Facile syntheses of conformationally constrained analogues of lysine and homoglutamic acid

Article abstract of DOI:10.1016/j.tetlet.2005.05.093

A facile divergent synthesis of the novel amino acid trans-4-aminoethyl-l- proline and trans-4-carboxymethyl-l-proline from commercially available trans-4-hydroxy-l-proline was developed. These conformationally constrained analogues of l-lysine and l-homoglutamic acid are useful proline templated amino acids (PTAAs) with potential applications in protein engineering and de novo protein design.

Full text of DOI:10.1016/j.tetlet.2005.05.093

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 4985–4987
Facile syntheses of conformationally constrained analogues of
lysine and homoglutamic acid
Salim Barkallah, Stephen L. Schneider and Dewey G. McCafferty*
Department of Biochemistry and Biophysics and the Johnson Research Foundation, The University of Pennsylvania School of Medicine,
Philadelphia, PA 19104-6059, USA
Received 10 March 2005; revised 19 May 2005; accepted 20 May 2005
Available online 14 June 2005
Abstract—A facile divergent synthesis of the novel amino acid trans-4-aminoethyl-L-proline and trans-4-carboxymethyl-L-proline
from commercially available trans-4-hydroxy-L-proline was developed. These conformationally constrained analogues of L-lysine
and L-homoglutamic acid are useful proline templated amino acids (PTAAs) with potential applications in protein engineering
and de novo protein design.
Ó 2005 Elsevier Ltd. All rights reserved.
The propensity of polyproline to adopt stable, helical
structures in both aqueous and nonaqueous environ-
ments makes it an attractive scaffolding element suitable
for exploitation in biological molecular recognition and
synthetic protein engineering. To increase the diversity
of proline structures available containing side chain
amine and acid functionalization, we report here the
development of a facile synthesis of trans-substituted
prolines 4-carboxymethyl-L-proline1 (Prc, 1) and the
novel amino acid 4-aminoethyl-L-proline (Pre, 2) from
receptor antagonist/agonist, trans-4-carboxymethyl-L-
proline, in six steps with a 26% overall yield from L-
pyroglutamic acid.3 Magdalengoitia and co-workers
have prepared 3-substituted aminoalkyl proline deriva-
tives (termed proline-templated amino acids or PTAAs)
for use in polyproline mimetics.4 Here, we describe an
alternative improved synthesis of trans-4-carboxy-
methyl-L-proline 1 from amino acid 3 with a 64% overall
yield. Also, we show that trans-4-aminoethyl-proline 2
could be obtained from the same starting material in
40% overall yield (Fig. 1).
commercially
hydrochloride 3.
available
trans-4-hydroxy-L-proline
4-Hydroxy-L-proline hydrochloride (3) was converted
into its corresponding Na-Boc-protected methylester5 4
by a two reaction, one pot treatment with methanolic
HCl followed by Boc-anhydride/diisopropylethylamine
in 91% overall yield. Dormoy and Castro have reported5
the biphasic oxidation of trans-Na-Boc-4-hydroxy-L-
proline methyl ester 4 to Na-Boc-4-oxo-L-proline methyl
ester 5 in 94% yield using ruthenium tetroxide and
In contrast to proline ring alkylation, few precedents
describing aminoalkyl or carboxymethyl modification
of the proline pyrrolidine side chain have been reported.
Koskinen and Rapoport have developed a general
method for synthesizing 4-substituted prolines from
N-(9-(9-phenylfluorenyl))-protected glutamic acid esters.2
Ezquerra et al. have also developed a general procedure
for the synthesis of 4-substituted prolines by the regio-
selective alkylation of L-pyroglutamic acid derivatives
followed by selective reduction of the lactam carbonyl.1
Langlois and Rojas have extended the latter approach to
the first stereoselective synthesis of the potential NMDA
Keywords: Aminoethyl; Proline; Carboxymethyl; Amino acid;
Polyproline.
*
Figure 1. Structures of proline analogues 1 and 2 and their relationship
to L-homoglutamic acid and L-lysine, respectively.
Corresponding author. Tel.: +1 215898 7619; fax: +1 215753
8052; e-mail: deweym@mail.med.upenn.edu
0040-4039/$ - see front matter Ó 2005Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.05.093

4986
S. Barkallah et al. / Tetrahedron Letters 46 (2005) 4985–4987
parison to chemical shifts of related compounds.1–3
The fixed geometric orientation of the anisotropic
shielding cone of the nitrile of 6a resulted in pro-
nounced chemical shift differences in the NMR reso-
nances of the C-3 and C-5ring methylenes relative to
the parent ketone that also facilitated discrimination
of the regiochemistry associated with each isomer (see
Supplementary data).
The exocyclic double bond of cyanoolefin 6a was hydro-
genated over sodium borohydride-reduced palladium
chloride to afford the cyanomethyl derivatives 7a and
7b in 90% yield as a mixture of cis and trans diastereo-
mers, with the trans isomer comprising ꢀ80% of the
mixture (62% diastereomeric excess). The stereochemical
assignments for anti isomer 7a and syn isomer 7b were
made based on comparison to literature examples of
L-proline ester derivatives substituted at C-4 and by
NMR chemical shift analyses. Depicted in Figure 2 is
the structure of the global energy minimum conforma-
tion of 7a and 7b as calculated using AM1 semi-empir-
ical methods. For compound 7b, the energy-minimized
structure clearly shows the proximal orientation of the
nitrile to the methyl ester. Due to their syn relationship,
not only are these two functional groups in potential van
der Waals contact as evidenced by the presence of an
ensemble of methyl ester resonances (conformers), but
the resonance of the methyl ester is significantly down-
field shifted due to the deshielding effect of nitrile aniso-
tropy cone. In contrast, an anti stereochemical
configuration for 7a was deduced as the methyl ester res-
onance is neither influenced by the nitrile anisotropy
cone, nor appears as multiple conformers on the
NMR timescale.
˚
Scheme 1. Reagents and conditions: (a) MeOH/HCl, 4 A mol. sieves,
D (Soxhlet); (b) Boc2O, NMM, DMF; (c) NMMO, TPAP (5mol %),
˚
CH2Cl2, 4 A mol. sieves; (d) (EtO)2P(O)CH2CN, LiHMDS, THF,
0 °C; (e) chromatographic separation; (f) NaBH4, PdCl2, H2 (45psi),
MeOH.
sodium periodate. However, we were able to achieve
similar yields (90%) of ketone 5 using the catalytic oxi-
dant tetrapropylammonium perruthenate6 (TPAP) with
the stoichiometric co-oxidant N-methyl-morpholine-N-
˚
oxide (NMMO) in the presence of 4 A molecular sieve
dust (Scheme 1). Use of TPAP simplified workup as
the crude product was freed of oxidant by passing the
reaction mixture through a pad of silica gel. In addition
to advantages with workup and purification, the poten-
tial hazard of performing preparative scale reactions
with volatile RuO4 was avoided.
The conversion of ketone 5 into cyanoolefins 6a and 6b
was sluggish using stabilized Wittig conditions7 with the
reagent cyanomethyltriphenylphosphonium bromide
and the bases sodium hydride, lithium diisopropyl
amide, or lithium hexamethydisilazide. However, olefin-
ation under Horner–Wadsworth–Emmons conditions8
(Scheme 1) proceeded smoothly at 0 °C to afford cyano-
olefin 6 in 79% yield as a 2:1 mixture of (E) 6a and (Z)
1
6b isomers (based on H NMR analysis of the crude
product).
Unfortunately, two-dimensional NMR difference-NOE
experiments could not be used to unequivocally deter-
mine the stereochemistry of compounds in this series
as the protons that would normally exhibit observable
NOE enhancements (H-3a, H-4, H-6) coincidentally
resonated at 2.5ppm. However, 1H NMR and 13C
NMR analyses of compounds 5, 6a, and 6b in CDCl3
revealed that all three compounds exist as a 45:55 mix-
tures of, respectively, cis and trans urethane rotamers
at room temperature. Furthermore, the stereochemistry
assignment of compounds 6a and 6b were made based
on coupling constant analysis of the vinyl proton and
the diastereotopic methylene protons on C-3 and com-
Figure 2. AM1 energy-minimized structure of 7a (bottom) and 7b
(top) depicting the relative orientations of the cyanomethyl and methyl
ester functional groups.

S. Barkallah et al. / Tetrahedron Letters 46 (2005) 4985–4987
4987
Acknowledgments
This work was generously supported by NIH research
grants GM 65539 and AI 46611 and a grant from the
McCabe fund.
Scheme 2. Reagents and conditions: (a) 2 N NaOH; (b) TFA/DCM;
(c) H2/Pt2O, EtOH/AcOH/H2O (4:1:1 v/v/v); (d) 6 N HCl, propylene
oxide, Dowex-50 ion-exchange chromatography.
Supplementary data
Experimental procedures and characterization data for
compounds are included. Supplementary data associ-
ated with this article can be found, in the online version,
at doi:10.1016/j.tetlet.2005.05.093.
Cyanomethyl derivative 7a was converted quantitatively
to the Boc-deprotected diacid 1 by alkaline hydrolysis
1 N NaOH and subsequent treatment with TFA (52%
overall yield of the TFA salt from 3) (Scheme 2). This
compound is a constrained version of L-homoglutamic
acid, and has identical spectroscopic data with reported
values.3 Moreover, the TFA salt of the novel trans 4-
aminoethyl-L-proline 2 conformationally constrained
analogue of L-lysine was also obtained through hydro-
genation of 7a on platinum oxide in 63% yield (40%
overall yield from 3).
References and notes
1. Esquerra, J.; Pedregal, C.; Rubio, A.; Yruretagoyena, B.;
Escribano, A.; Sanchez-Ferrando, F. Tetrahedron 1993, 49,
8665–8678.
2. Koskinen, A. M. P.; Rapoport, H. J. Org. Chem. 1989, 54,
1859–1866.
3. Langlois, N.; Rojas, A. Tetrahedron Lett. 1993, 34, 2477–
2480.
4. Zhang, R.; Brownewell, F.; Madalengoitia, J. S. J. Am.
Chem. Soc. 1998, 120, 3894–3902.
5. Dormoy, J. R.; Castro, B. Synthesis 1986, 81–82.
6. Griffith, W.; Ley, S.; Whitcombe, G.; White, A. Chem.
Commun. 1987, 1625–1627.
7. Wittig, G.; Schollkopf, U. Chem. Ber. 1954, 97, 1318–1330.
8. Horner, L.; Hoffmann, H.; Wippel, H. G.; Klaure, G.
Chem. Ber. 1959, 92, 2499–2505.
In summary, we have described new and facile syntheses
to both 4-carboxymethyl-L-proline 1 and 4-aminoethyl-
L-proline 2 in four steps starting from the same commer-
cially available 4-hydroxy-L-proline precursor 3 with
excellent overall yields. These conformationally con-
strained analogues of L-homoglutamic acid and L-lysine
will be of great utility in designing new polypeptides
derived from the proline II motif.