Indium chloride catalyzed intramolecular cyclization of N-aryl imines: synthesis of pyrrolo[2,3-d]pyrimidine annulated tetrahydroquinoline derivatives

Article abstract of DOI:10.1016/j.tetlet.2008.02.105

The intramolecular aza Diels-Alder cyclization reaction of aldimines derived from aromatic amines and N-prenyl/cinnamyl derivatives of pyrrolo[2,3-d]pyrimidine were efficiently catalyzed by InCl₃ to afford the corresponding tetrahydroquinoline

Full text of DOI:10.1016/j.tetlet.2008.02.105

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2583–2587
Indium chloride catalyzed intramolecular cyclization
of N-aryl imines: synthesis of pyrrolo[2,3-d]pyrimidine
annulated tetrahydroquinoline derivatives
Ekambaram Ramesh, Raghavachary Raghunathan *
Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India
Received 30 November 2007; revised 9 February 2008; accepted 18 February 2008
Available online 11 March 2008
Abstract
The intramolecular aza Diels–Alder cyclization reaction of aldimines derived from aromatic amines and N-prenyl/cinnamyl deriva-
tives of pyrrolo[2,3-d]pyrimidine were efficiently catalyzed by InCl3 to afford the corresponding tetrahydroquinoline derivatives in good
yields.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Imines; Tetrahydroquinoline; Pyrrolo[2,3-d]pyrimidine; Indium chloride
1. Introduction
Hence, new and efficient syntheses of such compounds are
still important.
The intramolecular aza Diels–Alder reaction is a useful
synthetic tool for constructing N-containing six-membered
heterocycles such as tetrahydroquinolines, octahydroacri-
dines, tetrahydrochromanoquinolones and dihydro-4-pyr-
idones.1–5 Of these, the tetrahydroquinoline skeleton is
found in a number of alkaloids and derivatives thereof
and is found to exhibit a wide range of biological activi-
ties,5–7 for example, indersine, oricine and veprisine and
their derivatives exhibit psychotropic, anti-allergic, anti-
inflammatory and estrogenic activities.8–11
Pyrrolo[2,3-d]pyrimidine derivatives are reported to
possess various biological activities such as anti-HCV,
anti-HIV type 1, anti-HSV, adenosine kinase inhibition,
Aurora-A kinase inhibition and cAMP phosphodiesterase
inhibition.12–15 Many naturally occurring compounds such
as mycalisine A, cadeguomycin and 2-deoxycadeguomycin
are found to possess a pyrrolo[2,3-d]pyrimidine moiety.16,17
In continuation of our research18–20 on the development
of highly expedient methods for the synthesis of hetero-
cyclic compounds of biological importance, we disclose
here our preliminary investigations on the indium chloride
promoted synthesis of novel pyrrolo[2,3-d]pyrimidine
annulated tetrahydroquinoline derivatives through intra-
molecular aza Diels–Alder reaction of the N-alkenylimines
of 1,3-dimethyl-1-pyrrolo[2,3-d]pyrimidine-2,4-dione-6-carb-
aldehyde 1.
The substrate N-alkenyl 1,3-dimethyl-1-pyrrolo[2,3-d]-
pyrimidine-2,4-dione-6-carbaldehydes 3a–b were prepared in
good yields (70–80%) from 1,3-dimethyl-1-pyrrolo[2,3-d]-
pyrimidine-2,4-dione-6-carbaldehyde 1 by treatment with
1-bromo-3-methyl-but-2-ene 2a or cinnamyl bromide 2b
in dry DMF in the presence of K2CO3 (Scheme 1).21 The
same reaction when carried out in 10% aqueous sodium
hydroxide in the presence of a catalytic amount of PTC
resulted in a lower yield of the products (40–58%).
Reaction of N-prenyl aldehyde 3a with aromatic amines
4a–f in the presence of indium trichloride generated the
corresponding imine in situ which then underwent
intramolecular aza Diels–Alder cycloaddition in one-pot
*
Corresponding author. Fax: +91 44 22300488.
E-mail address: ragharaghunathan@yahoo.com (R. Raghunathan).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.105

2584
E. Ramesh, R. Raghunathan / Tetrahedron Letters 49 (2008) 2583–2587
R1
Br
2a, 2b
O
O
N
R2
N
N
O
N
N
CHO
R2
K2CO3, DMF, rt, 12 h
O
N
CHO
H
1
a) R1, R2 =CH3
b) R1=H; R2=Ph
R1
3a, 3b
Scheme 1.
reaction to yield a mixture of cis- and trans-tetrahydro-
quinoline derivatives in moderate yield (55–65%) (route a).
However, in the absence of the catalyst reaction of alde-
hyde 3a with aniline derivatives 4a–f in refluxing ethanol
yielded an isolable imine which could be cyclized in the
presence of InCl3 to give better yields of the products in
short reaction times (85–93%) (route b) (Scheme 2).
The intermediate imines were characterized on the basis
Fig. 1. Ortep diagram of compound 5e.
Further, to examine the effect of substitution on the
dienophile on the cycloaddition process, we prepared N-
cinnamyl-pyrrolo[2,3-d]pyrimidine-6-carbaldehyde 3b as
previously described (Scheme 1). Aldehyde 3b underwent
intramolecular cycloaddition with 4a–f in the presence of
indium trichloride to afford the corresponding cis- and
trans-cycloadducts 9a–f and 10a–f in good yields (87–90%).
The structures of cycloadducts 9a and 10a were esta-
1
of H NMR, 13C NMR and elemental analysis. The struc-
ture of imine 5e was further confirmed by X-ray crystal
analysis22 (Fig. 1).
In an effort to demonstrate the utility of the InCl3 cata-
lyzed aza Diels–Alder cyclization, the reaction was carried
out with anilines with varied substituents and the results
are summarized in Table 1.
1
blished by the examination of their H NMR spectra and
NOE experiments. The enhanced yields and short reaction
times for the formation of products 9a–f and 10a–f relative
to the cycloaddition of 3a with 4a–f in acetonitrile, clearly
showed the greater reactivity of the phenyl-substituted die-
nophile in 3b (Scheme 3).
In all cases the products were obtained as a mixture of
cis- and trans-isomers, which were separated by column
chromatography using silica gel. The ratios of the products
were determined from the H NMR spectra of the crude
products.
1
From the above results, it can be seen that the cycli-
zation proceeds by a stepwise mechanism as shown in
Scheme 4.23–25 However, under thermal conditions without
any Lewis acid, cyclization of the N-arylimines was not
observed. This further confirms the stepwise mechanism.
To study the scope and limitations of the cycloaddition
reaction, the same reactions were carried out with the
Lewis acid catalysts, namely, BF3ÁOEt2, Yb(OTf)3,
Sc(OTf)3 and InCl3. Indium trichloride proved to be very
efficient as we found the overall yield of the products was
high (90%) when compared to the other Lewis acid cata-
lysts. The results obtained for the cycloaddition of 3a
The structures assigned to cycloadducts 6a–f and 7a–f
were confirmed by the examination of their respective H
1
NMR spectra. The Ha proton of 6a appeared as a doublet
at d 4.77 (J = 6.3 Hz). This small coupling constant is con-
sistent with a cis-diaxial relationship for these two protons.
Furthermore, the stereochemistry of Ha was confirmed by
the observation of a strong NOE enhancement of Ha upon
the irradiation of Hb (8.1%).
In a similar way, the Ha proton of 7a exhibited a dou-
blet at d 4.63 (J = 10.6 Hz) indicating the trans stereochem-
istry of Ha with respect to Hb (Scheme 2).
O
O
N
O
N
NH2
InCl3 20 mol %
CH3CN
N
N
N
Ha
NH
Ha
NH
O
N
N
CHO
O
N
O
N
route a
R
3a
Hb
Hb
4a-f
EtOH
reflux
R
cis
R
trans
6a-f
7a-f
O
N
route b
InCl3 20 mol %
CH3CN
O
N
N
R = H, Me, OMe, Cl, Br, NO2
N
R
5a-f
Scheme 2.

E. Ramesh, R. Raghunathan / Tetrahedron Letters 49 (2008) 2583–2587
2585
Table 1
Indium chloride catalyzed reaction of 3a/3b with 4a–f in the presence of InCl3
O
O
N
N
Ha
NH
Ha
NH
Entry
R
R1
R2
Time (h)
Ratio cis:trans
Overall yield (%)
O
N
N
O
N
N
Hb
R1
Hb
R1
R2
R2
R
R
1
2
3
4
5
6
7
8
9
H
CH3
CH3
CH3
CH3
CH3
H
H
H
H
H
CH3
CH3
CH3
CH3
CH3
CH3
Ph
Ph
Ph
Ph
Ph
6a
6b
6c
6d
6e
6f
7a
7b
4.0
3.0
2.5
3.5
4.0
5.0
3.0
2.5
3.0
3.0
2.5
3.5
73:27
80:20
90:10
84:16
86:14
76:24
88:12
84:16
82:18
90:10
92:8
90
85
87
92
94
93
87
91
85
93
94
90
Me
OMe
Cl
Br
NO2
H
Me
OMe
Cl
Br
NO2
7c
7d
7e
7f
9a
9b
9c
9d
9e
9f
10a
10b
10c
10d
10e
10f
10
11
12
H
H
Ph
90:10
O
O
O
N
NH2
N
InCl3 20 mol %
CH3CN
N
Ha
NH
Ha
NH
O
N
N
CHO
O
N
N
O
N
N
route a
R
4a-f
Hb
Ph
3b
Hb
Ph
H
Ph
H
cis
trans
R
R
EtOH
re
9a-f
10a-f
flux
O
N
route b
InCl3 20 mol %
CH3CN
O
N
N
H
R = H, Me, OMe, Cl, Br, NO2
N
Ph
Scheme 3.
R
8a-f
O
N
O
O
N
N
N
InCl3
InCl3
InCl3
N
O
N
O
N
N
N
O
N
N
N
R1
R2
R1
R2
R1
R2
12
11
O
N
O
N
O
N
N
-InCl3
InCl3
H
N
O
N
O
N
NH
N
H
InCl3
O
N
N
N
H
R1
R1
R2
R2
R1
2 H
R
15
14
13
Scheme 4. Proposed mechanism for the formation of the products.
and 4a in the presence of various Lewis acid catalysts are
summarized in Table 2.
The cis-isomer is prevalent in all of the cases studied. In
the cycloaddition step of the reaction the cis products result

2586
E. Ramesh, R. Raghunathan / Tetrahedron Letters 49 (2008) 2583–2587
Table 2
(ethyl acetate–hexane) to afford cis- and trans-tetrahydro-
quinolines in good overall yields.
Influence of Lewis acids on the reaction of 3a with 4a
Entry
Lewis acid
Reaction time (h)
Overall yield (%)
3.1. 7a,13b-cis-8,8-dimethyl pyrrolo[2,3-d]pyrimidine-
[2,1-a]pyrrolo[4,3:2,3]-7a,8,13,13b-tetrahydroquinoline 6a
1
2
3
4
BF3ÁOEt2
Yb(OTf)3
Sc(OTf)3
InCl3
8
5
6
3
62
78
72
90
1
Mp: 177 °C, H NMR (300 MHz, CDCl3): d 1.39 (s,
3H), 1.45 (s, 3H), 3.10 (dt, J = 9.0, 10.0 Hz, 1Hb), 3.38
(s, 3H), 3.57 (s, 3H), 3.70 (t, J = 10.0 Hz, 1Hd), 4.25 (dd,
J = 7.7, 9.0 Hz, 1Hc), 4.77 (d, J = 6.3 Hz, 1Ha), 5.30 (s,
1H), 6.69 (dd, J = 1.2, 6.9 Hz, 1H), 7.00 (d, J = 1.5 Hz,
1H), 7.16 (dd, J = 1.5, 7.8 Hz, 1H), 13C NMR (75 MHz,
CDCl3): d 25.62, 26.71, 29.51, 32.36, 35.16, 36.42, 55.69,
61.01, 100.52, 105.37, 120.13, 120.78, 123.95, 124.65,
131.39, 132.45, 138.19, 140.37, 154.52, 159.81 ppm. MS:
m/z: 350 (M+). Anal. Calcd for C20H22N4O2: C, 68.55;
H, 6.33; N, 15.99. Found: C, 68.76; H, 6.46; N, 15.81.
from the favourited endo transition states, imine is in the
favoured E-configuration and the reaction is influenced
by the bulky nature of the two alkyl groups.
Similarly, the yield proved to be strongly solvent depen-
dent and with acetonitrile as solvent, the overall yields of
the tetrahydroquinolines 6a–f and 7a–f were good.
In summary, this work describes our first attempt to use
N-alkenyl pyrrolo[2,3-d] pyrimidine derivatives as useful
precursors for the synthesis of pyrrolo[2,3-d]pyrimidine-
annulated tetrahydroquinolines through InCl3 mediated
aza Diels–Alder reactions.
3.2. 7a,13b-trans-8,8-dimethylpyrrolo[2,3-d]pyrimidine-
[2,1-a]pyrrolo[4,3:2,3]-7a,8,13,13b-tetrahydroquinoline 7a
Mp: 174 °C, IR (KBr): 1678, 3394 cmÀ1 1H NMR
;
2. General procedure for the synthesis of imines 5a–f and
8a–f
(300 MHz, CDCl3): d 1.47 (s, 3H), 1.53 (s, 3H), 3.20 (td,
J = 9.7, 10.4 Hz, 1Hb), 3.41 (s, 3H), 3.52 (s, 3H), 3.55 (t,
J = 10.4 Hz, 1Hd), 4.11 (dd, J = 7.5, 9.7 Hz, 1Hc), 4.63
(d, J = 10.6 Hz, 1Ha), 5.20 (s, 1H), 6.49–7.10 (m, 4H).
13C NMR (75 MHz, CDCl3): d 24.72, 25.18, 27.82, 30.14,
34.59, 37.31, 52.77, 60.00, 99.32, 106.89, 121.15, 121.79,
122.13, 125.25, 130.30, 133.72, 139.33, 141.10, 152.77,
160.93 ppm. MS: m/z: 350 (M+). Anal. Calcd for
C20H22N4O2: C, 68.55; H, 6.33; N, 15.99. Found: C,
68.71; H, 6.42; N, 15.84.
A solution of 3a,b (1 mmol) in ethanol (10 mL) was
added dropwise to a solution of p-substituted aniline 4a–f
(1 mmol) in ethanol (5 mL). The mixture was heated at
reflux for 4 h. The resulting solution was allowed to cool
to room temperature and then was cooled in an ice-water
bath for 0.5 h. Filtration provided the corresponding imi-
nes 5a–f and 8a–f.
2.1. 1,3-Dimethyl-7-(3-methylbut-2-enyl)-6-phenylimino-
pyrrolo[2,3-d]pyrimidine-2,4-dione 5a
3.3. 7a,13b-cis-8,8-dimethyl-10-methylpyrrolo[2,3-d]-
pyrimidine[2,1-a]pyrrolo[4,3:2,3]-7a,8,13,13b-
tetrahydroquinoline 6b
1
Mp: 162 °C, H NMR (300 MHz, CDCl3): d 1.56 (s,
Mp: 178 °C, IR (KBr): 1689, 3389 cmÀ1 1H NMR
,
3H), 1.59 (s, 3H), 3.39 (s, 3H), 3.52 (s, 3H), 5.17 (d,
J = 5.0 Hz, 2H), 5.59 (t, J = 5.0 Hz, 1H), 6.94–7.35 (m,
5H), 6.93 (s, 1H), 8.14 (s, 1H). 13C NMR (75 MHz,
CDCl3): d 18.31, 24.52, 25.69, 29.31, 30.30, 104.84,
105.83, 112.52, 123.67, 123.78, 125.86, 130.37, 132.98,
133.00, 133.49, 136.85, 152.56, 158.24, 159.80. MS: m/z:
350.13 (M+). Anal. Calcd for C20H22N4O2: C, 68.55; H,
6.33; N, 15.99. Found: C, 68.68; H, 6.40; N, 15.86.
(300 MHz, CDCl3): d 1.39 (s, 3H), 1.41 (s, 3H), 2.24 (s,
3H), 3.10 (dt, J = 9.7, 10.0 Hz, 1Hb), 3.27 (s, 3H), 3.32
(s, 3H), 3.47 (t, J = 10.0 Hz, 1Hd), 4.17 (dd, J = 7.8,
9.7 Hz, 1Hc), 4.63 (d, J = 6.5 Hz, 1Ha), 5.20 (s, 1H), 6.27
(d, J = 8.1 Hz, 1H), 6.53 (s, 1H), 6.71 (d, J = 8.1 Hz, 1H)
13C NMR (75 MHz, CDCl3): d 25.39, 26.14, 26.75, 30.13,
31.32, 36.77, 37.40, 53.91, 62.76, 102.25, 107.53, 121.01,
121.75, 123.32, 124.81, 130.00, 131.05, 137.14, 141.52,
152.13, 160.34 ppm. MS: m/z: 364.19 (M+). Anal. Calcd
for C21H24N4O2: C, 69.21; H, 6.64; N, 15.37. Found: C,
68.39; H, 6.51; N, 15.48.
3. General procedure for the synthesis of tetrahydroquinoline
derivatives
InCl3 (20 mol %) was added to a mixture of substituted
anilines (1 mmol) and 3a or 3b (1 mmol) in acetonitrile
(5 mL). The reaction mixture was stirred at 55–60 °C until
the completion of the reaction as indicated by TLC; the
mixture was then quenched with water and extracted with
ethyl acetate. The organic layer was washed with water and
dried over MgSO4. The solvent was evaporated in vacuo
and the crude product was chromatographed on silica gel
3.4. 7a,13b-trans-8,8-dimethyl-10-methylpyrrolo[2,3-d]-
pyrimidine[2,1-a]pyrrolo[4,3:2,3]-7a,8,13,13b-
tetrahydroquinoline 7b
Mp: 180 °C, IR (KBr): 1673, 3381 cmÀ1 1H NMR
(300 MHz, CDCl3): d 1.42 (s, 3H), 1.45 (s, 3H), 2.29 (s,
3H), 3.17 (td, J = 9.6, 10.1 Hz, 1Hb), 3.30 (s, 3H), 3.35

E. Ramesh, R. Raghunathan / Tetrahedron Letters 49 (2008) 2583–2587
2587
15. Klumpp, S.; Frey, M.; Kleefeld, G.; Sauer, A.; Eger, K. Biochem.
Pharmacol. 1989, 38, 949–953.
16. Wade, A. E.; Krawczyk, S. H.; Townsend, B. L. Tetrahedron Lett.
1988, 29, 4073–4076.
17. Edstrom, E. D.; Wei, Y. Tetrahedron Lett. 1994, 35, 8989–8990.
18. Rathna Durga, R.; Jayashankaran, J.; Raghunathan, R. Tetrahedron
Lett. 2006, 48, 7571–7574.
19. Rathna Durga, R.; Jayashankaran, J.; Raghunathan, R. Tetrahedron
Lett. 2007, 48, 4139–4142.
(s, 3H), 3.51 (t, J = 10.1 Hz, 1Hd), 4.20 (dd, J = 7.8,
9.6 Hz, 1Hc), 4.55 (d, J = 11.5 Hz, 1Ha), 5.25 (s, 1H),
6.31 (d, J = 8.2 Hz, 1H), 6.59 (s, 1H), 7.01 (d, J = 8.2 Hz,
1H). 13C NMR (75 MHz, CDCl3): d 24.14, 27.81, 28.65,
31.32, 31.98, 36.25, 36.93, 54.13, 60.27, 100.22, 102.75,
122.10, 123.17, 124.33, 124.48, 129.00, 130.10, 136.17,
143.15, 151.21, 159.03 ppm. Anal. Calcd for C21H24N4O2:
C, 69.21; H, 6.64; N, 15.37. Found: C, 68.29; H, 6.48; N,
15.29.
20. Ramesh, E.; Elamparuthi, E.; Raghunathan, R. Synth. Commun.
2006, 36, 1431–1436.
21. Experimental procedure for compounds 3a/3b: 1,3-Dimethyl-2,4-dioxo-
1H-pyrrolo[2,3-d]pyrimidine-6-carbaldehyde 1 (10 mmol) in DMF
(20 mL) was treated with solid K2CO3 (16 mmol) and 3,3-dimethyl
allylbromide/cinnamyl bromide (12 mmol). The mixture was stirred
overnight at 20 °C, then water (50 mL) was added. The aqueous layer
was extracted with ethyl acetate (4 Â 20 mL), the combined organic
layers were dried (MgSO4) and the solvent was removed in vacuo and
the crude product subjected to column chromatography (100–200
mesh) using hexane–ethyl acetate (8:2) as eluent. Compound 3a: Pale
yellow solid, mp: 152 °C; 1H NMR (400 MHz, CDCl3): 1.75 (s, 3H),
2.01 (s, 3H), 3.41 (s, 3H), 3.78 (s, 3H), 5.18 (d, J = 4.5 Hz, 2H), 5.33
(t, J = 4.5 1H), 7.38 (s, 1H), 9.49 (s, 1H); 13C NMR (100 MHz,
CDCl3): d 20.2, 24.3, 30.4, 30.1, 32.4, 104.8, 117.7, 117.9, 130.9, 132.2,
150.4, 158.8, 147.5, 178.6; mass m/z: 275 (M+); Anal. Calcd for
C14H17N3O3: C, 61.09; H, 6.18; N, 15.27. Found: C, 60.77; H, 6.14; N,
15.23. Compound 3b: Pale yellow solid, mp: 170 °C; 1H NMR
(400 MHz, CDCl3): d 3.42 (s, 3H), 3.82 (s, 3H), 5.54 (d, J = 4.8 Hz,
2H), 6.22 (d, J = 16.1 Hz, 1H), 6.35 (dt, J = 16.1 Hz, 1H), 7.26–7.30
(m, 5H), 7.45 (s, 1H), 9.52 (s, 1H); 13C NMR (100 MHz, CDCl3): d
29.5, 33.6, 31.5, 104.5, 117.7, 126.4 128.0, 128.7, 128.9, 130.5, 131.0,
133.2, 133.4, 135.2, 146.6, 150.1, 157.3, 178.3 ppm; mass m/z: 323
(M+); Anal. Calcd for C18H17N3O3: C, 66.87; H, 5.26; N, 13.00.
Found: C, 66.87; H, 5.26; N, 13.04.
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