 (s, 1H, pyran H4), 6.01 (d, 1H,
pyrrole H3), 6.41 (s, 2H, D2O exch. NH2), 7.01 (d, 2H, Ar–H), 6.65 (d, 1H, pyrrole
H2), 7.20 (d, 2H, Ar–H), 7.33 (s, 1H, pyrrole NH). 13C NMR (250 MHz, CDCl3) dC
(ppm): 31.0 (pyran C4), 62.7 (pyran C3), 100.9 (pyran C6), 108.9 (pyrrole C3), 116.7
(pyrrole, C1), 126.0 (CN), 130.0 (pyran C5), 122.6, 128.4, 133.7, 137.6 (Ar–C),
177.5 (pyran C2).
5-Amino-7-(2-bromophenyl)-1,7-dihydropyrano[3,2-b]pyrrole-6-carbonitrile
(5g)
Anal Calcd for C14H10BrN3O (316.15): C, 53.19; H, 3.19; N, 13.29 %. Found: C,
51.98; H, 3.16; N, 13.14 %. IR (KBr, mmax, cm-1): 3415 and 3298 (asym. and sym.
str. of –NH2), 3411 (NH), 2187 (-CN str.), 1257 (asym. str. of cyclic ArC–O–C
ether). 1H NMR (400 MHz, CDCl3) dH (ppm): 5.58 (s, 1H, pyran H4), 6.19 (d, 1H,
pyrrole H3), 6.73 (d, 1H, pyrrole H2), 6.79 (s, 2H, D2O exch., NH2), 6.97-7.11
(m, 4H, Ar–H), 7.58 (s, 1H, pyrrole NH). 13C NMR (250 MHz, CDCl3) dC (ppm):
28.3 (pyran C4), 57.5 (pyran C3), 108.6 (pyran C6), 108.9 (pyrrole C3), 121.6
(pyrrole, C1), 127.5 (CN), 131.5 (pyran C5), 122.8, 122.4, 128.9, 132.7, 133.0, 141.6
(Ar–C), 179.0 (pyran C2).
5-Amino-7-(4-chlorophenyl)-1,7-dihydropyrano[3,2-b]pyrrole-6-carbonitrile
(5h)
Anal Calcd for C14H10ClN3O (271.7): C, 61.89; H, 3.71; N, 15.47 %. Found C,
61.34; H, 3.69; N, 15.43 %. IR (KBr, mmax, cm-1): 3,414 and 3,259 (asym. and sym.
str. of –NH2), 3,415 (NH), 2,156 (–CN str.), 1,256 (asym. str. of cyclic ArC–O–C
ether). 1H NMR (400 MHz, CDCl3) dH (ppm): 5.48 (s, 1H, pyran H4), 6.16 (d, 1H,
pyrrole H3), 6.42 (s, 2H, D2O exch. NH2), 6.64 (d, 1H, pyrrole H2), 7.06 (d, 2H,
Ar–H), 7.21 (d, 2H, Ar–H), 7.29 (s, 1H, pyrrole NH). 13C NMR (250 MHz, CDCl3)
dC (ppm): 30.7 (pyran C4), 60.23 (pyran C3), 106.5 (pyran C6), 108.4 (pyrrole C3),
119.0 (pyrrole, C1), 124.8 (CN), 136.1 (pyran C5), 129.5, 129.4, 130.5, 132.8
(Ar–C), 179.7 (pyran C2).
123

Synthesis of functionalized pyrano[3,2-b]pyrrole derivatives
5-Amino-7-(2-chlorophenyl)-1,7-dihydropyrano[3,2-b]pyrrole-6-carbonitrile
(5i)
Anal Calcd for C14H10ClN3O (271.7): C, 61.89; H, 3.71; N, 15.47 %. Found C,
61.23; H, 3.66; N, 15.18 %. IR (KBr, mmax, cm-1): 3,458 and 3,256 (asym. and sym.
str. of –NH2), 3397 (NH), 2167 (–CN str.), 1248 (asym. str. of cyclic ArC–O–C
ether). 1H NMR (400 MHz, CDCl3) dH (ppm): 5.51 (s, 1H, pyran H4), 6.28 (d, 1H,
pyrrole H3), 6.64 (d, 1H, pyrrole H2), 6.84 (s, 2H, D2O exch. NH2), 7.09–7.19 (m,
4H, Ar–H), 7.43 (s, 1H, pyrrole NH). 13C NMR (250 MHz, CDCl3) dC (ppm): 26.2
(pyran C4), 55.6 (pyran C3), 113.9 (pyran C6), 114.2 (pyrrole C3), 118.6 (pyrrole,
C1), 129.1 (CN), 137.2 (pyran C5), 125.4, 127.3, 128.9, 132.7, 133.0, 139.6 (Ar–C),
173.7 (pyran C2).
5-Amino-7-(4-cyanophenyl)-1,7-dihydropyrano[3,2-b]pyrrole-6-carbonitrile
(5j)
Anal Calcd for C15H10N4O (262.09): C, 68.69; H, 3.84; N, 21.36 %. Found C,
67.84; H, 3.75; N, 20.90 %. IR (KBr, mmax, cm-1): 3,401 and 3,248 (asym. and sym.
str. of –NH2), 3,418 (NH), 2,210 (–CN str.), 2,181 (–CN str.), 1,257 (asym. str. of
cyclic ArC–O–C ether). 1H NMR (400 MHz, CDCl3) dH (ppm): 5.51 (s, 1H, pyran
H4), 6.10 (d, 1H, pyrrole H3), 6.48 (s, 2H, D2O exch. NH2), 6.60 (d, 1H, pyrrole
H2), 7.24 (s, 1H, pyrrole NH), 7.26 (d, 2H, Ar–H), 7.38 (d, 2H, Ar–H),. 13C NMR
(250 MHz, CDCl3) dC (ppm): 34.6 (pyran C4), 62.78 (pyran C3), 111.7 (pyran C6),
114.5 (pyrrole C3), 122.4 (pyrrole, C1), 124.8 (CN), 126.5 (CN), 139.4 (pyran C5),
114.8, 129.9, 133.5, 139.5 (Ar–C), 176.3 (pyran C2).
Results and discussion
In a preliminary experiment we found that the three-component condensation
reaction of 3-hydroxypyrrole, malononitrile, and benzaldehyde in CH3CN using
10 mol % Zr catalyst as promoter in the presence of ultrasonic irradiation worked
very well and the corresponding pyrano[3,2-b]pyrrole was obtained in good yield. In
this paper, therefore, an efficient and rapid procedure for one-pot synthesis of
pyrano[3,2-b]pyrrole derivatives by using a Zr catalyst in the presence of ultrasonic
waves is described (Scheme 1).
When toluene and benzene, as apolar media, were used for the model reaction,
chemical yields and reaction times were poor compared with those achieved by use
of acetonitrile as polar solvent.
The generality of this process was demonstrated by the wide range of substituted
aromatic aldehydes used to synthesize a series of functionalized pyrano[3,2-
b]pyrrole derivatives (Table 1, entries 1–8). It was found that the electronic nature
of substituents on the aromatic aldehyde could affect the reactions in terms of
reaction times and chemical yields. Yield of the reactions were increased by
changing the substituent groups on the benzaldehyde from hydroxyl and methoxy to
Br and F.
123

S. Damavandi et al.
Cl
N
Zr
Cl
O
2
OH
O
NH2
O
Ph
CN
CN
Ph
+
+
H
Ar
N
CH3CN,
US irradiation
N
H
N
H
Ar
Scheme 1 Zr-catalyzed synthesis of functionalized pyrano[3,2-b]pyrrole derivatives
As illustrated in Scheme 2, the catalyst could be coordinated with carbonyl
oxygen increasing the reactivity of the carbonyl compound. Moreover, the catalyst
is capable of binding with the nitrogen atom to facilitate heterocyclization to afford
the corresponding pyrano[3,2-b]pyrrole product.
On the basis of recent reports which have proposed ortho-quinone methides
(oQMs) as in-situ intermediates in one-pot three-component synthesis of
naphthopyran derivatives [16], we envisaged a mechanism with a similar
intermediate (A; Scheme 2). Michael-type addition of malononitrile to A is
followed by attack of a hydroxyl group on one of two nitrile groups to afford the
final product.
1
In the H NMR spectrum of compound 2 the aromatic hydrogens give rise to
multiplet signals in the aromatic region of the spectrum (dH = 6.11–7.05 ppm)
and two broad singlet peaks are seen at 6.75 and 7.50 ppm that disappear at the
presence of D2O. On the basis of their integration, they are attributed to NH2 and
NH groups, respectively. The proton next to the nitrogen atom of the pyrrole ring
appears as a doublet peak at dH = 6.68 ppm. Conjugation with the N atom of
pyrrole results in lower chemical shifts of the H2 and H3 hydrogens compared
with five hydrogens of phenyl ring, being present at approximately 7.1 ppm. H4 of
the pyran ring is substantially down-field, because of conjugation with the phenyl
and pyrroles ring, and it is present at approximately 5.51 ppm. Two doublets are
present at 6.25 and 6.73 ppm that split each other by the same coupling constant;
these are ascribed to H2 and H3 of pyrrole. Also, sharp singlets at dH = 3.37 and
dH = 5.43 ppm are assigned to the methoxy group and CH of the pyran ring,
respectively. Resonance of the aromatic protons of the substituted aromatic ring
on the pyran moiety possessing a methoxy group are observed at dH = 6.63 and
7.05 ppm. In addition, the 13C NMR spectrum of compound 1 contained 13
distinct resonances, in agreement with the proposed structure. Five signals
(dC = 28.6, 59.5, 101.5, 126.1, and 171.4 ppm) are assigned to the pyran carbon
atoms. Two signals assigned to the pyrrole ring can be observed at dC = 104.9
and 119.3 ppm. Resonance of aromatic carbons of the substituted aromatic ring on
the pyran moiety possessing methoxy group (Ar–C), are observed at dC = 119.2,
127.4, 130.3, and 154.6 ppm. Also, the resonance of the functional nitrile and
methoxy groups could be carefully assigned in the range dC = 55.23 and
122 ppm.
123

Synthesis of functionalized pyrano[3,2-b]pyrrole derivatives
Table 1 One-pot, three-component synthesis of 5-amino-7-aryl-6-cyano-4H-pyrano[3,2-b]pyrroles
Entry
1
Aldehyde
Product
Time (min)
58
Yield (%)
75
O
NH2
OH
N
H
N
O
OH
O
2
55
77
NH2
OMe
O
N
H
N
O
O
3
4
52
50
85
76
NH2
N
H
N
O
O
NH2
N
H
N
O
5
45
82
Br
O
NH2
N
H
N
O
Br
6
45
88
O
NH2
F
N
H
N
O
F
123

S. Damavandi et al.
Table 1 continued
Entry
7
Aldehyde
Product
Time (min)
50
Yield (%)
82
O
O
NH2
N
H
Br
N
Br
O
8
48
85
NH2
Cl
N
H
N
O
Cl
9
55
80
O
NH2
N
H
Cl
N
Cl
O
10
50
72
O
NH2
CN
N
H
N
O
CN
Reaction conditions: 1.0 equiv. 3-hydroxypyrrole, 1.0 equiv. aldehyde, 1.0 equiv. malononitrile,
10 mol % Zr catalyst, 4 ml CH3CN as solvent, 50 °C
Zr
CN
CN
Zr
CN
CN
Ar
Ar
Ar
Ar
H
O
CN
C
O
CN
NH
CN
NH2
Ar
HN
HN
HN
HN
HN
O
N
OH
O
Zr
OH
A
Scheme 2 The suggested mechanism for the one-pot synthesis of pyrano[3,2-b]indoles
Conclusion
In conclusion, we have described a novel and efficient method for synthesis of
pyrano[3,2-b]pyrrole derivatives by use of bis[N-(3,5-dicumylsalicylidene)-anthra-
cylaminato]zirconium(IV) dichloride as catalyst with the aid of ultrasonic irradi-
ation. The method has several advantages including good to high yields, short
123

Synthesis of functionalized pyrano[3,2-b]pyrrole derivatives
reaction times, and broad applicability of substrates. Hence, it is a useful addition to
the existing methods.
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