Asymmetric synthesis of ω-bromo-2(S)-methyl acids as precursors for novel arginine, lysine, and mercapto residues

Article abstract of DOI:10.1081/SCC-200061668

A series of ω-bromo-2(S)-methyl acids has been synthesized as precursors of novel arginine (Arg), lysine (Lys), and mercapto analogues. These intermediates contain α-methyl groups and are designed to mask the N-terminal amine when incorporated in pharmaceutically relevant peptides. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/SCC-200061668

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Asymmetric Synthesis of
ω‐Bromo‐2(S)‐Methyl Acids as
Precursors for Novel Arginine,
Lysine, and Mercapto Residues
M. Kyle Hadden a , Kyle P. Kokko a & Thomas A. Dix a
b
a Department of Pharmaceutical Sciences , Medical
University of South Carolina , Charleston, South
Carolina, USA
b Argolyn Biosciences Inc. , North Charleston, South
Carolina, USA
Published online: 16 Aug 2006.
To cite this article: M. Kyle Hadden , Kyle P. Kokko & Thomas A. Dix (2005)
Asymmetric Synthesis of ω‐Bromo‐2(S)‐Methyl Acids as Precursors for Novel Arginine,
Lysine, and Mercapto Residues, Synthetic Communications: An International Journal
for Rapid Communication of Synthetic Organic Chemistry, 35:12, 1675-1680, DOI:
10.1081/SCC-200061668
To link to this article: http://dx.doi.org/10.1081/SCC-200061668
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Synthetic Communicationsw, 35: 1675–1680, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1081/SCC-200061668
Asymmetric Synthesis of v-Bromo-2(S)-
Methyl Acids as Precursors for Novel
Arginine, Lysine, and Mercapto Residues
M. Kyle Hadden and Kyle P. Kokko
Department of Pharmaceutical Sciences, Medical University of
South Carolina, Charleston, South Carolina, USA
Thomas A. Dix
Department of Pharmaceutical Sciences, Medical University of
South Carolina, Charleston, South Carolina, USA and Argolyn
Biosciences Inc., North Charleston, South Carolina, USA
Abstract: A series of v-bromo-2(S)-methyl acids has been synthesized as precursors
of novel arginine (Arg), lysine (Lys), and mercapto analogues. These intermediates
contain a-methyl groups and are designed to mask the N-terminal amine when
incorporated in pharmaceutically relevant peptides.
Keywords: Peptidomimetic, arginine, lysine, aminopeptidase, non-natural amino acid,
rational drug design
INTRODUCTION
A surge of interest in the design and synthesis of small peptides and
peptidomimetics as pharmaceutically relevant compounds has occurred in
recent years. One strategy, designed to improve the pharmaceutical properties
of native peptides, is manipulation of the N-terminal amine to mask the
peptide from aminopeptidase degradation. Notably, Na-methyl,[1]
Received in the USA February 18, 2005
Address correspondence to Thomas A. Dix, Department of Pharmaceutical
Sciences, Medical University of South Carolina, 280 Calhoun Street, Charleston,
South Carolina, 29425, USA. Tel.: 843-876-5092; Fax: 843-792-1617; E-mail:
dixta@musc.edu
1675

1676
M. K. Hadden, K. P. Kokko, and T. A. Dix
Na-acetyl,[2] and the azido group[3] have all been used as N-terminal protect-
ing groups. Previous work in our laboratory has defined a route for the asym-
metric synthesis of v-bromo-2(S)-azido acids as precursors for nonnatural
analogues of arginine (Arg), lysine (Lys),[4] and mercapto derivatives.[5]
This route utilizes the azido moiety as a protected form of the a-amine that
can undergo catalytic hydrogenation at the end of the synthesis to produce
the enantiomerically pure Arg, Lys or, mercapto analogues.
We also demonstrated that incorporation of the 2(S)-azido acids for the
N-terminal amino acid in peptides not only improved the peptides’ blood
stability, but also increased their lipophilicity through removal of the charged
N-terminal amine. In particular, substitution for Arg8 in the C-terminal
fragment of neurotensin, neurotensin (8–13) yielded peptide analogues with
potential as novel antipsychotic compounds.[3] For further evaluation, a large-
scale synthetic route is necessary to produce quantities sufficient for preclinical
and clinical trials. However, large-scale synthesis of compounds containing
azido groups is avoided whenever possible because of potentially explosive
side-products.[6] It was hypothesized that replacement of the azido functionality
with a methyl group would maintain the steric and lipophilic character associ-
ated with the azide while utilizing a synthetic route involving an inert moiety.
Starting materials 1a–c were derivatized at the carboxylic acid with
Evans’ chiral auxiliary (S)-(-)-4-benzyl-2-oxazolidinone (Xps) as previously
described[4,7] (Scheme 1). Addition of potassium bis(trimethylsilyl) amide
(KHMDS) (5 eq.) to 2a–c at 2788C under N2 effected enolate formation in
situ. After 30 min, methyl iodide (2 eq.) was added via cannulation and the
solution was stirred at 2788C for 1 h to give 3a–c. Removal of the chiral
auxiliary with H2O2 and LiOH gave the respective v-bromo-2(S)-methyl
acids (4a–c).
Of note, quenching the methylation reaction after 2 min as previously
described[7] produced poor yields of 3a–c. The rapid quenching employed
with trisyl azide is warranted because it is highly reactive, thus electrophilic
addition at the a-carbon procedes at a rapid rate. However, an appreciable
yield of 3a–c was achieved only when the reaction was allowed to proceed
for 1 h after methyl iodide addition. In addition, we demonstrated previously
that enolization of 2b results in immediate displacement of the v-bromo group
to form the five-membered cyclopentane carboximide.[4] To compete with the
intramolecular cyclization reaction, 5 eq. of methyl iodide was added directly
following the KHMDS addition. This modified procedure still gave the
cyclopentane as the major product. This may be due to the less reactive
nature of the methyl iodide. 3b was separated routinely from the cyclized
by-product on silica gel eluted with 100% dichloromethane. Overall, 3a–c
1
were produced in 95% diastereomeric excess as determined by H NMR.
The v-bromo group of 4a–c is easily displaced with excess nucleophile to
produce a wide range of Lys analogues. In addition, this route can be
applied to our synthesis of Arg analogues to create a full library of nonnatural
amino acids containing a methyl group at the a-carbon.

v-Bromo-2(S)-Methyl Acids
1677
Scheme 1. Synthesis of v-bromo-2(S)-methyl acids.
EXPERIMENTAL FOR “C” SERIES
To a round-bottom flask containing 5 g (23.9 mmol) of 1c was added 75 ml
of anhydrous tetrahydrofuran (THF). This flask was purged with a continuous
stream of argon gas and cooled to 2788C. To this flask was added 413 ml
(29.6 mmol) of triethylamine (TEA) followed by dropwise addition of
trimethylacetyl chloride (3.17 ml; 25.8 mmol). The flask was subsequently
stirred at 08C for 1 h. In a separate flask, 5.08 g (28.7 mmol) of (S)-(-)-4-
benyl-2-oxazolidinone (Xps) was dissolved in 30 ml of anhydrous THF,
cooled to 2788C, and purged with a continuous stream of argon gas. To
this flask was added 11.2 ml of n-butyllithium (2.5 M solution in hexanes)
and allowed to stir for 5 min. At this time, the flask containing the activated
bromoacid was cooled again to 2788 C and the activated Xps solution was
transferred to this flask via cannula and allowed to stir for 1 h. The reaction
vessel was then warmed to room temperature and the solvent removed in
vacuo. The resulting residue was partitioned between 75 ml dichloromethane
(DCM) and 75 ml of 0.1 M phosphate buffer (pH ¼ 7.5). The organic fraction
was saved and the phosphate buffer was extracted with additional DCM
(75 ml  2). The organic fractions were pooled, dried with MgSO4, filtered,
and evaporated in vacuo. The resulting oil was dissolved in ethyl acetate
(EtOAc) and purified via silica-gel column chromatography eluted with 3:1
hexane–EtOAc to produce 8 g of 2c as clear oil (91% yield).
To a round-bottom flask containing 100 ml of THF cooled to 2788C and
purged with a continuous stream of argon gas was added KHMDS (24.1 ml;

1678
M. K. Hadden, K. P. Kokko, and T. A. Dix
12 mmol). In a separate flask purged with a continuous stream of argon gas
was added 3.42 g (9.3 mmol) of 2c dissolved in 20 ml of THF and cooled to
2788C. The solution containing 2c was transferred via cannula to the
KHMDS solution and stirred for 30 min at which time 1.16 ml (18.6 mmol)
of methyl iodide was added dropwise and allowed to stir an additional 1 h.
At this time, 2.3 ml of acetic acid was added to quench the reaction and the
mixture was allowed to warm to room temperature over 2 h at which time
the solvent was removed in vacuo. The resulting residue was partitioned
between 75 ml of DCM and 75 ml of half-saturated brine. The organic
fraction was saved and the half-saturated brine was extracted with additional
DCM (75 ml  2). The organic fractions were pooled, dried with MgSO4,
filtered, and evaporated in vacuo. The resulting oil was dissolved in EtOAc
and purified via silica-gel column chromatography eluted with 3:1 hexane–
EtOAc to produce 1.9 g of 3c as clear oil (53% yield).
To 7 ml of THF and 3 ml of dH20 in a round-bottom flask was added
0.33 g (0.86 mmol) of 3c and cooled to 08C. To this was added 355 ml of
hydrogen peroxide (30 wt. %), followed by 71 mg (1.7 mmol) of lithium
hydroxide, and the mixture was allowed to stir for 50 min at which time the
reaction was quenched with 3 ml of Na2SO3 (0.55 g in 3 ml of H2O)
followed by 8.5 ml of 0.5 N NaHCO3. The reaction was stirred for an
additional 5 min at which time the solvent was removed in vacuo. The
resulting residue was partitioned between 5 ml of DCM and 5 ml of dH20.
The aqueous layer was saved and additionally washed with DCM
(5 ml  2). The aqueous layer was acidified to pH ¼ 2 using 4 N HCl and
extracted with EtOAc (5 ml  3). The organic fractions were pooled, dried
with MgSO4, filtered, and evaporated in vacuo to produce 167 mg of 4c
(87% yield). Purity of all compounds was assessed by 1H NMR and
compounds 4a–c were subsequently incorporated into a series of neuro-
tensin (8–13) analogues as described in a subsequent manuscript.[8]
To summarize, a method for the asymmetric synthesis of novel v-bromo-
2(S)-methyl acids is described. The a-methyl group constitutes a new strategy
for protecting peptides from aminopeptidase degradation. Notably, N-terminal
methyl analogues of neurotensin (8–13) retained the receptor-binding affinity
of the parent peptide while demonstrating significant increases in CNS activity
in animal models when administered peripherally or orally.[8]
ANALYTICAL DATA
[3(2S),4S]-3-(2-Methyl-5-bromo-1-oxovaleryl)-4-(phenylmethyl)-2-oxazo-
lidinone (3a): 1H NMR (400 MHz, CDCl3) d 7.38–7.15 (m, 5H), 4.71–4.63
(m, 1H), 4.18–4.15 (d, J ¼ 5.0 Hz, 2H), 3.71–3.65 (m, 1H), 3.41–3.33
(m, 2H), 3.27–3.20 (dd, J ¼ 4.0, 13.8 Hz, 1H), 2.89–2.81 (dd, J ¼ 10.0,
14.2 Hz, 1H), 1.90–1.55 (m, 4H), 1.24–1.20 (d, J ¼ 7.4, 3H); 13C NMR

v-Bromo-2(S)-Methyl Acids
1679
(100 MHz, CDCl3) d 176.8, 153.2, 135.2, 129.6, 129.1, 127.6, 66.4, 55.6, 38.3,
37.6, 33.8, 32.2, 31.8, 17.9.
[3(2S),4S]-3-(2-Methyl-6-bromo-1-hexanoyl)-4-(phenylmethyl)-2-oxazoli-
1
dinone (3b): H NMR (400 MHz, CDCl3) d 7.36–7.19 (m, 5H), 4.72–4.65
(m, 1H), 4.25–4.16 (d J ¼ 4.2 Hz, 2H), 3.77–3.67 (m, 1H), 3.46–3.36
(t, J ¼ 7.0 Hz, 2H), 3.29–3.22 (dd, J ¼ 4.0, 14.0 Hz, 1H), 2.82–2.74 (dd,
J ¼ 9.0, 14.0 Hz, 1H), 1.92–1.74 (m, 3H), 1.50–1.42 (m, 3H), 1.25–1.21
(d, J ¼ 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 176.9, 153.2, 135.3,
129.6, 129.1, 66.4, 55.6, 38.1, 37.8, 34.1, 32.8, 32.5, 26.1, 18.7.
[3(2S),4S]-3-(2-Methyl-7-bromo-1-oxoheptyl)-4-(phenylmethyl)-2-oxazo-
lidinone (3c): 1H NMR (400 MHz, CDCl3) d 7.41–7.22 (m, 5H), 4.74–4.66
(m, 1H), 4.25–4.19 (d, J ¼ 4.0 Hz, 2H), 3.77–3.70 (m, 1H), 3.45–3.39
(t, J ¼ 7.0 Hz, 2H), 3.31–3.23 (dd, J ¼ 3.7, 13.7 Hz, 1H), 2.84–2.77 (dd,
J ¼ 10.0, 12.5 Hz, 1H), 1.91–1.77 (m, 3H), 1.50–1.32 (m, 5H), 1.25–1.20
(d, J ¼ 7.0 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 177.2, 153.1, 135.4,
129.7, 129.1, 127.5, 66.4, 55.7, 38.2, 37.9, 34.3, 33.4, 32.8, 28.4, 27.7, 17.8.
2(S)-Methyl-5-bromovaleric Acid (4a): 1H NMR (400 MHz, CDCl3) d
3.46–3.38 (t, J ¼ 6.0 Hz, 2H), 2.56–2.46 (m, 1H), 1.95–1.60 (m, 4H),
1.25–1.20 (d, J ¼ 7.0 Hz, 3H); 13C NMR (100 MHz, CDCl3) d 183.1, 38.9,
33.6, 32.1, 30.5, 17.3.
1
2(S)-Methyl-6-bromohexanoic Acid (4b): H NMR (400 MHz, CDCl3) d
3.45–3.38 (t, J ¼ 6.2 Hz, 2H), 2.55–2.45 (m, 1H), 1.92–1.85 (m, 2H),
1.77–1.68 (m, 1H), 1.55–1.46 (m, 3H), 1.24–1.19 (d, J ¼ 7.8 Hz, 3H); 13C
NMR (100 MHz, CDCl3) d 183.5, 39.5, 33.8, 32.9, 32.7, 26.0, 17.2.
1
2(S)-Methyl-7-bromoheptanoic Acid (4c): H NMR (400 MHz, CDCl3) d
3.43–3.36 (t, J ¼ 6.8 Hz, 2H), 2.51–2.42 (m, 1H), 1.90–1.64 (m, 3H),
1.49–1.32 (m, 5H) 1.20–1.14 (d, J ¼ 7.0 Hz, 3H); 13C NMR (100 MHz,
CDCl3) d 183.6, 39.5, 34.1, 33.5, 32.7, 28.2, 26.6, 17.1.
ACKNOWLEDGMENTS
This research was supported by NIMH-65099 and DA-16033 grants to
Argolyn Bioscience Inc. (T.A.D., P.I.). M.K.H. is an American Foundation
for Pharmaceutical Education predoctoral scholar.
REFERENCES
1. Tyler-McMahon, B. M.; Stewart, J. A.; Farinas, F.; McCormick, D. J.; Richelson, E.
Highly potent neurtotensin analog that causes hypothermia and antinociception.
Eur. J. Pharmacol. 2000, 390, 107–111.

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M. K. Hadden, K. P. Kokko, and T. A. Dix
2. Bednarek, M. A.; Macneil, T.; Kalyani, R. N.; Tang, R.; Van der Ploeg, L. H. T.;
Weinberg, D. H. Analogs of MTII, lactam derivatives of alpha-melanotropin,
modified at the N-terminus, and their selectivity at human melanocortin receptors
3, 4, and 5. Biochem. Biophys. Res. Comm. 1999, 261, 209–213.
3. Kokko, K. P.; Hadden, M. K.; Orwig, K. S.; Mazella, J.; Dix, T. A. In vitro analysis
of stable, receptor-selective neurotensin(8-13) analogues. J. Med. Chem. 2003, 46,
4141–4148.
4. Lundquist, J. T., IV; Dix, T. A. Asymetric synthesis of v-bromo-2(S)-azido acids as
precursors for the synthesis of novel amino acids. Tetrahedron Lett. 1998, 39,
775–778.
5. Lundquist, J. T., IV; Bu¨llesbach, E. E.; Dix, T. A. Synthesis of protected
v-mercapto amino acids: precursors for incorporation of elongated cysteines into
peptides. Tetrahedron Asymmetry 1998, 9, 2739–2743.
6. Dermer, O. C.; Edmison, M. T. Orientation in aromatic substitution by the benze-
nesulfonimido radical. J. Am. Chem. Soc. 1955, 77, 70–73.
7. Evans, D. A.; Britton, T. C.; Ellman, J. C.; Dorow, R. L. The asymmetric synthesis
of a-amino acids. Electrophilic azidation of chiral imide enolates, a practical
approach to the synthesis of (R)- and (S)-a-azido carboxylic acids. J. Am. Chem.
Soc. 1990, 112, 4011–4030.
8. Hadden, M. K.; Orwig, K. S.; Dix, T. A. Neuropharmacology, in press.