Simple efficient synthesis of pyranoquinoline alkaloids: Flindersine, khaplofoline, haplamine and their analogues

Article abstract of DOI:10.3184/030823407X191859

An efficient two step synthesis of pyranoquinoline alkaloids is described. Direct treatment of isoprene with 4-hydroxyquinolin-2(1H)-one in the presence of polyphosphoric acid furnished dihydroflindersine in good yield, and which on dehydrogenation led to a new synthesis of flindersine. Khaplofoline, a linear pyranoquinoline alkaloid, was obtained as a minor product. The syntheses of derivatives are also documented.

Full text of DOI:10.3184/030823407X191859

124 RESEARCH PAPER
FEBRuARY, 124–126
JOuRNAL OF CHEMICAL RESEARCH 2007
Simple efficient synthesis of pyranoquinoline alkaloids: flindersine,
khaplofoline, haplamine and their analogues
Dhanabal Thangavela, Sangeetha Ravindrana, Gengan Robert Moonsamyb and
Mohan Subramaniam Palathuraia*
aDepartment of Chemistry, Bharathiar University, Coimbatore 641 046, India
bDepartment of Chemistry, Durban University of Technology, Durban, South Africa
An efficient two step synthesis of pyranoquinoline alkaloids is described. Direct treatment of isoprene with
4-hydroxyquinolin-2(1H)-one in the presence of polyphosphoric acid furnished dihydroflindersine in good yield,
and which on dehydrogenation led to a new synthesis of flindersine. Khaplofoline, a linear pyranoquinoline alkaloid,
was obtained as a minor product. The syntheses of derivatives are also documented.
Keywords: isoprene, 4-hydroxyquinolin-2(1H)-one, PPA, DDQ
The last few decades have seen a significant rise in the
phytochemical exploration of Rutaceous species, which produce
more than 130 quinoline alkaloids including furo-, pyrano- and
prenylquinolines. Flindersine1 and N-methylflindersine2 were
isolated from the same species. Yunusov and his collaborators
isolated the alkaloids khaplofoline3a and haplamine3b,c from
the genus Haplophyllum.
quinolin-2(1H)-one using a variety of α,β-unsaturated
aldehydes. Khaplofoline was synthesised, either on oxidative
cyclisation of 4-hydroxy-3-(3'-methylbut-1'-enyl)quinolin-
2(1H)-one with DDQ12 or the Prevost reaction of 3-prenyl-2-
quinolones.13 All these synthetic routes require several steps
and are costly.
Over the last two decades, our laboratory has been involved
in synthesising these alkaloids.14 Recently we have reported15
the synthesis of flindersine via (4 + 2) cycloaddition reaction
using quinone methides.
Significance of flindersine, N-methylflindersine and haplamine
Recently, it has been reported4 that haplamine and flindersine
showed excellent cytotoxic activities. Flindersine is
important due to its antifungal activity5,6 and photoactivity in
photochemotherapy,7 whilst N-methylflindersine is reported
to be an insect antifeedant principle of Fagara chalybaea,
F. holstiiandXylocarpusgranatum. Haplamineshowsselective
inhibition6 against the odor-producing cyanobacterium.
Based on the wide range of biological activities of these
pyranoquinoline alkaloids, we have also synthesised some
derivatives.
A number of methods for the synthesis of 2,2-dimethyl-
pyranoquinoline alkaloids have been reported8 since
their isolation. Flindersine and its N-methyl derivative
were synthesised by treatment of 4-hydroxy-N-methyl-3-
prenylquinolin-2(1H)-one with DDQ,9 or from the 3-isoprenyl-
2,4-dimethoxyquinoline epoxides by treatment with
potassium hydroxide in aqueous dimethylsulfoxide.10 Lee
et al.11 synthesised these alkaloids with moderate yields
by a ytterbium(III)triflate-catalysed reaction of 4-hydroxy-
Results and discussion
In this context, we have followed a simple and efficient
two-step procedure for deriving both linearly and angularly
fused pyranoquinoline alkaloid systems. To the best of our
knowledge, there has been no report on the direct reaction of
isoprene with quinolines. We used isoprene, which might act
as a diene, with 4-hydroxyquinolin-2-one in the presence of
PPA in xylene. Isoprene might have attached at C3-position
of the compound 1a which resulted in angularly cyclised
product 2a (dihydroflindersine) in good yield with the small
amount of linearly cyclised product 3a (khaplofoline).
The linear cyclisation is not favoured due to shortening
of the corresponding N–C=O bond length and it less
susceptible to reaction. The dihydroflindersine is subjected
for dehydrogenation using DDQ. Thus, we synthesised
the alkaloid flindersine(4a) with the overall yield of 57%
(Scheme 1). Further, we have extended it to synthesise its
OH
O
OH
R2
R2
R2
Isoprene, PPA
Xylene
+
N
O
N
R
2
O
N
R
O
R1
R1
R1
1
3
DDQ/Benzene
O
Where 1, 2, 3 & 4,
a) R = R1 = R2 = H;
R2
b) R =CH3, R1 = R2 = H;
c) R = H, R1 = CH3, R2 = H;
d) R = H, R1 =H , R2 = CH3;
e) R = H, R1 = OCH3, R2 = H;
N
R
O
R1
f) R = H, R1 = H, R2 = OCH3; (4f: HAPLAMINE)
4
Scheme 1
* Correspondent. E-mail: ps_mohan_in@yahoo.com
PAPER: 06/4440

JOuRNAL OF CHEMICAL RESEARCH 2007 125
NMR(CDCl3) δ/ppm:17.25(C2–(CH3)2),17.54(C3),27.03(C4),32.32(C7–
CH3), 105.33(C2–CH3)2), 116.20(C4a and C8), 120.90(C9), 121.81(C10),
128.72 (C7–CH3), 131.51(C10a), 135.67(C6a), 158.02(C10b), 164.31(C=O);
MS (m/z) 243; Analysis (%): Calcd. C 74.05, H 7.04, N 5.76; Found C
74.32, H 7.21, N 5.78 (C15H17NO2).
analogues for the simple reason that they might also attract
pharmacological value.
Conclusion
2,2,9-Trimethyl-3,4-dihydropyrano[3,2-c]quinolin-5(6H)-one (2d):
M.p. 228°C, yield 62%; IR (KBr, nmax) cm-11650 cm-1(C=O), 3247;
1H NMR (CDCl3) δ/ppm: 1.41(6H, s, 2 × CH3), 1.78(2H, t, J = 6.60 Hz,
C3–CH2,), 2.45(3H, s, C9–CH3), 2.62 (2H, t, J = 6.60 Hz, C4–CH2),
7.31(1H, d, J = 7.2 Hz, C8–H), 7.70(1H, s, C10–H), 7.92(1H, d,
J = 7.2 Hz, C7–H), 9.45(1H, bs, NH); 13C NMR(CDCl3) δ/ppm:
17.14(C2–(CH3)2), 17.60(C3), 26.78(C4), 32.56(C9–CH3), 107.00(C2–
CH3)2), 116.15(C4a), 117.11(C8), 128.08(C9), 121.93(C10), 122.68
(C7), 132.88(C10a), 134.98(C6a), 159.17(C10b), 165.70(C=O); MS (m/z)
243; Analysis (%): Calcd. C 74.05, H 7.04, N 5.76; Found C 74.43,
H 7.14, N 5.81 (C15H17NO2).
We have developed a facile route to pyranoquinolone alkaloids
using PPA catalysed reaction of 2,4-dihydroxyquinolines
with isoprene as the key step. Since relatives of these
two starting compounds are conveniently available, the
present protocol allowed us assembling a wide range
of pyranoquinoline alkaloids/derivatives for biological
evaluation.
Experimental
2,2-Dimethyl-7-methoxy-3,4-dihydropyrano[3,2-c]quinolin-5(6H)-one
(2e): M.p. 165°C; yield 61%; IR (KBr, nmax) cm-1 1652, 3230; 1H
NMR (CDCl3) δ/ppm 1.52(6H, s, 2 × CH3), 1.67(2H, t, J = 6.40 Hz,
C3–CH2), 2.54(2H, t, J = 6.40 Hz, C4–CH2), 3.85(3H, s, C7–OCH3),
7.28–7.86(3H, m,Ar–H), 10.17(1H, bs, NH); 13C NMR (CDCl3) δ/ppm:
17.25 (C2–(CH3)2), 17.59(C3), 27.55(C4), 56.56(C7–O–CH3), 97.81(C2–
CH3)2), 115.23(C4a), 119.87(C8), 121.31(C9), 123.58(C10), 131.56(C10a),
133.47(C7–O–CH3), 135.19(C6a), 158.66(C10b), 164.90(C=O); MS (m/z)
259; Analysis (%): Calcd. C 69.48, H 6.61, N 5.40; Found C 69.35,
H 6.68, N 5.47 (C15H17NO3).
2,2-Dimethyl-9-methoxy-3,4-dihydropyrano[3,2-c]quinolin-5(6H)-
one (2f): M.p. 171°C; yield 63%; IR (KBr, nmax) cm-1 1667, 3210;
1H NMR (CDCl3) δ/ppm 1.49(6H, s, 2 × CH3), 1.63(2H, t, J = 6.60 Hz,
C3–CH2), 2.47(2H, t, J = 6.60 Hz, C4–CH2), 3.87(3H, s, C9–OCH3),
7.24–7.89(3H, m, Ar–H), 9.84(1H, bs, NH); 13C NMR(CDCl3) δ/ppm:
16.98(C2–(CH3)2), 17.45(C3), 26.75(C4), 58.91(C9–O–CH3), 99.27(C2–
CH3)2), 117.36(C4a), 120.32(C8), 124.67(C10), 127.41(C7), 132.17(C10a),
133.44(C9–O–CH3), 136.82(C6a), 157.90(C10b), 167.81(C=O); MS (m/z)
259; Analysis (%): Calcd. C 69.48, H 6.61, N 5.40; Found C 69.73,
H 6.65, N 5.44 (C15H17NO3).
Thin layer chromatography was used to follow the progress of the
reaction and purity of products. Melting points were determined
on a Boetius Microheating Table (Japan) and Mettler-FP5 Melting
apparatus and are uncorrected. IR spectra were recorded in Shimadzu–
8201 FT instrument (Japan) in KBr disc and only noteworthy
1
absorption levels (reciprocal centimeter) are listed. H & 13C NMR
spectra were recorded in Bruker-300 and 75 MHz spectrometer in
CDCl3 solution; Chemical shifts are expressed in ppm (δ) relative
to TMS. Satisfactory microanalyses were obtained on Vario EL-III
CHN analyser(Germany). Mass spectra were recorded on a Jeol-300
mass spectrometer(70eV) (Japan). DDQ and isoprene were purchased
from Aldrich and Acros Organics respectively and used as received.
General procedure
Synthesis of dihydropyranoquinoline alkaloids (2 and 3): Equal
moles of 4-hydroxy-1-methylquinolin-2(1H)-one (3 mmol) (1a) and
isoprene (3 mmol) with polyphosphoric acid (approximately 2–3 g)
in xylene (40 ml) were mechanically stirred at room temperature
for 2 h and then heated at 145°C for 6 h with cold water circulation.
Further 2 mmol of isoprene was added and heated for about 2 h.
After completion of reaction, inferred through TLC, the xylene was
distilled off and the crude reaction mixture was poured into 300 g of
crushed ice and extracted with ethyl acetate. The organic layer was
washed successively with saturated aq. sodium carbonate solution
(2 × 20 ml) and then brine and dried over anhydrous sodium sulfate
and concentrated. Column chromatography (silica gel 60–120 mesh)
of the residue using a gradient mixture of petroleum ether and
ethylacetate afforded dihydroproducts (2 and 3). Same procedure was
applied for the derivatives 1b–f.
Khaplofoline (3a): M.p. 272°C; yield 10%; Spectral data were
identical with the earlier reported.10,14
2,2,10-Trimethyl-5-hydroxy-3,4-dihydropyrano[2,3-b]quinoline
(3b): yield – Nil.
2,2,9-Trimethyl-5-hydroxy-3,4-dihydropyrano[2,3-b]quinoline
(3c):
M.p. 186°C, yield 6%; IR (KBr, nmax) cm-1 1610 cm-1(C=N), 3425–
3550(O–H); 1H NMR(CDCl3) δ/ppm: 1.37(6H, s, 2 × CH3), 1.48(2H,
t, J = 6.40 Hz, C3–CH2), 2.26(3H, s, C9–CH3), 2.58(2H, t, J = 6.40 Hz,
C4–CH2), 7.41(1H, t, J = 7.8 Hz, C7–H), 7.62(1H, d, J = 7.76 Hz,
C8–H), 7.84(1H, d, J = 7.8 Hz, C6–H), 10.17(1H, bs, OH); 13C
NMR(CDCl3) δ/ppm:17.22(C2–(CH3)2),18.73(C3),29.55(C4),32.81(C9–
CH3), 110.64(C2–CH3)2), 120.32(C8), 121.06(C7), 125.58(C6), 128.86(C4a),
129.17(C9), 130.56(C5a), 136.47(C9a), 149.43(C5), 171.57(C10a); MS (m/z)
243; Analysis (%): Calcd. C 74.05, H 7.04, N 5.76; Found C 74.09,
H 6.98, N 5.79 (C15H17NO2).
Synthesis of flindersine and its analogues
A mixture of 2a (1 mmol), and DDQ (1 mmol) in benzene (50 ml)
was refluxed for 12 h. The solution was then evaporated to dryness.
The residue was taken in ethyl-acetate and washed successively
with aq. sodium carbonate (10%) and water. The combined organic
extracts when subjected to silica gel column chromatography
(PE: EtOAc = 80: 20) gave the desired product 4a.The same was
extended to derivatives 4b–f.
2,2,7-Trimethyl-5-hydroxy-3,4-dihydropyrano[2,3-b]quinoline
(3d):
M.p. 228°C, yield 6%; IR (KBr, nmax) cm-1 1605 cm-1(C=N), 3420–
3550(O–H); 1H NMR(CDCl3) δ/ppm: 1.38(6H, s, 2 × CH3), 1.48(2H,
t, J = 6.60 Hz, C3–CH2), 2.29(2H, t, J = 6.60 Hz, C4–CH2), 2.52(3H,
s, C7–CH3), 7.21(1H, d, J = 8.0 Hz, C8–H), 7.80(1H, s, C6–H),
7.95(1H, d, J = 8.0 Hz, C9–H), 10.0(1H, bs, OH); 13C NMR(CDCl3)
δ/ppm: 16.95(C2–(CH3)2), 17.67(C3), 27.06(C7–CH3), 30.75(C4),
103.81(C2–CH3)2), 120.35(C8), 125.48(C6), 126.77 (C9), 128.83(C7),
129.96(C4a), 130.56(C5a), 138.42(C9a), 149.21(C5), 169.33(C10a); MS (m/z)
243; Analysis (%): Calcd. C 74.05, H 7.04, N 5.76; Found C 74.29,
H 7.11, N 5.83 (C15H17NO2).
2,2-Dimethyl-3,4-dihydropyrano[3,2-c]quinolin-5(6H)-one (2a) (di-
hydroflindersine): M.p. 232°C; yield 67%; IR (KBr, nmax) cm-1 1640,
1
3248; H NMR (CDCl3) δ/ppm: 1.43(6H, s, 2 × CH3), 1.65(2H, t,
J = 6.40 Hz, C3–CH2), 2.34 (2H, t, J = 6.40 Hz, C4–CH2), 7.1–7.9(4H,
m, Ar–H), 10.87(1H, bs, NH); 13C NMR(CDCl3) δ/ppm 17.05(C2–
(CH3)2), 17.61(C3), 26.70(C4), 104.67(C2–CH3)2), 116.46(C4a), 122.93(C8),
120.83(C9), 121.59(C10), 123.85(C7), 130.42 (C10a), 134.69(C6a),
159.04(C10b), 166.66(C=O); MS (m/z) 229;Analysis (%): Calcd. C 73.34,
H 6.59, N 6.11; Found C 73.32, H 6.65, N 6.05 (C14H15NO2).
Flindersine (4a): M.p. 195°C; yield 85%; Spectral data were
2,2,6-Trimethyl-3,4-dihydropyrano[3,2-c]quinolin-5-one
M.p. 176°C; yield 72%; IR(KBr, nmax) cm-1, 1668 cm-1(C=O);
1H NMR(CDCl3) δ/ppm 1.42(6H, s,
CH3), 1.62(2H, t,
(2b):
identical with earlier reported1.
N-Methylflindersine (4b): M.p. 85°C; Yield 85%; IR (KBr, nmax
)
1
2
×
cm-1 1662, 3213; H NMR(CDCl3) δ 1.45(s, 6H, 2 × CH3), 3.66(s,
3H, N–CH3), 5.87(d, 1H, C3–H, J = 6.42 Hz), 6.8–7.8(m, 5H,
Ar–H & C4–H); MS (m/z) 241; Analysis (%): Calcd. C 74.67, H 6.27,
N 5.80; Found C 74.70, H 6.22, N 5.84 (C15H15NO2).
J = 6.60 Hz, C3–CH2), 2.35(2H, t, J = 6.60 Hz, C4–CH2), 3.83(3H, s,
N–CH3), 7.1–8.0(4H, m, Ar–H); 13C NMR(CDCl3) δ/ppm: 17.07(C2–
(CH3)2), 17.67(C3), 28.91(C4), 37.22(N–CH3), 102.84(C2–CH3)2),
117.36(C4a), 120.32(C8), 121.00(C9), 122.16(C10), 122.72 (C7), 130.48
(C10a), 137.88(C6a), 159.37(C10b), 167.94(C=O); MS (m/z) 243; Analysis
(%): Calcd. C 74.05, H 7.04, N 5.76; Found C 74.41, H 7.22, N 5.83
(C15H17NO2).
7-Methylflindersine (4c): M.p. 189°C, yield 85%, IR(KBr, nmax)cm-1
1651(C=O), 3220(NH); 1H NMR (CDCl3) δ/ppm: 1.41(s, 6H, 2 × CH3),
2.44(s, 3H, C7–CH3), 5.83(d, 1H, C3–H, J = 6.4 Hz), 6.80(d, 1H,
C4–H, J = 6.4 Hz), 7.11(t, 1H, C9–H, J = 7.96 Hz), 7.35(d, 1H, C8–H,
J = 8.0 Hz), 7.80(d, 1H, C10–H, J = 8.0 Hz), 9.10(bs, 1H, NH); 13C
NMR(CDCl3) δ/ppm: 19.31(C2–(CH3)2), 30.09(C7–CH3), 115.90(C2–
(CH3)2), 132.78(C3), 118.35(C8), 120.33(C4), 121.23(C9), 124.75(C10),
126.35(C4a), 128.84(C7–CH3), 130.78(C10a), 141.32(C6a), 155.68(C10b),
164.73(C=O); MS (m/z) 241; Analysis (%): Calcd. C 74.67, H 6.27,
N 5.81; Found C 74.72, H 6.30, N 5.79 (C15H15NO2).
2,2,7-Trimethyl-3,4-dihydropyrano[3,2-c]quinolin-5(6H)-one
(2c): M.p. 221°C, yield 64%; IR (KBr, nmax) cm-11648 cm-1(C=O),
1
3250; H NMR (CDCl3) δ/ppm: 1.42(6H, s, 2 × CH3), 1.86(2H, t,
J = 6.60 Hz, C3–CH2), 2.46(3H, s, C7–CH3), 2.65(2H, t, J = 6.60 Hz,
C4–CH2), 7.09 (1H, t, J = 7.6 Hz, C9–H), 7.30(1H, d, J = 7.6 Hz,
C8–H), 7.78(1H, d, J = 7.72 Hz C10–H), 9.13(1H, bs, NH); 13C
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126 JOuRNAL OF CHEMICAL RESEARCH 2007
9-Methylflindersine (4d): M.p. 176°C, yield 90%, IR (KBr, nmax
D. Lavie, N. Danieli, R. Weitman and E. Glotter, Tetrahedron, 1968, 24,
3011; (e) M.A. Munoz, M.R. Torres, B.K. Cassels, J. Nat. Prod., 1982,
45, 367; (f) A. ulubelen, Phytochemistry, 1984, 23, 2123; (g) S. Ahmad,
J. Nat. Prod., 1984, 47, 391, (h) S. Mitaku, A.L. Skaltsounis, F. Tillequin
and M. Koch, J. Nat. Prod., 1985, 48, 772.
)
cm-11660(C=O), 3224(NH); 1H NMR (CDCl3) δ/ppm: 1.40(s, 6H,
2 × CH3), 2.35(s, 3H, C9–CH3), 5.76(d, 1H, C3–H, J = 6.80 Hz),
6.53(d, 1H, C4–H, J = 6.80 Hz), 7.44(d, 1H, C8–H, J = 7.8 Hz),
7.90(d, 1H, C7–H, J = 7.8 Hz), 7.80(s, 1H, C10–H), 9.18(bs, 1H,
NH); 13C NMR(CDCl3) δ/ppm: 20.07(C2–(CH3)2), 29.26(C9–CH3),
116.77(C2–CH3)2), 117.10(C8), 131.09(C3), 126.14(C10), 126.34(C7),
126.53(C4), 128.50(C9–CH3), 128.61(C4a), 133.06(C10a), 136.00(C6a),
150.41(C10b) 164.98(C=O); MS (m/z) 241; Analysis (%): Calcd. C 74.67,
H 6.27, N 5.81; Found C 74.67, H 6.29, N 5.81 (C15H15NO2).
2
3
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7-Methoxyflindersine (4e): M.p. 180°C; Yield 82%; IR (KBr, nmax
)
cm-1 1660(C=O), 3265(NH); H NMR (CDCl3) δ/ppm: 1.47(s, 6H,
2 × CH3), 3.91(s, 3H, –OCH3), 5.82(d, 1H, C3–H, J = 6.2 Hz),
6.76(d, 1H, C4–H, J = 6.2 Hz), 11.03 (s, 1H, NH), 7.2–7.8(m, 3H,
Ar–H); 13C NMR (CDCl3) δ/ppm: 20.78(C2–(CH3)2), 56.87(C7–O–
CH3), 117.20(C2–CH3)2), 118.45(C8), 120.25(C4), 121.73(C9), 125.49(C10),
128.31(C4a), 130.80(C10a), 134.45(C3), 136.83(C7), 138.64(C6a),
160.01(C10b), 168.52(C=O); MS (m/z) 257;Analysis (%): Calcd. C 70.02,
H5.88, N 5.44; Found C 70.14, H 5.86, N 5.47 (C15H15NO3).
1
4
9-Methoxyflindersine (4f) (Haplamine): M.p. 210°C; Yield 80%;
5
6
IR (KBr,
n
max)cm-1 1656(C=O), 3215(NH); 1H NMR (CDCl3)
δ/ppm 1.47(s, 6H, 2 × CH3), 3.90(s, 3H, C9–OCH3), 5.34(d, 1H,
C3–H, J = 6.20 Hz), 6.71(d, 1H, C4–H, J = 6.20 Hz), 7.30–7.95(m,
3H, Ar–H), 9.58(bs, 1H, NH); 13C NMR(CDCl3) δ/ppm: 19.42(C2–
(CH3)2), 57.71(C9–O–CH3), 118.09(C2–CH3)2), 120.32(C8), 124.57(C10),
125.91(C4), 127.40(C7), 129.34(C4a), 131.04(C10a), 131.67(C3), 137.82(C9–
O–CH3), 141.22(C6a), 152.43(C10b), 166.71(C=O); MS (m/z) 257;Analysis
(%): Calcd. C 70.02, H 5.88, N 5.44; Found C 70.23, H 5.93, N 5.51
(C15H15NO3).
7
8
TD thanks CSIR, New Delhi for the award of Senior Research
Fellowship. Authors thank SIF, IISc, Bangalore and IICT,
Hyderabad for providing the spectral and analytical data.
PSM thanks CSIR, New Delhi for providing financial
assistance.
9
10 M.R. Bowman, M.F. Grundon and K.J. James, J. Chem. Soc., Perkin
Trans 1, 1973, 1055.
11 Y.R. Lee, H.I. Kweon, W.S. Koh, K.R. Min, Y. Kim and S.H. Lee,
Synthesis, 2001, 1851.
12 F. Piozzi, P. Venturella and A. Bellino, Gazz. Chim. Ital., 1969, 99, 711.
13 M. Subramaniam, P.S. Mohan, P. Shanmugam and K.J. Rajendra Prasad,
Z.Naturforsch, 1992, 47b, 1016.
14 (a) M. Ramesh, V. Arisvaran, S.P. Rajendran and P. Shanmugam,
Tetrahedron Lett 1982, 23, 967; (b) M. Ramesh, P.S. Mohan and
P. Shanmugam, Tetrahedron, 1984, 40, 4041; (c) M. Sekar and
K.J. Rajendra Prasad, J. Nat. Prod., 1998, 61, 294; (d) R.N. Kumar,
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15 R.N. Kumar, S.T. Selvi, T. Suresh and P.S. Mohan, Ind. J. Chem. (Sect-B),
2003, 42B, 187.
Received 31 January 2007; accepted 19 February 2007
Paper 07/4440
doi:10.3184/030823407X191859
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PAPER: 06/4440