Synthesis of cyclic and acyclic Nα-methyl-Nω-alkyl-L-arginine analogues

Article abstract of DOI:10.1016/j.tetlet.2004.01.047

A series of Nα-methyl-alkyl-L-arginine (Arg) analogues have been synthesized from inexpensive, commercially available starting materials. These analogues, once incorporated into pharmaceutically relevant peptides, are expected to increase binding affinity, receptor selectivity, lipophilicity, and stability as demonstrated with analogues of similar design and structure.

Full text of DOI:10.1016/j.tetlet.2004.01.047

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2151–2153
Synthesis of cyclic and acyclic Na-methyl-Nx-alkyl-
L
-arginine
analoguesq
Kyle P. Kokko,a H. Brooks Hoopera and Thomas A. Dixa,b,
*
aDepartment of Pharmaceutical Sciences, The Medical University of South Carolina, 280 Calhoun Street, Box 250140,
Charleston, SC 29425, USA
bArgolyn Biosciences Inc., 710 Johnnie Dodds Blvd., Suite 202, Mt. Pleasant, SC 29464, USA
Received 8 August 2003; revised 6 January 2004; accepted 12 January 2004
Abstract—A series of Na-methyl-alkyl-L-arginine (Arg) analogues have been synthesized from inexpensive, commercially available
startingmaterials. These analogues, once incorporated into pharmaceutically relevant peptides, are expected to increase binding
affinity, receptor selectivity, lipophilicity, and stability as demonstrated with analogues of similar design and structure.
ꢀ 2004 Published by Elsevier Ltd.
In recent years, our laboratory has designed and
synthesized a library of nonnatural analogues of the
cationic amino acids Argand lysine (Lys) in which the
a-amines are modified with an array of alkyl groups.1–5
These analogues were designed to improve fundamental
parameters relevant to developingpeptide-based phar-
maceuticals, which include increasingthe parent pep-
tideÕs bindingaffinity and selectivity to its requisite
receptor, increasingthe peptide Õs lipophilicity to facili-
tate membrane partitioning, and increasing peptide
stability to serum and intracellular proteases. Recent
efforts have focused on incorporatingthese analogues
affinity, selectivity,11 stability,11;10 and lipophilicity9;6
14
(includingblood brain barrier partitioning)
increased.
were all
In this paper, techniques developed in this laboratory
and others15 are combined in order to synthesize a new
series of x-alkyl-L-Arganalouges in which a methyl
group attached to the Na amine is incorporated. This
serves to expand the structures and chemistry of the
nonnatural Argand Lys series.
Synthesis of the series of Na-methyl-alkyl-
logues began with enantiomerically pure, commercially
available Boc- -glutamine (Gln). The first two steps of
this synthesis followed the methods of Xue and DeG-
rado (1995) as illustrated in Scheme 1. Briefly, Boc-
Gln was stirred overnight with 1.2 equiv of acetic
L-Argana-
into numerous pharmaceutically relevant peptides
6–11
includingneurotensin (NT),
bradykinin,12 and tri-
L
peptide based thrombin inhibitors.13 Modification of
NTÕs C-terminal hexapeptide fragment, NT(8–13),
demonstrated full proof of concept, in that binding
L
-
O
NH
2
N
N
NH
2
O
O
CH3-I, NaH
THF
1) PtO2, MeOH
2) 4N HCl
pyrOidine
t-BOC
t-BOC
N
H
COOH
t-BOC
HN
COOH
N
H
COOH
N
COOH
H
H
H
H
BOC-Gln-OH
1
2
3
Scheme 1.
Keywords: Amino acids; Arginine; Peptides.
q Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.tetlet.2004.01.047
* Correspondingauthor. Tel.: +1-8-43-876-5092; fax: +1-8-43-792-0759; e-mail: dixta@musc.edu
0040-4039/$ - see front matter ꢀ 2004 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2004.01.047

2152
K. P. Kokko et al. / Tetrahedron Letters 45 (2004) 2151–2153
S
N
NH
(CH2)n
O
HN
(CH2)n 4, n = 1
HO
HO
N
N
5, n = 2
H
NH
NH
O
2 N NaOH
HO
NH2
NH
S
'R
6, R' = Me, R'' = H
7, R' = Me, R'' = Me
8, R' = Et, R'' = H
9, R' = Et, R'' = Et
R''
R'
3
O
NH
N
H
N
R''
N
H
N
Scheme 2.
anhydride in pyridine. Followingcleanup and column
chromatography eluting with 10% MeOH/CH2Cl2,
compound 1 was obtained in 80% yield as a viscous
yellow oil. Compound 1 was then combined with 8 equiv
of iodomethane in THF and cooled to 0 ꢁC in an ice
bath. Three equivalents of a 60% sodium hydride/min-
eral oil emulsion was added slowly to the mixture over
5 min. Followingcleanup, compound 2 was crystallized
from a 1:1 mixture of petroleum ether and ethyl acetate
resultingin a 50% yield. Compound 2 (8.0 g, 33.0 mmol)
(5) Na-Methyl-Nx-(CH2CH2CH2)-Nx-L
-Arg: 1H NMR
(400 MHz, D2O) d 3.71 (dd, 1H, J ¼ 1:72, 5.12), 3.14 (t,
4H, J ¼ 6:04), 2.99 (t, 2H, J ¼ 7:01), 2.56 (s, 3H), 1.9–
1.63 (m, 2H), 1.73 (t, 2H, J ¼ 6:04) 1.57–1.36 (m, 2H);
13C NMR (100 MHz, D2O, gHSQC, gHMBC) d 172.18,
152.91, 61.77, 39.67, 38.30, 31.59, 25.97, 23.62, 19.56;
20
½aꢀ 25ꢁ (c 1, 6 N HCl).
1
(6) Na-Methyl-Nx-methyl-
L
-Arg: H NMR (400 MHz,
D2O) d 3.62 (dd, 1H, J ¼ 1:84, 5.11), 3.03 (t, 2H,
J ¼ 7:09), 2.61 (s, 3H), 2.53 (s, 3H), 1.85–1.67 (m, 2H),
1.56–1.36 (m, 2H); 13C NMR (100 MHz, D2O, gHSQC,
was dissolved in 100 mL MeOH to which 1.0 gof PtO
2
was added under nitrogen. The hydrogenation flask was
positioned on the Parr apparatus and subject to 30 psi of
H2 gas and shaken overnight. The PtO2 was removed
from the mixture via filtration and the resultingsolution
was evaporated to dryness under reduced pressure. The
resultingoil was dissolved in 4 N HCl and stirred for 4 h
at which time the resultingsolution was evaporated to
dryness under reduced pressure to produce compound 3.
Compound 3 (1.72 g, 9.4 mmol) was combined with
1 equiv of the requisite iodothiouronium salt and 9.4 mL
of 2 N NaOH and stirred for 9 days as illustrated in
Scheme 2. The resultant solution was chromatographed
on a strongly acidic 50 · 8 Dowex ion exchange resin
with sequential elution with 0.5 and 1 M NH4OH. The
resultant fractions were analyzed by TLC usinga 3:1
phenol/water solvent system. The pure fractions con-
tainingthe desired product were combined and con-
gHMBC) d 172.77, 156.51, 62.41, 40.35, 38.35, 31.61,
20
27.34, 26.13, 23.68; ½aꢀ 29ꢁ (c 1, 6 N HCl).
(7) Na-Methyl-Nx,Nx-dimethyl-L
-Arg: 1H NMR
(400 MHz, D2O) d 3.76 (dd, 1H, J ¼ 1:59, 5.25), 3.06 (t,
2H, J ¼ 7:12), 2.61 (s, 6H), 2.55 (s, 3H), 1.89–1.72 (m,
2H), 1.59–1.37 (m, 2H); 13C NMR (100 MHz, D2O,
gHSQC, gHMBC) d 172.4, 155.79, 62.01, 40.14, 31.6,
20
27.25, 26.02, 23.61; ½aꢀ 23ꢁ (c 1, 6 N HCl).
(8) Na-Methyl-Nx-ethyl-L
-Arg: 1H NMR (400 MHz,
D2O) d 3.10 (t, 1H, J ¼ 6:35), 3.01 (q, 4H, J ¼ 7:01),
2.29 (s, 3H), 1.70–1.36 (m, 4H), 0.97 (t, 2H, J ¼ 7:01);
13C NMR (100 MHz, D2O, gHSQC, gHMBC) d 177.55,
155.55, 63.89, 40.52, 36.09, 32.17, 28.21, 24.10, 13.12;
20
½aꢀ 20ꢁ (c 1, 6 N HCl).
centrated to yield the desired Na-methyl-alkyl-L-Arg
(40–90% yield). Confirmation of structure was deter-
(9) Na-Methyl-Nx,Nx-diethyl-L
-Arg: 1H NMR
1
mined with H, gHMBC, and gHSQC NMR analysis.
(400 MHz, D2O) d 3.66 (t, 1H, J ¼ 5:8), 3.07 (t, 2H,
J ¼ 6:91), 3.03 (q, 4H, J ¼ 7:23), 2.54 (s, 3H), 1.86–1.68
(m, 2H), 1.58–1.36 (m, 2H), 0.97 (t, 6H, J ¼ 7:23); 13C
NMR (100 MHz, D2O, gHSQC, gHMBC) d 171.97,
Enantiomeric purity is >95%. NMR data and optical
rotation measurements of the novel amino acids are
provided below:
154.05, 61.57, 40.07, 36.18, 31.55, 25.89, 23.54, 13.41;
½aꢀ 26ꢁ (c 1, 6 N HCl).
20
(3) Na-Methyl-
L
-Orn: 1H NMR (400 MHz, D2O) d 4.06
(dd, 1H, J ¼ 2:3, 4.83), 3.02 (t, 2H, J ¼ 7:57), 2.71 (s,
3H), 2.15–1.74 (m, 4H); 13C NMR (100 MHz, D2O,
gHSQC, gHMBC) d 170.8, 61.4, 40.2, 37.6, 27.3, 24.3.
Acknowledgements
(4) Na-Methyl-Nx-(CH2CH2)-Nx-L
-Arg: 1H NMR
This research was supported by a South Carolina
Research Initiative grant to MUSC (T.A.D., P.I.),
NIMH 65099 to Argolyn Bioscience Inc. (T.A.D., P.I.)
and by a summer research award from the Summer
Health Professionals Program at MUSC (to H.B.H.).
(400 MHz, D2O) d 3.52 (s, 4H), 3.44 (dd, 1H, J ¼ 1:94,
5.06), 3.08 (t, 2H, J ¼ 7:07), 2.54 (s, 3H), 1.83–1.64 (m,
2H), 1.57–1.38 (m, 2H); 13C NMR (100 MHz, D2O,
gHSQC, gHMBC) d 173.6, 160.13, 63.26, 42.88, 42.62,
20
31.85, 26.50, 24.09; ½aꢀ 21ꢁ (c 1, 6 N HCl).

K. P. Kokko et al. / Tetrahedron Letters 45 (2004) 2151–2153
2153
6. Lundquist, J. T. I.; Dix, T. A. J. Med. Chem. 1999, 42,
4914–4918.
7. Lundquist, J. T. I.; Dix, T. A. Bioorg. Med. Chem. Lett.
1999, 9, 2579–2582.
We would like to thank Dr. John Oatis for assistance
obtaining gHSQC and gHMBC NMR data.
8. Lundquist, J. T. I.; Bullesbach, E. E.; Dix, T. A. Bioorg.
Med. Chem. Lett. 2000, 10, 453–455.
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