S. Vshyvenko et al. / Bioorg. Med. Chem. Lett. 24 (2014) 4236–4238
4237
Br
Br
BocNHOH, NaIO
OR
4
Table 1
O
Al(Hg)
OH DMP, TsOH
OH
MeOH, H
4%
over 2 steps)
O
Br
O
NBoc
O
O
2
O
O
O
THF, H
2
O
Conditions attempted for the Heck cyclization
7
89%
(
Conditions
Outcome
9
10
11
NHBoc
1
2
R = H
Pd(dppf)
2
Cl
, dppe, Ag
, PPh , Et
, dppe, AgNO
, BINAP, Ag PO
2
, dppf, Ag
PO
N, AgNO
, Cs CO3, toluene, 24 h, 110 °C
, Et
3
PO
4
dioxane, 60 °C, 24 h
N toluene, 80 °C, 18 h
, MeCN, 24 h, rt to 80 °C
No reaction
Traces of 18
No reaction
18, 35% (45% brsm)
No reaction
TBSCl, imidazole
CH Cl , 81%
2
2
Pd(OAc)
Pd(OAc)
Pd(OAc)
Pd(OAc)
2
3
4
, Et
3
13 R=TBS
2
2
2
3
3
3
3
2
Scheme 1.
3
4
3
N toluene, 80 °C, 18 h
1
. LTMP/nBuLi
2
I
Amberlyst 15
MeOH
I
2
. I
N
CO
2
H
90%
N
CO2
-
Li
+
40%
N
2
CO H
OH
OH
OH
14
15
7
OH
OH
O
O
OH
OH
mCPBA
CH Cl :MeOH
10:1
2
2
HCl, MeOH
quant.
Scheme 2.
NBoc
NH
45%
NH
N
N
N
O
O
O
5
O
1
9
6
oxidation of t-butyloxycarbohydroxamic acid, Scheme 1. Reduction
of bicyclic product 11 with aluminum amalgam led to alcohol 12.
This sequence was performed on a multigram scale and did not
require the isolation of any intermediates.
Scheme 4.
Protection of alcohol 12 with t-butylsilyl group led to protected
conduramine 13. Second coupling fragment, iodo acid 7, was pre-
pared by a simple direct ortho-metalation of picolinic acid with
The summary of the various conditions attempted is shown in
Table 1.
No product of the Heck reaction was ever observed under
similar conditions when the unprotected amide 16 was used.
Deprotection of the silyl group with TBAF led to alcohol 19.
The synthesis was completed by acid-catalyzed deprotection of
the Boc carbamate 19 to furnish the HCl salt of 5, which upon chro-
matography with basic eluent was isolated as a free base 5. Oxida-
tion of 5 to its respective N-oxide 6 was performed, as shown in
Scheme 3, and subjected to screening in two cancer cell lines to
compare the cytotoxicity results to those of the recently tested
2
lithium tetramethylpiperidide (LTMP) and I via a known literature
procedure, Scheme 2.6 The free acid 7 was not obtained in good
yield and proved somewhat unstable upon prolonged storage.
Therefore the lithium salt 15 was ultimately used as a coupling
partner because of its increased stability.
Coupling of the protected conduramine 13 with 3-iodopicolinic
acid 7 was initially attempted to produce 17 directly. Unfortu-
0
nately, under a variety of coupling conditions: 1,1 -carbonyldiimid-
azole (CDI), N,N’-dicyclohexylcarbodiimide/hydroxybenzotriazole
2j
pancratistatin C-1 homologs.
0
0
(
DCC/HOBt),
or
N,N,N ,N -tetramethyl-O-(1H-benzotriazol-1-
2r,2t
In our previous study it has been shown
that C-1 analogs 20 and
yl)uronium hexafluorophosphate (HBTU), no desired product was
observed, Scheme 3. Therefore conduramine 13 was selectively
deprotected with trifluoroacetic acid to afford the free amine 8.
Coupling of amine 8 and lithium salt 15 provided amide 16, in a
moderate yield that can probably be explained by steric hindrance
of the ortho-iodo group. Reprotection of the amide with di-tert-
butyl dicarbonate in the presence of DMAP provided carbamate
2
1 also displayed pronounced activity against pancreatic (BxPC-3),
prostate (DU-145), and lung (NCI-H460) cancer cell lines. Scheme 4
The respective IC50 values for these three lines for C-1 acetate 20 were
0
.07, 0.06 and 0.07
For the latter compound, IC50 values exceeded those of the natural
product narciclasine (3): 0.05, 0.03, 0.05 M, respectively.
lM and for C-1 benzoate 21: 0.01, 0.01. 0.03 lM.
l
The activity of pancratistatin and narciclasine against cancer
1
7. All of the currently existing Heck approaches to narciclasine
cell lines is known to be ꢀ100 times higher than that of the
4
,7
skeleton require tertiary amide or imide moiety for successful
transformation; most of them also use toxic thallium(I) salts as a
base. Nevertheless, both products 14 and 15 were submitted to a
range of standard Heck coupling conditions. We decided to avoid
the use of thallium salts and therefore silver phosphate was used
7
-deoxy-derivatives. It appears that the 7-hydroxy group is crucial
for maintaining effective inhibition of cancer cell growth. Table 2
The notion that the 7-N-oxide functionality would somehow
mimic this requirement was shown not to be correct and the
aza-derivatives were found to be inactive. The compounds tested
also lacked the methylene dioxy group, which is a known contrib-
utor to the pharmacophore of these compounds. Our next goal will
be the preparation of aza-derivatives of narciclasine that retain the
7
c
as a substitute. The only conditions that led to the desired
product 18, involved the reaction of 17 in the presence of
Pd(OAc)
2
/1,2-bis(diphenylphosphino)ethane and silver phosphate.
7
-hydroxy moiety as well as the methylene dioxy unit. The results
of these endeavors will be reported in due course.
OTBS
attempted
coupling
with 7
OTBS
OTBS
Acknowledgments
15, HBTU, DIPEA
O
O
TFA, CH
2
Cl
2
O
DMF, CH Cl
2
2
17
N
O
O
86%
O
66%
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,
the Ontario Partnership for Innovation and Commercialization
(OPIC), The Advanced Biomanufacturing Centre (Brock University),
and by grants from the National Institute of General Medical Sci-
ences (P20GM103451). We also appreciate the skillful assistance
of Razvan Simionescu and Tim Jones for their help with NMR and
mass spectrometry analysis.
NH
NHBoc
NH
2
8
I
O
13
16
2
Boc O, DMAP
92%
MeCN
OTBS
OTBS
O
OH
O
Pd(OAc)
2
, dppe
O
O
TBAF, THF
7%
Ag PO , PhMe
I
3
4
O
O
7
110°C, 31% [46%]
NBoc
NBoc
NBoc
N
N
N
O
18
O
17
O
19
Scheme 3.