we have recently shown that homooligomers (7-9 mers) of
L-arginine upon conjugation to various probe molecules (e.g.,
fluorescein) or drugs (e.g., cyclosporin A (CsA)) provide
highly water-soluble conjugates that rapidly enter cells (e.g.,
human Jurkat).1,2 In addition, drug conjugates of these
arginine transporters have been shown to exhibit novel and
significant penetration into human skin and to release their
drug cargo in targeted T cells.8
phase segment doubling synthesis13-15 of the same octamer
would require only 9 steps (three coupling and six depro-
tection steps). In the specific case of arginine-based peptides,
solution-phase synthesis offers the additional advantage of
avoiding expensive protecting groups for the guanidinium
subunit (e.g., Mtr,16 Pmc,17 and Pbf18) required in solid-phase
synthesis. We report now the first segment doubling synthesis
of the arginine oligomer 1 that is both cost-effective and
scalable.
The enormous potential of arginine-based molecular
transporters is limited for several applications only by their
availability and cost. Such homooligopeptides are usually
prepared using solid-phase peptide synthesis.1,2,9-11 Although
this approach is readily automated and allows for the
synthesis and purification of long peptides, it suffers
drawbacks including high cost, limited scalability, and the
need for resin attachment and cleavage. In contrast, solution-
phase synthesis avoids the scale and cost restrictions of resins
(the cost of the resin-based synthesis is more than an order
of magnitude greater than that of the solution-phase synthe-
sis) and, in the particular case of certain oligomers, can be
conducted using a step-saving segment doubling strategy
(Figure 1).12
Our first strategy directed at the synthesis of 1 involved
coupling arginine monomers (Figure 2). Of the commercially
Figure 2. Retrosynthetic analysis.
available arginine monomers that contain differentially
protected functionalities, FmocNH-Arg(Pbf)-CO2Me and
BocNH-Arg(NO2)-CO2Me, we chose to utilize the latter
because of the ease of Boc deprotection in solution and the
stability of the intermediate amine salts (which are easy to
handle and less susceptible to intramolecular lactamization).
The Boc group can be deprotected using either HCl or TFA,19
the methyl ester of the carboxyl terminus is base-labile
(NaOH),20 and the nitro protecting group of the guanidine
can be removed by hydrogenation.21 Isobutyl chloroformate
was used for coupling commercially available BocNH-Arg-
(NO2)-CO2H (2) and HCl‚NH2-Arg(NO2)-CO2Me (3) as the
reagent can be removed in vacuo and has been used to couple
Figure 1. Step count comparison between solid-phase and solution-
phase segment doubling strategies.
Illustrative of the latter point, the unidirectional synthesis
of an octamer employing solid-phase synthesis requires 17
steps (one coupling and deprotection step for each added
monomer and one resin cleavage step), whereas a solution-
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