Synthesis of podophyllotoxin and its derivatives via nicl2/nabh4 reduction of isoxazoline ring

Article abstract of DOI:10.14233/ajchem.2013.15073

A novel synthesis of podophyllotoxin was carried out in four steps. The key step was reduction of isoxazoline ring followed by diazotization and the resultant alkene was treated with Mn(OAc)3/CH3COOK to form podophyllotoxin.

Full text of DOI:10.14233/ajchem.2013.15073

Asian Journal of Chemistry; Vol. 25, No. 17 (2013), 9555-9557
http://dx.doi.org/10.14233/ajchem.2013.15073
Synthesis of Podophyllotoxin and its Derivatives via NiCl2/NaBH4 Reduction of Isoxazoline Ring
*
MEDURI SURESH BABU and KURIA MADHAVU LOKANATHA RAI
Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore-570 006, India
*Corresponding author: E-mail: kmlrai@yahoo.com
(Received: 31 December 2012;
Accepted: 1 October 2013)
AJC-14227
A novel synthesis of podophyllotoxin was carried out in four steps. The key step was reduction of isoxazoline ring followed by diazotization
and the resultant alkene was treated with Mn(OAc)3/CH3COOK to form podophyllotoxin.
Key Words: Nickel chloride, Isoxazoline, Podophyllotoxin, Manganese acetate.
Only five approaches to the asymmetric synthesis of podo-
phyllotoxin or analogues have been achieved using asymmetric
conjugate addition or Diels-Alder reactions. The key step in
the first route was reported byWard et al.8b involves asymmetric
conjugate addition of anion to chiral butenolide to yield adduct
which has been further elaborated to (-) deoxy-iso-podophyl-
lotoxin.
Meyers et al.9 achieved asymmetric synthesis of (-) podo-
phyllotoxin in 24 steps. The key step in the asymmetric conju-
gate addition of a trimethoxyphenyl anion to chiral oxazoline
to yield chiral adduct.
INTRODUCTION
Podophyllotoxin (Fig. 1)1,a natural lignan that is currently
used as a precursor to semi synthetic anticancer drugs like
etoposide, teniposide, etopophos, etc. Podophyllotoxin has
been extracted from two important medicinal plants named
Podophyllum emodi an Indian species and Podophyllum
peltatum, a North American species which belongs to the
family of berbideracea2. It has also been extracted from other
medicinal plants called Podophyllum pleianthum by Jackson
et al.3. The dried roots and rhizomes of these plants are known
as podophyllum, which on extraction from alcohol and after
drying gives a resin termed podophyllin.
EXPERIMENTAL
OH
Melting points were taken in open capillaries using
Thomus Hoover melting point apparatus and are uncorrected.
The compounds were routinely checked for their purity by
TLC using silica gel G. IR spectra were recorded on a nujol
mull on Shimadzu 8300 spectrometer. 1H NMR were recorded
either on a Bruker 300 MHz or Bruker 200 MHZ or Jeol 60
MHZ Hitachi Perkin Elmer spectrometer in CDCl3 solution.
Tetramethyl silane (TMS) was used as an internal standard
and the chemical shifts are expressed in ppm (δ). The 13C NMR
spectra were measured either on Jeol GSX 400(100 MHz) or
75 MHz instrument and the values are in parts per million
downfield from the tetramethyl silane. Mass spectra were
obtained either on Maspec MSW 9629 or Fennigen 4021 spectro-
meter and the important fragments are given with the relative
intensities in the bracket.
O
A
B
C O
O
O
H3CO
OCH3
OCH3
Fig. 1. Podophyllotoxin
Four general approaches to the synthesis of podophyllo-
toxin derivatives4 have so far been developed although several
variations and innovations have been introduced within each
of these overall schemes; namely (i) the oxoester route5, (ii)
dihydroxy acid route6, (iii) Diels-Alder approach7, (iv) tandem
conjugate addition8.
Thin layer chromatograms were obtained on a silica gel
(HF, 254, Sd Fine Chem) coated on glass slides. Silica gel-G
(TLC grade) slurry was prepared by dissolving 50 g of silica

9556 Babu et al.
Asian J. Chem.
gel in 100 mL of the solvent mixture (chloroform:methanol-
2:1) and the TLC plates prepared as per the methods of Piefer10.
Visualization of the spots on the chromatogram was relied on
adsorption of iodine. Chromatographic separations were carried
out on a silica gel (70-230 mesh, Merck) column. The proportion
of the solvents for the chromatography was given as volume/
volume.
pooled together, evaporated to dryness under reduced pressure
gave 5-phenyl-5,6-dihydro-naphtho[2,3-d][1,3]dioxole (5d) as
colourless oily mass (0.93 g, 75 %), b.p. 174-175 ºC; IR
(Nujol): ν 1600 cm-1 (-C=C-); 1H NMR (CDCl3): δ 2.58 (m,
2H, CH2), 4.07 (d, 1H, CH), 5.70 (m, 1H, CH), 5.92 (s, 2H,
OCH2O), 6.61 (d, 1H, CH), 6.75-7.02 (m, 2H, Ar’H), 7.30-
7.60 (m, 5H,ArH); MS (relative abundance) m/z: for C17H14O2,
250 (M+, 18), 174(100), 173(40), 144(30), 130 (35), 78(25).
A mixture of 5-phenyl-5,6-dihydronaphtho[2,3-d][1,3]-
dioxol (5d, 1.25 g, 5 mmol), Mn(III) acetate dihydrate (1.34 g,
5 mmol) and potassium acetate (0.58 g, 6 mmol) in glacial
acetic acid (15 mL) were refluxed under nitrogen atmosphere
till the brown colour was disappeared. The resulting reaction
mixture was cooled and diluted with water (15 mL). It was
then extracted in diethyl ether, washed with water (2 × 20 mL),
10 % sodium bicarbonate solution (2 × 20 mL) and finally
with water (20 mL). Ether layer after drying over anhydrous
sodium sulphate was completely evacuated to get thick residue.
The pure lactone was isolated by passing through column
packed with 70-320 mesh silica gel using petroleum ether:ethyl
acetate mixture (8:2) as eluent. Evaporation of major fraction
yields 5-phenyl-3a,4,5,10b-tetrahydro-3H-1,7,9-trioxa-dicyclo-
penta[a,g]naphthalene-2-one (6d) (0.61 g, 40 %) as an oily
liquid, b.p. 143-44 ºC. IR (Nujol): ν 1750 cm-1 (lactone C=O);
1H NMR (CDCl3): δ 1.85 (m, 2H, CH2), 2.26 (d, 2H, CH2),
2.82 (m, 1H, CH), 4.13 (t, 1H, CH), 5.31 (d, 1H, CH), 5.92 (s,
2H, OCH2O), 6.43 (s, 2H, Ar’H), 7.3-7.59 (s, 5H, ArH); MS
(relative abundance) m/z: for C19H16O4, 308 (M+, 35), 264(100),
232(40), 231(35), 202 (26), 188(48), 78(25).
Typical experimental procedure for compound (6d):
The required aldoximes (2) were prepared by standard method
by warming a solution of aromatic aldehydes in ethanol with
hydroxylamine hydrochloride in presence of sodium acetate.
Mixture of safrole (1, 0.810 g, 5 mmol) and benzaldoxime
(2d, 0.605 g, 5 mmol) were taken in alcohol in presence of
chloramines-T (1.43 g, 5.1 mmol) and refluxed for 3 h and
the progress of the reaction was monitered by TLC. After the
completion of the reaction, alcohol was evaporated under
vaccum. The reaction mixture was extracted with diethyl ether
(25 mL) and the organic layer was washed with 10 % NaOH
solution (25 mL × 2). The organic layer was dried over anhydrous
sodium sulphate and evaporated under vacuum to get required
5-benzo[1,3]dioxol-5-ylmethyl-3-phenyl-4,5-dihydro-isoxazole
(3d) (1.25 g, 90 %), m.p. 170-71 ºC; IR (Nujol): ν 1690 cm-1
(C=N); 1H NMR (CDCl3): δ 2.65 (d, 2H, CH2), 2.83 (d, 2H,
CH2), 3.6 (m, 1H, CH), 5.91 (s, 2H, OCH2O), 6.51-6.62 (m,
3H, Ar’H), 7.2-7.65 (m, 5H, ArH).
Mixture of 5-benzo[1,3]dioxol-5-ylmethyl-3-phenyl-4,5-
dihydro-isoxazole (3d, 1.40 g, 5 mmol) and NiCl2·6H2O (2.37
g, 10 mmol) were taken in methanol and cooled to -10 ºC.
Sodium borohydride (0.74 g, 20 mmol) was added in small
quantities for 0.5 h by maintaining the same temperature. It
was then kept at -10 ºC for further 0.5 h to complete the reaction.
After completion of the reaction, the reaction mixture was
filtered through celite bed. The TLC of the organic layer
showed a major spot with Rf value 0.54 and one minor spot
with Rf value 0.72. The organic layer after drying over anhy-
drous sodium sulphate was concentrated under vacuum to get
a oily mass. It was then subjected to column chromatography
using chloroform and acetone (9:1) as eluent. The major fraction
was then evaporated under vacuum to gave 4-amino-1-
benzo[1,3]dioxol-5-yl-4-phenyl-butan-2-ol (4d) as pale brown
solid (0.993 g, 70 %). m.p. 154-55 ºC; IR (Nujol): ν 3650
RESULTS AND DISCUSSION
We report herein a short and convenient synthesis of (-)
podophyllotoxin in 4 steps (Scheme-I) through 1,3-dipolar
cycloaddition of safrole with substituted benzaldoxoime in
presence of chloramine-T, followed by reduction of isoxazoline
ring with NiCl2/NaBH4 in presence of methanol as a solvent.
The β-amino alcohols produced on diazotization with acid
catalyst cyclizes the ring and on the elimination of water on
the ring to form a tetraline. Here we observed that the isomers
produced and they were separated by column chromatography.
This tetraline on reaction with Mn(OAc)3/CH3COOK in acetic
acid as a solvent form the lignan lactone ring which resembles
the podophyllotoxin.
1
(broad OH), 3350 (sharp NH), 1600 cm-1 (C=C); H NMR
(CDCl3): δ 1.98 (m, 2H, CH2), 2.72 (d, 2H, CH2), 3.62 (m,
1H, OCH), 3.85 (t, 1H, NCH), 5.92 (s, 2H, OCH2O), 6.51-
6.62 (m, 3H, Ar’H), 7.10-7.22 (m, 5H, ArH).
Most commonly employed method for the synthesis of
isoxazolines involving 1,3-dipolar cycloaddition reaction of
nitrile oxides to olefin. Rai et al.11-13 and others14 did consider-
able work on 1,3-dipolar cycloaddition using chloramine-T
as oxidant for generating 1,3-dipoles. The production of β-amino
alcohols requires that both the C=N and N-O bond of the
isoxazoline be reduced. This can be achieved through
reduction of isoxazoline either with Me3OBF4/NaBH4 or
LAH15. Annuziata et al.16 synthesized optically active amino
alcohols by stereoselective reduction of isoxazoline derivatives
by using NiCl2 and NaBH4 at low temperature. It is well known
that reaction of primary aliphatic amines with nitrous acid
liberate corresponding primary alcohols17. During this process
nitrogen is evolved. When nature of the alkyl permits it, the
products are mixture of isomeric alcohols, isomeric alkanes
Solution of 4-amino-1-benzo[1,3]dioxol-5-yl-4-phenyl-
butan-2-ol (4d, 1.42 g, 5 mmol) in 5 % aqueous sulphuric
acid (50 mL) was cooled to 0 ºC. Sodium nitrite (0.35 g, 5.2
mmol) was added in small quantities for 0.5 h by maintaining
the same temperature. It was then kept at 0 ºC for further 0.5 h
to complete the reaction. After completion of the reaction, the
reaction mass was extracted with diethylether (25 mL). The
organic layer was washed with distilled water (10 mL), dried
over anhydrous sodium sulphate and evaporated under vacuum
to get a oily mass. TLC of this shows a major spot at Rf value
0.5 in chloroform:acetone (7:1) as eluent. It was then passed
through a column packed with silica gel using chloroform:
acetone (7:1) as eluting solvent. The required fraction was

Vol. 25, No. 17 (2013)
Synthesis of Podophyllotoxin and its Derivatives via NiCl2/NaBH4 Reduction of Isoxazoline Ring 9557
R
1
Conclusion
R
2
H
N
OH
This is a new and modified route for the synthesis of podo-
phyllotoxin via reduction of isoxazoline ring with NiCl2 and
NaBH4 reagent.
O
O
R
O
3
Chloramine-T
Ethanol
+
O
N
O
R
1
R
3
3
NiCl /NaBH
2
R
4
R
2
1
1
2
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R
2
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R
O
O
3
O
O
NaNO /H SO
2
2
4
OH NH
2
4
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O
O
R
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1
R
3
O
R
2
5
O
Mn(OAc). 2H O
3
2
AcOH, CH COOK
3
R
1
R
3
R
2
6
OCH , b)R =R =OCH ,R =H c)R =R =H,R =OCH
3
a)R =R =R =
3
1
2
3
3
1
2
3
1
2
3
d)R =R =H,R =CH , e)R =R =H,R =Cl, f)R =R =R =H
1
3
2
3
1
3
2
1
2
3
Scheme-I
and cyclized structures. This prompted us to treat β-amino
alcohols prepared by the isoxazoline with nitrous acid with a
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Finkbeiner19 in provides the basis for a general approach to
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radical cyclizations was initially investigated by Corey and
Kang21, Ernst and Fristad22 and Snider et al.23. Heating the one
electron oxidant, Mn(III) acetate in acetic acid at reflux tempe-
rature 115 ºC generates the α-carboxymethyl radical. This
radical adds to double bonds of the alkenes with the formation
of 3-caboxy alkyl radical, which is oxidized by second equivalent
of Mn(III) acetate affords γ-lactone (Scheme-II).
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Mn(III)
Mn(III)
M n(III)
M n(III)
O
O
AcO
Slow
M n(III)
O
Mn(III)
O
CH2
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Fast
R
R
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M n(III)
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Mn(III)
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CH2
Mn(III)
O
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O
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O
R
Scheme-II

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