Synthesis of (2S,3S)-β-methyltryptophan

Article abstract of DOI:10.1016/S0040-4039(01)00718-3

N^α-Fmoc-protected (2S,3S)-β-methyltryptophan has been synthesized by asymmetric synthesis through the aid of an oxazolidinone auxiliary. Coupling of the mixed anhydride of Nⁱⁿ-Boc-protected indoleacrylic acid to the oxazolidinone auxiliary was achieved by lithium bromide and N,N-dimethylpyridine (DMAP). This reaction is milder and more convenient to run than the traditional conditions using butyl lithium.

Full text of DOI:10.1016/S0040-4039(01)00718-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4601–4603
Synthesis of (2S,3S)-b-methyltryptophan†
Guoxia Han, Arwel Lewis and Victor J. Hruby*
Department of Chemistry, University of Arizona, Tucson, AZ 85721, USA
Received 13 February 2001; revised 25 April 2001; accepted 27 April 2001
Abstract—Na-Fmoc-protected (2S,3S)-b-methyltryptophan has been synthesized by asymmetric synthesis through the aid of an
oxazolidinone auxiliary. Coupling of the mixed anhydride of Nin-Boc-protected indoleacrylic acid to the oxazolidinone auxiliary
was achieved by lithium bromide and N,N-dimethylpyridine (DMAP). This reaction is milder and more convenient to run than
the traditional conditions using butyl lithium. © 2001 Elsevier Science Ltd. All rights reserved.
Incorporation of x-constrained amino acids into bioac-
tive compounds has great potential for understanding
ligand–receptor interactions. Since such amino acids
limit the number of low energy conformations available
to a bioactive compound, use of the appropriate con-
strained amino acids can improve receptor selectivities,
potencies and stabilities.1 In the past, many peptides
with high potency and high receptor selectivities have
been designed and synthesized using highly constrained
amino acids in our laboratory. One example is
c[DPen2, DPen5]enkephalin (DPDPE, Fig. 1), a d-opi-
oid receptor selective and potent opioid analogue.2,3
This five-residue peptide contains two constrained D-
penicillamine residues—b,b-dimethyl substituted Cys.
Another example is SHU-9119,4 Ac-Nle4-c[Asp5,
DNal(2%)7, Lys10]a-MSH(4–10)-NH2 (Fig. 1), an a-
MSH analogue with some receptor selectivity among
the melanocortin receptors, with high potency and sta-
bility. This seven-residue peptide has a side-chain lac-
tam bridge between Asp and Lys, and contains a bulky
naphthyl group in the side-chain of position 7. Though
this lead compound showed some receptor selectivity, it
is intriguing to consider whether modification of con-
straints at other positions in a-MSH analogues would
provide constrained analogues with better receptor
selectivities and/or higher potency. One way to accom-
plish this goal is by modifying Trp9, a position which
has not been studied in detail in the past. As a first step,
Trp9 was constrained with b-methyl substitution. The
resulting four a-MSH analogues showed both increased
and decreased potency,5 and most interestingly variable
prolonged biological activity. This has prompted us to
look for better ways to synthesize b-methyltryptophans
in large quantities since our previous syntheses6 suf-
fered from a long synthetic pathway, an overall low
yield, and a final product that only was useful for
Na-Boc chemistry.
A more efficient synthetic strategy has now been carried
out in our laboratory. We hereby report this revised
synthetic route to achieve one of the four b-methyltryp-
tophan isomers using fewer synthetic steps, with moder-
ate to high yields in most steps, and a protecting group
for the nitrogen in the indole ring which is easy to
incorporate and deprotect.
Figure 1. The annotated structures of two constrained pep-
tides (see text for discussion).
The synthesis (Scheme 1) started with indole-3-trans-
acrylic acid 1 that is protected by Boc at the indole
nitrogen to make it compatible with Fmoc chemistry.
Nin-Boc is inert to basic reaction conditions, and yet
can be easily removed. It is very important that the
indole nitrogen is properly protected since an unpro-
tected indole nitrogen will otherwise give side reactions
in later steps in the synthesis.6
Keywords: constrained amino acids; auxiliary assisted asymmetric
synthesis; b-methyltryptophan; lithium bromide (LiBr); N,N-dimethyl-
aminopyridine (DMAP).
* Corresponding author. Tel.: (520) 621-6332; fax: (520) 621-8407;
e-mail: hruby@u.arizona.edu
†
A preliminary report of this work was presented at the 1998 Spring
ACS meetings in Dallas, TX, USA.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00718-3

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G. Han et al. / Tetrahedron Letters 42 (2001) 4601–4603
Scheme 1. Synthesis of (2S,3S)-b-methyltryptophan.
product 10 (Fig. 2) which was obtained in almost equal
amounts, even when more than 2 equiv. of NBS was
used with extended reaction time at room temperature.
Nonetheless, the desired product 5 was easily isolated
by flash chromatography (10 was isolated and
recycled).
In our initial efforts to prepare the indole nitrogen-pro-
tected Boc intermediate 2 (Scheme 1) using standard
conditions, we obtained numerous byproducts depend-
ing on the conditions applied. Traditional methods7 to
protect the indole ring of 1 with Boc in acetonitrile as
solvent was examined first. However, no reaction was
detected because the amine salt of indole-3-trans-acrylic
acid does not dissolve in acetonitrile. Replacing aceto-
nitrile with other common organic solvents did not
help. Finally, by adding a few drops of water, the
reaction went smoothly with less than 3% byproducts
that could be easily washed away (Scheme 1), and
Nin-Boc-indole-3-trans-acrylic acid 2 could be used
directly in the next step without further purification.
Coupling the chiral oxazolidinone to 2 can be trouble-
some. After making the mixed anhydride 3 of 2 with
pivaloyl chloride (PivCl), we tried many conditions, and
many common Lewis bases, acids and other promoters,
to couple 3 with the chiral auxiliary. The lithium salt8,9
was found best for this coupling reaction, which pro-
ceeded under mild conditions with excellent yields. The
compound 4 obtained was used directly without further
purification.
Azide formation of 5 was simpler than expected. Using
sodium azide in DMSO, the azide 6 was obtained after
flash chromatography (in order to reduce the possible
danger of explosion, the reaction was run under argon
atmosphere). The SN2 substitution occurred rapidly
(6 h) and no starting material was detected by either
TLC or HPLC. [Azide formation by using tetra-
methylguanidine azide (TMGA) requires a much longer
time (about 5 days).]
Hydrolysis of the chiral auxiliary azide 6 by LiOH with
H2O2 in an ice water bath was complete after 1 h
(monitored by TLC and confirmed by HPLC). High
yields of the a-azide acid 7 were achieved, and separa-
tion was rather simple using organic aqueous extraction
after acidification. No purification was necessary and
product 7 was used directly for hydrogenation. Unlike
Next, Michael addition of 4 with methyl magnesium
bromide and CuBr–Me2S in methyl sulfide was carried
out using reported optimized conditions. The trans
Michael addition product was the almost exclusive
product (as monitored by RP-HPLC), and then NBS
was added directly into the reaction mixture. However,
the yield was lower than expected in this step, though
only one diastereomer 5 was detected by HPLC. The
other product was the trans added Michael addition
Figure 2. trans Michael addition adduct 10.

G. Han et al. / Tetrahedron Letters 42 (2001) 4601–4603
4603
when the Nim-Mes (1,3,5-trimethylsulfonyl) protecting
group was used,6 hydrogenation of the Nim-Boc-pro-
tected azide acid (7) was short and clean. Extension of
the hydrogenation introduces some byproducts, proba-
bly over-hydrogenated compounds (not analyzed).
However, by monitoring the hydrogenation reaction,
product 8 can be obtained in nearly quantitative yield.
Finally, protection of the a-amino group was per-
formed in MeOH with 1 equiv. triethylamine (TEA) as
base and Fmoc-OSu as the Fmoc source to give 910 in
almost quantitative yield.
2. Mosberg, H. I.; Hurst, R.; Hruby, V. J.; Gee, K.;
Akiyama, K.; Yamamura, H. I.; Galligan, J. J.; Burks, T.
F. Life Sci. 1983, 33, 447–450.
3. Mosberg, H. I.; Hurst, R.; Hruby, V. J.; Gee, K.; Yama-
mura, H. I.; Galligan, J. J.; Burks, T. F. Proc. Natl.
Acad. Sci. USA 1983, 80, 5871–5874.
4. Hruby, V. J.; Lu, D. S.; Sharma, S. D.; Castrucci, A. D.;
Kesterson, R. A.; Al-Obeidi, F. A.; Hadley, M. E.; Cone,
R. D. J. Med. Chem. 1995, 38, 3454–3461.
5. Haskell-Luevano, C.; Boteju, L. W.; Miwa, H.; Dickin-
son, C.; Gantz, I.; Yamada, T.; Hadley, M. E.; Hruby, V.
J. J. Med. Chem. 1995, 38, 4720–4729.
A revised synthesis of (2S,3S)-b-methyltryptophan has
been achieved. This revised route is a significant
improvement over that reported previously,6 including
higher efficiency—high yields in most steps, easier pro-
tection and deprotection, more simple reaction condi-
tions, and easier separation of products. The other
three isomers of b-methyltryptophan can readily be
prepared by similar methods via either changing the
chiral center of the oxazolidinone and/or the azidation
path.
6. Boteju, L. W.; Wegner, K.; Qian, X.; Hruby, V. J.
Tetrahedron 1994, 50, 2391–2404.
7. Franzen, H.; Grehn, L.; Ragnarsson, U. J. Chem. Soc.,
Chem. Commun. 1984, 1699–1700.
8. Ho, G. J.; Mathre, D. J. J. Org. Chem. 1995, 60, 2271–
2273.
9. Fonquerna, S.; Moyano, A.; Pericas, M. A.; Riera, A.
Tetrahedron: Asymmetry 1997, 8, 1685–1691.
10. Spectra of new compound 9. 1H NMR (500 MHz,
DMSO-d6): l 8.05–8.03 (1H, d), 7.74–7.71 (2H, t), 7.67–
7.65 (1H, d), 7.52–7.50 (1H, d), 7.47 (1H, s), 7.37–7.18
(7H, m), 7.11–7.08 (1H, t), 4.59–4.58 (1H, d), 4.42–4.39
(1H, q), 4.15–4.12 (1H, t), 4.08–4.04 (1H, q), 3.59–3.54
(1H, m), 1.53 (9H, s), 1.38–1.36 (3H, d); 13C NMR (500
MHz, DMSO-d6): l 173.1, 157.0, 149.6, 143.7, 141.1,
135.5, 129.8, 127.3, 126.7, 124.9, 124.7, 123.9, 122.5,
122.1, 121.6, 119.5, 119.1, 114.8, 83.3, 66.6, 58.1, 33.4,
26.9, 17.0; FAB-MS calculated for C32H32N2O6: 540.6.
Found: m/e 541.7 [M+H+]. A small amount of fully
deprotected (2S,3S)-b-methyltryptophan hydrogen chlo-
ride was obtained by stirring 8 in HCl (6N); [h]D=+33°
(c 1, MeOH); FAB-MS: calculated for C12H14N2O2·HCl:
219.3 [M−Cl−]. Found: m/e 219.2. Lit: [h]D=+36° (c 1.55,
MeOH).11
Acknowledgements
Financial support from the US Public Health Service
(USPHS) Grant DK-17420 is greatly appreciated. A. L.
is grateful for the 1997 Ettie Steele Prize from the
University of St. Andrews, Scotland. The views
expressed herein are those of the authors and do not
necessarily reflect those of the USPHS.
References
11. Bruncko, M.; Crich, D. J. Org. Chem. 1994, 59, 4239–
4249.
1. Hruby, V. J.; Li, G.; Haskell-Luevano, C.; Shenderovich,
M. Biopolymers 1997, 43, 219–266.
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