Synthetic studies towards complex diterpenoids. Part 20.1 Total synthesis of the linear abietane o-quinone umbrosone

Article abstract of DOI:10.1039/a809579e

A simple total synthesis of the rearranged linear abietane diterpenoid o-quinone, umbrosone 1, has been accomplished through fraH5-l,2, 3, 4, 9, 9a-hexahydro-l,l-dimethyl-6-methoxyanthracen-10(4a//)-one 12t.

Full text of DOI:10.1039/a809579e

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Synthetic studies towards complex diterpenoids. Part 20.1
Total synthesis of the linear abietane o-quinone umbrosone
Keya Ghosha and Usha Ranjan Ghatak*b
a Department of Organic Chemistry, Indian Association for the Cultivation of Science,
Jadavpur, Calcutta - 700 032, India
b Indian Institute of Chemical Biology, Jadavpur, Calcutta - 700 032, India
Received (in Cambridge) 8th December 1998, Accepted 12th March 1999
A simple total synthesis of the rearranged linear abietane diterpenoid o-quinone, umbrosone 1, has been
accomplished through trans-1,2,3,4,9,9a-hexahydro-1,1-dimethyl-6-methoxyanthracen-10(4aH)-one 12t.
The unusual rearranged linear abietane diterpenoid o-quinone,
umbrosone 1,†2 isolated from Hyptis umbrosa Slazm
failed to give the acids 11. Cyclization of 11 with polyphos-
phoric acid gave a mixture of the epimeric ketones 12t and 12c
in a ratio of ca. 80:20. As expected from earlier results12 on
related systems, the epimerisation of this mixture with sodium
methoxide in methanol gave the stable trans-ketone 12t, mp
118 ЊC as the only isolable product in a good yield. The
assigned stereochemistry of 12t has been recently confirmed13
through its transformation to the known trans-1,2,3,4,4a,9,
9a,10-octahydro-1,1-dimethyl-6-methoxyanthracene. The con-
densation of the ketone 12t with methylmagnesium iodide
followed by dehydration of the resulting alcohol gave the
olefin 13. This underwent smooth dehydrogenation leading to
the tetrahydroanthracene ether 14. Friedel–Crafts acylation of
14 gave the oxoether 15 in excellent yield, which on condens-
ation with MeMgI afforded the alcohol 16. Attempted depro-
tection of the O-methyl ether 16 with NaH–EtSH in boiling
DMF1,14 failed to give the desired phenolic alcohols. Demeth-
ylation of 15 with AlCl3–EtSH15 proceeded smoothly to afford
the oxophenol 17. This on condensation with an excess of
MeMgI gave the relatively unstable phenolic alcohol 18, which
on oxidation with freshly prepared Fremy’s salt16 afforded
umbrosone 1 in good yield.
Me
O
Me
O
O
O
HO
O
Me
OH
Me
OH
Me
Me
Me
Me
Me
Me
1
2
Me
O
Me
O
O
O
OH
Me
Me
Me
Me
Me
Me
O
3
4
OH
(Lamiaceae), exhibited2 significant activity against Gram-
positive bacteria. A few other structurally related quinones,
pygmaeocine E 2,3 aegyptinone A 34 and aegyptinone B 44 have
been isolated from a number of traditional medicinal plants,
some of which are linear homologous structures4 related to the
cytotoxic tanshinones.5 The potential bioactivity of these new
diterpenoid anthraquinones makes them attractive synthetic
targets. In parallel to our communication6 on the first total
synthesis of umbrosone, the synthesis of aegyptinones A and B
were reported.7 We present in this paper a detailed account of
the synthesis of 1, employing a simple, flexible and convergent
route,1 suitable for the preparation of the other members of this
group and their analogues.
In conclusion, the first synthesis of the rearranged diter-
penoid umbrosone, has been realised through a simple con-
vergent route.
Experimental
Compounds described are all racemates. Unless otherwise
stated, IR spectra of solids (KBr), and liquid (film) were
recorded on a Perkin-Elmer model PE 298. UV spectra were
recorded on a Beckman DU instrument; absorption coef-
1
ficients, ε, were measured in dm3 molϪ1 cmϪ1. The H NMR
Results and discussion
spectra were taken at 60 MHz on a Varian EM-360L and at 200
MHz on a Varian Gemini-200 with SiMe4 as internal standard
and J values given in Hz. Column chromatography was
performed on neutral alumina [Brockmann Grade 1, of BDH
(India)] or silica gel [Glaxo Laboratories (India) Ltd.]. Light
petroleum refers to the fraction of bp 60–80 ЊC. Ether refers to
diethyl ether.
The gem-dimethylcyclohexane 7 (Scheme 1), obtained in excel-
lent yield from Hagemann’s ester 5, via the known8 cyclo-
hexenone 6, was transformed to the alkene 8 by a Wittig
reaction. Hydroboration of the alkene 8, followed by oxid-
ation1,9 with alkaline hydrogen peroxide gave an epimeric
mixture of the alcohol 9. Further oxidation of 9 with Jones’
reagent1,10 afforded an epimeric mixture of acids 11 as the
major product. From the neutral fraction a crystalline γ-lactone
10 was isolated in a moderate yield, the structure of which is
based upon spectral data and elemental analyses. The
attempted hydrogenolysis of 10 under various conditions11
2-(p-Methoxybenzyl)-3,3-dimethylcyclohexanone 7
This compound was prepared using a procedure described
earlier.1 To a stirred suspension of CuI (8.2 g, 43.15 mmol) in
dry ether (30 cm3) under N2 at Ϫ25 ЊC, was added MeLi in dry
ether (1.5 mol dmϪ3, 58 cm3, 78 mmol). The resulting yellow
suspension was cooled to Ϫ50 ЊC and BF3ؒEt2O (6.1 g, 43.26
mmol) was added to it. After 5 min, the enone 6 (3.2 g, 14.33
† IUPAC name: 1,2,3,4,5,6-hexahydro-7-(2-hydroxy-2-propyl)-1,1,10-
trimethylanthracene-5,6-dione.
J. Chem. Soc., Perkin Trans. 1, 1999, 1359–1362
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chromatographed on neutral alumina (25 g). Elution with light
petroleum afforded the ketone 7 as a light yellow liquid (2.88 g,
82%) (Found: C, 77.9; H, 8.9. C16H22O2 requires C, 78.0; H,
9.0%), νmax/cmϪ1 1710 (CO); δH (60 MHz, CCl4) 0.84 (3H, s,
Me), 1.20 (3H, s, Me), 1.46–2.54 (7H, m, methylenes and
methine), 2.75–3.25 (2H, m, ArCH2), 3.81 (3H, s, ArOMe) and
6.83 (4H, ABq, J 9, ArH).
OMe
O
O
OMe
OMe
ref.8
+
EtO2C
5
6
CH2Cl
OMe
Me
Me
Me
i
O
CH2
ii
2-(p-Methoxybenzyl)-3,3-dimethyl-1-methylenecyclohexane 8
A procedure described1 earlier was adopted. A suspension of
methyl(triphenyl)phosphonium iodide (4.5 g, 11.6 mmol) in
toluene (3 cm3) and a toluene solution of freshly prepared
sodium 2,2-dimethylpropoxide (1.5 mol dmϪ3 solution, 7 cm3)
was stirred at room temperature (ca. 25 ЊC) for 20 min. The
ketone 7 (1.2 g, 4.8 mmol) in toluene (4 cm3) was added drop-
wise to the mixture after which it was refluxed for 3 h and then
quenched with saturated aqueous NH4Cl and extracted with
ether. The extract was washed with aqueous NH4Cl and water,
dried (Na2SO4) and evaporated to yield an oil which was dis-
solved in light petroleum (100 cm3) and immediately filtered
through a short column of silica gel (10 g). MeI (3 cm3) was
added to the filtrate and left for 1 h at room temperature. The
precipitated salt was filtered off and the filtrate was evaporated
in vacuo to give the pure alkene 8 (950 mg, 80%). The analytical
sample was prepared by evaporative distillation (bp 160–165 ЊC
at 0.2 mm Hg) (Found: C, 83.3; H, 9.8. C17H24O requires C,
Me
Me
7
8
Me
iii,iv
CH2OH
O
OMe
v
O
H
Me
H
Me
10
Me
Me
9
OMe
+
O
CO2H
OMe
OMe
4a
vi
H
H
Me
Me Me
Me
83.6; H, 9.9%); νmax/cmϪ1 1635 (C᎐C); δ (60 MHz, CCl ), 0.98
᎐
H
4
11
12c 4aα
12t 4aβ
(6H, s, CMe2), 0.98–2.75 (9H, m), 3.69 (3H, s, ArOMe), 4.22
(1H, br d, J 4, C᎐CH ), 4.54 (1H, br d, J 4, C᎐CH ) and 6.88
(4H, ABq, J 9, ArH).
᎐
᎐
2
2
Me
Me
vii
OMe
OMe
viii,ix
x
12t
2-(p-Methoxybenzyl)-3,3-dimethyl-1-hydroxymethylcyclo-
hexane 9
H
14
Me
Me
Me
13
Me
The procedure described earlier1,9 was adopted. Diborane gas
[prepared from NaBH4 (3 g, 78.9 mmol), BF3ؒEt2O (9.9 cm3,
80.7 mmol) in diglyme (15 cm3)] was passed through a cold
(0 ЊC) solution of the alkene 8 (3.25 g, 13.33 mmol) in dry THF
(15 cm3) for 22 h under a continuous slow stream of N2. The
mixture was kept in refrigeration overnight and then carefully
decomposed with ice and transferred into aqueous NaOH
(3 mol dmϪ3; 60 cm3). To a well stirred cooled mixture (ca.
0–10 ЊC) was added H2O2 (30% v/v; 50 cm3) dropwise. Stirring
was continued for an additional 30 min after which further
H2O2 (25 cm3) was added to the mixture and then set aside
overnight. It was extracted with ether and the extract washed
with water and evaporated to afford the epimeric mixture of the
alcohol 9 (3.11 g, 89%) which was directly used for Jones’ oxid-
ation. The analytical sample was prepared by evaporative
distillation, bp 180–185 ЊC (0.2 mmHg) (Found: C, 77.7; H,
9.8. C17H26O2 requires C, 77.8; H, 10.0%); νmax/cmϪ1 3440 (br,
OH); δH (60 MHz, CCl4), 0.84 (3H, br s, Me), 1.01 (3H, br s,
Me), 1.15–2.75 (11H, m), 3.28–3.65 (2H, m, –CH2OH), 3.68
(3H, s, OMe) and 6.60–7.22 (m, 4H, ArH).
xi
Me
Me
OMe
OH
xiii
COMe
COMe
15
Me
Me
17
Me
Me
viii
viii
Me
Me
OH
Me
OMe
Me
xii
X
OH
OH
18
xiv
Me
Me
Me
Me
Me
Me
16
1
Scheme 1 Reagents and conditions: i, LiMe2Cu–BF3, Et2O (Ϫ50 ЊC, 1
h); ii, Ph3PϩMeϪ ϩ tC5H11ONa–toluene (60–65 ЊC, 3 h); iii, B2H6–THF;
iv, NaOH–H2O2; v, Jones’ reagent–CH3COCH3 (0 ЊC, 1 h); vi, PPA
(85 ЊC, 2 h); vii, NaOMe–MeOH (rt, 3.5 h); viii, MeMgI–Et2O; ix,
KHSO4 (140 ЊC, 30 min); x, Pd/C (10%)–xylene (reflux, 7 h); xi,
CH3COCl (1.5 eq.)–AlCl3 (1.6 eq.)–CH2Cl2 (0 ЊC, 1 h, rt, overnight);
xii, NaH–EtSH–DMF (heat 4 h); xiii, AlCl3 (1.2 eq.)–EtSH (2.5 eq.)–
CH2Cl2 (0 ЊC, 1 h, rt, overnight); xiv, Fremy’s salt (2 eq.)–KH2PO4
(2 eq.)–MeOH (0 ЊC, 1 h, rt, overnight).
trans-1,2,3,4,9,9a-Hexahydro-1,1-dimethyl-6-methoxy-
anthracen-10(4aH)-one 12t
The procedure described1 earlier was adopted. The cooled
(5–10 ЊC) alcohol 9 (3.56 g, 13.7 mmol) in acetone (30 cm3)
was stirred with an excess of Jones’ reagent10 (5.5 cm3, 15
mmol) for 1 h. After dilution with water, the mixture was
extracted with ether. The ether extract was washed with
aqueous KOH (0.36 mol dmϪ3, 60 cm3), water, dried (Na2SO4)
and evaporated to give a thick yellow gum, which on
chromatography over neutral alumina (25 g) and elution with
ether–light petroleum (1:19) afforded 2-(1-hydroxy-p-meth-
oxybenzyl-3,3-dimethylcyclohexane-1-carboxylic acid lactone
10 (780 mg, 21%), mp 84 ЊC (ether–light petroleum) (Found:
C, 74.3; H, 7.9. C17H22O3 requires C, 74.4; H, 8.1%); νmax/cmϪ1
mmol) in dry ether (5 cm3) was added dropwise during 15 min
and the resulting suspension was stirred at Ϫ30 ЊC for 15 min.
Additional BF3ؒEt2O (6.1 g, 43.26 mmol) was added and the
mixture was stirred at Ϫ30 ЊC for 1 h. The reaction mixture was
allowed to warm slowly to 0 ЊC and then quenched with satur-
ated aqueous NH4Cl. It was extracted with ether, washed with
aqueous Na2S2O3, water and dried (Na2SO4). Evaporation of
solvent in vacuo afforded a thick yellow liquid which was
1360
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1780 (γ-lactone); δH (60 MHz, CCl4) 0.74 (3H, s, Me), 1.01 (3H,
s, Me), 1.14–2.64 (8H, m, methylenes and methine), 3.74 (3H, s,
ArOMe), 5.21 (1H, d, J 6, CHAr) and 6.92 (4H, ABq, J 9,
ArH). The solution was acidified with HCl (6 mol dmϪ3) and
work-up afforded the acid 11 (2.82 g) as a thick light yellow
glass [νmax/cmϪ1 1710 (CO2H)], which was directly subjected to
cyclization. To a well stirred homogeneous solution of PPA,
prepared from P2O5 (40 g) and H3PO4 (20 cm3), was added the
acid 11 (2.82 g) and the mixture was heated at 80–85 ЊC for 2 h.
The dark red mixture was cooled, decomposed with ice and
extracted with ether. The ether extract was washed with aque-
ous NaOH (0.5 mol dmϪ3, 20 cm3), water, dried (Na2SO4) and
then evaporated to dryness to afford a stereoisomeric mixture
of ketones 12t and 12c in a ratio of ca. 8:2 (1H NMR) δH (60
MHz, CCl4) 0.99 and 1.08 (each CMe2 for major isomer), 0.99
and 1.05 (each s, CMe2 for the minor isomer). The stereoiso-
meric mixture of the ketones was treated with methanolic
MeONa (0.1 mol dmϪ3, 15 cm3) at room temperature under N2
for 3.5 h, diluted with water acidified with HCl (6 mol dmϪ3)
and worked up to give a solid which on chromatography over
silica gel (25 g) using ether–light petroleum (1:19) afforded the
trans ketone 12t (1.8 g, 50%), mp 118 ЊC (Found: C, 78.9; H,
8.5. C17H22O2 requires C, 79.0; H, 8.6%); νmax/cmϪ1 1685 (CO);
δH (60 MHz, CCl4) 0.99 (3H, s, Me), 1.08 (3H, s, Me), 1.2–2.8
(10H, m), 3.82 (3H, s, OMe) and 6.74–7.4 (3H, m, ArH).
and extracted with ether. Work-up of the extracts afforded
the crude product which was purified by chromatography over
alumina (20 g) and eluted with ether–light petroleum (1:10) to
give the oxoether 15 as light yellow needles (390 mg, 83%) mp
91 ЊC (ether–light petroleum) (Found: C, 80.9; H, 8.0. C20H24O2
requires C, 81.0; H, 8.6%); νmax/cmϪ1 1680 (CO); δH (200 MHz,
CDCl3) 1.34 (6H, s, CMe2), 1.62–2.34 (4H, m, CH2), 2.48 (3H,
s, ArMe), 2.70 (3H, s, COMe), 2.84–3.24 (3H, m, ArCH2), 3.99
(2H, s, ArOMe), 7.20 (1H, s, ArH), 7.72 (1H, s, ArH) and 8.14
(1H, s, ArH).
1,2,3,4-Tetrahydro-7-(1-hydroxy-1-methylethyl)-6-methoxy-
1,1,10-trimethylanthracene 16
To an ice-cold stirred solution of MeMgI, prepared from Mg
(36 mg, 1.5 g atom) in ether (15 cm3), a solution of the oxoether
15 (207 mg, 0.7 mmol) in ether (10 cm3) was added dropwise.
The stirring was continued for an additional 1 h and refluxed
for 30 min. The chilled reaction mixture was decomposed with
aqueous NH4Cl and extracted with ether. Work-up followed by
filtration through a short column of silica gel using ether–light
petroleum (1:10) as the eluent afforded the alcohol 16 as a
thick colourless liquid (180 mg, 82%) (Found: C, 80.6; H, 9.0.
C21H28O2 requires C, 80.7; H, 9.0%); νmax/cmϪ1 3300 (br OH); δH
(60 MHz, CCl4) 1.26 (6H, s, CMe2), 1.31 (6H, s, C(Me2)OH),
1.0–3.1 (6H, m), 2.48 (3H, s, ArMe), 3.99 (3H, s, ArOMe), 7.14
(1H, br s, ArH) and 7.52–7.64 (2H, m, ArH).
6-Methoxy-1,1,10-trimethyl-1,2,3,4,9,9a-hexahydroanthracene
13
7-Acetyl-6-hydroxy-1,1,10-trimethyl-1,2,3,4-tetrahydro-
anthracene 17
To an ice-cooled stirred solution of MeMgI, prepared from
Mg (240 mg, 10 mg atom) in ether (30 cm3), a solution of ketone
12t (1.82 g, 7.05 mmol) in ether (20 cm3) was added dropwise.
After the addition was completed stirring was continued for 1 h
at room temperature and finally refluxed for 1 h. The chilled
reaction mixture was decomposed with aqueous NH4Cl and
extracted with ether. Work-up of the extract afforded the
respective crude alcohol which was directly subjected to
dehydration by heating with fused KHSO4 (2.1 g, 15.27 mmol)
at 140–160 ЊC for 45 min in a short-path distillation flask. The
resulting mixture on evaporative distillation gave the olefin 13
as a colourless oil (1.12 g, 62%) bp 135–138 ЊC (0.12 mmHg)
(Found: C, 84.2; H, 9.4. C18H24O requires C, 84.3; H, 9.4%);
The procedure1 described earlier was adopted. Anhydrous
AlCl3 (160 mg, 1.2 mmol) was added to a stirred solution of the
oxoether 15 (300 mg, 1 mmol) and EtSH (1.4 cm3) in CH2Cl2
(12 cm3) with cooling in an ice-bath. The mixture was stirred at
0 ЊC for an additional 1 h and then left overnight. It was then
poured into aqueous HCl (6 mol dmϪ3) and extracted with
ether. Work-up of the extract afforded the crude oxophenol 17.
The residue was chromatographed on silica gel (15 g) and eluted
with ether–light petroleum (1:5 to 1:3) to afford the pure
oxophenol 17 (205 mg, 68%) as a light yellow crystalline solid,
mp 210 ЊC (ether–light petroleum) (Found: C, 80.6; H, 7.7.
C19H22O requires C, 80.8; H, 7.9%); νmax/cmϪ1 3150 (phenolic
OH), 1660 (CO) and 1600; δH (200 MHz, CDCl3) 1.35 (6H, s,
CMe2), 1.45–1.82 (4H, m, CH2), 2.45 (3H, s, ArMe), 2.77 (3H,
s, COMe), 2.81–2.89 (2H, m, ArCH2), 7.39 (1H, s, ArH), 7.70
(1H, s, ArH), 8.28 (1H, s, ArH) and 11.46 (1H, s, OH).
νmax/cmϪ1 1635 (C᎐C); δ (60 MHz, CCl ) 0.62 (3H, s, Me), 1.01
᎐
H
4
(3H, s, Me), 1.41–1.92 (7H, m), 2.01 (3H, s, C᎐C–Me),
᎐
2.62–3.12 (2H, m, ArCH2), 3.72 (3H, s, ArOMe) and 6.4–7.01
(3H, m, ArH).
6-Methoxy-1,1,10-trimethyl-1,2,3,4-tetrahydroanthracene 14
Umbrosone 1
A solution of the olefin 13 (820 mg, 3.2 mmol) in dry xylene
(20 cm3) was refluxed for 7 h with Pd/C (110 mg, 10%). The
catalyst was filtered off from the cooled reaction mixture and
washed with ether. The combined filtrate and washings were
evaporated and the residual light yellow solid on chrom-
atography over alumina (30 g) using ether–light petroleum as
eluent afforded the tetrahydroanthracene ether 14 as a colour-
less solid (1.08 g, 60%) mp 110 ЊC (ether–light petroleum)
To an ice-cold stirred solution of MeMgI, prepared from Mg
(100 mg, 4.16 mg atom) in ether (15 cm3), a solution of the
oxophenol 17 (105 mg, 0.35 mmol) in ether (10 cm3) was added
dropwise. The resulting white suspension was refluxed for 2 h.
The chilled reaction mixture was decomposed with aqueous
NH4Cl and extracted with ether. Work-up of the extract gave
the crude phenolic alcohol (110 mg), [νmax/cmϪ1 3300 (phenolic
OH), 3150 (OH) and 1600] which was dissolved in anhydrous
MeOH (16 cm3) and cooled in an ice-bath. To the cooled stirred
solution, a freshly prepared solution of Fremy’s salt (240 mg,
0.85 mmol) and KHPO4 (60 mg, 0.44 mmol) in H2O (16 cm3)
was added dropwise. The resulting violet solution was stirred at
room temperature for 14 h. The reaction mixture was diluted
with water and extracted with ether. Work-up of the extract
afforded a brown solid which was carefully chromatographed
on silica gel (10 g) and eluted as follows: (i) ether–light petrol-
eum (1–1.5 : 8.5–9, 30 cm3) gave the unchanged 18 (15 mg); (ii)
ether–light petroleum 2:3, 60 cm3) afforded umbrosone 1 as a
dark red solid (75 mg, 67%) mp 160–162 ЊC. Recrystallization
from ether–light petroleum gave 1 as a shining dark red needles,
mp 163 ЊC (lit.2 mp 163–165 ЊC) (Found: C, 76.8; H, 7.7.
(Found: 84.9; H, 8.6. C18H22O requires C, 85.0; H, 8.7%); νmax
/
cmϪ1 1645, 1610; δH (200 MHz, CDCl3) 1.38 (6H, s, CMe2),
1.5–1.92 (4H, m, CH2), 2.52 (3H, s, ArMe), 2.58–2.92 (2H, m,
ArCH2), 3.91 (3H, s, ArOMe), 7.04 (1H, dd, J 8, 2-ArH), 7.31
(1H, d, J 2, ArH) and 7.62–7.68 (2H, m, ArH).
7-Acetyl-6-methoxy-1,1,10-trimethyl-1,2,3,4-tetrahydro-
anthracene 15
Anhydrous AlCl3 (360 mg, 2.69 mmol) was added to a stirred
solution of the ether 14 (400 mg, 1.57 mmol) in dry CH2Cl2 (5
cm3) and CH3COCl (0.4 cm3), with cooling in an ice-bath. The
mixture was stirred for an additional 1 h and left overnight at
room temperature. It was then poured into HCl (6 mol dmϪ3)
J. Chem. Soc., Perkin Trans. 1, 1999, 1359–1362
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C20H24O3 requires C, 76.9; H, 7.7%); νmax (CHCl3)/cmϪ1 3510,
1680, 1650 and 1580 [lit.2 νmax (CHCl3)/cmϪ1 3520, 1680, 1650
and 1580]; λmax (EtOH)/nm 431 (log ε 3.48), 364 (log ε 3.56) and
270 (log ε 4.47) [lit.2 λmax (MeOH)/nm 430 (log ε 3.38), 366 (log
ε 3.58) and 268 (log ε 4.45); δH (200 MHz, CDCl3) 1.31 (6H, s,
CMe2), 1.54 (6H, s, CMe2), 1.64 (2H, m, CH2), 1.84 (2H, m,
CH2), 2.57 (3H, s, ArMe), 2.68 (2H, t, J 6, CH2), 3.20 (1H, br s,
OH), 7.17 (1H, s, ArH) and 7.39 (1H, s, ArH) [lit.2 δH (100.6
MHz, CDCl3), 1.32 (6H, s, CMe2), 1.55 (6H, s, CMe2), 1.65
(2H, m, CH2), 1.85 (2H, m, CH2), 2.58 (3H, s, ArMe), 2.70 (2H,
t, J 6, CH2), 3.20 (1H, br s, OH), 7.18 (1H, s, ArH and 7.41 (1H,
s, ArH).
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Acknowledgements
We thank CSIR, New Delhi for the award of SRF to K. G. and
financial support. U. R. G. gratefully acknowledges the Indian
National Science Academy for the Senior Scientistship and the
Director, I.I.C.B. for the facilities. We thank Mr S. Sarkar and
B. Pathak of the I.A.C.S. for elemental analyses.
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