Synthesis of substituted isoxazoles and pyrazoles from α-α Dioxoketen dithioacetals under solvothermal conditions

Article abstract of DOI:10.1080/00397910903459312

Some new 3,4,5-substituted isoxazoles, 3,4,5 and 1,3,4,5-substituted pyrazoles were prepared by reaction of,-oxoketen dithioacetals with hydroxylamine hydrochloride; phenyl hydrazine and hydrazine hydrate respectively under solvothermal conditions involving an ecofriendly method without any environmental pollution. The yields are in the range of 71-91%. The structure of the new compounds were established upon their elemental analysis, IR, ¹H NMR, and ¹³C NMR. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903459312

This article was downloaded by: [Southern Methodist University]
On: 27 March 2015, At: 15:33
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
Synthetic Communications: An
International Journal for Rapid
Communication of Synthetic Organic
Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/lsyc20
Synthesis of Substituted Isoxazoles
and Pyrazoles from α-α-Dioxoketen
Dithioacetals Under Solvothermal
Conditions
Ebraheem Abdu Musad a & K. M. Lokanatha Rai a
a Department of Studies in Chemistry , University of Mysore,
Manasagangotri , Mysore, India
Published online: 05 Nov 2010.
To cite this article: Ebraheem Abdu Musad & K. M. Lokanatha Rai (2010) Synthesis of Substituted
Isoxazoles and Pyrazoles from α-α-Dioxoketen Dithioacetals Under Solvothermal Conditions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
40:23, 3569-3576, DOI: 10.1080/00397910903459312
To link to this article: http://dx.doi.org/10.1080/00397910903459312
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

Synthetic Communications1, 40: 3569–3576, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903459312
SYNTHESIS OF SUBSTITUTED ISOXAZOLES AND
PYRAZOLES FROM a-a-DIOXOKETEN DITHIOACETALS
UNDER SOLVOTHERMAL CONDITIONS
Ebraheem Abdu Musad and K. M. Lokanatha Rai
Department of Studies in Chemistry, University of Mysore, Manasagangotri,
Mysore, India
Some new 3,4,5-substituted isoxazoles, 3,4,5 and 1,3,4,5-substituted pyrazoles were
prepared by reaction of a,a-oxoketen dithioacetals with hydroxylamine hydrochloride;
phenyl hydrazine and hydrazine hydrate respectively under solvothermal conditions involving
an ecofriendly method without any environmental pollution. The yields are in the range of
71–91%. The structure of the new compounds were established upon their elemental analysis,
IR, 1H NMR, and 13C NMR.
Keywords: Isoxazoles; a,a-oxoketen dithioacetals; pyrazole; solvothermal
INTRODUCTION
Aromatic heterocycles are valuable synthetic templates for the preparation of
new compounds with specific biological, pharmaceutical and material properties.
The pursuit of these properties requires efficient synthetic routes that allow rapid
construction of diverse aromatic heterocycles with defined substitution patterns.[1]
Isoxazole and pyrazole derivatives used as key intermediate in synthesis, and have
been investigated intensively for the last several years because of their biological,
pharmaceutical activities and industrial applications.[2–18] One of the useful methods
employed for the synthesis of substituted isoxazoles and pyrazoles that involves the
cyclocondensation of hydroxyl amines and hydrazine’s as bifunctional nucleophiles
with a-oxoketen dithioacetals as 1,3-bielectrophilic synthon.[19–23] Therefore
a-oxoketen dithioacetals as the organic synthetic intermediates have been widely
used in the formation of aromatic, heterocycles compounds. A lot of syntheses of
isoxazole and pyrazole derivatives have been developed. However, these syntheses
are usually carried out in organic solvents and many of these reactions require more
reaction times, high temperatures, or inconvenient reaction condition for their
execution and are often accompanied by painful chromatographic separation to
remove spent reagent from desired products. Recently Rai et al. have developed
the solvothermal method for the thionation of esters or amid[24] and for the
Received August 13, 2009.
Address correspondence to Dr. K. M. Lokanatha Rai, Department of Chemistry, University of
Mysore, Manasagangotri, Mysore 560 006, India. E-mail: kmlrai@yahoo.com
3569

3570
E. A. MUSAD AND K. M. L. RAI
Scheme 1.
conversion of oxdiazoles to thiodizoles.[25] The development of solvothermal
reaction is of interest because they offer the possibility of environmentally benign
reaction conditions by reducing the burden of organic solvent disposal. As part of
our programme related to the application of solvothermal reaction in organic
synthesis, we herein report a novel and convenient method for the synthesis of some
new substituted isoxazoles and pyrazoles in excellent yields from a,a-oxoketen
dithioacetals, which were taken as 1,3-electrophiles. It is clear in these system that
the highly polarized push (dialkylthio)- pull (dicarbonyl) interaction on the C–C
double bond (Scheme 1), therefore we have become interested in investigating the
reactivity of a,a-oxoketen dithioacetals (2a–d) towards bifunctional nucleophiles
such as hydroxylamine hydrochloride, phenyl hydrazine and hydrazine hydrate
under solvothermal condition.
RESULTS AND DISCUSSION
The desired a,a-dioxoketen dithioacetals (2a–d) were prepared in high yield by
reacting the corresponding active methylene compounds with carbon disulphide in
the presence of sodium hydride as base in benzene followed by alkylation’s with
methyl bromide or methyl, ethyl iodide in one pot reaction[26] (Scheme 1). Thus,
when a-a-dioxoketen dithioaetals (2a–c) were reacted with hydroxylamine hydro-
chloride in the presence of pyridine as base under solvothermal condition, afforded
the isoxazoles (3a–c) in 71–91% yield as shown in Scheme 2.
Scheme 2.

SYNTHESIS OF SUBSTITUTED ISOXAZOLES/PYRAZOLES
3571
Scheme 3.
The probable pathway for the formation of isoxazoles from a-a-dioxoketen
dithioaetals is as shown in Scheme 3.
Under identical condition, a,a-dioxoketen dithioacetals (2a–c) react with
phenylhdrrazine in neutral media afforded to products (4a–c) 77–91% yield as shown
in Scheme 4.
The probable mechanism for the formation of products (4a–c) involves the
attack of the unsubstituted nitrogen of the phenyl hydrazine to the carbonyl carbon,
followed by cyclization the other nitrogen to the methylenic carbon of the methylthio
(Scheme 5).
Finally the treatment of a,a-dioxoketen dithioacetals (2a–d) with hydrazine
hydrate under same conditions afforded to the compounds (5a–d) in 76–87% as
shown in Scheme 6.
The structures of products isolated were established with the help of spectral
and analytical data.
Scheme 4.

3572
E. A. MUSAD AND K. M. L. RAI
Scheme 5.
Scheme 6.
EXPERIMENTAL
All reagents were purchased from commercial sources and used without
1
treatment, H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded
in CDCl3 or DMSO-d6 and TMS is used as internal reference. IR spectra (KBr
and nujol) were recorded on an FTIR spectrophotometer in the range of 400–
4000 cmÀ1. Elemental analyses were carried out on an Elementor vairo-EL
instrument. Melting points are uncorrected.
The a,a-dioxoketen dithioacetals (2a–d) were prepared according to the
method cited in literature.(27)

SYNTHESIS OF SUBSTITUTED ISOXAZOLES/PYRAZOLES
3573
Synthesis of 3,4,5-Substituted Isoxazoles (3a–c)—General
Procedure
A mixture of a,a-dioxoketen dithioacetals (2a–c) (9.9 mmol), hydroxylamine
hydrochloride (0.69 gm, 9.9 mmol) and 3 to 4 drops of pyridine were taken in the
autoclave reaction container (Teflon Liner). The lid was placed and was lowered into
the autoclave, the plates were kept over it and the autoclave was closed and tigh-
tened. It was kept in the oven at 150 ꢀC for 4 hrs. The products after cooling, were
extracted into ethyl acetate (20 ml), washed thoroughly with water. TLC of the sol-
ution shows single spots, which are different from the starting materials. The organic
layer was dried over anhydrous Na2SO4, solvent was evaporated and the product
obtained as white crystals while 3b as viscous brown oil and 3c as orange crystals.
1-(3-Methyl-5-(methylthio)isoxazole-4-yl) ethanone (3a). White crystals
(1.2 g, 71%); mp 95–97 ꢀC; IR cmÀ1 (KBr): 2965, 1634, 1485, 1369; 1H NMR
(400 MHz, CDCl3); d 2,28 (s,3H,-CH3), 2.40 (s,3H,-COCH3), 2.42 (s,3H,-SCH3),
13C NMR (100 MHz CDCl3): d 11.07, 12.32, 13.83, 109.03, 148.30, 158.65, 168.06.
Anal. calcd. for C7H9NO2S: C, 49.10%, H, 5.30%; N, 8.18%: Found: C, 49.31%;
H, 5.20%; N, 8.21%.
Ethyl 3-ethoxy-5-(methylthio) isoxazole-4-carboxylate (3b). Brown oil
1
(2.7 g, 91%); IR cmÀ1 (neat): 2983, 2931, 1723, 1530, 1455; H NMR (400 MHz
CDCl3): d 1.25 (t,3H,-CO2CH2CH3), 1.32 (t,3H,-COCH2CH3), 2.44 (s,3H,-SCH3);
4.19 (q,2H-CO2CH2CH3), 4.29 (q,2H,-COCH2CH3), 13C NMR (100 MHz CDCL3):
d 14.06, 18.48, 61.44, 127.45, 158.68, 164.03, 168.06. Anal. calcd. for C9H13NO4S: C,
46.74%; H, 5.67%; N, 6.06%, Found: C, 46.69%, H, 5.62%, N, 6.10%.
Ethyl 3-methyl-5-(methythio) isoxazole-4-carboxylate (3c). Orange
crystals (1.6 g, 87%); mp 36–38 ꢀC; IR cmÀ1 (KBr): 2988, 2926, 1711, 1632, 1554,
1429; 1H NMR (400 MHz, DMSO-d6): d1.28 (t,3H,-CO2CH2CH3); 2.32 (s, 3H,
-CH3); 2.63 (s, 3H,-SCH3); 4.24 (q,2H,-CH2CH3), 13C NMR (100 MHz, DMSO-d6):
d13.02, 14.77, 9.33, 85.33, 162.45, 162.88, 168.81, Anal. calcd. for C8H11NO3S: C,
47.75%; H, 5.51%; N, 6.69%, Found C, 47.77%; H, 5.46%, N, 6.91%.
Synthesis of 3,4,5-Substituted Pyrzoles (4a–c)—General Procedure
A mixture of a,a-dioxoketen dithioacetals (2a–c) (9.9 mmol), and phenyl
hydrazine (1.07 ml, 9.9 mmol) were taken in the autoclave reaction container
(TeflonTM liner). The lid was placed and was lowered into the autoclave, the plates
were kept over it and the autoclave was closed and tightened. It was kept in the oven
at 150 ꢀC for 3 h. The products after cooling, were extracted into ethylacetate (20 ml),
washed thoroughly with water. TLC of the solution shows single spots, which are
different from the starting materials. The solvent reduced to the minimum. Then
petroleum ether (20 ml) was added. The solid product so formed in each case was
collected by filtration, and recrystallized from aqueous ethanol or ethylacetate=
petroleum ether.
1-(3-Methyl-5-(methylthio)-1-phenyl-1H-pyrazol-4-yl)ethanone (4a). White
crystals (2.2 g, 81%); mp 122–124 ꢀC IR cmÀ1 (KBr): 3056, 2999, 2923, 1640, 1510,

3574
E. A. MUSAD AND K. M. L. RAI
1
1419, H NMR (400 MHz DMSO-d6): d 2.42 (s,3H,-CH3); 2.43 (s, 3H, -COCH3);
2.49 (s,3H,-SCH3); 7.49–7.59 (m,5H,ArH); 13C NMR (100 MHz, DMSO-d6): d
13.55, 13.75, 30.75, 117.65, 126.26, 126.27, 129.81, 138.72, 144.83, 150.51, 192.17.
Anal. calcd. for C13H14N2OS, C, 63.39%, H, 5.73%; N, 11.37%; Found: C,
63.21%, H, 5.82%; N, 10.89%.
Ethyl-3-ethoxy-5-(methylthio)-1-phenyl-1-H-pÀy1razole-4-carboxylate (4b).
White crystals (2.8 g, 77%): mp 150–152 ꢀC; IR cm (Nujol) 3202, 2922, 2857,
1636, 1553, 1458, 1H NMR (400 MHz, DMSO-d6); d1.15 (t.3H,-CO2CH2CH3);
1.20 (t,3H,-COCH3); 2.31 (s,3H,-SCH3); 3.97 (q,2H,-CO2CH2); 4.04 (q, 2H,
-COCH2CH3); 6.91–7.27 (m,5H,ArH); 13C NMR (100 MHz, DMSO-d6): d 12.03,
15.33, 57.37, 90.46, 117.72, 121.87, 128.51, 141.72, 147.8, 164.44, 164.82. Anal. calcd.
for C15H18N2O3S: C, 58.80%; H, 5.92%; N, 9.14%. Found: C, 58.76%; H, 5.78%; N,
9.09%.
Ethyl-3-methyl-5-(methylthio)-1-1phenyl-1-H-pyrazole-4-carboxylate
(4c). Yellow crystals (21 g, 89%): mp74–76 ꢀC IR cm-1 (KBr): 3056, 2978, 2918,
1688, 1532, 1423; 1H NMR (400 MHz, DMSO-d6); d1.27 (t,3H,-CO2CH2CH3);
2.43 (s,3H,-CH3); 2.50 (s,2H,-SCH3): 7.48À7.58 (m, 5H, ArH); C13 NMR
(100 MHz CDCl3); d 12.58, 13.62, 14.41, 60.10, 110.26, 125.555, 128.49, 129.23,
138.82, 145.36, 151.29, 163.77. Anal. calcd. for C14H16N2O2S: C, 60.85%; H,
5.84%, N, 10.14%. Found: C, 60.69%; H, 5.79%; N, 10.32%.
Synthesis of 3,4,5 Substituted Pyrazoles (5a–d)—General Procedure
A mixture of a,a-dioxoketen dithioacetals (2a–d) (9.9 mmol), and hydrazine
hydrate (0.49 ml, 9.9 mmol) were taken in the autoclave reaction container (TeflonTM
liner). The lid was placed and was lowered into the autoclave, the plates were kept
over it and the autoclave was closed and tightened. It was kept in the oven at
100 ꢀC for 1 hr. After cooling the semi-solid mass was obtained (monitored by
TLC). The product extracted to the petroleum ether (60–80 ꢀC) (20 ml) at solvent
boiling point for several times. The solvent was cooled on ice bath, the product so
formed, in each case was collected by filtration, dried and recrystalized from ethanol.
1-(3-Methyl-5-(methythio)-1-H-pyrazol-4À-y1l) ethanone (5a). White crys-
tals (1.3 g, 76%) unstable mp 122–124 ꢀC; IR cm (KBr): 3189, 3099, 2979, 2914,
1
1684, 1569, 1433 H NMR (400 MHz, d6) d 1.70 (s,3H,CH3); 1.99 (s,3H,-COCH3);
2.48 (s,3H,-SCH3); 13C NMR (100 MHz, CDCl3): d 13.76, 14.96, 31.56, 114.11,
144.22, 158.59, 192.17. Anal. calcd. for C7H10N2OS: C, 49.39%; H 5.92%; N,
16.46%. Found: C, 49.10%; H, 5.86%; N, 16.58%.
Ethyl 3-ethoxy-5-(methythio)-1H-pyrÀa1zole-4-carboxylate (5b). White
crystals (1.4 g, 86%); mp 127–129 ꢀC; IR cm
(KBr): 3214, 2932, 1665, 1576,
1
1480; H NMR (100 MHz, DMSO-d6) d 1.14 (t, 3H, -CO2CH2CH3); 1.19 (t, 3H,
-OCH2CH3); 2.21 (s, 3H, -SCH3); 3.99 (q, 2H, -CO2CH2CH3); 4.01 (q, 2H,
-OCH2CH3); 7.09 (br, 1H, NH); 13C NMR (100 MHz, DMSO-d6): d 11.9, 14.79,
57.26, 89.7, 145.5, 164.3, 166.4. Anal. calcd. for C9H14N2O3S: C, 46.94%; H,
6.13%; N, 12.16%; Found: C 46%; N 6.01%, N, 12.46%.

SYNTHESIS OF SUBSTITUTED ISOXAZOLES/PYRAZOLES
3575
Ethyl 3-methyl-5-(methylthio)-1H-pyrazole-4-carboxylate (5c). Yellow
crystals (1.5 g, 87%); mp 99–101 ꢀC; IR cmÀ1 (KBr): 3254, 3102, 2980, 2921, 1663,
1492, 1375; 1H NMR (400 MHz, DMSO-d6): d 1.21, (t,3H,-OCH2CH3); 2.37
(s,3H,-CH3); 2.49 (s,3H,-SCH3); 4.18 (q,2H,-OCH2CH3); 13 (br,1H,NH); 13C
NMR (100 MHz, DMSo-d6): d, 11.93, 13.43, 14.70, 59.70, 107.70, 145.99, 163.43.
Anal. calcd. for C8H12N2O2S: C, 47.98%; H, 6.04% N, 13.99, Found: C, 47.78%,
H, 6.18%; N, 13.4%.
Ethyl 5-(ethylthio)-3-methyl-1H-Àp1yrazole-4-carboxylate (5d). Orange
crystals (1.4 g, 76%); mp 80–83 ꢀC; IR cm (Nujol): 3235, 2926, 2854, 1677, 1614,
1520, 1462, 1H NMR (400 MHz, DMSO-d6): d1.13 (t, 3H, -SCH2CH3); 1.23
(t,3H,- CO2CH2CH3); 2.48 (S, 3H, -CH3); 2.78 (q, 2H, -SCH2CH3); 3.9 (q, 2H,
-CO2CH2CH3); 9.1 (br,1H,NH), 13C NMR (100 MHz, DMSO-d6): d 51.21, 15.27,
22.92, 89.98, 145.12, 164.74, 166.93. Anal. calcd. for C9H14N2O2S: C, 50.45%, H,
6.59%, N, 13.07%; Found: C, 49.99%; H, 6.22%; N, 13.62%.
REFERENCES
1. Garcia, J. L.; Fajordo, G.; Martin, M. R. Synthesis of alkyl 4-(diethoxymethyl)-3-pyridin-
3-ylisoxazole-5-carboxylates. Tetrahedron 2005, 61, 4363–4371.
2. Maczynski, M.; Zimeck, M.; Ryng, S. A new class of isoxazole derivatives: The M 1–9
series of compounds eith immunotropic activity. Acta Pol-Pharm. 2008, 65, 241–244.
3. Xue, C.; Roderick, J.; Mousa, S.; Olson, R. E., DeGrado, W. F. Synthesis and antiplatelet
effects of an isoxazole series of glycoprotein IIb=IIIa antagonists. Biorg. Med. Chem. Lett.
1998, 8, 3499–3504.
4. Lee, Y.; Kim, B. H. Heterocyclic nucleoside analogues: Design and synthesis of antiviral,
modified nucleosides containing isoxazole heterocycles. Biorg. Med. Chem. Lett. 2002, 12,
1395–1397.
5. Kang, Y. K.; Shin, K. J.; Yoo, K. H.; Seo, K. J.; Hong, C. Y.; Lee Park, S. Y.; Kim, D. J.;
Park, S. W. Synthesis and antibacterial activity of new carbapenems containing isoxazole
moiety. Biorg. Med. Chem. Lett. 2000, 10, 95–99.
6. Martin, L.; Polo, G.; Ramos, V.; Torroba, T., Marcaccini, S. A short synthesis of poten-
tial juvenoids based on the isoxazole chemistry. Heterocycles 1993, 36, 2259–2265.
7. Elguero, J. In Comprehensive Heterocyclic Chemistry II; A. Katritzky, C. W. Rees,
E. F. V. Scriven (Eds.); Pergamon Press: Oxford, 1996; vol. 3, p. 1.
8. Kess, K. L.; Fitzgerald, J. J., Jr.; Steiner, K. E.; Mattes, J. F.; Mihan, B.; Tosi, T.;
Mandor, D.; McCaleb, M. L. J. Synthesis and structure-activity relationship studies of
(4-substituted benzyl) (trifluoromethyl)pyrazoles and -pyrazolones. Med. Chem. 1996,
39, 3920.
9. Jungheim, L. Bicyclic pyrazolidinone antibacterial agents. Synthesis of side chain analo-
gues of carbapenems. PS-5 and thienamycin. Tetrahedron Lett. 1989, 30, 1889.
10. Peruncheralathan, S.; Yadav, A. K.; Ila, H.; Jungappa, H. J. Highly regioselective
synthesis of 1-aryl-3(or 5)-alkyl=aryl-5 (or3)-(N-cycloamino)pyrazoles. Org. Chem. 2005,
70, 9644–9647.
11. Goda, F. E.; Maroul, A. R.; El-Bendory, E. R. Synthesis and antimicrobial evaluation of
new isoxazole and pyrazole derivatives. Saudi Pharma. J. 2003, 3, 11.
12. Katritzky, A. R.; Wang, M.; Zhang, S.; Voronkov, M. V. Regioselective synthesis of
polysubstituted pyrazoles and isoxazoles. J. Org. Chem. 2001, 66, 6787.

3576
E. A. MUSAD AND K. M. L. RAI
13. Sugiura, S.; Ohno, S.; Ohtani, O.; Izumi, K.; Kitamilado, T.; Asa, H.; Kato, K.; Hori, M.;
Fujimura, H. Synthesis and antinflammatory and hypnotic activity of 5-alkoxy-3-
(N-substituted carbamoyl)-1-phenylpyrazoles. J. Med. Chem. 1977, 20, 80.
14. Umesha, K. B.; Lokanatha, R. K. M.; Kumar, K. A. A new approach for the synthesis of
pyrazols via 1,3-dipolar cycloaddition of nitrile imines to acetyl acetone. Indian J. Chem.
2002, 418, 1405–1453.
15. Lee, K. Y.; Kim, J. M.; Kim, J. N. Regioselective synthesis of 1,3,4,5-tetrasubstituted
pyrazoles from Baylis-Hillman adducts. Tetrahedron Lett. 2003, 44, 6737.
16. Sakya, S. M.; Rast, B. Efficient synthesis of 5-alkyl amino and thioether substituted
pyrazoles. Tetrahedron Lett. 2003, 44, 7629.
17. Wang, X. Z.; Qin, L. H. Solventless syntheses of pyrazole derivatives. Green Chem. 2004,
6, 90–92.
18. Kost, A. N.; Grandberg, I. I. In Advances in Heterocyclic Chemistry; A. R. Katritzky, A. J.
Buoulton (Eds.), Academic Press: New York, 1966; Vol. 6, p. 347.
19. Wang, X. M.; Liu, Y.; Huang, T. Z. Novel and convenient synthesis of polyfunctionalized
quinolines, quinolones and their annulation reactions. Tetrahedron Lett. 2001, 42, 2553.
20. Liu, Y.; Dong, D.; Liu, Q.; Qi, Y.; Wang, Z. A novel and facile synthesis of dienals
and substituted 2H-pyrans via the Vilsmeier reaction of a-oxo-ketenedithioacetals. Org.
Baiomol. Chem. 2004, 2, 28.
21. Junjappa, H.; Ila, H.; Asokan, C. V. a-Oxo-ketene-S,S-, N,S- and N,N-acetals: Versatile
intermediates in organic synthesis. Tetrahedron 1990, 16, 5423–5506.
22. Dieter, R. K. a-Oxo ketene dithioacetals and related compounds: Versatile three-carbon
synthons. Tetrahedron 1986, 42, 3029–3096.
23. Huang, Z. T.; Shi, X. Synthesis of heterocyclic ketene-N,S-acetals and their reactions with
esters of a,b-unsaturated acids. Synthesis 1989, 162.
24. Aparna, E.; Rai, K. M. L.; Suresh Babu, M.; Jagadeesh, R. L.; Gaonkar, S. L.;
Byrappa, K. Synthesis of thioesters and thioamides under solvothermal condition using
thiourea as thionating agent. J. Mater. Sci. 2006, 41, 1391.
25. Rai, K. M. L.; Linganna, N. Transformation of 1,3,4-oxadiazoles to 1,3,4-thiadiazoles
using thiourea. Synth. Commun. 1998, 28, 4611.
26. Sandstorm, J.; Wennerbeck, I. Barriers to rotation around the carbon–carbon double
bond in 1,1-bis-alkylthio-ethylenes. Acta Chem. Scand. 1970, 24, 1191–1200.