Synthesis of C-1 homologues of pancratistatin and their preliminary biological evaluation

Article abstract of DOI:10.1016/j.bmcl.2011.06.068

The synthesis of two C-1 analogues of pancratistatin has been accomplished in 17 steps from bromobenzene. The key steps involved the enzymatic dihydroxylation, regioselective opening of epoxyaziridine 9 with the alane derived from 8, a solid-state silica-

Full text of DOI:10.1016/j.bmcl.2011.06.068

Bioorganic & Medicinal Chemistry Letters 21 (2011) 4750–4752
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Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Synthesis of C-1 homologues of pancratistatin and their preliminary
biological evaluation
Sergey Vshyvenko a, Jon Scattolon a, Tomas Hudlicky a, , Anntherese E. Romero b, Alexander Kornienko b,
⇑
⇑
a Chemistry Department and Centre for Biotechnology, Brock University, 500 Glenridge Avenue, St. Catharines, Ontario, Canada L2S 3A1
b Chemistry Department, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of two C-1 analogues of pancratistatin has been accomplished in 17 steps from bromoben-
zene. The key steps involved the enzymatic dihydroxylation, regioselective opening of epoxyaziridine 9
with the alane derived from 8, a solid-state silica-gel-catalyzed intramolecular opening of aziridine to
produce phenanthrene 13 whose oxidative cleavage and recyclization provided the full skeleton of the
Amaryllidaceae constituents. The new analogues 5 and 6 exhibited promising activity in several human
cancer cell lines.
Received 27 May 2011
Revised 13 June 2011
Accepted 15 June 2011
Available online 22 June 2011
Keywords:
Ó 2011 Elsevier Ltd. All rights reserved.
Intramolecular aziridine opening
Amaryllidaceae alkaloids
Pancratistatin analogs
Anti-cancer activity
Pancratistatin 1a, discovered by Pettit in 1984,1 exhibits potent
cytotoxicity against a variety of human cancer cell lines, Figure 1.2
Together with its congener, narciclasine 2a and the corresponding
7-deoxy analogues, this natural product represents the most prom-
ising Amaryllidaceae anticancer constituent and has been the sub-
ject of numerous biological studies. Pancratistatin’s low
availability from natural sources and the lack of water solubility,
however, limit its potential for development into a therapeutic
agent. The published SAR studies reveal that the 7-hydroxy group
is an important part of the cytotoxic pharmacophore and com-
pounds 1b and 2b are about 10 times less potent than their 7-hy-
droxy analogues in the same assays.3 Over the years many
unnatural or truncated derivatives of the Amaryllidaceae constitu-
ents have been prepared by Pettit,4 McNulty,5 Kornienko,6 Chapl-
eur,7 Marion,8 Banwell,9 Gonzalez,10 and us,11 and subjected to
biological evaluation. In addition, many creative syntheses have
been reported for these natural products.12
derivatives 3 and 4, respectively, exhibiting activities similar to
those of 1b.12a Because 1a is more potent than 1b, we therefore
assumed that the corresponding analogues containing the required
C-7 hydroxyl should also possess higher activity and perhaps
better solubility than those of the natural product itself. In this
Letter we report the synthesis and preliminary evaluation of
pancratistatin analogues 5 and 6.
The synthesis of 5 and 6 was executed, with some modifica-
tions, in a manner similar to that used for the preparation of the
corresponding analogues of 7-deoxypancratisatin. The acetylene
fragment 8, possessing a masked 7-hydroxyl group, was prepared
as shown in Scheme 1. Hydroxy aldehyde 5 was prepared from
piperonal by a literature procedure13 involving the metalation of
the corresponding imine with nBuLi, conversion to boronate ester,
oxidation with hydrogen peroxide, and hydrolysis in 71% overall
yield. Methylation provided aldehyde 6,14 which was converted
to acetylene 8 via Corey–Fuchs protocol as shown.
Based on the extensive studies it would appear that the essen-
tial pharmacophore of 1a must retain the C-7 hydroxyl as well as
the full amino inositol motif. The space at C-1 appears amenable
to modifications, as evidenced by the high activity of C-1 benzoate
ester prepared and tested by Pettit.4c Based on this observation we
have prepared several C-1 analogues of 7-deoxypancratistatin
and subjected these to biological evaluation. The most active
compounds proved to be the hydroxymethyl and acetoxymethyl
Conversion of 8 to its alane was accomplished as shown in
Scheme 2. Addition of the epoxyaziridine 9 to the alane produced
acetylene 10, which was hydrogenated under Lindlar’s conditions
to provide the cis-olefin 11. The intramolecular aziridine opening
was accomplished by heating the TBS-protected ether 12 adsorbed
on silica-gel, according to a solid phase protocol developed in our
laboratory for aziridine opening.15 It was discovered serendipi-
tously that traces of quinoline (ca. 5%) adsorbed on the silica
surface led to an improved yield. Phenanthrene 13 was attained
in 74% yield and was then subjected to oxidative cleavage and
recyclization to produce, after acylation, hemiaminal 16, possess-
ing the complete skeleton of the natural product. Oxidation of this
⇑
Corresponding authors.
E-mail addresses: thudlicky@brocku.ca (T. Hudlicky), akornien@nmt.edu (A.
Kornienko).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.06.068

S. Vshyvenko et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4750–4752
4751
OH
OH
OH
OH
HO
7
OH
OH
OH
OH
OH
OH
OH
OH
RO
RO
1
1
1
1
O
O
O
O
O
O
O
O
NH
NH
NH
NH
7
OH O
R
O
R
O
O
3, R = H
narciclasine 2a, R = OH
5, R = H
pancratistatin 1a, R = OH
4, R = Ac
lycoricidine 2b, R = H
6, R = Ac
7-deoxypancratistatin 1b, R = H
Figure 1. Amaryllidaceae constituents and their C-1 analogues.
Me2SO4, K2CO3,
acetone, reflux, 4h
O
O
O
O
O
O
O
O
Br
Br
CBr4, PPh3, DCM
79%
n
BuLi, THF, -78 °C
82%
CHO
OMe
CHO
OMe
OMe
OH
98%
8
5
7
6
Scheme 1.
material to phenanthridone 17 was followed by reductive detosy-
lation to 1815c and generation of the free hydroxyl at C-7 in 19 by a
modification of Trost‘s procedure, Scheme 2.16
To complete the synthesis of the required derivatives, 19 was
subjected to desilylation to afford alcohol 20, which on treatment
with HCl and methanol furnished the fully hydroxylated analogue
5, Scheme 3. Alternatively, a partial deprotection with TFA pro-
vided the C-1 acetoxymethyl analogue 6.17
The hydroxymethyl and acetoxymethyl analogues 5 and 6, to-
gether with their 7-deoxy counterparts 3 and 4, were evaluated
in a small panel of cancer cell lines diversely representing several
types of human malignancy (Table 1). Narciclasine, which is about
6 times as potent as pancratistatin in the NCI 60 cell line screen,2
was used as a reference compound because of its better availabil-
ity. As expected, the antiproliferative potencies of the 7-hydroxy
compounds were higher, again underscoring the beneficial effect
of the 7-hydroxy substituent. For example, both 5 and 6 were
about 10 times more potent than their 7-deoxy analogues 3 and
4 against the prostate DU-145 cells and this difference in activity
is similar to that between 1a and 1b.2,3 In addition, the double-di-
git nanomolar potency of the acetoxymethyl analogue 6 is note-
worthy and it approaches that of narciclasine. This finding is
consistent with the previous observations that the presence of
large hydrophobic C-1 substituents does not lead to a substantial
decrease in activity and in some cases actually leads to activity
enhancement.12a,4c
In conclusion, these results bode well for the possibility of a li-
brary construction with focus on the C-1 space. Of interest will be
especially the evaluation of C-1 benzoxymethyl analogue and the
comparison of its activity with the C-1 benzoate of pancratistatin,
a compound which exhibited greater activity than pancratistatin
itself, as reported by Pettit.4c In addition, compounds 5 and 6 will
be prepared on medium scale and subjected to a pharmacokinetics
evaluation.
1. nBuLi, toluene, -50 °C
2. Me2AlCl, -50 °C - rt
3. 9, -25 °C, 44%;
O
O
OR
O
O
O
OH
H2/Lindlar cat., MeOH, ~quant.
O
O
MeO
OMe
O
N
OMe
8
N
Ts
O
Ts
O
10
11, R = H
TBSOTf, Et3N, DCM, 0 °C, 97%
O
12, R = TBS
O
O
SiO2, quinoline, 120 °C, 74%
N
Ts
9
OTBS
O
OTBS
OH OTBS
O
1. O3/MeOH, Sudan Red 7b, -80 °C
2. NaBH4, -80 °C - rt;
HO
MeO
O
HO
MeO
O
O
O
O
NHTs
O
O
NHTs
+
NTs
O
O
MeO OH
15 30 %
O
14 61%
13
MnO2, DCM, 2 d, 87%;
Ac2O, pyridine, DCM, 1 d, 87%
OTBS
O
OTBS
O
OTBS
O
AcO
AcO
AcO
LiCl, DMF, 120 °C, 67%
TPAP, NMO, DCM, 84%
O
O
O
O
O
O
NTs
MeO OH
16
O
NH
NR
O
O
MeO
OH O
O
19
17, R = Ts
18, R = H
Na,napthalene, DME, -50 °C, 62%
Scheme 2.

4752
S. Vshyvenko et al. / Bioorg. Med. Chem. Lett. 21 (2011) 4750–4752
OH
NH
OH
OTBS
O
OH
OH
O
AcO
AcO
AcO
TFA, DCM, MeOH, 90%
TBAF, THF, 0 °C, 97%
O
O
O
O
O
NH
O
NH
O
O
OH O
OH O
OH O
20
6
19
OH
NH
OH
OH
HO
2% HCl, DCM, MeOH, 97%.
O
O
OH O
5
Scheme 3.
Table 1
Activity of C-1 analogues with narciclasine as a standard [IC50 (l
M)a]
OH
OH
OH
OH
NH
OH
NH
OH
NH
OH
OH
OH
OH
OH
OH
OH
OH
HO
AcO
HO
AcO
O
O
O
O
O
O
O
O
O
OH
NH
Cancer type
Cell line
NH
O
OH
O
O
O
O
OH O
OH
3
4
6
5
Narciclasine 2a
Pancreatic
Prostate
Lung
CRL-1687/BXPC-3
HTB-81/DU-145
HTB-177/NCI-H460
HTB-22/MCF-7
0.05 0.01
0.05 0.04
0.04 0.01
0.04 0.01
0.77 0.01
1.10 0.20
0.40 0.01
0.86 0.06
0.34 0.05
0.72 0.27
0.53 0.01
1.81 1.20
0.22 0.01
0.09 0.01
0.09 0.01
0.24 0.10
0.07 0.01
0.06 0.01
0.07 0.01
0.52 0.47
Breast
a
Concentration required to reduce the viability of cells by 50%, after 48 h of treatment with indicated compounds, relative to DMSO control; SD from two independent
experiments, each performed in four replicates, determined by MTT assay.
J. R. L.; Brennan, J. D.; Bastida, J. Bioorg. Med. Chem. Lett. 2010, 20, 5290; (e)
McNulty, J.; Nair, J. J.; Singh, M.; Crankshaw, D. J.; Holloway, A. C. Bioorg. Med.
Chem. Lett. 2010, 20, 2335.
Acknowledgements
The authors are grateful to the following agencies for financial
support of this work: Natural Sciences and Engineering Research
Council of Canada (NSERC) (Idea to Innovation and Discovery
Grants); Canada Research Chair Program, Canada Foundation for
Innovation (CFI), TDC Research, Inc., TDC Research Foundation,
and Brock University. A.E.R. and A.K. thank the National Institutes
of Health(P20 RR016480) for financial support and Professor Snez-
na Rogelj for her kind assistance with the cell culture.
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17. The full experimental and spectral data for all compounds reported in this
manuscript will be published in detail in an upcoming full paper.