Studies in aza-Claisen rearrangement: Synthesis of dimedone-annelated unusual heterocycles

Article abstract of DOI:10.1016/S0040-4020(01)00405-7

A number of 3-N-(4′-aryloxybut-2′-ynyl)N-methyl amino-5,5-dimethyl cyclohex-2-enones are synthesized in 62-65% yield by refluxing 3-chloro-5,5-dimethyl cyclohex-2-enone with a number of 4-aryloxy-4-chlorobut-2-ynes in ethanol for 4 h. The amines are then

Full text of DOI:10.1016/S0040-4020(01)00405-7

TETRAHEDRON
Pergamon
Tetrahedron 57 '2001) 4955±4958
Studies in aza-Claisen rearrangement: synthesis of
dimedone-annelated unusual heterocycles
K. C. Majumdarp and S. K. Samanta
Department of Chemistry, University of Kalyani, Kalyani 741 235, West Bengal, India
Received 19 January 2001; revised 14 March 2001; accepted 5 April2001
AbstractÐA number of 3-N-'40-aryloxybut-20-ynyl)N-methyl amino-5,5-dimethyl cyclohex-2-enones are synthesized in 62±65% yield by
re¯uxing 3-chloro-5,5-dimethyl cyclohex-2-enone with a number of 4-aryloxy-4-chlorobut-2-ynes in ethanol for 4 h. The amines are then
heated in re¯uxing chlorobenzene to give 7,7-dimethyl-1,2,3,6,7,8-hexahydro-4-aryloxymethylene-1-methyl quinoline-5-one in 75±80%
yields. q 2001 Elsevier Science Ltd. All rights reserved.
Since its discovery1 in 1912, the Claisen rearrangement of
allyl vinyl ethers has become a powerful tool for carbon±
carbon bond formation.2 Much of its current popularity is
due to the subsequent development of a series of new
variants3 of this [3,3] sigmatropic rearrangement as well
as its versatile application in the synthesis of various
heterocycles using oxygen,4 nitrogen,5 and sulfur6 Claisen
rearrangements. Our recent work on the synthesis of various
heterocycles by the application of [3,3] sigmatropic
rearrangement7 prompted us to undertake a study on the
synthesis of dimedone-annelated heterocycles. Here we
report the results of this investigation.
cyclohex-2-enone8 with a number of 1-aryloxy-4-N-methyl-
amino but-2-ynes '3a±f) in re¯uxing absolute ethanol for
4 h. 'Scheme 1). 3-Chloro-5,5-dimethyl cyclohex-2-enone
'2) in turn was obtained from the reaction of dimedone with
phosphorous trichloride.
The substrates '4a±f) are unique in the sense that the
molecules provide two different sites for the occurrence of
[3,3] sigmatropic rearrangement, the propargylvinylamine
moiety and the arylpropargylether moiety. Our past
observation in case of 5-N-[4-aryloxybut-2-ynyl],N-methyl
amino-1,3-dimethylpyrimidine-2,4-diones have shown that
the [3,3] sigmatropic rearrangement is preferred at the aryl
9
propargylether moiety over vinylpropargylamine moiety.
However, with other substrates such as 4-N-'4-aryloxybut-
2-ynyl),N-methylamino-6-methylpyran-2-one and 4- N-'4-
aryloxybut-2-ynyl),N-methylamino[1]benzopyran-2-one,
the [3,3] sigmatropic rearrangement occurred at the
1. Results and discussion
The starting materials '4a±f) for this study were synthesized
in 62±65% yield by the reaction of 3-chloro-5,5-dimethyl-
Scheme 1.
Keywords: aza-Claisen rearrangement; sigmatropic rearrangement; 3-chloro-5,5-dimethyl cyclohex-2-enone; 3,3-prototropic shift; 5-N-[4-aryloxybut-2-
ynyl],N-methylamino-1,3-dimethylpyrimidine-2,4-dione; 7,7-dimethyl-1,2,3,6,7,8-hexahydro-4-aryloxymethylenemethyl quinoline-5-one.
Corresponding author. Tel.: 133-582-7521; fax: 133-582-8282; e-mail: kcm@klyuniv.ernet.in
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020'01)00405-7

4956
K. C. Majumdar, S. K. Samanta / Tetrahedron 57 )2001) 4955±4958
Scheme 2.
10
propargylvinylamine part of the substrates.
The present
2. Experimental
substrates '4a±f), therefore, provide scope for studying the
competitive [3,3] sigmatropic rearrangement. With this end
in view the substrate 4a was heated in re¯uxing chloro-
benzene. TLC monitoring indicated the formation of a
new product. However, it took about 12 h to complete the
reaction. The product was characterized from its elemental
analysis and spectral data as 7,7-dimethyl-1,2,3,6,7,8-hexa-
hydro-4-phenoxymethylene-1-methylquinoline-5-one '5a).
Encouraged by this result other substrates were also
similarly treated to give products 5b±f 'Scheme 2)
Melting points were measured on a sulfuric acid bath and
are uncorrected. UV absorption spectra were recorded
[Perkin±Elmer UV±vis Spectrophotometer, Lamda 20,
1 nm] in EtOH. IR spectra were run on KBr disks on a
Perkin±Elmer 1330 apparatus. 1H NMR spectra were
determined for solutions in CDCl3 with TMS as internal
standard on a Bruker 300 '300 MHz) instrument. Elemental
analyses and recording of mass spectra were carried out by
RSIC'CDRI) Lucknow on a [JEOL D-300 'El)] instrument.
Silica gel ['60±120 mesh), Spectrochem, India] was used
for chromatographic separation. Petroleum ether refers to
the fraction boiling between 60 and 808C.
The formation of products 5a±f from 4a±f may be
explained by an initial [3,3] sigmatropic shift at the pro-
pargyl vinyl amine moiety to give the allene intermediate
6, followed by tautomerism, [1,5] H shift and electrocyclic
ring closure to give unstable endocyclic intermediate 9 'not
isolated). A [1,3] prototropic shift in 9 may give ®nal
product 5 '`pathway a'). Support in favour of this pathway
came from our recent observation that the thermal
rearrangement of 4-N-'4-aryloxybut-2-ynyl),N-methyl-
amino-6-methyl-2-pyrone gave both exocyclic and endo-
cyclic products and the endocyclic products are converted
to exocyclic products under the same reaction condition.10a
The TLC of the crude reaction mixture indicated the forma-
tion of the endocyclic products. However, we were unable
to isolate them despite our best efforts. Another pathway
may be via one more [1,5] H shift in 8 followed by a 6-endo-
cyclization '`pathway b') to give 5 'Scheme 2).
2.1. Preparation of 1-'aryloxy)-4-N-methylaminobut-2-
yne
1-Aryloxy-4-chlorobut-2-yne11 '1 g, 5.5 mmol) in 20 mL of
EtOH was added dropwise at room temperature to a well
stirred solution of '23 mL, 296.8 mmol) of methylamine
'40% aqueous solution) for a period of 10 min. Stirring
was continued for another 2 h at room temperature. Methyl-
amine, water and EtOH were removed under reduced
pressure and the residualmass was dissovled in CHCl
3
'25 mL). This was washed with brine '2£25 mL) and
dried over anhydrous Na2SO4. Removalof CHCl 3 afforded
a gummy mass. Attempts to purify this gummy mass by
column chromatography failed. It was therefore used
directly in the subsequent reactions.
It is interesting to note that the [3,3] sigmatropic rearrange-
ment occurred at the propargylvinylamine part of althe
substrates '4a±f) studied so far. The formation of unusual
exocyclic products instead of normal endocyclic products is
noteworthy.
2.2. General procedures for the synthesis of compounds
4'a±f)
A
mixture of 1-aryloxy-4-N-methylaminobut-2-yne

K. C. Majumdar, S. K. Samanta / Tetrahedron 57 )2001) 4955±4958
4957
'5 mmol) and 3-chloro-5,5-dimethylcyclohex-2-enone
'0.50 g, 3.2 mmol) in anhydrous EtOH '50 mL) was
re¯uxed for 4 h on a water bath. Alcohol was removed by
distillation and the residual mass was dissolved in CHCl3
'50 mL). This was washed with brine '2£25 mL) and dried
over anhydrous Na2SO4. Evaporation of CHCl3 afforded a
gummy mass which was subjected to column chromato-
graphy over silica gel. Elution of the column with benzene±
ethylacetate '1:1) gave compounds 4'a±f).
's, 1H, vCH), 6.8±6.9 'm, 2H, Ph), 7.1±7.2 'm, 2H, Ph);
MS m/z 311 'M1); Anal. calcd for C20H25 NO2: C, 77.17; H,
8.03; N, 4.50; found C, 77.35; H, 8.29; N, 4.62%.
2.2.6. 3-N-[40-'40-Methylphenoxy)but-20-ynyl]N-methyl
amino-5,5-dimethyl cyclohex-2-enone '4f). Yield 64%;
viscous liquid; lmax: 'l oge) 221 '4.98), 296 '5.40) nm; IR
1
'KBr) nmax: 1100, 1240, 1410, 1550, 1610, 2960 cm21; H
NMR 'CDCl3, 300 MHz): d 1.05 's, 6H, 2Me), 2.14 's, 2H,
CH2CO), 2.26 's, 2H, vCCH2), 2.28 's, 3H, MePh), 2.92 's,
3H, NMe), 4.01 'brs, 2H, NCH2), 4.66 'brs, 2H, OCH2), 5.20
's, 1H, vCH), 6.81±6.84 'm, 2H, Ph), 7.06±7.09 'm, 2H,
Ph); MS m/z 311 'M1); Anal. calcd for C20H25 NO2: C,
77.17; H, 8.03; N, 4.50; found C, 77.48; H, 8.25; N, 4.67%.
2.2.1. 3-N-'40-Phenoxybut-20-ynyl)N-methyl amino-5,5-
dimethyl cyclohex-2-enone '4a). Yield 62%; viscous
liquid; lmax: 'log e) 219 '3.94), 297 '4.36) nm; IR 'KBr)
1
n
max: 1100, 1240, 1410, 1540, 1610, 2960 cm21; H NMR
'CDCl3, 300 MHz): d 1.05 's, 6H, 2Me), 2.14 's, 2H,
CH2CO), 2.26 's, 2H, vCCH2), 2.92 's, 3H, NMe), 4.02
'brs, 2H, NCH2), 4.70 'brs, 2H, OCH2), 5.21 's, 1H,
vCH), 6.92±6.99 'm, 3H, Ph), 7.26±7.32 'm, 2H, Ph);
MS m/z 297 'M1); Anal. calcd for C19H23NO2: C, 76.76;
H, 7.74; N, 4.71; found C, 76.98; H, 7.89; N, 4.87%.
2.3. General procedures for the synthesis of compounds
5'a±f)
Compounds 4a±f '0.2 g, 0.67 mmol) were re¯uxed in
chlorobenzene '2 mL) for 12 h. The reaction mixture was
then cooled and directly subjected to column chromato-
graphy over silica gel. Chlorobenzene was eluted out with
petroleum ether. All the products 5'a±f) were obtained
when the column was eluted with benzene±ethyl acetate
'4:1). Unchanged starting materials 4'a±f) were also care-
fully eluted out with benzene±ethyl acetate '1:1). The yields
were calculated on the basis of actual conversion of starting
materials.
2.2.2. 3-N-[40-'20-Chlorophenoxy)but-20-ynyl]N-methyl
amino-5,5-dimethyl cyclohex-2-enone '4b). Yield 64%;
viscous liquid; lmax: 'log e) 220 '3.98), 295 '4.41) nm; IR
1
'KBr) nmax: 1100, 1230,1510, 1700, 2910 cm21; H NMR
'CDCl3, 300 MHz): d 1.05 's, 6H, 2Me), 2.04 's, 2H,
CH2CO), 2.26 's, 2H, vCCH2), 2.9 's, 3H, NMe), 4.01 't,
Jˆ1.5 Hz, 2H, NCH2), 4.79 't, Jˆ1.5 Hz, 2H, OCH2), 5.19
's, 1H, vCH), 6.94±7.03 'm, 1H, Ph), 7.18±7.21 'm, 1H,
Ph), 7.35±7.39 'm, 2H); MS m/z 331, 333 'M1); Anal. calcd
for C19H22ClNO2: C, 68.77; H, 6.63; N, 4.22; found C,
68.92; H, 6.81; N, 4.31%.
2.3.1.
7,7-Dimethyl-1,2,3,6,7,8-hexahydro-4-phenoxy-
methylene-1-methyl quinoline-5-one '5a). Yield 77%;
white crystal, mp 138±1408C; lmax: 'log e) 219 '3.97),
273 '4.06) nm; IR 'KBr)
nmax: 1230, 1510, 1700,
2.2.3. 3-N-[40-'40-Chlorophenoxy)but-20-ynyl]N-methyl
amino-5,5-dimethyl cyclohex-2-enone '4c). Yield 65%;
viscous liquid; lmax: 'log e) 221 '3.98), 294 '4.08) nm; IR
2910 cm21; H NMR 'CDCl3, 300 MHz): d 1.08 's, 6H,
2Me), 2.17 's, 2H, CH2CO), 2.36 's, 2H, vCCH2), 2.71
't, Jˆ6 Hz, 2H, NCH2CH2), 3.06 's, 3H, NMe), 3.32 't,
Jˆ6 Hz, 2H, NCH2), 6.95±7.08 'm, 3H, Ph), 7.26±7.30
'm, 2H, Ph), 8.00 's, 1H, vCHOAr); MS m/z 297 'M1);
Anal. calcd for C19H23NO2: C, 76.76; H, 7.74; N, 4.71;
found C, 76.98; H, 7.87; N, 4.83%.
1
1
'KBr) nmax: 1100, 1240, 1410, 1550, 1610, 2960 cm21; H
NMR 'CDCl3, 300 MHz): d 1.05 's, 6H, 2Me), 2.15 's, 2H,
CH2CO), 2.25, 's, 2H, vCCH2), 2.92 's, 3H, NMe), 4.01
'brs, 2H, NCH2), 4.67 'brs, 2H, OCH2), 5.21 's, 1H, vCH),
6.85±6.88 'm, 2H, Ph), 7.22±7.26 'm, 2H, Ph). MS m/z 331,
333 'M1); Anal. calcd for C19H22ClNO2: C, 68.77; H, 6.63;
N, 4.22; found C, 68.97; H, 6.78; N, 4.33%.
2.3.2. 7,7-Dimethyl-1,2,3,6,7,8-hexahydro-4-'20-chloro-
phenoxy)methylene-1-methylquinoline-5-one '5b). Yield
78%; viscous liquid; lmax: 'l oeg) 220 '4.08), 275
2.2.4. 3-N-[40-'20,40-Dichlorophenoxy)but-20-ynyl]N-methyl
amino-5,5-dimethyl cyclohex-2-enone '4d). Yield 63%;
white crystalmp 108 8C; lmax: 'log e) 222 '3.90), 294
'4.08) nm; IR 'KBr) nmax: 1100, 1240, 1410, 1540, 1600,
'3.73) nm; IR 'KBr) nmax: 1240, 1520, 1700, 2920 cm21
;
1H NMR 'CDCl3, 300 MHz): d 1.08 's, 6H, 2Me), 2.17 's,
2H, CH2CO), 2.36 's, 2H, vCCH2), 2.66 't, Jˆ6 Hz, 2H,
NCH2CH2), 3.07 's, 3H, NMe), 3.34 't, Jˆ6 Hz, 2H, NCH2),
6.91±7.26 'm, 4H, Ph), 8.03 's, 1H, vCHOAr); MS m/z
331, 333 'M1); Anal. calcd for C19H22ClNO2: C, 68.77;
H, 6.63; N, 4.22; found C, 68.98; H, 6.81; N, 4.33%.
1
2940 cm21; H NMR 'CDCl3, 300 MHz): d 1.05 's, 6H,
2Me), 2.15 's, 2H, CH2CO), 2.25 's, 2H, vCCH2), 2.92
's, 3H, NMe), 4.01 'brs, 2H, NCH2), 4.77 'brs, 2H,
OCH2), 5.21 's, 1H, vCH), 6.93±6.96 'm, 1H, Ph), 7.17±
7.20 'm, 1H, Ph), 7.37±7.38 'm, 1H, Ph); MS m/z 365, 369,
367 'M1); Anal. calcd for C19H21Cl2NO2: C, 62.29; H, 5.73;
N, 3.82; found C, 62.52; H, 5.96; N, 3.86%.
2.3.3. 7,7-Dimethyl-1,2,3,6,7,8-hexahydro-4-'40-chloro-
phenoxy)methylene-1-methyl quinoline-5-one '5c).
Yield 75%; viscous liquid; lmax: 'l oge) 223 '4.11), 276
'4.02) nm; IR 'KBr) nmax: 1240, 1490, 1700, 2930 cm21
;
1H NMR 'CDCl3, 300 MHz): d 1.08 's, 6H, 2Me), 2.26 's,
2H, CH2CO), 2.36 's, 2H, vCCH2), 2.68 't, Jˆ6 Hz, 2H,
NCH2CH2), 3.07 's, 3H, NMe), 3.32 't, Jˆ6 Hz, 2H, NCH2),
6.8±7.2 'm, 2H, Ph), 7.2±7.3 'm, 2H, Ph), 7.95 's, 1H,
vCHOAr); MS m/z 331, 333 'M1); Anal. calcd for
C19H22ClNO2: C, 68.77; H, 6.63; N, 4.22; found C, 68.99;
H, 6.85; N, 4.37%.
2.2.5. 3-N-[40-'20-Methylphenoxy)but-20-ynyl]N-methyl
amino-5,5-dimethyl cyclohex-2-enone '4e). Yield 65%;
viscous liquid; lmax: 'log e) 221 '4.84), 296 '5.37) nm; IR
1
'KBr) nmax: 1100, 1240, 1400, 1550, 1600, 2950 cm21; H
NMR 'CDCl3, 300 MHz): d 1.05 's, 6H, 2Me), 1.93 's, 2H,
CH2CO), 2.19 's, 3H, MePh), 2.27 's, 2H, vCCH2), 2.92 's,
3H, NMe), 4.01 'brs, 2H, NCH2), 4.71 'brs, 2H, OCH2), 5.20

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K. C. Majumdar, S. K. Samanta / Tetrahedron 57 )2001) 4955±4958
2.3.4. 7,7-Dimethyl-1,2,3,6,7,8-hexahydro-4-'20,40-dichloro-
phenoxy)methylene-1-methyl quinoline-5-one '5d). Yield
78%; viscous liquid; lmax: 'log e) 219 '4.14), 282
'3.84) nm; IR 'KBr) nmax: 1230, 1410, 1550, 1600,
2. 'a) Blechert, S. Synthesis 1989, 71±82. 'b) Ziegler, F. E.
Chem. Rev. 1988, 88, 1423±1452. 'c) Moody, C. J. Adv.
Heterocycl. Chem. 1987, 42, 203±244. 'd) Kallmertn, J.;
Wittman, M. D. Stud. Nat. Prod. Chem. 1989, 3, 233±285.
'e) Lutz, R. P. Chem. Rev. 1984, 84, 205±247. 'f) Bennett,
G. B. Synthesis 1977, 589±606.
1
2940 cm21; H NMR 'CDCl3, 300 MHz): d 1.08 's, 6H,
2Me), 2.17 's, 2H, CH2CO), 2.34 's, 2H, vCCH2), 2.73
't, Jˆ6 Hz, 2H, NCH2CH2), 3.08 's, 3H, NMe), 3.34 't,
Jˆ6 Hz, 2H, NCH2), 6.94±7.26 'm, 2H, Ph), 7.31±7.34
'm, 1H, Ph), 8.02 's, 1H, vCHOAr); MS m/z 365, 369,
367 'M1); Anal. calcd for C19H21Cl2NO2: C, 62.29; H,
5.73; N, 3.82; found C, 62.47; H, 5.89; N, 3.91%.
3. 'a) Carroll, M. F. J. Chem. Soc. 1940, 704±706. Carroll, M. F.
J. Chem. Soc. 1940, 1266±1268. Carroll, M. F. J. Chem. Soc.
1941, 507±511. 'b) Kimel, W.; Cope, A. C. J. Am. Chem. Soc.
1943, 65, 1992±1998. 'c) Marbet, R.; Saucy, G. Chimia 1960,
14, 362±363. Saucy, G.; Marbet, R. Helv. Chim. Acta 1967,
50, 1158±1167. 'd) Johnson, W. S. J. Am. Chem. Soc. 1970,
92, 741±743. Johnson, W. S.; Gravestock, M. B.; Parry, R. J.;
Myers, R. F.; Bryson, T. A.; Miles, H. J. Am. Chem. Soc. 1971,
93, 4330±4332. 'e) Felix, D.; Gschwend-Steen, K.; Wick,
A. E.; Eschensoser, A. Helv. Chim. Acta 1969, 52, 1030±
1042. 'f) Ireland, R. E.; Mueller, R. H. J. Am. Chem. Soc.
1972, 94, 5897±5898. Pereira, S.; Srebnik, M. Aldrichim.
Acta 1993, 26, 17±29.
2.3.5. 7,7-Dimethyl-1,2,3,6,7,8-hexahydro-4-'20-methyl-
phenoxy)methylene-1-methylquinoline-5-one '5e). Yield
78%; viscous liquid; lmax: 'log e) 220 '4.04), 273
'3.84) nm; IR 'KBr) nmax: 1235,1510, 1700, 2930 cm21
;
1H NMR 'CDCl3, 300 MHz): d 1.08 's, 6H, 2Me), 2.19 's,
2H, CH2CO), 2.26 's, 3H, MePh), 2.36 's, 2H, vCCH2),
2.71 't, Jˆ6 Hz, 2H, NCH2CH2), 3.06 's, 3H, NMe), 3.33 't,
Jˆ6 Hz, 2H, NCH2), 6.81±6.86 'm, 1H, Ph), 6.87±7.04 'm,
3H, Ph), 7.99 's, 1H, vCHOAr); MS m/z 311 'M1); Anal.
calcd for C20H25NO2: C, 77.17; H, 8.03; N, 4.50; found C,
77.41; H, 8.19; N, 4.71%.
4. Majumdar, K. C.; Balasubramanian, K. K.; Thyagarajan, B. S.
J. Heterocycl. Chem. 1973, 10, 159±164.
5. Scheurer, H.; Zsindely, J.; Schmid, H. Helv. Kim. Acta. 1973,
56, 478±489.
6. Kwart, H.; George, T. J. J. Chem. Soc., Chem. Commun. 1970,
433±434.
2.3.6. 7,7-Dimethyl-1,2,3,6,7,8-hexahydro-4-'40-methyl-
phenoxy)methylene-1-methylquinoline-5-one '5f). Yield
80%; viscous liquid; lmax: 'log e) 221 '4.20), 272
7. 'a) Majumdar, K. C.; Biswas, P. Tetrahedron 1998, 54,
11,603±11,612. 'b) Majumdar, K. C.; Biswas, P. Tetrahedron
1999, 55, 1449±1456. 'c) Majumdar, K. C.; Chatterjee, P.;
Saha, S. Tetrahedron Lett. 1998, 39, 7147±7148.
'd) Majumdar, K. C.; Das, U. J. Org. Chem. 1998, 63,
9997±10,000. 'e) Majumdar, K. C.; Das, U.; Jana, N. K.
J. Org. Chem. 1998, 63, 3550±3553.
'4.00) nm; IR 'KBr) nmax: 1230, 1500, 1700, 2920 cm21
;
1H NMR 'CDCl3, 300 MHz): d 1.08 's, 6H, 2Me), 2.19 's,
2H, CH2CO), 2.28 's, 3H, MePh), 2.41 's, 2H, vCCH2),
2.70 't, Jˆ6 Hz, 2H, NCH2CH2), 3.06 's, 3H, NMe), 3.32 't,
Jˆ6 Hz, 2H, NCH2), 6.7±7.08 'm, 4H, Ph), 7.92 's, 1H,
vCHOAr); MS m/z 311 'M1); Anal. calcd for
C20H25NO2: C, 77.17; H, 8.03; N, 4.50; found C, 77.39;
H, 8.27; N, 4.63%.
8. Dalgaard, L.; Lawesson, S. O. Acta. Chem. Scand. 1974, B28,
1077±1090.
9. Majumdar, K. C.; Jana, N. K. Synth. Commun. 2000, 30,
2613±2623.
10. 'a) Majumdar, K. C.; Ghosh, S. Tetrahedron 2001, 57, 1589±
1592. 'b) Majumdar, K. C.; Bhattacharyya, T. Tetrahedron
Lett. 2001, 42, 4231±4233.
Acknowledgements
We are thankfulto the CSIR 'New Dehli) for ®nancial
assistance. One of us 'S. K. S.) is gratefulto the CSIR
'New Delhi) for a fellowship.
11. Majumdar, K. C.; Thyagarajan, B. S. Int. J. Sulfur. Chem.
1972, 2A, 93±103.
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