Glutamic acid as a new linker for attachment of alcohols to solid support

Article abstract of DOI:10.1016/S0040-4039(02)02209-8

We took advantage of pyroglutamic cyclization of glutamic acid to investigate the use of this amino acid as a linker for the attachment of alcohols. As a first example, we reported the solid-phase synthesis of dimers on a 3,3′-diaminodiphenylmethanol.

Full text of DOI:10.1016/S0040-4039(02)02209-8

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9221–9223
Glutamic acid as a new linker for attachment of alcohols to
solid support
Gilles Subra,* Muriel Amblard and Jean Martinez
Laboratoire des Aminoacides Peptides et Prote´ines (LAPP), UMR5810-CNRS, Universite´s Montpellier I et II,
Faculte´ de Pharmacie, 15 Avenue C. Flahault, 34060 Montpellier, Ce´dex, France
Received 18 September 2002; revised 2 October 2002; accepted 3 October 2002
Abstract—We took advantage of pyroglutamic cyclization of glutamic acid to investigate the use of this amino acid as a linker
for the attachment of alcohols. As a first example, we reported the solid-phase synthesis of dimers on a 3,3%-diaminodiphenyl-
methanol. © 2002 Elsevier Science Ltd. All rights reserved.
The use of solid-phase synthesis for the generation of
diverse libraries of small organic compounds have been
extensively used in the search of leading compounds for
drug discovery.1,2 As part of a project to synthesize on
solid support libraries of dimers possessing an alcohol
functionality, we focused our efforts towards the design
of a new linker generation for such alcohols.3 Numer-
ous linkers have been already reported for the alcohol
attachments;4 nevertheless they have to be cleaved in
well defined conditions dictating the protecting condi-
tions strategy. A safety catch system, involving first
activation of the linker and cleavage in a second
step, appeared to be efficient for versatile library strat-
egy.
followed is presented in Scheme 1. The alcohol moiety
is attached to the glutamic acid side chain through an
ester linkage. The a-amino group is protected as an
urethane. At the end of the solid-phase synthesis,
removal of the urethane protection followed by basic
treatment, induced the g-lactam cyclization and subse-
quent release of the alcohol function. The pyroglutamic
moiety remains on the solid support with the desired
product into solution.
To validate this approach, we used this strategy to
synthesize dimers of a 3,3%-diaminodiphenylmethanol
(Scheme 2). As a first proof of concept, we decided to
use Boc-protected amino acids for dimer elongation
and the orthogonal Fmoc protection of the linker
amino group.
Atrash et al.5 have described a safety catch linker
strategy for carboxylic acid based on diketopiperazine
formation. To design a new linker,6 we took advantage
of another well know side reaction of peptide synthesis,
namely pyroglutamic cyclization. The strategy we have
Fmoc-Glu(OtBu)-OH was coupled to the polystyrene
MBHA resin (0.8 mmol/g) using BOP as coupling
reagent in presence of DIEPA in dimethylformamide.
Scheme 1. Stategy of glutamic acid linker cleavage.
Keywords: glutamic acid; linker; solid-phase synthesis; combinatorial chemistry.
* Corresponding author. Tel.: 33 4 67 54 86 55; fax: 33 4 67 54 86 54; e-mail: subra@colombes.pharma.univ-montp1.fr
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02209-8

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G. Subra et al. / Tetrahedron Letters 43 (2002) 9221–9223
Scheme 2. Synthesis of 3,3%-diaminodiphenylmethanol dimers using the glutamic acid linker.
After capping the unreacted amino functions using acetic
anhydride, OtBu side-chain protection of glutamic acid
was removed in acidic conditions using TFA/DCM 1/1
mixture to generate the free carboxylic acid on the resin.
Diaminoalcohol moiety is anchored on solid support as
3,3%-dinitrophenylmethanol (1). This compound is
directly obtained from reduction of commercially avail-
able dinitrobenzophenone using sodium borohydride
(NaBH4) in methanol for 60 min (88% yield). Loading
of 1 on the resin was performed by diisopropylcarbodi-
imide (DIC) in the presence of dimethylaminopyridine
(DMAP) in dichloromethane for 12 h. Nitro functions
were reduced into the corresponding amino groups using
tin(II) chloride (SnCl2) in N-methyl pyrrolidone (NMP)
at 50°C for 2 days to yield resin 2.
The synthesis was performed simultaneously on the two
amino function of 2 using standard Boc-strategy. BOP/
DIEA were used for coupling steps and N-Boc protec-
tions were removed using TFA/DCM 1/1 solution.
Nihydrine test7 was used for the monitoring of coupling
reactions.
The cleavage from the resin was performed in two steps.
First, the Fmoc protecting group of the amino function
of glutamic acid was removed by DMF/piperidine (80/
20) solution treatment for 20 min. Then the resin was
quickly washed with DMF, methanol and DCM to
minimize premature lactam formation in basic condi-
tions. Afterward, the cyclization was done in methanol/
DIEA 95/5 at 60°C for 12 h. Resin was removed by
Table 1. Yield and purity of dimers 3–5
a
Percentage calculated on the basis of theoretical resin loading.b Percentage calculated from reversed-phase HPLC spectra at 220 nM.

G. Subra et al. / Tetrahedron Letters 43 (2002) 9221–9223
9223
filtration and solvent was concentrated under vacuum.
Dimers 3–5 were obtained with at least 65% yield
calculated from resin 2 theoretical loading and with
purity ranging from 72 to 92%8 (Table 1). Compounds
were characterized by mass spectrometry ES+ on a
Micromass Platform II spectrometer.
evaluation of other protecting groups for glutamic acid
is under investigation.
References
1. Gordon, E. M.; Barret, R. W.; Dower, W. J.; Fodor, S. P.
A.; Gallop, M. A. J. Med. Chem. 1994, 24, 1385–1401.
2. Terret, N. K.; Gardner, M.; Gordon, D. W.; Kobylecki,
R. J.; Steele, J. Tetrahedron 1995, 51, 8135–8173.
3. Subra, G. Synthese d’agonistes et d’antagonistes du TGF-
b, Ph.D., Universite´ de Montpellier I, France, 1999; pp.
73–76.
4. James, I. W. Tetrahedron 1999, 55, 4855–4946.
5. Atrash, B.; Bradley, M. Chem. Commun. 1997, 1397–1398.
6. While the writing of this paper, we were advised of a
poster communication presented at the 27th European
Peptide Symposium in Sorrento (Italy) August 31, Septem-
ber 6, 2002, by T. Wakamiya et al. about the use of
glutamic acid linker.
In conclusion, we have developed a new linker strategy
based on lactam formation of glutamic acid to anchor
alcohols on solid support. The supported linker is easily
and quickly prepared from commercially available pro-
tected glutamic acid and MBHA resin. The alcohol is
removed from the resin by a two-step treatment ini-
tiated with the amino group deprotection of glutamic
acid followed by pyroglutamic cyclization. Our strategy
has been successfully applied to the synthesis of dimers
using Boc SPPS and Fmoc protection of the amino
group of glutamic acid. Nevertheless, the principle of
glutamic linker is versatile enough to be applied to
other synthetic conditions by using appropriate N-pro-
tection of glutamic acid (Alloc, Boc,…).
7. Kaiser, E.; Collescot, R. L.; Bossinger, C. D.; Cook, P. I.
Anal. Biochem. 1967, 40, 2164–2167.
8. Purity was estimated by HPLC analysis at 220 nm based
The application of this strategy to synphase™ lanterns
for the generation of libraries of alcohols as well as the
on peak areas.