Carbon-carbon bond formation by radical cyclization: Regioselective synthesis of spiro heterocyclic compounds by nBu3SnH- mediated reaction

Article abstract of DOI:10.1080/00397910600602602

Several spiro heterocyclic compounds have been regioselectively synthesized in excellent yield by n Bu 3 SnH-AIBN-mediated radical cyclization of 4-(2′-bromoaryloxymethyl)-1-methylquinolin-2(1 H )-ones in refluxing benzene under nitrogen for 4 h. Copyrigh

Full text of DOI:10.1080/00397910600602602

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Carbon–Carbon Bond Formation by Radical
Cyclization: Regioselective Synthesis
n
of Spiro Heterocyclic Compounds by
Bu3SnH‐Mediated Reaction
K. C. Majumdar a & N. Kundu a
a Department of Chemistry , University of Kalyani , Kalyani, India
Published online: 18 Aug 2006.
To cite this article: K. C. Majumdar & N. Kundu (2006) Carbon–Carbon Bond Formation by Radical Cyclization:
Regioselective Synthesis of Spiro Heterocyclic Compounds by n Bu3SnH‐Mediated Reaction, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:13,
1879-1886, DOI: 10.1080/00397910600602602
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Synthetic Communicationsw, 36: 1879–1886, 2006
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910600602602
Carbon–Carbon Bond Formation
by Radical Cyclization: Regioselective
Synthesis of Spiro Heterocyclic Compounds
by nBu3SnH-Mediated Reaction
K. C. Majumdar and N. Kundu
Department of Chemistry, University of Kalyani, Kalyani, India
Abstract: Several spiro heterocyclic compounds have been regioselectively
synthesized in excellent yield by nBu3SnH-AIBN-mediated radical cyclization of
4-(20-bromoaryloxymethyl)-1-methylquinolin-2(1H)-ones in refluxing benzene under
nitrogen for 4 h.
Keywords: 4-bromomethyl-1-methyl quinolone, 5-exo-trig, organotin reagent, radical
cyclization, spiro heterocycles
INTRODUCTION
In the course of our study of the application of radical cyclization for the
synthesis of heterocyclic compounds, we recently reported the formation
of a six-membered pyran ring in the case of a substrate containing a 4-hydro-
xycoumarin[1] and a 4-hydroxyquinolin-2-one.[2] We also reported the 5-exo
cyclization of 3-(20-bromobenzyloxy)quinolin-2-ones,[3] 3-(20-bromobenzy-
loxy)benzopyran-7-ones,[3] and 6-(20-bromophenoxymethyl)-1,3-dimethyl
uracil[4] with tri-n-butyl tin chloride and sodium cyanoborohydride. As
demonstrated by Zhang,[5] in the case of an aryl radical attached at the
b-carbon of an a, b-unsaturated lactone system, intramolecular free
radical Michael-type addition facilitates the spiro cyclization process.
Received in India December 28, 2005
Address correspondence to K. C. Majumdar, Department of Chemistry, University
of Kalyani, Kalyani 741 235, India. Tel.: þ 91-33-2582-7521; Fax: þ91-33-25828282;
E-mail: kcm_ku@yahoo.co.in
1879

1880
K. C. Majumdar and N. Kundu
Stabilization of the intermediate radical by the carbonyl group controls this
regioselectivity.[5] However, in the case of an aryl radical tethered (via an
ether linkage) to the a-carbon of an a, b-unsaturated lactone system, both
5-exo and 6-endo cyclization can occur.[6] It is well known that a
5-hexenyl radical cyclizes preferentially to the cyclopentylmethyl radical
via 5-exo cyclization and not to the more stable cyclohexyl radical via
6-endo cyclization.[7] In light of this, we became interested in undertaking
a study on the radical cyclizations of 4-(20-bromoaryloxymethyl)-1-methyl-
quinolin-2(1H)-ones where these very criteria are present. Herein we
report the regioselective formation of spiro quinolone heterocycles by
nBu3SnH-mediated radical cyclization.
RESULTS AND DISCUSSION
We started our investigation by examining the cyclization of several 4-(20-bro-
moaryloxymethyl)-1-methylquinolin-2(1H)-ones 4a–f. These were readily
prepared in 90–92% yields from various o-bromophenols or bromonapthols
3a–f and 4-bromomethyl-1-methylquinolin-2(1H)-one
2 in refluxing
acetone in the presence of anhydrous potassium carbonate. 4-Bromomethyl-
1-methylquinolin-2(1H)-one 2 was prepared by allylic bromination of 1,4-
dimethylquinolin-2(1H)-one with N-bromosuccinimide in CCl4. This result
is different from an earlier report. Cook, Bowen, Sorter and Daniels
reported[8] obtaining 3-bromo-1,4-dimethyl carbostyril instead of 2. The
n
ethers were treated with Bu3SnH-Azobisisobutyronitrile (AIBN) to induce
radical cyclization. Compound 4a was refluxed in degassed benzene under
n
nitrogen with Bu3SnH in the presence of a catalytic amount of AIBN for
4 h to afford the cyclic product 5a in 95% yield (Scheme 1).
The 1H NMR spectrum of the product 5a revealed a one-proton doublet at
d 2.89 (J ¼ 16 Hz) and another one-proton doublet at d 2.95 (J ¼ 16 Hz) due
to –COCH2, and other –OCH2 protons appear as two one-proton doublets at d
4.30 and 4.53 (J ¼ 9 Hz). Final confirmation for structure 5a came from its
COSY, HETCOR, and 13C NMR spectra. The Homonuclear Correlation or
HOMCOR (COSY) spectrum of compound 5a showed that –COCH2
protons at d 2.89 and 2.95 correlate with each other and –OCH2 protons at
d 4.30 and 4.53 correlate with each other. The 13C chemical shifts of the
compound 5a are assigned by DEPT experiment. Distortionless Enhancement
by Polarization Transfer (DEPT) showed 11 protonated carbons, two –CH3,
seven .CH–, and two .CH2 groups. Protonated carbon resonances are
established by direct correlation with proton resonance by the Heteronuclear
Correlation (HETCOR) experiment (normal one bond C–H coupling).
Methyl proton (NCH3 and OCH3) resonance at d 3.42 and 3.75 is related to
carbon resonance at d 28.97 and 55.44, respectively. Methylene proton
resonance at d 2.89 (CH2CO) and 4.30 (OCH2) is related to carbon
resonance at d 41.78 and 81.59, respectively. Mass spectrum of the

Carbon–Carbon Bond Formation by Radical Cyclization
1881
Scheme 1. Reagents and conditions: i) NBS, CCl4, AlBN, 4 h; ii) Acetone, K2CO3,
reflux, 8 h; iii) nBu3SnH, benzene, N2, 4 h.
compound 5a showed a molecular ion peak at m/z ¼ 295 (Mþ). All these
experiments clearly indicated that a 5-exo cyclization has taken place. To
test the generality of the reaction, compounds 4b–f were similarly treated
to afford 5b–f in 90–95% yields (Scheme 1).
The regioselective formation of the five-membered heterocyclic ring can
be explained by the application of the Frontier Molecular Orbital (FMO)
theory. Aryl radicals are high-energy species and hence are nucleophilic in

1882
K. C. Majumdar and N. Kundu
character. The presence of highly electron-withdrawing carbonyl group
confers considerable elecrophilic character to the C-4 position of the
quinolone moiety. Thus, in the case of the nucleophilic radical 6, FMO
theory suggests that the mode of ring closure is largely determined by the
interaction between the radical Singly Occupied Molecular Orbital (SOMO)
;;
(
Highest Occupied Molecular Orbital (HOMO)) and the alkene Lowest
Unoccupied Molecular Orbital (LUMO) of the acceptor (electron-deficient
center) and accordingly more favorable bond formation occurs between the
radical centre (nucleophilic) and C-4 of the quinolone ring for 5-exo
product 5a–f (Scheme 2).
In conclusion, we can say that the exclusive formation of five-membered
spiro heterocyclic furan ring in excellent yield occurs because of two driving
forces: first, the stabilization radical intermediate, and second, the stereoelec-
tronically favored 5-exo pathway. This gives a simple straightforward
synthesis for spiro heterocyclic compounds. The methodology described
here is simple, mild, and attractive because of its simplicity.
EXPERIMENTAL
Melting points were determined in an open capillary and are uncorrected. IR
spectra were recorded on a Perkin-Elmer L 120-000A spectrometer (nmax in
cm21) using samples as neat liquids, and solid samples were recorded on
KBr disks. UV absorption spectra were recorded in EtOH on a Shimadzu
UV-2401PC spectrophotometer (lmax in nm). 1H NMR (300 MHz,
400 MHz, 500 MHz) and 13C NMR (125 MHz) spectra were recorded on
Scheme 2. Mechanism.

Carbon–Carbon Bond Formation by Radical Cyclization
1883
Bruker DPX-300, Bruker DPX-400, and Bruker DPX-500 spectrometers in
CDCl3 (chemical shift in d) with TMS as internal standard. Elemental
analyses and mass spectra were recorded on a Leco 932 CHNS anlyser and
on a JEOL JMS-600 instrument respectively. Silica gel [(60–120 mesh),
Spectrochem, India] was used for chromatographic separation. Silica gel G
[E-Merck (India)] was used for TLC. Petroleum ether refers to the fraction
boiling between 60 and 808C.
Preparation of 4-Bromomethyl-N-methylquinolone (2)
A mixture of 4-methyl-N-methylquinolone (1, 5 mmol), N-bromosuccinimide
(5 mmol), and AIBN (20mg) were heated under reflux in dry CCl4 (125mL)
for 4 h. The reaction mixture was cooled, filtered, and concentrated. The
residual mass was extracted with CH2Cl2 (3 ꢀ 50mL), washed with 10%
NaHSO3 solution (2 ꢀ 25mL) and brine (3 ꢀ 50mL), and dried (Na2SO4).
Removal of the solvent (CH2Cl2) followed by column chromatography on
silica gel by using petroleum ether–EtOAc (7: 3) as eluant gave 2, recrystallized
from CHCl3–petroleum ether (70%); mp 165–1678C; IR (KBr): 2921, 1667,
1639, 751 cm21
;
1H NMR (CDCl3, 400 MHz): dH ¼ 3.72 (s, 3H, NCH3),
4.57 (s, 2H, CH2Br), 6.81 (s, 1H, 55CH), 7.30–7.85 (m, 4H, ArH); MS:
m/z ¼ 251, 253 (Mþ); UV (EtOH): lmax ¼ 218, 279, 338 nm. Anal. calcd,
for C11H10BrNO: C, 52.40; H, 3.99; N, 5.55. Found: C, 52.65; H, 4.19; N, 5.27.
General Procedure for the Preparation of 4a–f
A mixture of 4-bromomethyl-N-methylquinolone (2, 5 mmol), and either
2-bromophenols or bromonapthols (3a–f, 5 mmol), and anhydrous
potassium carbonate (5 g) was heated under reflux in dry acetone (125 mL)
for 8 h. The reaction mixture was cooled, filtered, and concentrated. The
residual mass was extracted with CH2Cl2 (3 ꢀ 50 mL), washed with 10%
Na2CO3 solution (2 ꢀ 25 mL) and brine (3 ꢀ 50 mL), and dried (Na2SO4).
The residual mass after the removal of solvent was subjected to column
chromatography on silica gel using petroleum ether–EtOAc (4 : 1) as eluant
to give compounds 4a–f, recrystallized from CHCl3–petroleum ether.
Compound 4a. Yield: 85%; white solid; mp 130–1328C; IR (KBr): 2829,
1655, 1595, 1457, 739 cm21
;
1H NMR (CDCl3, 500 MHz): dH ¼ 3.74
(s, 3H, –NCH3), 3.76 (s, 3H, OCH3), 5.28 (s, 2H, –OCH2), 6.78–6.91
(m, 2H, ArH), 6.99 (s, 1H, 55CH), 7.14-7.78 (m, 5H, ArH). MS:
m/z ¼ 373, 375 (Mþ); UV (EtOH): lmax ¼ 228, 277, 302, 331 nm. Anal.
calcd. for C18H16BrNO3: C, 57.77; H, 4.31; N, 3.74. Found: C, 58.07; H,
4.52; N, 3.97.
Compound 4b. Yield: 90%; white solid; mp 105–1078C; IR (KBr): 2927,
1659, 1595, 1454, 745 cm21
;
1H NMR (CDCl3, 500 MHz): dH ¼ 3.75

1884
K. C. Majumdar and N. Kundu
(s, 3H, NCH3), 5.36 (s, 2H, OCH2), 7.23 (s, 1H, 55CH), 7.28–8.06 (m, 10H,
ArH); MS: m/z ¼ 393, 395 (Mþ); UV (EtOH): lmax ¼ 230, 274, 279, 330 nm.
Anal. calcd for C21H16BrNO2: C, 63.97; H, 4.09; N, 3.55. Found: C, 64.26; H,
3.93; N, 3.74.
Compound 4c. Yield: 90%; white solid; mp 190–1928C; IR (KBr): 2917,
1646, 1593, 1455, 742 cm21
;
1H NMR (CDCl3, 300 MHz): dH ¼ 3.74
(s, 3H, NCH3), 5.48 (s, 2H, OCH2), 7.08 (s, 1H, 55CH), 7.28–8.25
(m, 10H, ArH); MS: m/z ¼ 393, 395 (Mþ); UV (EtOH): lmax ¼ 231, 273,
279, 331 nm. Anal. calcd. for C21H16BrNO2: C, 63.97; H, 4.09; N, 3.55.
Found: C, 64.25; H, 4.24; N, 3.32.
Compound 4d. Yield: 80%; white solid; mp 138–1408C; IR (KBr): 2922,
1670, 1593, 1455, 749 cm21
;
1H NMR (CDCl3, 500 MHz): dH ¼ 2.24
(s, 3H, ArCH3), 2.44 (s, 3H, ArCH3), 3.76 (s, 3H, NCH3), 5.15 (s, 2H,
OCH2), 7.08 (s, 1H, 55CH), 7.27–7.77 (m, 6H, ArH); MS: m/z ¼ 371, 373
(Mþ); UV (EtOH): lmax ¼ 226, 270, 275, 330 nm. Anal. calcd for
C19H18BrNO2: C, 61.30; H, 4.87; N, 3.76. Found: C, 61.02; H, 5.09; N, 3.58.
Compound 4e. Yield: 80%; white solid; mp 130-1328C; IR (KBr): 2954,
1664, 1593, 1455, 741 cm21
;
1H NMR (CDCl3, 400 MHz): dH ¼ 2.30
(s, 3H, ArCH3), 3.74 (s, 3H, –NCH3), 5.20 (s, 3H, OCH2), 7.10 (s, 1H,
55CH), 7.27–7.88 (m, 7H, ArH); MS: m/z ¼ 357, 359 (Mþ); UV (EtOH):
lmax ¼ 208, 225, 275, 330 nm. Anal. calcd. for C18H16BrNO2: C, 60.35; H,
4.50; N, 3.91. Found: C, 60.10; H, 4.65; N, 4.11.
Compound 4f. Yield: 80%; white solid; mp 126–1288C; IR (KBr): 2958,
1652, 1594, 1456, 740 cm21
;
1H NMR (CDCl3, 500 MHz): dH ¼ 3.76
(s, 3H, NCH3), 5.25 (s, 2H, OCH2), 6.92–6.97 (m, 1H, ArH), 7.12 (s, 1H,
55CH), 7.26–7.90 (m, 7H, ArH); MS: m/z ¼ 343, 345 (Mþ); UV (EtOH):
lmax ¼ 208, 225, 273, 330 nm. Anal. calcd for C17H14BrNO2: C, 59.32; H,
4.10; N, 4.07. Found: C, 59.62; H, 4.25; N, 3.82.
General Procedure for the Preparation of Compounds 5(a–f)
Tributyltin hydride (1.1 mmol) was added to a stirred solution of (4a–f,
1 mmol) and azobisisobutyronitrile (0.5 mmol) in dry degassed benzene
(5 mL) under nitrogen. The mixture was heated under reflux for 4 h and con-
centrated. The residue was dissolved in ether (10 mL) and stirred with a 10%
aq. potassium fluoride solution (10 mL) for 45 min. The white precipitate was
filtered, and the aqueous phase extracted with ether (10 mL). The combined
ether extract was washed with brine and dried over anhyd. Na2SO4. The
residual mass after the removal of solvent was subjected to column chromato-
graphy using petroleum ether–ethyl acetate (4 : 1) as eluant to give cyclized
products 5a–f.

Carbon–Carbon Bond Formation by Radical Cyclization
Data
1885
Compound 5a. Yield: 95%; white solid; mp 85–878C; IR (KBr): 2923,
1676, 1488, 754 cm21
;
1H NMR (CDCl3, 500 MHz): dH ¼ 2.89 (d, 1H,
J ¼ 16 Hz, –COCH), 2.95 (d, 1H, J ¼ 16 Hz, –COCH), 3.42 (s, 3H,
–NCH3), 3.75 (s, 3H, –OCH3), 4.30 (d, 1H, J ¼ 9 Hz, –OCH), 4.53
(d, 1H, J ¼ 9 Hz, –OCH), 6.66-7.33 (m, 7H, ArH); 13C NMR (CDCl3,
125 MHz): dC ¼ 28.97 (NCH3), 41.78 (CH2CO), 48.27 (CH2C), 55.44
(OCH3), 81.59 (OCH2), 109.31 (ArCH), 109.90 (ArCH), 114.06 (ArCH),
114.62 (ArCH), 122.97 (ArCH), 125.99 (ArCH), 127.98 (ArCH), 129.61
(ArC), 130.74 (ArC), 139.18 (ArC), 153.71 (ArC), 154.22 (ArC), 167.18
(CO); MS: m/z ¼ 295 (Mþ); UV (EtOH): lmax ¼ 302, 253, 210 nm. Anal.
calcd. for C18H17NO3: C, 73.20; H, 5.80; N, 4.74. Found: C, 73.49; H,
6.02; N, 4.53.
Compound 5b. Yield: 95%; white solid; mp 170–1728C; IR (KBr): 2922,
1678, 1451, 766 cm21
;
1H NMR (CDCl3, 400 MHz): dH ¼ 2.94 (d, 1H,
J ¼ 16 Hz, –COCH), 3.05 (d, 1H, J ¼ 16 Hz, –COCH), 3.45 (s, 3H,
–NCH3), 4.25 (d, 1H, J ¼ 9 Hz, –OCH), 4.78 (d, 1H, J ¼ 9 Hz, –OCH),
6.94–8.01 (m, 10H, ArH); 13C NMR (CDCl3, 125 MHz): dC ¼ 29.03
(NCH3), 42.24 (CH2CO), 48.77 (CH2C), 82.29 (OCH2), 114.62 (ArCH),
120.23 (ArC), 120.54 (ArCH), 120.98 (ArCH), 121.06 (ArCH), 122.38
(ArC), 122.97 (ArCH), 125.24 (ArCH), 125.99 (ArCH), 126.23 (ArCH),
127.39 (ArCH), 127.98 (ArCH), 130.21 (ArC), 134.11 (ArC), 139.21
(ArC), 155.51 (ArC), 167.31 (CO); MS: m/z ¼ 315 (Mþ); UV (EtOH):
lmax ¼ 234, 211 nm. Anal. calcd for C21H17NO2: C, 79.98; H, 5.43; N,
4.44. Found: C, 79.72; H, 5.73; N, 4.54.
Compound 5c. Yield: 95%; white solid; mp 120–1228C; IR (KBr): 2923,
1677, 1460, 758 cm21
;
1H NMR (CDCl3, 500 MHz): dH ¼ 2.94 (d, 1H,
J ¼ 16 Hz, –COCH), 3.40 (d, 1H, J ¼ 16 Hz, –COCH), 3.46 (s, 3H,
–NCH3), 4.34 (d, 1H, J ¼ 9 Hz, –OCH), 4.80 (d, 1H, J ¼ 9 Hz, –OCH),
6.94–7.87 (m, 10H, ArH); MS: m/z ¼ 315 (Mþ); UV (EtOH): lmax ¼ 240,
213 nm. Anal. calcd for C21H17NO2: C, 79.98; H, 5.43; N, 4.44. Found: C,
80.23; H, 5.58; N, 4.19.
Compound 5d. Yield: 92%; viscous liquid; IR (neat): 2920, 1679, 1479,
1
754 cm21; H NMR (CDCl3, 400 MHz): dH ¼ 2.22 (s, 3H, ArCH3), 2.25
(s, 3H, ArCH3), 2.85 (d, 1H, J ¼ 16 Hz, –COCH), 2.92 (d, 1H, J ¼ 16 Hz,
–COCH), 3.42 (s, 3H, –NCH3), 4.29 (d, 1H, J ¼ 9 Hz, –OCH), 4.51
(d, 1H, J ¼ 9 Hz, –OCH), 6.67–7.29 (m, 6H, ArH); MS: m/z ¼ 293 (Mþ);
UV (EtOH): lmax ¼ 286, 254, 217 nm. Anal. calcd for C19H19NO2: C,
77.79; H, 6.53; N, 4.77. Found: C, 78.09; H, 6.31; N, 4.88.
Compound 5e. Yield: 90%; viscous liquid; IR (neat): 2920, 1678, 1474,
1
755 cm21; H NMR (CDCl3, 400 MHz): dH ¼ 2.27 (s, 3H, ArCH3), 2.90
(d, 1H, J ¼ 16 Hz, –COCH), 2.96 (d, 1H, J ¼ 16 Hz, –COCH), 3.42

1886
K. C. Majumdar and N. Kundu
(s, 3H, –NCH3), 4.41 (d, 1H, J ¼ 9 Hz, –OCH), 4.62 (d, 1H, J ¼ 9 Hz,
–OCH), 6.81–7.34 (m, 7H, ArH); MS: m/z ¼ 279 (Mþ); UV (EtOH):
lmax ¼ 292, 255, 212 nm. Anal. calcd for C18H17NO2: C, 77.40; H, 6.13; N,
5.01. Found: C, 77.66; H, 5.88; N, 5.20.
Compound 5f. Yield: 90%; viscous liquid; IR (neat): 2925, 1677, 1469,
755 cm21
;
1H NMR (CDCl3, 300 MHz): dH ¼ 2.87 (d, 1H, J ¼ 16 Hz,
–COCH), 2.94 (d, 1H, J ¼ 16 Hz, –COCH), 3.43 (s, 3H, –NCH3), 4.44
(d, 1H, J ¼ 9 Hz, –OCH), 4.65 (d, 1H, J ¼ 9 Hz, –OCH), 6.83–7.43
(m, 8H, ArH); MS: m/z ¼ 265 (Mþ); UV (EtOH): lmax ¼ 280, 257,
213 nm. Anal. calcd. for C17H15NO2: C, 76.96; H, 5.70; N, 5.28. Found: C,
77.25; H, 5.92; N, 5.09.
ACKNOWLEDGMENT
We thank the Council of Scientific and Industrial Research (CSIR) (New
Delhi, India) for financial assistance. One of us (N. K.) is grateful to CSIR
for a junior research fellowship.
REFERENCES
1. Majumdar, K. C.; Basu, P. K.; Mukhopadhyay, P. P.; Sarkar, S.; Ghosh, S. K.;
Biswas, P. Tetrahedron 2003, 59, 2151.
2. Majumdar, K. C.; Mukhopadhyay, P. P. Synthesis 2003, 97.
3. Majumdar, K. C.; Mukhopadhyay, P. P.; Basu, P. K. Synth. Commun. 2005, 35,
1291.
4. Majumdar, K. C.; Mukhopadhyay, P. P. Synthesis 2004, 1864.
5. Zhang, W. Tetrahedron Lett. 2001, 42, 2523.
6. Ponaras, A. A.; Zaim, O. Tetrahedron Lett. 1993, 34, 2879.
7. (a) Beckwith, A. L. J.; Ingold, K. U. Rearrangement in Ground and Excited States,
de Mayo, P., (Ed.); Academic Press: New York, 1980; Vol. 1, 161;
(b) Beckwith, A. L.J. Tetrahedron 1981, 37, 3073.
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