A novel, stereoselective and practical protocol for the synthesis of 4β-aminopodophyllotoxins

Article abstract of DOI:10.2478/s11696-010-0020-z

Ritter reaction of podophyllotoxins with chloroacetonitrile and subsequent cleavage of the chloroacetyl group in the resulting chloroacetamide with thiourea under both classical heating and ultrasonic conditions is an efficient procedure for the synthesis

Full text of DOI:10.2478/s11696-010-0020-z

Chemical Papers 64 (4) 533–536 (2010)
DOI: 10.2478/s11696-010-0020-z
SHORT COMMUNICATION
A novel, stereoselective and practical protocol for the synthesis
of 4β-aminopodophyllotoxins
b
c
a,bYing-Qian Liu, Lin-Hai Li, Liu Yang, a,bHong-Yu Li*
aMOE Key Laboratory of Arid and Grassland Ecology, School of Life Sciences, bSchool of Pharmacy, Lanzhou University,
Lanzhou 730000, China
cEnvironmental and Municipal Engineering School, Lanzhou Jiaotong University, Lanzhou 730000, China
Received 1 October 2009; Revised 26 November 2009; Accepted 2 December 2009
Ritter reaction of podophyllotoxins with chloroacetonitrile and subsequent cleavage of the
chloroacetyl group in the resulting chloroacetamide with thiourea under both classical heating and
ultrasonic conditions is an efficient procedure for the synthesis of 4β-aminopodophyllotoxins. In
general, significant improvements in the rates of reaction and yields of the sonochemical reactions
relative to the classical heating reactions were observed.
c
ꢀ 2010 Institute of Chemistry, Slovak Academy of Sciences
Keywords: 4β-aminopodophyllotoxins, anticancer drugs, Ritter reaction, ultrasound
Semisynthetic analogues of naturally occurring
podophyllotoxin (I ) have drawn renewed interest in
recent years as a result of the development of etopo-
side (VP-16, II ) and teniposide (VM-26, III ) as an-
ticancer drugs (Canetta et al., 1982; Bohlin & Rosen,
1996; Gordaliza et al., 2004). These semisynthetic glu-
coconjugates of epipodophyllotoxin block the catalytic
activity of DNA topoisomerase II and concurrent
enzyme-mediated production of lethal DNA strand
breaks leading to DNA damage and cytotoxicity
(Wilstermann et al., 2007; Berger et al., 1996; Hande,
1998; Burden & Osheroff, 1998). Their clinical success
as well as intriguing mechanism of action has greatly
stimulated the interest in further studies on the modi-
fication of the C-4 substituent of I for better antitumor
activity. Some nonsugar substituted analogues, par-
ticularly nitrogen containing congeners were found to
exhibit pharmacological properties superior to VP-16
and some of them were brought into clinical evalua-
tions (Cho et al., 1996a, 1996b; Kamal et al., 2003,
2005). In view of the important biological activities as
well as of the significant clinical role of N-substituted
derivatives, 4β-aminopodophyllotoxins are of consid-
erable interest as attractive synthetic targets since
they serve as important precursors to construct a va-
riety of biologically useful N-substituted derivatives
of 4β-aminopodophyllotoxins with powerful antineo-
plastic and insecticidal properties (Roulland et al.,
2002; Hansen et al., 1993; Li et al., 2006). Convenient
methods available for the conversion of podophyllo-
toxins to 4β-aminopodophyllotoxins and the key step
in the sequence involve the azidation of podophyllo-
toxins through a HN3 (or NaN3)/Lewis acids system
and further reduction of 4β-azidopodophyllotoxins by
catalytic hydrogenation, H2/HCO2NH4, TMSCl/NaI,
FeSO4/NH3, SmI2, Zn/HCO2NH4, and the biocat-
alytic method (Liu et al., 2007; Yu et al., 1999; Kamal
et al., 1997, 1998; Chen et al., 2000; Xu et al., 2008).
Although some good results were obtained using these
protocols, most of these methods often suffer from one
or more limitations with respect to general applica-
bility, stereoselectivity, ready availability, operational
convenience, the main disadvantage of these methods
being the use of hazardous and explosive azides (HN3
or NaN3) as reactive reagents, which makes them un-
suitable not only for large-scale synthesis of this class
*Corresponding author, e-mail: lihy@lzu.edu.cn

534
Y.-Q. Liu et al./Chemical Papers 64 (4) 533–536 (2010)
4β-chloroamidopodophyllotoxins (V, VI ); subsequent
O
R
O
cleavage of the chloroacetyl group with thiourea gave
the corresponding 4β-aminopodophyllotoxins (VII,
VIII ) under both classical heating and ultrasonic con-
ditions in moderate to good yields (Fig. 2). The de-
sired compounds (VII, VIII ) were characterized by
O
OH
O
HO
OH
O
O
O
O
O
O
O
O
1
m.p., IR, H and 13C NMR, MS, and HRMS analy-
ses.
Melting points were taken on a Kofler melting
point apparatus and are uncorrected. IR spectra were
obtained on an NIC-5DX spectrophotometer and mass
spectral analysis was performed on ZAB-HS and
Bruker Daltonics APEXII49e instruments. 1H (400
MHz) and 13C (100 MHz) NMR spectra were recorded
on a Bruker AM-400 spectrometer using TMS as
reference. Sonication was performed in a Shanghai
Branson-CQX ultrasonic cleaner with the frequency of
25 kHz and the nominal power of 500 W. The reaction
flask was located in the maximum energy area in the
cleaner, where the surface of reactants (reaction ves-
sel) is slightly lower than the level of water; addition or
removal of water was used to control the temperature
of the water bath. All chemicals were commercially
available (Sigma–Aldrich Co., USA) and were used
without further purification. Podophyllotoxin (I ) and
4ꢀ-demethylpodophyllotoxin (IV ) were isolated from
the Chinese medicinal herb Podophyllum emodi Wall
var. Chinesis Sprague.
OCH3
OCH3
H3CO
H3CO
OCH3
I
OH
II, III
Fig. 1. Structure of podophyllotoxin (I), etoposide (II, R =
methyl), and teniposide (III, R = 2-thienyl).
of biologically active molecules but for the synthesis of
screening libraries for drug discovery or for industrial
production.
Due to the above reasons and also as a part of
our ongoing research program for the synthesis of
podophyllotoxin congeners of biological significance,
a facile and efficient synthetic route for the prepara-
tion of stereoselective 4β-aminopodophyllotoxins by
a modified Ritter reaction and smooth cleavage of
the chloroacetyl group with thiourea under stan-
dard conditions using a simple ultrasonic technique
is described in this paper. It is worth to men-
tion that ultrasound as a non-thermal energy trans-
fer source is well known to enhance reaction rates,
yields, and selectivity in organic synthesis and has
found widespread application in synthetic organic
chemistry. Significant enhancement in the reaction
rates and yields of sonochemical reactions relative to
the classical heating reactions was observed. Thus,
the Ritter reaction with ClCH2CN was carried out
with podophyllotoxin (I ) or its derivative IV in the
presence of 60 mass % of MsOH/Al2O3 to give
General procedure for the preparation of 4β-
aminopodophyllotoxins (VII, VIII )
Method A (classical heating): Stirred mixture of
podophyllotoxin (I, 4 mmol) and homogeneous mix-
ture of MsOH/Al2O3 (60 mass %, 1 g) in ClCH2CN
(10 mL) were heated at 60◦C for 5 h. After completion
of the reaction as indicated by TLC, the solvent was
evaporated under reduced pressure and the residue
was poured onto ice water (20 mL) and extracted with
ethyl acetate (3 × 50 mL). Combined organic extracts
O
Cl
OH
NH2
HN
O
O
O
O
O
O
O
O
O
O
O
O
i
iv
v or vi
ii or iii
OCH3
OCH3
OCH3
H3CO
H3CO
H3CO
OR
I, IV
OR
V, VI
OR
VII, VIII
Fig. 2. Synthesis of 4β-aminopodophyllotoxins. Reaction conditions: i) ClCH2CN, MsOH/Al2O3; ii) classical heating, 60◦C, 5 h;
iii) sonication, 60◦C, 2 h; iv) thiourea, AcOH; v) classical heating, 80◦C, 12 h; vi) sonication, 80◦C, 4 h. R = CH3 for I,
V, and VII; R = H for IV, VI, and VIII.

Y.-Q. Liu et al./Chemical Papers 64 (4) 533–536 (2010)
535
were dried over anhydrous Na2SO4, the mixture was
filtered and the filtrate was evaporated under reduced
pressure to give the crude product which was purified
by flash column chromatography using CHCl3/MeOH
(ϕr = 9.5 : 0.5) as an eluent affording the pure com-
pound V in a 50 % yield. Subsequently, a solution
of amide V (5 mmol) and thiourea (0.46 g, 6 mmol)
in AcOH (50 mL) was heated at 80◦C for 12 h, neu-
tralized with 20 mass % of aqueous NaHCO3 and ex-
tracted with CHCl3 (3 × 30 mL). Combined extracts
were washed with brine (30 mL), dried (MgSO4), fil-
tered, and concentrated under diminished pressure.
The residue was purified by column chromatography
to give compound VII as a white solid in a 50 % yield.
Analogously, amide VI (52 % yield) and subsequently
amine VIII (55 % yield) were prepared starting from
the podophyllotoxin derivative IV.
146.3, 134.1, 133.9, 131.2, 131.1, 110.2, 108.6, 107.9,
101.3, 68.1, 56.4, 48.9, 43.7, 40.2, 38.02; EIMS (100
eV), m/z: 400 (M + 1); HRMS, m/z calculated for
C21H21NO7: 400.1822 [M + H]+, found: 400.1825 [M
+ H]+.
It is necessary to note that this modified Ritter
reaction is a highly stereoselective process as it af-
fords nearly pure 4β-aminopodophyllotoxins in quan-
titative yield in contrast to the already reported chem-
ical protocol wherein α and β isomers were obtained
in the ratio of 2 : 5, respectively. The β-selectivity
in the present method is probably due to the epimer
formed at C-4 which seems to be directed by steric
hindrance of the bulky pseudoaxial trimethoxyphenyl
E-ring acting on the α-side of the 4-carbocation of
podophyllotoxins. It is shown in literature (Keller-
Juslen et al., 1971; Tian et al., 1997; Wang et al.,
1990) that biological activity is generaly retained or
predominant in the case of 4β-substituted podophyl-
lotoxin congeners when compared to their α-isomers,
thus illustrating the importance of the β-isomers syn-
thesis. Assignment of the configuration at the C-4 po-
sition to 4β-aminopodophyllotoxins (VII, VIII ) was
based on the J3,4 coupling constants. According to
the Karplus dihedral angle rule in six-membered rings,
the C-4α-substituted compounds have J3,4 ≥ 8.5 Hz
as the H-3 atom is trans to the H-4 one, whereas C-
4β-substituted compounds have J3,4 < 4.5 Hz due to
the cis relationship between the H-3 and H-4 atoms
(Wang et al., 1990).
In conclusion, a novel, mild, and efficient synthe-
sis of 4β-aminopodophyllotoxins by a modified Rit-
ter reaction under both sonication and classical heat-
ing conditions with practical applicability has been
described. In general, significant enhancement in the
reaction rates and yields of the sonochemical reac-
tions relative to the classical heating reactions was
observed. This procedure provides a significant im-
provement over the existing methods for obtaining 4β-
aminopodophyllotoxins. As such, the modified version
of the Ritter reaction opens up an easy way of prepar-
ing various new biologically significant N-substituted
derivatives of 4β-aminopodophyllotoxins.
Method B (ultrasound irradiation): To a stirred
mixture of podophyllotoxins (4 mmol) and ClCH2CN
(10 mL) in a round-bottomed flask equipped with a
condenser, homogeneous mixture of MsOH/Al2O3 (60
mass %, 1 g) was added and the mixture was irradi-
ated by an ultrasonic generator in a water bath at
60◦C for 2 h. The progress of reaction was monitored
by TLC followed by the same work-up as described
in method A to give pure V in an 80 % yield. Subse-
quently, a solution of amide V (5 mmol) and thiourea
(0.46 g, 6 mmol) in AcOH (50 mL) was sonicated at
80◦C for 4 h followed by the same work-up as de-
scribed in method A to give compound VII as a white
solid in an 82 % yield. Analogously, amide VI and
subsequently amine VIII were prepared starting from
the podophyllotoxin derivative IV in 82 % and 85 %
yields, respectively.
For compound VII: m.p. 110–112◦C; IR (KBr),
νmax/cm−1: 3430 (NH2), 1775 (lactone), 1590, 1550,
and 1484 (C
aromatic), 935 (OCH2O); 1H NMR
—
C
—
(CDC13, 400 MHz), δ: 6.84 (s, 1H, H-5), 6.47 (s, 1H,
H-8), 6.36 (s, 2H, H-2ꢀ, H-6ꢀ), 5.97 (s, 2H, OCH2O),
4.58 (d, 1H, J = 5.1 Hz, H-1), 4.32 (m, 2H, H-11), 4.25
(d, 1H, J = 4.0 Hz, H-4), 3.80 (s, 3H, 4ꢀ-OCH3), 3.72
(s, 6H, 3ꢀ,5ꢀ-OCH3), 3.32 (q, 1H, H-2), 2.92–2.63 (m,
1H, H-3), 1.84 (d, 2H, 4-NH2); 13C NMR (CDC13, 100
MHz), δ: 175.4, 147.6, 147.2, 146.3, 134.1, 133.9, 131.2,
131.1, 110.2, 108.6, 107.9, 101.3, 68.1, 56.4, 48.9, 43.7,
40.2, 37.9; EIMS (100 eV), m/z: 414 (M + 1); HRMS,
m/z calculated for C22H23NO7: 414.1366 [M + H]+,
found: 414.1365 [M + H]+.
Acknowledgements. This work was financially supported by
the National Natural Science Foundation of China (30800720);
the Interdisciplinary Innovation Research Foundation For
Young Scholars, Lanzhou University (LZUJC2007018); Pro-
gram for Changjiang Scholars and Innovative Research Team
in University (PCSIRT).
For compound VIII: m.p. 227–229◦C; IR (KBr),
νmax/cm−1: 3360 (OH), 3290 (NH2), 1745 (lactone),
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