Sodium dodecyl sulfate-assisted synthesis of 1-(Benzothiazolylamino)methyl- 2-naphthols in water

Article abstract of DOI:10.1071/CH10209

An efficient synthesis of 1-(benzothiazolylamino)methyl-2-naphthols has been developed in water by one-pot condensation of 2-naphthol, aldehydes and 2-aminobenzothiazole catalyzed by sodium dodecyl sulfate. Advantages of the methodology include a very sho

Full text of DOI:10.1071/CH10209

Communication
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 1538–1540
www.publish.csiro.au/journals/ajc
Sodium Dodecyl Sulfate-assisted Synthesis of
1-(Benzothiazolylamino)methyl-2-naphthols
in Water
A B
,
A
A
Anil Kumar,
M. Sudershan Rao, and V. Kameswara Rao
A
Chemistry Group, Birla Institute of Technology and Science, Pilani 333 031, India.
Corresponding author. Email: anilkumar@bits-pilani.ac.in
B
An efficient synthesis of 1-(benzothiazolylamino)methyl-2-naphthols has been developed in water by one-pot condensa-
tion of 2-naphthol, aldehydes and 2-aminobenzothiazole catalyzed by sodium dodecyl sulfate. Advantages of the
methodology include a very short reaction time, excellent yields and catalytic use of the sodium dodecyl sulfate.
Manuscript received: 24 May 2010.
Manuscript accepted: 29 September 2010.
o-Quinone methides (o-QMs) are highly reactive, transient
species that have been applied as intermediates in the synthesis
of several natural products including flavonoids, isoflavans,
chromenes and benzopyrans.[1–4] Because of their synthetic
utility and biological importance, various methods have been
reported for generating these extremely reactive intermediates
that rely on the use of catalysts, acidic or basic conditions, high
temperatures or long reaction times.[5–8] The ortho aldolization
of phenols with aldehydes followed by Lewis acid-assisted
water elimination is probably the most convenient path available
up to now. The o-QM thus generated then participates in a
regiospecific, [4 þ 2] cycloaddition reaction with olefins, enol
ethers or enamines, along with formation of side products.[1a,9]
In recent years, trapping of o-QM by suitable nucleophiles has
been used to provide a rapid access to novel Michael addition
products.[10–11]
With growing concern over the environmental impact of
chemicals and tight legislation, development of greener chemi-
cal processes in synthetic chemistry has been advocated. At the
heart of green chemistry are alternative reaction media. They are
the basis of many of the cleaner chemical technologies that have
reached commercial development. Water is among the most
environmentally friendly and least expensive solvents, and thus
reactions in aqueous medium have gained considerable momen-
tum.[12–14] In many of these reactions, water not only acts as a
greener solvent but also accelerates reaction rates and enhances
reaction selectivity.[15] However, most organic substances are
insoluble in water and this hampers the complete exploitation of
water as a solvent in organic synthesis.
Sodium dodecyl sulfate (SDS) is a surfactant, and the
surfactant used in water can make organic materials soluble,
so it can solve the drawback of the reactions in water. The use of
SDS as a catalyst in aqueous medium for promoting various
transformations is well documented in the literature.[16] There-
fore, development of a new method that is efficient and leads to
better yields under environmental friendly conditions is well
appreciated.
Benzothiazole is an important heterocyclic unit that has been
widely used as a key building block for pharmaceutical agents.
Its derivatives are endowed with many pharmacological pro-
perties such as antitubercular, antiallergic, anti-inflammatory
and fungicidal activity.[17] Recently, two reports appeared for
the synthesis of 1-(benzothiazolylamino)methyl-2-naphthols
using LiCl and heteropolyacid (HPA).[18] In continuation of
our interest in green synthesis and the use of o-QM as an
intermediate in the synthesis of organic molecules with inter-
esting biological properties,[19] herein we report a green,
one-pot, efficient synthesis of 1-(benzothiazolylamino)methyl-
2-naphthols catalyzed by sodium dodecyl sulfate (SDS) in water
(Scheme 1).
Initially, the efficacy of various catalysts was tested for
the model reaction of 4-chlorobenzaldehyde, 2-nathphol and
2-aminobenzothiazole to give 1-((benzo[d]thiazol-2-ylamino)
(4-chloro-phenyl)-methyl)naphthalen-2-ol (4g) under different
CHO
S
R1
H
N
N
S
OH
SDS
N
OH
NH2
R
H2O, 100ЊC
R1
R
1
2
3
4
Scheme 1.
Ó CSIRO 2010
10.1071/CH10209
0004-9425/10/111538

Synthesis of 1-(Benzothiazolylamino)methyl-2-naphthols
1539
Table 1. Comparison of different reaction conditions for the synthesis
of 4g
Table 3. Synthesis of different of 1-(benzothiazolylamino)methyl-2-
naphthols using sodium docecyl sulfate (SDS) in water
Reaction conditions: 4-chlorobenzaldehyde (1.0 mmol), 2-naphthol
(1.0 mmol), 2-aminobenzothiazole (1.0 mmol), SDS (20 mol-%) and water
(3.0 mL)
Reaction conditions: 4-chlorobenzaldehyde (1.0 mmol), 2-naphthol
(1.0 mmol), 2-aminobenzothiazole (1.0 mmol) and water (3.0 mL). SDS,
sodium dodecyl sulfate; pTSA, p-toluenesulfonic acid
Sr. no.
Catalyst
Catalyst [mol-%]
Time [h]
Yield [%]A
Sr. no.
R
R1
ProductA Time [h] Yield [%]B
1
2
SDS
20
10
5
1
1
1
1
1
6
3
6
1
1
88
81
76
71
57
50
69
40
75
55
1
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
4a
4b
4c
4d
2
2
2
2
2
2
1
1
2
2
2
2
3
3
1
5
80
84
89
86
92
93
88
84
80
82
78
92
75
76
71
74
SDS
2
4-CH3O
3-CH3O
2-CH3O
3
SDS
3
4
SDS
30
50
–
4
5
SDS
5
2,4-(CH3O)2 4e
6
–
6
4-CH3
4-Cl
3-Cl
4-N
4f
7
pTSA
YbCl3
Yb(OTf)3
Sc(OTf)3
20
20
20
20
7
4g
4h
4i
8
8
9
9
10
10
11
12
13
14
15
16
4-F
4j
3-Br
2-F
4k
4l
AIsolated yield.
3-NO2
4-NO2
4m
4n
4o
4p
Table 2. The comparison of different solvents for synthesis of 4g using
SDS as catalyst
7-OCH3 4-Cl
5-Aza 4-Cl
Reaction conditions: 4-chlorobenzaldehyde (1.0 mmol), 2-naphthol
(1.0 mmol), 2-aminobenzothiazole (1.0 mmol), catalyst (20 mol-%) and
solvent (3.0 mL)
AStructure of all the products was well characterized by 1H and 13C NMR
and mass spectroscopy.
BIsolated yield after purification by column chromatography.
Sr. no.
Solvent
Time [h]
Yield [%]A
88
water using SDS as catalyst. It shows that water not only acts as a
reaction medium but also assists the reaction rate. The role of
SDS is not clear but it is expected that it provides a micellar
environment and acts as mild Brønsted acid in aqueous medium.
To explore the scope of this reaction, various substituted
aromatic aldehydes were reacted with 2-naphthol and 2-amino-
benzothiazole under optimized reaction conditions, and the
results are shown in Table 3. The reaction proceeded efficiently
with both aldehydes containing electron-withdrawing and
electron-releasing substituents. However, the yield of product
was lower with aldehydes containing electron-withdrawing
substituents compared with aldehydes with electron-donating
groups (Table 3, entries 9, 13, 14). The reaction of 4-chloro-
benzaldehyde with other activated phenols such as 7-methoxy-
2-naphthol, 6-hydroxyquinoline and 2-aminobenzothiazole gave
corresponding products in 71 and 74% yield respectively
(Table 3, entries 15, 16) but with 2-methoxyphenol, reaction
did not proceed.
All the synthesized compounds were characterized by
1H NMR and mass spectroscopy (see Accessory Publication).
Presumably the reaction proceeds through the formation of an
o-QM intermediate followed by Michael addition by 2-
aminobenzothiazole to give the desired product, as shown in
Scheme 2. Formation of the imine as an intermediate was ruled
out by examining the reaction of the corresponding imine
generated in situ with 2-naphthol, which did not result in the
formation of product. It is interesting to mention that under these
conditions, formation of xanthenes was not observed as a by-
product. Further studies for in situ trapping of o-QM generated
in aqueous media for the synthesis of different heterocylic
compounds are under progress in our laboratory.
1
2
H2O
1
6
6
6
6
6
6
6
6
6
B
DMF
–
B
3
DMSO
Acetonitrile
Toluene
Ethanol
Methanol
CHCl3
–
4
12
45
15
5
6
B
–
7
8
10
B
–
9
Acetone
THF
10
10
AIsolated yield.
BNo product formation was observed.
reaction conditions. The results, indicated in Table 1, showed
that among the screened catalysts such as p-toluenesulfonic acid
(pTSA), YbCl3, Yb(OTf)3, Sc(OTf)3 and SDS, SDS was super-
ior, with better yields isolated in comparable or shorter times.
In other cases, formation of xanthenes was observed as a by-
product along with the desired product. Recently, Kumar et al.
reported the synthesis of Betti base in water catalyzed by Triton
X-100.[20] They showed that among different surfactants used,
Triton X-100 was the most effective for this reaction.
Next, we optimized the catalyst loading for the model
reaction by varying the catalyst amount from 5 to 50 mol-%.
It was observed that increasing the catalyst loading up to
20 mol-% increased the yield of 4g, but beyond 20 mol-%, it
did not improve; rather, it decreased (Table 1). Thus, we took
20 mol-% of SDS as the optimum amount for further studies.
It should also be pointed out here that the reaction gave only
50% of 4g in the absence of SDS after 6 h (Table 1, entry 6),
confirming the effectiveness of the catalyst.
In conclusion, we have developed an efficient method for
the synthesis of 1-(benzothiazolylamino)methyl-2-naphthols
by trapping the o-QM generated by the reaction of a reactive
phenol with aldehyde in the presence of SDS in situ with
2-aminobenzothiazole as nucleophile in aqueous medium.
We also examined the influence of solvents on the reaction
yields. It was found that a very low yield of the desired product
4g was obtained in organic solvents, except toluene in which 4g
was obtained in 45% yield (Table 2). Yield of 4g was 88% in

1540
A. Kumar, M. S. Rao, and V. K. Rao
Cl
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O
–H2O
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S
o-QM
H2N
Michael addition
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S
N
H
N
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General Procedure for the Synthesis of
1-(Benzothiazolylamino)methyl-2-naphthols
To a mixture of 2-naphthol (0.2 g, 1.0 mmol), 4-chloro-
benzaldehyde (0.214 g, 1.0 mmol) and 2-aminobenzothiazole
(0.21 g, 1.0 mmol) in H2O (3.0 mL) was added SDS (0.2 equiv.).
The reaction mixture was heated at1008C for an appropriate
time (see Table 3). The reaction progress was followed by TLC
(hexane/ethyl acetate 8:2). On completion of the reaction, the
reaction mixture was cooled to room temperature and extracted
with ethyl acetate (2 ꢀ 20 mL). The organic layer was dried with
anhydrous sodium sulfate and evaporated under reduced pres-
sure, and the residue was percolated through a band of silica gel
(60–120 mesh) using hexane/ethyl acetate (9:1 v/v) as an eluent
to give pure compound 4g in 88% yield. All the compounds were
characterized by 1H NMR, 13C NMR, IR spectroscopy and mass
spectrometry.
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Accessory Publication
Spectral data (1H NMR, 13C NMR and electrospray ionization
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Acknowledgements
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We thank CSIR, New Delhi and University Grant Commission-Special
Assistance Programme (UGC-SAP), New Delhi for financial support.
M.S.R. thanks Birla Institute of Technology and Science (BITS), Pilani for
institutional fellowships.
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