Synthesis of 2,3-diaryl-3H-pyrrolo[2,3-c]pyridin-3-ol derivatives by the reaction of aryl(3-isocyanopyridin-4-yl)methanones with aryl Grignard reagents

Article abstract of DOI:10.1002/hlca.201100079

The reaction of aryl(3-isocyanopyridin-4-yl)methanones 1, easily prepared from commercially available pyridin-3-amine, with aryl Grignard reagents gave, after aqueous workup, 2,3-diaryl-3H-pyrrolo[2,3-c]pyridin-3-ols 2. These rather unstable alcohols were O-acylated with Ac₂O in pyridine in the presence of a catalytic amount of 4-(dimethylamino)pyridine (DMAP) to afford the corresponding 2,3-diaryl-3H-pyrrolo[2,3-c]pyridin-3-yl acetates 3 in relatively good yields. Copyright

Full text of DOI:10.1002/hlca.201100079

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Helvetica Chimica Acta – Vol. 94 (2011)
Synthesis of 2,3-Diaryl-3H-pyrrolo[2,3-c]pyridin-3-ol Derivatives by the
Reaction of Aryl(3-isocyanopyridin-4-yl)methanones with Aryl Grignard
Reagents
by Kazuhiro Kobayashi*, Taketoshi Kozuki, Manami Konishi, Teruhiko Suzuki, Miyuki Tanmatsu, and
Hisatoshi Konishi
Division of Applied Chemistry, Department of Chemistry and Biotechnology, Graduate School of
Engineering, Tottori University, 4-101 Koyama-minami, Tottori 680-8552, Japan
(phone/fax: þ 81-857-315263; e-mail: kkoba@chem.tottori-u.ac.jp)
The reaction of aryl(3-isocyanopyridin-4-yl)methanones 1, easily prepared from commercially
available pyridin-3-amine, with aryl Grignard reagents gave, after aqueous workup, 2,3-diaryl-3H-
pyrrolo[2,3-c]pyridin-3-ols 2. These rather unstable alcohols were O-acylated with Ac2O in pyridine in
the presence of a catalytic amount of 4-(dimethylamino)pyridine (DMAP) to afford the corresponding
2,3-diaryl-3H-pyrrolo[2,3-c]pyridin-3-yl acetates 3 in relatively good yields.
Introduction. – As part of our program to explore the potential of the reactions of
ortho-functionalized phenyl isocyanides with organometals for heterocycle synthesis
[1], we have recently demonstrated that 2-isocyanophenyl ketones can be converted
into 2,3-disubstituted 3H-indol-3-ols in generally good yields on treatment with
Grignard reagents [1e]. As an extension of this synthesis, we envisioned that a synthesis
of 2,3-disubstituted 3H-pyrrolo[2,3-c]pyridin-3-ols could be accomplished by reacting
3-isocyanopyridin-4-yl ketones with Grignard reagents. Here, we report the results of
such a study. The investigation has indicated that the reactions of aryl(3-isocyanopyr-
idin-4-yl)methanones 1 with aryl Grignard reagents provide a convenient synthetic
approach to 2,3-diaryl-3H-pyrrolo[2,3-c]pyridin-3-ol derivatives 3. Due to instability of
these alcohols during purification procedures, they were isolated as O-acetylated
derivatives. There have been so far no reports on the synthesis of this heterocyclic
skeleton, 3H-pyrrolo[2,3-c]pyridin-3-ol.
Results and Discussion. – The synthesis of 2,3-diaryl-3H-pyrrolo[2,3-c]pyridin-3-yl
acetates 3 from aryl(3-isocyanopyridin-4-yl)methanones 1 was performed as depicted
in the Scheme. The starting isocyano ketones 1 could be easily prepared from
commercially available pyridin-3-amine in a five-step sequence according to the
procedure reported previously by us [2]. First, (3-isocyanopyridin-4-yl)(phenyl)me-
thanone (1a) was chosen as a representative substrate and allowed to react with
PhMgBr. The reaction was carried out in THFat 08. The conditions were essentially the
same as reported previously for preparing 2,3-disubstituted 3H-indol-3-ols from 2-
isocyanophenyl ketones with Grignard reagents [1e]. The preferential initial addition
of the phenyl anion to the isocyano C-atom occurred almost exclusively, and the
subsequent cyclization by the intramolecular attack of the resulting imidoyl anion on
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Helvetica Chimica Acta – Vol. 94 (2011)
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the CO C-atom proceeded immediately and cleanly to give, after aqueous workup, 2,3-
diphenylpyrrolo[2,3-b]pyridin-3-ol (2a) in an almost quantitative yield, as judged by
1H-NMR spectra of the concentrated extracts. However, we found that this product was
quite unstable under purification procedures. Our attempts to isolate 2a by
chromatography on silica gel or alumina, or recrystallization were all unsuccessful.
Therefore, the crude 2a was quickly subjected to O-acylation. Thus, exposure of the
crude product to Ac2O in pyridine in the presence of a catalytic amount of 4-
(dimethylamino)pyridine (DMAP) at room temperature afforded 2,3-diphenyl-3H-
pyrrolo[2,3-c]pyridin-3-yl acetate (3a) in an excellent yield (93%).
Scheme
With this promising result in hand, the generality of this reaction was then
investigated. Using two other aryl(3-isocyanopyridin-4-yl)methanones, 1b and 1c, and
six arylmagnesium bromides, we prepared nine other 2,3-diaryl-3H-pyrrolo[2,3-
c]pyridin-3-yl acetates, 3b – 3j, in yields ranging from 73 to 95% as compiled in the
Table. These data indicate that somewhat decreased yields of the corresponding desired
products 3e and 3f using 1b (Entries 5 and 6) were obtained, compared with those using
the other starting materials 1a and 1c. The lower yields of these products might be
explained by the Cl substituents on the benzene nucleus, which may cause the initial
addition of Grignard reagent to the CO C-atom to some extent. It should be noted that
the use of alkyl Grignard reagents, such as MeMgBr and EtMgBr, under the reaction
conditions described above, resulted in the formation of complicated mixtures of
products, probably including those arising from initial addition of Grignard reagents to
Table. Preparation of 3H-Pyrrolo[2,3-c]pyridin-3-ol Derivatives 3
Entry
1
R
Ar in ArMgBr
3
Yielda) [%]
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1b
1b
1c
1c
1c
1c
H
H
H
H
Cl
Cl
MeO
MeO
MeO
MeO
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
93
86
86
95
73
73
87
89
95
93
3-ClꢀC6H4
3-MeOꢀC6H4
3,4-(MeO)2ꢀC6H3
Ph
4-ClꢀC6H4
Ph
4-MeꢀC6H4
4-ClꢀC6H4
4-MeOꢀC6H4
10
3j
a) Yields of isolated products based on 1.

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Helvetica Chimica Acta – Vol. 94 (2011)
the CO group of 1. No desired products could be isolated after acetylation under the
above mentioned conditions.
Finally, we planed to explore the reactions of alkyl(3-isocyanopyridin-4-yl)metha-
nones with Grignard reagents. Unfortunately, however, such a ketone, i.e., 1-(3-
isocyanopyridin-4-yl)propan-1-one, could not be prepared in satisfactory purity and
yield, and we could not carry out this exploration.
In conclusion, the presented results indicate that the reaction of aryl(3-isocyano-
pyridin-4-yl)methanones 1 with arylmagnesium bromides provides an efficient method
for the preparation of 2,3-diaryl-3H-pyrrolo[2,3-c]pyridin-3-ol derivatives 3 under mild
reaction conditions. This method should be of use in synthesizing this class of
heterocycles because of the simple manipulations and the ready availability of the
starting materials.
Experimental Part
General. (3-Isocyanopyridin-4-yl)(4-methoxyphenyl)methanones 1a and 1c were prepared from 2,2-
dimethyl-N-(pyridin-3-yl)propanamide [3] by the procedure reported previously by us [2]. All other
chemicals used in this study were commercially available. All of the org. solvents used in this study were
dried over appropriate drying agents and distilled prior to use. TLC: Kieselgel 60 PF254 (Merck). Column
chromatography (CC): Wako gel C-200E. M.p.: Laboratory Devices MEL-TEMP II; uncorrected. IR
Spectra: Shimadzu FTIR-8300 spectrometer; ˜n in cmꢀ1. 1H- and 13C-NMR Spectra: JEOL ECP500 FT-
1
NMR spectrometer at 500 and 125 MHz, resp.; H-NMR in CDCl3; d in ppm rel. to TMS as internal
standard, J in Hz. LR-EI-MS: JEOL JMS AX505 HA spectrometer at 70 eV; in m/z (rel. %).
(4-Chlorophenyl)(3-isocyanopyridin-4-yl)methanone (1b) was prepared from 2,2-dimethyl-N-(pyr-
idin-3-yl)propanamide [3], via the following precursors, under the conditions reported previously by us
for the preparation of 1a and 1c [2].
N-[4-(4-Chlorobenzoyl)pyridin-3-yl]-2,2-dimethylpropanamide. Yield 59%. Yellow solid. M.p. 141 –
1438 (hexane/Et2O). IR (KBr): 3358, 1668, 1655. 1H-NMR: 1.34 (s, 9 H); 7.33 (d, J ¼ 5.0, 1 H); 7.52
(AA’BB’, J ¼ 8.2, 2 H); 7.71 (AA’BB’, J ¼ 8.2, 2 H); 8.47 (d, J ¼ 5.0, 1 H); 9.90 (s, 1 H); 10.29 (br. s, 1 H).
Anal. calc. for C17H17ClN2O2 (316.10): C 64.46, H 5.41, N 8.84; found: C 64.45, H 5.53, N 8.63.
(3-Aminopyridin-4-yl)(4-chlorophenyl)methanone. Yield: 79%. Yellow solid. M.p. 134 – 1368
(hexane/THF). IR (KBr): 3447, 3306, 1633, 1607. 1H-NMR: 5.81 (br. s, 2 H); 7.19 (d, J ¼ 5.5, 1 H);
7.48 (AA’BB’, J ¼ 8.7, 2 H); 7.64 (AA’BB’, J ¼ 8.7, 2 H); 7.95 (d, J ¼ 5.5, 1 H); 8.30 (s, 1 H). Anal. calc. for
C12H9ClN2O (232.04): C 61.95, H 3.90, N 12.04; found: C 62.04, H 4.04, N 11.83.
N-[4-(4-Chlorobenzoyl)pyridin-3-yl]formamide. Yield 83%. Yellow oil. Rf (THF/hexane 1:1) 0.32.
1
IR (neat): 3308, 1699, 1668. H-NMR: 7.35 (d, J ¼ 5.0, 1 H); 7.52 (AA’BB’, J ¼ 8.7, 2 H); 7.73 (AA’BB’,
J ¼ 8.7, 2 H); 8.47 – 9.88 (m, 4 H). Anal. calc. for C13H9ClN2O2 (260.04): C 59.90, H 3.48, N 10.75; found:
C 60.03, H 3.52, N 10.51.
Compound 1b. Yield 79%. Yellow solid. M.p. 110 – 1148 (hexane/Et2O). IR (KBr): 2131, 1668.
1H-NMR: 7.41 (d, J ¼ 5.0, 1 H); 7.52 (AA’BB’, J ¼ 8.7, 2 H); 7.75 (AA’BB’, J ¼ 8.7, 2 H); 8.80 (d, J ¼ 5.0,
1 H); 8.83 (s, 1 H). Anal. calc. for C13H7ClN2O (242.02): C 64.34, H 2.91, N 11.54; found: C 64.47, H 2.87,
N 11.45.
2,3-Diaryl-3H-pyrrolo[2,3-c]pyridin-3-ol Derivatives 3 (General Procedure). To a stirred soln. of 1
(1.0 mmol) in THF (3 ml) at 08 was added one of the Grignard reagents (Et2O soln.; 1.0 mmol). After
15 min, sat. aq. NH4Cl (15 ml) was added, and the mixture was extracted with Et2O (3 ꢁ 10 ml). The
combined extracts were washed with brine, dried (anh. Na2SO4), and concentrated by evaporation. The
residue was dissolved in pyridine (3 ml), and to this soln. Ac2O (0.31 g, 3.0 mmol) and 4-
(dimethylamino)pyridine (12 mg, 0.10 mmol) were added. The resulting mixture was then allowed to
stir at r.t. overnight. Pyridine and excess Ac2O were removed under reduced pressure to give a residue,
which was purified by CC (SiO2; AcOEt/hexane 1:1) to give the desired product.

Helvetica Chimica Acta – Vol. 94 (2011)
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2,3-Diphenyl-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3a). White solid. M.p. 155 – 1568 (hexane/
CH2Cl2). IR (KBr): 1751. 1H-NMR: 2.14 (s, 3 H); 7.08 (dd, J ¼ 5.0, 0.9, 1 H); 7.26 – 7.33 (m, 3 H); 7.35 –
7.38 (m, 4 H); 7.41 (tt, J ¼ 7.3, 1.4, 1 H); 7.96 (dd, J ¼ 7.8, 1.4, 2 H); 8.43 (d, J ¼ 5.0, 1 H), 8.98 (s, 1 H).
13C-NMR (CDCl3): 20.92; 90.27; 116.39; 123.94; 128.69; 128.71; 128.78; 129.35; 130.42; 131.79; 135.26;
142.78; 148.09; 149.03; 149.25; 167.70; 177.69. MS: 328 (39, Mþ), 285 (100). Anal. calc. for C21H16N2O2
(328.12): C 76.81, H 4.91, N 8.53; found: C 76.60, H 4.90, N 8.43.
2-(3-Chlorophenyl)-3-phenyl-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3b). Pale-yellow solid. M.p.
1
185 – 1878 (hexane/CH2Cl2). IR (KBr): 1748. H-NMR: 2.15 (s, 3 H); 7.08 (dd, J ¼ 5.5, 0.9, 1 H); 7.25 –
7.36 (m, 6 H); 7.39 (dt, J ¼ 7.8, 0.9, 1 H); 7.73 (dd, J ¼ 7.8, 0.9, 1 H); 8.06 (t, J ¼ 0.9, 1 H); 8.45 (d, J ¼
5.5, 1 H); 8.99 (s, 1 H). 13C-NMR (CDCl3): 20.91; 90.15; 116.44; 123.90; 126.65; 128.48; 128.99; 129.47;
129.91; 131.73; 132.16; 134.66; 134.92; 143.02; 148.49; 148.97; 149.00; 167.74; 176.42. MS: 362 (19, Mþ),
319 (100). Anal. calc. for C21H15ClN2O2 (362.81): C 69.52, H 4.17, N 7.72; found: C 69.51, H 4.21, N 7.61.
2-(3-Methoxyphenyl)-3-phenyl-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3c). Pale-yellow solid. M.p.
211 – 2148 (hexane/CH2Cl2). IR (KBr): 1747, 1600. 1H-NMR: 2.15 (s, 3 H); 3.82 (s, 3 H); 6.98 (dd, J ¼ 7.8,
2.7, 1 H); 7.07 (dd, J ¼ 4.6, 0.9, 1 H); 7.23 (t, J ¼ 7.8, 1 H); 7.29 – 7.33 (m, 3 H); 7.35 – 7.37 (m, 2 H); 7.40 (d,
J ¼ 7.8, 1 H); 7.64 (t, J ¼ 1.4, 1 H); 8.43 (d, J ¼ 4.6, 1 H), 8.98 (s, 1 H). 13C-NMR (CDCl3): 20.95; 55.29;
90.23; 112.63; 116.40; 118.70; 121.45; 123.91; 128.77; 129.32; 129.58; 131.67; 135.28; 142.78; 148.13;
149.09; 149.19; 159.69; 167.72; 177.64. MS: 358 (25, Mþ), 315 (100). Anal. calc. for C22H18N2O3 (358.39): C
73.73, H 5.06, N 7.82; found: C 73.60, H 5.13, N 7.88.
2-(3,4-Dimethoxyphenyl)-3-phenyl-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3d). Pale-yellow solid.
1
M.p. 143 – 1458 (hexane/CH2Cl2). IR (KBr): 1755. H-NMR: 2.14 (s, 3 H); 3.88 (s, 3 H); 3.92 (s, 3 H);
6.75 (d, J ¼ 8.2, 1 H); 7.06 (dd, J ¼ 5.0, 0.9, 1 H); 7.28 – 7.33 (m, 4 H); 7.37 (dd, J ¼ 8.2, 1.8, 2 H); 7.77 (d,
J ¼ 2.3, 1 H); 8.40 (d, J ¼ 5.0, 1 H); 8.95 (s, 1 H). 13C-NMR (CDCl3): 20.97; 55.86; 55.89; 90.15; 110.40;
110.68; 116.37; 123.07; 123.36; 123.87; 128.70; 129.28; 135.83; 142.33; 147.61; 148.87; 149.21; 149.49;
152.34; 167.61; 177.24. MS: 388 (26, Mþ), 345 (100). Anal. calc. for C23H20N2O4 (388.42): C 71.12, H 5.19,
N 7.21; found: C 71.08, H 5.33, N 7.15.
3-(4-Chlorophenyl)-2-phenyl-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3e). White solid. M.p. 203 –
1
2058 (hexane/CH2Cl2). IR (KBr): 1751. H-NMR: 2.14 (s, 3 H); 7.06 (dd, J ¼ 5.0, 0.9, 1 H); 7.28 (d, J ¼
9.2, 2 H), 7.30 (d, J ¼ 9.2, 2 H); 7.37 (dd, J ¼ 7.8, 7.3, 2 H), 7.45 (tt, J ¼ 7.3, 1.4, 1 H); 7.94 (dd, J ¼ 7.8,
1.4, 2 H); 8.45 (d, J ¼ 5.0, 1 H); 8.98 (s, 1 H). 13C-NMR ((D6)DMSO): 20.58; 89.11; 116.90; 126.04;
128.11; 129.13; 129.51; 129.81; 132.23; 133.53; 134.21; 142.03; 148.43; 148.47; 148.58; 168.25; 176.74. MS:
362 (79, Mþ), 319 (100). Anal. calc. for C21H15ClN2O2 (362.81): C 69.52, H 4.17, N 7.72; found: C 69.39, H
4.09, N 7.65.
2,3-Bis(4-chlorophenyl)-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3f). White solid. M.p. 202 – 2038
1
(hexane/CH2Cl2). IR (KBr): 1753. H-NMR: 2.15 (s, 3 H); 7.06 (dd, J ¼ 4.6, 0.9, 1 H); 7.28 (d, J ¼ 9.2,
2 H); 7.29 (d, J ¼ 9.2, 2 H); 7.35 (d, J ¼ 8.7, 2 H); 7.87 (d, J ¼ 8.7, 2 H); 8.46 (d, J ¼ 4.6, 1 H); 8.97 (s, 1 H).
13C-NMR (CDCl3): 20.87; 89.70; 116.37; 125.41; 128.61; 129.21; 129.73; 129.84; 133.49; 134.99; 138.35;
142.85; 148.30; 148.61; 149.06; 167.48; 176.27. MS: 396 (74, Mþ), 353 (100). Anal. calc. for C21H14Cl2N2O2
(397.25): C 63.49, H 3.55, N 7.05; found: C 63.47, H 3.62, N 6.82.
3-(4-Methoxyphenyl)-2-phenyl-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3g). Pale-yellow amorphous
powder. Rf (AcOEt/hexane 1:1) 0.31. IR (neat): 1749, 1609. 1H-NMR: 2.12 (s, 3 H); 3.75 (s, 3 H); 6.82 (d,
J ¼ 8.7, 2 H); 7.01 (d, J ¼ 4.6, 1 H); 7.2 (d, J ¼ 8.7, 2 H); 7.37 (dd, J ¼ 7.8, 7.3, 2 H); 7.44 (t, J ¼ 7.3, 1 H); 7.97
(dd, J ¼ 7.8, 0.9, 2 H); 8.43 (d, J ¼ 4.6, 1 H); 8.96 (s, 1 H). 13C-NMR (CDCl3): 20.88; 55.18; 90.08; 114.66;
116.23; 125.37; 127.02; 128.65; 128.71; 130.49; 131.74; 142.69; 148.00; 149.06; 149.20; 159.79; 167.75;
177.69. MS: 358 (28, Mþ), 315 (100). Anal. calc. for C22H18N2O3 (358.39): C 73.73, H 5.06, N 7.82; found: C
73.55, H 5.09, N 7.55.
3-(4-Methoxyphenyl)-2-(4-methylphenyl)-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3h). White solid.
1
M.p. 187 – 1908 (hexane/CH2Cl2). IR (KBr): 1755, 1609. H-NMR: 2.12 (s, 3 H); 2.35 (s, 3 H); 3.74 (s,
3 H); 6.82 (d, J ¼ 9.2, 2 H); 7.08 (dd, J ¼ 4.6, 0.9, 1 H); 7.17 (d, J ¼ 7.8, 2 H); 7.28 (d, J ¼ 9.2, 2 H); 7.87 (d,
J ¼ 7.8, 2 H); 8.41 (d, J ¼ 4.6, 1 H); 8.94 (s, 1 H). 13C-NMR (CDCl3): 20.94; 21.60; 55.20; 90.04; 114.63;
116.25; 125.38; 127.22; 127.76; 128.75; 129.47; 142.36; 142.50; 147.66 (two overlapped C-atoms); 149.22;
159.75; 167.74; 177.72. MS: 372 (49, Mþ), 329 (100). Anal. calc. for C23H20N2O3 (372.42): C 74.18, H 5.41,
N 7.52; found: C 73.95, H 5.47, N 7.42.

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Helvetica Chimica Acta – Vol. 94 (2011)
2-(4-Chlorophenyl)-3-(4-methoxyphenyl)-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3i). White solid.
M.p. 161 – 1638 (hexane/CH2Cl2). IR (KBr): 1757, 1609. 1H-NMR: 2.13 (s, 3 H); 3.75 (s, 3 H); 6.82 (d, J ¼
9.2, 2 H); 7.07 (d, J ¼ 5.0, 1 H); 7.25 (d, J ¼ 8.7, 2 H); 7.35 (d, J ¼ 9.2, 2 H); 7.91 (d, J ¼ 8.7, 2 H); 8.44 (d,
J ¼ 5.0, 1 H); 8.95 (s, 1 H). 13C-NMR (CDCl3): 20.87; 55.20; 89.93; 114.79; 116.25; 125.32; 126.62; 128.96;
129.03; 129.90; 138.00; 142.81; 148.24; 128.92; 149.01; 159.89; 167.72; 176.57. MS: 392 (71, Mþ), 349 (100).
Anal. calc. for C22H17ClN2O3 (392.84): C 67.26, H 4.36, N 7.13; found: C 67.25, H 4.48, N 6.99.
2,3-Bis(4-methoxyphenyl)-3H-pyrrolo[2,3-c]pyridin-3-yl Acetate (3j). White solid. M.p. 191 – 1938
(hexane/CH2Cl2). IR (KBr). 1753, 1607. 1H-NMR: 2.12 (s, 3 H); 3.74 (s, 3 H); 3.81 (s, 3 H); 6.82 (d, J ¼
8.7, 2 H); 6.88 (d, J ¼ 9.2, 2 H); 7.06 (d, J ¼ 4.6, 1 H); 7.28 (d, J ¼ 8.7, 2 H); 7.94 (d, J ¼ 9.2, 2 H); 8.39 (d,
J ¼ 4.6, 1 H); 8.92 (s, 1 H). 13C-NMR (CDCl3): 20.94; 55.18; 55.30; 89.99; 114.15; 114.58; 116.15; 123.17;
125.35; 127.45; 130.66; 142.27; 147.51; 148.88; 149.31; 159.70; 163.43; 167.70; 177.12. MS: 388 (69, Mþ), 345
(100). Anal. calc. for C23H20N2O4 (388.42): C 71.12, H 5.19, N 7.21; found: C 71.06, H 5.22, N 7.14.
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Received February 21, 2011