Unprecedented SnCl2·2H2O-mediated intramolecular cyclization of nitroarenes via C-N bond formation: a new entry to the synthesis of cryptotackieine and related skeletons

Article abstract of DOI:10.1016/j.tetlet.2008.09.154

A mild, efficient, and one-pot protocol for the intramolecular cyclization of 2-substituted nitroarenes via C-N bond formation using SnCl₂·2H₂O is described. The versatility of the method has been demonstrated by synthesizing two sets of polycyclic structures based on privileged structures of indole and pyrrole, and of the alkaloid cryptotackieine associated with antimalarial activity. Our new approach provides a powerful entry into polycyclic structures related to an alkaloid.

Full text of DOI:10.1016/j.tetlet.2008.09.154

Tetrahedron Letters 49 (2008) 7062–7065
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Unprecedented SnCl2ꢀ2H2O-mediated intramolecular cyclization
of nitroarenes via C–N bond formation: a new entry to the synthesis
of cryptotackieine and related skeletons q
*
Sunil Sharma, Bijoy Kundu
Medicinal Chemistry Division, Central Drug Research Institute, Lucknow 226 001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A mild, efficient, and one-pot protocol for the intramolecular cyclization of 2-substituted nitroarenes via
C–N bond formation using SnCl2ꢀ2H2O is described. The versatility of the method has been demonstrated
by synthesizing two sets of polycyclic structures based on privileged structures of indole and pyrrole,
and of the alkaloid cryptotackieine associated with antimalarial activity. Our new approach provides a
powerful entry into polycyclic structures related to an alkaloid.
Received 14 August 2008
Revised 23 September 2008
Accepted 26 September 2008
Available online 1 October 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Cryptotackieine
Indoloquinoline
Pyrroloquinoline
SnCl2-Mediated reductive cyclization
In recent years, N-containing polycyclic structures based on
privileged templates1 have drawn significant interest owing to
their ability to act as natural product-like chemical probes2 that
may be useful for drug discovery. Besides structural analogy to
natural products, other interesting features involve: (1) rigid and
planar molecular frameworks; (2) hybrid structures derived from
two to three privileged structures; (3) annulation of heterocyclic
templates leading to new polycyclic motifs and (4) generation of
almost unlimited combinations of annulated heterocyclic struc-
tures, resulting in polycyclic skeletons with diverse physical,
chemical and biological properties. In the literature, synthetic
N-rich fused polyheterocyclic systems have been reported to be
associated with a wide range of biological activities3 including
antimalarial activity.4 Therefore, development of an efficient meth-
od for the construction of fused polyheterocyclic ring systems in
few steps is highly desirable, particularly in the field of medicinal
chemistry.
Recently, we disclosed a novel procedure for generating annu-
lated polycyclic structures using the modified Pictet–Spengler
reaction.5 The latter is a reaction where an aryl amine, when linked
covalently to either an activated or deactivated heterocyclic ring,
after forming an iminium ion with an aldehyde in the presence
of a Bronsted acid, furnishes intermediates having both nucleo-
philic (Nu) and electrophilic (El) centres. The resulting intermedi-
ate eventually leads to the formation of polycyclic structures via
intramolecular (endo) cyclization. We have successfully applied
this modified strategy5 for the synthesis of several novel N-con-
taining polycyclic motifs with combinations of 5-, 6- and 7-mem-
bered rings in a given skeleton.
During one such study, we proposed to apply our modified ap-
proach for the Pictet–Spengler reaction to bisindole derivatives
having a covalently linked aryl amine, based on substrate 1, with
the view to generate bisindole-based polycyclic structures 2
(Fig. 1) for our ongoing antimalarial programme. Bisindole motifs
are known to be widely distributed in nature and are important
building blocks for a variety of biologically active natural prod-
ucts.6 An attempt to obtain 2 was made by treating indole with
o-nitrobenzaldehyde to give 3 followed by reduction of the aryl
nitro group with SnCl2ꢀ2H2O. However, to our surprise, reduction
of the NO2 group in 3a furnished a new product 4a within an hour
in greater than 95% purity (HPLC) and in >85% isolated yield,
instead of the expected product 1.
Chemical characterization of the resulting product using NMR
and HRMS led to the identification of a polycyclic structure, 6H-
indolo[2,3-b]quinoline 4a. A literature search revealed a close
structural resemblance to cryptotackieine 5 (5-N-methyl-5H-indo-
lo[2,3-b]quinoline; Fig. 1), an alkaloid with antimalarial activity
isolated from the West African shrub Cryptolepis sanguinolenta.7
The alkaloid 5 was found to be a N-methyl derivative of the linear
indolo[2,3-b]quinoline ring system. A careful analysis on the for-
mation of our product 4a from 3a revealed that an intramolecular
cyclization via C–N bond formation may have occurred resulting in
q
CDRI communication number 7633.
* Corresponding author. Tel.: +91 522 2612411 18x4383; fax: +91 522 2623405.
E-mail address: bijoy_kundu@yahoo.com (B. Kundu).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.154

S. Sharma, B. Kundu / Tetrahedron Letters 49 (2008) 7062–7065
7063
us to study the scope and limitation of our strategy by carrying
out the synthesis of a mini-library based on 6H-indolo[2,3-b]quin-
olines 4 and pyrroloquinolines 7 followed by application of our
strategy for the synthesis of cryptotackieine 5.
Initially, we synthesized four congeners based on 4b–e by vary-
ing the indole and o-nitrobenzaldehyde (Scheme 1). The synthesis
commenced with the formation of 3a–e by treating indole with o-
nitrobenzaldehyde in the presence of iodine.10 The resulting nitro
derivatives 3b–e were then treated with SnCl2ꢀ2H2O to give 4b–e
in high yields.11
HN
NH
HN
NH
NH2
(ii)
R
N
HN
2
1
NH (i)
NO2
After successfully demonstrating the efficacy of our strategy on
indoles, we expanded the repertoire of substrates for generating
polycyclic structures by replacing indole (benzopyrrole) with pyr-
role (Scheme 2). The synthesis commenced with the formation of
nitro intermediates 6 by treating pyrrole with o-nitrobenzaldehyde
derivatives using the procedure reported in the literature.12 The
resulting nitro derivatives 6a–e were then treated with SnCl2ꢀ2H2O
in methanol under reflux to give 7a–e in moderate to excellent
yields.
HN
3a
9
10
1
N
11
N
H3C
8
2
7
3
N
N
54a
6
cryptotackieine 5
H
4
Figure 1. Reagents and conditions: (i) SnCl2ꢀ2H2O, MeOH, reflux and (ii) RCHO,
p-TsOH, toluene, reflux.
We also extended our strategy to the synthesis of the indole-
based natural product cryptotackieine 5 with the view to probe
the substitution at position 11 in 4a–e. Our synthesis (Scheme 3)
for 5 commenced with the synthesis of nitro intermediate 8 by
treating indole with 2-nitrobenzyl bromide in the presence of
Na2CO3.13 The resulting product 8 was then subjected to reduction
by refluxing in methanol in the presence of SnCl2ꢀ2H2O. The crude
product was a mixture of three components as was evident by both
HPLC and TLC. The product with tR = 14.035 min (Rf = 0.31, 9:1 hex-
ane/EtOAc) corresponded to the desired tetracyclic 6H-indolo-
[2,3-b]quinoline8c 9. The second component with tR = 13.194 min
(Rf = 0.28, 9:1 hexane/EtOAc) was characterized as the amine 10,
a reduced product of 8. The third component with tR = 19.434 min
(Rf = 0.56, 9:1 hexane/EtOAc) was characterized as spiroindole 11.
The isolation of stable spiroindole 11 in traces, during the reductive
cyclization, is probably the first experimental evidence in support
for the plausible mechanism14 where the C-3 in the indole has
been implicated as the most preferred position for initial intramo-
lecular electrophilic attack thereby forming an unstable ‘spiroindo-
lenin’,14a which then quickly undergoes rearrangement to the C-2
of the indole. Thus, reductive cyclization of 8 in the absence of
a tetracyclic structure. Interestingly, catalytic reduction of the nitro
substrate 3a using Pd–C furnished amine 1 (Fig. 1) as the only
product without any detectable cyclization (4a) as evident by
HPLC. This suggests that formation of product 4a could result from
either of the nitroso and hydroxylamine intermediates formed dur-
ing reduction of the nitro group using SnCl2. Although Pd-catalyzed
reduction (Pd/C) also proceeds via the same intermediate, it is pos-
sible that conversion of amines may be occurring on the catalyst’s
surface without concomitant release of intermediates at any stage,
thus furnishing amine 1 and not the cyclized product 4a.
A careful survey of the literature revealed a few reports dealing
either with the synthesis of 6H-indolo[2,3-b]quinolines8 (a precur-
sor to cryptotackieine) or with the synthesis of cryptotackieine 5.9
Timári et al.9e have described the synthesis of cryptotackieine in
five steps in which the key intermediate, 3-(2-aminophenyl)-N-
methyl-2-quinolone, was obtained via palladium-catalyzed cross-
coupling between a 3-bromo-2-quinolone derivative and (2-N-
pivaloylaminophenyl)boronic acid. Molina et al.9b reported an
eight-step synthesis of cryptotackieine 5 via a key intermediate,
1-methyl-3-(o-azidophenyl)quinolin-2-one. This intermediate
was directly converted into cryptotackieine via an intramolecular
aza-Wittig reaction with trimethylphosphine. Wang and co-work-
ers8c reported biradical-mediated thermolysis of N-[2-(1-alkyn-
yl)phenyl]-N0-phenylcarbodiimides, obtained via aza-Wittig
reaction between 4-methoxyphenyl isocyanate and iminophos-
phoranes, leading to the corresponding 6H-indolo[2,3-b]quinoline
(a precursor of cryptotackieine). Ila and co-workers9f described a
five-step synthesis of cryptotackieine 5 using conjugate addition
of an enolate anion from cyclohexanone (or 4-methylcyclohexa-
none) to bis[(methylsulfanyl)-methylene]-2-oxindole followed by
heterocyclization in the presence of ammonium acetate. The 11-
methylsulfanyl group in the initial precursor was then either
desulfurized (Raney Ni) or replaced by methyl/phenyl groups via
a nickel-catalyzed cross-coupling reaction using appropriate Grig-
nard reagents. Recently, Tilve and co-workers9g reported an inter-
esting method for the synthesis of cryptotackieine via a Perkin
reaction, a tandem double reduction-double cyclization reaction
followed by methylation at the quinoline nitrogen. Thus, the re-
ported strategies for cryptotackieine not only involved multi-step
synthetic routes for both precursors based on 4 and 5, but also
involved harsh and stringent reaction conditions. In contrast, our
method is based on a two-step procedure involving mild reaction
conditions for intramolecular cyclization. Nevertheless, 5, despite
exhibiting strong antimalarial activity, has not yet been subjected
to lead optimization studies through analogue synthesis. This led
R
CHO
R1
HN
R
NH
NO2
O2N
(i)
+
N
H
R
R1
3a-e
HN
R
R
(ii)
R1
N
N
H
4a-e
Entry
R
R1
Product
Yield
(%)
89
91
80
Mass
HPLC
tR=min
22.675
21.277
19.483
18.460
21.294
(M++H)
334.6
368.4
352.4
394.3
428.3
1
2
3
4
5
H
H
H
5-OMe
5-OMe
H
4a
4b
4c
4d
4e
5-Cl
5-F
H
82
84
5-Cl
Scheme 1. Reagents and conditions: (i) I2, CH3CN, 20 min; (ii) SnCl2ꢀ2H2O, MeOH,
reflux.

7064
S. Sharma, B. Kundu / Tetrahedron Letters 49 (2008) 7062–7065
CHO
Finally, for the synthesis of crytotackinene 5, 9 was subjected to
O2N
+
(i)
N-methylation using CH3I in toluene to give the desired N-methyl-
N
N
H
NO2
ated product9f 5 in excellent yield.
H
R
N
H
In conclusion, we have described the first example of
SnCl2ꢀ2H2O-mediated intramolecular cyclization of 2-substituted
nitroarenes via C–N bond formation under mild conditions. The
generality of the method has been established by synthesizing
two sets of polycyclic structures based on indole and pyrrole, and
the alkaloid cryptotackieine. Thus, our new approach provides a
powerful entry into polycyclic structures related to an alkaloid.
R
6a-e
NH
(ii)
H
N
N
R
7a-e
Acknowledgements
Entry
R
Product
Isolated
Yield
(%)
87
92
81
Mass
HPLC
tR=min
(M++H)
The authors are thankful to Dr. A.K. Mandwal and SAIF, for the
spectral data. SS is thankful to CSIR, New Delhi India for a senior
research fellowship.
1
2
3
4
5
H
5-Cl
5-F
7a
7b
7c
7d
7e
234.4
268.3
252.4
264.2
294.2
15.327
18.249
16.495
18.440
15.978
3-OMe
4,5-Di-OMe
53
51
References and notes
1. (a) Evans, B. E.; Rittle, K. E.; Bock, M. G.; DiPardo, R. M.; Freidinger, R. M.;
Whitter, W. L.; Lundell, G. F.; Veber, D. F.; Anderson, P. S.; Chang, R. S. L.; Lotti,
V. J.; Cerino, D. J.; Chen, T. B.; Kling, P. J.; Kunkel, K. A.; Springer, J. P.; Hirshfield,
J. J. Med. Chem. 1988, 31, 2235–2246; (b) Horton, D. A.; Bourne, G. T.; Smythe,
M. L. Chem. Rev. 2003, 103, 893–930; (c) Costantino, L.; Barlocco, D. Curr. Med.
Chem. 2006, 13, 763–771.
Scheme 2. Reagents and conditions: (i) Cat. TFA, N2, rt, 30 min; (ii) SnCl2ꢀ2H2O,
MeOH, reflux.
2. David, F.; Abderaouf, A.; Gerardo, D.; Pablo, C.; Marta, M.; Olga, P.; Carmen, C.;
Fernando, A.; John, A. J.; Mercedes, Á. Monatsh. Chem. 2004, 135, 615–627.
3. (a) Bräse, S.; Gil, C.; Knepper, K. Bioorg. Med. Chem. 2002, 10, 2415–2437; (b)
Balaban, A. T.; Oniciu, D. C.; Katritzky, A. R. Chem. Rev. 2004, 104, 2777–2812;
(c) Chen, J.-J.; Fang, H.-Y.; Duh, C.-Y.; Chen, I.-S. Planta Med. 2005, 71, 470–475;
(d) Nicolaou, K. C.; Baran, P. S.; Zhong, Y.-L.; Sugita, K. J. Am. Chem. Soc. 2002,
124, 2212–2220.
4. (a) Nyangulu, J. M.; Hargreaves, S. L.; Sharples, S. L.; Mackay, S. P.; Waigh, R. D.;
Duval, O.; Mberu, E. K.; Watkins, W. M. Bioorg. Med. Chem. Lett. 2005, 15, 2007–
2010; (b) Joshi, A. A.; Viswanathan, C. L. Bioorg. Med. Chem. Lett. 2006, 16, 2613–
2617; (c) Joshi, A. A.; Narkhede, S. S.; Viswanathan, C. L. Bioorg. Med. Chem. Lett.
2005, 15, 73–76.
Br
(i)
O2N
NH
NO2
+
N
H
8
(ii)
5. (a) Kundu, B.; Sawant, D.; Chhabra, R. J. Comb. Chem. 2005, 7, 317–321; (b)
Kundu, B.; Sawant, D.; Partani, P.; Kesarwani, A. P. J. Org. Chem. 2005, 70, 4889–
4892; (c) Duggineni, S.; Sawant, D.; Saha, B.; Kundu, B. Tetrahedron 2006, 62,
3228–3241; (d) Sharma, S.; Saha, B.; Sawant, D.; Kundu, B. J. Comb. Chem. 2007,
9, 783–792; (e) Saha, B.; Sharma, S.; Sawant, D.; Kundu, B. Tetrahedron 2008, 64,
8676–8684.
N
N
+
+
N
H
N
NH2
N
H
9
11
10
6. For a recent review, see: Yang, C.-G.; Huang, H.; Jiang, B. Curr. Org. Chem. 2004,
8, 1691–1720.
(iii)
7. Cimanga, K.; De Bruyne, T.; Pieters, L.; Vlietinck, A. J.; Turger, C. A. J. Nat. Prod.
1997, 60, 688–691.
5
8. (a) Molina, P.; Alajarin, M.; Vidal, A. J. Chem. Soc., Chem. Commun. 1990, 1277–
1279; (b) Molina, P.; Alajarin, M.; Vidal, A.; Sanchez-Andrada, P. J. Org. Chem.
1992, 57, 929–939; (c) Shi, C.; Zhang, Q.; Wang, K. K. J. Org. Chem. 1999, 64,
925–932; (d) Saito, T.; Ohmori, H.; Furuno, E.; Motoki, S. J. Chem. Soc., Chem.
Commun. 1992, 22–24; (e) Saito, T.; Ohmori, H.; Ohkubo, T.; Motoki, S. J. Chem.
Soc., Chem. Commun. 1993, 1802–1803.
9. (a) Peczynska-Czoch, W.; Pognan, F.; Kaczmarek, L.; Boratynski, J. J. Med. Chem.
1994, 37, 3503–3510; (b) Molina, P.; Fresneda, P. M.; Delgado, S. Synthesis 1999,
326–329; (c) Fresneda, P. M.; Molina, P.; Delgado, S. Tetrahedron Lett. 1999, 40,
7275–7278; (d) Alajarin, M.; Molina, P.; Vidal, A. J. Nat. Prod. 1997, 60, 747–
748; (e) Timári, G.; Soós, T.; Hajós, G. Synlett 1997, 1067–1068; (f) Sundaram, G.
S. M.; Venkatesh, C.; Syam Kumar, U. K.; Ila, H.; Junjappa, H. J. Org. Chem. 2004,
69, 5760–5762; (g) Parvatkar, P. T.; Parameswaran, P. S.; Tilve, S. G. Tetrahedron
Lett. 2007, 48, 7870–7872; (h) Dhanabal, T.; Sangeetha, R.; Mohan, P. S.
Tetrahedron 2006, 62, 6258–6263.
Product
HPLC
(%)
Isolated
Yield
(%)
35
27
10
Mass
HPLC
tR=min
(M++H)
9
10
11
45
35
15
219.5
223.0
219.5
14.035
13.194
19.434
Scheme 3. Reagents and conditions: (i) Na2CO3, 80% acetone in H2O, 70 °C, 36 h,
83%. (ii) SnCl2ꢀ2H2O, MeOH, reflux. 1 h. (iii) MeI, toluene, 130 °C, sealed tube, 4 h,
82%.
10. Bandgar, B. P.; Shaikh, K. A. Tetrahedron. Lett. 2003, 44, 1959–1961.
11. General procedure: A solution of 3 or 6 (1 mmol) and SnCl2ꢀ2H2O (5 mmol) in
methanol (4 mL) was refluxed for 1 h. The solution was allowed to cool and
was then poured into ice. The pH was made slightly basic (pH 8) by addition of
5% aqueous NaHCO3. EtOAc (50 mL) was added to the mixture which was
filtered through a bed of CeliteÒ. The organic layer was washed with water
(50 mL), brine (50 mL), dried over anhydrous Na2SO4 and evaporated to
dryness under reduced pressure. The residue was purified by silica gel column
chromatography using hexane/ethyl acetate to afford products 4 and 7.
11-(1H-Indol-3-yl)-6H-indolo[2,3-b]quinoline: Compound (4a) Yield: 89%;
yellow solid; mp > 250 °C; Rf 0.60 (7:3 hexane/EtOAc); IR (KBr) mmax 3399,
any substitution at the methylene leads to incomplete intramole-
cular cyclization and competes with the formation of 10 and 11.
The use of other reductive procedures15 involving Zn, NH4Cl,
H2O, EtOH and Zn, NaOH, H2O, EtOH, reflux failed to offer any
improvement in terms of yield/selectivity. This is in contrast to
our findings on substrates 3a–e and 6a–e with substituents at their
respective methines resulting in products 4a–e and 7a–e. The
surprising result of incomplete reductive cyclization in 8 is indica-
tive of the crucial role of the substituents at the methines in 3 and
6, respectively. Substituents at the methines could be playing a
role by restricting rotation of the C–C bond in a manner that brings
the nitro group in close proximity to C-2 of the indole thereby
facilitating complete cyclization via the formation of a C–N bond.
1607 cmꢁ1 1H NMR (300 MHz, DMSO-d6) d 11.80 (br s, 1H, NH), 10.86 (br s,
;
1H, NH), 8.39 (d, 1H, J = 7.7 Hz, ArH), 8.27 (d, 1H, J = 8.2 Hz, ArH), 7.96 (d, 1H,
J = 8.0 Hz, ArH), 7.87 (d, 1H, J = 2.5 Hz, ArH), 7.69–7.44 (m, 5H, ArH), 7.31–7.21
(m, 2H, ArH), 7.14 (d, 1H, J = 7.8 Hz, ArH), 7.03 (t, 1H, J = 7.4 Hz, ArH); 13C NMR
(50 MHz, DMSO-d6) d 145.6, 144.5, 144.4, 136.9, 132.0, 129.7, 129.6, 127.2,
127.0, 126.1, 125.9, 124.8, 122.1, 121.8, 121.6, 120.5, 120.0, 119.8, 119.6, 112.4,
112.2, 107.7; Mass (ES+) m/z 334.6 (M++1); HRMS (EI) m/z calcd for [M+]

S. Sharma, B. Kundu / Tetrahedron Letters 49 (2008) 7062–7065
7065
333.1266, found 333.1239; Anal. Calcd for C23H15N3: C, 82.86; H, 4.54; N,
12.60. Found: C, 82.78; H, 4.42; N, 12.80.
2-(1H-Indol-3-ylmethyl)aniline: 10 Yield: 27%; white solid; mp 89–90 °C; Rf
0.28 (9:1 hexane/EtOAc); IR (KBr) mmax 3225, 3198, 2946, 2852 cmꢁ1 1H NMR
;
9-(1H-Pyrrol-2-yl)-1H-pyrrolo[3,2-b]quinoline: (7a) Yield: 87%; yellow solid;
(300 MHz, CDCl3) d 7.95 (br s, 1H, NH); 7.58 (d, 1H, J = 7.9 Hz, ArH), 7.34 (d, 1H,
J = 8.1 Hz, ArH), 7.24–7.06 (m, 4H, ArH), 6.82–6.81 (m, 1H, ArH), 6.79–6.73 (m,
mp > 250 °C; Rf 0.67 (1:1 hexane/EtOAc); IR (KBr) mmax 3399, 1592 cmꢁ1 1H
;
NMR (300 MHz, DMSO-d6) d 11.57 (br s, 1H, NH), 11.49 (br s, 1H, NH), 8.32 (dd,
1H, J = 8.6, 0.9 Hz, ArH), 7.96 (dd, 1H, J = 8.4, 0.8 Hz, ArH), 7.73–7.70 (m, 1H,
ArH), 7.64–7.59 (m, 1H, ArH), 7.43–7.35 (m, 1H, ArH), 7.11–7.09 (m, 1H, ArH),
6.61–6.57 (m, 2H, ArH), 6.38–6.35 (m, 1H, ArH); 13C NMR (75 MHz, DMSO-d6) d
149.5, 144.7, 129.7, 127.3, 126.6, 125.7, 125.6, 122.0, 121.0, 119.9, 118.8, 110.9,
108.5, 99.3; mass (ES+) m/z 234.4 (M++1); Anal. Calcd for C15H11N3: C, 77.23; H,
4.75; N, 18.01. Found: C, 77.09; H, 4.78; N, 18.13.
1H, ArH), 6.67 (d, 1H, J = 7.8 Hz, ArH), 4.00 (s, 2H, CH2), 3.54 (br s, 2H, NH2); 13
C
NMR (75 MHz, DMSO-d6) d 146.0, 136.5, 129.2, 127.3, 126.5, 124.6, 123.3,
120.9, 118.7, 118.2, 116.2, 114.6, 112.3, 111.4, 26.8; Mass (ES+) m/z 223.0
(M++1); Anal. Calcd for C15H14N2: C, 81.05; H, 6.35; N, 12.60. Found: C, 81.12;
H, 6.41; N, 12.47.
12. Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea, D. F.; Boyle, P. D.;
Lindsey, J. S. J. Org. Chem. 1999, 64, 1391–1396.
Spiroindole (Compound 11): Yield: 10%; yellow solid; mp 103–105 °C; Rf 0.56
13. Westermaier, M.; Mayr, H. Org. Lett. 2006, 8, 4791–4794.
14. (a) Kowalski, P.; Bojarski, A. J.; Mokrosz, J. L. Tetrahedron 1995, 51, 2737–2742;
(b) Kusurkar, R. S.; Alkobati, N. A. H.; Gokule, A. S.; Puranik, V. G. Tetrahedron
2008, 64, 1654–1662.
(9:1 hexane/EtOAc); IR (KBr) m ;
max 1610 cmꢁ1 1H NMR (300 MHz, CDCl3) d 8.96
(s, 1H, ArH), 8.50–8.44 (m, 2H, ArH), 8.02–7.97 (m, 2H, ArH), 7.66–7.54 (m, 2H,
ArH), 7.49–7.42 (m, 2H, ArH), 7.17 (s, 1H, ArH); mass (ES+) m/z 219.5 (M++1);
HRMS (EI) m/z calcd for [M+] 218.0844, found 218.0833; Anal. Calcd for
C15H10N2: C, 82.55; H, 4.62; N, 12.84. Found: C, 82.43; H, 4.76; N, 12.81.
15. Kimbaris, A.; Varvounis, G. Tetrahedron 2000, 56, 9675–9682.