7920
D. Mahajan et al. / Tetrahedron Letters 47 (2006) 7919–7921
of the imines are hygroscopic in nature and decompose
at high temperature, they are difficult to purify by distil-
lation or column chromatography and therefore we con-
templated performing both imine formation and Diels–
86
84
8
8
2
0
Alder reaction with DHP in the presence of SbCl –HAP
3
in one-pot. Benzaldehyde (10 mmol), aniline (10 mmol)
and SbCl –HAP (1.6 mol %, 3.1 g) were refluxed in dry
3
˚
acetonitrile in the presence of 4 A molecular sieves with
78
stirring under N . Complete formation of benzylid-
1
2
3
4
5
6
7
8
9
10 11
2
eneaniline was observed in 30 min and at this point
DHP (14 mmol) was added and the reaction mixture
was heated at reflux. To our delight, the imino-Diels–
Alder reaction was completed in 2 h resulting in the
formation of trans product as described in Scheme 1.
No. of times catalyst used
Figure 1. Results of recycling the catalyst.
Table 2. SbCl
3
–HAP catalyzed one pot stereoselective aza-Diels–Alder
reaction
In another set of experiments, the reaction was carried
14
a
out with SbCl alone and with SbCl –Al O , when
Entry
Substituents
Products Time Yield Melting
3
3
2
3
b
3
(h)
(%)
point (ꢁC)
the formation of both cis and trans stereoisomers was
observed (Table 1). The reaction did not occur with
HAP alone in refluxing acetonitrile, even after 10 h.
1
2
R
R
1
2
3
4
5
6
7
8
9
H
H
2-CH
4-F
4-Br
4-OCH3
4-CH
H
H
a
b
c
d
e
f
2.5
2.5
3.5
3.0
3.5
2.0
3.0
2.5
3.0
85
83
83
82
80
82
82
82
81
Oil
122
125
Oil
Oil
98–100
Oil
169–170
130–132
4-Cl
4-CH
H
H
H
H
4-NO
H
However since the use of SbCl –HAP resulted in the
3
3
3
formation of only the trans isomer, it follows that
HAP probably plays some role in the stereochemical
outcome of the reaction. The mechanism of the reaction
seems to be SbCl –HAP catalyzed formation of imine
3
g
h
i
3
followed by imino-Diels–Alder reaction.
2
2-CH
3
a
A notable feature of the SbCl –HAP catalyst is its reus-
Isolated yields.
Melting points are uncorrected.
3
b
ability. The catalyst was recovered and used ten times
for the reaction between p-tolualdehyde, o-toluidine
and DHP without any significant decrease in its effi-
ciency as shown in Figure 1. Substituted anilines and
benzaldehydes have also been used and the results are
shown in Table 2.
General procedure: A mixture of the appropriate benz-
aldehyde (10 mmol), aniline (10 mmol) and SbCl –HAP
3
(1.6 mol %, 3.1 g) was refluxed in acetonitrile (20 mL)
under nitrogen for about 30–60 min till the formation
of imine was complete (TLC). The reaction mixture
was cooled, DHP (14 mmol) was added to it and then
refluxed for another 1.5–3.0 h till the completion of
the reaction (TLC). Acetonitrile was distilled off under
reduced pressure, the reaction mixture was diluted with
ethyl acetate (70 mL) and filtered. The filtrate was
washed with brine (2 · 25 mL), dried over anhydrous
Na SO , concentrated under reduced pressure and the
In summary, we have shown that SbCl –HAP is an effi-
3
cient catalyst for both the imino-Diels–Alder reaction
between imines and DHP and the three component
one-pot reaction of araldehydes, anilines and DHP to
afford only the trans-pyrano[3,2-c]quinolines in good
yields. The catalyst has also been shown to be recover-
able and reusable.
2
4
residue was added to a small silica gel (60–120 mesh)
column and eluted with 8:2 mixtures of petroleum ether
and ethyl acetate to afford pure trans-pyrano[3,2-c]quin-
olines (81–85%). The spectral data of all the known
compounds were found to be identical with those of
9
1
0
8
R1
H
NH2
CHO
1
O1
0b
7
2
3
O
SbCl -HAP
3
H
4a
R1
+
+
N6 H
5
10,11
CH CN
3
4
R2
H
the authentic samples.
1
2
R , R = alkyl, halo, OMe, NO
2
The product 3c trans-7-methyl-5-(p-tolyl)-3,4,4a,5,6,
R2
1
0b-hexahydro-2H-pyrano[3,2-c]quinoline has the fol-
lowing physical and spectral data.
Scheme 1. Three-component reaction sequence.
À1
White crystalline; mp: 125 ꢁC. IR, mmax/cm
(KBr):
1
3
396, 2950, 1610, 1515, 1475. H NMR: (CDCl3,
Table 1. Comparison of results with SbCl
3
and SbCl
3
–Al
2
O
3
400 MHz) d 1.33 (1H, m), 1.48 (1H, m), 1.65 (1H, m),
.86 (1H, m), 2.04 (3H, s), 2.08 (1H, m), 2.38 (3H, s),
3.73 (1H, dt, J = 11.6, 2.4 Hz), 3.86 (1H, br s), 4.10
1H, td, J = 11.2, 2.2 Hz), 4.40 (1H, d, J = 2.4 Hz),
1
Entry
Substituents Catalyst
Time 4a,5
Overall
trans/cis yield (%)
1
2
(h)
R
R
(
1
2
3
2-CH
2-CH
2-CH
3
3
3
4-CH
4-CH
4-CH
3
3
3
SbCl
SbCl
SbCl
3
3
3
5.0
4.5
53:47
65:35
62
70
83
4
7
.74 (1H, d, J = 10.8 Hz), 6.65 (1H, t, J = 7.2 Hz),
2 3
–Al O
–HAP 3.5 100:0
.00 (1H, d, J = 7.2 Hz), 7.12 (1H, d, J = 7.2 Hz), 7.20
13
(2H, d, J = 8.0 Hz), 7.35 (2H, d, J = 8.0 Hz).
C