Selenium dioxide reaction of substituted diphenacyl sulfides: Generation of α-ketoacids

Article abstract of DOI:10.1080/17415993.2013.769542

The attempted selenium dioxide oxidation of substituted diphenacyl sulfides in anticipation of further functionalization led to a series of α -ketoacids 3 via oxidation followed by CS bond cleavage. Two minor products, 5 and 6, have also been isolated and a mechanistic pathway for the formation of 3, 5 and 6 has been proposed.

Full text of DOI:10.1080/17415993.2013.769542

This article was downloaded by: [Linnaeus University]
On: 16 October 2014, At: 02:07
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Sulfur Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/gsrp20
Selenium dioxide reaction of
substituted diphenacyl sulfides:
generation of α -ketoacids
Muthupandi Nagaraja, Devanathan Perumala, Muthusamy
Boominathana, Shanmugam Muthusubramaniana & Nattamai
Bhuvaneshb
a Department of Organic Chemistry, School of Chemistry, Madurai
Kamaraj University, Madurai 625 021, India
b X-Ray Diffraction Lab, Department of Chemistry, Texas A&M
University, College Station, TX 77842, USA
Published online: 15 Mar 2013.
To cite this article: Muthupandi Nagaraj, Devanathan Perumal, Muthusamy Boominathan,
Shanmugam Muthusubramanian & Nattamai Bhuvanesh (2014) Selenium dioxide reaction of
substituted diphenacyl sulfides: generation of α -ketoacids, Journal of Sulfur Chemistry, 35:1,
24-30, DOI: 10.1080/17415993.2013.769542
To link to this article: http://dx.doi.org/10.1080/17415993.2013.769542
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &

Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

Journal of Sulfur Chemistry, 2014
Vol. 35, No. 1, 24–30, http://dx.doi.org/10.1080/17415993.2013.769542
Selenium dioxide reaction of substituted diphenacyl sulfides:
generation of α-ketoacids
Muthupandi Nagaraja, Devanathan Perumala, Muthusamy Boominathana, Shanmugam
Muthusubramaniana* and Nattamai Bhuvaneshb
aDepartment of Organic Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai 625 021,
India; bX-Ray Diffraction Lab, Department of Chemistry, Texas A&M University, College Station,
TX 77842, USA
(Received 19 November 2012; final version received 20 January 2013)
The attempted selenium dioxide oxidation of substituted diphenacyl sulfides in anticipation of further
−
functionalization led to a series of α-ketoacids 3 via oxidation followed by C S bond cleavage. Two
minor products, 5 and 6, have also been isolated and a mechanistic pathway for the formation of 3, 5 and
6 has been proposed.
O
O
O
3
O
S
R
R
OH
R
R
SeO2
SO2
R
S
O
O
O
O
O
O
1
2
O
R
H
O
R
S
5
O
O
SeO2
R
R
R
S
4
S
O
R
O2
O
O
6
Keywords: diphenacyl sulfides; α-ketoacids; oxidation; 2-oxo-2-arylacetic thioanhydride; selenium
dioxide
1. Introduction
Selenium dioxide, which is used as a key component in the chemistry reported in this manuscript,
is a very useful and versatile reagent for the synthesis of various types of organic compounds. The
reagent is particularly useful for allylic oxidation (1) and oxidation of reactive methylenes (2).
However, when coupled with other reagents such as acetic acid (3), acetic anhydride (4) or
chlorotrimethylsilane (5), it is also employed for other useful transformations. Triselenium
dicyanide is formed by the reaction of malononitrile and selenium dioxide, which is used as
a selenocyanating reagent (6). In addition, selenium-containing heterocyclic compounds are of
*Corresponding author. Email: muthumanian2001@yahoo.com
© 2013 Taylor & Francis

Journal of Sulfur Chemistry 25
interest due to their biological and synthetic applications. In particular, 1,2,3-selenadiazole and
its derivatives are well known and have attracted attention as versatile synthetic intermediates (7).
Recently, a series of α-ketoamides were synthesized via selenium dioxide-mediated oxidative
amidation between arylglyoxals and secondary amines, which was accelerated by microwave
irradiation (8).
α-Ketoacids, the products of the new reactions described in this manuscript, are versatile
compounds used as intermediates for the synthesis of variety of substances. They are of great
interest in medicinal chemistry, and this structural scaffold represents a key framework of many
biologically active agents in natural products (9). α-Ketoacids are used primarily as energy for
liver cells and in fatty acid synthesis. Oxoacid participates in an atom-economical alternative
cycle by which plants and bacteria convert fatty acids into carbohydrates (10).
2. Results and discussion
We are interested in the reaction of selenium dioxide with compounds having active methylene
group to generate new heterocycles (11). Previously, it has been shown that α-bromoketones
undergo selenium dioxide oxidation to yield reactive 2-oxo-2-arylacetyl bromides, which are
trapped by aryl-1,2-diamines, 1,2-aminophenol or 1,2-aminothiophenol to give quinoxalinones,
benzoxazinones and benzothiazinones in good yields (12). In this connection, it was planned to
investigate the reaction of diphenacyl sulfide with selenium dioxide. The intention was to prepare
2-oxo-2-arylacetic thioanhydride 2 as potential precursors for different heterocycles (Scheme 1).
To our surprise, however, instead of the anticipated 1,2,4,5-tetraketone 2, α-ketoacids 3 were
−
obtained as the major products via oxidation followed by C S bond cleavage.
O
O
O
O
S
S
SeO2
O
O
R
R
R
R
2
1
1a R= H
1b R=Cl,
1c R=Br,
1d R=OMe,
1e R=Ph
Scheme 1. Anticipated product.
To begin our studies, diphenacyl sulfide 1a (0.5 mmol), selenium dioxide (1.5 mmol) and diox-
ane (10.0 mL) were taken together in a 50.0 mL RB flask and refluxed for 2 h. The course of
the reaction was monitored by TLC. After 2 h, all of the reactants vanished and three new spots
were detected by TLC. Compound 3 was subsequently isolated in good yield, while two other
compounds, 5 and 6, were detected in trace amounts but could only be isolated in a few cases. In
the case of 1b, only compound 5 could be isolated. In the case of 1a, compounds 5 and 6 were
both isolated. However, in all the cases (1a–1e), compound 3 was isolated in very good yield
(more than 80%, Scheme 2).
Compounds 3, 5 and 6 were characterized by their nuclear magnetic resonance (NMR) spectra.
The 1H NMR spectrum of 3b in CDCl3 has two 2H doublets at 7.48 and 8.20 ppm. The absence
of peaks in the aliphatic region indicates that both the methylene groups of the sulfide 1b are
missing and presumably oxidized. A signal in the downfield region (nearly at 7 ppm), which gets
exchanged with D2O, was also noticed in all the cases. In the 13C NMR spectrum of 3b, only six

26 M. Nagaraj et al.
O
Ar
O
3
H
O
Ar
S
5
Ar
Ar
SeO2
O
OH
S
1
Ar
Ar
S
O
O
O
O
Ph
O2
6
Scheme 2. Reaction of substituted diphenacyl sulfides with selenium dioxide.
Figure 1. ORTEP diagram of 3b.
signals were observed, two of them are assignable to carbonyl carbon signals appearing at 183.7
and 162.1 ppm. In the DEPT-135 spectrum, two CH carbons and four quaternary carbons could be
located. The mass spectrum of compound 3b appears to have a molecular ion peak at m/e 183.0
(M−). The presence of two carbonyl groups, one aryl ring and replaceable hydrogen indicate that
the compound obtained could be an alpha keto acid. Unambiguous evidence for the formation of
an arylglyoxalic acid was obtained by single crystal X-ray analysis of 3b (Figure 1).1 This acid
exists in a dimeric hydrated form as revealed by the solid state structural analysis.
1
The structure of 5 has been confirmed by the spectral data (13). The H NMR spectrum of
6a in CDCl3 has a 1H singlet at 8.57 ppm, a 2H doublet at 8.14 ppm, a 1H doublet at 7.92 ppm,
a 1H triplet at 7.64 ppm, a 3H multiplet between 7.54 and 7.59 ppm, and a triplet of doublet at
7.32 ppm. The presence of one benzoyl group and one benzo group are clearly evident from this
data. The 13C NMR spectrum of 6a has 14 signals. The two carbonyl carbon signals appear at
189.3 and 192.1 ppm. In the DEPT-135 spectrum, eight CH carbons and six quaternary carbons
were observed and the HMBC spectrum of 6a has contours connecting the deshielded singlet
at 8.57 ppm with the carbon at 189.3 ppm revealing that this hydrogen is in β-position to the
benzoyl group. The doublet at 8.14 ppm gives HMBC contour with the carbons at 189.3, 133.7
and127.3 ppm.Theseconnectivitiesclearlysuggestthepossiblestructuresfor 6 showninFigure 2.
The infrared (IR) spectrum of 6a shows absorptions at 1132 and 1328 cm−1 due to symmetric
and asymmetric stretching vibrations of sulfone moiety. The mass spectrum of compound 6a has
the molecular ion peak at 316.25 (M− + H2O adduct), proving that the product is a sulfone and
not a sulfide or sulfoxide.
O
O
O
S
S
S
O
O2
O
O
O
Figure 2. Possible structures for 6a.

Journal of Sulfur Chemistry 27
Thus, oxidationsofsubstituteddiphenacylsulfidesbyseleniumdioxideproducedthreeproducts
3, 5 and 6. Unfortunately, none of these is the expected polyketone targeted during this inves-
tigation. Nevertheless, the oxidation to give the major product 3 is an interesting process and a
potentially important pathway to these compounds. The formation of 3 is best visualized as occur-
ring by initial oxidation of 1 to 1,2,4,5-tetraketone 2 followed by the selenium dioxide-induced
−
C S bond cleavage. The smell of sulfur dioxide noticed during the reaction also lends support to
−
an oxidative cleavage of the C S bond during the reaction.
The minor product5 observed in two cases has previously been obtained from 1 by a self-
condensation process catalysed by a base (13). In our laboratory, we have also noticed that
prolonged exposure of 1 to atmospheric oxygen also led to slow formation of 5. We suggest that
selenium dioxide oxidation of 1 at one methylene group generates diketone 4 that serves as a
precursor for 5 as shown in Scheme 3. The elimination of PhCOCHS noticed in this case has been
already observed in some other reactions carried out in our laboratory (14).
O
O
OH
O
S
S
O
O
O
H
O
Mono oxidised product
S
Ph
Ph
4
H
O
O
OH
S
[O]
1a
S
O
O
Ph
O
O
O2
S
4
O
OH
O
O
2
O
S
S
O
H
Ph
Ph
O
Ph
+ PhCOCHS
OH
O
O
Ph
OH
S
O
Ph
OH
SO2
COPh
2
COPh
O
OH
S
5a
PhOC
3a
-H2O
[O]
O
S
O2
6a
COPh
Scheme 3. Proposed mechanism for the formation of 3, 5 and 6.
The formation of 6a from 1a, though in a minor amount, is an interesting feature of the
investigation. We speculate that the formation of 6 can be explained by the mechanism shown in
Scheme 3. The key intermediate in this mechanism is the diketone 4. The tautomeric form of 4
could undergo an unusual intramolecular rearrangement involving a nucleophilic addition to the

28 M. Nagaraj et al.
aromatic ring by the sulfur and formation of a transient positively charged thiirane intermediate.
This rearrangement is presumably aided by the conjugation present and the electrophilicity of the
carbonyl. The final product could then be formed by rearomatization, cleavage of the thiirane,
dehydration and finally oxidation to the sulfone.
3. Conclusion
In conclusion, we have investigated the selenium dioxide mediated oxidation of substituted
diphenacyl sulfides. The investigation has led to a simple method for the synthesis of α-ketoacids.
4. Experimental
4.1. General information
NMR (1H and 13C NMR) spectra were recorded on a Bruker 300 MHz NMR spectrometer in
CDCl3 using TMS as internal standard. Chemical shifts are reported in parts per million (δ),
coupling constants (J values) are reported in Hertz (Hz). 13C NMR spectra were routinely run with
broadband decoupling. IR spectra were recorded on a Shimadzu FT-IR instrument (in KBr pellets).
Melting points were determined on a melting point apparatus equipped with a thermometer and
were uncorrected. Column chromatography was carried out in silica gel (60–120 mesh) using pet
ether–ethyl acetate as eluent. Elemental analyses were performed on a Perkin Elmer 2400 series
II Elemental CHNS analyzer. Mass analyses were performed on a liquid chromatography-ion trap
mass spectrometer (LCQ Fleet, Thermo Fisher Instruments Limited, USA). The samples were
introduced into the ion source by infusion method at flow rate 1 μL/min. The capillary voltage
of the mass spectrometer was 33V, with source voltage 4.98 kV for the mass scale (m/z 100–500
and 50–500).
4.2. General procedure for selenium dioxide oxidation of diphenyl sulfide
Selenium dioxide (1.5 mmol) was dissolved in dioxane (10.0 mL) and added to a 50 mL RB flask.
To this, diphenacyl sulfide derivative 1 (0.5 mmol) was added and the mixture refluxed for 2 h.
The course of the reaction was monitored by TLC. After completion, the reaction mixture was
filtered and the solvent was removed under vacuum. The crude product was purified by column
chromatography using petroleum ether and ethyl acetate mixture (90:10) as the eluent to get 3/5/6.
4.2.1. 2-Oxo-2-phenylacetic acid (15) (3a)
Colorless solid; isolated yield 71%; mp 64–66◦C; 1H NMR (300 MHz, CDCl3): δ = 7.50 (2H, t,
J = 7.2 Hz, CH), 7.64–7.66 (1H, m, CH), 8.16 (2H, d, J = 7.2 Hz, CH), 9.45 (1H, bs, COOH);
13
=
C NMR (75 MHz, CDCl3): δ = 128.8 (2CH), 130.5 (2CH), 131.9 (C), 135.2 (C), 163.7 (C O),
=
185.6 (C O).
4.2.2. 2-(4-Chlorophenyl)-2-oxoacetic acid (3b)
Colorless solid; isolated yield 78%; mp 72–74◦C; H NMR (300 MHz, CDCl3): δ = 7.15 (1H,
1
bs, COOH), 7.50 (2H, d, J = 8.4 Hz, CH), 8.20 (2H, d, J = 8.4 Hz, CH); 13C NMR (75 MHz,

Journal of Sulfur Chemistry 29
=
=
CDCl3): δ = 129.4 (2CH), 130.2 (2CH), 132.4 (C), 142.5 (C), 162.1 (C O), 183.7(C O); Mass
m/e 183 (M).
4.2.3. 2-(4-Bromophenyl)-2-oxoacetic acid (3c)
1
Colorless solid; isolated yield 82%; mp 76–78◦C; H NMR (300 MHz, CDCl3): δ = 6.31 (1H,
bs, COOH), 7.67 (2H, d, J = 8.4 Hz, CH), 8.12 (2H, d, J = 8.4 Hz, CH); 13C NMR (75 MHz,
=
=
CDCl3): δ = 129.4 (2CH), 130.2 (2CH), 132.4 (C), 142.5 (C), 162.1 (C O), 183.7 (C O).
4.2.4. 2-(4-Methoxyphenyl)-2-oxoacetic acid (3d)
1
Colorless solid; isolated yield 72%; mp 58–60◦C; H NMR (300 MHz, CDCl3): δ = 3.89 (3H,
s, OCH3), 6.96 (2H, d, J = 9.0 Hz, CH), 8.24 (2H, d, J = 9.0 Hz, CH) 9.85 (1H, bs, COOH);
13C NMR (75 MHz, CDCl3): δ = 55.6 (OCH3), 114.3 (2CH), 124.7 (2CH), 133.7 (C), 163.9 (C),
=
=
165.5 (C O), 183.2 (C O).
4.2.5. 2-(Biphenyl-4-yl)-2-oxoacetic acid (3e)
1
Colorless solid; isolated yield 76%; mp 78–80◦C; H NMR (300 MHz, CDCl3): δ = 7.06 (1H,
bs, COOH), 7.43–7.51 (3H, m, CH), 7.64 (2H, d, J = 7.5 Hz, CH), 7.74 (2H, d, J = 8.1 Hz, CH),
8.39 (2H, d, J = 8.1 Hz, CH); 13C NMR (75 MHz, CDCl3): δ = 127.4 (2CH), 127.5 (2CH), 128.5
=
=
(CH), 128.8 (2CH), 130.5 (2CH), 131.9 (C), 139.4 (C), 148.3 (C), 161.6 (C O), 189.8 (C O);
Anal. Calcd for C14H10O3 (226.23): C, 74.33; H, 4.46%; found: C, 74.11; H, 4.38%.
4.2.6. (3-Phenylthiophene-2,5-diyl)bis(phenylmethanone) (5a)
Colorless solid; isolated yield 10%; mp 152–154◦C (156◦C lit) (13); 1H NMR (300 MHz, CDCl3):
δ = 7.17–7.18 (2H, m, CH), 7.24–7.27 (4H, m, CH), 7.35–7.43 (2H, m, CH), 7.50–7.62 (3H, m,
CH), 7.70 (2H, d, J = 8.1 Hz, CH), 7.75 (1H, s, CH), 7.95 (2H, d, J = 8.1 Hz, CH); 13C NMR
(75 MHz, CDCl3): δ = 127.9, 128.0, 128.2, 128.5, 128.8, 129.6, 132.6, 132.9, 134.6, 135.6,
=
=
135.7, 136.7, 137.4, 142.6, 144.7, 145.6, 187.6 (C O), 189.8 (C O);Anal. Calcd for C24H16O2S
(368.45): C, 78.24; H, 4.38; S, 8.70%; found: C, 78.02; H, 4.31; S, 8.61%.
4.2.7. (3-(4-Chlorophenyl)thiophene-2,5-diyl)bis((4-chlorophenyl) methanone) (5b)
Colorless solid; isolated yield 11%; mp 170–172◦C (174◦C lit) (13); 1H NMR (300 MHz, CDCl3):
δ = 7.20–7.23 (4H ,m, CH), 7.27 (2H, d, J = 8.4 Hz, CH), 7.52 (2H, d, J = 8.4 Hz, CH), 7.65
(2H, d, J = 8.4 Hz, CH), 7.69 (1H, s, CH), 7.89 (2H, d, J = 8.4 Hz, CH); 13C NMR (75 MHz,
CDCl3): δ = 128.7, 128.8, 129.1, 130.1, 130.7, 131.2, 132.8, 134.7, 134.9, 135.4, 135.5, 139.7,
=
=
140.1, 142.4, 144.5, 144.8, 186.4 (C O), 188.4 (C O);Anal. Calcd for C24H13Cl3O2S (471.78):
C, 61.10; H, 2.78; S, 6.80%; found: C, 60.92; H, 2.69; S, 6.72%.
4.2.8. 2-Phenyl-4H-S,S-oxo-thiochromen-4-one (6a)
Red color semisolid; isolated yield 8%; IR (KBr): 3028, 1985, 1688, 1672, 1132, 1328, 880,
1
672 cm−1; H NMR (300 MHz, CDCl3): δ = 7.32 (1H, td, J = 8.4 Hz, 1.8 Hz, CH), 7.54–7.59
(3H, m, CH), 7.64 (1H, t, J = 7.5 Hz, CH), 7.92 (1H, d, J = 7.5 Hz, CH), 8.14 (2H, d, J = 7.2 Hz,
CH), 8.57 (1H, s, CH); 13C NMR (75 MHz, CDCl3): δ = 121.8, 126.6, 127.3, 128.2, 128.5,

30 M. Nagaraj et al.
=
=
128.9, 131.2, 133.7, 136.4, 137.2, 144.9, 149.9, 189.3 (C O), 192.1 (C O); Anal. Calcd for
C24H13Cl3O2S (298.31): C, 64.42; H, 3.38; S, 10.75%; found: C, 6.29; H, 3.30; S, 10.62%; Mass
m/e 316 (M + H2O).
Acknowledgements
The authors thank DST, New Delhi, for assistance under the IRHPA program for providing funds for creating an NMR
facility. Financial supports from CSIR-JRF, New Delhi, are gratefully acknowledged.
Note
1. Crystal data for 3b: Empirical formula C8H7ClO4, Formula weight 202.59, triclinic, space group P-1, Unit cell =
5.8696(9) Å = 78.149(9)◦ b = 7.8565(12) Å, = 89.719(12)◦ c = 18.475(3) Å= 87.365(8)◦, Volume 832.9(2) Å3,
Z = 4, f(000) = 416, R(reflections) = 0.0311(1826), wR2(reflections) = 0.0783( 2416), and Goodness-of-fit on
F2 1.089. Complete crystallographic data, as a CIF file, have been deposited with the Cambridge Crystallographic
Data Centre (CCDC number is 908749).
References
(1) (a) Sharpless, K.B.; Lauer, R.F. J. Am. Chem. Soc. 1972, 94, 7154–7155; (b)Arigoni, D.;Vasella,A.; Sharpless, K.B.;
Jensen, H.P. J. Am. Chem. Soc. 1973, 95, 7917–7919; (c) Stephenson, L.M.; Speth, D.R. J. Org. Chem. 1979, 44,
4683–4689.
(2) (a) Shirude, S.T.; Patel, P. Giridhar, R.; Yadav, M.R. Indian J. Chem. 2006, 45B, 1080–1085; (b) Goswami, S.P.;
Maity, A.C. Chem. Lett. 2007, 36, 1118–1119.
(3) Kariyone, K.; Yazawa, H. Tetrahedron Lett. 1970, 11, 2885–2888.
(4) Kovylyaeva, I.; Sharipova, R.R.; Strobykina, I.Y.; Militsina, O.I.; Musin, R.Z.; Beskrovnyi, D.V.; Gubaidullin, A.T.;
Alfonsov, V.A.; Kataev, V.E. Russ. J. Gen. Chem. 2009, 79, 2663–2667.
(5) Lee, J.G.; Park, S.; Seo, J.W. Bull. Korean Chem. Soc. 1995, 16, 349–355.
(6) Kachanov,A.V.;Slabko, O.Y.;Baranova, O.V.;Shilova, E.V.;Kaminskii,V.A.TetrahedronLett. 2004, 45, 4461–4463.
(7) Bakulev, V.A.; Dehaen, W. The Chemistry of 1,2,3-Thiadiazoles; John Wiley & Sons, Inc.: New York, NY, 2004;
pp. 193–228.
(8) Shaw, A.Y.; Denning, C.R.; Hulme, C. Tetrahedron Lett. 2012, 53, 4151–4153.
(9) (a) Cooper, A.J.L.; Ginos, J.Z.; Meister, A. Chem. Rev. 1983, 83, 321–536; (b) Mooney, B.P.; Miesnyk, J.P.;
Randall, D.D. Ann. Rev. Plant Biol. 2002, 53, 357–375.
(10) Kerber, R.C.; Fernando, M.S. J. Chem. Educ. 2010, 87, 1079–1084.
(11) (a) Saravanan, S.; Azath, I.A.; Muthusubramanian, S. J. Org. Chem. 2008, 73, 2323–2329; (b) Saravanan, S.;
Nithya, A.; Muthusubramanian, S. J. Heterocycl. Chem. 2006, 43, 149–155.
(12) Nagaraj, M.; Sathiyamoorthy, S.; Boominathan, M.; Muthusubramanian, S.; Bhuvanesh, N. J. Heterocycl. Chem.,
in press, doi:10.1002/jhet.1557.
(13) Ravindran, G.; Nidhin, P.; Muthusubramanian, S.; Perumal, S. J. Sulfur Chem. 2008, 29, 575–581.
(14) Paul, N.; Shanmugam, M.J.; Muthusubramanian, S. Synth. Commun. 2013, 43, 129–138.
(15) (a)Aly, M.F.; Grigg, R. Tetrahedron 1988, 44, 7271–7282; (b) Corson, B.B.; Dodge, R.A.; Harris, S.A.; Hazen, R.K.
Org. Synth. Coll. 1941, 1, 241–243.