SnCl22H2O: An efficient reagent for selective and direct oxidative desulfurization of phenylmethylene-2-thiohydantoins to corresponding hydantoins

Article abstract of DOI:10.1080/10426507.2010.525765

A new and efficient protocol is presented for selective and direct oxidative conversion of phenylmethylene-2-thiohydantoins to corresponding hydantoins. It involves the use of SnCl₂.2H₂O, which oxidatively desulfurizes the phenylmeth

Full text of DOI:10.1080/10426507.2010.525765

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SnCl2·2H2O: An Efficient Reagent
for Selective and Direct Oxidative
Desulfurization of Phenylmethylene-2-
thiohydantoins to Corresponding
Hydantoins
Ravi Kumar a , Shashi Pandey a , Shahnawaz Khan a & Prem M. S.
Chauhan a
a Medicinal and Process Chemistry Division , Central Drug Research
Institute , Lucknow , CSIR , India
Published online: 04 Aug 2011.
To cite this article: Ravi Kumar , Shashi Pandey , Shahnawaz Khan & Prem M. S. Chauhan
(2011) SnCl2·2H2O: An Efficient Reagent for Selective and Direct Oxidative Desulfurization of
Phenylmethylene-2-thiohydantoins to Corresponding Hydantoins, Phosphorus, Sulfur, and Silicon and
the Related Elements, 186:7, 1404-1410, DOI: 10.1080/10426507.2010.525765
To link to this article: http://dx.doi.org/10.1080/10426507.2010.525765
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Phosphorus, Sulfur, and Silicon, 186:1404–1410, 2011
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2010.525765
COMMUNICATION
SNCL2·2H2O: AN EFFICIENT REAGENT FOR SELECTIVE
AND DIRECT OXIDATIVE DESULFURIZATION OF
PHENYLMETHYLENE-2-THIOHYDANTOINS TO
CORRESPONDING HYDANTOINS
Ravi Kumar, Shashi Pandey, Shahnawaz Khan,
and Prem M. S. Chauhan
Medicinal and Process Chemistry Division, Central Drug Research Institute,
Lucknow, CSIR, India
GRAPHICAL ABSTRACT
O
S
HN
HN
R
NH
R
NH
O
O
R
S
HN
SnCl2.2H2O,
EtOH, reflux
H
N
×
NH
R
O
O
NR1
O
×
S
O
HN
HN
NR1
O
NH
R
R
O
Abstract A new and efficient protocol is presented for selective and direct oxidative con-
version of phenylmethylene-2-thiohydantoins to corresponding hydantoins. It involves the use
of SnCl2.2H2O, which oxidatively desulfurizes the phenylmethylene-2-thiohydantoins to their
corresponding hydantoin analogues in good to excellent yields while all other 2-thiohydantoins
remain unaffected.
Supplemental materials are available for this article. Go to the publisher’s online edition of
Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.
Keywords
Hydantoin; oxidative desulfurization; phenylmethylene-2-thiohydantoins;
SnCl2.2H2O
Received 28 July 2010; accepted 17 September 2010.
We acknowledge SAIF, CDRI, for providing analytical data. R.K. and S.P. thank the Council of Scientific
and Industrial Research, India, for Senior and Junior Research Fellowships, respectively. S.K. thanks UGC New
Delhi, India, for a Junior Research Fellowship. CDRI Communication No. 7906.
Address correspondence to Prem M. S. Chauhan, Medicinal and Process Chemistry Division, Central Drug
Research Institute, Lucknow 226001, CSIR, India. E-mail: prem chauhan 2000@yahoo.com
1404

DIRECT OXIDATIVE DESULFURIZATION
1405
OH
O
H
N
O
O
OMe
HN
O
HN
H
NH
HO
NH
N
H
O
H
N
HO
OH
O
Br
Br
O
+
1
( ) Hydantocidin ( )
3
4
H
H
Agesamide A ( ) =
Agesamide B ( ) =
NH
N
O
O
O
2
( )
HN
O
H
O
N
NH
O
HN
HN
HN
HN
O
NH
NH2
NH
N
O
N
H
O
R
R
H
5
Parazoanthine C ( )
8
(Z)-Axinohydantoin ( ) R = Br
6
(E)-Axinohydantoin ( ), R = Br
9
7
(Z)-Debromoaxinohydantoin ( ) R = H
(E)-Debromoaxinohydantoin ( ) R = H
Figure 1 Bioactive hydantoin natural products.
INTRODUCTION
Hydantoins and their derivatives have long been a focus of study due to their numerous
biological, therapeutic, and agrochemical applications.1 Recently, they have attracted more
attention for their antidiabetic,2 antiarrhythmics,3 anticonvulsant,4 antiallergic,5 serotonin
and fibrinogen receptor antagonists,6 and aldose reductase inhibitory7 activities. Antitu-
mor, antiangiogenic, and antimetastatic8 properties of hydantoins have also been reported.
Hydantoin or related moieties are also found in many bioactive natural products; for exam-
ple, hydantocidin (1),9 4-hydroxyphenylmethylene hydantoin (2),8a agesamides (3 & 4),10
parazoanthin C (5),11 and axinohydantoins (6–9)12 (Figure 1).
Desulfurization of 2-thiohydantoin for conversion to its corresponding hydantoins is
still the commonly used method in the synthesis of therapeutically relevant compounds,
and many efforts have been devoted toward development of new methodologies for
the synthesis of hydantoins. Existing methods for desulfurization of 2-thiohydantoins
use dimethyl formamide (DMF)/H2O2/CH3COOH,13 H2O2/NaHCO3/CH3CN-H2O,14
NaOMe/MeI/DMF,15 and S-alkylation of 2-thiohydantoin followed by treatment with
ethanolic HCl,16 bromoacetyl benzene, and Hg(OAc)2 with a microwave.17 All of these
methods suffer from no or poor selectivity and the use of hazardous chemicals, oxidizing
agents (H2O2), or strong acids. Hence, there is strong demand for an efficient reagent that
directly converts the 2-thiohydantoins to their corresponding hydantoins selectively.
SnCl2.2H2O is a good Lewis acid with a wide range of applications in synthetic
organic chemistry.18 It catalyzes the deprotection of tert-butyldimethylsilyl (TBS),19
esterification of oleic acid for biodiesel production,20 synthesis of β-acetamidoketones
and esters,21 hydroperoxides,22 hydroxyquinazolinones,23 coumarins,24 quinoxaline,25
chromans,26 dithiolanes,27 and allylation of aldehyde and ketones.28 In our en-
deavor to synthesize small molecule natural product mimics, we found that refluxing
2-nitrophenylmethylene-2-thiohydantoin with SnCl2.2H2O in ethanol yields the cor-
responding hydantoin derivative. In this article, we wish to report the selective and
direct oxidative desulfurization of phenylmethylene-2-thiohydantoins to corresponding

1406
R. KUMAR ET AL.
R
R
H
SnCl2.2H2O,
EtOH, reflux
H
N
N
S
O
NH
(10-22)
NH
O
O
(10a - 22a)
Scheme 1 General scheme for direct oxidative desulfurization of 2-thiohydantoins.
hydantoins by SnCl2.2H2O. To the best of our knowledge, this kind of selective and direct
oxidative conversion of phenylmethylene-2-thiohydantoins to hydantoins by SnCl2.2H2O
has not been reported.
RESULTS AND DISCUSSION
In an attempt to reduce the nitro group of o-nitrophenylmethylene-2-thiohydantoin
(10), we discovered that SnCl2.2H2O, under refluxing conditions in ethanol, oxidatively
desulfurizes it to the corresponding hydantoin (10a). Upon optimization, it was also found
that 5 equivalents of SnCl2.2H2O were required for complete conversion of compound (10)
to (10a) (Scheme 1).
Inspired by this finding, we also tried the same with phenylmethylene-2-thiohydantoin
(11) and obtained the corresponding hydantoin (11a) in 98% yield. To investigate whether
two water molecules of SnCl2.2H2O were essential for the reaction, compound (11) was
refluxed with anhydrous SnCl2, but it failed to desulfurize the thiohydantoin (11) to corre-
sponding hydantoin (11a). The addition of 2 equivalents of water to the reaction mixture
with anhydrous SnCl2 also failed to start the reaction, indicating that water molecules of
SnCl2.2H2O were essential for the oxidative desulfurization activity of SnCl2.2H2O. To
study the applicability of the reagent, a number of other phenylmethylene-2-thiohydantoins
(10–21) were subjected to oxidative desulfurization by SnCl2.2H2O and the results are
summarized in Table 1. All of the required phenylmethylene-2-thiohydantoins (PMHs)
were synthesized according to a literature procedure.29
Table 1 Direct oxidative desulfurization of 2-thiohydantoins to their corresponding hydantoins
Entry
R
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
2-Nitrophenyl (10)
Phenyl (11)
8
10
8
8
8
95
96
96
94
95
95
92
85
82
91
84
90
3-Bromophenyl (12)
4-Methylphenyl (13)
4-Ethylphenyl (14)
4-Propylphenyl (15)
4-Chlorophenyl (16)
4-Methoxyphenyl (17)
3,4-Dimethoxyphenylphenyl (18)
2-Bromophenyl (19)
4-Benzyloxyphenyl (20)
Naphthyl (21)
8
8
12
12
8
12
8
10
11
12

DIRECT OXIDATIVE DESULFURIZATION
1407
SnCl2.2H2O,
EtOH, reflux
SnCl2.2H2O,
EtOH, reflux
H
N
H
N
H
N
H
N
R
R
O
S
O
S
N
R
NH
NH
N
R
O
O
O
O
(24a & 25a)
(22 & 23)
For 22 R = benzyl,
23
(24 & 25)
For 24 R = methyl,
25
22a & 23a
R = phenyl
R = benzyl
Scheme 2 Reactions of N-3 substituted phenylmethylene-2-thiohydantoins and C-5 alkyl–substituted hydantoins.
R
O
R
R
O
R
O
H
N
N
H
N
H
N
N
S
OH
S
Sn
S
Cl
Cl
H
Sn
H
H2
O
O
O
Cl
H
O
N
N
H
N
H
O
H
Sn Cl
H
H
H
O
H
O
H
H
Scheme 3 Proposed mechanism of direct oxidative desulfurization of 2-thiohydantoins by SnCl2.2H2O.
Most of the PMHs oxidatively desulfurized in excellent yields except the PMHs with
methoxy substitutents (17, 18, and 20) in the phenyl ring, which were less reactive toward
this reaction. Unsubstituted PMH (11) reacted slowly in comparison to other substituted
PMHs, and methoxy-substituted PMHs were even less reactive. Efforts toward oxidative
desulfurization of 4-nitrophenylmethylene-2-thiohydantoin with SnCl2.2H2O yielded a
complex mixture of products. Spectroscopic data of phenylmethylene hydantoins (11a,
13a, 16a, 17a),30 18a,31 and 20a32 were identical to the reported data.
A few PMHs with benzyl or phenyl substituents at N-3 (22 and 23)33 were also
treated with SnCl2.2H2O but were not desulfurized (Scheme 2). Similarly, 2-thiohydantoins
with alkyl substituents at C-5 (24 and 25) were not reactive toward desulfurization by
SnCl2.2H2O, proving that it selectively desulfurizes the phenylmethylene-2-thiohydantoins.
The crystal structure of SnCl2.2H2O shows that one of the two water molecules
directly coordinate with the Sn2+ ion and the other water molecule remains hydrogen
bonded to the first one and is sandwiched between the layers of SnCl2.34 It may, however,
be possible that the first nucleophilic displacement of the chloro group of SnCl2 by the sulfur
nucleophile (Scheme 3) of 2-thiohydantoin and then attack of the second water molecule
may substitute the sulfur complex with an oxygen atom, resulting in the formation of
corresponding hydantoin analogues.
CONCLUSION
In summary, SnCl2.2H2O has been discovered as an efficient reagent for selective
and direct oxidative desulfurization of phenylmethylene-2-thiohydantoins. Our method has
many advantages over existing methods. Most noteworthy are the following: (a) It does not
involve the use of oxidizing agents, which makes it applicable to the PMHs with oxidation-
prone functional groups. (b) The use of strong bases and mineral acids is also avoided,
making it amenable to base- and acid-sensitive substrates. (c) Alkyl halides, which are

1408
R. KUMAR ET AL.
toxic, dangerous, carcinogenic, and nonselective, are not used.36 (d) Highly toxic mercuric
acetate is also avoided. Further studies regarding the application of this reagent toward
oxidative desulfurization of other chemical systems are in progress in our laboratory and
will be reported in due course.
EXPERIMENTAL
General Experimental Procedure for Oxidative Desulfurization
of Phenylmethylene-2-Thiohydantoins
The mixture of 2-thiohydantoin (0.5 mmol, 1 eq.), SnCl2.2H2O (2.5 mmol, 5 eq.),
in ethanol (30 mL) was refluxed. After thin-layer chromatography (TLC) analysis showed
completion of the reaction, the reaction mixture was cooled and a white precipitate was
obtained. The white precipitate that formed was filtered and washed with (3 × 10 mL)
chilled alcohol. Solid residue was crystallized from ethanol. In the case of entry 9, in which
a precipitate did not appear, the reaction solvent was removed in vacuo and the crude
product was quickly column chromatographed on silica gel using chloroform as the eluent.
Representative Analytical Data for (Z)-5-(2-Nitrobenzylidene)
imidazolidine-2,4-dione (10a)35
Mp: > 250 ◦C; IR (KBr) ν = 3423, 3218, 1763, 1711, 1638, 1539, 1343, 1273 cm−1
;
1H-NMR (300 MHz, CD3OD): 7.74–7.70 (m, 1H), 7.56–7.46 (m, 3H), 6.62 (s, 1H) ppm;
13C-NMR (75 MHz, DMSO-d6): δ = 163.9, 154.5, 149.7, 130.1, 129.5, 128.6, 127.3, 126.3,
122.2, 109.2 ppm. Anal. Calcd. for C10H7N3O4: C, 51.51; H, 3.03; N, 18.02%. Found: C,
51.32; H, 3.10; N, 18.11%.
◦ (Z)-5-(3-Bromobenzylidene)imidazolidine-2,4-dione (12a)4. Mp: 241–
243 C; IR (KBr) ν = 3440, 3145, 3042, 1779, 1730, 1661, 1470, 1382, 1246 cm−1
;
1H-NMR (300 MHz, DMSO-d6): δ = 11.36 (s, 1H), 10.65, (s, 1H), 7.70–7.66 (m, 2H),
7.42 (t, 1H, J = 7.6 Hz), 7.29–7.23 (m, 1H), 6.51 (s, 1H) ppm; 13C-NMR (75 MHz,
DMSO-d6): δ = 164.7, 155.1, 132.3, 132.1, 129.7, 129.5, 129.5, 127.6, 123.5, 105.2 ppm.
Anal. Calcd. for C10H7BrN2O2: C, 44.97; H, 2.64; N, 10.49%. Found: C, 44.78; H, 2.71;
N, 10.56%.
4
(Z)-5-(4-Ethylbenzylidene)imidazolidine-2,4-dione (14a). Mp: > 250 ◦C; IR
(KBr) ν = 3291, 1771, 1727, 1659, 1434, 1381, 1255 cm−1; 1H-NMR (300 MHz, DMSO-
d6): δ = 11.19 (s, 1H), 10.45, (s, 1H), 7.54 (d, 2H, J = 8.1 Hz), 7.23 (d, 2H, J = 8.1 Hz),
6.38 (s, 1H), 2.59 (q, 2H, J = 7.5 Hz), 1.18 (t, 3H, J = 7.5 Hz) ppm; 13C-NMR (75 MHz,
DMSO-d6): δ = 166.0, 156.0, 144.9, 130.8, 129.9, 128.6, 127.6, 109.0, 28.4, 15.8 ppm.
Anal. Calcd. for C12H12N2O2: C, 66.65; H, 5.59; N, 12.96%. Found: C, 66.40; H, 5.66; N,
13.03%.
(Z)-5-(4-Propylbenzylidene)imidazolidine-2,4-dione (15a). Mp: 186–188
◦C; IR (KBr) ν = 3438, 3235, 2937, 1770, 1720, 1660, 1434, 1376, 1291, 1169 cm−1
;
1H-NMR (300 MHz, DMSO-d6): δ = 11.18 (s, 1H), 10.46, (s, 1H), 7.53 (d, 2H, J = 8.1
Hz), 7.21 (d, 2H, J = 8.1 Hz), 6.38 (s, 1H), 2.57 (t, 2H, J = 7.3 Hz), 1.65–1.63 (m, 2H),
0.89 (t, 3H, J = 7.3 Hz) ppm; 13C-NMR (75 MHz, DMSO-d6): δ = 165.0, 155.1, 142.2,
129.9, 128.8, 128.2, 126.7, 108.0, 28.4, 23.3, 13.0 ppm. Anal. Calcd. for C13H14N2O2: C,
67.81; H, 6.13; N, 12.17%. Found: C, 67.64; H, 6.21; N, 12.15%.

DIRECT OXIDATIVE DESULFURIZATION
1409
4
(Z)-5-(2-Bromobenzylidene)imidazolidine-2,4-dione (19a). Mp: decom-
◦
poses at 220 C; IR (KBr) ν = 3440, 3145, 3042, 1779, 1730, 1661, 1470, 1382, 1246,
1097, 1027 cm−1; H-NMR (300 MHz, DMSO-d6): δ = 11.36 (s, 1H), 10.65, (s, 1H),
1
7.70–7.66 (m, 2H), 7.42 (t, 1H, J = 7.6 Hz), 7.29–7.23 (m, 1H), 6.51 (s, 1H) ppm; 13C
NMR (75 MHz, DMSO-d6): δ = 164.7, 155.1, 132.3, 132.1, 129.7, 129.5, 129.5, 127.6,
123.5, 105.2 ppm. Anal. Calcd. for C10H7BrN2O2: C, 44.97; H, 2.64; N, 10.49%. Found:
C, 44.72; H, 2.74; N, 10.47%.
4
(Z)-5-(Naphthalen-1-ylmethylene)imidazolidine-2,4-dione (21a). Mp: >
250 ◦C; IR (KBr) ν = 3422, 3146, 3041, 1776, 1728, 1663, 1382, 1230 cm−1; 1H-NMR (300
MHz, DMSO-d6): δ = 11.23 (s, 1H), 10.52, (s, 1H), 7.61 (d, 1H, J = 7.2 Hz), 7.42–7.32
(m, 6H), 6.41 (s, 1H) ppm; 13C-NMR (75 MHz, DMSO-d6): δ = 164.7, 155.1, 132.7,
130.5, 129.7, 128.8, 128.1, 126.7, 126.3, 125.7, 125.2, 123.1, 103.9 ppm. Anal. Calcd. for
C14H10N2O2: C, 70.58; H, 4.23; N, 11.76%. Found: C, 70.45; H, 4.22; N, 11.64%.
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