Solid-phase synthesis of Agel 416; a novel approach to modified polyamines

Article abstract of DOI:10.1016/S0040-4039(00)00995-3

The synthesis of Agel 416, an acylpolyamine derived from the Spider Agenolopsis aperta, has been achieved on solid support. Stepwise synthesis of the polyamine chain was accomplished using N-protected amino alcohols and the Fukuyama-Mitsunobu reaction. (C) 2000 Published by Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00995-3

Tetrahedron Letters 41 (2000) 6149±6152
Solid-phase synthesis of Agel 416; a novel approach to
modi®ed polyamines
Neal D. Hone* and Lloyd J. Payne
Cambridge Discovery Chemistry Ltd, The Merri®eld Centre, Rosemary Lane, Cambridge CB1 3LQ, UK
Received 18 April 2000; accepted 14 June 2000
Abstract
The synthesis of Agel 416, an acylpolyamine derived from the spider Agenolopsis aperta, has been
achieved on solid support. Stepwise synthesis of the polyamine chain was accomplished using N-protected
amino alcohols and the Fukuyama±Mitsunobu reaction. # 2000 Published by Elsevier Science Ltd.
Keywords: solid-phase synthesis; polyamine; combinatorial chemistry.
The acylpolyamines derived from certain spiders and wasps exhibit neuromodulatory activity
within the mammalian central nervous system. They are useful pharmacological tools for
investigating diseases such as stroke, Alzheimer's disease and epilepsy.1 These compounds
characteristically consist of a linear chain of consecutive alkylamine units, and are acylated at one
or both ends. Although structurally simple, construction of the often unsymmetrical polyamine
core and selective functionalisation of the desired terminus traditionally requires many synthetic
and puri®cation steps.2 Several approaches have been used for the preparation of polyamine
chains in solution,3 and some examples have utilised solid supports,3e,4 but no general procedure
has emerged. We envisaged that stepwise polyamine synthesis on solid support using simple
monomers and a repetitive reaction cycle would allow both automated and combinatorial
synthesis, in a manner analogous to solid-phase peptide synthesis. This was achieved using N-
Dde5 protected amino alcohols and the Fukuyama±Mitsunobu condensation,6 and the procedure
exempli®ed by the synthesis of Agel 416 (1),1a,7 a toxin produced by the spider Agenolopsis aperta.
Immobilisation of 1,3-diaminopropane as the Wang carbamate 3 was achieved from resin 2,8
followed by 2,4-dinitrobenzenesulfonylation of the free amino group. Acidolytic cleavage of 4 at
this stage (5% TFA in DCM, 5 min) yielded the monosulfonylated diamine and approximately
1
5 mol% diaminopropane (by H NMR). This percentage was una€ected after a repeated
sulfonylation, and appears to result from diaminopropane bis-Wang carbamate. Reaction of the
resin bound sulfonamide 4 with N-Dde 4-amino-1-butanol under Mitsunobu conditions9 gave the
* Corresponding author.
0040-4039/00/$ - see front matter # 2000 Published by Elsevier Science Ltd.
PII: S0040-4039(00)00995-3

6150
fully protected amine 5. A 2,4-dinitrobenzenesulfonamide was used instead of a 2-mononitro-
benzene alternative due to the greater rate of both sulfonamide formation and the subsequent
Mitsunobu alkylation. Under the conditions outlined in Scheme 1, sulfonylation of 3 with 2,4-
dinitrobenzenesulfonyl chloride to form 4 was complete within 2 h when monitored by ninhydrin.10
The analogous reaction with 2-nitrobenzenesulfonyl chloride was incomplete after 3 h, and
required the addition of pyridine for completion. The Mitsunobu alkylation of 4 was monitored
Scheme 1. (a) 1,3-Diaminopropane (5 equiv.), DCM, 1 h; (b) 2,4-dinitrobenzenesulfonyl chloride (4 equiv.), 2,6-lutidine
(4 equiv.), DCM, 3 h; (c) N-Dde butanolamine (3 equiv.), triphenylphosphine (3 equiv.), diethylazodicarboxylate (3
equiv.), THF, 3 h; (d) mercaptoacetic acid (10 equiv.), N,N-diisopropylethylamine (10 equiv.), DCM, 30 min, then
.
Boc2O (5 equiv.), N,N-diisopropylethylamine (2 equiv.), DCM, 3 h; (e) hydrazine H2O (2% in DMF), 30 min; (f)
repeat steps b to e using N-Dde propanolamine; (g) N-Boc indole-3-acetic acid (5 equiv.), HOBt (5 equiv.), DIC (5
equiv.), DCM, 1 h; (h) TFA (5% in DCM), 5 min; (i) 4 M HCl in dioxane, 3 h

6151
by periodic resin cleavage, with product analysis by LC-MS and was complete within 2 h.
The analogous reaction using a 2-nitrobenzenesulfonamide was 50% complete after 2 h, and
incomplete after 16 h. Dde protection was employed due to the facile preparation of amino
alcohol derivatives11 and its removal on solid support under non-acidic conditions.
Sulfonamide cleavage was achieved by brief treatment with mercaptoacetic acid, and treatment
with di-tert-butyl dicarbonate gave the reprotected amine 6. This deprotection±reprotection cycle
was employed to allow selective derivatisation of each newly formed secondary amine if required.
For example, some natural polyamines are N-alkylated or hydroxylated1 at certain positions, and
this ¯exibility is attractive for combinatorial synthesis. Hydrazinolysis of the Dde group allowed
re-entry of the resultant primary amine 7 into a second sulfonylation, alkylation, deprotection,
reprotection cycle, using N-Dde 3-amino-1-propanol, to give the amine 8. A third cycle, again
using N-Dde 3-amino-1-propanol gave the resin bound Agel 416 polyamine chain 9, with 2ꢀ
amine protection intact. After each synthetic step, a quantity of resin was cleaved (5% TFA in
DCM, 5 min) and the product con®rmed by 1H NMR and LC-MS. Acylation of 9 with indole-3-
acetic acid, monitored by ninhydrin, was complete within 1 h although on cleavage (5% TFA in
DCM, 5 min) a negligible yield of product was obtained. This was attributed to indole quenching
of the Wang derived cation, and so N-Boc indole-3-acetic acid2a,12 was substituted. This coupling
was again complete within an hour and cleavage of 10 gave crude tetra-Boc Agel 416 (11) (88%
based on original loading). LC-MS con®rmed the presence of this compound, comprising >70%
of the crude product (by evaporative light scattering). Puri®cation of 11 was readily achieved
through column chromatography, and ®nal deprotection using HCl in dioxane2a furnished Agel
416 (1).13
In conclusion, we have described a novel and general strategy for the synthesis of modi®ed
polyamines on solid support from simple building blocks. The use of robust reactions performed
at room temperature and without stringent inert control allows reliable automation, and com-
binatorial synthesis.
Acknowledgements
We wish to thank Paula Bryans for HPLC analysis.
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the amino alcohol in ethanol to re¯ux, followed by cooling and evaporation. The products were recrystallised from
ethyl acetate±hexane. For N-Dde-3-amino-1-propanol ꢀH (400 MHz, CDCl3) 1.10 (6H, s), 2.00 (2H, app. quintet,
J=6.3 Hz), 2.43 (4H, s), 2.52 (1H, br s), 2.64 (3H, s), 3.64 (2H, app. q, J=6.0 Hz), 3.85 (2H, t, J=5.9 Hz), 13.52
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13. Agel 416 eluted as a single peak at 11.4 min (94% purity). RPHPLC was performed using a Phenomenex C8 Luna
column (0.46Â10 cm) at a constant ¯ow rate of 1.0 ml min^1. Mobile phase: (A) 0.05 M NH4OAc(aq); and (B)
0.05 M NH4OAc in MeOH; gradient: 20% B isocratic for 2 min, 20 to 100% B in 10 min linearly, 100% B
isocratic 7 min. ES-MS m/z (ES+ in D2O) 418.3423 (M+D, C23H39N6D requires m/z 418.3420) was in accordance
with the reported spectroscopic data.