Dimethyl homophthalates to naphthopyrans: The total synthesis of arnottin i and the formal synthesis of (-)-arnottin II

Article abstract of DOI:10.1039/c3ra45312j

A simple and efficient 3-step synthetic protocol has been reported for dimethyl homophthalates to naphthopyrans. Starting from dimethyl 2,3-dimethoxyhomophthalate, a practical synthesis of arnottin I has been described via a base catalyzed mono-alkylation, the selective hydrolysis of an aliphatic ester moiety, two consecutive intramolecular cyclizations and an oxidative aromatization pathway with a very good overall yield. The involved intramolecular acylation followed by an associated enolative lactonization was the decisive step. The synthesis of dihydroarnottin I also completes the formal synthesis of (-)-arnottin II.

Full text of DOI:10.1039/c3ra45312j

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Dimethyl homophthalates to naphthopyrans: the
total synthesis of arnottin I and the formal synthesis
of (ꢀ)-arnottin II†
Cite this: RSC Adv., 2014, 4, 5531
*
Ravi Jangir and Narshinha P. Argade
A simple and efficient 3-step synthetic protocol has been reported for dimethyl homophthalates to
naphthopyrans. Starting from dimethyl 2,3-dimethoxyhomophthalate, a practical synthesis of arnottin I
has been described via a base catalyzed mono-alkylation, the selective hydrolysis of an aliphatic ester
moiety, two consecutive intramolecular cyclizations and an oxidative aromatization pathway with a very
good overall yield. The involved intramolecular acylation followed by an associated enolative
lactonization was the decisive step. The synthesis of dihydroarnottin I also completes the formal
synthesis of (ꢀ)-arnottin II.
Received 24th September 2013
Accepted 18th November 2013
DOI: 10.1039/c3ra45312j
www.rsc.org/advances
The coumarin-based natural products such as gilvocarcins, cyclized products 5a,b in nearly quantitative yields (98%) via the
ravidomycins, chrysomycins and defucogilvocarcins are an corresponding unisolable tetralone intermediates 4a,b. As per
important class of antibiotics.1a–e Analogous arnottin I and the the planned strategy, an acid-promoted regioselective intra-
related (ꢀ)-arnottin II were isolated as minor components from molecular Friedel–Cras acylation utilizing an aliphatic ester
the bark of Xanthoxylum arnottianum Maxim. (Rutaceae) by Ish- moiety, the instantaneous enolization of the thus formed tet-
ikawa and co-workers in 1977.2 They established their structures ralone intermediates 4a,b using an acidic a-methine proton and
in 1993 and 1995 on the basis of spectral data and synthesis.3a,b the concurrent d-lactonization employing
a less reactive
Since then several well-designed synthetic routes involving new aromatic ester unit took place in one-pot to deliver the aimed
carbon–carbon bond forming strategies have been reported for products 5a,b. Compounds 5a,b on treatment with SeO2 in
these signicant targets.4a–i A careful scrutiny of the arnottins I reuxing acetic acid provided the corresponding expected
and II structures and their retrosynthetic analysis revealed that
the multifunctional methyl 6-(4-(benzo[d][1,3]dioxol-5-yl)-1-
methoxy-1-oxobutan-2-yl)-2,3-dimethoxybenzoate would be a
potential precursor to provide convergent access to both the
target compounds (Scheme 1). More specically, the selective
intramolecular acylation of the diester followed by a concomi-
tant enolization–lactonization would be the strategic step in
generating rings B and C to obtain the desired advanced
common intermediate of arnottins I and II. In continuance of
our efforts to synthesize structurally interesting and biologically
important natural and unnatural products from cyclic anhy-
drides and their derivatives as the potential precursors,5a–e we
herein report a concise and efficient access to naphthopyrans,
arnottin I and arnottin II (Scheme 2).
Dimethyl homophthalates 1a,b on base promoted alkylation
with alkyl iodides 2b,c exclusively furnished the requisite mono-
alkylated coupling products 3a–c in 82–85% yields.6a,b The
reaction of appropriate precursors 3a,b with methanesulfonic
acid at room temperature directly formed the desired double
Division of Organic Chemistry, National Chemical Laboratory (CSIR), Pune 411 008,
India. E-mail: np.argade@ncl.res.in
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3ra45312j
Scheme 1 The retrosynthetic analysis of arnottins I and II.
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Scheme 2 A general approach to naphthopyrans and the total synthesis of arnottin I.
aromatized products 6a,b in 2 h with quantitative yield (98%). formed a reactive mixed anhydride intermediate and delivered a
The aromatization plausibly took place via a regioselective mixture of the corresponding simple acylation product; the
introduction of an acetate function at the relatively more reac- known tetralone intermediate 4c and the desired double
tive allylic site followed by its in situ elimination by abstracting cyclized advanced intermediate 5c in quantitative yield4e (ꢂ7 : 3
the adjacent benzylic proton.7 Thus starting from dimethyl ratio, by 1H NMR). Herein the second cyclization step was
homophthalates we have developed a new practical approach to relatively slow due to the mesomeric deactivation of an aromatic
the naphthopyran systems and it has advantages in terms of ester function by the corresponding ortho-methoxy group. An
number of steps involved and obtained yields.
increase in the reaction temperature and/or extending the
In the second part of our studies, we planned the synthesis of reaction time again resulted in some decomposition. The above
arnottin I and (ꢀ)-arnottin II by using the below dened specied mixture of the tetralone intermediate 4c and
synthetic protocol. The pivotal arnottin antecedent compound compound 5c was silica gel column chromatographically
3c on similar treatment with methanesulfonic acid underwent inseparable. Thus to ensure the complete transformation into
an unfortunate instantaneous decomposition under the several the essential compound 5c, the above mixture of products was
sets of reaction conditions. We also tried various conditions to further treated with Cs2CO3 in reuxing toluene (93% yield).
effect the transformation of 3c to 5c utilizing reagents such as Starting from the advanced common intermediate 5c, the
acetic acid, triuoroacetic acid and p-TSA, but this always synthesis of arnottin I by employing DDQ-oxidation in benzene
resulted in the isolation of the starting material and/or and the synthesis of (ꢀ)-arnottin II via the Sharpless asym-
decomposition. The cause for the decomposition of compound metric dihydroxylative spiro-lactonization route have been
3c under acidic conditions was the presence of a labile dioxy- known in the literature.4e Similarly, we repeated the DDQ-
methylene bridge attached to ring D. To circumvent the above oxidation of 5c in reuxing toluene and obtained the natural
specied difficulty, we initially performed a base catalyzed product arnottin I (6c) in a quantitative yield (98%). However,
regioselective mono-hydrolysis of an aliphatic ester moiety in analogous to the 5a,b to 6a,b transformation, the SeO2 oxida-
compound 3c and obtained the product 7c in a 91% yield, as the tion of compound 5c in reuxing acetic acid resulted in the
synthesis of the corresponding dicarboxylic acid followed by decomposition of the reaction mixture. Alternatively, the per-
treatment with dehydrating agents would form the cyclic formed SeO2 oxidation of compound 5c in reuxing benzene,
anhydride and demand sequential synthetic steps to transform toluene and freshly distilled acetic anhydride was very slow and
compound 3c into the essential product 5c. The acid-ester 7c on provided the silica gel column chromatographically inseparable
treatment with triuoroacetic anhydride at ꢀ50 ꢁC to 25 ꢁC mixture of the starting material 5c and arnottin I (6c) in 48 h.
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The 1H NMR spectra of above specied mixtures indicated only 147.2, 148.7, 167.8, 174.3; ESIMS (m/z) 395 [M + Na]+; HRMS
10 to 20% conversions into the desired product 6c. As antici- (ESI) calcd for C21H25O6 373.1646, found 373.1639; IR (CHCl3)
pated, the neat SeO2 (10.00 equiv.) induced oxidative aromati- nmax 1732, 1721, 1602 cmꢀ1
.
ꢁ
zation of dihydronaphthopyran 5c at 200 C took place in 2 h
The products 3b and 3c were similarly obtained by using the
without any decomposition and provided the desired natural above specied procedure.
naphthopyran 6c in a 96% yield. The analytical and spectral
data obtained for arnottin I was in complete agreement with the
reported data.3a,4e Arnottin I was obtained in four steps by using
two different oxidizing agents at the ultimate step with 71% and
69% overall yields, respectively.
In summary, we have demonstrated a bio-inspired protec-
tion-free concise and efficient total synthesis of arnottin I and
the formal synthesis of (ꢀ)-arnottin II. The present transition
metal free diversity oriented robust 3-step new approach to the
imperative naphthopyran architectures is general in nature and
will be useful to design several focused mini-libraries of their
natural and unnatural analogues and congeners for SAR
studies. Our present approach also provides an efficient access
to several corresponding isoquinoline alkaloids.6b,8
Methyl 6-(4-(3,4-dimethoxyphenyl)-1-methoxy-1-oxobutan-2-yl)-
2,3-dimethoxybenzoate (3b)
1
Thick oil (1.33 g, 83%); H NMR (CDCl3, 200 MHz) d 1.90–2.15
(m, 1H), 2.25–2.68 (m, 3H), 3.55 (t, J ¼ 8 Hz, 1H), 3.67 (s, 3H),
3.82 (s, 3H), 3.85 (s, 6H), 3.86 (s, 3H), 3.88 (s, 3H), 6.62–6.74 (m,
2H), 6.79 (d, J ¼ 8 Hz, 1H), 6.94 (d, J ¼ 8 Hz, 1H), 7.15 (d, J ¼ 8
Hz, 1H); 13C NMR (CDCl3, 50 MHz) d 33.2, 35.3, 46.5, 52.0, 52.1,
55.7, 55.8 (2C), 61.4, 111.1, 111.6, 113.7, 120.2, 123.1, 128.3,
129.4, 133.7, 145.8, 147.1, 148.7, 151.5, 167.5, 173.9; ESIMS (m/z)
455 [M + Na]+; HRMS (ESI) calcd for C23H29O8 433.1857, found
433.1851; IR (CHCl3) nmax 1735, 1610 cmꢀ1
.
Methyl 6-(4-(benzo[d][1,3]dioxol-5-yl)-1-methoxy-1-oxobutan-2-yl)-
2,3-dimethoxybenzoate (3c)
Experimental section
1
Thick oil (1.32 g, 85%); H NMR (CDCl3, 200 MHz) d 1.65–2.10
(m, 1H), 2.20–2.65 (m, 3H), 3.54 (t, J ¼ 8 Hz, 1H), 3.65 (s, 3H),
3.85 (s, 9H), 5.89 (s, 2H), 6.58 (dd, J ¼ 8, 2 Hz, 1H), 6.64 (d, J ¼ 2
Hz, 1H), 6.71 (d, J ¼ 8 Hz, 1H), 6.94 (d, J ¼ 10 Hz, 1H), 7.13 (d, J ¼
10 Hz, 1H); 13C NMR (CDCl3, 50 MHz) d 33.3, 35.3, 46.5, 51.9,
52.1, 55.8, 61.4, 100.6, 108.0, 108.8, 113.8, 121.1, 123.1, 128.2,
129.4, 135.0, 145.6, 145.8, 147.4, 151.5, 167.5, 173.8; ESIMS (m/z)
439 [M + Na]+; HRMS (ESI) calcd for C22H25O8 417.1544, found
The melting points are uncorrected. The mass spectra were
taken on an MS-TOF mass spectrometer. HRMS (ESI) were taken
on an Orbitrap (quadrupole plus ion trap) and TOF mass
analyzer. The IR spectra were recorded on an FT-IR spectrom-
eter. The starting materials 1a,b and 2b,c were prepared by
using known literature procedures.4h,9 Commercially available
chemicals and reagents were used.
417.1537; IR (CHCl3) nmax 1733, 1604 cmꢀ1
.
Methyl 2-(4-(3,4-dimethoxyphenyl)-1-methoxy-1-oxobutan-2-
yl)-benzoate (3a)
2,3-Dimethoxy-11,12-dihydro-6H-dibenzo[c,h]chromen-6-one
(5a)
A fresh solution of LDA was prepared from diisopropylamine
(0.87 mL, 6.24 mmol) and n-BuLi (1.60 M in hexane, 4.20 mL,
6.72 mmol) in THF (5 mL) under an argon atmosphere at 0 ꢁC.
This was added to a solution of compound 1a (1.00 g,
4.80 mmol) in THF (10 mL) and HMPA (10 mL) mixture at
To compound 3a (372 mg, 1.00 mmol) CH3SO3H (4 mL) was
added at room temperature under an argon atmosphere and the
reaction mixture was stirred for 30 min. The reaction mixture
was poured on crushed ice and the obtained precipitate was
ltered, washed with water and 10% aqueous NaHCO3 and
dried using a vacuum pump. The silica gel (60–120 mesh)
column chromatographic purication of the resulting
compound using ethyl acetate–petroleum ether (3 : 7) as an
ꢁ
ꢀ78 C under an argon atmosphere and the reaction mixture
was further stirred at the same temperature for 30 min. To the
above reaction mixture a solution of compound 2b (1.54 g, 5.28
mmol) in THF (5 mL) was added in a dropwise fashion. The
reaction mixture was allowed to gradually attain room temper-
ature in 7 h. The reaction was quenched with a saturated NH4Cl
solution and the solvent was removed in vacuo. The obtained
residue was dissolved in ethyl acetate and the organic layer was
washed with water, brine and dried over Na2SO4. The concen-
tration of the organic layer in vacuo followed by silica gel (60–
120 mesh) column chromatographic purication of the result-
ing residue using ethyl acetate–petroleum ether (1 : 3) as an
eluent gave the pure product 3a as a thick oil (1.46 g, 82%).
1H NMR (CDCl3, 200 MHz) d 1.95–2.25 (m, 1H), 2.35–2.70 (m,
3H), 3.68 (s, 3H), 3.87 (s, 6H), 3.88 (s, 3H), 4.64 (t, J ¼ 8 Hz, 1H),
6.69 (s, 1H), 6.72 (t, J ¼ 8 Hz, 1H), 6.78 (t, J ¼ 8 Hz, 1H), 7.27–7.42
(m, 1H), 7.42–7.60 (m, 2H), 7.92 (dd, J ¼ 8, 2 Hz, 1H); 13C NMR
eluent gave the pure product 5a as a yellow solid (302 mg, 98%).
1
ꢁ
ꢁ
Mp 167–169 C (ref. 10, 105 C); H NMR (CDCl3, 400 MHz) d
2.85–3.00 (m, 4H), 3.92 (s, 3H), 3.96 (s, 3H), 6.75 (s, 1H), 7.38 (s,
1H), 7.45 (t, J ¼ 8 Hz, 1H), 7.57 (d, J ¼ 8 Hz, 1H), 7.75 (t, J ¼ 8 Hz,
1H), 8.33 (d, J ¼ 8 Hz, 1H); 13C NMR (CDCl3, 100 MHz) d 21.2,
26.9, 56.0, 56.3, 106.1, 107.6, 110.9, 120.4, 121.3, 121.8, 127.1,
129.7, 130.2, 134.7, 137.6, 148.0, 148.3, 149.8, 162.3; ESIMS (m/z)
331 [M + Na]+; IR (CHCl3) nmax 1733, 1722, 1631, 1603 cmꢀ1
.
The product 5b was similarly obtained by using the above
specied procedure.
2,3,7,8-Tetramethoxy-11,12-dihydro-6H-dibenzo[c,h]chromen-
6-one (5b)
(CDCl3, 50 MHz) d 33.4, 35.2, 46.5, 52.0, 52.1, 55.7, 55.9, 111.1, Yellow solid (360 mg, 98%); mp 172–174 ꢁC (ref. 11, 171–172 ꢁC);
111.7, 120.2, 126.9, 128.7, 129.8, 130.7, 132.2, 134.0, 140.2, 1H NMR (CDCl3, 200 MHz) d 2.80–3.02 (m, 4H), 3.93 (s, 3H), 3.96
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(s, 3H), 3.98 (s, 3H), 4.00 (s, 3H), 6.75 (s, 1H), 7.30 (d, J ¼ 10 Hz, 7.14 (d, J ¼ 8 Hz, 1H); 13C NMR (CDCl3, 50 MHz) d 33.1, 34.5,
1H), 7.38 (s, 1H), 7.39 (d, J ¼ 10 Hz, 1H); 13C NMR (CDCl3, 50 46.4, 52.4, 55.9, 61.4, 100.7, 108.1, 108.9, 114.2, 121.2, 123.2,
MHz) d 21.6, 27.0, 56.0, 56.3, 56.6, 61.5, 105.8, 106.9, 110.9, 127.7, 129.3, 134.8, 145.7, 146.1, 147.5, 151.8, 168.2, 177.9;
115.0, 117.7, 119.9, 121.3, 129.1, 132.2, 146.6, 147.9, 149.3, ESIMS (m/z) 425 [M + Na]+; HRMS (ESI) calcd for C21H22O8Na
151.5, 152.1, 158.7; ESIMS (m/z) 391 [M + Na]+; IR (CHCl3) nmax 425.1207, found 425.1204; IR (CHCl3) nmax 2700–2500, 1731,
1730 cmꢀ1
.
1709, 1606 cmꢀ1
.
1,2-Dimethoxy-13H-[1,3]dioxolo[40,50:4,5]benzo[1,2-h]benzo[c]
2,3-Dimethoxy-6H-dibenzo[c,h]chromen-6-one (6a)
chromen-13-one (5c)
To a stirred solution of compound 5a (154 mg, 0.50 mmol) in
AcOH (5 mL) SeO2 (165 mg, 1.50 mmol) was added and the
reaction mixture was reuxed for 2 h under an argon atmo-
sphere. It was allowed to reach room temperature and was
concentrated in vacuo. The obtained residue was dissolved in
ethyl acetate (20 mL) and the organic layer was washed with
water, a saturated solution of NaHCO3 and brine and dried over
Na2SO4. The concentration of the organic layer in vacuo followed
by the silica gel (60–120 mesh) column chromatographic puri-
cation of the resulting residue using ethyl acetate–petroleum
ether (3 : 7) as an eluent gave the pure product 6a as a faint
yellow solid (150 mg, 98%). Mp 217–220 ꢁC (ref. 10, 213 ꢁC); 1H
NMR (CDCl3, 200 MHz) d 4.02 (s, 3H), 4.09 (s, 3H), 7.10 (s, 1H),
7.54 (t, J ¼ 8 Hz, 1H), 7.56 (d, J ¼ 8 Hz, 1H), 7.74 (s, 1H), 7.82 (dt,
J ¼ 8, 2 Hz, 2H), 7.87 (d, J ¼ 10 Hz, 1H), 8.11 (d, J ¼ 10 Hz, 1H),
8.41 (d, J ¼ 8 Hz, 1H); 13C NMR (CDCl3, 50 MHz) d 55.8, 56.2,
100.7, 106.1, 111.5, 117.2, 118.6, 120.3, 121.5, 122.6, 127.8,
130.1, 130.2, 134.6, 135.4, 146.0, 149.9, 150.5, 161.2; ESIMS (m/z)
To compound 7c (200 mg, 0.49 mmol) TFAA (2 mL) was added at
ꢀ50 ꢁC and the reaction mixture was stirred under an argon
atmosphere at ꢀ50 ꢁC to 25 ꢁC for 3 h. The reaction mixture was
concentrated in vacuo and the obtained residue was dried using
a vacuum pump. To the residue toluene (5 mL) and Cs2CO3 (326
mg, 1.00 mmol) were added, and the stirred reaction mixture
was reuxed for 2 h. It was allowed to reach room temperature
and was concentrated in vacuo. The obtained residue was dis-
solved in ethyl acetate (30 mL) and the organic layer was washed
with water and brine and dried over Na2SO4. The concentration
of the organic layer in vacuo followed by the silica gel (60–120
mesh) column chromatographic purication of the resulting
residue using ethyl acetate–petroleum ether (3 : 7) as an eluent
gave the pure product 5c as a yellow solid (162 mg, 93%). Mp
1
ꢁ
ꢁ
245–248 C (ref. 4e, 250–251 C); H NMR (CDCl3, 500 MHz) d
2.81 (dd, J ¼ 10, 2 Hz, 1H), 2.82 (d, J ¼ 10 Hz, 1H), 2.91 (d, J ¼ 10
Hz, 1H), 2.92 (dd, J ¼ 10, 2 Hz, 1H), 3.95 (s, 3H), 3.99 (s, 3H), 5.97
(s, 2H), 6.70 (s, 1H), 7.28 (d, J ¼ 10 Hz, 1H), 7.35 (s, 1H), 7.36 (d,
J ¼ 10 Hz, 1H); 13C NMR (CDCl3, 125 MHz) d 21.6, 27.6, 56.6,
61.5, 101.2, 103.5, 107.1, 108.3, 115.2, 117.8, 119.9, 122.7, 130.8,
132.1, 146.6, 146.7, 147.9, 151.6, 152.2, 158.5; ESIMS (m/z) 375
306 [M]+; IR (CHCl3) nmax 1732, 1629, 1607 cmꢀ1
.
The product 6b was similarly obtained by using the above
specied procedure.
[M + Na]+; IR (CHCl3) nmax 1734, 1700, 1670 cmꢀ1
.
2,3,7,8-Tetramethoxy-6H-dibenzo[c,h]chromen-6-one (6b)
Faint yellow solid (179 mg, 98%); mp 230–232 ꢁC (ref. 12, 218–
1
1,2-Dimethoxy-13H-[1,3]dioxolo[40,50:4,5]benzo[1,2-h]benzo[c]
chromen-13-one (arnottin I, 6c)
ꢁ
220 C); H NMR (CDCl3, 200 MHz) d 4.00 (s, 3H), 4.05 (s, 6H),
4.12 (s, 3H), 7.15 (s, 1H), 7.46 (d, J ¼ 8 Hz, 1H), 7.58 (d, J ¼ 8 Hz,
1H), 7.80 (s, 1H), 7.86 (d, J ¼ 8 Hz, 1H), 7.91 (d, J ¼ 8 Hz, 1H); 13
C
A neat mixture of compound 5c (70 mg, 0.20 mmol) and SeO2
(220 mg, 2.00 mmol) was heated in the sealed tube at 200 ꢁC for
2 h. It was allowed to reach room temperature and the obtained
residue was dissolved in ethyl acetate (20 mL). The organic layer
was washed with water, a saturated solution of NaHCO3 and
brine and dried over Na2SO4. The concentration of the organic
layer in vacuo provided the pure product 6c as a yellow solid (67
mg, 96%). The analytically pure sample of 6c was obtained by
recrystallization from chloroform. Mp 296–298 ꢁC (ref. 3a, 293–
NMR (CDCl3, 125 MHz) d 55.9, 56.3, 56.4, 61.5, 100.9, 106.3,
111.5, 115.1, 117.4, 117.7, 118.6, 119.5, 122.5, 129.70, 129.74,
145.4, 150.0, 150.4, 151.5, 152.8, 157.9; ESIMS (m/z) 389 [M +
Na]+; IR (CHCl3) nmax 1735, 1633 cmꢀ1
.
4-(Benzo[d][1,3]dioxol-5-yl)-2-(3,4-dimethoxy-2-
(methoxycarbonyl)phenyl)butanoic acid (7c)
To a stirred solution of compound 3c (1.25 g, 3.00 mmol) in
MeOH (25 mL) 2% aqueous KOH (25 mL) was added at 0 ꢁC. The
reaction mixture was allowed to gradually attain room temper-
ature and was further stirred for 24 h. It was acidied with 2 N
HCl and the formed product was extracted in ethyl acetate (25
mL ꢃ 2). The organic layer was washed with water and brine
and dried over Na2SO4. The concentration of the organic layer in
vacuo followed by the silica gel (60–120 mesh) column chro-
matographic purication of the resulting residue using ethyl
acetate–petroleum ether (4 : 6) as an eluent gave the pure
product 7c as a thick oil (1.10 g, 91%). 1H NMR (CDCl3,
1
ꢁ
297 C); H NMR (CDCl3, 200 MHz) d 4.00 (s, 3H), 4.04 (s, 3H),
6.11 (s, 2H), 7.15 (s, 1H), 7.46 (d, J ¼ 10 Hz, 1H), 7.55 (d, J ¼ 10
Hz, 1H), 7.86 (d, J ¼ 10 Hz, 1H), 7.86 (s, 1H), 7.90 (d, J ¼ 10 Hz,
1H); 13C NMR (CDCl3, 125 MHz) d 56.6, 61.6, 99.1, 101.5, 104.0,
112.1, 115.5, 117.7, 117.8, 119.7, 120.2, 123.2, 129.8, 131.2,
146.1, 148.5, 148.9, 151.7, 153.1, 157.7; ESIMS (m/z) 373 [M +
Na]+; IR (CHCl3) nmax 1734,1651 cmꢀ1
.
Acknowledgements
200 MHz) d 1.90–2.15 (m, 1H), 2.20–2.60 (m, 3H), 3.52 (t, J ¼ 8 R. J. thanks CSIR, New Delhi, for the award of research fellow-
Hz, 1H), 3.86 (s, 9H), 5.89 (s, 2H), 6.57 (dd, J ¼ 8, 2 Hz, 1H), 6.62 ship. N. P. A. thanks the Department of Science and Technology,
(d, J ¼ 2 Hz, 1H), 6.70 (d, J ¼ 8 Hz, 1H), 6.96 (d, J ¼ 10 Hz, 1H), New Delhi for nancial support.
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