Figure 1. Structures of natural toxins and hybrid synthetic targets 3a,b. The numbering in ArgTX-659 denotes the molecular mass. The
digits in the name PhTX-433 denote the number of methylene groups that separate the amino groups, counting from the head-group.
previously been used as the point of attachment in SPS on
Merrifield-OH9 and Wang-OH10 resins via the Mitsunobu
reaction, as well as on Wang-Br,11 dihydropyran,12 and
2-chlorotrityl-Cl13 resins.
The protocol described here employs a trityl bromide resin
for mild immobilization of the active pentafluorophenyl (Pfp)
ester, NR-Fmoc-Tyr-OPfp (4), via its free phenolic function-
ality. Thus, concise bidirectional SPS, including synthesis
of novel hybrid spider-wasp neurotoxin analogues (e.g.,
3a,b, Figure 1), is feasible from this internal residue, while
the linker is compatible with TFA cleavage and concomitant
Boc/Teoc deprotection.
Polyamine-containing toxins isolated from the venoms of
spiders (e.g., argiotoxin-659, 1)14 and wasps (philanthotoxin-
433, 2)15 are known to be noncompetitive inhibitors of
various types of ionotropic receptors in the central nervous
system (CNS).16 Natural and synthetic polyamine derivatives
have therefore proved to be valuable as probes in structural
and functional studies of ligand-gated ion channels such as
ionotropic glutamate receptors (iGluRs) and nicotinic ace-
tylcholine receptors (nAChRs).17 Recent developments in
SPS methods have greatly facilitated the preparation of these
types of compounds,18 as they allow omission of several
tedious purification steps inherent to solution-phase methods.
As a result of recent synthetic efforts, structure-activity
relationship (SAR) studies have been performed with com-
pounds having altered position and number of NH groups
in the polyamine chain,19 branched sites in the polyamine
chain,20 bulky acyl21 or amino acid22 moieties in the head-
group, variation of the stereochemistry,23 and rigidity of the
headgroup.22 The present method enables SPS of novel
philanthotoxins elongated with amino acid tail moieties24
through peptide SPS using Fmoc-25 and 2-(trimethylsilyl)-
ethoxycarbonyl (Teoc)-protected26 building blocks. Since the
tail group is introduced in the last synthetic step prior to
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