Very short and efficient syntheses of the spermine alkaloid kukoamine A and analogs using isolable succinimidyl cinnamates

Article abstract of DOI:10.1246/Cl.2005.264

Direct selective acylation of the primary amino functions of spermine and spermidine with a variety of isolable succinimidyl cinnamates, followed by catalytic hydrogenation, gave high yields of the spermine alkaloid kukoamine A and analogs suitable for structure-activity relationship studies. Suitable succinimidyl cinnamates were readily obtained through Wittig reaction of aromatic aldehydes with the ylides Ph₃P=CRCO₂Me, followed by saponification and activation with N-hydroxysuccinimide in the presence of N,N'-dicyclohexylcarbodiimide. Copyright

Full text of DOI:10.1246/Cl.2005.264

264
Chemistry Letters Vol.34, No.2 (2005)
Very Short and Efficient Syntheses of the Spermine Alkaloid Kukoamine A and Analogs
Using Isolable Succinimidyl Cinnamates
Thomas Garnelis, Constantinos M. Athanassopoulos, Dionissios Papaioannou,ꢀ Ian M. Eggleston,y and Alan H. Fairlambyy
Department of Chemistry, University of Patras, GR-265 00 Patras, Greece
yDivision of Biological Chemistry and Molecular Microbiology, Faculty of Life Sciences, Carnelley Building,
University of Dundee, Dundee DD1 4HN UK
yyDivision of Biological Chemistry and Molecular Microbiology, Faculty of Life Sciences, Wellcome Trust Biocentre,
University of Dundee, Dundee DD1 4HN UK
(Received November 11, 2004; CL-041357)
Direct selective acylation of the primary amino functions of
H
N
H
NH2
N
NH2
H2N
H2N
N
H
spermine and spermidine with a variety of isolable succinimidyl
cinnamates, followed by catalytic hydrogenation, gave high
yields of the spermine alkaloid kukoamine A and analogs suita-
ble for structure-activity relationship studies. Suitable succini-
midyl cinnamates were readily obtained through Wittig reaction
of aromatic aldehydes with the ylides Ph3P=CRCO2Me, fol-
lowed by saponification and activation with N-hydroxysuccini-
mide in the presence of N,N0-dicyclohexylcarbodiimide.
SPD
SPM
O
R2
R1
H
H
N
N
R1
R2
N
N
H
H
O
1
R2
R1
O
H
N
H
N
R1
R2
N
H
O
2
R2
R1
Kukoamine A (KukA, 1a), a spermine alkaloid isolated
from the dried root bark of Lycium chinense, is a clinically effec-
tive hypotensive agent1 and a potent and selective inhibitor of
trypanothione reductase (TryR),2 a crucial enzyme for the sur-
vival of pathogenic trypanosomatid parasites.3 KukA therefore
constitutes an interesting lead for the development of effective
antiparasitic agents. Several multi-step synthetic protocols, in
liquid4,5 or on solid phase,6 for the preparation of KukA have
been published, along with the preparation and anti-TryR activ-
ities of a series of mono- and disubstituted spermine (SPM) and
spermidine (SPD) analogs of KukA, such as the SPD analog 2a
(SkukA) and the SPM analogs 1k and 3a (Figure 1).2 Many other
spermine or spermidine derivatives bearing arylalkyl or arylacyl
units on their primary or secondary amino functions respectively
have been prepared based on KukA7,8 and have also been shown
to exhibit potent anti-TryR activity. Preparation of such com-
pounds also typically requires several steps of sequential orthog-
onal protection/deprotection of the primary and secondary ami-
no functions. We have recently shown that all four possible re-
gioisomers of KukA, namely KukA-D, and all three possible
SPD regioisomers (SkukA-C) can be readily obtained, albeit in
multi-step syntheses, by using suitably protected SPM and
SPD derivatives and O,O0-bisbenzylcaffeoyl chloride to intro-
duce the required dihydrocaffeoyl unit into the polyamine skel-
eton.9 However, in the course of a different study we have ob-
served that the corresponding isolable, crystalline, succinimidyl
ester 8b is a more stable alternative to the aforementioned chlo-
ride, and very selective towards the primary amino functions of
polyamines.10 Although the selective reactivity of certain succi-
nimidyl11 and alkylphenyl12 carbonates has been previously ap-
plied in the preparation of selectively protected polyamine deriv-
atives, the use of succinimidyl esters for the direct synthesis of
polyamine conjugates appears to have received little attention.
We now wish to report on a very short, simple, and efficient syn-
thetic protocol which exploits this selectivity and provides easy
access to KukA, SkukA, and a series of KukA analogs (Figure 1)
O
R
H
H
R1
R2
N
N
N
N
H
H
O
R
3 ; R=H
4 ; R=Me
R2
R1
O
H
N
H
N
R1
R2
N
H
O
5
i ; R1=H, R2=OBn
j ; R1=H, R2=OH
k ; R1=H, R2=OMe
a ; R1=R2=OH
b ; R1=R2=OBn
c ; R1=R2=OAc
d ; R1=R2=OMe
e ; R1=OBn, R2=OMe
; R1=OH, R2=OMe
m ; R1=H, R2=NO2
f
g ; R1=OMe, R2=OBn
h ; R1=OMe, R2=OH
l
; R1=R2=H
R1
R2
6
7
8
9
; R=H, X=OH
O
O
; R=Me, X=OH
; R=H, X=OSu
; R=Me, X=OSu
R1
R2
X
R
13 ; R=H, X=Cl
10
Figure 1. Structures of compounds encountered in this work.
suitable for structure-activity relationship studies.
The key-step in the present synthetic protocol is the direct
condensation of succinimidyl esters 8 or 9 (suitably protected,
where necessary), with the commercially available polyamines
SPM and SPD. Most of the cinnamic acids required were readily
obtained through a two-step sequence involving Wittig reaction
with an aromatic aldehyde 10, followed by saponification.
Where necessary, (6b, 6e, 6f, 6i) these were appropriately O-pro-
tected with the hydrogenolytically cleavable benzyl (Bn) group
or the hydrolytically cleavable acetyl (Ac) group, if retention
of the cinnamoyl double bond was required. Cinnamic acid 6c
was obtained through a Perkin reaction of the aldehyde 10a with
excess Ac2O which resulted in simultaneous bisacetylation
(Scheme 1). From the thus obtained cinnamic acids 6 or 7d,
the corresponding crystalline active esters 8 and 9d, respective-
ly, were readily obtained through reaction with N-hydroxysucci-
nimide (HOSu) in the presence of N,N0-dicyclohexylcarbodii-
mide (DCC) (Table 1).
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.2 (2005)
265
night refrigeration. The free bases 3 or 4d were obtained in ex-
cellent (70–89%) yields through partitioning of these salts be-
tween CHCl3 and a 5% aq NaHCO3 solution, followed by wash-
ings with H2O, drying and evaporation (Table 1). In the case of
the SPD conjugates 5, 2.5 mmol of iPr2NEt was also included to
neutralize HOSu and the reaction mixture was directly processed
to aqueous work-up as for conjugates 3 to give free bases 5 in
70–75% yields, following trituration with Et2O (Table 1). Final
catalytic hydrogenation of the double bonds and hydrogenolysis
of the Bn groups, where necessary, was effected as follows.
Fully protected conjugates (1 mmol), e.g. 3b, 3e, 3g, 3i and
5b, in MeOH (16 mL)/AcOH (4 mL)/H2O (0.4 mL) were hydro-
genolyzed over 0.1 g of 20% Pd on C under an atmosphere of H2
for 2–10 h at ambient temperature. Filtration through celite,
washings with MeOH/AcOH (4:1) and finally evaporation of
the filtrates under reduced pressure left an oily residue. This
was treated with a ca. 2 M solution of HCl in anhydrous MeOH,
triturated with dry Et2O and refrigerated to give the correspond-
ing bishydrochloride salts of compounds 1a(KukA), 1f, 1h, 1j
and 2a (SkukA) in 73–89% yields.
O
R1
i, ii
iv
X
10
10a
R
R2
6 ; R-H, X=OH
7 ; R=Me, X=OH
8 ; R=H, X=OSu
9 ; R=Me, X=OSu
iii
Ph3P=CRCO2Me
iii
11 ; R=H
12 ; R=Me
8 or 9
v
vi
1 / 2
3 / 4 / 5
Scheme 1. Synthesis of KukA and analogs. Reagents and con-
ditions: (i) Ph3P=CHCO2Me or Ph3P=C(Me)CO2Me, MeCN,
reflux 10 h, 70–97%; (ii) 4N NaOH, DMSO/MeOH (2:1),
0 ꢁC, 30 min then 25 ꢁC, 12 h, 85–96%; (iii) HOSu/DCC,
THF/DMF (3:1), 0 ꢁC, 1 h then 25 ꢁC, 12 h, 72–88%; (iv) Ac2O,
Et3N, reflux 12 h, 72%; (v) SPM or SPD/iPr2NEt, CH2Cl2, 0 ꢁC,
30 min to 2 h; (vi) H2 (1 atm)/20% Pd–C, MeOH/AcOH/H2O
(4:1:0.1), 25 ꢁC, 2–10 h, 73–89%.
The present protocol constitutes a significantly more direct
and cost-effective approach to such SPM and SPD conjugates
than other methodologies reported to date, avoiding cumber-
some protection/deprotections of the polyamine moiety and im-
portantly, without the need for chromatographic purifications.
Table 1. Selected data for succinimidyl esters (SUEs), the cor-
responding polyamine conjugates (PACs) and the fully depro-
tected KukA analogsa
Yield
/%
Yield
/%
KukA
analogs
Yield
/%
Entry SUEsb
PACs
References
1
8b
83
3b
5b
3cd
3d
5d
3e
3g
3i
3k
3l
85
70
—
77
75
82
70
72
83
80
89
86
1a
2a
3ad
1d
2d
1f
1h
1j
1k
1l
89
81
—
84
86
82
73
78
83
81
1
2
3
S. Funayama, K. Yoshida, C. Konn, and H. Hikino, Tetrahe-
dron Lett., 21, 1355 (1980).
2
3
8cc
8d
72
75
J. A. Ponasik, C. Strickland, C. Faerman, S. Savvides, P. A.
Karplus, and B. Ganem, Biochem. J., 311, 371 (1995).
D. S. Fries and A. H. Fairlamb, in ‘‘Burger’s Medicinal
Chemistry and Drug Discovery,’’ 6th ed., ed. by D. J.
Abraham, John Wiley Inc., London (2003), Vol. 5, Chemo-
therapeutic Agents, Chap. 19.
a) K. Chantrapromma and B. Ganem, Tetrahedron Lett., 22,
23 (1981). b) B. Ganem, Acc. Chem. Res., 15, 290 (1982).
T. Moriwake, S. Saito, H. Tamai, and M. Inata, Heterocy-
cles, 23, 277 (1985).
P. Page, S. Burrage, L. Baldock, and M. Bradley, Bioorg.
Med. Chem. Lett., 8, 1751 (1998).
M. C. O’Sullivan, Q. Zhou, Z. Li, T. B. M. Durham, D.
Rattendi, S. Lane, and C. J. Bacchi, Bioorg. Med. Chem.,
5, 2145 (1997).
a) B. Bonnet, D. Soullez, S. Girault, L. Maes, V. Landry, E.
Davioud-Charvet, and C. Sergheraert, Bioorg. Med. Chem.
Lett., 8, 95 (2000). b) B. Chitkul and M. Bradley, Bioorg.
Med. Chem. Lett., 10, 2367 (2000).
a) G. Karigiannis, P. Mamos, G. Balayiannis, I.
Katsoulis, and D. Papaioannou, Tetrahedron Lett., 39,
5117 (1998). b) S. Vassis, G. Karigiannis, G. Balayiannis,
M. Militsopoulou, P. Mamos, G. W. Francis, and D.
Papaioannou, Tetrahedron Lett., 42, 1579 (2001).
4
5
6
7
8
8e
8gc
8i
8k
8l
88
72
84
82
80
86
87
4
5
6
7
9
10
8m
9d
3m
4d
aThe structures of new compounds described in this communi-
cation were determined by a combination of spectroscopic tech-
niques (UV, IR, ESI-MS, NMR) and microanalysis. Final prod-
ucts, i.e. KukA analogs, were characterized by MALDI-TOF/
TOF HR-MS. SUEs showed characteristic C=O IR bands at
1768 and 1738 cmꢂ1 and the following typical characteristic
b
8
9
1
(compound 8b) H NMR (400 MHz, CDCl3) data: ꢀ 7.79 (1H,
d, J 16 Hz), 7.14 (1H, d, J 2 Hz), 7,12 (1H, dd, J 8.2 and
2 Hz), 6.93 (1H, d, J 8.2 Hz), 6.36 (1H, d, 16 J Hz), 2.86 (4H,
br. s). cIsolated yield following FCC purification using as eluant
PhMe/EtOAc (1:1) and (7:3), respectively. dThe reaction of ac-
tive ester 8c with SPM leads directly to KukA analog 3a togeth-
er with the corresponding mono- and di-N-acetylated derivative
and succinimidyl acetate as shown by HPLC, ESI–MS, and
1H NMR.
10 M. Militsopoulou, N. Tsiakopoulos, Ch. Chochos, G.
Magoulas, and D. Papaioannou, Tetrahedron Lett., 43,
2593 (2002).
11 M. Adamczyk, J. R. Fishpaugh, and K. J. Heuser, Org. Prep.
Proced. Int., 30, 339 (1998).
Treatment of a solution of 1.7 mmol of SPM in 25 mL
CH2Cl2 with 5 mmol of esters 8 or 9d for 1 h at 0 ꢁC lead to pre-
cipitation of the conjugates 3 or 4d as the corresponding bishy-
droxysuccinimidate salts, which were obtained pure by simple
filtration and washing with ice-cold CH2Cl2, following over-
12 M. Pittelkow, R. Lewinsky, and J. B. Christensen, Synthesis,
2002, 2195.
Published on the web (Advance View) January 26, 2005; DOI 10.1246/cl.2005.264