Synthetic studies towards the benzophenone precursor for balanol

Article abstract of DOI:10.1016/j.tet.2003.12.029

Synthesis of 4-carboxy-2,6-dimethoxyphenyl 2′-carboxy-6′- methoxyphenyl ketone, an important precursor for balanol's benzophenone portion, has been achieved via a short and efficient route in three steps using ortho-lithiation as the key step. In another

Full text of DOI:10.1016/j.tet.2003.12.029

Tetrahedron 60 (2004) 1869–1873
Synthetic studies towards the benzophenone precursor for balanol
*
Mahesh L. Patil, Vishnu H. Deshpande, S. Ramlingam and Hanumant B. Borate
Organic Chemistry Technology, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411 008, India
Received 26 September 2003; revised 13 November 2003; accepted 11 December 2003
Abstract—Synthesis of 4-carboxy-2,6-dimethoxyphenyl 20-carboxy-60-methoxyphenyl ketone, an important precursor for balanol’s
benzophenone portion, has been achieved via a short and efficient route in three steps using ortho-lithiation as the key step. In another
approach aromatization of 2-(20-methoxy-60-methylbenzoyl)-5-methyl-1,3-cyclohexanedione afforded benzophenone precursor 2,6-
dimethoxy-4-methylphenyl 20-methoxy-60-methylphenyl ketone along with the formation of substituted xanthone.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
The structure–activity relationship studies3 on balanol (1)
proved the critical importance of the benzophenone portion
for the efficacy of balanol. Any attempted change in this
tetra-ortho-substituted benzophenone portion resulted in a
decrease in its activity. Adams et al.2c used benzophenone
precursor 2 in the total synthesis of balanol.
Balanol (1), a novel protein kinase ‘C’ inhibitor, was
isolated in 1993 by Kulanthaivel et al.1 as an unusual
metabolite produced by the fungus Verticillium balanoides.
Balanol shows remarkable activity against cancer, HIV
infection, rheumatoid arthritis, diabetes, central nervous
system disorder etc. A wide range of biological activities
associated with this compound has attracted researchers to
undertake synthesis of balanol and its intermediates.2
The steric hindrance, which makes the synthesis of
this tetra-ortho-substituted benzophenone difficult,
prompted us to undertake its synthesis. Earlier reports on
the synthesis of balanol benzophenone precursors are
based on the direct utilization of aromatic rings. We
envisaged that use of alicyclic systems where the steric
hindrance would be less compared to the aromatic systems
and later aromatization of these alicyclic systems to the
desired benzophenone portion would be a convenient route
for the synthesis of this sterically hindered benzophenone
portion.
The synthetic approaches mainly began with disconnection
of balanol at its ester linkage to yield the hexahydroazepine
and the benzophenone carboxylic acid. The retrosynthesis
of balanol is as shown in Scheme 1. Total synthesis of
balanol has been achieved by coupling the protected
benzophenone domain and hexahydroazepine moiety by
esterification using modified Mukaiyama procedure.
Scheme 1. Retrosynthetic analysis for balanol (1).
Keywords: Balanol; Benzophenone; Dicarboxylic acid; Xanthone.
*
Corresponding author. Tel.: þ91-20-5893300; fax: þ91-20-5893614; e-mail address: borate@dalton.ncl.res.in
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.12.029

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M. L. Patil et al. / Tetrahedron 60 (2004) 1869–1873
Scheme 2. Retrosynthetic analysis for the synthesis of benzophenone precursor 2.
Retrosynthetic plan for benzophenone domain 2 (Scheme 2)
suggested the triketone 4 as an important intermediate
which on aromatization and oxidation would afford
benzophenone precursor 3. Our synthetic route started
with synthesis of acyl cyanide 6 from carboxylic acid 5.4
Thus carboxylic acid 5 was converted into corresponding
acid chloride followed by treatment with trimethylsilyl
cyanide in the presence of catalytic stannic chloride to
afford the acyl cyanide 6 in 85% yield. The synthesis of
triketone 4 was achieved by the treatment of acyl cyanide 6
with 5-methylcyclohexane-1,3-dione (7) in presence of
triethylamine to give the triketone 4 in 70% yield as shown
in Scheme 3.5,6
of the triketone 4 under these conditions resulted in the
formation of aromatized product 8 in 20% yield. Formation
of compound 8 was confirmed by converting it into the
known intermediate 3, an important benzophenone pre-
cursor for balanol. Spectral data for compound 3 were
identical to those reported by Adams et al.2c
Aromatization of triketone 4 in the presence of mercuric
acetate and sodium acetate in acetic acid afforded only 20%
of the required product 8 along with formation of xanthone
1
99 in 40% yield. The H NMR spectrum of xanthone 9
exhibited singlets at d 2.41 and 2.89 for aromatic methyls,
singlets at d 6.55 and 6.65 each integrating for one proton
and a singlet at d 12.86 for chelated –OH group. This
xanthone 9 was then methylated using dimethyl sulphate
in the presence of potassium carbonate in refluxing
acetone to give the corresponding methyl ether 10 whose
1H NMR spectrum showed the presence of only one
methoxy group at d 4.01 confirming the assigned structure
of xanthone 9.
After successful synthesis of triketone 4, our next aim was
to convert compound 4 into the desired benzophenone 3.
Unfortunately the attempted aromatization of the compound
4 using I2/MeOH,7 DDQ in benzene or Pd/C failed to give
the desired compound 3 and resulted either in the recovery
of starting material or a complex reaction mixture. Finally
aromatization of compound 4 was achieved using mercuric
acetate and sodium acetate in acetic acid under the
conditions reported by Oliver et al.8 for the aromatization
of 2-acyl-3-hydroxy-2-cyclohexane-1-ones. Aromatization
Probable reason for the formation of xanthone would be the
loss of methanol during aromatization due to the presence of
Lewis acidic mercuric acetate as shown in Scheme 4. Burger
Scheme 3. Synthesis of benzophenone precursor 3.

M. L. Patil et al. / Tetrahedron 60 (2004) 1869–1873
1871
Scheme 4. Formation of xanthone 9.
and Montes5 reported similar results of dehydration during
the synthesis of the antibiotic pyoluteorin.
scopic data in good agreement with those reported by
Adams et al.2c
Thus aromatization of triketone 4 afforded the desired
benzophenone 8, the precursor for balanol, in 20% yield
along with the formation of xanthone 9 in 40% yield.
Though this reaction yielded the desired intermediate 8 in
low yield, the formation of xanthone 9 in this reaction could
be explored for the synthesis of substituted xanthones using
appropriately substituted triketone 4.
Oxidation of 3 to the dicarboxylic acid 2, an important
precursor for benzophenone portion of balanol (1), has been
reported by Adams et al.2c Thus oxidation of the compound
3 using potassium permanganate and pyridine in the
presence of phase transfer catalyst tetrabutylammonium
bromide in water afforded the desired benzophenone
precursor for balanol 2 in 45% yield.
Low yield for aromatization of triketone 4 to the desired
benzophenone 8 in the earlier approach prompted us to
explore another route for the synthesis of benzophenone
precursor 3 for balanol.
Thus in conclusion we have achieved synthesis of an
important intermediate 2 for balanol’s benzophenone
portion by using two different approaches. In the first
approach we have developed a new method for the synthesis
of benzophenone 3 starting with an alicyclic system
followed by aromatization to give sterically hindered
benzophenone 8 along with formation of xanthone 9. This
method would be useful for the synthesis of substituted
xanthones. In another approach, an improved synthesis of
dicarboxylic acid 3 has been achieved via a short and
efficient route starting from commercially available chemi-
cals in three steps using ortho-lithiation as the key step.
Hollinshead et al.2b reported two efficient syntheses of the
benzophenone portion of balanol. The key step of this
synthesis was the utilization of ortho-lithiation reactions to
generate a carbinol, which was then oxidized to the
benzophenone portion of balanol. We anticipated that
coupling of aldehyde 12, which can be easily prepared
from 2,3-dimethylanisole, and commercially available 3,5-
dimethoxytoluene under these conditions followed by
oxidation would afford the desired benzophenone 2 easily
and effectively. Accordingly, 3,5-dimethoxytoluene (11)
was ortho-lithiated10 using n-BuLi in the presence of
TMEDA at 0 8C and coupled with the aldehyde 12 to afford
alcohol 13 in 70% yield (Scheme 5). Further oxidation of
the benzylic alcohol 13 to the desired intermediate 3 was
carried out using MnO2 in dichloromethane at room
temperature in 90% yield. This compound showed spectro-
2. Experimental
2.1. General
1H NMR and 13C NMR spectra were recorded on Bruker
AC-200 spectrometer in CDCl3 containing TMS as an
internal standard. Infrared spectra (nmax in cm21) were
Scheme 5. Synthesis of benzophenone precursor 2.

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M. L. Patil et al. / Tetrahedron 60 (2004) 1869–1873
recorded as either nujol mull or in CHCl3 on Perkin–Elmer
Infrared 683 B or 160S FT-IR spectrometer with sodium
chloride optics. All solvents and reagents were purified and
dried by standard procedures. TLC was carried out on silica
gel plates prepared by spreading the slurry (in CCl4) and
drying at room temperature. The plates were analyzed by
keeping in iodine chamber. Column chromatography was
performed on silica gel (60–120 mesh). Petroleum ether
refers to the fraction boiling in the range of 60–80 8C.
at 120–125 8C so that the clear liquid turned to a
voluminous precipitate that was again redissolved to give
a brown coloured liquid with separation of mercury (2–3 h).
The mixture was cooled and then 1 N HCl (5 mL) was
added and allowed to boil for 30 min. Ethyl acetate was
added after cooling and the mixture was filtered through a
pad of Celite. The ethyl acetate layer was separated, washed
with brine (10 mL), dried over sodium sulphate and
concentrated in vacuo. TLC of the reaction mixture
indicated the formation of two compounds. Evaporation of
the solvent and purification by column chromatography
over silica gel (eluent 10–20% ethyl acetate in petroleum
ether) afforded the xanthone 9 (96 mg, 40%) and the
required compound 8 (55 mg, 20%).
2.1.1. 2-(20-Methoxy-60-methylbenzoyl)-5-methyl-1,3-
cyclohexanedione (4). To a stirred solution of 2-methoxy-
6-methylbenzoic acid (5) (498 mg, 3 mmol) in dry benzene
(10 mL), thionyl chloride (0.43 mL, 6 mmol) and a drop of
dimethylformamide were added. The resulting mixture was
then refluxed for 6 h. After completion of the reaction
(checked by paper chromatography), benzene was removed
by distillation to leave the acid chloride as a brownish
semisolid. IR (neat): 1800 cm21 (acid chloride). Acid
chloride obtained above was used for further reaction
without purification. To a stirred solution of the acid
chloride (554 mg, 3 mmol) and cyanotrimethylsilane
(0.48 mL, 3.6 mmol) in dry dichloromethane (5 mL) under
nitrogen atmosphere was added stannic chloride (0.2 mL) at
room temperature. The stirring was continued for a further
2 h. After the reaction, the mixture was poured into ice-cold
water (10 mL) and extracted with dichloromethane. The
combined dichloromethane layer was washed with water
(20 mL) followed by brine (20 mL) and dried over sodium
sulphate. Evaporation of the solvent under reduced pressure
afforded crude cyanide 6 as a brown oil (335 mg, 85%). IR
2.2.1. 2,6-Dihydroxy-4-methylphenyl 20-methoxy-60-
methylphenyl ketone (8). Pale yellow solid, mp 270–
1
273 8C. H NMR (CDCl3, 200 MHz): d 2.22 (s, 3H, Ar-
CH3), 2.25 (s, 3H, Ar-CH3), 3.76 (s, 3H, –OMe), 6.26 (s,
2H, aromatic), 6.86 (d, J¼8 Hz, 1H, aromatic), 6.92 (d,
J¼8 Hz, 1H, aromatic), 7.36 (t, J¼8 Hz, 1H, aromatic). 13C
NMR (CDCl3, 50 MHz): d 18.71, 22.24, 56.02, 108.49,
109.45 (2C), 123.52 (2C), 128.59, 131.57 (2C), 136.02,
149.88 (2C), 155.68, 198.18 (CvO). Mass (m/z): 272 (Mþ).
2.2.2. 1-Hydroxy-3,8-dimethyl-xanthen-9-one (9). Pale
1
yellow solid, mp 146–149 8C (lit.9 mp 149–151 8C). H
NMR (CDCl3, 200 MHz): d 2.41 (s, 3H, Ar-CH3), 2.89 (s,
3H, Ar-CH3), 6.55 (s, 1H, aromatic), 6.65 (s, 1H, aromatic),
7.07 (d, J¼8 Hz, 1H, aromatic), 7.25 (d, J¼8 Hz, 1H,
aromatic), 7.52 (t, J¼8 Hz, 1H, aromatic), 12.86 (s, 1H,
–OH). 13C NMR (CDCl3, 50 MHz): d 22.55, 23.30, 106.76,
107.50, 110.99, 115.79, 118.99, 126.67, 134.10, 141.66,
148.13, 155.36, 157.35, 161.61, 183.92 (CvO). Mass (m/z):
240 (Mþ, 100), 222 (12), 211 (30).
(CHCl3): 2150, 1680 cm21
.
Triethylamine (1 mL) was added to a stirred solution of the
above acyl cyanide 6 (472 mg, 2.7 mmol) and 5-methyl-
cyclohexane-1,3-dione (7) (378 mg, 3 mmol) in dry aceto-
nitrile (5 mL) at room temperature and stirring was
continued overnight. After completion of the reaction,
acetonitrile was evaporated and the mixture was poured into
ice-cold 1 N HCl (10 mL). The reaction mixture was then
extracted with dichloromethane and the combined organic
layer was washed with water (20 mL) followed by brine
(20 mL) and dried over sodium sulphate. Evaporation of the
solvent and purification by column chromatography over
silica gel (eluent 20% ethyl acetate in petroleum ether)
afforded the triketone 4 (574 mg, 70%) as a pale yellow
thick oil. 1H NMR (CDCl3, 200 MHz): d 1.12 (d, J¼5.8 Hz,
3H), 2.10–2.88 (m including singlet for benzylic methyl at
d 2.17, 9H), 3.72 (s, 3H, –OCH3), 6.65–6.90 (m, 2H,
aromatic), 7.15–7.35 (m, 1H, aromatic). 13C NMR (CDCl3,
50 MHz): d 19.25, 21.05, 26.86, 41.09, 46.53, 55.97, 108.50,
114.79, 122.91, 130.04, 135.08, 155.96, 193.56 (CvO),
197.16 (CvO), 199.04 (CvO). Mass (m/z): 274 (Mþ, 5), 259
(M215, 8), 243 (90), 166 (50), 148 (100). Anal. calcd for
C16H18O4: C 70.06, H 6.60; found C 70.00, H 6.61.
2.3. 1-Methoxy-3,8-dimethyl-xanthen-9-one (10)
To the stirred solution of compound 9 (90 mg, 0.37 mmol)
and potassium carbonate (76 mg, 0.55 mmol) in dry acetone
(2 mL) was added dimethyl sulphate (52 mg, 0.40 mmol).
The resulting reaction mixture was refluxed for 8 h. The
acetone was then removed under reduced pressure and the
residue was diluted with ice-cold water (5 mL) followed by
extraction with ethyl acetate. The combined ethyl acetate
layer was washed with water (5 mL), brine (5 mL) and dried
over sodium sulphate. Evaporation of the solvent afforded
the xanthone 10 as a white solid (81 mg, 85%), mp 169–
171 8C. 1H NMR (CDCl3, 200 MHz): d 2.45 (s, 3H, Ar-CH3),
2.90 (s, 3H, Ar-CH3), 4.01 (s, 3H, –OCH3), 6.56 (s, 1H,
aromatic), 6.81 (s, 1H, aromatic), 7.06 (d, J¼7.2 Hz, 1H,
aromatic), 7.24 (d, J¼7.2 Hz, 1H, aromatic), 7.46 (t, J¼
7.2 Hz, 1H, aromatic). Mass (m/z): 254 (Mþ, 100), 240 (70).
2.3.1. 2,6-Dimethoxy-4-methylphenyl 20-methoxy-60-
methylphenyl ketone (3). A mixture of the compound 8
(54 mg, 0.2 mmol) and dimethyl sulphate (0.05 mL,
0.55 mmol) and potassium carbonate (100 mg, 0.72 mmol)
in dry acetone (5 mL) was refluxed for 6 h. The acetone was
then evaporated under reduced pressure and the residue was
diluted with water (5 mL), extracted with ethyl acetate and
concentrated to give the benzophenone 3 as a white solid
(54 mg, 90%), mp 133–135 8C (lit.2c mp 132–133 8C). IR
2.2. Aromatization of triketone 4
The triketone 4 (274 mg, 1 mmol) was taken in glacial
acetic acid (3 mL) in a round bottom flask attached with a
short path condenser under argon atmosphere. To this were
added mercuric acetate (956 mg, 3 mmol) and sodium
acetate (246 mg, 3 mmol). The reaction mixture was heated

M. L. Patil et al. / Tetrahedron 60 (2004) 1869–1873
1873
1
(CHCl3): 1675 cm21. H NMR (CDCl3, 200 MHz): d 2.32
(5 mL) and the filtrate was acidified with conc. HCl (5 mL).
The aqueous solution was saturated with sodium chloride and
extractedwithethyl acetate. Thecombined extractsweredried
oversodiumsulphate and evaporated to givedicarboxylic acid
2 as colourless solid (97 mg, 45%); mp 253–255 8C (lit.2c
(s, 3H, Ar-CH3), 2.34 (s, 3H, Ar-CH3), 3.58 (s, 3H,
–OCH3), 3.66 (s, 6H, 2£ –OCH3), 6.35 (s, 2H, aromatic),
6.68 (d, J¼8 Hz, 1H, aromatic), 6.79 (d, J¼8 Hz, 1H,
aromatic), 7.17 (t, J¼8 Hz, 1H, aromatic). Mass (m/z): 300
(Mþ, 15), 269 (100). Anal. calcd for C18H20O4: C 71.99, H
6.70; found C 72.23, H 6.90.
253–256 8C). IR (CHCl3): 1690, 3400 cm21 1H NMR
.
(acetone d6, 200 MHz): d 3.76 (s, 3H, –OCH3), 3.82 (s, 6H,
2£ –OCH3), 7.33 (d, J¼8 Hz, 1H, aromatic), 7.38 (s, 2H,
aromatic), 7.50 (d, J¼8 Hz, 1H, aromatic), 7.57 (t, J¼8 Hz,
1H, aromatic). Mass (m/z): 360 (Mþ, 10), 342 (8), 315 (25),
299 (30), 285 (90), 209 (98), 195 (100). Anal. calcd for
C18H16O8: C 60.01, H 4.47; found C 60.07, H 4.60.
2.3.2. 1-{(2,6-Dimethoxy-4-methyl)phenyl}hydroxy-
methyl-20-methoxy-60-methylbenzene (13). n-Butyl-
lithium (1.5 mL of a 1.4 M solution in hexane, 2.1 mmol)
was added dropwise to a stirred solution of 3,5-dimethoxy-
toluene (11) (304 mg, 2 mmol) and TMEDA (0.31 mL,
2.1 mmol) in anhydrous THF (5 mL) at room temperature
under argon atmosphere. The mixture was stirred for 5 h
whereupon it was added to a solution of the aldehyde 12
(345 mg, 2.3 mmol) in anhydrous THF (5 mL) at 0 8C. The
resulting light yellow solution was stirred at 0 8C for 2 h and
at room temperature overnight (the colour changes from
light yellow to dark brown). The reaction mixture was then
quenched with a saturated solution of ammonium chloride,
ice-cold water (5 mL) was added and extracted with ethyl
acetate. The ethyl acetate layer was washed with water
(10 mL), brine (10 mL) and dried over sodium sulphate.
Evaporation of the solvent and purification by column
chromatography over silica gel (eluent 15% ethyl acetate in
petroleum ether) afforded alcohol 13 (423 mg, 70%) as
yellow solid; mp 125–128 8C. IR (CHCl3): 1580,
Acknowledgements
M.L.P. thanks CSIR, New Delhi for the award of senior
research fellowship.
References and notes
1. Kulanthaivel, P.; Hallock, Y. F.; Boros, C.; Hamilton, S. M.;
Janzen, W. P.; Ballas, L. M.; Loomis, C. R.; Jiang, J. B.; Katz, B.;
Steiner,J.R.;Clardy,J.J.Am.Chem.Soc.1993,115, 6452–6453.
2. (a) Nicolaou, K. C.; Koide, K.; Bunnage, M. E. Chem. Eur. J.
1995, 1, 454–456. (b) Hollinshead, S.; Nichols, J.; Wilson, J.
J. Org. Chem. 1994, 59, 6703–6709. (c) Adams, C.; Fairway,
S.; Hardy, C.; Hibbs, D.; Hursthouse, M.; Morley, A.; Sharp,
B.; Vicker, N.; Warner, I. J. Chem. Soc. Perkin Trans. 1 1995,
2355–2362. (d) Miyabe, H.; Torieda, M.; Kiguchi, T.; Naito,
T. Synlett 1997, 580–582. (e) Denieul, M.-P.; Skrydstrup, T.
Tetrahedron Lett. 1999, 40, 4901–4904. (f) Storm, J. P.;
Andersson, C.-M. Organic Lett. 1999, 1, 1451–1453.
3. Lai, Y.-S.; Mendoza, J.; Jagdmann, G. E., Jr.; Menaldino, D.;
Biggers, C.; Heerding, J.; Wilson, J.; Hall, S.; Jiang, J.; Janzen,
W.; Ballas, L. J. Med. Chem. 1997, 40, 226–235.
3450 cm21. H NMR (CDCl3, 200 MHz): d 2.30 (s, 3H,
1
Ar-CH3), 2.32 (s, 3H, Ar-CH3), 3.75 (s, 6H, 2£ –OCH3),
3.78 (s, 3H, –OCH3), 5.80 (d, J¼10 Hz, 1H, exchanges with
D2O, –OH), 6.37 (s, 2H, aromatic), 6.43 (d, J¼8.2 Hz, 1H,
aromatic), 6.75 (d, J¼8.2 Hz, 1H, aromatic), 7.07 (t,
J¼8.2 Hz, 1H, aromatic). 13C NMR (CDCl3, 50 MHz): d
19.96, 22.16, 56.16 (3C), 67.69, 105.99 (2C), 109.96,
117.13, 123.69, 127.36, 131.05, 137.61, 138.41, 158.40,
159.00 (2C). Mass (m/z): 302 (Mþ, 25), 284 (85), 269 (60),
253 (40), 179 (100). Anal. calcd for C18H22O4: C 71.50, H
7.32; found C 71.58, H 7.24.
4. Carter, S.; Wallace, T. Synthesis 1983, 1000–1002.
5. Burger, U.; Montes, I. F. Tetrahedron Lett. 1996, 37, 1007–1010.
6. The cyanide catalyzed rearrangement of enol ester 14 under
conditions reported by Burger and Montes5 failed to give
triketone 4. However, when unsubstituted benzoate ester 15
was subjected to cyanide catalyzed rearrangement, it afforded
triketone 16
2.3.3. 2,6-Dimethoxy-4-methylphenyl 20-methoxy-60-
methylphenyl ketone (3). Manganese dioxide (1.3 g,
15 mmol) was added in portions to a stirred solution of
the alcohol 13 (302 mg, 1 mmol) in anhydrous dichloro-
methane at room temperature and mixture was stirred
overnight. The catalyst was removed by filtration through
Celite and the residue was washed with dichloromethane.
The filtrate was evaporated to provide the benzophenone 3
(270 mg, 90%) as white crystals; mp 133–135 8C (lit.2c
132–133 8C). The spectral data were identical to the
product obtained by methylation of compound 8.
2.3.4. 4-Carboxy-2,6-dimethoxyphenyl 20-carboxy-60-
methoxyphenyl ketone (2). To a stirred solution of
potassium permanganate (284 mg, 1.8 mmol) in water
(3 mL) and pyridine (1 mL), a solution of compound 3
(180 mg, 0.6 mmol) in pyridine (2 mL) was added followed
by water (1 mL) and a pinch of tetrabutylammonium
bromide. The mixture was then heated at 100 8C. After 1 h
further quantities of potassium permanganate (in lots of
284 mg, 1.8 mmol) were added at intervals of 1 h until a total
of 23.4 equiv. had been added. The reaction mixture was then
filtered through Celite. The residue was washed with water
7. Mphahlele, M.; Modro, T. J. Chem. Soc. Perkin Trans. 1 1996,
2261–2264.
8. Oliver, J.; Wilzer, K.; Waters, R. Synthesis 1990, 1117–1119.
9. Aghoramurthy, K.; Sarma, K. G.; Seshadri, T. R. J. Sci. Ind.
Res. 1961, 20B, 437–440.
10. Snieckus, V. Chem. Rev. 1990, 90, 879–933.