4β-Isocyanopodophyllotoxins: Valuable precursors for the synthesis of new podophyllotoxin analogues

Article abstract of DOI:10.2478/s11696-011-0062-x

A new and efficient method for the synthesis of novel 4β- isocyanopodophyllotoxins as a valuable building block for the synthesis of versatile bioactive podophyllotoxin analogues under both classical and ultrasonic conditions was developed. In general, significant improvements in rates of reaction and yields of sonochemical reactions relative to the classical ones were observed.

Full text of DOI:10.2478/s11696-011-0062-x

Chemical Papers 65 (5) 739–742 (2011)
DOI: 10.2478/s11696-011-0062-x
SHORT COMMUNICATION
4β-Isocyanopodophyllotoxins: valuable precursors for the synthesis
of new podophyllotoxin analogues
a
a
b
c
aYing-Qian Liu*, Lin-Hai Li, Wen-Qun Li, Gang Feng, Liu Yang
aSchool of Pharmacy, Lanzhou University, Lanzhou 730000, China
bEnvironment and Plant Protection Institute, Chinese Academy of Tropical Agricultural Science, Danzhou 571737, China
cEnvironmental and Municipal Engineering School, Lanzhou Jiaotong University, Lanzhou 730000, China
Received 16 March 2011; Accepted 31 March 2011
A new and efficient method for the synthesis of novel 4β-isocyanopodophyllotoxins as a valuable
building block for the synthesis of versatile bioactive podophyllotoxin analogues under both classical
and ultrasonic conditions was developed. In general, significant improvements in rates of reaction
and yields of sonochemical reactions relative to the classical ones were observed.
c
ꢀ 2011 Institute of Chemistry, Slovak Academy of Sciences
Keywords: 4β-isocyanopodophyllotoxins, anticancer drugs, multicomponent reactions, ultrasound
Isocyanides constitute a special group of start-
ing materials useful for producing combinatorial li-
braries in isocyanide based multicomponent reactions
(IMCRs) due to their unique reactivity involving the
formation of α-adducts with nucleophiles and elec-
trophiles (D¨omling, 2006; Shaabani et al., 2008; El
Kaim et al., 2008; Bremner & Organ, 2007; Bon et
al., 2005; Nair et al., 2001). Nowadays, they are in-
creasingly used as valuable building blocks for various
biologically active molecule based multicomponent re-
actions in organic synthesis and medicinal chemistry
(Touré & Hall, 2009; Dolle et al., 2009). Despite
their promising applications in drug discovery, there
is a very limited number of applicable diverse iso-
cyanide building blocks. Thus, finding or designing
efficient and practical bioactive isocyanide building
blocks seems to be important.
O
R
O
O
O
OH
HO
HO
O
O
O
O
O
O
O
O
H3CO
OCH3
OCH3
H3CO
OH
OCH3
I
II R= CH3
III R=
S
Fig. 1. Structures of podophyllotoxin (I), etoposide (II ), and
teniposide (III).
Podophyllotoxin (I, (10R,11R,15R,16R)-16-hydro-
xy-10-(3,4,5-trimethoxyphenyl)-4,6,13-trioxatetra-
cyclo[7.7.0.03,7.011,15]hexadeca-1,3(7),8-trien-12-one,
Fig. 1), a naturally occurring bioactive lignan used as
a molecular precursor for the development of potent
antineoplastic drugs and antiviral agents (Bohlin &
Rosen, 1996; Gordaliza et al., 2004; Liu et al., 2007),
exhibits promising insecticidal activity (Liu et al.,
2008; Xu et al., 2009). Extensive chemical transforma-
tions of podophyllotoxin led to the clinical introduc-
tion of two C-4β-glucoconjugate analogues of etopo-
side (VP-16, II ) and teniposide (VM-26, III ), their
clinical success has greatly stimulated the interest in
further study of the modification of C-4 substituent in
*Corresponding author, e-mail: yqliu@lzu.edu.cn

740
Y.-Q. Liu et al./Chemical Papers 65 (5) 739–742 (2011)
NC
NHCHO
NH2
O
O
O
O
O
O
i
ii
O
O
O
O
O
O
OCH3
OCH3
OCH3
H3CO
H3CO
H3CO
OR
OR
OR
IV R = CH3
V R = H
VI R = CH3
VII R = H
VIII R = CH3
IX R = H
Fig. 2. Synthesis of 4β-isocyanopodophyllotoxins. Reaction conditions: i) ultrasonic reflux, 1 h, HCO2C2H5/CH2Cl2; ii) p-
TsCl/pyridine, ultrasonic, room temperature, 2 h.
I to achieve better antitumour activity. However, typ-
ical replacement of the C-4 position in I has been lim-
ited mainly to simple groups such as 4β-substituted
ethers, esters and N-linked congeners. To enrich the
limited set of podophyllotoxin derivatives typically
employed in drug discovery, an efficient synthesis of
4β-isocyanopodophyllotoxins is reported herein; such
key building blocks increase the range of available iso-
cyanides and furthermore allow the synthesis of a ver-
satile bioactive podophyllotoxin fragment in the MCR
products, libraries created in the MCRs reactions and
heterocycles built from isocyanides.
rates/yields/selectivity in organic synthesis and that it
has found wide application in synthetic organic chem-
istry. Significant enhancement in the reaction rates
and yields was observed when using the sonochemi-
cal reactions relative to the classical ones.
In a typical procedure, the starting material of
4β-aminopodophyllotoxins (IV, V ) is prepared from
podophyllotoxin by employing the above procedures
(Yu et al.,1999; Liu et al., 2010), then, formylation
of 4β-aminopodophyllotoxins with ethyl formate pro-
ceeds followed by the dehydration of 4β-formamido-
4-deoxypodophyllotoxins (VI, VII ) with p-toluene
sulfonyl chloride in combination with pyridine un-
der both classical and ultrasonic conditions to give
the corresponding 4β-isocyanopodophyllotoxins (VIII,
XI ) in moderate to good yields. In general, signif-
icant improvement in the reaction times and yields
was observed when ultrasonic conditions were applied.
The desired compounds (VIII, XI) were characterized
by m.p., IR, 1H NMR, 13C NMR, MS, and HRMS
analyses. A noteworthy aspect of this reaction is that
the γ-lactone ring is intact under ultrasonic conditions
though this reagent is known to form the correspond-
ing cis-fused lactone (Roulland et al., 2002). More-
over, the assignment of configuration at the C-4 po-
sition in 4β-isocyanopodophyllotoxins (VIII, XI ) was
based on the J3,4 coupling constants. According to the
Karplus dihedral angle rule in six-membered rings, in
C-4-α-substituted compounds, J3,4 ≥ 8.5 Hz as H-3 is
in trans position to H-4, whereas in C-4β-substituted
compounds, J3,4 < 4.5 Hz due to the cis relationship
between H-3 and H-4 (Wang et al., 1990).The overall
sequence is shown in Fig. 2.
Available methods for the preparation of iso-
cyanides mainly involve the dehydration of N-substitu-
ted formamides using phosgene, diphosgene, phos-
phorus oxychloride in combination with bases, and
the reaction of primary amines with chloroform in
the presence of strong bases (Katritzky et al., 1993;
Doemling & Ugi, 2000). Although some good results
were obtained, most of these methods often suffer
from one or more limitations with respect to gen-
eral applicability, chemoselectivity, ready availability,
operational convenience, and the use of toxic phos-
gene and diphosgene as reactive reagents. These lim-
itations make their application difficult not only in
large-scale synthesis of isocyanides and screening li-
braries for drug discovery but disadvantage them
also from a general industrial standpoint. Due to
the above reasons and also as a part of our ongo-
ing project aiming at the development of novel multi-
component reactions based on podophyllotoxin con-
geners of biological significance, a facile and reli-
able access to isocyanopodophyllotoxins is required.
After the screening of various dehydrating agents,
including POCl3/NEt3, CHCl3/KOH, SOCl2/DMF,
phenyl chlorothionoformate, Tf2O/Hu¨nig’s base, the
easily available and cheap p-toluene sulfonyl chloride
(TsCl) in combination with pyridine was selected as
a mild and high-yielding alternative to the synthe-
sis of isocyanopodophyllotoxins under classical con-
ditions in the simple ultrasonic technique The results
of our study are reported in this paper. It is worth
to mention that ultrasound as a non-thermal energy
transfer source is well known to enhance the reaction
In conclusion, a novel, mild and efficient synthe-
sis of 4β-isocyanopodophyllotoxins by dehydration of
the corresponding formamide under both ultrasonic
and classical conditions with practical applicability
is described for the first time. In general, signifi-
cant enhancement in the reaction rates and yields
for the sonochemical reactions relative to the classi-
cal ones were observed. This procedure provides valu-
able building blocks for the synthesis of versatile bio-
logically significant podophyllotoxin fragments in the
MCR products, libraries created in the MCRs reac-

Y.-Q. Liu et al./Chemical Papers 65 (5) 739–742 (2011)
Table 1. Spectral data of newly prepared compounds
741
Compound
Spectral data
IR, ν˜/cm−1: 2130 (NC), 1779 (lactone), 1588, 1506, and 1485 (aromatic C C), 930 (OCH2O)
—
VIII
—
1H NMR (CDCl3) δ: 6.91 (s, 1H, H-5), 6.56 (s, 1H, H-8), 6.26 (s, 2H, H-2ꢀ,6ꢀ), 6.03 (dd, J = 7.9 Hz, J = 1.1 Hz, 2H,
OCH2O), 4.99 (d, J = 4.4 Hz, 1H, H-4), 4.68 (d, J = 5.1 Hz, 1H, H-1), 4.39 (m, 2H, H-11), 3.81 (s, 3H, 4ꢀ-OCH3),
3.75 (s, 6H, 3ꢀ,5ꢀ —OCH3), 3.20 (q, 1H, H-2), 2.96 (m, 1H, H-3)
13C NMR: (CDC13) δ: 173.01, 160.01, 152.78, 149.27, 147.95, 137.64, 134.32, 131.59, 125.13, 110.53, 108.71, 108.33,
101.95, 67.69, 60.73, 56.35, 53.53, 43.53, 41.37, 35.24
MS(EI) (m/z): 424 (M+1)
HRMS (m/z) for C23H21NO7 [M+NH4]+: calc. 441.1656, found 441.1652
IR, ν˜/cm−1: 3447 (OH), 2130 (NC), 1745 (lactone), 1600, 1500, and 1480 (aromatic C C), 933 (OCH2O)
—
XI
—
1H NMR (CDCl3) δ: 6.88 (s, 1H, H-5), 6.50 (s, 1H, H-8), 6.32 (s, 2H, H-2ꢀ,6ꢀ), 5.98 (dd, J = 7.9 Hz, J = 1.1 Hz, 2H,
OCH2O), 4.92 (d, J = 4.4 Hz, 1H, H-4), 4.68 (d, J = 5.1 Hz, 1H, H-1), 4.37 (m, 2H, H-11), 3.78 (s, 6H, 3ꢀ,5ꢀ-OCH3),
3.15 (q, 1H, H-2), 2.80 (m, 1H, H-3)
13C NMR: (CDC13) δ: 175.40, 160.01, 152.78, 149.27, 146.30, 137.64, 134.12, 133.95, 125.13, 110.53, 108.71, 108.33,
101.95, 67.69, 60.73, 56.35, 53.53, 43.70, 41.37, 38.02
MS(EI) (m/z): 410 (M+1)
HRMS (m/z) for C22H19NO7 [M+NH4]+: calc. 427.1843, found 427.1825
tions and heterocycles built from isocyanides.
5 min, the ice-bath was removed and the mixture was
stirred at room temperature for 8 h. The yellow solu-
tion slowly turned brown. The solution was quenched
by an addition of water (10 mL) and extracted with
CH2Cl2 (3 × 15 mL). The organic layer was washed
with a saturated aqueous solution of NaHSO4 (2 ×
15 mL); the extracts were dried (MgSO4), filtered, the
reaction solvent was evaporated and chromatographed
on silica-gel (hexane/ethyl acetate, ϕr = 9 : 1, then 8
: 2, and 7 : 3) to give compounds VIII and XI as white
solids in ≈ 35–37 % yields.
Melting points were taken on a Kofler melting
point apparatus (Triumph Company, Germany) and
are uncorrected; IR spectra were obtained on a NIC-
5DX spectrophotometer (Nicote Company, USA);
mass spectral analysis was performed on a ZAB-HS
and Bruker Daltonics APEXII49e instrument (Bruker
Company, USA). Optical rotations were determined
on a Perkin–Elmer Model 341 spectropolarimeter
(Taike Company, China). NMR spectra were recorded
on a Bruker AM-400 spectrometer (Bruker Com-
pany, USA) at 400 MHz using TMS as the reference.
Sonication was performed in a Shanghai Branson-
CQX (Shanghai Branson Company, China) ultrasonic
cleaner at the frequency of 25 kHz and nominal power
of 500 W. The reaction flask was located in the maxi-
mum energy area of the cleaner, where the surface of
reactants (reaction vessel) is slightly lower than the
level of water; addition or removal of water was used
to control the temperature of the water bath. Chem-
icals are commercially available and were used with-
out further purification. The starting materials of 4β-
aminopodophyllotoxins (IV, V) were prepared by the
above procedures in our laboratory (Yu et al., 1999;
Liu et al., 2010).
General procedure for 4β-isocyanopodophylloto-
xins (VIII, XI ) preparation: in method A (classi-
cal method), to a solution of 0.24 mmol of 4β-
aminopodophyllotoxins (IV, V ) in 10 mL of CH2Cl2,
5 mL of ethyl formate were added and the reac-
tion mixture was refluxed for 4 h. The solvents
were then evaporated under reduced pressure, and
the residue was purified by flash chromatography
(CH2Cl2/MeOH, ϕr = 98 : 2) to give 4β-formamido-4-
deoxypodophyllotoxins (VI, VII ) in ≈ 45–47 % yields.
Subsequently, to a cooled and stirred solution of 0.31
mmol of formamide (VI, VII ) in pyridine (5 mL), 0.77
mmol of p-toluenesulfonyl chloride was added. After
In method B (ultrasound irradiation), to a solution
of 0.24 mmol of 4β-aminopodophyllotoxins (IV, V ) in
10 mL of CH2Cl2, 5 mL of ethyl formate were added
and the above mixture was irradiated by an ultrasonic
generator in a water bath at 30◦C for 1 h. Progress of
the reaction was monitored by TLC. After completion
of the reaction, the solvents were evaporated under re-
duced pressure, and the residue was purified by flash
chromatography (CH2Cl2/MeOH, ϕr = 98 : 2) to give
4β-formamido-4-deoxypodophyllotoxins (VI, VII ) in
≈ 78–80 % yields. Subsequently, to a cooled and
stirred solution of 0.31 mmol of formamide (VI, VII )
in 5 mL of pyridine, 0.77 mmol of p-toluenesulfonyl
chloride was added. After 5 min, the ice-bath was re-
moved and the mixture was sonicated at room temper-
ature for 2 h. The yellow solution slowly turned brown.
The solution was quenched by an addition of water
(10 mL) and extracted with CH2Cl2 (3 × 15 mL).
The organic layer was washed with a saturated aque-
ous solution of NaHSO4 (2 × 15 mL), the extracts
were dried (MgSO4), filtered, concentrated, and chro-
matographed on silica-gel (hexane/ethyl acetate, ϕr =
9 : 1, then 8 : 2, and 7 : 3) to give compounds VIII
and XI as white solids in ≈ 65–67 % yields. Charac-
terization data of newly prepared compounds: VIII,
yield: 67 %, m.p. = 115–117◦C; XI, yield: 65 %, m.p.
= 187–189◦C. Spectral data are listed in Table 1.

742
Y.-Q. Liu et al./Chemical Papers 65 (5) 739–742 (2011)
Acknowledgements. This work was financially supported by
Liu, Y.-Q., Li, L.-H., Yang, L., & Li, H.-Y. (2010). A novel,
stereoselective and practical protocol for the synthesis of 4β-
aminopodophyllotoxins. Chemical Papers, 64, 533–536. DOI:
10.2478/s11696-010-0020-z.
Liu, Y.-Q., Liu, Y., Xiao, H., Gao, R., & Tian, X. (2008).
Synthesis and insecticidal activities of novel derivatives
of podophyllotoxin: Part XII. Pesticide Biochemistry and
Physiology, 91, 116–121. DOI: 10.1016/j.pestbp.2008.03.002.
Liu, Q. Y., Liu, Y., & Xuan, T. (2007). Podophyllotoxin: Cur-
rent perspectives. Current Bioactive Compounds, 3, 37–66.
DOI: 10.2174/157340707780126499.
the National Natural Science Foundation of China (30800720);
the Post-Doctor Research Foundation (20090450142); the Fun-
damental Research Funds for the Central Universities (lzujbky-
2009-98); the Key Scientific Research Programme of EPPI
(2010hzsZDZX003) of the Chinese Academy of Tropical Agri-
cultural Science, the Ministry of Agriculture, and the Natural
Science Foundation of the Gansu Province (0916RJZA033).
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