Solid-phase synthesis of polyamines using a Dde-linker: Philanthotoxin- 4.3.3 via an on-resin Mitsunobu reaction

Article abstract of DOI:10.1016/S0040-4039(99)02239-X

A Dde linker/solid-phase procedure for the stepwise construction of polyamines is illustrated with the synthesis of philanthotoxin-4.3.3. Resin- bound 4-aza-1,7-heptanediamine was N⁴-protected and N⁷-activated in a single operation with 2-nitrobenzenesulphonyl chloride. Alkylation with N- Fmoc-4-aminobutanol under Mitsunobu conditions completed the polyamine backbone. Deprotection of the sulphonamide groups was readily achieved with thiolates in DMF at room temperature. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(99)02239-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1099–1102
Solid-phase synthesis of polyamines using a Dde-linker:
philanthotoxin-4.3.3 via an on-resin Mitsunobu reaction
Siri Ram Chhabra, Azra N. Khan and Barrie W. Bycroft ∗
School of Pharmaceutical Sciences, University of Nottingham, University Park, Nottingham NG7 2RD, UK
Received 30 September 1999; accepted 29 November 1999
Abstract
A Dde linker/solid-phase procedure for the stepwise construction of polyamines is illustrated with the synthesis
of philanthotoxin-4.3.3. Resin-bound 4-aza-1,7-heptanediamine was N4-protected and N7-activated in a single
operation with 2-nitrobenzenesulphonyl chloride. Alkylation with N-Fmoc-4-aminobutanol under Mitsunobu
conditions completed the polyamine backbone. Deprotection of the sulphonamide groups was readily achieved
with thiolates in DMF at room temperature. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: solid-phase synthesis; polyamines; Mitsunobu reaction; Dde-linker.
In the preceding paper,1 we described a further application of our Dde-based primary amine linker
to the synthesis of polyamines and illustrated the procedure with the synthesis of polyamine derivative
philanthotoxin-3.4.3 (PhTX-3.4.3) 1 and the structural isomer PhTX-3.3.4 2. Philanthotoxins and other
naturally occurring polyamine toxins, as well as their synthetic analogues, are valuable investigative
tools in neuropharmacology and show some promise as lead entities for the development of therapeutic
agents.2–5
The synthesis of 1 was readily achieved starting with spermine but the polyamine backbone of 2 was
constructed by two successive rounds of reductive alkylation of resin bound 1,4-butanediamine with
N-Fmoc-3-aminopropanal.† We sought, by a similar procedure, to synthesise the remaining analogue
PhTX-4.3.3 3 in order to complete the series, and further demonstrate the flexibility of the approach.
∗
Corresponding author. Tel: +44 115 9515101; fax: +44 115 9515049; e-mail: barrie.bycroft@nottingham.ac.uk (B. W.
Bycroft)
†
Abbreviations: Amino acids and peptides follow the IUPAC-IUB nomenclature where applicable (Eur. J. Bio-
chem. 1984, 9-37); Boc, t-butoxycarbonyl; tBu, tert-butyl; DCM, dichloromethane, DEAD, diethyl azodicarboxylate;
DIEA, N,N-diisopropylethylamine; DMF, N,N-dimethylformamide; ES-MS, electrospray mass spectrometry; Fmoc, 9-
fluorenylmethoxycarbonyl; HOBt, 1-hydroxybenzotriazole; Nbs, 2-nitrobenzenesulfonyl; RPHPLC, reverse phase high per-
formance liquid chromatography; TBTU, O-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate; THF, tetra-
hydrofuran; TFA, trifluoroacetic acid; TIPS, triisopropylsilane.
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02239-X
tetl 16167

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Although a solution phase synthesis of this naturally occurring toxin has been described,6 no solid phase
methods have been reported.
We envisaged that the reductive alkylations would be straightforward and the approach would involve
anchoring 1,3-propanediamine to the solid support followed by stepwise elongation of the polyamine
with N-Fmoc-3-aminopropanal and N-Fmoc-4-aminobutanal. The first step, as anticipated from the
previous work, proceeded smoothly but implementing the latter was thwarted by our inability to achieve
a convenient synthesis of N-Fmoc-4-aminobutanal. Neither reduction of the corresponding acid nor the
oxidation of the corresponding alcohol led to clean products. Since similar difficulties were experienced
with the corresponding C5 analogues, we assumed that a predisposition for both these aldehydes to
undergo intramolecular cyclisation to hemiaminal derivatives and subsequently to other products might
be the reason. Rather than pursue this line of investigation for introducing C4 and C5 units into
polyamines, we decided to explore the alternative possibility of using the readily available Fmoc-amino
alcohols for on-resin N-alkylation via a Mitsunobu type reaction.
Generally this procedure requires that the primary amine is converted into a derivative in which
the NH is strongly acidic.7 This can be achieved through the formation of toluenesulfonamide,8
trifluoroacetamide,9 trifluoromethanesulfonamide10 and 2,2,5,7,8-pentamethylchroman-6-sulfonyl
(pmc)11 derivatives but the conditions required for their deprotection render them unsuitable
for our solid-phase approach. However, Fukuyama et al.12 have recently reported that 2- and 4-
nitrobenzenesulfonamide derivatives can be conveniently prepared, and following alkylation, efficiently
deprotected at room temperature under neutral conditions using sodium thiophenolate in DMF.13 These
conditions have been recently employed for the synthesis of N-methyl amino acids14 and the site-specific
N-alkylation of peptides on solid-phase.14,15
We have now successfully completed the synthesis of PhTX-4.3.3 using 2-nitrobenzenesulfonyl
derivatives by the two routes summarised in Scheme 1. Either the triamine 5a or its N4-Boc derivative
5b16 was directly ligated onto the Dde-linker 417 to afford 6a and 6b respectively, or alternatively built up
on the resin via reductive alkylation of bound 1,3-propanediamine with N-Fmoc-3-aminopropanal in the
manner previously described.1 The primary amine groups of both reacted with 2-nitrobenzenesulfonyl
chloride in the presence of DIEA to give 7a or 7b, respectively. It is noteworthy that in the process
N4 in 6a is also sulfonylated and thereby protected with a 2-Nbs group. Alkylation of 7a and 7b with
excess of N-Fmoc-4-aminobutanol18 in the presence of triphenylphosphine and diethyl azodicarboxylate
completed the polyamine backbones 8a or 8b. Fmoc loading assays at this stage indicated resin

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Scheme 1. Reagents and conditions. (i) 2-NbsCl (4 equiv.), DIEA (6 equiv.) in THF, 5 h; (ii) FmocNH(CH2)4OH
(10 equiv.), Ph3P (5 equiv.), DEAD (5 equiv.) in dry THF, 20 h; (iii) 20% v/v piperidine in DMF, 10 min; (iv)
Fmoc-Tyr(tBu)-OH/TBTU/HOBt/DIEA, 4 h; (v) 20% v/v piperidine in DMF, 10 min; (vi) n-butyric acid/TBTU/HOBt/DIEA,
4 h; (vii) 50% TFA in DCM/TIPS/H2O, 3 h; (viii) 1 M NaSPh in DMF (2×1 ml) per 100 mg of resin 2×1 h; (ix) 10% v/v
n-propylamine in DMF, 6×30 min
substitutions of approximately 80%. Removal of the Fmoc group and successive acylations with activated
Fmoc-Tyr(OtBu)-OH and n-butyric acid gave the resin-bound protected toxin 9a or 9b. Both acylation
steps were monitored by TNBS test and the efficiency of the first acylation step was also determined by
a Fmoc loading assay. After removal of the Boc and tBu groups by acidolysis with 50% TFA in DCM,
the deprotection of the Nbs group was achieved by exposing the resin to 1 M solution of PhSNa in DMF.
Finally cleavage from the support in both cases, using 10% v/v n-propylamine in DMF, afforded PhTX-
4.3.3 as an oil in 75% overall yield and with excellent purity as determined by a single peak eluting at 12.3
min in RPHPLC analysis.19 The isolated product gave satisfactory ES-MS, m/z 436 (M+H, C23H41N5O3
requires m/z 435) and appropriate spectral data.
In conclusion, Mitsunobu alkylation complements the reductive alkylation procedure reported pre-
viously but importantly allows the convenient introduction of C4 and C5 units into polyamines. These

1102
protocols can now be exploited, either individually or together for the elaboration of a polyamine
backbone of any sequence. Furthermore the high yield obtained further demonstrates the stability of
Dde-linker not only towards acid, base and reductive alkylation but also to the reagents employed for
Mitsunobu alkylation. In addition, the compatibility of the linker with the introduction and deprotection
of the 2-Nbs group adds another level of protective orthogonality and thus further extends the utility of
this linker.
Acknowledgements
We thank the Commonwealth Scholarship Commission, UK for a research studentship to A. N. Khan.
References
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19. RPHPLC conditions — see preceding paper.