Regioselective synthesis of coumarin and quinolone-annulated spiro heterocycles via aryl radical cyclization

Article abstract of DOI:10.1081/SCC-200057236

3-(2′-Bromobenzyloxy)quinolin-2-ones and 3-(2′-bromobenzyloxy) benzopyran-7-ones undergo aryl radical cyclization in the presence of "Bu₃SnCl-Na(CN)BH₃-AIBN to give spiro-quinolone and coumarin derivatives. Copyright Taylor & Francis

Full text of DOI:10.1081/SCC-200057236

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Regioselective Synthesis
of Coumarin and
Quinolone‐Annulated Spiro
Heterocycles via Aryl Radical
Cyclization
K. C. Majumdar a , P. P. Mukhopadhyay a & P. K. Basu
a
a Department of Chemistry, University of Kalyani,
Kalyani, India
Version of record first published: 16 Aug 2006.
To cite this article: K. C. Majumdar , P. P. Mukhopadhyay & P. K. Basu (2005):
Regioselective Synthesis of Coumarin and Quinolone‐Annulated Spiro Heterocycles
via Aryl Radical Cyclization, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 35:10, 1291-1299
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Synthetic Communicationsw, 35: 1291–1299, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200057236
Regioselective Synthesis of Coumarin and
Quinolone-Annulated Spiro Heterocycles
via Aryl Radical Cyclization
K. C. Majumdar, P. P. Mukhopadhyay, and P. K. Basu
Department of Chemistry, University of Kalyani, Kalyani, India
Abstract: 3-(20-Bromobenzyloxy)quinolin-2-ones and 3-(20-bromobenzyloxy)benzo-
pyran-7-ones undergo aryl radical cyclization in the presence of nBu3SnCl-
Na(CN)BH3-AIBN to give spiro-quinolone and coumarin derivatives.
Keywords: 5-Exo-trig, organotin reagent, radical cyclization, regioselection, spiro
heterocycles
INTRODUCTION
Aryl radical cyclization reactions have become an important tool in the devel-
opment of modern carbo- and heterocyclic chemistry[1] and the synthesis of
polycyclic natural products. In particular, the intramolecular addition of aryl
radicals to double bonds is well documented in literature[2] and the addition
of aryl radicals to cyclic rings to form spiro heterocycles followed by
reduction has also been reported.[3] In contrast to carbocycles, there are rela-
tively few reports dealing with the intramolecular addition of aryl radicals to
heterocyclic rings. Several examples, by Da Mata et al.,[4] Shankaran et al.,[5]
and Harrowven,[6] involve the addition of aryl radicals onto quinoline,
thiophene, and pyridine derivatives. One reported example of an indony[7]
radical was offered by Sundberg[7c] in the synthesis of iboga alkaloids.
Furo[3,2-c]quinolin-4(5H)-one and 2H-pyrano[3,2-c]quinolin-5(6H)-one
Received in India December 28, 2004
Address correspondence to K. C. Majumdar, Department of Chemistry, University
of Kalyani, Kalyani 741235, W. B., India. Tel.: þ91-33-2582-7521; Fax: þ91-33-
25828282; E-mail: kcm@klyuniv.ernet.in
1291

1292
K. C. Majumdar, P. P. Mukhopadhyay, and P. K. Basu
derivatives are abundantly distributed in nature[8] and a number of syntheses[9]
of these heterocycles, including those from our own work,[10] have been
reported. Coumarins and quinolones fused with other heterocycles are
known to have interesting biological and photodynamic properties,[11] that
in turn, have encouraged research with regard to procedures for the prep-
aration of families of these compounds. Thus, a number of methodologies
have been reported[12] for the synthesis of various 3,4-fused furo- and pyrano-
coumarins, as some members belongs to these two families have shown[13]
useful levels of biological activities such as antihelminthic, hypnotic, insecti-
cidal, and antifungal activities; anticoagulant effects on blood; diuretic prop-
erties; and also their occurrence in nature.[14] As not much work has been
published on the generation of radicals in heteroatomic systems, we became
interested in undertaking, the synthesis of various heterocycles incorporating
3-hydroxyquinolone and 3-hydroxycoumarin moieties by aryl radical
cyclization.
RESULTS AND DISCUSSION
3-Hydroxyquinolin-2(1H)-ones 1a and 2-bromobenzyl bromide 2 in refluxing
acetone in the presence of anhydrous potassium carbonate afforded 3-(20-bro-
mobenzyloxy)quinolin-2-ones 3a (Scheme 1). Similar results were obtained
when the other 3-hydroxy compounds (1b,c) and different 2-bromobenzyl
bromides (2a,b) are subjected to same reaction conditions to produce the
radical precursors 3b–f.
Scheme 1. Reagents and conditions: acetone, K2CO3, reflux, 6–8 h.

Regioselective Synthesis of Coumarin and Spiro Heterocycles
1293
Compounds 3a–f were characterized from their elemental analyses and
spectroscopic data. The substrate 3a was then refluxed in dry degassed
benzene under a nitrogen atmosphere with tri-n-butyltin chloride and
sodium cyanoborohydride in the presence of azobisisobutyronitrile (AIBN)
for 4 h to give the cyclic product 4a (Scheme 2).
The 13C NMR spectrum supports the proposed structure. The 13C NMR
chemical shift as well as the multiplicity of compound 4a was established
by distortionless enhancement by polarization transfer (DEPT) experiment.
DEPT shows twelve protonated carbons: one CH3, three CH2, and eight
CH. To test the generality of the reaction, similar treatment of compounds
n
3b–f with Bu3SnCl and Na(CN)BH3 in the presence of AIBN afforded the
corresponding cyclized products 4b–f in 80–85% yield.
The mechanism for the radical transformation is outlined in Scheme 3.
The regioselectively exclusive formation of spiroheterocyclic ring in
products 4a–f from the starting material 3a–f may be explained by the gener-
ation of aryl radical 5. Subsequent 5-exo cyclization may give spirohetero-
cyclic radical 6, which then readily abstracts the hydrogen atom from the in
n
situ formed Bu3SnH to produce spiroheterocycles 4a–f. At this instance,
the exclusive formation of five-membered heterocyclic compounds 4a–f
from the substrates 3a–f suggests that the benzylic radicals 6a–f generated
by 5-exo-trig ring closure of radicals 5 might be more stable than the radical
7, generated from 6-endo-trig ring closure. Inspection of the molecular
models indicates that the radical intermediate 6 should be much more stabil-
ized because of the excellent overlapping of the p-orbital of the radical
center with the neighboring aromatic p-system.
Scheme 2. Reagents and conditions: nBu3SnCl, Na(CN)BH3, AIBN, benzene, reflux,
4–5 h, N2 atms.

1294
K. C. Majumdar, P. P. Mukhopadhyay, and P. K. Basu
Scheme 3.
In conclusion, we can say that the exclusive formation of 5-membered
spiro heterocyclic furan ring in excellent yield occurs because of two
driving forces: first, the stabilized radical intermediate and second, the stereo-
electronically favored 5-exo pathway. This gives a simple straightforward
synthesis for spiro heterocyclic compounds. The methodology described
here is mild and is attractive because of its simplicity. Thus, we have demon-
strated that the combined alkylation and radical cyclization is a viable strategy
for the synthesis of some potentially bioactive furanopyran-annulated
coumarins and quinolones.
EXPERIMENTAL
Melting points were determined in open capillaries and are uncorrected. IR
spectra were recorded on a Perkin-Elmer L 120-000A spectrometer (nmax in
cm21) using samples as neat liquids, solid samples were recorded in KBr
disks, and UV absorption spectra were, recorded in EtOH on a Shimadzu
UV-2401PC spectrophotometer (lmax in nm). 1H NMR (300 MHz,
500 MHz) and 13C NMR (125.7 MHz) spectra were recorded on a Bruker
DPX-300 and Bruker DRX-500 spectrometer in CDCl3 (chemical shift in d)
with TMS as internal standard. Elemental analyses and mass spectra were

Regioselective Synthesis of Coumarin and Spiro Heterocycles
1295
recorded on a Leco 932 CHNS analyser and on a JEOL JMS600 instrument
1
respectively. H NMR and 13C NMR spectra were recorded at the Indian
Institute of Chemical Biology, Kolkata, India, and Bose Institute, Kolkata,
India. Silica gel [(60–120 mesh), Spectrochem, India] was used for chromato-
graphic separation. Silica gel G[E-Merck (India)] was used for TLC.
Petroleum ether refers to the fraction boiling between 608 and 808C.
General Procedure for the Preparation of 3-(20-Bromobenzyloxy)
quinolin-2-ones (3a–d) or 3-(20-Bromobenzyloxy)benzopyran-
7-ones (3e–f)
3-hydroxy compounds (1) (4 mmol) and 2-bromobenzyl bromide (2) (1.0 g,
4 mmol) were refluxed in acetone (100 mL) in the presence of anhydrous
potassium carbonate (4 g) for 8 h. The reaction mixture was then cooled and
filtered, and the solvent was removed. The residual mass was extracted with
CH2Cl2 (3 ꢀ 50 mL) and the extract was washed with 10% Na2CO3
solution to remove unreacted 3-hydroxycompound, then with brine
(3 ꢀ 50 mL), and dried (Na2SO4). The residual mass after removal of the
solvent (CH2Cl2) was subjected to column chromatography over silica gel
using petroleum ether–ethyl acetate (19 : 1) as eluent to give compounds 3,
which were then recrystallized from chloroform–petroleum ether.
1
Compound 3a: Yield: 90%; white solid; mp: 1408C; H NMR (CDCl3,
300 MHz): dH ¼ 1.37 (t, 3H, J ¼ 7 Hz, CH3), 4.40 (q, 2H, J ¼ 7 Hz,
NCH2), 5.25 (s, 2H, OCH2), 6.93 (s, 1H, 55CH), 7.16 (m, 2H, ArH), 7.31
(m, 2H, ArH), 7.41 (m, 2H, ArH), 7.57 (m, 1H, ArH), 7.67–7.70 (m, 1H,
ArH) ppm; IR (KBr): n ¼ 3050, 1703, 1644, 1461 cm21; UV/Vis (EtOH):
lmax ¼ 331 (log1 ¼ 3.47), 286 (log1 ¼ 3.46), 276 (log1 ¼ 3.49), 222
(log1 ¼ 4.28) nm; MS: m/z ¼ 357, 359 (Mþ); Anal. calcd. for
C18H16NO2Br: C, 60.35; H, 4.50; N, 3.91; found: C, 60.24; H, 4.39; N, 4.02.
1
Compound 3b: Yield: 88%; white solid; mp: 1208C; H NMR (CDCl3,
300 MHz): dH ¼ 1.36 (t, 3H, J ¼ 7 Hz, CH3), 3.76 (s, 3H, NCH3), 4.39
(q, 2H, J ¼ 7 Hz, NCH2), 5.15 (s, 2H, OCH2), 6.71 (m, 2H, ArH), 6.95
(s, 1H, 55CH), 7.17 (m, 1H, ArH), 7.22 (m, 1H, ArH), 7.30 (m, 1H, ArH),
7.42 (m, 2H, ArH) ppm; IR (KBr): n ¼ 3045, 1702, 1640, 1446 cm21
;
UV/Vis (EtOH): lmax ¼ 332 (log1 ¼ 3.39), 286 (log1 ¼ 3.49), 277
(log1 ¼ 3.44), 223 (log1 ¼ 4.24) nm; MS: m/z ¼ 387, 389 (Mþ); Anal.
calcd. for C19H18NO3Br: C, 58.78; H, 4.67; N, 3.61; found: C, 58.84; H,
4.58; N, 3.52.
1
Compound 3c: Yield: 80%; white solid; m.p. 1488C; H NMR (CDCl3,
300 MHz): dH ¼ 3.80 (s, 3H, NCH3), 5.27 (s, 2H, OCH2), 6.92 (s, 1H,
55CH), 7.16 (m, 2H, ArH), 7.31 (m, 2H, ArH), 7.42 (m, 2H, ArH), 7.57
(m, 1H, ArH), 7.63 (m, 1H, ArH) ppm; IR (KBr): n ¼ 3050, 1700, 1620,
1440 cm21; UV/Vis (EtOH): lmax ¼ 332 (log1 ¼ 3.59), 286 (log1 ¼ 3.55),

1296
K. C. Majumdar, P. P. Mukhopadhyay, and P. K. Basu
276 (log1 ¼ 3.58), 222 (log1 ¼ 4.4) nm; MS: m/z ¼ 343, 345 (Mþ); Anal.
calcd. for C17H14NO2Br: C, 59.32; H, 4.09; N, 4.07; found: C, 59.49; H,
4.20; N, 3.98.
1
Compound 3d: Yield: 85%; white solid; mp: 1248C; H NMR (CDCl3,
300 MHz): dH ¼ 3.77 (s, 3H, NCH3), 3.80 (s, 3H, OCH3), 5.24 (s, 2H,
OCH2), 6.73 (m, 1H, ArH), 6.91 (s, 1H, 55CH), 7.19–7.24 (m, 2H, ArH),
7.32–7.34 (m, 1H, ArH), 7.42 (m, 3H, ArH) ppm; IR (KBr): n ¼ 3052,
1710, 1620, 1445 cm21; UV/Vis (EtOH): lmax ¼ 332 (log1 ¼ 3.36), 286
(log1 ¼ 3.45), 277 (log1 ¼ 3.45), 222 (log1 ¼ 4.2) nm; MS: m/z ¼ 373,
375 (Mþ); Anal. calcd. for C18H16NO3Br: C, 57.77; H, 4.31; N, 3.74;
found: C, 57.84; H, 4.20; N, 3.80.
Compound 3e: Yield: 80%; white solid; mp: 1108C; 1H NMR (CDCl3,
300 MHz): dH ¼ 5.19 (s, 2H, OCH2), 6.90 (s, 1H, 55CH), 7.28 (m, 2H,
ArH), 7.35 (m, 3H, ArH), 7.41 (m, 2H, ArH), 7.60 (m, 1H, ArH) ppm; IR
(KBr): n ¼ 3030, 1720, 1620, 1350 cm21; UV/Vis (EtOH): lmax ¼ 293
(log1 ¼ 3.65), 228 (log1 ¼ 3.87), 209 (log1 ¼ 4.16) nm; MS: m/z ¼ 330,
332 (Mþ); Anal. calcd. for C16H11O3Br: C, 58.03; H, 3.35. Found: C,
58.09; H, 3.26.
Compound 3f: Yield: 85%; white solid; mp: 1228C; 1H NMR (CDCl3,
300 MHz): lH ¼ 3.79 (s, 3H, OCH3), 5.20 (s, 2H, OCH2), 6.75–6.79 (dd,
J ¼ 9, 3 Hz, 1H, ArH), 6.89 (s, 1H, 55CH), 7.15 (d, J ¼ 3 Hz, 1H, ArH),
7.22 (m, 2H, ArH), 7.36 (m, 2H, ArH), 7.45 (d, J ¼ 9 Hz, 1H, ArH) ppm;
IR (KBr): n ¼ 3030, 1724, 1620, 1380 cm21; UV/Vis (EtOH): lmax ¼ 290
(log1 ¼ 3.91), 227 (log1 ¼ 4.00), 206 (log1 ¼ 4.36) nm; MS: m/z ¼ 360,
362 (Mþ); Anal. calcd. for C17H13O4Br: C, 56.53; H, 3.63. Found: C,
56.36; H, 3.79.
General Procedure for the Preparation of Compounds 4a–f
A suspension of the compounds 3 (0.08 mmol), nBu3SnCl (0.08 mL,
0.296 mmol), Na(CN)BH3 (250 mg, 3.98 mmol), and AIBN (15 mg) in dry
degassed benzene (7 mL) were refluxed for 4–5 h under N2 atmosphere.
Solvent was evaporated under reduced pressure and the residue was taken
in H2O (10 mL) and was extracted with CH2Cl2 (3 ꢀ 10 mL). The
combined organic extract was washed with 1% aqueous NH4OH
(2 ꢀ 10 mL), brine, and dried (Na2SO4). The residual mass after removal of
the solvent was magnetically stirred with saturated solution of potassium
fluoride for 24 h. It was then extracted with CH2Cl2 (3 ꢀ 10 mL), washed
several times with water, and dried (Na2SO4). The residual mass after
removal of the solvent (CH2Cl2) was subjected to column chromatography
using petroleum ether–ethyl acetate (4 : 1) as eluent to give cyclized
products 4, which were then recrystallized from chloroform–petroleum ether.

Regioselective Synthesis of Coumarin and Spiro Heterocycles
1297
1
Compound 4a: Yield: 80%; white solid; mp: 1128C; H NMR (CDCl3,
300 MHz): dH ¼ 1.20 (t, J ¼ 7 Hz, 3H), 3.18 (d, J ¼ 15.5 Hz, 1H), 3.49
(d, J ¼ 15.5 Hz, 1H), 3.92 (q, J ¼ 7 Hz, 2H), 5.21 (d, J ¼ 12 Hz, 1H), 5.42
(d, J ¼ 12 Hz, 1H), 6.58 (m, 1H), 7.11 (m, 3H), 7.27–7.37 (m, 4H) ppm;
13C NMR (125.7 MHz, CDCl3): dC ¼ 12.87 (CH3), 38.20 (-NCH2), 39:08
(-CH2), 73.98 (OCH2), 85.92 (COC), 114.82, 114.89, 121.61, 121.70,
123.58, 127.65, 128.53, 129.07, 129.88, 139.15, 140.28, 140.68 (ArC),
170.09 (-CO) ppm; IR (KBr): n ¼ 3050, 1703, 1608, 1444 cm21; UV/Vis
(EtOH): lmax ¼ 334 (log1 ¼ 3.62), 278 (log1 ¼ 3.58), 268 (log1 ¼ 3.81),
210 (log1 ¼ 4.34) nm; MS: m/z ¼ 279 (Mþ); Anal. calcd. for C18H17NO2:
C, 77.39; H, 6.13; N, 5.01. Found: C, 77.58; H, 5.89; N, 4.88.
Compound 4b: Yield: 82%; white solid; mp: 828C; 1H NMR (CDCl3,
300 MHz): dH ¼ 1.20 (t, J ¼ 7 Hz, 3H), 3.13 (d, J ¼ 15.5 Hz, 1H), 3.48
(d, J ¼ 15.5 Hz, 1H), 3.84 (s, 3H), 3.95 (q, J ¼ 7 Hz, 2H), 5.16
(d, J ¼ 12 Hz, 1H), 5.38 (d, J ¼ 12 Hz, 1H), 6.41 (m, 1H), 6.59 (m, 3H),
7.27 (m, 3H) ppm; IR (KBr): n ¼ 3052, 1709, 1610, 1450 cm21; UV/Vis
(EtOH): lmax ¼ 331 (log1 ¼ 3.58), 275 (log1 ¼ 3.77), 269 (log1 ¼ 3.76),
222 (log1 ¼ 4.44) nm; MS: m/z ¼ 309 (Mþ); Anal. calcd. for C19H19NO3:
C, 73.77; H, 6.19; N, 4.53. Found: C, 73.65; H, 6.31; N, 4.62.
1
Compound 4c: Yield: 84%; white solid; mp: 1188C; H NMR (CDCl3,
500 MHz): dH ¼ 3.24 (d, J ¼ 15.5 Hz, 1H), 3.40 (s, 3H), 3.50
(d, J ¼ 15.5 Hz, 1H), 5.21 (d, J ¼ 12 Hz, 1H), 5.42 (d, J ¼ 12 Hz, 1H), 6.58
(m, 2H), 7.09 (m, 3H), 7.28 (m, 3H) ppm; IR (KBr): n ¼ 3052, 1704, 1610,
1440 cm21; UV/Vis (EtOH): lmax ¼ 334 (log1 ¼ 3.65), 283 (log1 ¼ 3.67),
265 (log1 ¼ 3.82), 224 (log1 ¼ 4.30) nm; MS: m/z ¼ 265 (Mþ); Anal. calcd.
for C17H15NO2: C, 76.96; H, 5.69; N, 5.28. Found: C, 76.71; H, 5.91; N, 5.42.
1
Compound 4d: Yield: 80%; white solid; mp: 1068C; H NMR (CDCl3,
500 MHz): dH ¼ 3.21 (d, J ¼ 15.5 Hz, 1H), 3.26 (s, 3H), 3.48
(d, J ¼ 15.5 Hz, 1H), 3.76 (s, 3H), 5.16 (d, J ¼ 12 Hz, 1H), 5.39
(d, J ¼ 12 Hz, 1H), 6.55 (m, 2H), 7.00 (m, 3H), 7.27 (m, 2H) ppm; 13C
NMR (125.7 MHz, CDCl3): dC ¼ 30.79 (NCH3), 39.04 (CH2), 55.88
(OCH3), 73.85 (OCH2), 86.92 (COC), 109.19, 113.55, 115.62, 123.52,
123.93, 125.91, 128.80, 129.46, 130.58, 135.39, 139.53, 159.14 (ArC),
167.69 (-CO) ppm; IR (KBr): n ¼ 3045, 1700, 1615, 1435 cm21; UV/Vis
(EtOH): lmax ¼ 335 (log1 ¼ 3.65), 279 (log1 ¼ 3.54), 266 (log1 ¼ 3.87),
228 (log1 ¼ 4.30) nm; MS: m/z ¼ 295 (Mþ); Anal. calcd. for C18H17NO3:
C, 73.20; H, 5.80; N, 4.74. Found: C, 73.45; H, 5.51; N, 4.62.
Compound 4e: Yield: 85%; white solid; mp: 1428C; 1H NMR (CDCl3,
300 MHz): dH ¼ 2.90 (d, J ¼ 15 Hz, 1H), 3.41 (d, J ¼ 15 Hz, 1H), 5.13
(d, J ¼ 12 Hz, 1H), 5.20 (d, J ¼ 12 Hz, 1H), 6.93 (m, 1H), 7.08 (m, 2H),
7.31 (m, 3H) 7.39 (m, 2H) ppm; IR (KBr): n ¼ 3035, 1722, 1615,
1380 cm21; UV/Vis (EtOH): lmax ¼ 279 (log1 ¼ 3.34), 271 (log1 ¼ 3.40),

1298
K. C. Majumdar, P. P. Mukhopadhyay, and P. K. Basu
265 (log1 ¼ 3.32), 205 (log1 ¼ 4.27) nm; MS: m/z ¼ 252 (Mþ); Anal. calcd.
for C16H12O3: C, 76.18; H, 4.79. Found: C, 76.05; H, 4.89.
Compound 4f: Yield: 84%; white solid; mp: 1498C; 1H NMR (CDCl3,
300 MHz): dH ¼ 3.02 (d, J ¼ 15 Hz, 1H), 3.31 (d, J ¼ 15 Hz, 1H), 3.77
(s, 3H), 5.03 (d, J ¼ 12 Hz, 1H), 5.09 (d, J ¼ 12 Hz, 1H), 6.65 (m, 2H),
6.80 (m, 3H), 7.34 (m, 2H) ppm; IR (KBr): n ¼ 3030, 1730, 1622,
1375 cm21; UV/Vis (EtOH): lmax ¼ 279 (log1 ¼ 3.38), 274 (log1 ¼ 3.37),
268 (log1 ¼ 3.32), 204 (log1 ¼ 4.23) nm; MS: m/z ¼ 282 (Mþ); Anal.
calcd. for C17H14O4: C, 72.33; H, 4.99. Found: C, 72.46; H, 4.84.
ACKNOWLEDGMENTS
We thank the CSIR (New Delhi) for financial assistance. P. P. Mukhopadhyay
is grateful to CSIR (New Delhi) for a Senior Research Fellowship. We also
thank the DST (New Delhi) for providing the UV-VIS spectrophotometer
and FT-IR spectrometer under DST-FIST.
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