Synthesis of coumarin derivatives from 4-oxo-4H-1-benzopyran-3- carboxaldehyde via 3-(arylaminomethylene)chroman-2,4-dione

Article abstract of DOI:10.1016/S0040-4020(00)00269-6

K-10 Montmorillonite mediated condensation of aldehyde 1 with arylamine 2 affords chroman-2,4-dione 5 which gives tricoumarol 8 by acid hydrolysis, 4-arylamino-3-formylcoumarin 11 and 1-benzopyrano[4,3-b]quinoline 12 on heating with POCl₃. 2000

Full text of DOI:10.1016/S0040-4020(00)00269-6

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 3583±3587
Synthesis of Coumarin Derivatives from 4-Oxo-4H-1-benzopyran-
3-carboxaldehyde via 3-(Arylaminomethylene)chroman-2,4-dione
*
Chandrakanta Bandyopadhyay, Kumar Ranabir Sur, Ranjan Patra and Arunabha Sen
Department of Chemistry, Ramakrishna Mission Vivekananda Centenary College, Rahara, North 24 Parganas, West Bengal 743186, India
Received 4 October 1999; revised 6 March 2000; accepted 23 March 2000
AbstractÐK-10 Montmorillonite mediated condensation of aldehyde 1 with arylamine 2 affords chroman-2,4-dione 5 which gives tri-
coumarol 8 by acid hydrolysis, 4-arylamino-3-formylcoumarin 11 and 1-benzopyrano[4,3-b]quinoline 12 on heating with POCl3. q 2000
Elsevier Science Ltd. All rights reserved.
The diverse biological activity of coumarin derivatives1 as
anticoagulants and antithrombotics is well known. Some 3-
substituted-4-hydroxycoumarins2 and tricoumarol3 possess
some HIV inhibitory potency. Some 1-benzopyrano[4,3-b]-
quinolines are antispasmodic and antihistaminic4 whereas
1-benzopyrano[3,4-f]quinolines are nonsteroidal human
progesterone receptor (HPR) agonists.5 Heterocycles fused
at 3,4-position of coumarin also draw special attention.6 We
report herein new syntheses of some coumarin derivatives
from easily accessible 4-oxo-4H-1-benzopyran-3-carbox-
aldehyde.7
and recyclisation gives the intermediate 4 which needs
oxidation to form 5 (Scheme 1). This oxidation may be
accomplished by clay itself or by Schiff-base 3. Oxidation
of 4-t-butylphenol by K-10 montmorillonite doped with
Fe(III) ion,12 that of secondary as well as benzylic alcohols
by clayfen13,14 and aromatisation of 1,4-dihydropyridine by
claycop15 are known. In all these cases of oxidation by
impregnated clay, NO3± ion was responsible for oxidation
though NaNO3 impregnated clay fails to perform oxi-
dation14 and the role of metal ion remained unclear. The
clay used in our reaction was found to contain Fe(III) ion
but no NO3± or manganese ion was detected. So Fe(III) may
be considered to be responsible for this oxidation. When the
clay, made almost free from iron by leaching with 4N HCl at
room temperature, was employed in the above reaction,
a sharp decrease in yield of 5 (to ,5%) was noted. The
residual mixture obtained after isolation of 5 could not be
properly analysed but on treatment with fresh K-10
montmorillonite again produced enaminone 5 in moderate
yield.
Clay-mediated reaction of aldehyde 1 with arylamine 2 in
benzene produced the enaminone
5 (stereoisomeric
mixture) instead of the corresponding simple Schiff-base
3. The Z:E ratios of 5 were determined from their 1H
NMR spectra. The relatively higher deshielding effect on
the b-H which is cis to the ester function of an a,b-unsatu-
rated ester compared to that of an a,b-unsaturated ketone8
helps to distinguish the E and Z isomer. The additional
support is from comparable d values for NH protons in
E-isomers of 5a±e with that for NH proton of 6. The high
d value (d 11.52) for NH proton in 6 is due to intramolecular
H-bonding in its Z-con®guration. The presently reported
preparation of 5 involving a short reaction time, easy work-
ing procedure and devoid of any chromatographic sepa-
ration is advantageous over its earlier syntheses (a) by
reaction of 4-hydroxycoumarin with triethyl orthoformate
and aniline,6 (b) by oxidation of 3-aryliminomethylchro-
mone 3 with MnO29 or (c) as a side product from the thermal
rearrangement of N-phenylnitrones of 1.10
Treatment of 1 (RˆH and Me) with arylamine 2 (XˆMe) in
the presence of K-10 montmorillonite produced respectively
6 (10%) and 7 (15%) in addition to the corresponding
enaminone 5. The formation of these two reduced Schiff-
bases 6 and 7 indicates oxidation to some extent of 4 to 5 by
the appropriate Schiff-base intermediates. The anil 3 when
prepared by p-toluenesulfonic acid catalysed condensation
of 1 and 2 is always associated with other compounds,
presumably arising due to the oxidising property of the
anil 3 itself. It should be mentioned here that the reaction
of 1 with 2 in the presence of anhydrous FeCl3 in benzene
fails to produce 5.
The attack of the primary amino group at the 2-position of
aldehyde 111 with concomitant opening of the pyran ring
Compound 5, a vinylogous benzanilide, survived re¯uxing
in ethanol containing aqueous HCl but produced tri-
coumarol 816,17 on heating with 70% H2SO4 at 1008C,
presumably via 3-formyl-4-hydroxycoumarin 9 (Scheme 2).
Keywords: coumarin derivatives; montmorillonite; heterocycles; benzo-
pyran.
* Corresponding author. E-mail: prasanta@cal3.vsnl.net.in
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00269-6

3584
C. Bandyopadhyay et al. / Tetrahedron 56 (2000) 3583±3587
Scheme 1.
Scheme 2.
Two con¯icting publications on the Vilsmeier±Haack reac-
tion of 4-hydroxycoumarin have appeared, one reporting the
formation of 817 and the other 4-chloro-3-formylcoumarin
10.18 We found that the above reaction was dependent on
reaction temperature; it gave mainly 10 at room temperature
and 8 at 60±708C; the former on prolonged (2±3 days)
standing in aqueous-acetone gradually transformed into
the latter.
Scheme 3.

C. Bandyopadhyay et al. / Tetrahedron 56 (2000) 3583±3587
3585
The enaminones 5a, b, d, e on treatment with POCl3 at
1008C gave the 1-benzopyranoquinolines 12a,4 b,19 d, e
but not the anticipated 10. The regioselectivity of the
reaction of 2 (XˆMe) with 10 leading to 12b19 is the
same as observed in that of 2 with b-chloroacraldehyde.20
The conversion of 5 to 12 involves the rearrangement of the
former to 4-arylamino-3-formylcoumarin 11 followed by its
cyclisation. When heated with POCl3 at 60±708C 5a±d
indeed formed 11a±d, the latter on further treatment with
POCl3 at 1008C yielding 12. The plausible mechanism for
the transformation of 5 into 11 is depicted in Scheme 3. The
compound 5 reacts with POCl3 to form 13 which rearranges
to 15 possibly via azetine 14. Subsequent hydrolysis of 15
during aqueous work-up produces 11. The conversion of 5
to 12 is analogous to thermal as well as photochemical
cyclisation of 4-chloro-3-aryliminomethyl-2H-chromene to
1-benzopyrano[4,3-b]quinoline involving the rearranged
intermediate 4-arylamino-3-formyl-2H-chromene though
its isolation or independent synthesis was not achieved.21
5a:9 Yield (600 mg, 45%); mp 204±2088C; 5b: Yield
(560 mg, 40%); mp 193±1968C (Found: C, 73.0; H, 4.5;
N, 5.2. C17H13NO3 requires C, 73.1; H, 4.7; N, 5.0%); 5c:
Yield (680 mg, 46%); mp 164±1668C (Found: C, 69.3; H,
4.2; N, 4.9 C17H13NO4 requires C, 69.1; H, 4.4; N, 4.7%);
5d: Yield (590 mg, 42%); mp 214±2188C (Found: C, 73.2;
H, 4.5; N, 5.1. C17H13NO3 requires C, 73.1; H, 4.7; N, 5.0%)
5e: Yield (510 mg, 35%); mp 188±1928C (Found: C, 73.5;
H, 5.2; N, 4.9. C18H15NO3 requires C, 73.7; H, 5.1; N,
4.8%).
In the reaction of 1a (RˆH) and 1b (RˆMe) with 2
(XˆMe), the ®ltrate after recovery of the corresponding
enaminone 5 was chromatographed over silica gel (100±
200) using 50% benzene-light petroleum mixture as eluent
to produce 6 and 7, respectively.
3-( p-Tolylaminomethylene)2H-chromen-4-one (6). The
title compound 6 (130 mg, 10%) was obtained as a yellow
amorphous solid, mp 1588C (Found: C, 77.1; H, 5.8; N, 5.5.
C17H15NO2 requires C, 77.0; H, 5.7; N, 5.3%); IR: 3220,
1
1650, 1590, 1470 cm21; H NMR: d 2.32 (3H, s, ArMe),
Experimental
4.93 (2H, s, OCH2), 6.94 (1H, dd, Jˆ7.8 Hz, 1.9, 8-H), 6.98
(2H, d, Jˆ8.2 Hz, 20-H and 60-H), 7.05 (1H, m, 6-H), 7.14
(2H, d, Jˆ8.2 Hz, 30-H and 50-H), 7.31 (1H, d, Jˆ
12 Hz, vCH), 7.41 (1H, m, 7-H), 7.94 (1H, dd, Jˆ
7.7 Hz, 1.5, 5-H) and 11.52 (1H, exchangeable, d, Jˆ
12 Hz, NH).
The recorded melting points are uncorrected. IR spectra
were recorded in KBr on a Beckman IR-20A and NMR
spectra in CDCl3 with SiMe4 as internal reference on a
300 MHz spectrophotometer unless otherwise stated; J
values are given in Hz. Light petroleum refers to the fraction
with bp 60±808C.
6-Methyl-3(p-tolylaminomethyl)chromen-4-one (7). The
title compound 7 (200 mg, 15%) was obtained as a yellow
crystalline solid, mp 1668C (Found: C, 77.3; H, 6.0; N, 5.1.
C18H17NO2 requires C, 77.4; H, 6.1; N, 5.0%); IR: 3200,
1700, 1660, 1620 cm21; 1H NMR: d 1.9 (1H, exchangeable,
bs, NH), 2.21 (3H, s, ArMe), 2.48 (3H, s, ArMe), 4.28 (2H,
s, CH2), 6.57 (2H, d, Jˆ8.2 Hz, 20-H and 60-H), 6.97 (2H, d,
Jˆ8.2 Hz, 30-H and 50-H), 7.32 (1H, d, Jˆ8.5 Hz, 8-H),
7.46 (1H, dd, Jˆ8.5 Hz, 2.0, 7-H), 7.89 (1H, s, 2-H) and
8.00 (1H, d, Jˆ2.0 Hz, 5-H); 13C NMR (75 MHz): d 20.3
(CH3), 20.8 (CH3), 40.8 (CH2), 113.7 (20-C and 60-C),
117.9 (8-C), 121.4 (40-C), 123.6 (3-C), 125.0 (7-C),
127.3 (4a-C), 129.8 (30-C and 50-C), 134.8 (5-C),
135.0 (6-C), 145.3 (10-C), 153.0 (2-C), 154.9 (8a-C)
and 177.9 (CO).
General procedure for the preparation of 3-(arylamino-
methylene)chroman-2,4-diones (5a±e)
A mixture of K-10 montmorillonite (2 g), 3-formylchromone
1 (5 mmol) and arylamine 2 (5 mmol) in dry benzene
(60 ml) was heated under re¯ux with constant stirring for
2 h and the resulting mixture ®ltered hot. The solid obtained
after concentration of the ®ltrate was crystallised from
benzene to afford 5 as faint yellow ¯uffy solid whose IR
1
and H NMR spectral data are recorded in Table 1. The
reported melting points are of the isomer mixtures shown
in Scheme 1. The yields and CHN analytical data are given
below.
Table 1. 3-(Arylaminomethylene)chroman-2,4-dione (5a±e)
Product
IR (cm21
)
d values (CDCl3) (J values in Hz)
N±H
OCvO
CvO
NHa
vCH
H-5
H-7
ArH
Meb
Z1E
±
Z
E
E
Z
E
Z
Z1E
Z1E
5a
5b
5c
5d
5e
3250
3220
3240
3250
3230
1690
1700
1720
1700
1700
1660
1670
1640
1660
1670
13.80
(13.6)
13.68
(13.7)
13.73
(13.6)
13.71
(13.5)
13.62
(13.7)
11.95
(14.5)
11.93
(14.6)
11.95
(14.5)
11.96
(14.5)
11.92
(14.5)
9.05
(14.5)
9.00
8.90
(13.6)
8.86
8.12
(7.8, 1.6)
8.13
(8,1.5)
8.10
(7.7, 1.5)
7.91
(1.3)
7.85
(1.3)
8.06
(7.8, 1.6)
8.06
(8,1.5)
8.03
(7.7, 1.5)
7.84
(1.2)
7.77
(1.3)
7.60±7.58
(m)
7.63±7.56
(m)
7.61±7.54
(m)
7.50±7.46
(m)
7.50±7.27
(m)
7.32±7.25
(m)
7.33±6.93
(m)
7.45±7.14
(m)
2.38
(s)
3.83
(s)
2.42
(s)
2.31, 2.29
(s) (s)
(14.6)
8.90
(13.7)
8.76
(14.5)
9.03
(13.6)
8.89
(14.5)
8.93
(14.5)
(13.5)
8.79
(13.7)
7.35±7.31
(m)
7.29±7.14
(m)
a High d values of NH protons are due to intramolecular H-bonding in both isomers.
b Me groups of the isomers appear as a singlet.

3586
C. Bandyopadhyay et al. / Tetrahedron 56 (2000) 3583±3587
General procedure for the synthesis of tricoumarol
(8a±b) from 5
12a (RˆXˆH): Yield (200 mg, 80%); mp 1758C (lit.4
1678C). 12b (RˆH; XˆMe): Yield (220 mg, 85%); mp
1
2348C (lit.19 2408C); H NMR: d 2.57 (3H, s, Me), 7.35±
An enaminone 5 (1 mmol) was heated in 70% H2SO4
(20 ml) at 1008C for 3 h. The reaction mixture was cooled
and poured into crushed ice (150 g). The separated white
solid was ®ltered off, washed with water, dried in air, and
crystallised from CHCl3±MeOH to afford the corresponding
tricoumarol 8 as a white solid.
7.44 (2H, m, ArH), 7.57 (1H, m, ArH), 7.70±7.73 (2H, m,
ArH), 8.08 (1H, d, Jˆ9.3 Hz, 11-H), 8.73 (1H, dd,
Jˆ7.8 Hz, 1.5, 1-H) and 9.06 (1H, s, 7-H). 12d (RˆMe;
XˆH): Yield (235 mg, 90%); mp 2248C (Found: C, 78.3;
H, 4.0; N, 5.2. C17H11NO2 requires C, 78.1; H, 4.2; N,
1
5.4%); IR: 2920, 1710, 1500 1190 cm21; H NMR: d 2.52
(3H, s, Me), 7.28 (1H, d, Jˆ8.4 Hz, 4-H), 7.39 (1H, dd,
Jˆ8.4 Hz, 1.7, 3-H), 7.63 (1H, m, 10-H), 7.92 (1H, m,
9-H), 8.01 (1H, dd, Jˆ8.2 Hz, 1.2, 8-H), 8.23 (1H, dd,
Jˆ8.6 Hz, 1.2, 11-H), 8.56 (1H, d, Jˆ1.7 Hz, 1-H) and
9.21 (1H, s, 7-H). 12e (RˆMe; XˆMe): Yield (205 mg,
75%); mp 2148C (Found: C, 78.3; H, 4.7; N, 5.0.
C18H13NO2 requires C, 78.5; H, 4.8; N, 5.1%); IR: 2920,
8a (RˆH): Yield (150 mg, 95%); mp .3008C (lit.16
1
.3008C); IR: 2960±3200 (br), 1725, 1670, 1610 cm21; H
NMR: d (DMSO-d6) 5.41 (1H, s, methine H), 7.16 (1H, dd,
Jˆ8.2 Hz, 1.9, 80-H), 7.22 (1H, m, 60-H), 7.36 (2H, dd,
Jˆ8.3 Hz, 2.0, 2£HD), 7.40 (2H, m, 2£HC), 7.46 (1H, m,
70-H), 7.62 (2H, m, 2£HB), 7.88 (1H, dd, Jˆ7.7 Hz, 1.8,
50-H), 8.25 (2H, dd, Jˆ7.8 Hz, 1.9, 2£HA) and 12.18 (1H,
exchangeable, bs, OH). Compound 8a (RˆH) had super-
imposable IR with the authentic sample.16 8b (RˆMe):
Yield (160 mg, 95%); mp .3008C (Found: C, 71.6; H,
3.8. C31H20O8 requires C, 71.5; H, 3.9%); IR: 3000±3240
(br), 1725, 1660, 1615 cm21; 1H NMR: d (DMSO-d6) 2.39
(3H, s, 60-Me), 2.49 (6H, s, 2£Me), 5.53 (1H, s, methine H),
7.19 (1H, d, Jˆ8.5 Hz, 80-H), 7.37 (2H, d, Jˆ8.5 Hz,
2£HD), 7.41 (1H, dd, Jˆ8.5 Hz, 1.9, 70-H), 7.53 (2H, dd,
Jˆ8.5 Hz, 1.8, 2£HB), 7.82 (1H, d, Jˆ1.9 Hz, 50-H), 8.13
(2H, d, Jˆ1.8 Hz, 2£HA) and 12.15 (1H, exchangeable, bs,
OH).
1
1720, 1585. 1180 cm21; H NMR: d 2.52 (3H, s, 2-Me),
2.60 (3H, s, 9-Me), 7.29 (1H, d, Jˆ8.3 Hz, 4-H), 7.38
(1H, dd, Jˆ8.3 Hz, 2.1, 3-H), 7.74-7.78 (2H, m, 8-H and
10-H), 8.14 (1H, d, Jˆ8.2 Hz, 11-H), 8.56 (1H, d,
Jˆ2.1 Hz, 1-H) and 9.14 (1H, s, 7-H).
General procedure for the preparation of 4-arylamino-
3-formylcoumarines 11 from 5
An enaminone 5 (1 mmol) was heated with POCl3 (15 ml) at
60±708C for 10 h. The reaction mixture was cooled, poured
in ice water, kept overnight, deposited solid ®ltered off,
washed with water, dried and recrystallised from benzene-
light petroleum to afford the corresponding coumarin 11 as a
faint yellow ®ne crystalline solid.
Reaction of 4-hydroxycoumarin with DMF-POCl3
To a clear solution of 4-hydroxycoumarin (810 mg, 5 mmol)
in DMF (4 ml, 50 mmol), POCl3 (0.5 ml, 5 mmol) was
added at room temperature, producing a semi-solid mass.
A clear solution appeared after stirring for 4 h at room
temperature. It was further stirred for 6 h. The reaction
mixture was poured into crushed ice (200 g) with stirring.
The separated solid was ®ltered off and washed thoroughly
with water. A portion of this solid went into solution in
boiling acetone, the insoluble portion was ®ltered off and
recrystallised from CHCl3±MeOH to give tricoumarol 8a
(RˆH) (200 mg, 25%). It is identical in all respects to that
produced from 5a by acid hydrolysis. The acetone solution
on concentration and subsequent dilution with water
produced 4-chloro-3-formylcoumarin 10 (400 mg, 40%),
mp 1228C (lit.18 120±228C). When the above reaction was
performed at 60±708C for 6 h instead of 10 h at room
temperature, tricoumarol (400 mg, 50%) and 10 (50 mg,
5%) were obtained.
11a (RˆXˆH): Yield (225 mg, 85%); mp 1858C (Found: C,
72.6; H, 4.3; N, 5.1. C16H11NO3 requires C, 72.4; H, 4.2; N,
5.3%); IR (CHCl3): 3220, 3000, 2880, 2850, 1720, 1690,
1
1615 cm21; H NMR: d 6.87 (IH, m, 6-H), 7.13 (1H, dd,
Jˆ8.6 Hz, 1.8, 8-H), 7.29±7.50 (7H, m, other ArH), 10.27
(1H, s, CHO) and 13.21 (1H, exchangeable, s, NH). 11b
(RˆH; XˆMe): Yield (225 mg, 80%); mp 1758C; (Found:
C, 73.3; H, 4.9; N, 5.1. C17H13NO3 requires C, 73.1; H, 4.7;
N, 5.0%); IR (CHCl3): 3200, 3010, 2880, 2860, 1720, 1700,
1
1620, 1610 cm21; H NMR: d 2.43 (3H, s, Me), 6.88 (1H,
m, 6-H), 7.14±7.18 (3H, m, ArH), 7.24±7.27 (2H, m, ArH),
7.29 (1H, dd, Jˆ8.4 Hz, 2.0, 5-H), 7.50 (1H, m, 7-H), 10.25
(1H, s, CHO) and 13.16 (1H, exchangeable, s, NH). 11c
(RˆH; XˆOMe): Yield (265 mg, 90%); mp 1658C
(Found: C, 69.0; H, 4.4; N, 4.5. C17H13NO4 requires C,
69.1; H, 4.4; N, 4.7%); IR (CHCl3): 3200, 3000, 2925,
2880, 1720, 1700, 1620, 1610 cm21 1H NMR: d 3.86
;
(3H, s, OMe), 6.87±7.63 (8H, m, ArH), 10.30 (1H, s,
CHO) and 13.20 (1H, exchangeable, s, NH). 11d (RˆMe;
XˆH): Yield (230 mg, 82%); mp 226±2288C (Found: C,
73.0; H, 4.6; N, 4.9. C17H13NO3 requires C, 73.1; H, 4.7;
General procedure for the synthesis of 6-oxo-6H-1-
benzopyrano[4,3-b]quinolines 12 from 5
N, 5.0%); IR (CHCl3): 3220, 3000, 2870, 1710, 1620 cm21
;
An enaminone 5 (1 mmol) was heated with POCl3 (15 ml) at
1008C with stirring for 10 h. The reaction mixture was
cooled, poured in crushed ice (150 g) and kept overnight
at room temperature. The separated solid was ®ltered off,
washed with water and dried. The crude product was eluted
through a small column of silica-gel (60±120) using 50%
benzene-light petroleum mixture to afford the corresponding
quinoline 12 as a white ¯uffy solid. The compounds 12a
(RˆXˆH) and 12b (RˆH; XˆMe) were identical (mp,
mixed mp and IR) with the respective authentic samples.4,19
1H NMR: d 2.01 (3H, s, ArMe), 6.83 (1H, d, Jˆ1.8 Hz, 5-
H), 7.19 (1H, d, Jˆ8.4 Hz, 8-H), 7.29±7.50 (6 H, m, other
ArH), 10.26 (1H, s, CHO) and 13.20 (1H, exchangeable, s,
NH).
Conversion of 11 into 12
Aminoaldehydes 11a, b, d (50 mg) were heated with POCl3
(5 ml) at 1008C for 4 h. Each reaction mixture after usual

C. Bandyopadhyay et al. / Tetrahedron 56 (2000) 3583±3587
3587
D. E.; Gottardis, M. M.; Jones, T. K. J. Med. Chem. 1998, 41, 291±
302.
work up as in the previous cases produced a solid. Crude
compounds were eluted through a very small column of
silica gel (60±120) using 50% benzene-light petroleum
mixture to afford white compounds 12a, b, d, identical in
all respects to those produced directly from 5a, b, d, respec-
tively.
6. Reviews: Darbarwar, M.; Sundaramurthy, V. Synthesis 1982,
337±388; Ghosh, C. K. J. Indian Chem. Soc. 1990, 67, 5±15; 1991,
68, 21±28 (and references therein).
7. Harnisch, H. Justus Liebigs Ann. Chem. 1972, 765, 8±14.
8. Pascual, C.; Meier, J.; Simon, W. Helv. Chim. Acta 1966, 49,
164±168.
Acknowledgements
9. Fitton, A. O.; Frost, J. R.; Houghton, P. G.; Suschitzky, H.
J. Chem. Soc., Perkin Trans. 1 1979, 1691±1694.
10. Ishar, M. P. S.; Kumar, K.; Singh, R. Tetrahedron Lett. 1998,
39, 6547±6550.
Thanks are due to UGC, New Delhi for ®nancial assistance.
We are grateful to the learned referee and the editor for their
valuable comments, to Prof. C. K. Ghosh, Department of
Biochemistry, Calcutta University, Calcutta 700019, for his
valuable advice and to Dr S. B. Maiti, Department of Chem-
istry, Bangabasi College, Calcutta, for his help in drawing
the structures.
11. Ghosh, C. K.; Bandyopadhyay, C.; Tewari, N. J. Org. Chem.
1984, 49, 2812±2814 (and references therein).
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