Synthesis of thiochromen-4-one-annelated heterocycles: Regioselective cyclization of 3-hydroxy-2-cyclohex-2′-enylthiochromen-4-one

Article abstract of DOI:10.1081/SCC-120015569

2-Cyclohex-2′-enyl-3-hydroxythiochromen-4-one (4) was synthesized in 80% yield by the thermal [3,3] sigmatropic rearrangement of 3-cyclohex-2′-enyloxythiochromen-4-one (3). Treatment of 4 with palladium (bisbenzonitrile) chloride in refluxing benzene afforded linearly cyclized product, tetrahydrobenzofuro[3,2-b]thiochromen-6-one (5) in 98% yield which on dehydrogenation with palladized charcoal gave benzofuro[3,2-b]thiochromen-6-one (6). Substrate 4 on reaction with hexamethylenetetramine hydrotribromide gave a mixture of three products, linearly cyclized heterocycle 10 (35%), bicyclic heterocyclic 8 (15%), and dibromide 9 (20%). The same reaction when conducted with pyridine hydrotribromide furnished only the dibromide 9 which was then cyclized in acetone-potassium carbonate to give the bicyclic compound 8 in 85% yield. The dibromide 9 was cyclized by heating with palladized charcoal in diphenyl ether to give a mixture of linearly cyclized product 5 (7%) and bicyclic product 8 (63%). Substrate 4 was also cyclized in cold conc. H₂SO₄ to give a bicyclic product 11 in 95% yield.

Full text of DOI:10.1081/SCC-120015569

This article was downloaded by: [California Institute of Technology]
On: 08 June 2013, At: 08:08
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic
Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lsyc20
Synthesis of Thiochromen-4-one-annelated
Heterocycles: Regioselective Cyclization of 3-
Hydroxy-2-cyclohex-2′-enylthiochromen-4-one
K. C. Majumdar a b & B. Roy a
a Department of Chemistry, University of Kalyani, Kalyani, India
b Department of Chemistry, University of Kalyani, Kalyani, 741 235, India
Published online: 20 Aug 2006.
To cite this article: K. C. Majumdar & B. Roy (2003): Synthesis of Thiochromen-4-one-annelated Heterocycles: Regioselective
Cyclization of 3-Hydroxy-2-cyclohex-2′-enylthiochromen-4-one, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 33:1, 133-142
To link to this article: http://dx.doi.org/10.1081/SCC-120015569
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to
anyone is expressly forbidden.
The publisher does not give any warranty express or implied or make any representation that the contents
will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should
be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims,
proceedings, demand, or costs or damages whatsoever or howsoever caused arising directly or indirectly in
connection with or arising out of the use of this material.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 1, pp. 133–142, 2003
Synthesis of Thiochromen-4-one-annelated
Heterocycles: Regioselective Cyclization of
3-Hydroxy-2-cyclohex-20-enylthiochromen-4-one
*
K. C. Majumdar and B. Roy
Department of Chemistry, University of
Kalyani, Kalyani, India
ABSTRACT
2-Cyclohex-20-enyl-3-hydroxythiochromen-4-one (4) was synthesized
in 80% yield by the thermal [3,3] sigmatropic rearrangement of
3-cyclohex-20-enyloxythiochromen-4-one (3). Treatment of 4 with
palladium (bisbenzonitrile) chloride in refluxing benzene afforded
linearly cyclized product, tetrahydrobenzofuro[3,2-b]thiochromen-
6-one (5) in98% yield which ondehydrogenationwith palladized
charcoal gave benzofuro[3,2-b]thiochromen-6-one (6). Substrate 4 on
reaction with hexamethylenetetramine hydrotribromide gave a mix-
ture of three products, linearly cyclized heterocycle 10 (35%), bicyclic
heterocyclic 8 (15%), and dibromide 9 (20%). The same reaction
when conducted with pyridine hydrotribromide furnished only the
*Correspondence: K. C. Majumdar, Department of Chemistry, University of
Kalyani, Kalyani 741 235, India. E-mail: kcm@klyuniv.ernet.in.
133
DOI: 10.1081/SCC-120015569
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com
Copyright & 2003 by Marcel Dekker, Inc.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
134
Majumdar and Roy
dibromide 9 which was then cyclized in acetone–potassium carbonate
to give the bicyclic compound 8 in85% yield. The dibromide 9 was
cyclized by heating with palladized charcoal in diphenyl ether to give
a mixture of linearly cyclized product 5 (7%) and bicyclic product
8 (63%). Substrate 4 was also cyclized incold conc. H 2SO4 to give a
bicyclic product 11 in95% yield.
INTRODUCTION
The thieno[2,3-b]benzothiapyran-4-one skeleton is an intermediate in
the synthesis of a series of drugs[1] used against psychotic disturbances.
Recently we reported[2] a simple synthesis for this heterocyclic system.
The importance of the thieno[2,3-b]benzothiapyran-4-one system
prompted us to undertake a study on the structural modification of the
lead compound by altering the ring fusion and replacing the thiophene
ring by a furan ring. We have recently reported some results.[3–5] We
now wish to report the results of our investigation on the synthesis of
0
related heterocycles by the cyclizationof 3-hydroxy-2-(cyclohex-2 -enyl)
thiochromen-4-one (4).
RESULTS AND DISCUSSION
Thermal [3,3] sigmatropic rearrangement of 3-cyclohex-20-enyloxy-
thiochromen-4-one (3) in refluxing chlorobenzene for 13 h afforded
2-cyclohex-20-enyl-3-hydroxythiochromen-4-one (4), the starting material
for this study. Compound 3 was inturnprepared from the reaction
of 3-bromocyclohexene (2) with 3-hydroxythiochromen-4-one (1) in
refluxing acetone in the presence of anhydrous potassium carbonate
(Sch. 1).
Three different approaches were considered for the cyclization of
substrate 4. Recently there has been a lot of interest in the palladium
catalyzed[6] synthesis of heterocyclic compounds. This aroused our
interest to apply this reaction to the synthesis of heterocycle from
substrate 4. Accordingly, substrate 4 was treated with palladium chloride
bisbenzonitrile in refluxing benzene in the presence of sodium methoxide
for 5 h to give a linear heterocyclic compound 5 in98% yield. Compound
5 ontreatment with palladized charcoal indiphenyl ether at 160
1.5 h gave aromatized product 6 (Sch. 2).
ꢀC for

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
Thiochromen-4-one-annelated Heterocycles
135
Scheme 1.
Scheme 2.
Our second approach for the cyclization of substrate 4 involved
its reaction with pyridine hydrotribromide and hexamethylenetetramine
hydrotribromide. Recently these two reagents have been used for
the synthesis of heterocycles from o-alkenyl phenols and enols.[7]
Substrate 4 was treated with hexamethylenetetramine hydrotribromide
ꢀ
inchloroform at 0–5 C for 45 minto give a mixture of three different
products viz, linear heterocycle 10 (35%), bicyclic heterocycle 8 (15%),
and the dibromide 9 (20%). The linear heterocycle 10 was dehydroge-
nated with palladized charcoal in diphenyl ether at 160ꢀC for 2 h to give
aromatic compound 6 (95%) (Sch. 3). However, substrate 4 ontreat-
ment with pyridine hydrotribromide in chloroform at 0–5ꢀC for 2 h
furnished only the dibromide 9 in90% yield. Dibromide 9 when
refluxed in acetone with anhydrous potassium carbonate for 1 h gave
the bicyclic heterocycle 8 in85% yield. However, whenthe dibromide
was refluxed with palladized charcoal indiphenyl ether for 1.5 h gave a
mixture of two products viz, compound 8 (63%) and compound
9

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
136
Majumdar and Roy
Scheme 3.
5 (7%). Compound 5 was dehydrogenated to give aromatic compound
6 (Sch. 3).
Our third approach for the cyclizationof substrate 4 towards the
synthesis of heterocyclic compound involved the use of conc. H2SO4.
There are numerous examples of cyclization of o-alkenyl phenols and
enols in recent literature for the synthesis of heterocyclic compounds.[8]
Accordingly, substrate 4 was treated with cold conc. H2SO4 at 0–5ꢀC to
give the bicyclic heterocycle 11 inalmost quantitative yield. Product 11
remained unaffected when refluxed with palladized charcoal in
diphenyl ether for 2 h (Sch. 4). This indicates the bicyclic nature of
this product.
Cyclizationof 3-hydroxy-2-cyclohex-2 0-enylthiochromen-4-one (4)
with palladium chloride bisbenzonitrile and conc. H2SO4 and is
regioselective and the cyclization of the dibromide 9 with acetone–
potassium carbonate is also regioselective. Only one product in each
case is obtained in excellent yield. However, the cyclization of 4
with hexamethylenetetramine hydrotribromide is not regioselective. The

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
Thiochromen-4-one-annelated Heterocycles
137
Scheme 4.
initially formed bromonium ion gives three different products viz, linear
heterocycle 10 by ‘‘5-exo’’ process, bicyclic heterocycle 8 by a ‘‘6-endo’’
cyclization, and the dibromide 9 by SN2 displacement by BrÀ.
EXPERIMENTAL
Melting points were determined in a sulfuric acid bath and are uncor-
rected. UV absorptionspectra were recorded ona Shimadzu UV-160A
spectrophotometer (absolute ethanol). IR spectra were run on a Perkin-
1
Elmer 1330 apparatus. H-NMR spectra were recorded inCDCl with
3
TMS as internal standard on a Bruker 300 MHz spectrometer.
Elemental analyses data (data in accord with the calculated values) and
mass spectra were recorded by RSIC (CDRI), Lucknow. Silica
(60–120 mesh) was obtained from Spectrochem (India). Extracts were
dried over anhydrous Na2SO4.
Preparation of 3-(20-Cyclohexenyloxy)-thiochromen-4-one (3)
A mixture of 3-hydroxythiochromen-4-one (1) (0.5 g, 2.25 mmol) and
3-bromocyclohexene (2) (2.25 mmol) indry acetone (50 mL) was refluxed
inpresence of K 2CO3 (2 g) for 6 h. The solvent was evaporated and the
residual mass was extracted with CHCl3 (3 Â 25 mL) and dried (Na2SO4).
Chloroform was distilled off and the residue was then purified by column
chromatography over silica gel (60% starting material remained
unchanged). The product 3 was eluted with benzene.
Compound 3: Yield 70%; viscous liquid; ꢀmax/nm 225, 266, 358;
ꢁ
max/cmÀ1 3040, 2930, 1620, 1590, 1400; ꢂH (300 MHz): 1.67–2.17 (m,
6H), 4.84 (s, 1H), 5.89–6.00 (m, 2H), 7.19 (s, 1H), 7.47–7.67 (m, 3H),

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
138
Majumdar and Roy
8.59 (d, J ¼ 7.86 Hz, 1H); m/z 258 (Mþ); (Found: C, 69.63; H, 5.49;
S, 12.35. C15H14O2S requires C, 69.77; H, 5.43; S, 12.40%).
Thermal Rearrangement of Compound 3 to Compound 4
Compound 3: (0.51 g, 2 mmol) in chlorobenzene (3 mL) was refluxed
for 12–14 h. The reactionmixture was thencolumnchromatographed
over silica gel. The product 4 was eluted with benzene:petroleum ether
(1:1) and recrystallized from petroleum ether.
Compound 4: Yield 80%; m.p.: 146ꢀC; ꢀmax/nm 241, 351; ꢁmax/cmÀ1
3295, 3020, 2920, 1580, 1440; ꢂH (300 MHz): 1.71–1.85 (m, 3H), 2.12–2.14
(m, 3H), 4.18–4.22 (m, 1H), 5.72–5.76 (m, 1H), 6.04–6.08 (m, 1H),
7.49–7.64 (m, 3H), 7.67 (brs, 1H, D2O exchangeable), 8.53 (d, J ¼
7.73 Hz, 1H); m/z 258 (Mþ); (Found: C, 69.86; H, 5.60; S, 12.52.
C15H14O2S requires C, 69.77; H, 5.43; S, 12.40%).
Palladium (Bisbenzonitrile) Chloride Cyclization of
Compound 4 to Compound 5
To compound 4 (250 mg, 1 mmol) in thiophene free dry benzene
(20 mL), sodium methoxide (55 mg, 1 mmol) and palladium (bisbenzoni-
trile) chloride (425 mg, 1 mmol) were added and refluxed on water bath for
5 h. Then the reaction mixture was filtered and benzene was distilled
off from the filtrate. The residual mass was purified by column
chromatography over silica gel. The product 5 was eluted with ethyl
acetate:benzene (1:9) and recrystallized from chloroform–petroleum ether.
Compound 5: Yield 98%; m.p.: 190ꢀC; ꢀmax/nm 270, 309, 348;
ꢁ
max/cmÀ1 3060, 2930, 1620, 1560, 1500, 1450; ꢂH (300 MHz): 1.86–1.99
(m, 4H), 2.56 (m, 2H), 2.83 (m, 2H), 7.52–7.71 (m, 3H), 8.74 (d,
J ¼ 8.22 Hz, 1H); m/z 256 (Mþ); (Found: C, 70.54; H, 5.46; S, 12.59.
C15H14O2S requires C, 70.34; H, 4.69; S, 12.50%).
Aromatization of Compound 5 to Compound 6
The compound 5 (125 mg, 0.5 mmol) indiphenyl ether was heated at
160ꢀC with palladized charcoal for 1.5 h and then the reaction mixture was
columnchromatographed over silica gel. The compound 6 was eluted with
ethyl acetate:benzene (1:9) and recrystallized from petroleum ether.

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
Thiochromen-4-one-annelated Heterocycles
139
Compound 6: Yield 98%; m.p.: 204–206ꢀC; ꢀmax/nm 216, 270, 312,
363; ꢁmax/cmÀ1 3060, 1620, 1520, 1440; ꢂH (300 MHz): 7.42–7.47
(m, 1H), 7.61–7.85 (m, 6H), 8.79 (m, 1H); m/z 252 (Mþ); (Found:
C, 71.62; H, 3.25; S, 12.52. C15H8O2S requires C, 71.43, H, 3.17, S,
12.70%).
Preparation of Compound 9 from Compound 4
The compound 4 (250 mg, 1 mmol) in chloroform (20 mL) was
cooled at 0–5ꢀC and pyridine hydrotribromide (320 mg, 1 mmol)
was added slowly. The reactionmixture was stirred for 2 h adn
washed with water and dried (Na2SO4). Chloroform was distilled off
and the residual mass was purified by column chromatography. The
product 9 was eluted with benzene–petroleum ether (1:1) and recrystal-
lized from petroleum ether.
Compound 9: Yield 90%; m.p.: 178ꢀC; ꢀmax/nm 218, 266, 371;
ꢁ
max/cmÀ1 3300, 3040, 2920, 1575, 1440; ꢂH (300 MHz): 1.87–2.26
(m, 5H), 2.50–2.59 (m, 1H), 4.37 (d, J ¼ 12 Hz, 1H), 4.82 (s,
1H), 5.21 (s, 1H), 7.51–7.69 (m, 3H), 7.82 (brs, 1H, D2O exchangeable),
8.54 (d, J ¼ 7.26 Hz, 1H); m/z 420, 418, 416 (Mþ); (Found: C,
43.34; H, 3.49; S, 7.42. C15H14O2SBr2 requires C, 43.06; H, 3.34;
S, 7.65%).
Cyclization of Compound 9 to Compound 8
The compound 9 (100 mg, 0.25 mmol) indry acetone (10 mL) was
refluxed with potassium carbonate (0.2 g) for 1 h. Acetone was distilled
off and the residue was extracted with chloroform (3 Â 15 mL). The
extract was washed with water and dried (Na2SO4). Chloroform was
distilled off and the residual mass was purified by column chromatog-
raphy. The product 8 was eluted with ethyl acetate:benzene (1:9) and
recrystallized from chloroform–petroleum ether.
Compound 8: Yield 85%; m.p.: 260ꢀC; ꢀmax/nm 218, 263, 366;
ꢁ
max/cmÀ1 3030, 2900, 1580, 1440; ꢂH (300 MHz): 1.60–1.62 (m,
2H), 1.78–1.80 (m, 1H), 1.99–2.10 (m, 1H), 2.15–2.17 (m, 1H),
2.38–2.43 (m, 1H), 3.15 (m, 1H), 4.44–4.60 (m, 1H), 4.85–4.86 (m, 1H),
7.47–7.58 (m, 3H), 8.63 (m, 1H); m/z 338, 336 (Mþ); (Found: C, 53.55;
H, 4.03; S, 9.34. C15H13O2SBr requires C, 53.31; H, 3.86; S, 9.49%).

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
140
Majumdar and Roy
Cyclization of Compound 9 with Palladized Charcoal
The compound 9 (100 mg, 0.25 mmol) indipheynl ether (2 mL)
was refluxed with 10% palladized charcoal (200 mg) for 2 h. The reac-
tionmixture was chromatographed over silica gel. Yield of the reaction
was 70 mg (70%) of which 7% was compound 5 and 63% was
compound 8.
Reaction of Compound 4 with
Hexamethylenetetramine Hydrotribromide
The compound 4 (520 mg, 2 mmol) inchloroform (20 mL) was
cooled at 0–5ꢀC and hexamethylenetetramine hydrotribromide (770 mg,
2 mmol) was added slowly. The reactionmixture was stirred for
45 min and washed with water and dried (Na2SO4). Chloroform was
distilled off and residual mass was purified by column chromatography
over silica gel. The product 9 was eluted with benzene–petroleum
ether (1:1) and the product 8 and 10 were eluted with ethyl acetate–
benzene (1:9).
Compound 10: Yield 35%; m.p.: 185ꢀC: ꢀmax/nm 223, 272, 307, 353;
ꢁ
max/cmÀ1 3040, 2920, 1600, 1580, 1440; ꢂH (300 MHz): 1.90–2.75 (m, 7H),
5.39 (t, J ¼ 3.39, 1H), 7.53–7.71 (m, 3H), 8.73 (m, 1H); m/z 256 (Mþ);
(Found: C, 70.26; H, 4.78; S, 12.57. C15H12O2S requires C, 70.34;
H, 4.69; S, 12.50%).
Cyclization of Compound 4 to Compound 11 with Sulphuric Acid
The compound 4 (250 mg, 1 mmol) was slowly added to a cold conc.
H2SO4 (3 mL) at 0–5ꢀC and stirred for 2 h. The reaction mixture was
poured into crushed ice and extracted with chloroform (3 Â 25 mL).
The chloroform was washed with Na2CO3 solution(2 Â 25 mL), water
(2 Â 25 mL) and dried (Na2SO4). Chloroform was distilled off and the
residual mass was purified with columnchromatograph over silica gel.
The product 11 was eluted with ethyl acetate–benzene (1:9) and recrys-
tallized from benzene.
Compound 11: Yield 95%; m.p.: 150ꢀC; ꢀmax/nm 218, 264, 367;
ꢁ
max/cmÀ1 3030, 2910, 1580, 1440; ꢂH (300 MHz): 1.81–2.28 (m, 8H),
2.87 (s, 1H), 4.86 (s, 1H), 7.43–7.55 (m, 3H), 8.60 (d, J ¼ 8.19, 1H); m/z

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
Thiochromen-4-one-annelated Heterocycles
141
258 (Mþ); (Found: C, 69.66; H, 5.63; S, 12.62. C15H14O2S requires
C, 69.77; H, 5.43; S, 12.40%).
ACKNOWLEDGMENTS
We thank the CSIR, New Delhi for financial assistance and
University of Kalyani for providing laboratory facilities.
REFERENCES
1. 7-(R-Substituted)-4-oxothieno[2,3-b][1]benzothiapyrans. Neth Appl
6, 411, 477 (Cl, C 07d). April 9, 1965, Swiss Appl Oct 8, 1963 and
July 29, 1964. Sandoz Ltd.
2. (a) Majumdar, K.C.; Khan, A.T.; Saha, S. Facile regioselective alky-
lative cyclisation: synthesis of 2-methyl-4H-thieno[2,3-b][1]benzothio-
pyran-4-one and thiopyrano[2,3-b][1]benzothiopyran-5(4H)-one.
Syn. Lett. 1991, 595; (b) Majumdar, K.C.; Khan, A.T.; Saha, S.
Regioselective synthesis of 4-aryloxymethyl thiopyrano[2,3-b]benzo-
pyran-5(2H )-ones. Synth. Commun. 1992, 22, 901; (c) Majumdar,
K.C.; Saha, S.; Khan, A.T. Facile alkylative cyclisation: regioselec-
tivity in the synthesis of thieno[2,3-b]benzothiopyran-5(4H )-one.
Indian J. Chem. 1994, 33, 216.
3. Majumdar, K.C.; Roy, B. Studies in sigmatropic rearrangement:
thermal rearrangement of 3-prop-2-ynyloxybenzothiapyran-4-ones.
Synth. Commun. in press.
4. Majumdar, K.C.; Roy, B.; Biswas, P. Thiochromenone-annelated
heterocycles regioselective synthesis of furo[3,2-b]thiochromen-9-
ones. Monatsh. Chem. in press.
5. Majumdar, K.C.; Roy, B. Studies in sigmatropic rearrangement:
thermal rearrangement of 3-(40-aryloxybut-2-ynyloxy)thiochromen-
4-ones. J. Chem. Res. (S). in press.
6. (a) Hegedus, L.S.; Allen, G.F.; Waterman, E.L. Palladium-assisted
intramolecular amination of olefins. A new synthesis of indoles.
J. Amer. Chem. Soc. 1976, 98, 2674–2676; (b) Hegedus, L.S.; Allen,
G.F.; Bozell, J.J.; Waterman, E.L. Palladium-assisted intramolecular
amination of olefins. Synthesis of nitrogen heterocycles. J. Amer.
Chem. Soc. 1978, 100, 5800–5807; (c) Jagadish Kumar, R.; David
Krupadanam, G.L.; Srimannarayana, G. A new approach to

MARCEL DEKKER, INC. • 270 MADISON AVENUE • NEW YORK, NY 10016
©2003 Marcel Dekker, Inc. All rights reserved. This material may not be used or reproduced in any form without the express written permission of Marcel Dekker, Inc.
142
Majumdar and Roy
the synthesis of 2-methyl-4H-furo[3,2-c][1]benzopyran-4-ones and
2H,5H-[3,2-c][1]benzopyran-5-ones. Synthesis 1990, 535–538.
7. (a) Majumdar, K.C.; Kundu, A.K. Pyridine hydrotribromide
mediated regioselective cyclizationof ortho-cyclohexenyl phenols.
Ind. J. Chem. 1993, 32B, 605; (b) Majumdar, K.C.; Kundu, A.K.
Pyridine hydrobromide perbromide mediated regioselective heterocy-
clizationof ortho-cyclohexenyl phenols. Can. J. Chem. 1995, 73, 1727.
8. (a) Hurd, C.D.; Hoffman, W.A. Directed ring closure in the synthesis
of chromans and coumarans from o-allylphenols. J. Org. Chem.
1940, 5, 212–222; (b) Frater, Gy.; Schmid, H. Uber die photoche-
mische cyclisierung von 2-allylphenolen. Helv. Chim. Acta 1967, 50,
255–262.
Received inthe UK November 28, 2001