Asymmetric synthesis of 1-aza-4-deoxypicropodophyllotoxin

Article abstract of DOI:10.1055/s-0033-1338414

In our search for new easily accessible analogues based on the natural product podophyllotoxin, we synthesized 1-aza-4-deoxypicropodophyllotoxin in good overall yield and excellent enantioselectivity. The synthesis was centered around a direct asymmetric Mannich reaction using d-proline as the key step for introduction of the chiral centres. Our synthesis of 1-aza-4- deoxypodophyllotoxin was hindered through the increased instability towards epimerization of the C2 position. We did, however, synthesized a new scaffold based on the opened lactone analogue. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0033-1338414

LETTER
▌1097
lAettserymmetric Synthesis of 1-Aza-4-deoxypicropodophyllotoxin
1-Aza-4-deoxypicropodophyllotoxin
Benoit M. Aigret, Jeroen Jacobs, Luc Van Meervelt, Wim M. De Borggraeve*
Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium
Fax +32(16)327990; E-mail: Wim.DeBorggraeve@chem.kuleuven.be
Received: 10.02.2013; Accepted after revision: 25.03.2013
merase. It does, however, possess immunosuppressive
and anti-inflammatory properties in vitro,6 and therefore
could be used as a starting point for the development of
immunomodulating agents with therapeutic relevance. It
Abstract: In our search for new easily accessible analogues based
on the natural product podophyllotoxin, we synthesized 1-aza-4-de-
oxypicropodophyllotoxin in good overall yield and excellent enan-
tioselectivity. The synthesis was centered around a direct
asymmetric Mannich reaction using D-proline as the key step for in- is clear that the basic skeleton of podophyllotoxin can be
troduction of the chiral centres. Our synthesis of 1-aza-4-deoxy-
podophyllotoxin was hindered through the increased instability
towards epimerization of the C2 position. We did, however, synthe-
seen as a privileged scaffold with potential applications in
many fields of medicine.
Several aza analogues have been synthesized in order to
find new potent antineoplasic agents with lower toxicity
and improved stability.7 Most noteworthy are those based
on 4-aza-2,3-didehydropodophyllotoxin, a scaffold that
has retained much of the cytotoxicity of the parent com-
pound but which is much more accessible for SAR studies
due to its simplified structure and absence of the labile C2
stereocenter.8 Their synthesis can be performed in a single
step with the use of a multicomponent reaction.9
sized a new scaffold based on the opened lactone analogue.
Key words: Mannich bases, asymmetric catalysis, amino alde-
hydes, enantioselectivity, total synthesis
4-Deoxypodophyllotoxin (DPT, 1) is a naturally occur-
ring lignan, which was first isolated from Anthriscus syl-
vestris.1 It is closely related to the better known
podophyllotoxin (3) that is used for the production of the
anticancer drugs etoposide and teniposide (Figure 1).2
Also DPT has shown potent cytotoxicity against a series
of tumor-cell lines.3 It induces cell-cycle arrest through
tubulin polymerase inhibition,4 which is complementary
to the topoisomerase inhibitory activity of etoposide and
teniposide. Furthermore, it shows a broad spectrum of
other biological activities.5
We envisaged the synthesis of new D(P)PT analogues,
more specifically 1-aza-substituted D(P)PT derivatives.
Our original goal was to synthesize 1-aza-DPT (4) from
which 1-aza-DPPT (5) could easily be synthesized by a
base-induced epimerization of the 2-position, as this is the
thermodynamically more stable cis-fused product. Cen-
tral to our reaction strategy is the direct organocatalytic
asymmetric Mannich reaction of an unmodified aldehyde
as first reported by Barbas.10 Herein, D-proline acts as the
chiral auxiliary that controls the formation of the two con-
tiguous stereocenters upon carbon–carbon bond forma-
tion between a preformed imine and an unmodified
aldehyde precursor.
OH
4
3
O
O
O
O
O
O
1
2
O
O
In order to synthesize the aldehyde precursor, a six-step
reaction sequence was optimized starting from commer-
cially available piperonal (6, Scheme 1). Condensation
with malonic acid following a literature procedure11 gave
the cinnamic acid 7 in excellent yield. This was converted
into the corresponding cinnamic ester 8 by refluxing in
acidic methanol. Subsequent catalytic reduction with 10%
Pd/C gave the propionic ester 9, which was regioselec-
tively brominated in dichloromethane at 0 °C in excellent
yield. The thus obtained ester 10 was reduced in high
yield to the corresponding alcohol 11 with the use of lith-
ium aluminium hydride at 0 °C. Earlier attempts avoiding
the extra esterification step led to much lower yields in
this reduction step due to solubility problems of the acid
in ethereal solvents (64% vs. 97%). Up to this point, all
the reactions are high yielding and do not need any addi-
tional purification after workup. Furthermore, the reac-
tions are easily adapted to large-scale synthesis. The
alcohol 11 was finally converted into the aldehyde 12 in
MeO
OMe
MeO
OMe
O
O
OMe
OMe
3
O
1 2-α
2 2-β
2
N
O
MeO
OMe
OMe
4 2-α
5 2-β
Figure 1 Structures of (deoxy)(picro)podophyllotoxin and the pro-
posed aza analogues thereof
4-Deoxypicropodophyllotoxin (DPPT, 2, Figure 1) shows
less toxicity as it is not a good inhibitor of tubulin poly-
SYNLETT 2013, 24, 1097–1100
Advanced online publication: 12.04.2013
DOI: 10.1055/s-0033-1338414; Art ID: ST-2013-B0132-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

1098
B. M. Aigret et al.
LETTER
good yield by the classical Swern oxidation protocol using phenyl-derived imines does not have a negative influence
oxalyl chloride and DMSO.12
on the stereoselectivity of the reaction. This in contrast to
2-methoxy substituents, which gave a very low enantio-
meric excess (ee <10%) under these conditions.16
The imine 14 was synthesized starting from 3,4,5-trime-
thoxyaniline (13) which was condensed with ethyl glyox-
ylate using an adaptation of a literature procedure and is
also depicted in Scheme 1.13
O
MeO
MeO
N
O
O
OEt
O
+
O
O
O
Br
HO
OH
12
O
O
OMe
14
O
O
4-methylpiperidine
OH
O
pyridine, 95%
D-proline (30 mol%)
THF, 4 °C
6
7
Br
Br
MeOH, H2SO4
98%
O
O
O
O
OEt
HN
1. NaBH4
0 °C
HN
O
O
O
O
H2, Pd/C
OMe
O
O
OMe
O
O
O
O
2. ZnCl2
79%
MeOH, 96%
8
9
MeO
OMe
MeO
OMe
Br2, 0°C
CH2Cl2, 92%
OMe
16
OMe
15
Pd2(dba)3
Sphos
Cs2CO3
toluene, 80 °C
51%
O
1. LiAlH4
2. H3O+
OMe
O
O
O
O
OH
THF, 0 °C
97%
Br
Br
O
O
10
11
O
1. DMSO, (COCl)2
2. Et3N
N
CH2Cl2, 90%
O
O
O
O
MeO
OMe
OMe
5
Br
12
O
O
Scheme 2 Synthesis of 1-aza-4-deoxypicropodophyllotoxin
O
MeO
MeO
NH2
MeO
MeO
N
OEt
Na2SO4
OEt
Ring closure to the tetracyclic ring system was accom-
plished in moderate yield using a Buchwald–Hartwig
cross-coupling reaction employing Pd2(dba)3 as palladi-
um source and Sphos as a ligand (Scheme 2). Other li-
gands such as Xphos, Ruphos, Brettphos, and t-BuXphos
gave lower yields.17 Copper-catalyzed reactions also gave
toluene
OMe
13
OMe
14
95%
Scheme 1 Synthesis of the aldehyde and imine precursors
The thus formed achiral precursors where combined in an very low yields.18 Even when using Cs2CO3 as a base,
asymmetric Mannich reaction. Both aldehyde 12 and im- epimerization of the chiral center next to the ester moiety
ine 14 were dissolved in THF and placed in a cryostat at could not be avoided, and the product of this reaction was
4 °C. D-Proline (30 mol%)14 was added as the chiral cata- 1-aza-4-deoxypicropodophyllotoxin (5), which was con-
lyst, and the reaction was stirred for 16 hours at 4 °C, after firmed by X-ray crystallography (Figure 2).
which the temperature was lowered to 0 °C and three
Gensler reported in his original synthesis of podophyllo-
equivalents of sodium borohydride were added to the re-
toxin that picropodophyllotoxin could be partially
action mixture, to reduce the aldehyde moiety in situ.15
epimerized through the enolization of the ester with tri-
The partial intramolecular ring closure of the thus ob-
phenylmethyl sodium and subsequent quench with AcOH
tained alcohol with the ester to afford lactone 16 was driv-
at –78 °C.19 This gave him up to 38% of the desired prod-
en to completion by treatment with zinc chloride, and the
uct. When we performed a similar experiment using LDA
product was isolated in a yield of 79% (Scheme 2). The
as the base, no epimerized product could be detected upon
stereoselectivity of the reaction was monitored by chiral
reprotonation with AcOH.20 The different cation could not
HPLC using a Chiralpak-IB column. The enantiomeric ra-
have been the source of this difference, as LDA was also
tio was found to be 99:1 and the diasteromeric ratio 95:5,
used by Kende et al. in their epimerization of picropodo-
showing that the presence of two extra methoxy groups
phyllotoxin.21
compared to the more frequently employed 4-methoxy-
Synlett 2013, 24, 1097–1100
© Georg Thieme Verlag Stuttgart · New York

LETTER
1-Aza-4-deoxypicropodophyllotoxin
1099
Br
Br
OTBS
O
1. NaBH4
0 °C
OEt
OEt
HN
HN
O
O
2. TBSCl
imidazole
O
O
O
O
71%
MeO
OMe
MeO
OMe
OMe
15
OMe
17
Pd2(dba)3
Xphos
Cs2CO3
toluene, 80 °C
61%
OTBS
O
O
OEt
N
Figure 2 X-ray structure of 1-aza-DPPT (50% probability of the
thermal ellipsoids)
O
MeO
OMe
In order to avoid the epimerization occurring during the
Buchwald–Hartwig ring closure, which is probably due to
the high strain present in the trans-fused lactone, we chose
OMe
18
to delay the ring closure of the lactone until after the Bu- Scheme 3 In situ TBS protection and subsequent ring closure
chwald–Hartwig cross-coupling, as to avoid the presence
of a base when working with the strained system. A strat-
egy was thus needed to protect the alcohol in situ after the
OTBS
O
O
O
O
TBAF
buffered
pH (7.1)
reduction of the aldehyde, to avoid any lactonization. A
literature procedure using pyridinium zinc borohydride
reduction in the presence of ethyl acetate to generate the
acylated alcohol was tried with good results on a model
system,22 but this strategy gave poor results when incor-
porated into our Mannich reaction. Although no literature
report had been made on a consecutive reduction–si-
lylation reaction, we thought that the intermediate alkoxy-
borate formed after a sodium borohydride reduction
would be reactive enough to generate a silyl ether when
quenched with a silyl electrophile.
OH
OEt
OEt
N
N
74%
O
O
MeO
OMe
MeO
OMe
OMe
18
OMe
19
ZnCl2
MS
THF
O
O
O
O
Therefore, after the reduction step of the Mannich reac-
tion, the reaction was quenched by the addition of TBSCl
and imidazole to avoid any spontaneous cyclization to the
lactone. This gave the TBS-protected alcohol 17 with only
slightly diminished yield of 71% (er = 98:2) compared to
the synthesis of lactone 16 and is presented in Scheme 3.
N
MeO
OMe
OMe
4
The TBS-protected alcohol 17 could be cyclized using the
same Buchwald–Hartwig amination protocol except that
in this case Xphos proved to be the best ligand, giving
product 18 in 61% yield (Scheme 3).
Scheme 4 TBS deprotection and attempted cyclization
presence of ZnCl2 and 4 Å molecular sieves, almost no re-
action took place after 24 hours (Scheme 4). When heat-
ing was continued for several days, eventually the C2-
epimerized cis-fused lactone 5 was again formed. This
method was, however, already used successfully as the fi-
nal step of the total synthesis of podophyllotoxin reported
by Bush et al.24 Together with the epimerization occurring
during the Buchwald–Hartwig reaction, these result
shows that the 1-aza analogue of DPPT (5) is more unsta-
ble towards epimerization then DPPT (2) itself. When we
attempted to hydrolyze the ester in order to cyclize it using
a coupling reagent, 19 did not hydrolyze after refluxing in
Removal of the TBS protecting group of 18 was done in
neutral conditions using TBAF in a phosphate buffer. De-
protection proceeded smoothly when at least one equiva-
lent of TBAF was used. We could not get complete
conversion when performing the reaction using substoi-
chiometric amounts of TBAF, as stated in the original ar-
ticle.23 After workup this gave the ethyl ester 19 in 74%
yield (Scheme 4).
Ring closure to the trans-fused lactone did not proceed as
expected. When the ester 19 was refluxed in THF in the
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1097–1100

1100
B. M. Aigret et al.
LETTER
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composed.
Although its presence generally results in an increased ac-
tivity, the lactone ring is not a prerequisite for biological
activity,25 so we envisaged continuing with compound 19
as the basis for further studies as the alcohol and ester
groups allow many possibilities of derivatization for fur-
ther SAR studies.
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To conclude we have shown a straightforward synthesis
towards a new 1-aza-substituted scaffold of DPPT (5).
During our search for a synthesis of 1-aza-DPT (4) we
were faced with epimerization problems that could not be
overcome. We have therefore shifted our attention to-
wards 1-aza-DPPT (5) and the 1-aza analogues of the es-
ter 19. Derivatization of these new scaffolds and broad
screening of their biological activities is currently in prog-
ress and will be reported in due time. We have also shown
a novel reduction–silylation strategy which might encour-
age more research towards examining the scope of this re-
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(15) This lower temperature is necessary to avoid epimerization
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9%, 4-hydroxyproline: 12%.
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Acknowledgment
This study was supported by the IWT Flanders and KU Leuven via
OT/11/047.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
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Synlett 2013, 24, 1097–1100
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