Simple syntheses of N-alkylated spermidine fragments and analogues of the spermine alkaloid kukoamine A

Article abstract of DOI:10.1016/S0040-4039(00)02308-X

Acylation of a variety of amines with succinimidyl N-trityl-β-alanyl-γ-aminobutyrate and N-trityl-γ-aminobutyryl-β-alaninate, readily obtained through coupling of succinimidyl N-trityl-β-alaninate with trimethylsilyl γ-aminobutyrate and of N-trityl-γ-aminobutyric acid with methyl β-alaninate, respectively, followed by LiAlH₄ reduction, produced N-monoalkylated spermidine fragments and analogues of the spermine alkaloid kukoamine A. The applicability of this methodology on the solid phase was also demonstrated.

Full text of DOI:10.1016/S0040-4039(00)02308-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1579–1582
Simple syntheses of N-alkylated spermidine fragments and
analogues of the spermine alkaloid kukoamine A
Stratos Vassis,a George Karigiannis,a George Balayiannis,a Maria Militsopoulou,a Petros Mamos,b,*
George W. Francisc and Dionissios Papaioannoua
aDepartment of Chemistry, University of Patras, GR-265 00 Patras, Greece
bDepartment of Medicine, University of Patras, GR-265 00 Patras, Greece
cDepartment of Chemistry, University of Bergen, Allegaten 41, N-5007 Bergen, Norway
Received 25 September 2000; revised 29 November 2000; accepted 14 December 2000
Abstract—Acylation of a variety of amines with succinimidyl N-trityl-b-alanyl-g-aminobutyrate and N-trityl-g-aminobutyryl-b-
alaninate, readily obtained through coupling of succinimidyl N-trityl-b-alaninate with trimethylsilyl g-aminobutyrate and of
N-trityl-g-aminobutyric acid with methyl b-alaninate, respectively, followed by LiAlH4 reduction, produced N-monoalkylated
spermidine fragments and analogues of the spermine alkaloid kukoamine A. The applicability of this methodology on the solid
phase was also demonstrated. © 2001 Elsevier Science Ltd. All rights reserved.
Synthetic N-alkylated analogues and homologues of the
naturally occurring linear polyamines (PAs) spermidine
(SPD) and spermine (SPM) and natural or synthetic PA
conjugates (PACs) show interesting biological proper-
ties and thus constitute attractive synthetic targets.1 We
have recently shown that a variety of PA analogues and
analogues of the alkaloid kukoamine A (KukA) can be
readily prepared, employing the N-triphenylmethyl
(trityl, Trt)- or polymeric o-chlorotrityl (PCtr)-pro-
tected amino acids b-alanine (bAla) and g-aminobutyric
acid (gAba) in the form of the corresponding active
esters (1–3, Fig. 1) with N-hydroxysuccinimide (HOSu)
or 1-hydroxybenzotriazole (HOBt), to acylate suitable
amino components, followed by LiAlH4 reduction of
the amides.2 The required dihydrocaffeyl (Dhc) units
were introduced in the SPM skeleton of KukA through
the acyl chloride 4 (Dbc-Cl).2b Using a similar protocol,
Nordlander et al. employed other derivatives of bAla
and gAba to obtain N-polyalkylated SPDs.3 We now
wish to report on a simple fragment synthesis protocol
which allows the efficient preparation of selectively
N-alkylated SPDs and SPD analogues of KukA using
active esters 1–3 and the dipeptide active esters 5, 6 and
14 (Schemes 1 and 2).
Figure 1. Structures of compounds used to prepare SPD and SPM analogues and conjugates.
Keywords: amino acids and derivatives; spermidine derivatives; fragment synthesis; solid-phase synthesis; protecting groups.
* Corresponding author. E-mail: d.a.papaioannou@chemistry.upatras.gr
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02308-X

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S. Vassis et al. / Tetrahedron Letters 42 (2001) 1579–1582
upon routine activation with HOSu or HOBt in the
presence of DCC or diisopropylcarbodiimide (DIC),
respectively, produced the corresponding isolable active
Thus, acylation of the in situ generated TMS ester of
gAba with either 1 or 2 and then methanolysis gave the
dipeptide acids Trt- and PCtr-bAla-gAba-OH which,
G
i, ii or iii,
iv, v or vi
O
H
N
vii
HN
γAba
X
5 : 69%
6 : 93%
7a-g : 60-80%
7h-j : 93-97%
O
5 ; G=Trt, X=OSu
6 ; G=PCtr, X=OBt
R3
N
G
G
O
H
N
R1
R1
viii or ix
HN
HN
N
N
80-95%
R2
crude yields
R2
O
7a-f ; G=Trt, R1=H, R2=H, Me, Et, Hex, cHex, Bn
7g ; G=Trt, R1=R2=Bn
7h-i ; G=PCtr, R1=H, R2=Et, Bn
7j ; G=PCtr, R1=R2=Bn
8a-d ; G=Trt, R1=R3=H, R2=H, Et, Hex, Bn
8e ; G=Trt, R1=R2=Bn, R3=H
8f-g ; G=PCtr, R1=R3=H, R2=Et, Bn
8h ; G=PCtr, R1=R2=Bn, R3=H
xii, x, xi
8b-e
x
65-80%
G
R3
N
H
N
R1
R1
xi
H2N
HN
N
N
60-74%
R2
R2
9a-c ; G=Trt, R1=R3=Boc, R2=Et, Hex, Bn
9d ; G=Trt, R1=R2=Bn, R3=Boc
10a-c ; R1=H, R2=Et, Hex, Bn
10d ; R1=R2=Bn
G
Et
N
G
HN
H
N
xiii, viii or ix
51%
HN
Bn
R1
N
N
R2
Bn
8e ; G=Trt
8h ; G=PCtr
11a ; G=Trt, R1=R2=Bn
11b ; G=PCTr, R1=R2=Bn
11c ; G=R2=H, R1=Bn
11d ; G=R1=R2=H
xiv
xv
95%
R2
G
Et
HN
Boc
x
R1
xvi, xii,
xiii, viii
HN
N
N
8e
N
NR2
64%
45%
R2
13a ; G=Trt, R=Bn
13b ; G=R=H
12a ; R1=R2=Bn
12b ; R1=Bn, R2=H
12c ; R1=R2=H
xiv
90%
xiv
xv
93%
Scheme 1. Fragment synthesis of selectively N-alkylated SPDs. Reagents and conditions: (i) Me3SiCl, CH2Cl2, 25°C, 10 min; (ii)
1/Et3N, 0–25°C, 20 min; (iii) 2/Et3N, 0, 10 min then 25°C, 6 h; (iv) MeOH; (v) DCC/HOSu, THF/DMF (3:1), 0°C, 1 h then 25°C,
12 h; (vi) DIC/HOBt–THF, 0°C, 1 h then 25°C, 12 h; (vii) R1R2NH/Et3N, DMF (CH2Cl2 for resins 7i,j), 0°C, 10 min then 25°C,
2 h (12 h for resins 7h–j); (viii) LiAlH4, THF, reflux 1–2 days; (ix) BH3, THF, reflux, 1–2 days; (x) Boc2O/Et3N, CHCl3, 0°C, 15
min then 25°C, 2 h; (xi) TFA/CH2Cl2 (1:1), 25°C, 30 min; (xii) TFA–CH2Cl2 (1:4), 25°C, 30 min; (xiii) Ac2O/Et3N, CHCl3, 0°C,
15 min then 25°C, 1 h; (xiv) H2/10% Pd–C, EtOAc/MeOH (1:1) then AcOH/MeOH/H2O (1:1:0.05), 25°C, 1 day; (xv) H2/20%
Pd(OH)2 on C, MeOH, 25°C, 1 day, 3 atm; (xvi) Bz-Cl/Et3N, CHCl3, 0°C, 15 min then 25°C, 1 h.

S. Vassis et al. / Tetrahedron Letters 42 (2001) 1579–1582
1581
R1
R2
N
H
N
v, vi
N
OSu
R3
HN
G
HN
Trt
O
O
14
15a-b ; G=Trt, R1=R2=H, R3=H,Et
16 ; G=Trt, R1=R2=Boc, R3=Et
12c ; G=R1=R2=H, R3=Et
vii
i, ii, iii, iv
viii
βAla
15a ix, x
Dhc
Dhc
Dbc
Dbc
N
xi, ix
x
N
HN
8e
HN
NBn2
NH
R
17
19 ; R=Bn
SkukB ; R=H
H
N
H
ix, x
Dhc
N
13b
Dhc
N
H
SkukA
Trt
HN
Dhc
N
Dbc
ix
x
R
R
H2N
N
8a, d
N
N
20 ; R=Bn
SkukC ; R=H
18 ; R=Bn
21 ; R=H
Dhc
Dbc
Scheme 2. Synthesis of N1-alkylated SPDs and the KukA analogues SkukA–C. Reagents and conditions: (i) MeOH/SOCl2, 0°C,
30 min then 25°C, 1 day, 97%; (ii) Trt-gAba/DCC/HOBt/Et3N, DMF, 0°C, 30 min then 25°C, 1 day, 30%; (iii) 2N NaOH,
MeOH, reflux, 30 min, 98%; (iv) DCC/HOSu, DMF, 0°C, 30 min then 25°C, 12 h, 97%; (v) R1R2NH/Et3N, DMF, 0°C, 10 min
then 25°C, 1 h, 55–65%; (vi) LiAlH4, THF, reflux, 1–2 day, 58–82%; (vii) Boc2O/Et3N,CHCl3, 0°C, 15 min then 25°C, 2 h, 92%;
(viii) TFA/CH2Cl2 (1:1), 25°C, 1 h, 97%; (ix) Dbc-Cl (4)/Et3N, CH2Cl2, 0°C, 30 min then 25°C, 1–3 h, 50–70%; (x) H2/10% Pd–C,
AcOH/MeOH/H2O (1:1:0.05), 25°C, 3 h; (xi) TFA/DCM (1:4), 25°C, 30 min; (xii) 2N HCl in MeOH, 25°C, 12 h.
esters 5 and 6 (Scheme 1). Aminolysis of esters 5 and 6
with a variety of concentrated aqueous (aq.) amines,
such as 30% NH3, 40% MeNH2 and 33% EtNH2, or
neat amines, such as hexylamine, cyclohexylamine, ben-
zylamine and dibenzylamine, gave the corresponding
crystalline amides 7a–g4 and the polymeric amides 7h–j.
Reduction of amides 7a–g with LiAlH4 and of 7h–j
with BH3·THF produced the corresponding crude SPD
derivatives 8a–h.5 Although compounds 8a–e could be
routinely purified by chromatography, pure N8-alky-
lated SPDs (10a–d) were better obtained through rou-
tine Boc protection of crude compounds, producing
compounds 9a–d, followed by purification and com-
plete deprotection with trifluoroacetic acid (TFA).
tial detritylation, acetylation and LiAlH4 reduction,
gave the intermediate 12a. Complete deprotection of
11a and 12a could only be realized using Pearlman’s
catalyst at 3 atm of hydrogen pressure. On the con-
trary, compound 13a, readily obtained from crude 8e
through tert-butoxycarbonylation, was readily hy-
drogenolyzed in the presence of 10% Pd–C producing
N4-Boc-SPD (13b). Compound 13b is a useful interme-
diate in the synthesis of SPD conjugates.5a,6 Alterna-
tively, N1-monoalkylated SPDs, such as N1-Et-SPD
(12c), were obtained using active ester 14 (Scheme 2) as
key intermediate. Compound 14 was prepared through
a four-step protocol involving esterification of H-bAla,
followed by coupling with Trt-gAba, saponification and
finally activation. Aminolysis of 14 with either 30%
NH3 or 40% EtNH2, followed by LiAlH4 reduction,
produced the SPD derivatives 15a and b. Compound
15b, purified as the diBoc derivative 16, gave pure 12c
upon deprotection with TFA.
On the other hand, acetylation of 8e, followed by
reduction of the acetamide with either diisobutylalu-
minium hydride (DIBALH) or LiAlH4 produced a mix-
ture of the desired N4-ethylated product 11a and 8e
(11a:8e=6:4), from which 11a was obtained in pure
form through chromatography. Similar results were
obtained on the solid phase using 8h as starting mate-
rial and BH3·THF to reduce the polymeric acetamide.
Furthermore, benzoylation of 8e, followed by sequen-
Taking into consideration that the SPD analogue
SkukA (Scheme 2) of the alkaloid KukA has similar
antiparasitic properties with KukA,7 we decided to

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S. Vassis et al. / Tetrahedron Letters 42 (2001) 1579–1582
apply intermediates 8d, 8e and 13b to the synthesis of
the three isomers SkukA–C. Indeed, bisacylation of 13b
with Dbc-Cl (4), followed by hydrogenolysis and Boc
group removal, produced SkukA in 55% overall yield.
On the other hand, detritylation of 8e with TFA,
followed by bisacylation with 4, produced the SkukB
precursor 17, whereas bisacylation of 8d with 4 gave the
SkukC precursor 18. Hydrogenolyses of precursors 17
and 18, under the conditions used to obtain N4-Boc-
SPD, produced N8-Bn-SkukB 19 and -SkukC 20,
respectively, along with small amounts of the corre-
sponding SkukB and SkukC. Finally, bisacylation of
the alternative intermediates N8-Trt-SPD 15a and N1-
Trt-SPD 8a with 4, followed by hydrogenolysis, pro-
duced SkukB and SkukC, respectively, in ca. 60%
overall yields.
2. (a) Mamos, P.; Karigiannis, G; Athanassopoulos, C.;
Bichta, S.; Kalpaxis, D.; Papaioannou, D.; Sindona, G.
Tetrahedron Lett. 1995, 36, 5187–5190; (b) Karigiannis,
G.; Mamos, P.; Balayiannis, G.; Katsoulis, I.; Papaioan-
nou, D. Tetrahedron Lett. 1998, 39, 5117–5120.
3. Nordlander, J. E.; Payne, M. J.; Balk, M. A.; Gress, J. L.;
Harris, F. D.; Scott Lane, J.; Lewe, R. F.; Marshall, S. E.;
Nagy, D.; Rachlin, D. J. J. Org. Chem. 1984, 49, 133–138.
4. The structures of the new compounds were determined by
IR, ESI-MS, NMR and microanalysis. The course of the
hydrogenolyses routinely used to split-off the Trt and Bn
protecting groups was followed by ESI-MS.
5. For a most recent application of the polyamide reduction
strategy in the synthesis of PA toxins see: Wang, F.;
Manku, S.; Hall, D. G. Org. Lett. 2000, 2, 1581–1583. For
sensitive polyamides, Raney nickel-mediated desulfuriza-
tion of the corresponding thioamides has been used: Blag-
brough, I. S.; Moya, E. Tetrahedron Lett. 1994, 35,
2057–2060. Recently, the alternative reductive amination
strategy has been frequently used to assemble the PA
skeleton of biologically interesting PACs. For examples
see: (a) Carrington, S.; Renault, J.; Tomasi, S.; Corbel,
J.-C.; Uriac, P.; Blagbrough, I. S. J. Chem. Soc., Chem.
Commun. 1999, 1341–1342; (b) Geall, A. J.; Blagbrough, I.
S. Tetrahedron 2000, 56, 2449–2460; (c) Blagbrough, I. S.;
Geall, A. J.; David, S. A. Bioorg. Med. Chem. Lett. 2000,
10, 1959–1962; (d) Kim, H.-S.; Choi, B.-S.; Kwon, K.-C.;
Lee, S.-O.; Kwak, H. J.; Lee, C. H. Bioorg. Med. Chem.
2000, 8, 2059–2065.
In conclusion, the present methodology provides easy
access to N-monoalkylated and N,N%-bisacylated SPDs.
It can also be applied to the solid phase and is therefore
amenable to combinatorial synthesis applications. Fur-
ther applications of the new SPD derivatives 8a and e,
13b and 15a in the synthesis of other medicinally inter-
esting SPD conjugates are currently in progress.
Acknowledgements
6. For selected papers, reporting the use of the Boc group to
block the secondary amine functions of Pas, see: (a) Car-
rington, S.; Fairlamb, A. H.; Blagbrough, I. S. J. Chem.
Soc., Chem. Commun. 1998, 2335–2336; (b) Nazih, A.;
Cordier, Y.; Bischoff, R.; Kolbe, H. V. J.; Heissler, D.
Tetrahedron Lett. 1999, 40, 8089–8091; (c) Hone, N. D.;
Payne, L. J. Tetrahedron Lett. 2000, 41, 6149–6152; (d)
Garrett, S. W.; Davies, O. R.; Milroy, D. A.; Wood, P. J.;
Pouton, C. W.; Treadgill, M. D. Bioorg. Med. Chem. 2000,
8, 1779–1797; (e) Chhabra, S. R.; Khan, A. N.; Bycroft, B.
W. Tetrahedron Lett. 2000, 41, 1099–1102.
The European Commission and the Greek Ministry of
Education are gratefully acknowledged for financial
support in the form of fellowships (S.V. and M.M.) and
consumables.
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