Synthesis of furo[2,3-b]pyridine

Article abstract of DOI:10.3987/COM-11-12305

Two synthetic routes toward furo[2,3-b]pyridine (1) starting with the known β,γ-unsaturated ester (3) are described.

Full text of DOI:10.3987/COM-11-12305

HETEROCYCLES, Vol. 83, No. 10, 2011
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HETEROCYCLES, Vol. 83, No. 10, 2011, pp. 2373 - 2380. © 2011 The Japan Institute of Heterocyclic Chemistry
Received, 13th July, 2011, Accepted, 17th August, 2011, Published online, 17th August, 2011
DOI: 10.3987/COM-11-12305
SYNTHESIS OF FURO[2,3-b]PYRIDINE
Meng-Yang Chang* and Hang-Yi Tai
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University,
Kaohsiung 807, Taiwan. Email: mychang@kmu.edu.tw
Abstract – Two synthetic routes toward furo[2,3-b]pyridine (1) starting with the
known β,γ-unsaturated ester (3) are described.
Furo[2,3-b]pyridine (1) is a key biologically active moiety in drug discovery libraries.1 An important
example is the furo[2,3-b]pyridine (1) contained in L-754,394, a derivative reported as a potent HIV
protease enzyme inhibitor (Figure 1).2 There are a number of processes available to generate the core
skeleton of furo[2,3-b]pyridine with bicyclic fused 5,6-ring system,3 but generally, two major approaches
have been developed. One is the annulation of a furan ring onto a substituted pyridine, and the other is the
cyclization of a functionalized pyridine ring from a furan derivative. Some other synthetic routes are
described as follows: (1) palladium-catalyzed annulation of 3-alkynyl 2-hydroxypyridine or 2-pyridone,4
(2) base-induced cyclization of 2-fluoropyridine with α-hydroxyester,5 (3) one-pot condensation of
2-aminofuran with 1,3-dicarbonyl synthon,6 and (4) multi-component approach.7
N
OH
H
N
OH
N
O
O
N
O
O
N
N
H
L-754,394
1
Figure 1. Structures of furo[2,3-b]pyridine (1) and L-754,394
Recently, we described the synthesis of homokainoid from N-benzenesulfonylpiperidin-3-one (2).8a In
previous studies, furo[2,3-c]pyridine was discussed as the regioisomer of furo[2,3-b]pyridine (1); it had
been synthesized from N-benzenesulfonylpiperidin-4-one.8b To continue our investigation, two synthetic

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HETEROCYCLES, Vol. 83, No. 10, 2011
routes to furo[2,3-b]pyridine (1) were reported (see Scheme 1). The known compound (3) was prepared in
two-step protocol via Wittig olefination and deconjugation.8a Because 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU) could be used as the same base in the preparation of Wittig ylide and the deconjugation reaction,
the combination of two procedures required further study. DBU-mediated one-pot reaction of ketone (2)
with Wittig salt in THF under reflux produced β,γ-unsaturated ester (3) as the sole product. In comparison
with two routes, the one-pot reaction provided a higher yield (85%) than the two-step reaction process did
(61%).8a
HO
CO Et
2
Br
Br
CO Et
Ph
Ph
O

P
2
NBS, BF -OEt (71%)


3
2
DIBALH, THF
(83%)
Ph
O
N
S
N
S
N
S
N
S
DBU, THF (84%)
H
DBU, THF (85%)
O
O
O
O
O
O
O
O
2
3
4
5
DIBALH, THF
then 2N HCl (72%)
t-BuOK, THF
(22%)
H
H
I , DIB, Cu(OAc)
DDQ (ex.), MeCN
(51%)
2
2
O
O
N
S
N
O
N
MeCN (33%)
S
O
O
O
O
1
6
7
Scheme 1. Synthetic approach toward furo[2,3-b]pyridine (1)
Furthermore, reduction of compound (3) with three equivalents of diisobutylaluminum hydride (DIBALH,
1.0 M in THF) at rt afforded alcohol (4) with 83% yield. Compound (5) was prepared as a sole isomer in
71% yield by N-bromosuccinimide (NBS)-mediated intramolecular addition reaction of compound (4) in
the presence of boron trifluoride etherate (BF3·OEt2) at rt for 5 h. With the skeleton of
octahydrofuro[2,3-b]pyridine obtained, the aromatization reaction was next examined. However, when
compound (5) was treated with DBU under reflux for 4 h, compound (4) was generated in 84% yield
without the formation of dehydrobromination products. This is an interesting reversible transformation.
We expected DBU to function as a key promoter for the debrominative ring-opening reaction.9 As shown
in Equation 1, DBU should abstract the bromo group to form the intermediate (A). Furthermore,
compound (4) was obtained by the ring-opening of intermediate (B) and protonation of intermediate (C).

HETEROCYCLES, Vol. 83, No. 10, 2011
2375
Br
DBU
H O
2
DBU, THF
O
O
O
N
5
4
N
S
N
H
H
S
S
O
O
O
O
O
O
DBU-Br
+
A
B
C
Equation 1. A possible mechanism for DBU-promoted reaction of compound (5)
Attempts to perform the reaction with different bases (i.e., sodium hydride, sodium methoxide or lithium
diisopropylamide) and under different reaction conditions (rt or reflux) afforded the complex mixture.
When compound (5) was treated with potassium t-butoxide (t-BuOK) in THF under reflux for 1 h,
tetrahydrofuro[2,3-b]pyridine (7) was obtained with only 22% yield. Literature reports on aromatization
reaction,10 show that 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a good oxidant for the
formation of furan and pyridine ring. Under the reported condition, the reaction of compound (7) with
five equivalents of DDQ in toluene at reflux afforded furo[2,3-b]pyridine (1) with a 51% yield. This
five-step route provided a poor total yield (5.6%) from ketone (2).
In another approach, compound (6) was synthesized as a single stereoisomer with 72% yield via reduction
of compound (3) with DIBALH under the abovementioned conditions and the subsequent acidic work-up
with hydrochloric acid. The structural framework of compound (6) with cis-fused bicyclic ring was
determined by single-crystal X-ray analysis (Figure 2).11
Figure 2. X-Ray structure of compound (6)

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HETEROCYCLES, Vol. 83, No. 10, 2011
Finally, furo[2,3-b]pyridine (1) was produced in a 33% yield by the Cu(OAc)2-mediated aromatization
reaction of octahydrofuro[2,3-b]pyridine (6) with (diacetoxyiodo)benzene (DIB) and I2 in MeCN under
reflux for 42 h.12,13 Mechanically, it is not clear if the reaction followed the same pathway as shown in
Equation 2: (a) the initial event might be considered as the radical formation at C-7a position, (b) the next
step should be the iodination reaction of intermediate (I), followed by the dehydroiodination reaction of
intermediate (II); (c) furthermore, intermediate (IV) was generated by the radical formation at C-6
position of intermediate (III), and (d) furo[2,3-b]pyridine (1) was afforded via the aromatization reaction
with the desulfonation reaction of intermediate (IV) and the double dehydrogenative reaction of
intermediate (V). Notably, the second methodology provided a rapid process to form the ring system of
furan and pyridine. The total yield of three-step route was 20% from ketone (2).
H
H
7a
6
I , DIB,
-PhSO H
-2H
2
2
2
-HI
O
O
O
O
N
N
S
N
N
S
1
6
I
O
Cu(OAc)
N
S
S
2
O
O
O
O
O
O
O
O
V
I
II
III
IV
Equation 2. A possible mechanism for synthesis of furo[2,3-b]pyridine (1)
In summary, we have successfully presented two new synthetic methodologies for producing the
furo[2,3-b]pyridine (1). Further studies on the synthetic application of β,γ-unsaturated ester (3) are
actively underway in the laboratory.
EXPERIMENTAL
General. Tetrahydrofuran (THF) was distilled prior to use. All other reagents and solvents were obtained
from commercial sources and used without further purification. Reactions were routinely carried out
under an atmosphere of dry nitrogen with magnetic stirring. Products in organic solvents were dried with
anhydrous magnesium sulfate before concentration in vacuo. 1H and 13C NMR spectra were recorded on a
Varian INOVA-400 spectrometer operating at 200/400 and at 50/100 MHz, respectively. Chemical shifts
() are reported in parts per million (ppm) and the coupling constants (J) are given in Hertz. High
resolution mass spectra (HRMS) were measured with a mass spectrometer Finnigan/Thermo Quest MAT
95XL. X-ray crystal structures were obtained with an Enraf-Nonius FR-590 diffractometer (CAD4,
Kappa CCD). Elemental analyses were carried out with Heraeus Vario III-NCSH, Heraeus
CHN-OS-Rapid Analyzer or Elementar Vario EL III.

HETEROCYCLES, Vol. 83, No. 10, 2011
2377
(1-Benzenesulfonyl-1,4,5,6-tetrahydropyridin-3-yl)acetic acid ethyl ester (3). A solution of DBU (760
mg, 5.0 mmol) in THF (10 mL) was added to a rapidly stirred solution of ethoxycarbonylmethyl-
triphenylphosphonium bromide (2.15 g, 5.0 mmol) in THF (30 mL) at rt. The reaction mixture was stirred
at reflux for 30 min and cooled to rt. A solution of 2 (1.0 g, 4.18 mmol) in THF (10 mL) was added to the
resulting reaction mixture at rt. The reaction mixture was reflux for 4 h and cooled to rt. A solution of
DBU (3.1 g, 20.4 mmol) in THF (10 mL) was added to the reaction mixture at rt. The reaction mixture
was reflux for 4 h and cooled to rt again. The resulting mixture was concentrated under reduced pressure.
Water (20 mL) was added to the residue and extracted with EtOAc (3 x 50 mL). The combined organic
layers were washed with brine, dried, filtered and evaporated to afford crude product under reduced
pressure. Purification on silica gel (hexanes/EtOAc = 10/1) afforded 3 (1.1 g, 85%). Compound (3) is a
known compound and the analytical data are consistent with those in the literature.8a Colorless oil; HRMS
(ESI, M++1) calcd for C15H20NO4S 310.1113, found 310.1112; 1H NMR (200 MHz): δ 7.80-7.74 (m, 2H),
7.68-7.49 (m, 3H), 6.61 (s, 1H), 4.10 (q, J = 6.8 Hz, 2H), 3.33 (t, J = 5.8 Hz, 2H), 2.94 (s, 2H), 1.94 (t, J
= 5.8 Hz, 2H), 1.67-1.60 (m, 2H), 1.22 (t, J = 6.8 Hz, 3H).
2-(1-Benzenesulfonyl-1,4,5,6-tetrahydropyridin-3-yl)ethanol (4). A solution of DIBALH (1.0 M in
THF, 3.0 mL, 3.0 mmol) was added to a stirred solution of 3 (310 mg, 1.0 mmol) in THF (20 mL) at 0 oC.
o
The reaction mixture was stirred for 5 h at rt. The reaction was quenched with water (1 mL) at 0 C and
the mixture was concentrated under reduced pressure. The residue was extracted with EtOAc (3 x 50 mL).
The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude
product under reduced pressure. Purification on silica gel (hexanes/EtOAc = 4/1~2/1) afforded 4 (222 mg,
83%). Colorless oil; HRMS (ESI, M++1) calcd for C13H18NO3S 268.1007, found 268.1008; 1H NMR (400
MHz): δ 7.80-7.76 (m, 2H), 7.60-7.55 (m, 1H), 7.54-7.49 (m, 2H), 6.54 (s, 1H), 3.62 (t, J = 6.4 Hz, 2H),
3.33 (t, J = 5.6 Hz, 2H), 2.21 (dt, J = 0.8, 6.4 Hz, 2H), 2.15 (br s, 1H), 1.87 (t, J = 6.0 Hz, 2H), 1.68-1.61
(m, 2H); 13C NMR (100 MHz): δ 137.75, 132.76, 129.10 (2x), 126.95 (2x), 121.63, 117.35, 60.36, 43.50,
38.38, 24.40, 20.76; Anal. Calcd for C13H17NO3S: C, 58.40; H, 6.41; N, 5.24. Found: C, 58.72; H, 6.74; N,
5.43.
7-Benzenesulfonyl-3a-bromo-2,3,3a,4,5,6,7,7a-octahydrofuro[2,3-b]pyridine (5). NBS (180 mg, 1.0
mmol) was added to a solution of 4 (200 mg, 0.75 mmol) in DCM (10 mL) at rt. The reaction mixture
was stirred at rt for 10 min. A solution of BF3-OEt2 (0.2 mL) in DCM (1 mL) was added to a stirred
solution of the reaction mixture at ice bath. The reaction mixture was stirred at rt for 5 h. Saturated
NaHCO3(aq) (2 mL) was added to the reaction mixture and the solvent was concentrated under reduced
pressure. The residue was extracted with EtOAc (3 x 30 mL). The combined organic layers were washed
with brine, dried, filtered and evaporated to afford crude product under reduced pressure. Purification on

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HETEROCYCLES, Vol. 83, No. 10, 2011
silica gel (hexanes/EtOAc = 8/1~4/1) afforded 5 (185 mg, 71%). Colorless oil; HRMS (ESI, M++Na)
1
calcd for C13H16BrNO3SNa 367.9932, found 367.9930; H NMR (400 MHz): δ 7.93-7.90 (m, 2H),
7.59-7.54 (m, 1H), 7.51-7.47 (m, 2H), 5.64 (s, 1H), 3.97-3.85 (m, 2H), 3.54-3.50 (m, 1H), 2.79 (dt, J =
2.8, 12.0 Hz, 1H), 2.64 (dt, J = 9.6, 12.8 Hz, 1H), 2.34 (ddd, J = 3.2, 7.6, 12.8 Hz, 1H), 2.08-2.03 (m, 1H),
1.96-1.88 (m, 1H), 1.86-1.78 (m, 1H), 1.60-1.58 (m, 1H); 13C NMR (100 MHz): δ 138.99, 132.74, 128.74
(2x), 127.94 (2x), 89.39, 63.73, 57.97, 40.98, 39.63, 33.99, 20.97.
A representative procedure for synthesis of compound (4) via the DBU-mediated debrominative
ring-opening of compound (5) is as follows: A solution of DBU (150 mg, 1.0 mmol) in THF (5 mL)
was added to a rapidly stirred solution of 5 (110 mg, 0.32 mmol) in THF (10 mL) at rt. The reaction
mixture was stirred at reflux for 30 min and cooled to rt. The resulting mixture was concentrated under
reduced pressure. Water (5 mL) was added to the residue and extracted with EtOAc (3 x 20 mL). The
combined organic layers were washed with brine, dried, filtered and evaporated to afford crude product
under reduced pressure. Purification on silica gel (hexanes/EtOAc = 10/1) afforded 4 (72 mg, 84%).
7-Benzenesulfonyl-4,5,6,7-tetrahydrofuro[2,3-b]pyridine (7). t-BuOK (560 mg, 5.0 mmol) was added
to a solution of 5 (178 mg, 0.52 mmol) in THF (5 mL) at rt. The reaction mixture was stirred at reflux for
1 h and cooled to rt. Water (1 mL) was added to the reaction mixture and the solvent was concentrated.
The reaction mixture was extracted with EtOAc (3 x 15 mL). The combined organic layers were washed
with brine, dried, filtered and evaporated to afford crude product. Purification on silica gel
(hexanes/EtOAc = 3/1~2/1) afforded 7 (30 mg, 22%). Pale yellow oil; HRMS (ESI, M++1) calcd for
1
C13H14NO3S 264.0694, found 264.0696; H NMR (400 MHz): δ 7.86-7.84 (m, 2H), 7.72-7.70 (m, 1H),
7.69-7.60 (m, 2H), 7.25 (br t, J = 1.2 Hz, 1H), 6.20 (d, J = 1.2 Hz, 1H), 3.69 (t, J = 7.2 Hz, 2H), 2.19 (t, J
13
= 7.6 Hz, 2H), 1.44-1.40 (m, 2H); C NMR (100 MHz): δ 144.40, 140.19, 136.20, 134.50, 129.95 (2x),
127.28 (2x), 116.46, 107.62, 44.06, 22.08, 19.94; Anal. Calcd for C13H13NO3S: C, 59.30; H, 4.98; N, 5.32.
Found: C, 59.51; H, 5.23; N, 5.50.
Furo[2,3-b]pyridine (1). A solution of DDQ (206 mg, 0.9 mmol) was added to a stirred solution of 7 (48
mg, 0.18 mmol) in toluene (3 mL) at rt. The reaction mixture was stirred at reflux for 20 h. Saturated
NaHCO3(aq) (2 mL) was added to the reaction mixture and the solvent was concentrated under reduced
pressure. The residue was extracted with EtOAc (3 x 30 mL). The combined organic layers were washed
with brine, dried, filtered and evaporated to afford crude product under reduced pressure. Purification on
silica gel (hexanes/EtOAc = 4/1~1/1) afforded 1 (11 mg, 51%). Compound (1) is a known compound and
1
1
the H NMR data is consistent with those in the literature.1d Yellowish oil; H NMR (400 MHz): δ 8.29

HETEROCYCLES, Vol. 83, No. 10, 2011
2379
(dd, J = 1.2, 7.2 Hz, 1H), 7.23 (d, J = 1.2, 7.2 Hz, 1H), 7.75 (d, J = 2.0, 7.6 Hz, 1H), 7.20 (d, J = 7.8 Hz,
13
1H), 6.79 (d, J = 2.0 Hz, 1H); C NMR (100 MHz): δ 160.45, 158.69, 142.26, 128.73, 121.98, 116.89,
101.33.
7-Benzenesulfonyl-2,3,3a,4,5,6,7,7a-octahydrofuro[2,3-b]pyridine (6). A solution of DIBALH (1.0 M
in THF, 3.0 mL, 3.0 mmol) was added to a stirred solution of 3 (310 mg, 1.0 mmol) in THF (30 mL) at
ice bath. The mixture was further stirred for 5 h at rt. The reaction mixtre was quenched with HCl(aq) (2N,
5 mL) at rt and stirred for 30 min. The reaction mixture was concentrated under reduced pressure. The
residue was extracted with EtOAc (3 x 50 mL). The combined organic layers were washed with brine,
dried, filtered and evaporated to afford crude product under reduced pressure. Purification on silica gel
o
(hexanes/EtOAc = 10/1~6/1) afforded 6 (193 mg, 72%). White solid; Mp = 105-106 C (recrystallized
1
from hexanes and EtOAc); HRMS (ESI, M++1) calcd for C13H18NO3S 268.1007, found 268.1011; H
NMR (400 MHz): δ 7.94-7.91 (m, 2H), 7.57-7.46 (m, 3H), 5.58 (d, J = 4.0 Hz, 1H), 3.89 (dd, J = 8.4,
16.0 Hz, 1H), 3.82 (dt, J = 2.8, 8.4 Hz, 1H), 3.53 (dt, J = 4.0, 12.0 Hz, 1H), 2.67 (dt, J = 2.8, 12.0 Hz, 1H),
2.16-2.06 (m, 2H), 1.71-1.54 (m, 4H), 1.26-1.18 (m, 1H); 13C NMR (100 MHz): δ 139.33, 132.46, 128.61
(2x), 128.02 (2x), 84.97, 64.74, 40.18, 35.55, 30.40, 24.75, 23.58. Anal. Calcd for C13H17NO3S: C, 58.40;
H, 6.41; N, 5.24. Found: C, 58.68; H, 6.59, N, 5.46. Single-crystal X-ray diagram: crystal of 6 was grown
by slow diffusion of EtOAc into a solution of 6 in DCM to yield colorless prism. The compound
crystallizes in the monoclinic crystal system, space group P 1 21/c 1, a = 13.6992(7) Å, b = 8.8534(4) Å,
c = 11.0552(6) Å, V = 1304.23(11) Å3, Z = 4, dcalcd = 1.361 g/cm3, F(000) = 568, 2θ range 1.53~26.60o, R
indices (all data) R1 = 0.0601, wR2 = 0.0935.
Furo[2,3-b]pyridine (1). Cu(OAc)2 (182 mg, 1.0 mmol), DIB (322 mg, 1.0 mmol) and iodine (254 mg,
1.0 mmol) were added to a solution of 6 (106 mg, 0.4 mmol) in MeCN (8 mL) at rt. The reaction mixture
was stirred at reflux for 42 h. Na2S2O3(aq) solution (10%, 2 x 5 mL) was added to the reaction mixture and
the solvent was concentrated under reduced pressure. The residue was extracted with ethyl acetate (3 x 30
mL). The combined organic layers were washed with brine, dried, filtered and evaporated to afford crude
product under reduced pressure. Purification on silica gel (hexanes/EtOAc = 4/1~1/1) afforded 1 (16 mg,
33%).
ACKNOWLEDGEMENTS
The authors would like to thank the National Science Council of the Republic of China for its financial
support (NSC 99-2113-M-037-006-MY3). The project is also supported by a grant from the Kaohsiung
Medical Research Foundation (KMU-Q100004).

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