Synthesis of (2S)-2-amino-3-(1H-4-indolyl)propanoic acid, a novel tryptophan analog for structural modification of bioactive peptides

Article abstract of DOI:10.1016/S0957-4166(98)00445-5

A convenient, multigram synthesis of a novel α-amino acid (2S)-2- amino-3-(1H-4-indolyl)propanoic acid (la), is reported. An Fmoc-t-Boc derivative of this novel regioisomer of the natural aromatic amino acid tryptophan could be readily incorporated into b

Full text of DOI:10.1016/S0957-4166(98)00445-5

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 9 (1998) 4127–4134
Synthesis of (2S)-2-amino-3-(1H-4-indolyl)propanoic acid, a
novel tryptophan analog for structural modification of bioactive
peptides
∗
†
∗
Abdul H. Fauq, Feng Hong, Bernadette Cusack, Beth M. Tyler, Yuan Ping-Pang and
Elliott Richelson
Mayo Clinic Jacksonville, 4500 San Pablo Road, Jacksonville, FL 32224, USA
Received 2 September 1998; accepted 23 October 1998
Abstract
A convenient, multigram synthesis of a novel α-amino acid (2S)-2-amino-3-(1H-4-indolyl)propanoic acid (1a),
is reported. An Fmoc–t-Boc derivative of this novel regioisomer of the natural aromatic amino acid tryptophan
could be readily incorporated into bioactive synthetic peptides using standard solid phase synthesis. The synthesis
featured the use of Schöllkopf chiral auxiliary reagents for chirality induction during a key step. © 1998 Elsevier
Science Ltd. All rights reserved.
1. Introduction
Tryptophan (Trp) is one of the essential amino acids and plays a key role in the structure and function
1
2a
2b
2c
of many bioactive proteins and peptides, inter alia, serotonin, melatonin and dynorphin. Minor
structural modifications of these and other naturally occurring macromolecules provide important probes
for determining their function in biological systems. Other uses of the targeted structural modifications
include the development of analogs and therapeutic drugs that modulate specific biological activities
of receptors and enzymes. Certain analogs of tryptophan, for example, have been extensively used in
time-resolved fluorescence spectroscopy, which is used to monitor molecular interactions and motions
3
that occur in the picosecond–nanosecond time range. During our work on the development of novel
4
neurotensin receptor ligands, we required neurotensin analogs with reduced sizes that would better
fit the slightly smaller human neurotensin receptor relative to the rat receptor, as well as impart cell
∗
†
Corresponding authors.
Current address: Mayo Cancer Center, Department of Pharmacology, May Foundation for Medical Education and Research,
200 First Street SW, Rochester, MN 55905, USA.
0957-4166/98/$ - see front matter © 1998 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(98)00445-5

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A. H. Fauq et al. / Tetrahedron: Asymmetry 9 (1998) 4127–4134
permeability and resistance to peptidases. Herein, we report the synthesis of a novel, enantiopure Trp
isostere, (2S)-2-amino-3-(1H-4-indolyl)propanoic acid (1a) which was successfully incorporated into
some brain-penetrating, hydrolysis resistant, and highly potent neurotensin analogs.5
2. Results and discussion
The enantiomeric synthesis of 1a and 1b (see Scheme 1) began with 2-methyl-3-nitrobenzoic acid
2
which, after esterification, was followed by reaction with N,N-dimethylformamide dimethylacetal to
6
furnish the enamine 4. Reductive cyclization using H /Pd–C gave the 4-substituted indole methyl ester
2
7
5. Protection of the indole nitrogen of 5 with the tert-butoxycarbonyl (Boc) group and reduction of
the resulting ester with DIBAL proceeded uneventfully to give N-Boc-4-hydroxymethyl indole 6. Next,
conversion to the benzylic bromide 7 with phosphorus tribromide in ether was followed by the key SN2
displacement of the bromide with the carbanion derived from the commercially available (R)-Schöllkopf
8
reagent, (2R)-2-isopropyl-3,6-dimethoxy-2,5-dihydropyrazine to provide the diastereomeric bislactim
products 8a and 8b in 10:1 diastereomeric ratio, in 67% yield of 8a. The desired diastereomer 8a could
be easily isolated by flash silica gel chromatography and then subjected to mild acid (0.1 M TFA/CH CN)
3
treatment which only hydrolyzed the bislactim, leaving the Boc group unaffected. The resulting amino
9
ind
ester 9 was saponified to the N -t-Boc amino acid 10. Finally, the amino nitrogen was protected as its
fluorenylmethoxycarbonyl (Fmoc) derivative 1b for convenient incorporation of this novel amino acid
into peptides by the commonly employed solid phase synthesis methods. For spectral, chemical, and
optical characterization, the t-Boc group of compound 10 was removed to furnish the free amino acid 1a.
Following the above protocol, multigram synthesis of enantiopure 1b was achieved in our laboratories
in fairly short order. Since the S-enantiomer of the Schöllkopf reagent is commercially available, the
(R)-2-amino-3-(1H-4-indolyl)propanoic acid can be similarly synthesized.
To determine the enantiomeric purity of the final Fmoc–Boc derivatives, 1b and ent-1b were con-
verted to the diastereomeric (S,S) amides 11 and 12 by EDC/HOBT-mediated coupling with (S)-α-
methylbenzylamine. The (R,S) diastereomer 12 was obtained by carrying the minor (R,R) diastereomer
of 8, obtained during the Schöllkopf reaction, through the acidic and basic hydrolysis steps, followed
by fluorenylmethoxycarbonylation, to the Fmoc–Boc derivative ent-1b. The diastereomers (S,S)-11 and
1
(
S,R)-12 exhibited important chemical shift differences in two regions of their H NMR spectra, obtained
1
for pure as well as for premixed samples of defined composition. Specifically, the H NMR spectra
of the pure samples of (S,S)-11 and (R,S)-12 gave sharp signals for the methyl protons of the (S)-α-
methylbenzylamine portion at δ 1.31 (d, J=6.9 Hz, 3H) and 1.11 (d, J=6.9 Hz, 3H), respectively. Also,
(
S,S)-11 showed a peak at δ 8.00 (d, J=8.3 Hz, 1H) for one of the aromatic protons of the (S)-amino acid
portion, whereas the same peak was found at δ 8.11 (d, J=8.3 Hz, 1H) for the (R)-amino acid portion of
1
the (S,R)-12. The analysis of the H NMR spectra for both the pure diastereomers 11 and 12 showed only
a trace diastereomeric impurity, and amounted to an estimated 98% ee for the final Fmoc–Boc-protected
1b or ent-1b. Thus, the diastereomers 8 were obtained not only in almost pure form during the silica gel
chromatography following the coupling reaction with the Schöllkopf reagent, but also the mild alkaline
conditions employed during the hydrolysis of the methyl ester 9 and ent-9 did not cause any significantly
detectable racemization of the α-chiral center.
The doubly protected amino acid 1b was readily incorporated into some novel biologically active
5
neurotensin analogs by conventional Fmoc-related automated solid phase chemistry. Interestingly, these
analogs have not only provided more potent analogs for the cloned human neurotensin receptor, but they

A. H. Fauq et al. / Tetrahedron: Asymmetry 9 (1998) 4127–4134
4129
Scheme 1. Reagents and conditions: (a) K
tal, DMF, 120°C; (c) H , 10% Pd–C (cat), MeOH, RT, 50–55 psi, benzene (67% over 2 steps); (d) (BOC)
CH CN, DMAP (cat), RT (100%); (e) DIBAL-H, CH Cl , ether:CH Cl (95%); (g)
:ether, −78°C (88%); (f) PBr
2R)-2-isopropyl-3,6-dimethoxy-2,5-dihydropyrazine:BuLi, THF, −78°C, then 7 (67%); (h) 0.1 M aq. TFA, CH CN, RT (100%
overall); (i) LiOH·H O, THF:H O, RT (62%); (j) TFA:CH Cl , RT; (k) Fmoc–Suc, 10% NaHCO , acetone, 0°C–RT (72%); (l)
α-methylbenzylamine, EDC, HOBT, DMF, RT (88%)
2
CO
3
, MeI, DMF, RT (100%); (b) N,N-dimethylformamide dimethylace-
2
2
O,
3
2
2
3
2
2
(
3
2
2
2
2
3
also appear to cross the blood–brain barrier and cause considerable resistance to the peptidases known to
5
cleave neurotensin.
Many potential applications of the apparently hitherto unknown amino acid 1a can be envisioned. This
novel amino acid could, in principle, be incorporated into molecules of biological and therapeutic interest
in which tryptophan, or possibly tyrosine, appears to play a key role. These modified molecular species
may then be re-evaluated for better potency and reduced toxicity. Our research in some of these sharply
defined areas is continuing and will be reported in due course.

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A. H. Fauq et al. / Tetrahedron: Asymmetry 9 (1998) 4127–4134
3. Experimental
3.1. General methods
1
13
1
Nuclear magnetic spectra ( H, C) were measured with a Bruker WH-300 instrument ( H frequency
13
1
3
00 MHz, C frequency 75 MHz) in the solvent noted. H chemical shifts are expressed in parts per
million, downfield from Me Si used as internal standard. Melting points were taken with a GallenKamp
4
instrument and are uncorrected. The column chromatographic separations were performed with ‘J. T.
Baker’ Silica gel (40 µm). Anhydrous DMF was obtained from Aldrich Chemicals. Tetrahydrofuran
(THF) and diethyl ether were distilled over sodium benzophenone ketyl before use. Methylene chloride
was distilled over calcium hydride or P O . Acetonitrile was reagent grade from E. M. Science and was
2
5
used without further drying. Ethyl acetate and hexane were reagent grade and were used as received. The
1
13
purity of all compounds was shown to be >95% by TLC and high field H NMR and C NMR (300
and 75 MHz Brucker instrument). Optical rotations were taken with a 241 Perkin–Elmer polarimeter
(Na lamp). IR spectra were measured with a 2020 Galaxy Series FT-IR (Mattson Instruments). Mass
spectra analyses were performed at the Mayo Clinic Rochester Mass Spectroscopy Facility, Rochester,
Minnesota.
3.2. Methyl 2-methyl-3-nitrobenzoate (3)
To well mixed 2-methyl-3-nitrobenzoic acid 2 (50 g, 0.28 mol) and KHCO (84 g, 0.84 mol) was added
3
DMF (130 ml). Since the mixture became highly viscous, it was heated to 40°C with manual shaking.
Iodomethane (79 g, 0.56 mol) was added via syringe after the gas evolution had ceased. The resulting
orange coloured solution was stirred for 12 h at room temperature. The reaction mixture was poured into
water (800 ml) and the resulting precipitate was collected by filtration and dried over P O to give pure
2
5
1
product 3 (56 g, 100%) as a white solid: mp 64.2–65.5°C. H NMR (CDCl ) δ 8.00 (d, J=7.8 Hz, 1H),
3
−
1
7
.85 (d, J=8.0 Hz, 1H), 7.39 (t, J=8.0 Hz, 1H), 3.95 (s, 3H), 2.63 (s, 3H); IR (KBr, cm ) 1724, 1548,
+
1279; MS (EI): 195 (M ).
3.3. Methyl 1H-4-indolecarboxylate (5)
A solution of 3 (20 g, 0.1 mol), N,N-dimethylformamide dimethylacetal (40 ml, 0.3 mol) in DMF (50
ml) was stirred at 120°C under nitrogen for 12 h. The solution became deep red. The excess amount
of N,N-dimethylformamide dimethylacetal and DMF was distilled under reduced pressure to give crude
enamine 4 that was directly used in the next step without purification.
The crude enamine 4 was dissolved in anhydrous benzene (250 ml). To this solution was added Pd–C
(10%, 2.8 g). The resulting mixture was hydrogenated at 55 psi. Warming was observed at the start of the
reaction. The deep red mixture became dark gray after 12 h at room temperature. Pd–C was filtered off
over Celite and the filtrate was concentrated under reduced pressure. Chomatography on silica gel (ethyl
acetate:hexanes: 30:70 v/v, R =0.55) afforded 5 (11.8 g, 67%) as a light yellow solid: mp 67.5–69.0°C.
f
1
H NMR (CDCl ) δ 8.40 (s, 1H), 7.93 (d, J=8.3 Hz, 1H), 7.60 (d, J=8.9 Hz, 1H), 7.36 (t, J=3.0 Hz, 1H),
.26 (t, J=3.7 Hz, 1H), 7.26–7.18 (m, 1H), 4.0 (s, 3H): IR (KBr, cm ) 3322, 1705, 1279; MS (ESI): 176
3
−1
7
+
(M +1).

A. H. Fauq et al. / Tetrahedron: Asymmetry 9 (1998) 4127–4134
4131
3.4. 1-(tert-Butyl) 4-methyl 1H-1,4-indoledicarboxylate
To a solution of 5 (11.8 g, 67.4 mmol) in acetonitrile (50 ml) was added di-tert-butyl dicarbonate (14.7
g, 67.4 mmol) and DMAP (0.2 g). The mixture was stirred at room temperature for 12 h. Some bubbling
was observed. The solvent was removed under reduced pressure to give a residue that was redissolved in
ethyl acetate (200 ml). The solution was washed sequentially with cold 1 N HCl (80 ml), water (50 ml),
and brine (50 ml), and then dried (MgSO ). The solvent was removed under reduced pressure to give the
4
1
pure product (18.5 g, 100%) as a light yellow oil. H NMR (CDCl ) δ 8.41 (d, J=8.2 Hz 1H), 7.98 (d,
3
J=7.7 Hz, 1H), 7.71 (d, J=3.7 Hz, 1H), 7.36 (t, J=7.9 Hz, 1H), 7.28 (d, J=3.8 Hz, 1H), 3.98 (s, 3H), 1.68
1
3
(
5
s, 9H); C NMR (CDCl ) δ 187.3, 149.4, 135.9, 130.5, 127.8, 125.4, 123.5, 121.9, 119.7, 107.8, 84.1,
3
1
−
+
1.8, 28.1; IR (KBr, cm ) 1703, 1603, 1283, 1146; MS (ESI): 276 (M +1).
3.5. tert-Butyl 4-(hydroxymethyl) 1H-1-indolecarboxylate (6)
To a solution of the Boc-protected indole (18.5 g, 67 mmol, obtained in the previous step) in ether (150
ml), was added DIBAL (1.0 M in CH Cl , 163 mmol) at −78°C in 30 min under nitrogen. Stirring was
2
2
continued at this temperature for another 30 min at which point the reaction was quenched with saturated
citric acid at −78°C. A precipitate immediately formed which, after warming to room temperature, was
acidified to pH 1 with 1 N HCl and extracted with ethyl acetate (3×200 ml). The combined extracts were
washed with water (100 ml), and brine (200 ml), and then dried (MgSO ). The solvent was evaporated
4
under vacuum to give a residue that was purified by chomatography on silica gel (ethyl acetate:hexanes
1
3
0:30 v/v, R =0.45) to yield 6 (14.5 g, 88%) as a light yellow solid: mp 62.9–64.1°C. H NMR (CDCl )
f
3
δ 8.09 (d, J=8.1 Hz 1H), 7.6 (d, J=3.7 Hz, 1H), 7.28 (t, J=7.5 Hz, 1H), 7.20 (d, J=7.3 Hz, 1H), 6.69 (d,
J=3.7 Hz, 1H), 4.90 (s, 2H), 2.01 (s, 1H), 1.67 (s, 9H); 13C NMR (CDCl ) δ 149.7, 135.3, 132.7, 128.8,
3
−1
1
25.9, 124.2, 121.2, 114.8, 105.2, 83.7, 63.4, 28.1; IR (KBr, cm ) 3364, 1732, 1130; MS (ESI): 248
+
(M +1).
3.6. tert-Butyl 4-{[(2S,5R)-5-isopropyl-3,6-dimethoxy-2,5-dihydro-2-pyrazinyl]methyl}-1H-1-indole-
carboxylate (8a)
To a stirred solution of the alcohol 6 (6.0 g, 24.3 mmol) in ether (80 ml) and CH Cl (20 ml) under
2
2
nitrogen was added dropwise PBr (2.4 ml, 25.8 mmol) at 0°C under nitrogen. The reaction was complete
3
3
0 min after the addition. The mixture was poured into cold aq. NaHCO solution (100 ml) and extracted
3
with ethyl acetate (3×80 ml). The combined extract was washed with water (80 ml) and brine (80
ml), then dried (MgSO ), filtered, and finally concentrated to provide tert-butyl 4-(bromomethyl)-1H-
4
1-indolecarboxylate (7) (6.3 g, 84%) as an oil which was immediately taken to the next step.
To a solution of (2R)-2-isopropyl-3,6-dimethoxy-1,5-dihydropyrazine (3.8 g, 20.4 mmol) in THF (70
ml) under nitrogen at −78°C was added dropwise n-BuLi (2.5 M in hexane) via syringe. The carbanion
was allowed to form for 10 min at the same temperature, at which point a solution of the bromide 7 in
THF (40 ml) was added in a dropwise fashion. The reaction proceeded to completion in 1 h at −78°C.
Saturated NH Cl aq (80 ml) was added at −78°C and the THF was evaporated under reduced pressure.
4
The aqueous phase was extracted with ethyl acetate (3×80 ml). The combined extracts were washed with
brine (100 ml), dried (MgSO ), and concentrated. The residue was purified on a silica gel column (ethyl
4
acetate:hexanes 5:95 then 10:90 v/v, R =0.60 for the major product) to afford the desired product 8a (5.8
f
25
1
g, 67%) as a colourless oil: [α]D +26.7 (c 14.8 mg/ml, CHCl ); H NMR (CDCl ) δ 7.98 (d, J=8.2 Hz
H), 7.54 (s, 3H), 7.17 (t, J=7.8 Hz, 1H), 6.96 (d, J=7.3 Hz, 1H), 6.69 (d, J=4.0 Hz, 1H), 4.45–4.38 (m,
3
3
1

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A. H. Fauq et al. / Tetrahedron: Asymmetry 9 (1998) 4127–4134
1
3
1
1
H), 3.69 (s, 3H), 3.61 (s, 3H), 3.45–3.20 (m, 3H), 2.15–2.05 (m, 1H), 1.67 (s, 9H), 0.91 (d, J=6.8 Hz,
13
H), 0.58 (d, J=6.8 Hz, 3H); C NMR (CDCl ) δ 163.7, 162.2, 149.8, 134.9, 131.0, 130.0, 125.0, 124.0,
3
−1
21.8, 113.3, 106.5, 83.4, 60.1, 56.7, 52.3, 52.1, 36.9, 31.1, 28.2, 18.9, 16.4; IR (KBr, cm ) 1734, 1696,
+
346,1128; MS (ESI): 414 (M +1).
3.7. tert-Butyl 4-[(2S)-2-amino-3-methoxy-3-oxopropyl]-1H-1-indolecarboxylate (9)
To a solution of 8 (3.5 g, 8.6 mmol) in acetonitrile (95 ml) was added TFA (0.15 N, 24 mmol).
The mixture was purged with nitrogen and stirred for 12 h at room temperature. The acetonitrile was
evaporated and the water phase was extracted with CH Cl (5×60 ml). The combined extract was washed
2
2
with water (3×100 ml) and brine (80 ml), then dried (MgSO ). Filtration and evaporation of the solvent
4
left the amino ester 9 as a colourless oil (2.7 g, 98%). [α]D2 +17.6 (c 12.8 mg/ml, CHCl ); H NMR
5
1
3
(
CDCl ) δ 8.06 (d, J=8.3 Hz 1H), 7.61 (d, J=3.7 Hz, 1H), 7.26 (t, J=7.6 Hz, 1H), 7.06 (d, J=3.7 Hz, 1H),
.84 (dd, J=5.1, 8.1 Hz, 1H), 3.70 (s, 3H), 3.36 (dd, J=5.1, 13.5 Hz, 1H), 3.06 (dd, J=8.2, 13.6 Hz, 1H),
.67 (s, 9H), 1.46 (s 2H); C NMR (CDCl ) δ 175.4, 149.7, 135.2, 130.2, 129.5, 125.8, 124.3, 114.0,
05.3, 83.7, 55.5, 52.0, 38.5, 28.1; IR (neat, cm ) 3383, 1732, 1346, 1155; MS (ESI): 319 (M +1).
3
3
1
1
13
3
−1
+
3.8. (2S)-2-Amino-3-[1-(tert-butoxycarbonyl)-1H-4-indolyl]propanoic acid (10)
To a solution of 9 (2.7 g, 8.5 mmol) in THF (200 ml), was added LiOH·H O (980 mg, 26 mmol)
2
dissolved in H O (100 ml) at room temperature. The reaction was judged complete after 10 min (close
2
monitoring by TLC). After neutralizing with 1N HCl (30 ml), the THF and most of the water were
evaporated in vacuo. The precipitated product was collected by filtration and dried over P O under high
2
5
vacuum (1.8 g, 62%). Mp 169.5–171.2°C (dec). [α]D25 −9.8 (c 6.6 mg/ml, EtOH); H NMR (DMSO-d )
1
6
δ 7.89 (d, J=8.2 Hz 1H), 7.69 (d, J=3.8 Hz, 1H), 7.27 (t, J=7.7 Hz, 1H), 7.14 (d, J=7.5 Hz, 1H), 6.96 (d,
J=3.8 Hz, 1H), 3.99 (t, J=6.6 Hz, 1H), 3.45–3.26 (m, 2H), 1.63 (s, 9H); 13C NMR (DMSO-d ) δ 170.3,
6
−1
1
49.1, 134.6, 130.0, 128.0, 126.0, 124.3, 123.8, 113.7, 105.8, 83.8, 53.3, 33.5, 27.6; IR (KBr, cm
)
+
3432, 3179, 1734, 1603, 1051; MS (ESI): 305 (M +1).
3.9. (2S)-3-[1-(tert-Butoxycarbonyl)-1H-4-indolyl]-2-{[(9H-fluorenylmethoxy)carbonyl]amino}-
propanoic acid (1b)
A mixture of 10 (1.8 g, 5.28 mmol) in 10% aq. NaHCO (30 ml) was stirred for 1 h at room
3
temperature. To this mixture was added a solution of Fmoc–Suc (1.9 g, 5.55 mmol) in acetone (30 ml).
The resulting mixture was stirred for 12 h at room temperature. Acetone was evaporated under reduced
pressure. The aqueous phase was acidified to pH 5 with 1N HCl, and extracted with ethyl acetate (3×60
ml). The combined extracts were washed with brine (80 ml), then dried (Na SO ), and concentrated.
2
4
The resulting residue was purified on silica gel (MeOH:CH Cl 5:95 v/v. R =0.3) as a white solid: mp
2
2
f
25
1
9
(
2.1–93.8°C. [α]D +1.9 (c 3.6 mg/ml, CHCl ); H NMR (DMSO-d ) δ 7.93 (d, J=8.2 Hz 1H), 7.87
d, J=7.6 Hz, 2H), 7.78 (d, J=8.7 Hz, 1H), 7.67 (d, J=6.0 Hz, 1H), 7.59 (dd, J=7.5, 10.6 Hz, 2H),
3
6
7
1
1
1
.45–7.36 (m, 2H), 7.36–7.19 (m, 3H), 7.15 (d, J=7.2 Hz, 1H), 6.85 (d, J=3.6 Hz, 1H), 4.34–4.23 (m,
13
H), 4.23–4.10 (m, 2H), 3.41–3.32 (m, 2H), 3.20–3.08 (m, 1H), 1.62 (s, 9H); C NMR (DMSO-d ) δ
6
73.4, 156.0, 149.2, 143.9, 140.8, 134.6, 130.7, 129.9, 127.8, 127.2 126.0, 125.4, 124.3, 123.6, 120.2,
−
1
13.3, 105.8, 84.0, 65.8, 55.2, 46.6, 34.2, 27.8; IR (KBr, cm ) 3308, 1703, 1346, 1128; MS (ESI): 563
+
+
+
(M+K ), 549 (M+Na ); HRMS (FAB) calcd for (M +1) C H N O 527.2182, found 527.2212. Anal.
31 31 2 2

A. H. Fauq et al. / Tetrahedron: Asymmetry 9 (1998) 4127–4134
4133
calcd for C H N O ·0.5H O: C, 69.52; H, 5.83; N, 5.23. Found: C, 69.54; H, 6.40; N, 5.05. The ent-1b
3
1
30
2
2
2
25
showed [α]D −1.4 (c 10.0 mg/ml, CHCl ).
3
3.10. (2S)-2-Amino-3-(1H-4-indolyl)propanoic acid (1a)
A solution of 10 (10 mg, 0.03 mmol) in TFA (1 ml) and CH Cl (2 ml) was stirred for 90 min. The
2
2
solvent was evaporated under reduced pressure. The residue was purified on reverse phase HPLC on a
Vydak C column (15–20 µm particle size, 250×22 mm i.d) using a gradient of 10% B to 90% B in
8
3
0 min (buffer A 0.1% TFA in H O, buffer B 80% CH CN in buffer A; UV detection at λ
220 nm;
max
2
3
25
flow rate 8 ml/min) to give the desired product 12 as a trifluoroacetate salt: mp 110.0–111.8°C. [α]D
+
1
31.8 (c 1.1 mg/ml, H O); H NMR (DMSO-d ) δ 11.22 (s, 1H), 8.27 (s, 3H), 7.42–7.34 (m, 2H), 7.04
2
6
−1
(t, J=7.6 Hz, 1H), 6.87 (d, J=7.1 Hz, 1H), 6.53 (s, 1H), 4.17 (s, 1H), 3.41–3.26 (m, 2H); IR (KBr, cm )
+
3399, 1736; MS (ESI): 205 (M +1); HRMS (EI) calcd for C H N O 204.0899, found 204.0976.
31 31 2 2
3.11. tert-Butyl 4-((2S)-2-{[(9H-9-fluorenylmethoxy)carbonyl]amino}-3-oxo-3-{[(1S)-1-phenylethyl]-
amino}propyl)-1H-1-indolecarboxylate (11)
To a solution of 1b (300 mg, 0.57 mmol), HOBT (48 mg, 0.62 mmol) and EDCI (120 mg, 0.62 mmol)
in DMF (10 ml), was added (S)-1-phenylethyl amine (65 mg, 0.54 mmol) at room temperature. After
1
2 h stirring, the mixture was poured into saturated NaHCO (40 ml), and then extracted with ethyl
3
acetate (3×50 ml). The combined extract was washed with water (2×40 ml) and brine (50 ml), then
dried (Na SO ), concentrated, and the residue was purified on silica gel column (ethyl acetate:hexanes
2
4
1
3
0:70 v/v. R =0.4) to afford 11 (315 mg, 88%) as a white solid: mp 170.5–171.9°C. H NMR (CDCl )
f
3
δ 8.00 (d, J=8.3 Hz, 1H), 7.76 (d, J=7.4 Hz, 2H), 7.60–7.48 (m, 3H), 7.40 (t, J=7.3 Hz, 2H), 7.30 (t,
J=7.5 Hz, 2H), 7.25–7.16 (m, 3H), 7.10 (t, J=7.7 Hz, 1H), 6.95–6.85 (m, 3H), 6.82–6.73 (m, 1H), 5.66
(d, J=6.7 Hz, 1H), 5.52 (d, J=7.4 Hz, 1H), 4.97 (t, J=7.1 Hz, 1H), 4.52–4.32 (m, 3H), 4.20 (t, J=6.8 Hz,
1
H), 3.45–3.36 (m, 1H), 3.15 (dd, J=9.3, 13.4 Hz, 1H), 1.68 (s, 9H), 1.31 (d, J=6.9 Hz, 3H); IR (KBr,
−1
+
cm ) 3297, 1732, 1651, 1537, 1032; MS (ESI): 630 (M+H ).
3.12. tert-Butyl 4-((2R)-2-{[(9H-9-fluorenylmethoxy)carbonyl]amino}-3-oxo-3-{[(1S)-1-phenylethyl]-
amino}propyl)-1H-1-indolecarboxylate (12)
This amide was prepared in 80% yield as a white solid by the same procedure as that used for amide
1
1
3
1: mp 89.9–91.0°C. H NMR (CDCl ) δ 8.11 (d, J=8.3 Hz, 1H), 7.77 (d, J=7.5 Hz, 2H), 7.64–7.53 (m,
3
H), 7.40 (t, J=7.3 Hz, 2H), 7.36–7.15 (m, 6H), 7.12–7.02 (m, 3H), 7.11–6.95 (m, 1H), 5.65 (d, J=7.1
Hz, 1H), 5.32 (d, J=7.7 Hz, 1H), 4.91 (t, J=7.1 Hz, 1H), 4.52–4.31 (m, 3H), 4.19 (t, J=7.0 Hz, 1H),
−1
3
.56–3.48 (m, 1H), 3.14 (dd, J=9.5, 13.3 Hz, 1H), 1.68 (s, 9H), 1.12 (d, J=6.8 Hz, 3H); IR (KBr, cm )
+
33037, 1732, 1653, 1346, 1047; MS (ESI): 630 (M+H ).
Acknowledgements
This work was supported by NIH (Grant no. MH27692, awarded to ER).

4134
A. H. Fauq et al. / Tetrahedron: Asymmetry 9 (1998) 4127–4134
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