Copper (II) bromide-catalyzed imino Diels-Alder reaction: Synthesis of pyrano[3,2-c]- and furo [3,2-c]tetrahydroquinolines

Article abstract of DOI:10.1080/00397910500334595

Copper(II) bromide is found to be an effective catalyst for the imino Diels-Alder reaction between an imine (generated in situ) and an activated alkene in acetonitrile at the ambient temperature. The reaction proceeds smoothly to afford the corresponding adduct as a mixture of chromatographically separable cis- and trans-isomers in moderate to good yields. Copyright Taylor & Francis LLC.

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Copper(II) Bromide–Catalyzed
Imino Diels–Alder Reaction:
Synthesis of Pyrano[3,2‐c]‐ and Furo
[3,2‐c]tetrahydroquinolines
Abha Semwal a & Sandip K. Nayak a
a Bio‐organic Division, Bhabha Atomic Research Centre, Mumbai, India
Version of record first published: 15 Aug 2006
To cite this article: Abha Semwal & Sandip K. Nayak (2006): Copper(II) Bromide–Catalyzed Imino Diels–Alder
Reaction: Synthesis of Pyrano[3,2‐c]‐ and Furo [3,2‐c]tetrahydroquinolines, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 36:2, 227-236
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Synthetic Communicationsw, 36: 227–236, 2006
Copyright # Taylor & Francis LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500334595
Copper(II) Bromide–Catalyzed
Imino Diels–Alder Reaction: Synthesis
of Pyrano[3,2-c]- and Furo
[3,2-c]tetrahydroquinolines
Abha Semwal and Sandip K. Nayak
Bio-organic Division, Bhabha Atomic Research Centre, Mumbai, India
Abstract: Copper(II) bromide is found to be an effective catalyst for the imino Diels–
Alder reaction between an imine (generated in situ) and an activated alkene in
acetonitrile at the ambient temperature. The reaction proceeds smoothly to afford
the corresponding adduct as a mixture of chromatographically separable cis- and
trans-isomers in moderate to good yields.
Keywords: Copper(II) bromide, furoquinolines, imino Diels–Alder reaction,
pyranoquinolines
Substituted tetrahydroquinolines are core structures in many important
pharmaceutical agents and drug molecules.[1] They possess a wide range of
biological activities such as psychotropic,[2] anti-inflammatory,[3] anti-arrhyth-
mic,[4] estrogenic,[5] immunosupressive,[6] and anticancer properties.[7]
Therefore, many efforts have been made to synthesize substituted tetrahydro-
quinoline derivatives.
The Diels–Alder reaction is one of the most powerful methods for the
regio- and stereospecific preparation of carbocyclic and heterocyclic rings.
The imino Diels–Alder (IDA) reaction between N-aryl imines and nucleophi-
lic olefins provides an easy access to substituted tetrahydroquinolines.
Received in India July 11, 2005
Address correspondence to Sandip K. Nayak, Bio-organic Division, Bhabha Atomic
Research Centre, Mumbai 400 085, India. Fax: þ91(22) 25505151; E-mail: sknayak@
magnum.barc.ernet.in
227

228
A. Semwal and S. K. Nayak
[8]
.
Initially, BF3 OEt2 was used as the Lewis acid catalyst for this reaction.
However, in many cases, the Lewis acid–catalyzed IDA reaction cannot be
carried out with aromatic amines, aldehydes, and alkenes in one pot
because the water formed during imine formation leads to deactivation/
decomposition of the catalyst, requiring more than a stoichiometric amount
of the catalyst for the in situ reaction.[9] Subsequently, water-comptaible
Lewis acids such a InCl3,[10] GdCl3,[11] Yb(OTf)3,[12] Dy(OTf)3,[13] and so
forth, and metal carbonyls [(Co2(CO)8, Ni(CO)4][14] were found to be
efficient for this purpose. Still, there is a need to develop a new synthetic
recipe for IDA reaction using cheap and nontoxic catalysts. In a recent
report,[15] an asymmetric Diels–Alder reaction of 2-azadienes was carried
out with a chiral copper(II) complex, derived from copper(II) triflate and a
C2-symmetric bis (oxazoline) ligand. Keeping this in mind, we wanted to
explore the influence of copper halides as catalyst in IDA reaction because
they are usually cheap and nontoxic. CuBr2 has been extensively used in
a-bromination of carbonyls[16] and bromination of olefins and aromatics.[17]
However, its use in imino Diels–Alder reaction has not been reported
so far. Herein, we describe an efficient one-pot CuBr2-catalyzed
imino Diels–Alder reaction between the imine (prepared in situ) and 3,4-
dihydro-2H-pyran (DHP)/2,3-dihydrofuran (DHF) to furnish pyrano- and
furo-tetrahydroquinolines (Scheme 1) (Scheme 2).
In the initial experiment, the reaction of N-benzylideneaniline
(2.0 mmol), DHP (2.1 mmol), and CuBr2 (45.0 mg, 10 mol%) was carried
out in dry acetonitrile at room temperature. However, the expected pyrano-
tetrahydroquinoline adduct was obtained in only 30% yield as a mixture of
cis- and trans-isomers (3a and 4a) along with a large amount of unreacted
imine (63%). The structural assignment of cis- and trans-isomer was done
on the basis of the coupling constant between protons H4a and H5,[11]
because cis-isomer J(4a,5) ¼ 5.5 Hz, which is significantly smaller than the
coupling constant for trans-isomer J(4a,5) ¼ 10.7 Hz. After several exper-
iments, the optimum amount of the catalyst needed for the completion
Scheme 1.

Imino Diels–Alder Reaction
229
Scheme 2.
of the reaction was found to be 50 mol%, which furnished the adducts
(3a/4a ¼ 51/49) in 74% total yield within 30 min.
Most of the imines are hygroscopic, unstable at higher temperatures, and
difficult to purify by distillation or column chromatography. From a synthetic
point of view, it is therefore desirable to generate the imine in situ for its
subsequent reaction with dienophiles. In this context, the CuBr2
(50 mol%)–catalyzed one-pot reaction of benzaldehyde and aniline with
DHP was carried out in acetonitrile at an ambient temperature (258C) and a
smooth reaction was observed. The product was isolated in a comparable
yield (76%) to that obtained with preformed imine (Table 1, entry 1) prefer-
entially as the trans-isomer. However, the in situ reaction required more
time to reach completion. This result shows retention of the activity of the
CuBr2 under the reaction conditions although deactivation of copper(II)
salts by aromatic amines (due to complexation) is well known.
The effect of the solvent and additive on the yield and stereoisomeric ratio
of the in situ reaction was also investigated. Use of MgSO4 as an additive in
the reaction of benzaldehyde, aniline, and DHP showed a marginal improve-
ment in the yield. Among the different solvents investigated, acetonitrile was
found to be the best in terms of both yield and stereoselectivity (Table 1).
Table 1. Effect of solvents and additive on the CuBr2–catalyzed imino Diels–Alder
reaction of benzaldehyde, aniline, and 3,4-dihydro-2H-pyrana
Time
(h)
Yield (%)
(3a þ 4a)
Entry
Solvent
Additive
Cis/trans
1
2
3
4
5
6
Acetonitrile
Acetonitrile
Dichloromethane
Tetrahydrofuran
Dioxan
None
2
6
76
78
39
68
55
36
21/79
34/66
21/79
52/48
33/67
29/71
MgSO4
MgSO4
None
24
20
72
72
None
None
Toluene
aAll the reactions were carried out with 50 mol% of CuBr2 at ambient temperature.

230
A. Semwal and S. K. Nayak
To check the generality of the protocol, a variety of aromatic amines,
aromatic aldehydes, and two electron-rich olefins (DHP and DHF) were
used, and the results are summarized in Table 2. In all the cases, the
reaction proceeded smoothly to furnish the desired tetrahydroquinoline
derivatives in 46–76% yield as mixtures of cis- and trans-isomers, which
were easily separated by preparative or column chromatography. The struc-
tural assignment of cis- and trans-isomers was done on the basis of
coupling constant between protons H4a and H5 for pyrano-tetrahydroquino-
lines and between protons H3a and H4 for furo-tetrahydroquinolines;[11] the
coupling constant for cis-isomers were significantly smaller than the
coupling constant for trans-isomers.
In conclusion, we have developed an easy protocol for the synthesis of
pyrano- and furo-tetrahydroquinolines using CuBr2 as the catalyst under
mild reaction conditions. The method offers several advantages including
the use of a cheap and nontoxic catalyst. Moreover, the protocol does not
require the isolation of N-aryl imines. Deactivation of the catalyst by
amines and water is a major shortcoming in transition metal–catalyzed
organic reactions. In the present studies, the in situ generation of imines
from aromatic amines and aldehydes in the presence of CuBr2 clearly
indicated no such deactivation. Use of CuBr2 in other Lewis acid–catalyzed
organic reactions is currently in progress.
EXPERIMENTAL
Melting points were taken on a Fisher-Johns melting-point apparatus and
are uncorrected. The FT-IR spectra were scanned with a Jasco FT-IR
Table 2. CuBr2–catalyzed one-pot synthesis of pyrano- and furo-
tetrahydroquinolinesa
Time
(h)
Productsb
(total % yield)
Cis/
trans
Entry
Amine
Aldehyde
Alkene
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1a
1a
1b
1c
2a
2a
2a
2a
2a
2b
2a
2a
2a
DHP
DHP
DHP
DHP
DHP
DHP
DHF
DHF
DHF
2
3a þ 4a (76)
3b þ 4b (46)
3c þ 4c (52)
3d þ 4d (57)
3e þ 4e (71)
3f þ 4f (58)
5a þ 6a (68)
5b þ 6b (52)
5c þ 6c (49)
21/79
37/63
16/84
26/74
25/75
38/62
47/53
38/62
14/86
2.25
4.75
4.5
3
3
2.5
4.8
4.2
aAll the reactions were carried out with 50 mol% of CuBr2 in acetonitrile at ambient
temperature.
bIsolated yields after preparative or column chromatography.

Imino Diels–Alder Reaction
231
spectrophotometer. The 1H NMR spectra were recorded with a Bruker AC 200
(200-MHz) spectrometer. Mass spectra were recorded with a Shimadzu mass
spectrometer. All reactions were carried out under an argon atmosphere.
Acetonitrile was dried and distilled over P2O5.
General Procedure for CuBr2–Catalyzed Imino
Diels–Alder Reaction
A mixture of the aromatic aldehyde (3 mmol), aromatic amine (3.1 mmol), and
CuBr2 (0.335 g, 1.5 mmol) in dry acetonitrile (15 mL) was stirred at room
temperature for the time specified in Tables 1 and 2. The reaction mixture
was concentrated in vacuo, the residue taken in Et2O, and the extract succes-
sively washed with water and brine and dried (Na2SO4). Removal of solvent
followed by purification of the crude product by preparative thin-layer chrom-
atography or column chromatography (silica gel, hexane/ethyl acetate)
yielded the respective cis- and trans-tetrahydroquinoline derivatives. In all
the cases, trace amounts (3–5%) of reduced imines were also isolated. All
the compounds were characterized with their reported spectral data.
(4aa,5b,10ba)-3,4,4a,5,6,10b-Hexahydro-5-phenyl-2H-pyrano[3,2-c]quinoline
(3a): yield 16%, solid. Mp 130–1318C (lit.[11] mp 128.8–1318C). IR
(CHC13): v ¼ 3375, 3325, 2941, 1608, 1479, 1089 cm21
.
1H NMR
(200 MHz, CDC13): d ¼ 1.23–1.56 (m, 4H), 2.09–2.28 (m, 1H), 3.35–3.69
(m, 2H), 3.93 (br s, 1H), 4.70 (td, J ¼ 2.3 Hz, 1H), 5.36 (d, J ¼ 5.5 Hz,
1H), 6.63 (d, J ¼ 7.9 Hz, 1H), 6.83 (t, J ¼ 7.3 Hz, 1H), 7.13 (t, J ¼ 7.5 Hz,
1H), 7.27–7.51 (m, 6H). 13C NMR (50 MHz, CDCl3): d ¼ 17.9, 25.3, 38.8,
59.2, 60.5, 72.6, 114.3, 118.1, 119.7, 126.7, 127.4, 128.0, 128.3, 141.0,
145.1. MS: 265 (Mþ).
(4aa,5a,10ba)-3,4,4a,5,6,10b-Hexahydro-5-phenyl-2H-pyrano[3,2-c]quinoline
(4a).[11] yield 60%, viscous oil. IR (CHC13): v ¼ 3379, 2939, 1611, 1492,
1
1071 cm21. H NMR (200 MHz, CDCl3): d ¼ 1.25–2.02 (m, 4H), 2.04–
2.21 (m, 1H), 3.76 (td, J ¼ 11.2, 2.4 Hz, 1H), 3.97–4.25 (m, 2H), 4.43
(d, J ¼ 2.5 Hz, 1H), 4.73 (d, J ¼ 10.7 Hz, 1H), 6.54 (d, J ¼ 8.0 Hz, 1H),
6.75 (t, J ¼ 7.1 Hz, 1H), 7.13 (t, J ¼ 7.0 Hz, 1H), 7.27 (d, J ¼ 7.5 Hz, 1H),
7.33–7.56 (m, 5H). 13C NMR (50 MHz, CDCl3): d ¼ 21.8, 23.8, 38.6, 54.5,
68.2, 74.1, 113.9, 117.0, 120.3, 127.5, 128.3, 129.0, 130.0, 131.1, 142.1,
144.5. MS: 265 (Mþ).
(4aa,5b,10ba)-3,4,4a,5,6,10b-Hexahydro-9-methoxy-5-phenyl-2H-pyrano[3,2-
c]quinoline (3b): yield 17%, solid. Mp 146–1478C (lit.[11] mp 144–1468C).
1
IR (CHCl3): v ¼ 3369, 2939, 1492, 1230, 1039 cm21. H NMR (200 MHz,
CDCl3): d ¼ 1.28–1.76 (m, 4H), 2.10–2.15 (m, 1H), 3.40–3.57 (m, 3H),
3.76 (s, 3H), 4.57 (s, 1H), 5.29 (d, J ¼ 5.3 Hz, 1H), 6.55 (d, J ¼ 8.6 Hz,

232
A. Semwal and S. K. Nayak
1H), 6.73 (d, J ¼ 8.6 Hz, 1H), 7.04 (s, 1H), 7.25–7.48 (m, 5H). 13C NMR
(50 MHz, CDCl3): d ¼ 17.7, 25.0, 38.8, 55.7, 59.3, 60.5, 72.6, 111.5, 114.5,
115.3, 121.0, 126.7, 127.1, 128.0, 139.0, 141.0, 152.5. MS: 295 (Mþ).
(4aa,5a,10ba)-3,4,4a,5,6,10b-Hexahydro-9-methoxy-5-phenyl-2H-pyrano[3,2-
c]quinoline (4b):[11] yield 29%, solid. Mp 99–1008C (lit.[11] mp 98–1008C).
1
IR (CHCl3): v ¼ 3365, 2939, 1505, 1228, 1033 cm21. H NMR (200 MHz,
CDCl3): d ¼ 1.20–2.19 (m, 5H), 3.56–4.20 (m, 6H), 4.36 (br s, 1H), 4.63
(d, J ¼ 10.4 Hz, 1H), 6.47 (d, J ¼ 8.4 Hz, 1H), 6.65–6.88 (m, 2H), 7.15–
7.60 (m, 5H). 13C NMR (50 MHz, CDCl3): d ¼ 21.5, 23.5, 38.4, 54.5, 55.1,
67.7, 73.8, 114.2, 115.0, 116.0, 120.7, 127.2, 127.9, 138.5, 141.9, 151.2.
MS: 295 (Mþ).
(4aa,5b,10ba)-3,4,4a,5,6,10b-Hexahydro-9-methyl-5-phenyl-2H-pyrano[3,2-c]
quinoline (3c): yield 8%, solid. Mp 141–1428C (lit.[18a] mp 142–1438C). IR
1
(CHCl3): v ¼ 3377, 2939, 1493, 1071 cm21. H NMR (200 MHz, CDCl3):
d ¼ 1.11–1.72 (m, 4H), 2.16 (m, 1H), 2.27 (s, 3H), 3.5–3.57 (m, 3H), 4.64
(s, 1H), 5.30 (d, J ¼ 5.4 Hz, 1H), 6.53 (d, J ¼ 8.0 Hz, 1H), 6.91
(d, J ¼ 8.3 Hz, 1H), 7.25–7.55 (m, 6H). 13C NMR (50 MHz, CDCl3):
d ¼ 17.5, 20.2, 25.0, 38.7, 59.0, 60.2, 72.4, 114.1, 119.6, 126.5, 127.0,
127.2, 127.4, 128.1, 128.3, 139.9, 142.4. MS: 279 (Mþ).
(4aa,5a,10ba)-3,4,4a,5,6,10b-Hexahydro-9-methyl-5-phenyl-2H-pyrano[3,2-c]
quinoline (4c):[18a] yield 44%, viscous oil. IR (CHCl3): v ¼ 3384, 2937, 1509,
1084 cm21
.
1H NMR (200 MHz, CDCl3): d ¼ 1.36–2.14 (m, 5H), 2.34
(s, 3H), 3.78 (t, J ¼ 10.4 Hz, 1H), 4.01 (br s, 1H), 4.16–4.21 (m, 1H), 4.42
(d, J ¼ 2.3 Hz, 1H), 4.71 (d, J ¼ 10.7 Hz, 1H), 6.48 (d, J ¼ 8.1 Hz, 1H),
6.99 (d, J ¼ 7.9 Hz, 1H), 7.14 (s, 1H), 7.40–7.48 (m, 5H). 13C NMR
(50 MHz, CDCl3): d ¼ 20.0, 21.6, 23.6, 38.5, 54.4, 67.9, 74.0, 113.8, 114.7,
120.1, 125.8, 127.3, 128.0, 129.5, 130.6, 131.5, 142.1. MS: 279 (Mþ).
(4aa,5b,10ba)-3,4,4a,5,6,10b-Hexahydro-9-bromo-5-phenyl-2H-pyrano[3,2-c]
quinoline (3d):[18b] yield 15%, solid. Mp 134–1368C. IR (CHCl3): v ¼ 3374,
3324, 2938, 1597, 1485, 1069 cm21. 1H NMR (200 MHz, CDC13): d ¼ 1.19–
1.62 (m, 4H), 2.02–2.26 (m, 1H), 3.32–3.72 (m, 2H), 3.97 (br s, 1H), 4.64
(d, J ¼ 2.4 Hz, 1H), 5.26 (d, J ¼ 5.5 Hz, 1H), 6.48 (d, J ¼ 8.5 Hz, 1H), 7.17
(dd, J ¼ 8.5, 1.9 Hz, 1H), 7.27–7.48 (m, 5H), 7.54 (d, J ¼ 1.3 Hz, 1H). 13C
NMR (50 MHz, CDCl3): d ¼ 17.9, 25.2, 38.4, 59.1, 60.7, 72.2, 110.0,
115.9, 121.9, 126.7, 127.6, 128.4, 130.7, 140.5, 144.1. MS: 344 (Mþ).
(4aa,5a,10ba)-3,4,4a,5,6,10b-Hexahydro-9-bromo-5-phenyl-2H-pyrano[3,2-c]
quinoline (4d):[18b] yield 42%, solid. Mp 129–1318C. IR (CHCl3): v ¼ 3406,
2941, 1603, 1489, 1216, 1069 cm21. 1H NMR (200 MHz, CDCl3): d ¼ 1.06–
2.28 (m, 5H), 3.70 (td, J ¼ 10.7, 2.4 Hz, 1H), 3.98–4.21 (m, 2H), 4.34
(d, J ¼ 2.7 Hz, 1H), 4.65 (d, J ¼ 10.6 Hz, 1H), 6.39 (d, J ¼ 8.6 Hz, 1H),

Imino Diels–Alder Reaction
233
7.16 (dd, J ¼ 8.6, 2.3 Hz, 1H), 7.28–7.48 (m, 6H). 13C NMR (50 MHz,
CDCl3): d ¼ 22.1, 25.2, 38.5, 54.7, 68.4, 73.7, 108.5, 115.6, 122.3, 127.6,
127.9, 128.6, 131.9, 133.6, 141.8, 143.7. MS: 344 (Mþ).
(4aa,5b,10ba)-3,4,4a,5,6,10b-Hexahydro-7-methyl-5-phenyl-2H-pyrano[3,2-c]
quinoline (3e): yield 18%, solid. Mp 144–1458C (lit.[11] mp 143–1448C). IR
(CHCl3): v ¼ 3404, 2940, 1600 cm21
.
1H NMR (200 MHz, CDCl3):
d ¼ 1.17–1.84 (m, 5H), 2.22 (s, 3H), 3.27–3.91 (m, 3H), 4.77
(d, J ¼ 2.0 Hz, 1H), 5.44 (d, J ¼ 5.4 Hz, 1H), 6.81 (t, J ¼ 7.5 Hz, 1H), 7.08
(d, J ¼ 7.2 Hz, 1H), 7.31–7.65 (m, 6H). 13C NMR (50 MHz, CDCl3):
d ¼ 17.4, 17.9, 25.3, 38.6, 59.1, 60.5, 72.8, 117.6, 119.3, 121.4, 125.3,
126.8, 127.5, 128.3, 129.0, 141.3, 143.1. MS: 279 (Mþ).
(4aa,5a,10ba)-3,4,4a,5,6,10b-Hexahydro-7-methyl-5-phenyl-2H-pyrano[3,2-c]
quinoline (4e): yield 53%, solid. Mp 130–1318C (lit.[11] mp 130–1328C). IR
(CHCl3): v ¼ 3409, 2934, 1604 cm21
.
1H NMR (200 MHz, CDCl3):
d ¼ 1.21–2.05 (m, 5H), 2.13 (s, 3H), 3.81 (td, J ¼ 11.4, 2.2 Hz, 1H), 3.99
(br s, 1H), 4.10–4.32 (m, 1H), 4.48 (d, J ¼ 2.5 Hz, 1H), 4.85
(d, J ¼ 10.9 Hz, 1H), 6.75 (t, J ¼ 7.4 Hz, 1H), 7.09 (d, J ¼ 7.2 Hz, 1H),
7.22 (d, J ¼ 7.4 Hz, 1H), 7.32–7.63 (m, 5H). 13C NMR (50 MHz, CDCl3):
d ¼ 17.1, 21.8, 23.9, 38.6, 54.7, 68.5, 74.7, 116.7, 119.9, 120.9, 127.7,
128.5, 128.6, 130.1, 142.5, 142.6. MS: 279 (Mþ).
(4aa,5b,10ba)-3,4,4a,5,6,10b-Hexahydro-5-(4-chlorophenyl)-2H-pyrano[3,2-c]
quinoline (3f): yield 22%, solid. Mp 168–1698C (lit.[18c] mp 168.4–1698C. IR
1
(CHCl3): v ¼ 3348, 2938, 1487, 1216 cm21. H NMR) (200 MHz, CDCl3):
d ¼ 1.20–1.66 (m, 4H), 2.00–2.20 (m, 1H), 3.33–3.70 (m, 2H), 3.95 (br s,
1H), 4.63 (d, J ¼ 1.5 Hz, 1H), 5.30 (d, J ¼ 5.3 Hz, 1H), 6.62 (d, J ¼ 7.9 Hz,
1H), 6.82 (t, J ¼ 7.4 Hz, 1H), 7.12 (t, J ¼ 7.5 Hz, 1H), 7.26–7.51 (m, 5H).
13C NMR (50 MHz, CDCl3): d ¼ 17.7, 25.2, 38.7, 58.6, 60.4, 72.4, 114.4,
118.3, 119.7, 127.4, 128.0, 128.3, 132.9, 139.5, 144.8. MS: 299 (Mþ).
(4aa,5a,10ba)-3,4,4a,5,6,10b-Hexahydro-5-(4-chlorophenyl)-2H-pyrano[3,2-c]
quinoline (4f): yield 36%, solid. Mp 139–1408C (lit.[18c] mp 139.2–140.18C).
IR (CHCl3): v ¼ 3405, 2941, 1488, 1216 cm21. 1H NMR (200 MHz, CDCl3):
d ¼ 1.33–1.49 (m, 2H), 1.60–1.85 (m, 2H), 2.01–2.06 (m, 1H), 3.74
(t, J ¼ 10.4 Hz, 1H), 4.08–4.14 (m, 2H), 4.39 (br s, 1H), 4.68
(d, J ¼ 10.7 Hz, 1H), 6.53 (d, J ¼ 8.0 Hz, 1H), 6.73 (t, J ¼ 7.3 Hz, 1H),
7.11 (t, J ¼ 7.6 Hz, 1H), 7.24 (d, J ¼ 7.3 Hz, 1H), 7.36 (s, 4H). 13C NMR
(50 MHz, CDCl3): d ¼ 21.7, 23.7, 38.5, 53.8, 68.0, 73.9, 113.9, 117.2,
120.2, 128.4, 128.8, 130.5, 132.9, 140.6, 144.3. MS: 299 (Mþ).
(3aa,4a,9ba)-2,3,3a,9b-Tetrahydro-4-phenyl-2H-furo[3,2-c]quinoline (5a):
yield 32%, solid. Mp 94–958C (lit.[11] 958C). IR (CHCl3): v ¼ 3369, 3326,
2876, 1611, 1482 cm21
(m, 1H), 2.18–2.23 (m, 1H), 2.76–2.80 (m, 1H), 3.74–3.78 (m, 3H), 4.71
.
1H NMR (200 MHz, CDCl3): d ¼ 1.41–1.54

234
A. Semwal and S. K. Nayak
(d, J ¼ 2.8 Hz, 1H), 5.28 (d, J ¼ 7.8 Hz, 1H), 6.60 (d, J ¼ 8.0 Hz, 1H), 6.80
(t, J ¼ 7.4 Hz, 1H), 7.10 (t, J ¼ 8.0 Hz, 1H), 7.25–7.46 (m, 6H). 13C NMR
(50 MHz, CDCl3): d ¼ 24.6, 45.8, 57.3, 66.7, 75.8, 114.8, 119.2, 122.6,
126.4, 127.4, 128.3, 128.5, 130.1, 142.1, 144.8. MS: 251 (Mþ).
(3aa,4b,9ba)-2,3,3a,9b-Tetrahydro-4-phenyl-2H-furo[3,2-c]quinoline (6a).[11]
yield 36%, viscous oil. IR (CHCl3): v ¼ 3366, 3334, 2874, 1612, 1482 cm21
.
1H NMR (200 MHz, CDCl3): d ¼ 1.68–1.81 (m, 1H), 1.94–2.12 (m, 1H),
2.42–2.53 (m, 1H), 3.78–3.92 (m, 2H), 3.93–4.21 (m, 2H), 4.62
(d, J ¼ 5.0 Hz, 1H), 6.64 (d, J ¼ 8.0 Hz, 1H), 6.83 (t, J ¼ 7.1 Hz, 1H), 7.16
(t, J ¼ 7.0 Hz, 1H), 7.3–7.46 (m, 6H). 13C NMR (50 MHz, CDCl3):
d ¼ 28.6, 43.2, 57.5, 65.0, 76.0, 114.5, 118.1, 119.8, 128.0, 128.1, 128.5,
128.8, 131.1, 141.5, 145.3. MS: 251 (Mþ).
(3aa,4a,9ba)-2,3,3a,9b-Tetrahydro-8-methoxy-4-phenyl-2H-furo[3,2-c]quino-
line (5b): yield 20%, solid. Mp 131–1338C (lit.[11] mp: 132–1338C. IR
1
(CHCl3): 3388, 2930, 1504, 1365, 1250 cm21. H NMR (200 MHz, CDCl3):
d ¼ 1.41–1.54 (m, 1H), 2.18–2.23 (m, 1H), 2.76–2.80 (m, 1H), 3.51–3.81
(m, 6H), 4.50 (br s, 1H), 5.18 (d, J ¼ 7.8 Hz, 1H), 6.51 (d, J ¼ 8.6 Hz, 1H),
6.70 (m, 1H), 6.95 (m, 1H), 7.25–7.46 (m, 5H). 13C NMR (50 MHz,
CDCl3): d ¼ 24.3, 45.7, 55.5, 57.7, 66.8, 76.1, 113.4, 115.7, 116.1, 123.2,
126.4, 127.4, 128.5, 138.8,142.2, 152.9; MS: 281 (Mþ).
(3aa,4b,9ba)-2,3,3a,9b-Tetrahydro-8-methoxy-4-phenyl-2H-furo[3,2-c]quino-
line (6b): yield 32%, solid. Mp 95–968C (lit.[11] mp 94–968C), IR (CHCl3):
3320, 2970, 1485 cm21. 1H NMR (200 MHz, CDCl3): d ¼ 1.64–1.77 (m, 1H),
1.91–2.09 (m, 1H), 2.42–2.53 (m, 1H), 3.69–4.08 (m, 7H), 4.59
(d, J ¼ 5.3 Hz, 1H), 6.57 (d, J ¼ 8.7 Hz, 1H), 6.76 (dd, J ¼ 8.7, 2.8 Hz,
1H), 6.98 (d, J ¼ 2.7 Hz, 1H), 7.35–7.51 (m, 5H). 13C NMR (50 MHz,
CDCl3): d ¼ 28.7, 43.5, 55.7, 58.3, 65.3, 76.4, 114.5, 115.9, 116.5, 120.8,
127.9, 128.1, 128.5, 139.4, 141.6, 152.4; MS: 281 (Mþ).
(3aa,4a,9ba)-2,3,3a,9b-Tetrahydro-8-methyl-4-phenyl-2H-furo[3,2-c]quinoline
(5c): yield 7%, solid. Mp 102–1038C (lit.[18d] mp 101–1028C). IR
(CHCl3): 3351, 2875, 1621, 1507 cm21 1H NMR (200 MHz, CDC13):
;
d ¼ 1.41–1.54 (m, 1H), 2.18–2.23 (m, 1H), 2.30 (s, 3H), 2.76–2.80
(m, 1H), 3.74–3.78 (m, 3H), 4.60 (d, J ¼ 2.8 Hz, 1H), 5.28
(d, J ¼ 8.0 Hz, 1H), 6.68 (m, 1H), 7.02 (m, 1H), 7.23 (d, J ¼ 7.5 Hz,
1H), 7.31–7.52 (m, 5H). 13C NMR (50 MHz, CDC13): d ¼ 20.8, 28.9,
44.0, 62.2, 64.5, 76.1, 114.8, 128.2, 129.0, 129.9, 130.0, 130.7, 131.8,
136.3, 140.5, 145.0; MS: 265 (Mþ).
(3aa,4b,9ba)-2,3,3a,9b-Tetrahydro-8-methyl-4-phenyl-2H-furo[3,2-c]quinoline
(6c): yield 42%, solid. Mp 91–928C (lit.[18d]mp 90–928C). IR (CHCl3): 3376,

Imino Diels–Alder Reaction
235
2850, 1622, 1507 cm21. 1H NMR (200 MHz, CDCl3): d ¼ 1.67–1.80 (m, 1H),
1.92–2.10 (m, 1H), 2.32 (s, 3H), 2.40–2.53 (m, 1H), 3.73–3.91 (m, 2H),
4.00–4.19 (m, 2H), 4.60 (d, J ¼ 5.1 Hz, 1H), 6.56 (d, J ¼ 8.1 Hz, 1H),
6.99 (dd, J ¼ 9.0, 1.7 Hz, 1H), 7.25–7.28 (m, 1H), 7.38–7.46 (m, 5H).
13C NMR (50 MHz, CDC13): d ¼ 20.5, 28.8, 43.5, 57.9, 65.1, 76.2,
114.8, 120.1, 127.3, 128.0, 128.3, 128.5, 129.6, 131.4, 142.0, 143.2; MS:
265 (Mþ).
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