Regioselective synthesis of 2H-benzothiopyrano[3,2-c]quinolin-7(8H)-ones by tri-n-butyltinhydride-mediated radical cyclization

Article abstract of DOI:10.1080/00397910701266182

A number of hitherto unreported 2H-benzothiopyrano[3,2-c]quinolin-7(8H)- ones have been regioselectively synthesized in 90-96% yield by tri-n-butyltinhydride-AIBN-mediated radical cyclization from 4-(2′-bromothioarylmethyl)-1-methylquinolin-2(1H)-ones and

Full text of DOI:10.1080/00397910701266182

Synthetic Communicationsw, 37: 1747–1756, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701266182
Regioselective Synthesis of 2H-
Benzothiopyrano[3,2-c]quinolin-7(8H)-ones
by Tri-n-butyltinhydride–Mediated Radical
Cyclization
K. C. Majumdar and N. Kundu
Department of Chemistry, University of Kalyani, Kalyani, India
Abstract: A number of hitherto unreported 2H-benzothiopyrano[3,2-c]quinolin-
7(8H)-ones have been regioselectively synthesized in 90–96% yield by
tri-n-butyltinhydride–AIBN–mediated radical cyclization from 4-(20-bromothioaryl-
methyl)-1-methylquinolin-2(1H)-ones and their corresponding sulfones. 4-(20-Bro-
mothioarylmethyl)quinolin-2(1H)-ones were in turn prepared from 4-bromomethyl-
quinolin-2(1H)-one and o-bromothiophenols by refluxing in acetone in the presence of
anhydrous K2CO3. These were converted to the corresponding sulfones by oxidation
with two equivalents of m-CPBA in refluxing dichloromethane for 1 h.
Keywords: 4-bromomethyl-1-methylquinolin-2(1H)-one, 6-endo-trig, organotin reagent,
radical cyclization, sulfur heterocycles
INTRODUCTION
Organic synthesis has benefitted enormously from reactions involving internal
addition of a radical center to a double bond. In particular, cyclization of
5-hexenyl radicals represents a process that, over the years, has provided
access to a large number of complex substances.[1] Contrary to predictions
based on thermodynamic criteria, cyclization of 5-hexenyl systems
generally gives cyclopentyl methyl radical via a prominent 5-exo mode of
closure[1c,2] and not the more stable cyclohexyl radical via 6-endo cyclization,
Received October 16, 2006
Address correspondence to K. C. Majumdar, Department of Chemistry, University
of Kalyani, Kalyani 741 235, W.B., India. E-mail: kcm_ku@yahoo.co.in
1747

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K. C. Majumdar and N. Kundu
a feature that has been rationalized[3] in terms of stereoelectronic control of the
reaction. Nevertheless, for stabilized radicals, the regiochemistry of ring
closure of 5-hexenyl radicals is reversed.[4]
Ring closure of hexenyl systems containing first-row atoms such as
oxygen and nitrogen have been studied intensively. However, a literature
search revealed only a few examples of the synthesis of sulfur-based hetero-
cycles via radical methods.[5] We recently reported the cyclization of 4-(20-
bromoaryloxymethyl)-1-methylquinolin-2(1H)-ones by nBu3SnH–AIBN,
where a 5-exo ring closure took place to give the spiro heterocyclic
compound.[6] We thus directed our attention to an analogous study of
sulfur-based heterocycle. Our objective was to examine, first, the effect of
longer C-S bonds for the cyclization of sulfides, sulfones, and sulfoxides,
and second, how the presence of these groups might influence the regiochem-
istry of ring closure. Ultimately, we were keen to determine whether this study
may lead to a viable procedure for the efficient synthesis of heterocyclic
compounds.
RESULTS AND DISCUSSION
The starting materials, 4-(20-bromothioarylmethyl)-1-methylquinolin-2(1H)-
ones 3 (X ¼ S), were prepared in 92–95% yield from 4-bromomethyl-1-
methylquinolin-2(1H)-one 1 and o-bromothiols 2a–c in refluxing acetone
for 8 h in the presence of anhydrous potassium carbonate and sodium
iodide. The corresponding sulfones 4 (X ¼ SO2) were synthesized from the
sulphides 3 by oxidizing with 2 equivalents of m-chloroperoxybenzoic acid
(m-CPBA) in refluxing dichloromethane for 1 h (Scheme 1). Compounds 3
and 4 were characterized from their elemental analysis and spectroscopic
data. The two-proton singlet due to -SCH2 protons of 3a at d 4.21 was
shifted by 0.67 ppm downfield in 4a. The substrate 3a was refluxed in dry
n
degassed toluene at 808C under a nitrogen atmosphere with Bu3SnH in the
presence of AIBN for 1 h to give the cyclic product 6a (X ¼ S) in 90%
yield. Exposure of the sulfone 4a to nBu3SnH under the same reaction
condition as described previously afforded the cyclized sulfone 6d
(X ¼ SO2) in 95% yield (Scheme 2).
The structures of the compounds 6 (X ¼ S, SO2) were readily elucidated
1
from H NMR spectroscopy. Compound 6a (X ¼ S) exhibited a one-proton
multiplet at d 3.42–3.46 and another one-proton doublet at d 3.89
(J ¼ 4.5 Hz) due to ring junction protons Hc and Hd respectively, whereas
the corresponding two protons in 6d (X ¼ SO2) exhibited a multiplet at
d 4.15–4.19 and a doublet at d 4.03 (J ¼ 5 Hz). The stereochemistry of the
ring fusion of the cyclic product can be surmised from the molecular model
(Dreiding model), which showed a strain-free cis rearrangement and also
from the small coupling constant values of the ring juncture protons of 6a
(X ¼ S, J ¼ 4.5 Hz) and 6d (X ¼ SO2, J ¼ 5 Hz). The 13C NMR spectrum

2H-Benzothiopyrano[3,2-c]quinolin-7(8H)-ones
1749
Scheme 1. Reagents and conditions: i) acetone, K2CO3, Nal, reflux, 8 h; ii) m-CPBA
(2 equiv.), CH2Cl2, reflux, 1 h; iii) m-CPBA (1 equiv.), CH2Cl2, stirring, 1 h, 0–58C.
of 6a,d (X ¼ S, SO2) also supported the proposed structure. The 13C NMR
chemical shifts as well as the multiplicity of compound 6a,d were established
by DEPT experiment. There are twelve protonated carbons, two CH3, one
CH2, and nine CH moieties. The mass spectra of compounds 6a and 6d
showed molecular ion peaks at m/z 295 and 327 (Mþ) respectively. All
these experiments clearly indicate that a 6-endo cyclization has taken place.
To test the generality of the reaction, substrates 3b,c and 4b,c were also
n
treated similarly with Bu3SnH and AIBN to give exclusively the 6-endo
cyclized product in 90–96% yield (Scheme 2).
We have also attempted to extend the radical cyclization reaction to the
sulphoxides 5a–c as well. Sulfoxides were prepared from the corresponding
sulfides by slow addition of m-CPBA at 0–58C in dichloromethane over
1 h. The formation of the sulfoxide was indicated by the disappearance of
starting material and appearance of a highly polar spot on thin-layer chromato-
graphy (TLC). The sulphoxides were found to be unstable during workup.

1750
K. C. Majumdar and N. Kundu
Scheme 2. Reagents and conditions: nBu3SnH, toluene, 808C, N2 atm, 1 h.
Therefore, no effort was made to characterize them. Dichloromethane was
removed under reduced pressure, and radical reaction under conditions
described previously was attempted. The starting materials, however, decom-
posed, and no cyclized product was obtained.
The formation of a six-membered sulfur heterocyclic ring can be
explained by the addition of a hydrogen radical to the intermediate radical
10, which in turn is formed from the aryl radical 7 by a 6-endo ring closure.
An alternative route, via 5-exo ring closure to generate the spiro heterocyclic
radical[7] 8 with a subsequent neophyl rearrangement,[8] has also been con-
sidered (Scheme 3).
However, the 5-exo cyclization to form the spiro heterocyclic radical 8
followed by a neophyl rearrangement is highly unlikely with the systems
studied at present. It is known that b-fragmentation of alkyl thiyl radicals
are very fast[9] (.108 s21) compared to neophyl-type rearrangements,
which are much slower[10] (about 103 s21–104 s21). Therefore, neophyl
rearrangement of radical 8 cannot compete with the b-fragmentation
reaction, which would lead to another product. The other reason is that the
intermediate tertiary radical 10 is more stable than the spiro heterocyclic
secondary radical 8. The stabilized conformational intermediate radical 10
gave preferably cis, the usual reduced product, and the dihydro heterocyclic
ring is isolated in good yield. All the starting materials regioselectively
gave the reduced six-membered heterocyclic ring by nBu3SnH–mediated
cyclization. The methodology described here is mild and interesting for its
simplicity.

2H-Benzothiopyrano[3,2-c]quinolin-7(8H)-ones
1751
Scheme 3.
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
(400 MHz, 500 MHz) and 13C NMR (125 MHz) spectra were recorded on
a
Bruker DPX-400 and Bruker DPX-500 spectrometer 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), Spectro-
chem, 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.
General Procedure for the Preparation of 3a–c
A mixture of 4-bromomethyl-N-methylquinolone (1, 5 mmol), 2-bro-
mothiols (2a–f, 5 mmol), anhydrous potassium carbonate (5 g), and
sodium iodide (20 mg) 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 removal of solvent was subjected to
column chromatography on silica gel using petroleum ether–EtOAc (7:3)
as eluant to give compounds 3a–c, which were recrystallized from
CHCl3–petroleum ether.

1752
K. C. Majumdar and N. Kundu
Data
Compound (3a)
Yield 94%, white solid, mp 108–1108C; IR (KBr) ymax: 2921, 2850, 1654,
1
746 cm21; UV (EtOH) lmax: 208, 333 nm; H NMR (400 MHz, CDCl3): dH
2.28 (s, 3H, ArCH3), 3.71 (s, 3H, NCH3), 4.21 (s, 2H, SCH2), 6.59 (s, 1H,
55CH), 6.97–7.90 (m, 7H, ArH); MS (m/z): 373, 375 (Mþ). Anal. calcd. for
C18H16BrNOS: C, 57.76; H, 4.31; N, 3.74%. Found: C, 57.48; H, 4.35; N, 3.82%.
Compound (3b)
Yield 92%, white solid, mp 99–1018C; IR (KBr) ymax: 2925, 1651, 747 cm21
;
1
UV (EtOH) lmax: 214, 229, 334 nm; H NMR (500 MHz, CDCl3): dH 1.21
(t, J ¼ 7.5 Hz, 3H, ArCH2CH3), 2.59 (q, J ¼ 7.5 Hz, 2H, ArCH2CH3), 3.72
(s, 3H, NCH3), 4.24 (s, 2H, SCH2), 6.57 (s, 1H, 55CH), 7.03–7.97 (m, 7H,
ArH); MS (m/z): 387, 389 (Mþ). Anal. calcd. for C19H18BrNOS: C, 58.77;
H, 4.67; N, 3.61%. Found: C, 59.06; H, 4.69; N, 3.27%.
Compound (3c)
Yield 95%, white solid, mp 98–1008C; IR (KBr) ymax: 2922, 2851, 1639,
1
743 cm21; UV (EtOH) lmax: 214, 333 nm; H NMR (400 MHz, CDCl3): dH
3.74 (s, 3H, NCH3), 4.29 (s, 2H, SCH2), 6.73 (s, 1H, 55CH), 7.10–7.91
(m, 8H, ArH); MS (m/z): 359, 361 (Mþ). Anal. calcd. for C17H14BrNOS:
C, 56.68; H, 3.92; N, 3.89%. Found: C, 56.99; H, 3.87; N, 4.03%.
General Procedure for the Preparation of 4a–c
The sulfide (3a–c, 5 mmol) and m-CPBA (10 mmol) were refluxed in CH2Cl2
(125 mL) for 1 h. The reaction mixture was cooled and washed with saturated
sodium bicarbonate solution, water, and finally brine. The CH2Cl2 extract was
then dried over anhydrous Na2SO4. The residual mass after removal of solvent
was subjected to column chromatography over silica gel using petroleum
ether–EtOAc (1:1) as eluant to give compounds 4a–c, which were recrystal-
lized from CHCl3–petroleum ether.
Data
Compound (4a)
Yield 95%, white solid, mp 147–1498C; IR (KBr) ymax: 2920, 1650, 1587,
1
752 cm21; UV (EtOH) lmax: 212, 228, 279, 336 nm; H NMR (500 MHz,

2H-Benzothiopyrano[3,2-c]quinolin-7(8H)-ones
1753
CDCl3): dH 2.41 (s, 3H, ArCH3), 3.68 (s, 3H, NCH3), 4.88 (s, 2H, SO2CH2),
6.43 (s, 1H,55CH), 7.18–7.97 (m, 7H, ArH); MS (m/z): 405, 407 (Mþ). Anal.
calcd. for C18H16BrNO3S: C, 53.21; H, 3.97; N, 3.45%. Found: C, 53.49; H,
3.86; N, 3.51%.
Compound (4b)
Yield 94%, white solid, mp 148–1508C; IR (KBr) ymax: 2928, 1646, 1588,
1319, 1154, 750 cm21; UV (EtOH) lmax: 231, 279, 340 nm; 1H NMR
(500 MHz, CDCl3): dH 1.23 (t, J ¼ 7.5 Hz, 3H, ArCH2CH3), 2.66
(q, J ¼ 7.5 Hz, 2H, ArCH2CH3), 3.68 (s, 3H, NCH3), 4.88 (s, 2H,
SO2CH2), 6.46 (s, 1H, 55CH), 7.20–7.95 (m, 7H, ArH); MS (m/z): 419,
421 (Mþ). Anal. calcd. for C19H18BrNO3S: C, 54.29; H, 4.32; N, 3.33%.
Found: C, 54.60; H, 4.20; N, 3.30%.
Compound (4c)
Yield 95%, white solid, mp 194–1968C; IR (KBr) ymax: 2922, 1642, 1584,
1
743 cm21; UV (EtOH) lmax: 206, 347, 343, 278 nm; H NMR (500 MHz,
CDCl3): dH 3.68 (s, 3H, NCH3), 4.91 (s, 2H, SO2CH2), 6.46 (s, 1H, 55CH),
7.30–7.95 (m, 8H, ArH); MS (m/z): 391, 393 (Mþ). Anal. calcd. for
C17H14BrNO3S: C, 52.05; H, 3.60; N, 3.57%. Found: C, 52.30; H, 3.47; N,
3.61%.
General Procedure for the Preparation of 6a–f
Tributyltin hydride (1.1 mmol) was added to a stirred solution of 3a–c and
4a–c (1 mmol) and azobisisobutyronitrile (0.5 mmol) in dry degassed
toluene at 808C (5 mL) under nitrogen. The mixture was heated under
reflux for 1 h and concentrated. 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 anhydrous Na2SO4. The residual mass after the removal of solvent was
subjected to column chromatography using petroleum ether–ethyl acetate
(9:1) and (7:3) as eluant to give cyclized product 6a–c and 6d–f respectively.
Data
Compound (6a)
Yield: 90%, viscous liquid; IR (neat) ymax: 2922, 2851, 1666, 758 cm21; UV
1
(EtOH) lmax: 211, 248 nm; H NMR (500 MHz, CDCl3): dH 2.29 (s, 3H,

1754
K. C. Majumdar and N. Kundu
ArCH3), 2.83 (dd, J ¼ 12.8, 3.4 Hz, 1H, SCH2), 3.14 (t, J ¼ 12.3 Hz, 1H,
SCH2), 3.40 (s, 3H, NCH3), 3.42–3.46 (m, 1H, CHc), 3.89 (d, J ¼ 4.5 Hz,
1H, CHd), 6.96–7.36 (m, 7H, ArH); 13C NMR (125 MHz, CDCl3): dC
20.84 (ArCH3), 27.76 (CHCH2), 30.07 (NCH3), 36.35 (SCH2), 45.55
(CHCO), 115.13 (ArCH), 123.43 (ArCH), 125.91 (ArCH), 127.80 (ArCH),
127.87 (ArCH), 127.92 (ArCH), 128.16 (ArCH), 128.52 (ArC), 128.94
(ArC), 133.33 (ArC), 133.80 (ArC), 139.23 (ArC), 169.10 (CO); MS (m/z):
295 (Mþ). Anal. calcd. for C18H17NOS: C, 73.19; H, 5.80; N, 4.74%.
Found: C, 72.90; H, 5.86; N, 4.61%.
Compound (6b)
Yield 92%, viscous liquid; IR (neat) ymax: 2925, 2870, 1674, 754 cm21; UV
(EtOH) lmax: 217, 249 nm; 1H NMR (500 MHz, CDCl3): dH 1.22
(t, J ¼ 7.5 Hz, 3H, ArCH2CH3), 2.60 (q, J ¼ 7.5 Hz, 2H, ArCH2CH3), 2.86
(dd, J ¼ 12.8, 3.4 Hz, 1H, SCH2), 3.17 (t, J ¼ 12.3 Hz, 1H, SCH2), 3.42
(s, 3H, NCH3), 3.46–3.49 (m, 1H, CHc), 3.94 (d, J ¼ 4.5 Hz, 1H, CHd),
7.00–7.37 (m, 7H, ArH); MS (m/z): 309 (Mþ). Anal. calcd. for
C19H19NOS: C, 73.75; H, 6.19; N, 4.53%. Found: C, 73.56; H, 6.21; N, 4.69%.
Compound (6c)
Yield 91%, viscous liquid; IR (neat) ymax: 2922, 2851, 1666, 758 cm21; UV
(EtOH) lmax: 211, 245 nm; 1H NMR (500 MHz, CDCl3): dH 2.85 (dd,
J ¼ 12.8, 3.4 Hz, 1H, SCH2), 3.16 (t, J ¼ 12.3 Hz, 1H, SCH2), 3.40 (s, 3H,
NCH3), 3.46–3.48 (m, 1H, CHc), 3.94 (d, J ¼ 4.5 Hz, 1H, CHd), 7.03–7.35
(m, 8H, ArH); MS (m/z) 281 (Mþ). Anal. calcd. for C17H15NOS: C, 72.57;
H, 5.37; N, 4.98%. Found: C, 72.82; H, 5.45; N, 4.95%.
Compound (6d)
Yield 95%, white solid, mp 214–2168C; IR (KBr) ymax: 2921, 1671, 1601,
1274, 772 cm21; UV (EtOH) lmax: 206, 254 nm; 1H NMR (500 MHz,
CDCl3): dH 2.44 (s, 3H, ArCH3), 3.29 (dd, J ¼ 13.8, 2.4 Hz, 1H, SO2CH2),
3.42 (s, 3H, NCH3), 3.51 (t, J ¼ 13.5 Hz, 1H, SO2CH2), 4.03 (d, J ¼ 5 Hz,
1H, CHd), 4.15–4.19 (m, 1H, CHc), 7.07–7.86 (m, 7H, ArH); 13C NMR
(125 MHz, CDCl3): dC 21.61 (ArCH3), 30.46 (NCH3), 36.06 (CHCH2),
45.29 (CHCO), 52.16 (SO2CH2), 115.54 (ArCH), 123.39 (ArCH), 124.20
(ArCH), 124.81 (ArCH), 127.89 (ArCH), 129.52 (ArCH), 130.26 (ArCH),
131.17 (ArC), 133.41 (ArC), 135.51 (ArC), 139.40 (ArC), 142.66 (ArC),
166.69 (ArCO); MS (m/z): 327 (Mþ). Anal. calcd. for C18H17NO3S: C,
66.03; H, 5.23; N, 4.28%. Found: C, 66.27; H, 5.29; N, 4.31%.

2H-Benzothiopyrano[3,2-c]quinolin-7(8H)-ones
Compound (6e)
1755
Yield: 96%, white solid, mp 210–2128C; IR (KBr) ymax: 2927, 1676, 1602,
1276, 758 cm21; UV (EtOH) lmax: 207, 254 nm; 1H NMR (400 MHz,
CDCl3): dH 1.24 (t, J ¼ 7.5 Hz, 3H, ArCH2CH3), 2.70 (q, J ¼ 7.5 Hz, 2H,
ArCH2CH3), 3.28 (dd, J ¼ 13.8, 2.4 Hz, 1H, SO2CH2), 3.41 (s, 3H, NCH3),
3.50 (t, J ¼ 13.5 Hz, 1H, SO2CH2), 4.04 (d, J ¼ 5 Hz, 1H, CHd), 4.15–
4.19 (m, 1H, CHc), 7.06–7.88 (m, 7H, ArH); MS (m/z): 341 (Mþ). Anal.
calcd. for C19H19NO3S: C, 66.84; H, 5.61; N, 4.10%. Found: C, 67.12; H,
5.24; N, 4.22%.
Compound (6f)
Yield 95%, white solid, mp 220–2228C; IR (KBr) ymax: 2921, 1667, 1601,
1274, 763 cm21; UV (EtOH) lmax: 209, 250 nm; 1H NMR (500 MHz,
CDCl3): dH 3.32 (dd, J ¼ 13.8, 2.4 Hz, 1H, SO2CH2), 3.42 (s, 3H, NCH3),
3.54 (t, J ¼ 13.5 Hz, 1H, SO2CH2), 4.09 (d, J ¼ 5 Hz, 1H, CHd), 4.18–
4.22 (m, 1H, CHc), 7.08–7.98 (m, 8H, ArH); MS (m/z): 313 (Mþ); Anal.
calcd. for C17H15NO3S: C, 65.16; H, 4.82; N, 4.47%. Found: C, 65.37; H,
4.86; N, 4.38%.
ACKNOWLEDGMENTS
N. Kundu is thankful to Council of Scientific and Industrial Research (CSIR)
(New Delhi) for a senior research fellowship. We also thank Department of
Science and Technology (DST) (New Delhi) for providing a UV-Vis spectro-
photometer and FT-IR spectrometer under the Department of Science and
Technology-Financial assistance for Infrastructural Development of Science
and Technology (DST-FIST) program.
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