Synthesis of nitrogen bridgehead heterocycles with phosphonates via a novel tandem process

Article abstract of DOI:10.3987/COM-12-12661

A novel and efficient method was developed for the synthesis of nitrogen bridgehead heterocycles with phosphonates. Nitrogen containing five-membered heterocyclic aldehyde and diethyl 3-bromoprop-1- enylphosphonate were used as substrates. Bridgehead nitrogen-containing arylphosphonates were obtained via one-pot reaction including four steps: SN2, deprotonation followed by electron flow, nucleophic additon and elimination of water.

Full text of DOI:10.3987/COM-12-12661

HETEROCYCLES, Vol. 87, No. 4, 2013
815
HETEROCYCLES, Vol. 87, No. 4, 2013, pp. 815 - 826. © 2013 The Japan Institute of Heterocyclic Chemistry
Received, 30th December, 2012, Accepted, 4th February, 2013, Published online, 20th February, 2013
DOI: 10.3987/COM-12-12661
SYNTHESIS OF NITROGEN BRIDGEHEAD HETEROCYCLES WITH
PHOSPHONATES VIA A NOVEL TANDEM PROCESS
Ya-Fei Xie,a Yan-Qing Ge,a,b Lei Feng,a Hua-Qiang Xu,a Song Meng,a
Gui-Long Zhao,c Wei-Ren Xu,c Jiong Jia,a* and Jian-Wu Wanga*
a
School of Chemistry and Chemical Engineering, Shandong University, Jinan,
Shandong 250100, P. R. China. E-mail: jwwang@sdu.edu.cn
b
School of Chemical Engineering, Taishan Medical University, Taian, Shandong
271016, P. R. China
c
Tianjin Key Laboratory of Molecular Design and Drug Discovery, Tianjin
Institute of Pharmaceutical Research, Tianjin 300193, P. R. China
Abstract – A novel and efficient method was developed for the synthesis of
nitrogen bridgehead heterocycles with phosphonates. Nitrogen containing
five-membered
heterocyclic
aldehyde
and
diethyl
3-bromoprop-1-
enylphosphonate were used as substrates. Bridgehead nitrogen-containing
arylphosphonates were obtained via one-pot reaction including four steps: SN2,
deprotonation followed by electron flow, nucleophic additon and elimination of
water.
INTRODUCTION
The importance of arylphosphonates in organic synthesis,1 materials,2 and medicinal chemistry3 has been
well documented for years. They are used, e.g., in the synthesis of heterocyclic compounds,4 designing
fuel cell membranes,5 materials with special optical properties,6 and building blocks in polymer sciences.7
There also has been a growing interest in these compounds in medicinal chemistry and nucleic acid
chemistry due to biological activity such as inhibitors of farnesyl protein transferase,8 progesterone
receptor antagonists3b and antiviral activity.9 Thus, the creation of new arylphosphonates could have a
great impact on various fields of chemical science.10
Bridgehead nitrogen-containing heterocycles are an important class of compounds due to their presence in
natural and non-natural products, which exhibit useful biological activities.11 Among such compounds,

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HETEROCYCLES, Vol. 87, No. 4, 2013
indolizine, pyrido[1,2-a]benzimidazole and pyrazolo[1,5-a]pyridine are particularly interesting because of
their similarities and diversions in structure to indole.12 Synthetic indolizines can be used as calcium entry
blockers,13 potential central nervous system depressants,14 5-HT3 receptor antagonist,15 histamine H3
receptor antagonists,16 cardiovascular agents,17 and PLA2 inhibitors.18 They have also drawn much
attention owing to their possible usage as dyes and chemosensors.19 Pyrido[1,2-a]benzimidazoles receive
much more attention because of their pharmaceutical applications as antifungal agents,20 antiviral agents21
and antineoplastic agents.22 Furthermore, some of them also exhibit unique fluorescent properties and
potential application in electroluminescence (EL) materials.23 Pyrazolo[1,5-a]pyridines display numerous
biological activities and pharmacological properties. For instance, some of them are used as dopamine
D2-like receptor antagonist in the treatment of neurological disorders.24 Some derivatives are synthesized
as adenosine A1 receptor antagonists25 and CRF1 receptor antagonist.26 Additionally, they are also useful
for the treatment of rheumatoid arthritis27 and herpes viral infection.28
Despite continued interest in these building blocks, synthetic methods in the literatures suffer strict
operation conditions or low yields.29 As far as we know, there remain no general synthetic routes to form
these nitrogen bridgehead heterocycles.
Due to the synthetic and practical importance of arylphosphonate derivatives and bridgehead
nitrogen-containing heterocycles, we are especially interested to extend the previously found tandem
reaction30 to synthesize the nitrogen bridgehead heterocycles with phosphonate. Here, we report a
convenient, transition metal-free and general method for the synthesis of indolizine,
pyrido[1,2-a]benzimidazole and pyrazolo[1,5-a]pyridine phosphonates. Pyrrole-2-carbaldehyde
(benzimidazole carbaldehyde or pyrazole carbaldehyde) and diethyl 3-bromoprop-1-enylphosphonate
were used as substrates, potassium carbonate as the base, and DMF as the solvent in one-pot.
RESULTS AND DISCUSSION
The reaction conditions were optimized by using 4-propionyl-1H-pyrrole-2-carbaldehyde 1a and diethyl
3-bromoprop-1-enylphosphonate 2 as the model substrates, and K2CO3 in DMF was found to be the most
efficient system (Table 1). It was started by screening various bases in DMF, and K2CO3 provided the
highest yield (Table 1 entries 1-6). Then the effect of solvents was investigated, and DMF was proved to
be the most efficient solvent (entries 7-11). The reaction was also carried out with NaH as the base and
MeCN as the solvent, but yield of the product was much lower. Yields of 3a under other conditions were
similar (entries 1, 3, 7, 11). Consequently, all following reactions were conducted with K2CO3 in DMF.
To explore the scope of this methodology, a variety of pyrrole-2-carbaldehyde were studied under the
reaction conditions which were optimized above. As observed in Table 2, a series of indolizine
phosphonates 3a-g were obtained in moderate to good yields under mild conditions. We tried aliphatic

HETEROCYCLES, Vol. 87, No. 4, 2013
817
substituent ethyl and a variety of phenyl to investigate the effect of R1 on the yield. When R1 was
4-fluorophenyl, the lowest yield of 65% was obtained. When R1 was ethyl, 4-methoxyphenyl,
4-chlorophenyl or 4-nitrophenyl, we got analogously good yield, no matter the substituent attached to
phenyl was electron-drawing or electron-donating. Thus, the yield was hardly dependent on the
substituent R1.
Table 1. Solvent and base effects on the novel tandem reaction
Entry
Base
Solvent
Time (h)
Isolated Yield (%)
1
2
3
4
5
6
7
8
9
K2CO3
KHCO3
Na2CO3
NaOH
MgO
DMF
DMF
DMF
DMF
DMF
DMF
MeCN
acetone
DCM
THF
8
60
6
84
0
61
22
0
0.5
60
60
16
16
60
60
16
Et3N
0
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
75
63
0
53
73
10
11
EtOH
Table 2. Synthesis of Indolizine Phosphonates
O
O
O
R1
K2CO3
R1
+
P
OEt
Br
OEt
OEt
DMF
OEt
CHO
N
P
N
H
O
3
2
1
Entry
R1
Product
Isolated Yield (%)
1
2
3
4
5
6
7
ethyl
phenyl
4-methylphenyl
4-methoxyphenyl
4-chlorophenyl
4-fluorophenyl
4-nitrophenyl
3a
3b
3c
3d
3e
3f
84
74
66
82
83
65
80
3g
13
The structures of adducts 3a-g were characterized by spectroscopic methods (1H and CNMR, IR, and
MS). The structure of 3g was further confirmed by X-ray crystallographic analysis as shown in Figure 1.

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HETEROCYCLES, Vol. 87, No. 4, 2013
Figure 1. Crystal structure of 3g
On the basis of the structures of the products, we propose a plausible reaction course for the one-pot
tandem reaction (Scheme 1). The first step is the formation of the intermediate A through an
intermolecular
SN2
reaction
between
pyrrole-2-carbaldehyde
1
and
diethyl
3-bromoprop-1-enylphosphonate 2. The second step is initiated by deprotonation of the intermediate A,
and then a series of electron flow makes the intermediate B formed. The third step is an intramolecular
nucleophilic addition between the formed β,γ-unsaturated α-carbanion of ester and the aldehyde group,
which afford intermediate C. In the fourth step, intramolecular Horner-Emmons type reaction does not
occur under the condition, but the final product 3 is formed by elimination of one water molecule; the
proposed reason is that the isolated product 3 is much more stable than the Horner-Emmons product 3’,
due to the presence of conjugation between the aromatic ring and the P=O system.
O
O
O
P
R
R
OEt
+ Br
CHO
CHO
N
OEt
O
N
base:
P
H
EtO
H
OEt
base:
O
2
1
A
O
O
R
R
O
-H2O
+H+
OH
P
R
H
O
N
O
N
N
P
OEt
EtO
EtO
OEt
3
EtO P O
EtO
O
Horner-Emmons
B
C
R
N
3'
Scheme 1. Proposed mechanism for the one-pot tandem reaction

HETEROCYCLES, Vol. 87, No. 4, 2013
819
To examine the applicability of the tandem reaction in synthesizing other nitrogen bridgehead
heterocycles bearing phosphonate moiety, we also used benzo[d]imidazole-2-carbaldehyde and
pyrazole-5-carbaldehyde as substrates.
R2
N
R2
R2
N
K2CO3, DMF
O
O
P
CHO
R2
N
O
N
Br
P(OEt)2
H
O
4a,b
2
2
5a:
R =H
57% yield
5b: R2=Me 63% yield
Scheme 2. Synthesis of pyrido[1,2-a]benzimidazole phosphonates 5
O
O
P
H
N
H
N
Br
P(OEt)2
O
N
PCC
DMF
N
N
N
O
CH2OH
CHO
DMF, K2CO3
R3
R3
R3
3
8a
8b
=
: R phenyl
48% yield
7a-c
6a-c
3
: R =4-methylphenyl 47% yield
8c: R3=4-fluorobenzoyl 53% yield
Scheme 3. Synthesis of pyrazolo[1,5-a]pyridine phosphonate 8
N
N
N
N
CHO
CHO
R3
R3
I
II
Scheme 4. Resonance structures of the deprotonated pyrazole carbaldehyde 7a-c
As showed in Scheme 2, benzo[d]imidazole-2-carbaldehyde proceeded well under the same reaction
conditions, and pyrido[1,2-a]benzimidazole phosphonates 5a, b were obtained in moderate yields.
When 1H-pyrazole-5-carbaldehyde 7 was treated with diethyl 3-bromoprop-1-enylphosphonate 2 under
the similar reaction conditions, a complex mixture was obtained. But raising the reaction temperature to
70 C produced a moderate yield (Scheme 3). We assume that, the two resonance structures (Scheme 4)
result in different reactivity from other two reactants of pyrrole-2-carbaldehyde and
benzo[d]imidazole-2-carbaldehyde. At low temperature, it is kinetically-controlled procedure. The
different position of electronegativity on the pyrazole carbaldehyde nitrogens gives different reaction
points in the first step, intermolecular SN2 reaction, in the proposed mechanism. This makes a complex
mixcture resulted. At 70 C, the reaction turns out to be thermodynamically controlled. Raising reaction
temperature may be favourable for the effective intermediate I, which makes the reaction proceed as
proposed reaction course.

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HETEROCYCLES, Vol. 87, No. 4, 2013
In summary, we have demonstrated a novel tandem process which can be applied to the construction of
nitrogen bridgehead heterocycles with phosphonates in moderate to good yields. Furthermore, the tandem
reaction can be widely applied in synthesizing nitrogen bridgehead arylphosphonates including indolizine,
pyrido[1,2-a]benzimidazole and pyrazolo[1,5-a]pyridine phosphonates. We believe that this method has
great potential applications in nitrogen bridgehead arylphosphonates synthesis. Further efforts to
investigate the reaction are ongoing in our group, and the results will be reported in the future.
O
OEt
P
Br
OEt
OEt NaH
Br
EtO
Br2
Br
P
Br
P
+
P(OEt)3
OEt
OEt
O
O
2
9
10
Scheme 5. Synthesis of diethyl 3-bromoprop-1-enylphosphonate 2
EXPERIMENTAL
All reagents were commercially available and used without further purification unless otherwise noted.
Starting materials were prepared according to literatures. Melting points were recorded on an XD-4digital
1
micro melting point apparatus and uncorrected. H and 13C NMR spectra were recorded on a Bruker
Avance 300 MHz spectrometer, using CDCl3 or DMSO-d6 as solvent and TMS as internal standard. IR
spectra were recorded with an IR spectrophotometer Avtar 370 FT-IR (Termo Nicolet). Elemental
analyses were performed on a Vario EL III (Elementar Analysensysteme GmbH) spectroanalyzer. HRMS
spectra were recorded on a Q-TOF6510 spectrograph (Agilent). Compounds 1a-g,31 4a-b,32 7a-c30a were
prepared according to the literatures.
General procedure for the synthesis of diethyl 3-bromoprop-1-enylphosphonate 2
Compound 2 was synthesized according to the literature method (Scheme 5).33,34 To a 250-mL
round-bottomed flask, triethyl phosphite (20 mL, 120 mmol) and allyl bromide (11 mL, 130 mmol) were
added and the mixture was heated at 140 C for 4 h. After evaporated under reduced pressure to remove
the remaining ally bromide, the product, compound 9, was purified by column chromatography. Dry
CH2Cl2 was added to dissolve compound 9, and the solution was cooled in an ice bath. Bromine (144
mmol) was added dropwise and the mixture was stirred for 2 h at rt. This mixture was poured into a
saturated aqueous solution of Na2SO3, extracted with CH2Cl2 and dried over MgSO4. After evaporation of
the solvent, compound 10 was obtained as a pale yellow oil, which was subjected to the following steps
without purification. NaH (40 mmol, 60%) was dispersed in 30 mL dry CH2Cl2 and the solution was
cooled in an ice bath. Diethyl 2,3-dibromopropylphosphonate 10 (30 mmol) in 20 mL dry CH2Cl2 was
added dropwise and the mixture was stirred for 24 h at rt. The temperature was raised to reflux for
additional 1 h. After filtrated over celite and evaporation of the solvent, pale yellow oil 2 was purified by

HETEROCYCLES, Vol. 87, No. 4, 2013
821
column chromatography in 40% yield.
General procedure for the synthesis and analytical data of 3a-g
The mixture of compound 1 (1.5 mmol), compound 2 (1.8 mmol), potassium carbonate (3.3 mmol) and
40 mL DMF was stirred at 40 C for 6-8 h. When TLC indicated the end of reaction, the reaction mixture
was poured to water (50 mL), and extracted by EtOAc (15 mL×3). The combined extracts were washed
by brine and dried over anhydrous MgSO4. After concentrated under reduced pressure, the crude products
were purified by column chromatography to afford 3a-g in 65-84% yield.
Diethyl 2-propionylindolizin-7-ylphosphonate (3a)
pale yellow solid; mp 102-104 C. 1H NMR (CDCl3, 300 MHz): δ 1.24 (t, J = 7.4 Hz, 3H), 1.34 (t, J = 7.1
Hz, 6H), 2.95 (q, J = 7.3 Hz, 2H), 4.04-4.24 (m, 4H), 6.74-6.80 (m, 1H), 7.03 (s, 1H), 7.88 (s, 1H), 7.91
(dd, J = 3.0 Hz, J = 7.2 Hz, 1H), 8.00 (d, J = 16.8 Hz, 1H); 13C NMR (CDCl3, 75 MHz): δ 8.4, 16.3, 16.4,
33.3, 62.27, 62.34, 103.5, 112.1, 112.2, 116.3, 116.6, 118.9, 125.3, 125.5, 127.6, 127.8, 129.2, 131.2,
131.5, 197.6; IR (KBr) ν = 3133, 3063, 2983, 2938, 2905, 1668, 1628, 1470, 1387, 1287, 1249, 1198,
1020, 965, 789 cm-1; ESI-MS m/z Calcd: 309.1. Found: 310.1 [M++1]. Anal. Calcd for C15H20NO4P: C,
58.25; H, 6.52; N, 4.53. Found: C, 58.27; H, 6.53; N, 4.55.
Diethyl 2-benzoylindolizin-7-ylphosphonate (3b)
brown solid; mp 103-105 C. 1H NMR (CDCl3, 300 MHz): δ 1.35 (t, J = 7.1 Hz, 6H), 4.05-4.25 (m, 4H),
6.77-6.84 (m, 1H), 7.09 (s, 1H), 7.48-7.54 (m, 2H), 7.58-7.63 (m, 1H), 7.88-7.92 (m, 2H), 7.92-7.96 (m,
2H), 8.03 (d, J = 16.8 Hz, 1H); 13C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 62.3, 62.4, 105.6, 112.2, 112.4,
116.6, 118.6, 119.2, 125.2, 125.4, 127.6, 127.8, 128.3, 128.4, 129.4, 131.1, 131.3, 132.2, 139.0, 191.4; IR
(KBr) ν = 3138, 3060, 2981, 2941, 2900, 1641, 1470, 1318, 1240, 1024, 962, 795 cm-1; ESI-MS m/z
Calcd: 357.1. Found: 358.1 [M++1]. Anal. Calcd for C19H20NO4P: C, 63.86; H, 5.64; N, 3.92. Found: C,
63.87; H, 5.64; N, 3.90.
Diethyl 2-(4-methylbenzoyl)indolizin-7-ylphosphonate (3c)
pale yellow-green solid; mp 132-134 C. 1H NMR (CDCl3, 300 MHz): δ 1.35 (t, J = 7.1 Hz, 6H), 2.46 (s,
3H), 4.05-4.25 (m, 4H), 6.77-6.82 (m, 1H), 7.07 (s, 1H), 7.31 (d, J = 7.8 Hz, 2H), 7.84 (d, J = 7.8 Hz, 2H),
7.87 (s, 1H), 7.92-7.95 (m, 1H), 8.03 (d, J = 16.8 Hz, 1H); 13C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 21.6,
62.3, 62.4, 105.6, 112.1, 112.3, 116.4, 118.5, 119.0, 125.2, 125.4, 127.6, 127.8, 128.5, 129.1, 129.6, 131.0,
131.3, 136.3, 143.0, 191.1; IR(KBr) ν = 3136, 3059, 2980, 2900, 1630, 1470, 1320, 1244, 1026, 969, 782
cm-1; ESI-MS m/z Calcd: 371.1. Found: 372.1 [M++1]. Anal. Calcd for C20H22NO4P: C, 64.68; H, 5.97;
N, 3.77. Found: C, 64.66; H, 5.98; N, 3.77.
Diethyl 2-(4-methoxybenzoyl)indolizin-7-ylphosphonate (3d)
1
white solid; mp 167-169 C. H NMR (CDCl3, 300 MHz): δ 1.35 (t, J = 7.1 Hz, 6H), 3.91 (s, 3H),

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HETEROCYCLES, Vol. 87, No. 4, 2013
4.05-4.25 (m, 4H), 6.77-6.83 (m, 1H), 6.98-7.02 (m, 2H), 7.06 (s, 1H), 7.87 (s, 1H), 7.92-7.99 (m, 3H),
13
8.03 (d, J = 16.8 Hz, 1H); C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 55.5, 62.26, 62.33, 105.5, 112.0,
112.2, 113.7, 118.2, 118.9, 125.2, 125.4, 127.6, 127.7, 128.6, 130.9, 131.2, 131.6, 131.8, 163.1, 190.1;
IR(KBr) ν = 3129, 2980, 2937, 2905, 1629, 1600, 1572, 1469, 1316, 1242, 1168, 1017, 953, 769 cm-1;
ESI-MS m/z Calcd: 387.1. Found: 388.1 [M++1]. Anal. Calcd for C20H22NO5P: C, 62.01; H, 5.72; N, 3.62.
Found: C, 62.03; H, 5.70; N, 3.59.
Diethyl 2-(4-chlorobenzoyl)indolizin-7-ylphosphonate (3e)
yellow-green solid; mp 136-138 C. 1H NMR (CDCl3, 300 MHz): δ 1.35 (t, J = 7.1 Hz, 6H), 4.07-4.23 (m,
4H), 6.79-6.85 (m, 1H), 7.04 (s, 1H), 7.47-7.51 (m, 2H), 7.86-7.87 (m, 2H), 7.89-7.90 (m, 1H), 7.94 (dd,
13
J = 7.1 Hz, J = 3.2 Hz, 1H), 8.03 (d, J = 16.5 Hz, 1H); C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 62.3,
62.4, 105.5, 112.4, 112.5, 116.9, 118.4, 119.5, 125.2, 125.4, 127.6, 127.7, 127.9, 128.7, 128.9, 130.8,
130.9, 131.2, 131.4, 137.3, 138.7, 190.1; IR(KBr) ν = 3137, 2983, 2929, 1631, 1584, 1472, 1400, 1241,
1167, 1088, 1019, 954, 746 cm-1; ESI-MS m/z Calcd: 391.1. Found: 392.1 [M++1]. Anal. Calcd for
C19H19ClNO4P: C, 58.25; H, 4.89; N, 3.58. Found: C, 58.26; H, 4.90; N, 3.59.
Diethyl 2-(4-fluorobenzoyl)indolizin-7-ylphosphonate (3f)
yellowish brown solid; mp 123-124 C. 1H NMR (CDCl3, 300 MHz): δ 1.36 (t, J = 7.1 Hz, 6H), 4.05-4.25
(m, 4H), 6.79-6.85 (m, 1H), 7.05 (s, 1H), 7.15-7.23 (m, 2H), 7.87 (s, 1H), 7.93-7.99 (m, 3H), 8.03 (d, J =
13
16.8 Hz, 1H); C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 62.3, 62.4, 105.5, 112.3, 112.5, 115.4, 115.7,
118.4, 125.2, 125.4, 127.6, 127.7, 128.1, 131.1, 131.4, 131.9, 132.0, 135.17, 135.21, 163.6, 167.0, 189.9;
IR(KBr) ν = 3136, 3064, 2983, 2939, 2901, 1640, 1594, 1504, 1470, 1387, 1318, 1240, 1155, 1082, 1024,
969, 886, 787 cm-1; ESI-MS m/z Calcd: 375.1. Found: 376.1 [M++1]. Anal. Calcd for C19H19FNO4P: C,
60.80; H, 5.10; N, 3.73. Found: C, 60.81; H, 5.08; N, 3.73.
Diethyl 2-(4-nitrobenzoyl)indolizin-7-ylphosphonate (3g)
yellow solid; mp 168-170 C. 1H NMR (CDCl3, 300 MHz): δ 1.36 (t, J = 7.1 Hz, 6H), 4.06-4.26 (m, 4H),
6.82-6.88 (m, 1H), 7.04 (s, 1H), 7.87 (s, 1H), 7.95 (dd, J = 7.2 Hz, J = 3.3 Hz, 1H), 8.01-8.07 (m, 3H),
13
8.36-8.38 (m, 2H); C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 62.36, 62.43, 105.3, 112.7, 112.9, 117.4,
118.7, 120.0, 123.6, 125.3, 125.5, 127.3, 127.6, 127.7, 130.1, 131.4, 131.6, 144.1, 149.8, 189.5; IR(KBr)
ν = 3138, 2992, 1646, 1600, 1525, 1467, 1347, 1319, 1236, 1084, 1048, 1018, 961, 796 cm-1; ESI-MS m/z
Calcd: 402.1. Found: 403.1 [M++1]. Anal. Calcd for C19H19N2O6P: C, 56.72; H, 4.76; N, 6.96. Found: C,
56.73; H, 4.77; N, 6.94.
General procedure for the synthesis and analytical data of 5a,b
Compounds 5a and 5b were obtained using the same method for 3a-g in 57%, 63% yields, respectively.
3-Diethoxyphosphorylpyrido[1,2-a]benzimidazole (5a)

HETEROCYCLES, Vol. 87, No. 4, 2013
823
yellow-green solid; mp 114-116 C. 1H NMR (CDCl3, 300 MHz): δ 1.38 (t, J = 7.1 Hz, 6H), 4.12-4.31 (m,
4H), 7.16-7.22 (m, 1H), 7.45-7.50 (m, 1H), 7.58-7.64 (m, 1H), 7.96 (d, J = 8.1Hz,1H), 8.03 (d, J = 8.1 Hz,
1H), 8.26 (d, J = 17.1 Hz, 1H), 8.54-8.58 (m, 1H); 13C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 62.86, 62.93,
110.8, 120.5, 122.6, 123.3, 123.5, 125.2, 125.4, 126.6, 128.5; IR(KBr) ν = 3032, 2979, 2924, 1502, 1484,
1467, 1391, 1320, 1244, 1089, 1031, 966, 792, 757 cm-1; ESI-MS m/z Calcd: 304.1. Found: 305.1 [M++1].
Anal. Calcd for C15H17N2O3P: C, 59.21; H, 5.63; N, 9.21. Found: C, 59.19; H, 5.64; N, 9.19.
3-Diethoxyphosphoryl-7,8-dimethylpyrido[1,2-a]benzimidazole (5b)
yellow-green solid; mp 146-147 C. 1H NMR (CDCl3, 300 MHz): δ 1.37 (t, J = 7.2 Hz, 6H), 2.48 (s, 3H),
2.50 (s, 3H), 4.10-4.33 (m, 4H), 7.07-7.13 (m, 1H), 7.67 (s, 1H), 7.75 (s, 1H), 8.16 (d, J = 16.8 Hz, 1H),
8.43-8.47 (m, 1H); 13C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 20.7, 20.8, 62.7, 62.8, 109.8, 110.0, 110.5,
120.3, 123.4, 123.6, 125.0, 125.2, 127.2, 127.3, 129.9, 132.3, 136.1, 143.9, 146.1, 146.3; IR(KBr) ν =
3047, 2985, 2942, 1503, 1467, 1388, 1297, 1249, 1081, 1023, 963, 841, 778 cm-1; ESI-MS m/z Calcd:
332.1. Found: 333.1 [M++1]. Anal. Calcd for C17H21N2O3P: C, 61.44; H, 6.37; N, 8.43. Found: C, 61.43;
H, 6.36; N, 8.44.
General procedure for the synthesis and analytical data of 8a-c
To a 50-mL round-bottomed flask were added 6a-c (2.0 mmol), powdered pyridinium chlorochromate
(0.86 g, 4 mmol) and DMF (10 mL). The mixture was stirred at rt for 3 h. Compound 2 (2.4 mmol) and
potassium carbonate (4.4 mmol) were added when TLC indicated that compound 6a-c was all oxidized to
7a-c. The mixture was stirred at 70 C for 8-12 h and then filtered. The filtrate was poured to water (50
mL) and extracted by EtOAc (15 mL×3). The combined extracts were washed by brine and then dried
over anhydrous MgSO4. After concentrated under reduced pressure, the crude products were purified by
column chromatography to afford 8a-c in 47-53% yield.
Diethyl 2-phenylpyrazolo[1,5-a]pyridin-5-ylphosphonate (8a)
1
white solid; mp 76-78 C. H NMR (CDCl3, 300 MHz): δ 1.36 (t, J = 7.1 Hz, 6H), 4.10-4.25 (m, 4H),
6.96-7.02 (m, 2H), 7.36-7.42 (m, 1H), 7.44-7.50 (m, 2H), 7.95-8.00 (m, 2H), 8.09-8.15 (m, 1H),
8.50-8.54 (m, 1H); 13C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 62.5, 62.6, 96.2, 111.7, 111.8, 122.4, 124.0,
124.2, 125.0, 126.6, 128.4, 128.6, 128.8, 128.9, 132.6, 140.1, 140.4, 154.6; IR(KBr) ν = 3100, 3019, 2924,
2854, 1907, 1667, 1619, 1548, 1507, 1453, 1346, 1314, 1285, 1178, 1129, 1081, 997, 900, 784, 599 cm-1;
ESI-MS m/z Calcd: 330.1. Found: 331.1 [M++1]. Anal. Calcd for C17H19N2O3P: C, 61.81; H, 5.80; N,
8.48. Found: C, 61.79; H, 5.81; N, 8.47.
Diethyl 2-p-tolylpyrazolo[1,5-a]pyridin-5-ylphosphonate (8b)
1
reddish oil, H NMR (CDCl3, 300 MHz): δ 1.36 (t, J = 7.1Hz, 6H), 2.40 (s, 3H), 4.09-4.25 (m, 4H),
6.93-7.00 (m, 2H), 7.27 (d, J = 7.8 Hz, 2H), 7.86 (d, J = 7.8 Hz, 2H), 8.10 (d, J = 16.2 Hz, 1H), 8.51 (dd,

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J = 3.3 Hz, J = 6.9 Hz, 1H); 13C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 21.3, 62.5, 62.6, 96.0, 111.5, 111.6,
122.2, 123.9, 124.1, 124.7, 126.5, 128.4, 128.6, 129.6, 129.8, 138.8, 140.1, 140.3, 154.7; IR(KBr) ν =
3110, 3028, 2976, 2924, 2858, 1924, 1725, 1620, 1546, 1518, 1462, 1338, 1284, 1244, 1091, 1019, 928,
784 cm-1; ESI-MS m/z Calcd: 344.1. Found: 345.1 [M++1]. Anal. Calcd for C18H21N2O3P: C, 62.78; H,
6.51; N, 8.14. Found: C, 62.80; H, 6.50; N, 8.14.
Diethyl 2-(4-fluorophenyl)pyrazolo[1,5-a]pyridin-5-ylphosphonate (8c)
1
white solid; mp 94-96 C. H NMR (CDCl3, 300 MHz): δ 1.36 (t, J = 7.1 Hz, 6H), 4.07-4.25 (m, 4H),
6.91 (s, 1H), 6.96-7.02 (m, 1H), 7.12-7.18 (m, 2H), 7.92-7.97 (m, 2H), 8.11 (d, J = 16.2 Hz, 1H), 8.50 (dd,
13
J = 3.0 Hz, J = 6.9 Hz, 1H); C NMR (CDCl3, 75 MHz): δ 16.3, 16.4, 62.5, 62.6, 95.9, 111.7, 111.9,
115.7, 116.0, 122.7, 124.0, 124.1, 125.3, 128.3, 128.4, 128.6, 128.85, 128.90, 140.2, 140.4, 153.6, 161.6,
164.9; IR(KBr) ν = 3077, 2984, 2935, 2905, 1890, 1603, 1474, 1255, 1157, 1097, 1027, 969, 811, 769,
586 cm-1; ESI-MS m/z Calcd: 348.1. Found: 349.1 [M++1]. Anal. Calcd for C17H18FN2O3P: C, 58.62; H,
5.21; N, 8.04. Found: C, 58.63; H, 5.21; N, 8.05.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the Science and Technology Special Major Project for Significant
New Drugs Formulation (2011ZX09401-009), the Shandong Natural Science Foundation (NO.
Y2008B40) and the Natural Science Foundation of Shandong Province (No. ZR2012BL04) for financial
support of this work. We also thank State Key Laboratory of Crystal Materials of Shandong University
for crystal data.
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