Molecular iodine: An efficient catalyst for the synthesis of tetrahydrobenzo[b]pyrans

Article abstract of DOI:10.1080/00397910701578578

An iodine/DMSO system is introduced as an excellent catalyst in three-component coupling reactions of tetrahydrobenzo[b]pyran synthesis. The reaction proceeds quickly under very mild reaction conditions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910701578578

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Molecular Iodine: An Efficient Catalyst for the
Synthesis of Tetrahydrobenzo[b]pyrans
Rajesh S. Bhosale a , Chandrakant V. Magar a , Kuldeep S. Solanke a ,
Sandeep B. Mane a , Sunil S. Choudhary a & Rajendra P. Pawar a
a Organic Chemistry Synthesis Laboratory, Dnyanopasak College, Parbhani,
India
Published online: 14 Dec 2007.
To cite this article: Rajesh S. Bhosale , Chandrakant V. Magar , Kuldeep S. Solanke , Sandeep B. Mane ,
Sunil S. Choudhary & Rajendra P. Pawar (2007): Molecular Iodine: An Efficient Catalyst for the Synthesis of
Tetrahydrobenzo[b]pyrans, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 37:24, 4353-4357
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Synthetic Communicationsw, 37: 4353–4357, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701578578
Molecular Iodine: An Efficient Catalyst for
the Synthesis of Tetrahydrobenzo[b]pyrans
Rajesh S. Bhosale, Chandrakant V. Magar,
Kuldeep S. Solanke, Sandeep B. Mane, Sunil S. Choudhary,
and Rajendra P. Pawar
Organic Chemistry Synthesis Laboratory, Dnyanopasak College,
Parbhani, India
Abstract: An iodine/DMSO system is introduced as an excellent catalyst in three-
component coupling reactions of tetrahydrobenzo[b]pyran synthesis. The reaction
proceeds quickly under very mild reaction conditions.
Keywords: aldehydes, malononitrile, molecular iodine, tetrahydrobenzo[b]pyran
INTRODUCTION
Tetrahydrobenzo[b]pyrans are an important class of organic compounds that
attracts considerable attention because of their wide range of biological activi-
ties. These compounds show anticoagulant, anticancer, spasmolytic, diuretic,
and anti-ancaphylactia activity[1] Polysubstituted tetrahydrobenzo[b]pyrans
constitute a structural unit of a series of natural products.[2]
The conventional methods used for the synthesis of 4H-benzo[b]pyrans
were reported in acetic acid or dimethyl formamide.[3,4] Some improved
methods are also reported for the synthesis of tetrahydrobenzo[b]pyran deriva-
tives using hexadecyltrimethyl ammonium bromide and sodium bromide
catalysts[5,6] and synthesis by utilizing the reactants in solid or molten states.[7]
In recent years, the use of molecular iodine in organic synthesis has
received considerable attention. Because of numerous advantages associated
with this ecofriendly element, iodine has been explored as a powerful
Received May 21, 2007
Address correspondence to Rajendra P. Pawar, Organic Chemistry Synthesis Lab-
oratory, Dnyanopasak College, Parbhani 431401, India. E-mail: rppawar@yahoo.com
4353

4354
R. S. Bhosale et al.
catalyst for various organic transformations.[8] The acidity of iodine makes it
capable of binding with the aldehyde carbonyl oxygen, increasing the reactiv-
ity of the parent carbonyl compounds.[9]
RESULTS AND DISCUSSION
To examine the catalytic activity of iodine in solvent dimethylsulphoxide, the
reaction conditions of benzaldehyde and dimedone with malononitrile were
optimized (Scheme 1). In the present communication, the products were syn-
thesized by adding iodine (10 mmol%) to a solution of dimedone, aromatic
aldehydes, and malononitrile in the solvent dimethylsulphoxide. The mixture
was stirred at 1208C. The results suggest that the optimized system of
10 mmol% of catalyst is active enough to complete the reaction in 3–4 h.
However, the same reaction does not proceed in the absence of iodine and
dimethylsulphoxide even after stirring the reaction mixture at 1208C for 10 h.
The excellent result led us to expand this catalytic system of iodine/
dimethylsulphoxide (DMSO). Almost all reactions were completed in 3–4 h
at 1208C, although the yields of the products were highly dependent on the
substrate used. The generality of the system was also confirmed by using
other aromatic aldehydes for the reactions (Table 1). Both electron-rich and
electron-deficient substituted aromatic aldehydes produced an excellent
yield of products. The iodine/DMSO system has good activity for a variety
of substrates. This procedure is significant from the viewpoint of avoiding
pollution and practical industrial process.
General Procedure for the Synthesis of 2-Amino-3-cyano-7,7-
dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-benzo[b]pyran (4a)
A mixture of 1,3-cyclic dimedones 1a (10 mmol), benzaldehyde 2a
(10 mmol), malononitrile 3a (10 mmol), and molecular iodine (10 mmol%)
in DMSO (10 ml) was stirred at 1208C for 3–4 h. The progress of reaction
was monitored by thin-layer chromatography (TLC). After completion of
the reaction, the reaction mixture was cooled and poured onto crushed ice
Scheme 1.

Molecular Iodine as Catalyst
4355
Table 1. Iodine catalyzed synthesis of tetrahydrobenzo[b]pyran derivatives
Yield
(%)a
Entry
Products
Time (h)
3.2
Mp (8C)
231 (233[10]
4a
92
)
4b
3.5
88
208 (207–209[10]
)
4c
3.5
90
199 (198–200[10]
)
4d
4e
4f
3.6
4.0
3.8
90
85
85
211 (208–210[10]
133 (130–132[10]
130 (130–132[10]
)
)
)
(continued)

4356
R. S. Bhosale et al.
Table 1. Continued
Yield
(%)a
Entry
Products
Time (h)
4.0
Mp (8C)
4g
86
215 (210–212[10]
)
)
4h
3.8
80
209 (206–208[11]
aRefers to isolated yield.
and further stirred for 10–15 min. The solid separated was filtered and washed
with aqueous sodium thiosulphate solution (to remove excess iodine) and
water. The crude product was crystallized in ethanol to afford pure tetrahydro-
benzo[b]pyran 4a (92% yield).
Entry 4a
IR (KBr) nmax 3395, 3287, 2932, 2202, 1685, 1600 cm21 1H NMR
.
(300 MHz): d 1.05 (s, 3H, CH3), 1.15 (s, 3H, CH3), 2.16–2.31 (m, 2H,
CH2), 2.47 (s, 2H, CH2), 4.38 (s, 1H, CH), 4.59 (s, 2H, NH2), 7.12–7.34
(m, 5H, Ar-H).
Entry 4c
IR (KBr) nmax 3376, 3316, 2199, 1683, 1140, 1035 cm21 1H NMR
.
(300 MHz): d 1.05 (s, 3H, CH3), 1.11 (s, 3H, CH3), 2.16–2.27 (m, 2H,
CH2), 2.45 (s, 2H, CH2), 3.78 (s, 3H, OCH3), 4.38 (s, 1H, CH), 4.59 (s, 2H,
NH2), 6.82 (d, J ¼ 9.6 Hz, 2H, Ar-H), 7.17 (d, J ¼ 8.0 Hz, 2H, Ar-H).
ACKNOWLEDGMENT
The authors are thankful to W. N. Jadhav, Head of the Chemistry Department,
Dnyanopasak College, Parbhani, India, for his valuable guidance.

Molecular Iodine as Catalyst
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