An efficient two-step synthesis of mono-, di- and triureas from resin- bound amides

Article abstract of DOI:10.1016/S0040-4039(00)00845-5

An efficient method for the solid-phase synthesis of mono-, di- and triureas from resin-bound mono-amines, diamines and triamines is described. The exhaustive reduction of solid support-bound amides generated the requisite amines, which, following treatme

Full text of DOI:10.1016/S0040-4039(00)00845-5

Tetrahedron Letters 41 (2000) 5441±5446
An ecient two-step synthesis of mono-, di- and triureas
from resin-bound amides
Adel Nefzi, Nhi A. Ong and Richard A. Houghten*
Torrey Pines Institute for Molecular Studies, 3550 General Atomics Court, San Diego, CA 92121, USA
Received 24 March 2000; revised 4 May 2000; accepted 25 May 2000
Abstract
An ecient method for the solid-phase synthesis of mono-, di- and triureas from resin-bound mono-
amines, diamines and triamines is described. The exhaustive reduction of solid support-bound amides
generated the requisite amines, which, following treatment with isocyanates and cleavage, provided the
corresponding ureas in high purity and good yields. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: combinatorial chemistry; solid-phase synthesis; peptidomimetics; urea; diurea; triurea.
Solid-phase parallel synthesis is used worldwide to generate libraries of small organic com-
pounds for the acceleration of the drug discovery process.1,2 The urea functionality has attracted
considerable attention due to its stability compared to the amide bond.3 Ureas are found in many
biological compounds.4 Included in such compounds are ureas that e€ect acid gastric secretion
and the healing of chronic gastric ulcers induced in rats,5 inhibitors of acyl-Co:cholesterol O-acyl
transferase (ACAT),6 and antioxidant derivatives.7 Ureas are typically prepared in solution or
solid-phase by treatment of isocyanates with amines.8 We describe here an ecient method for
the generation of mono-, di- and triureas from resin-bound amides.
Starting from p-methylbenzhydrylamine (MBHA) resin, and following neutralization and acyla-
tion of the amine, the resin-bound amide 1 was reduced with borane±THF. The resulting secondary
amine 2 is treated with an excess of isocyanate to a€ord, following HF cleavage, the desired N,N0-
ureas 3 in high purity (Scheme 1). The reduction of amide bonds on the solid support and the use
of the resulting amine for the generation of peptidomimetic acyclic and heterocyclic compounds
has been previously reported by our laboratory.9 The feasibility of the reaction was determined
by ®rst preparing a small number of individual compounds using the `tea-bag' method of parallel
synthesis.1b Twenty-three individual N,N0-ureas were prepared using nine di€erent carboxylic
acids and three di€erent isocyanates. The expected compounds were obtained in excellent purity
* Corresponding author. Tel: 858 455 3803; e-mail: rhoughten@tpims.org
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00845-5

5442
(>95%) and good yields10 (Table 1). Starting from the same solid support, and following amino
acid coupling and acylation, compound 5 was reduced with borane±THF; the resin-bound diamine
was then treated with excess of isocyanate to a€ord, following HF cleavage, the expected diureas
6 in high purity (>95%) (Scheme 1). Four individual diureas were ®rst prepared as controls. The
desired compounds were obtained in excellent purity and yields10 (Table 2). Similarly, treatment
of a reduced resin-bound acylated dipeptide 8 with isocyanate (Scheme 1) a€orded the corre-
sponding triureas 9 following HF cleavage. Seven individual compounds were synthesized in high
purity and good yields10 (Table 3). In Fig. 1, we show the LC±MS of the triurea 9c, which is
representative of the purities obtained in each case.
Scheme 1. (a) R1COOH, DIPCDI, HOBt; (b) BH3±THF; (c) Xaa±OH, DIPCDI, HOBt; (d) R2NCO, DMF; (e) HF/
anisole
Following the strategy described in Scheme 1, 135 di€erent triureas 9, in which the individual
building blocks were varied while ®xing the remaining two positions, were synthesized (48 dif-
ferent amino acids at R1, 48 di€erent amino acids at R2 and 39 di€erent carboxylic acids at R3).
Modi®cations occurring to the amino acid side chains during the reduction steps have been
carefully studied. These individual compounds will serve as controls for the synthesis of a large
individual or mixture-based triurea library. The library synthesis and screening results for the
identi®cation of highly active triureas will be reported elsewhere.
The typical procedure for the synthesis of urea derivatives is as follows: Solid-phase syntheses
were carried out using the `tea-bag' method in which the resin is contained within sealed poly-
propylene mesh packets.1b The completeness of amino acid coupling and N-acylation were veri-
®ed using the ninhydrin test.11 100 mg p-MBHA resin (0.1 meq/g, 100±200 mesh) was contained
within a sealed polypropylene mesh packet. Reactions were carried out in 10 ml polyethylene
bottles. Following neutralization with 5% diisopropylethylamine (DIPEA) in dichloromethane
(DCM), the resin was washed with DCM, and the free amine was either coupled to an amino acid
or acylated with a carboxylic acid.

5443
Table 1
Examples of monoureas synthesized. The products were run on a Vydac column, gradient 5 to 95% TFA
in CAN in 7 min. The purity was estimated on analytical traces at 214 nm
Table 2
Examples of diureas synthesized. The products were run on a Vydac column, gradient 5 to 95% TFA in
CAN in 7 min. The purity was estimated on analytical traces at 214 nm

5444
Table 3
Examples of triureas synthesized (R2=Ph). The products were run on a Vydac column, gradient 40 to
95% TFA in CAN in 7 min. The purity was estimated on analytical traces at 214 nm
Figure 1. LC±MS of triurea 9c

5445
Amino acid coupling: The amino acid (Boc-Xaa-OH, 6 equiv.) was coupled using the conven-
tional reagents hydroxybenzotriazole (HOBt, 6 equiv.) and diisopropylcarbodiimide (DIPCDI, 6
equiv.) in anhydrous DMF for 60 min. Following removal of the Boc group with 50% tri-
¯uoroacetic acid in DCM (2Â30 min) and washing with DCM (6Â) and 5% DIPEA in DCM
(2Â), the free amine is then coupled to a second amino acid in the same conditions or acylated
with a carboxylic acid.
Acylation: The free amine was N-acylated with a carboxylic acid (10 equiv.) in the presence of
DIPCDI (10 equiv.) and HOBt (10 equiv.) overnight in anhydrous DMF.
Exhaustive reduction of the amide groups: The reduction was performed in 50 ml kimax tubes
under nitrogen. The resin packet and boric acid (15-fold excess over each amide bond) were
added to each tube. Trimethyl borate (15-fold excess over each amide bond) was added, followed
by 1 M BH3±THF (40-fold excess over each amide bond). The tubes were heated at 65ꢀC for 72 h,
followed by quenching with MeOH at room temperature. The resin was then washed with
methanol (4Â) and the borane disproportionated by treatment with neat piperidine at 65ꢀC
overnight. The resin was then washed with methanol (2Â) and DMF (6Â) and dried. The com-
pleteness of the reaction was veri®ed by cleavage and analysis following reduction.
Urea formation: The resin-bound amines were treated with a seven-fold excess of isocyanate
over each amine (0.1 M) in anhydrous DMF overnight. Following cleavage from the resin with
anhydrous HF in the presence of anisole at 0ꢀC for 90 min, the desired product was extracted
with acetonitrile:water (50:50) and lyophilized.12
The described ecient two-step transformation of mono-, di- and triamides to the corresponding
mono-, di- and triureas is one of a series of transformations of peptides and peptidomimetics to
classic small molecules and heterocyclic compounds. As in earlier studies, the described chemistries
will be used to generate individual compounds and mixture-based combinatorial libraries.13
Acknowledgements
This work was supported by National Cancer Institute Grant No. CA 78040 (Houghten).
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