N-trityl- and N-phenylfluorenyl-N-carboxyanhydrides and their use in dipeptide synthesis

Full text of DOI:10.1021/jo981860p

2532
J . Org. Chem. 1999, 64, 2532-2536
Sch em e 1. Syn th esis of N-Tr ityl- a n d
N-P h en ylflu or en yl-N-ca r boxy An h yd r id es (NCAs)
N-Tr ityl- a n d
N-P h en ylflu or en yl-N-ca r boxya n h yd r id es
a n d Th eir Use in Dip ep tid e Syn th esis
Tae Bo Sim and Henry Rapoport*
Department of Chemistry, University of California,
Berkeley, California 94720
Received September 14, 1998
In tr od u ction
The N-carboxyanhydrides (Leuchs’ anhydrides, NCAs)
at one time appeared to be the panacea for peptide
synthesis. They are easily prepared in one step with
negligible waste stream, they couple rapidly at room
temperature with nitrogen nucleophiles, and the coupling
byproduct is carbon dioxide. Their value, however, was
quickly adumbrated. The same quality that made them
attractive, their extreme reactivity, limited their use,
with one notable exception, the synthesis of ribonuclease.1
Accompanying this reactivity is an inherent instability
and propensity for polymerization, as well as some loss
in enantiomeric integrity.
In attempting to overcome these deficiencies, the
nitrogen of the NCA was additionally substituted with
tosyl2 and nitrophenylsulfonyl groups3 with little im-
provement. The most promising results were achieved
recently with an alkoxycarbonyl group added to the
nitrogen, forming the urethane-protected-NCA (UNCA).4
While this modification does overcome their instability
and tendency to polymerize, there still remains a signifi-
cant loss of enantiomeric purity during peptide synthesis5
and some instability to base.6
were chosen as representative of a range of enantiomeric
stabilities,5 with the introduction of some additional
functionality as well. For example, O-benzylserine and
the â-ester of aspartic acid were included because of their
established epimerization when used as UNCAs.5
Shown in Scheme 1 is the preparation of the N-trityl
(Trt) derivatives, which is uneventful and follows re-
ported procedures,9 and the preparation of the N-phen-
ylfluorenyl (Pf) derivatives, which proceeded following
our general procedure.10 In all examples high yields were
readily obtained.
To convert these derivatives to the corresponding
NCAs, we have used triphosgene in some cases and
phosgene in others. The yields varied from 65% to 81%,
and both TNCAs and PFNCAs were easily isolated. They
are relatively stable (several months at room tempera-
ture), crystalline compounds.
We have addressed these problems by preparing the
N-trityl-NCAs (TNCAs) and the N-phenylfluorenyl-NCAs
(PFNCAs). Our intent was to increase stability and,
primarily, to inhibit any epimerization, a well-established
effect of substitution of a trityl or phenylfluorenyl residue
on the R-amino group.7
We then proceeded to use these TNCAs and PFNCAs
for dipeptide formation. The objective, of course, was to
use the mildest conditions commensurate with acceptable
yields and the absence of epimerization. Various condi-
tions, with and without catalysts, were explored. The best
reaction conditions we have found are summarized in
Table 1, entries 1-7. These are simply to heat the
reaction mixture in THF at 40 °C or at reflux. Yields are
generally 80-90%. In each example, dipeptide formation
took place with retention of complete enantiomeric
integrity. Analysis was by HPLC of the dipeptide using
standards prepared from DL-amino acids. Our limits of
detection are 0.5% of epimerization.
Since it might be advantageous to shorten the time and
lower the temperature, we examined alternative condi-
tions recently reported11 in dipeptide formation with some
limited NCAs. These reports describe the use of KCN or
NaN3 in DMF at room temperature for relatively shorter
times.
Resu lts a n d Discu ssion
Only one previous claim for synthesis of an N-trityl-
NCA was found. At a meeting in 1962 the preparation
was reported of the TNCAs of glycine and alanine; the
process failed with other amino acids.8
We now report the synthesis and use in peptide
formation of four N-trityl or N-phenylfluorenyl NCAs:
those of alanine, phenylalanine, O-benzylserine, and the
â-methyl ester of aspartic acid. These four amino acids
(1) Denkewalter, R. G.; Veber, D. F.; Holly, F. W.; Hirschmann, R.
J . Am Chem. Soc. 1969, 91, 502.
(2) Zaoral, M.; Rudinger, J . Collect. Czech. Chem. Commun. 1961,
26, 2316.
(3) (a) Kricheldorf, H. R. Agnew. Chem. 1973, 85, 86. (b) Halstrom,
J .; Brunfeldt, K.; Kovacs, K. Acta Chem. Scand., Ser. B 1979, B33,
685.
(4) Fuller, W. D.; Cohen, M. P.; Shabankareh, M.; Blair, R. K.;
Goodman, M.; Naider, F. R. J . Am. Chem. Soc. 1990, 112, 7414.
(5) Romoff, T. T.; Goodman, M. J . Peptide Res. 1997, 49, 281.
(6) Leban, J . J .; Colson, K. L. J . Org. Chem. 1996, 61, 228.
(7) Christie, B. D.; Rapoport, H. J . Org. Chem. 1985, 50, 1239.
(8) Block, H.; Cox, M. E. Proceedings of the 5th European Peptide
Symposium 1962; Pergamon: Oxford, 1963; pp 83-87.
(9) (a) Barlos, K.; Papaioannou, D.; Theodoropoulos, D. J . Org. Chem.
1982, 47, 1324. (b) Mutter, M.; Hersperger, R. Synthesis 1989, 198.
(10) J amison, T. F.; Rapoport, H. Org. Synth. 1992, 71, 226.
10.1021/jo981860p CCC: $18.00 © 1999 American Chemical Society
Published on Web 03/11/1999

Notes
J . Org. Chem., Vol. 64, No. 7, 1999 2533
Ta ble 1. Dip ep tid e F or m a tion fr om TNCAs a n d P F NCAs
conditions solvent/
temp, °C/time, h
enantiomeric ratio,
entry
R
R1
R2
additive
yield, %
6/6a
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
CH3
Bn
BnOCH2
CH3
BnOCH2
CH3O2CCH2
CH3O2CCH2
CH3
Bn
CH3
CH3
BnOCH2
CH3
Trt
Trt
Trt
Pf
Pf
Trt
Pf
Trt
Trt
Pf
Trt
Trt
Pf
Pf
Trt
Pf
Bn
CH3
Bn
Bn
Bn
Bn
Bn
Bn
CH3
Bn
Bn
Bn
Bn
Bn
Bn
Bn
Bn
THF/∆x/6
THF/∆x/7
THF/40/20
THF/∆x/6
THF/40/12
THF/∆x/9
THF/∆x/9
DMF/rt/3
DMF/rt/3
DMF/rt/3
DMF/rt/3
DMF/rt/15
DMF/rt/3
DMF/rt/9
DMF/rt/12
DMF/rt/12
DMF/rt/15
81
72
89
82
91
94
92
87
73
73
86
86
91
91
92
86
88
100
100
100
100
100
100
100
36/64
23/77
87/13
91/9
50/50
100
83/17
81/19
98/2
97/3
KCN
KCN
KCN
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
NaF
BnOCH2
CH3O2CCH2
CH3O2CCH2
CH3O2CCH2
Trt
a
b
Reflux is designated as ∆x. Enantiomeric ratio was determined by HPLC; 100 indicates no detectable epimer.
When we applied the KCN catalysis procedure to our
TNCAs and PFNCAs, we found significant epimerization
in all cases (Table 1). Thus, even the N-trityl and N-Pf
groups could not suppress epimerization. Similarly,
catalysis by NaN3 led to significant epimerization, with
one exception, the PFNCA of alanine, entry 13. Also, the
PFNCA of aspartic acid â-methyl ester, entry 16, showed
only 2% epimerization. The results in entries 13 and 16
are not surprising since they involve the most protective
nitrogen substituent, the Pf group, and the two amino
acids (of the four) less susceptible to epimerization. Of
interest is the one example of catalysis with NaF in which
only 3% of epimerization was found with the TNCA of
aspartic acid â-methyl ester, entry 17.
Exp er im en ta l Section
Gen er a l Meth od s. Melting points were determined on a
capillary apparatus and are uncorrected. THF and Et2O were
distilled from Na-benzophenone ketyl under nitrogen; CHCl3
was distilled from P2O5; DMF was dried over molecular sieves
(3 Å), and TrtCl was recrystallized from isooctane. Petroleum
ether and hexane were used from the supplier without further
purification. All final organic solutions were dried over Na2 SO4
before evaporation. NMR spectra were taken in CDCl3 and are
referenced to TMS. 1H-coupling constants, J , are reported in Hz.
Column chromatography was performed using 230-400 mesh
silica gel. HPLC analyses were conducted on a 4.6 × 250 mm 5
µm Si normal-phase silica column, monitoring at 254 nm.
Elemental analyses were obtained from the Microanalytical
Laboratory, Department of Chemistry, University of California,
Berkeley.
As one would have predicted, epimerization during
these couplings took place in the NCA component and
not in the coupling companion amino acid. Proof of this
conclusion was obtained by preparing authentic samples
of the epimeric dipeptides. These controls permitted
establishment of identity and limits of detection by HPLC.
All the dipeptides were easily isolated, crystalline com-
pounds; no diketopiperazine formation was detected.
These dipeptides have been completely stable on storage
at room temperature for 6 months.
Syn th esis of N-Tr ip h en ylm eth yl (Tr ityl) Der iva tives of
L-Ala n in e (1a ) a n d L-P h en yla la n in e (1b). To a solution of
trityl chloride (30.67 g, 110 mmol) in CHCl3/DMF (350 mL, 2/1)
was added the amino acid (50 mmol), the mixture was vigorously
stirred for 3 h, triethylamine (20.2 g, 200 mmol) in CHCl3/DMF
(50 mL, 2/1) was added slowly over a period of 1 h, and the
mixture was stirred for another 2 h. After addition of methanol
(250 mL), the reaction mixture was heated at 50-55 °C for 2 h,
the solvent was evaporated, and the residue was distributed
between Et2O (500 mL) and 10% aqueous citric acid (100 mL).
The organic layer was washed with 10% aqueous citric acid (2
× 100 mL) and H2O (3 × 100 mL), dried, and evaporated, and
the residue was chromatographed (hexane/EtOAc, 6/4) to afford
the N-tritylamino acid.
Con clu sion s
N-Tr ityl-L-a la n in e (2a ): 78% yield; 2a ‚HNEt2 mp 154-155
We have prepared the N-trityl-NCA and N-phenylfluo-
renyl-NCA derivatives of four representative amino acids.
In all cases, these TNCAs and PFNCAs are crystalline
solids, readily prepared in good yields, and are relatively
stable on storage. When heated in THF in the presence
of a companion amino acid, they are converted in good
yield to dipeptides with complete enantiomeric integrity.
These dipeptides are crystalline solids and are stable on
storage at room temperature.
1
°C (lit.13 mp 157 °C); H NMR δ 1.23 (d, J ) 7.1, 3H), 3.43 (q,
J ) 7.1, 1H), 7.20-7.30 (m, 10H), 7.39-7.41 (m, 5H).
N-Tr ityl-L-p h en yla la n in e (2b): eluting solvent, hexane/
EtOAc, 7/3; 87% yield; 2b‚HNEt2 mp 148-150 °C (lit.13 mp 150-
151 °C); 1H NMR δ 2.08 (dd, J ) 13.4, 5.8, 1H), 2.89 (dd, J )
13.4, 6.7, 1H), 3.56 (dd, J ) 6.7, 5.8), 7.09-7.36 (m, 20H).
Syn th esis of N-Tr ityl Der iva tives of O-Ben zyl-L-ser in e
(1c) a n d L-Asp a r tic Acid â-Meth yl Ester (1d ). To a solution
of the amino acid (12 mmol) in CHCl3/CH3CN (60 mL, 5/1) was
added chlorotrimethylsilane (1.52 mL, 12 mmol), and the
(11) (a) Palomo, C.; Aizpurua, J . M.; Urchegui, R.; Garcia, J . M. J .
Chem. Soc., Chem. Commun. 1995, 2327. (b) Palomo, C.; Aizpurua, J .
M.; Ganboa, I.; Odriozola, B.; Urchegui, R.; Go¨rls, H. Chem. Commun.
1996, 1269. (c) Palomo, C.; Aizpurua, J . M.; Cuevas, C.; Urchegui, R.;
Linden, A. J . Org. Chem. 1996, 61, 4400.
(12) Gmeiner, P.; Feldman, P. L.; Chu-Moyer, M. Y.; Rapoport, H.
J . Org. Chem. 1990, 55, 3068.
(13) Zervas, L.; Theodoropoulos, D. M. J . Am. Chem. Soc. 1956, 78,
1359.

2534 J . Org. Chem., Vol. 64, No. 7, 1999
Notes
mixture was vigorously stirred for 3 h at room temperature.
Triethylamine (3.68 mL, 26.4 mmol) was then added slowly to
maintain a gentle reflux, and the mixture was stirred for 15 min
after which TrtCl (4.01 g, 14.4 mmol) in 30 mL of CHCl3 was
added.
The mixture was vigorously stirred for 3 h at room temper-
ature; methanol (2.4 mL, 60 mmol) was added, and the mixture
was stirred for an additional 30 min. The mixture was evapo-
rated, and the residue was partitioned between ether (200 mL)
and aqueous 5% citric acid (60 mL). The aqueous layer was
extracted with ether (2 × 50 mL), the combined organic solution
was dried, filtered, and evaporated, and the residue was chro-
matographed.
O-Ben zyl-N-tr ityl-L-ser in e (2c): eluting solvent, hexane/
EtOAc, 8/2, then hexane/THF, 6/4; 82% yield; mp 180-181 °C;
1H NMR δ 2.33 (dd, J ) 9.1, 4.0, 1H), 3.59 (dd, J ) 9.1, 2.5,
1H), 3.49-3.51 (m, 1H), 4.21 (d, J ) 12.0, 1H), 4.32 (d, J ) 12.0,
1H), 7.17-7.28 (m, 20H). Anal. Calcd for C29H27NO3: C, 79.6;
H, 6.2; N, 3.2. Found: C, 79.7; H, 6.5; N, 3.4.
N-Tr ityl-L-a sp a r tic a cid â-m eth yl ester (2d ): from L-
aspartic acid â-methyl ester;12 after addition of TrtCl, the
reaction mixture was stirred for 5 h; eluting solvent, hexane/
EtOAc, 7/3, then THF; 85% yield; mp 191-192 °C; 1H NMR δ
1.22 (dd, J ) 17.5, 5.1, 1H), 2.68 (dd, J ) 17.5, 3.3, 1H), 3.59 (s,
3H), 3.60-3.61 (m, 1H), 7.23-7.45 (m, 15H). Anal. Calcd for
C24H23NO4: C, 74.0; H, 6.0; N, 3.6. Found: C, 73.9; H, 6.2; N,
3.4.
N-(9-P h en yl-9-flu or en yl)-L-a la n in e (3a ). To a solution of
L-alanine (1a , 4.45 g, 50 mmol) in CHCl3/DMF (150 mL, 5/1)
was added chlorotrimethylsilane (6.35 mL, 50 mmol), and the
mixture was heated at reflux for 2 h with vigorous stirring. The
mixture was cooled to room temperature under a stream of
nitrogen, triethylamine (15.3 mL, 110 mmol) was slowly added
to maintain a gentle reflux, and the mixture was stirred for 15
min after which Pb(NO3)2 (10.03 g, 33.3 mmol) was added,
followed by the addition of 9-bromo-9-phenylfluorene (19.3 g, 60
mmol) in 60 mL of CHCl3. The mixture was vigorously stirred
for 48 h at room temperature, methanol (5.1 mL, 125 mmol) was
then added, and the mixture was stirred for an additional 30
min. The mixture was filtered, the filter cake was washed with
CHCl3 (3 × 20 mL), and the dark orange filtrate was evaporated
to a residue that was partitioned between ether (300 mL) and
aqueous 5% citric acid (300 mL). The aqueous layer was
extracted with ether (4 × 100 mL), and the combined organic
solution was extracted with 1 M NaOH (100 mL). The aqueous
solution was washed with 100 mL of ether and cooled to 0 °C
with stirring, and the pH was adjusted to 7 by the dropwise
addition of glacial acetic acid. The mixture, containing an off-
white precipitate, was extracted with 2-propanol/CHCl3 (1/3, 5
× 100 mL). The combined organic solution was washed with 100
mL of brine, dried, filtered, and evaporated to a light yellow foam
to afford 3a (12.67 g, 77%): mp 153-155 °C (lit.10 mp 158-161
°C); 1H NMR δ 1.00 (d, J ) 7.1, 3H), 2.69 (q, J ) 7.1, 1H), 7.07-
7.40 (m, 11H), 7.64-7.70 (m, 2H).
O-Ben zyl-N-(9-p h en yl-9-flu or en yl)-L-ser in e (3c). To a
solution of L-serine (1c, 2.34 g, 12 mmol) in CHCl3/DMF (60 mL,
5/1) was added chlorotrimethylsilane (1.52 mL, 12 mmol), and
the mixture was vigorously stirred for 3 h at room temperature.
Triethylamine (3.68 mL, 26.4 mmol) was then added slowly to
maintain a gentle reflux, and the mixture was stirred for 15 min
after which Pb(NO3)2 (2.66 g, 8.0 mmol) was added, followed by
the addition of 9-bromo-9-phenylfluorene (4.36 g, 14.4 mmol) in
30 mL of CHCl3. The mixture was vigorously stirred for 48 h at
room temperature, methanol (1.2 mL, 30 mmol) was then added,
and the mixture was stirred for an additional 30 min. The
mixture was filtered, the filter cake was washed with CHCl3 (2
× 50 mL), and the dark orange filtrate was evaporated to a
residue that was partitioned between ether (200 mL) and
aqueous 5% citric acid (60 mL). The aqueous layer was extracted
with ether (2 × 50 mL), the combined organic solution was dried,
filtered, and evaporated, and the residue was chromatographed
(hexane/EtOAc, 8/2; hexane/THF, 6/4) to afford 3c (4.23 g, 81%)
as a white solid: mp 137-138 °C; 1H NMR δ 2.64-2.66 (m, 1H),
2.94 (dd, J ) 9.2, 4.1, 1H), 3.66 (dd, J ) 9.2, 2.2, 1H), 4.28 (d, J
) 12.0, 1H), 4.47 (d, J ) 12.0, 1H), 6.93-7.71 (m, 18H). Anal.
Calcd for C29 H25 NO3: C, 80.0; H, 5.8; N, 3.2. Found: C, 79.7;
H, 6.0; N, 3.1.
N-(9-P h en yl-9-flu or en yl)-L-a sp a r tic Acid â-Meth yl Ester
(3d ). To a solution of l-aspartic acid â-methyl ester (1d , 0.44 g,
3 mmol) in CHCl3/CH3CN (40 mL, 3/1) was added chlorotri-
methylsilane (0.38 mL, 3 mmol), and the mixture was vigorously
stirred for 4 h at room temperature. Triethylamine (0.92 mL,
6.6 mmol) was added slowly to maintain a gentle reflux, and
the mixture was stirred for 15 min after which Pb(NO3)2 (0.67
g, 2.0 mmol) was added followed by 9-bromo-9-phenylfluorene
(1.16 g, 3.6 mmol) in 10 mL of CHCl3. The mixture was
vigorously stirred for 72 h at room temperature, methanol (0.3
mL, 7.5 mmol) was then added, and the mixture was stirred for
an additional 30 min. The mixture was filtered, the filter cake
was washed with CHCl3 (2 × 20 mL), and the dark orange
filtrate was evaporated to a residue that was partitioned between
ether (100 mL) and 5% aqueous citric acid (50 mL). The aqueous
layer was extracted with ether (2 × 50 mL), and the combined
organic solution was washed with brine (20 mL), dried, and
filtered. Evaporation and chromatography of the residue (hex-
ane/EtOAc, 6/4; THF) gave 3d (0.95 g, 82%) as a white solid:
mp 163-164 °C (lit.12 mp 160-161 °C); 1H NMR δ 1.95 (dd, J )
17.2, 4.8, 1H), 2.76 (dd, J ) 17.2, 3.8, 1H), 2.86-2.88 (m, 1H),
3.65 (s, 3H), 7.22-7.76 (m, 13H).
Syn th esis of N-Tr ityl- a n d N-P h en ylflu or en yl(P f)-N-
ca r boxya n h yd r id es (NCAs). P r oced u r e A. To a solution of
triphosgene (1.51 g, 5.6 mmol) in EtOAc (280 mL) was added
the N-tritylamino acid (14 mmol). 1-Ethylpiperidine (1.74 g, 15.4
mmol) in EtOAc (20 mL) was added dropwise to the solution
over a period of 40 min, and the mixture was stirred for another
2 h. The reaction mixture was filtered and evaporated, and the
residue was chromatographed through a short column (EtOAc).
Evaporation and crystallization of the residue gave the N-trt-
NCA.
N-Tr ityl-L-a la n in e NCA (4a ): crystallized from EtOAc/
petroleum ether; 71% yield; 205-206 °C (lit.8 mp 208-210 °C);
[R]21 +34.4° (c 0.5, EtOAc) [lit.8 [R]20 +34.7° (c 0.5, EtOAc)];
D
D
1H NMR δ 0.94 (d, J ) 6.8, 3H), 4.51 (q, J ) 6.8, 1H), 7.22-7.25
(m, 5H), 7.34-7.44 (m, 10H); 13C NMR δ18.8, 57.7, 74.7, 128.0,
128.1, 129.7, 140.9, 150.9, 169.7. Anal. Calcd for C23H19NO3: C,
77.3; H, 5.4; N, 3.9. Found: C, 77.2; H, 5.4; N, 3.5.
N-Tr ityl-L-p h en yla la n in e NCA (4b): eluting solvent, hex-
ane/Et2O, 3/7; crystallized from Et2O/hexane; 68% yield; mp 92-
95 °C; [R]23D +53.3° (c 1.0, EtOAc); 1H NMR δ (dd, J ) 14.3, 8.3,
1H), 2.88 (dd, J ) 14.3, 2.8, 1H), 4.46 (dd, J ) 8.3, 2.8, 1H),
6.84-6.85 (m, 2H), 7.20-7.37 (m, 18H); 13C NMR δ 38.0, 62.4,
75.6, 127.7, 128.1, 128.2, 128.6, 129.8, 130.0, 133.2, 140.9, 151.5,
168.4. Anal. Calcd for C29H23NO3: C, 80.3, H, 5.4; N, 3.2.
Found: C, 80.3; H, 5.6; N, 3.0.
P r oced u r e B. To a solution of the N-trityl or N-Pf amino acid
(1.0 mmol) in 1,4-dioxane (26 mL) was added phosgene (0.20 g,
2.0 mmol) in benzene (0.8 mL) followed by the dropwise addition
of 1-ethylpiperidine (0.453 g, 4.0 mmol) in 1,4-dioxane (4 mL),
and the mixture was stirred for another 3 h at room tempera-
ture. The reaction mixture was passed through a short column
of SiO2, with 1,4-dioxane/hexane, 7/3, and rapidly eluted with
an additional 100 mL of 1,4-dioxane/hexane, 7/3. The organic
solution was evaporated, and the residue was recrystallized to
give the NCA.
N-P f-L-a la n in e NCA (5a ): crystallized from the THF/hexane;
81% yield; mp 107-108 °C; [R]21D +636° (c 0.5, CHCl3); 1H NMR
δ 0.75 (d, J ) 6.9, 3H), 4.40 (q, J ) 6.9, 1H), 7.20-7.42 (m, 9H),
7.49-7.53 (m, 1H), 7.70-7.76 (m, 2H), 7.97 (d, J ) 7.5, 1H); 13
C
NMR δ 17.4, 57.6, 72.1, 120.2, 120.6, 124.6, 125.6, 127.1, 127.8,
128.3, 128.7, 129.1, 129.7, 129.8, 139.8, 139.9, 140.1, 143.9, 147.3,
151.3, 169.3. Anal. Calcd for C23H17NO3: C, 77.7; H, 4.8; N, 3.9.
Found: C, 77.5; H, 5.0; N, 4.1.
O-Ben zyl-N-P f-L-ser in e NCA (5c): 74% yield; crystallized
from THF/hexane; mp 86-87 °C; [R]22 +450° (c 1.0, CHCl3);
D
1H NMR δ 2.56 (dd J ) 10.2, 2.2, 1H), 3.32 (dd, J ) 10.2, 1.8,
1H), 3.59 (d, J ) 12.1, 1H), 3.71 (d, J ) 12.1, 1H), 4.38-4.39
(m, 1H), 6.91-6.93 (m, 2H), 7.21-7.50 (m, 13H), 7.69-7.78 (m,
2H), 7.97 (d, J ) 7.7, 1H); 13C NMR δ 62.6, 66.6, 72.1, 72.6, 120.0,
120.7, 124.7, 125.8, 127.1, 127.5, 127.9, 128.2, 128.4, 128.7, 129.2,
129.6, 129.7, 136.8, 139.6, 140.4, 144.1, 146.8, 152.0, 167.4. Anal.
Calcd for C30H23NO4: C, 78.1; H, 5.0; N, 3.0. Found: C, 77.8; H,
5.3; N, 3.2.
O-Ben zyl-N-tr ityl-L-ser in e NCA (4c): initial eluting solvent,
EtOAc/hexane, 1/1, followed by 1,4-dioxane/hexane, 7/3; crystal-

Notes
J . Org. Chem., Vol. 64, No. 7, 1999 2535
lized from THF/hexane; 66% yield; mp 163-164 °C; [R]22D +14.3°
(c 1.5, CHCl3); 1H NMR δ 2.08 (dd, J ) 10.2, 1.2, 1H), 3.41 (dd,
J ) 10.2, 1.2, 1H), 4.27 (d, J ) 11.7, 1H), 4.41 (d, J ) 11.7, 1H),
4.48-4.50 (m, 1H), 7.25-7.50 (m, 20H); 13C NMR δ 62.8, 67.3,
73.5, 74.9, 127.8, 127.9, 128.0, 128.1, 128.5, 129.9, 136.5, 144.1,
151.4, 167.9. Anal. Calcd for C30H25NO4: C, 77.7; H, 5.4; N, 3.0.
Found: C, 77.5; H, 5.6.; N, 2.9.
N-Tr ityl-L-a sp a r tic a cid â-m eth yl ester NCA (4d ): eluting
solvent, EtOAc/hexane, 1/1; crystallized from EtOAc/hexane;
71% yield; mp 156-157 °C; [R]22D +52.8° (c 1.0, CHCl3); 1H NMR
δ 1.56 (dd, J ) 17.8, 5.2, 1H), 2.54 (dd, J ) 17.8, 2.7, 1H), 3.72
(s, 3H), 4.56 (dd, J ) 5.2, 2.7, 1H), 7.21-7.37 (m, 15H); 13C NMR
δ 35.3, 52.4, 58.2, 75.2, 128.1, 128.2, 129.7, 140.6, 151.7, 168.5,
38.1, 52.2, 52.4, 73.1, 120.0, 120.1, 124.4, 125.9, 149.2, 171.9,
174.9. Anal. Calcd for C32H30N2O3: C, 78.3; H, 6.2; N, 5.7.
Found: C, 78.1; H, 6.2; N, 5.6.
5. O-Ben zyl-N-(9-p h en yl-9-flu or en yl)-L-Ser -L-P h e-OMe:
eluting solvent, hexane/EtOAc, 7.5/2.5; 91% yield; HPLC (EtOAc/
hexane, 1.5/8.5, 1.5 mL/min) tR 21.0 min; mp 48-50 °C; [R]22
D
+175.6° (c 1.5, CHCl3); 1H NMR (CHCl3 + 1 drop of D2O) δ 2.53
(dd, J ) 4.3, 2.5, 1H), 2.93 (dd, J ) 9.0, 4.3, 1H), 3.07 (dd, J )
13.6, 5.7, 1H), 3.20 (dd, J ) 13.6, 4.4, 1H), 3.70 (dd, J ) 9.0, 2.5,
1H), 3.74 (s, 3H), 4.24 (d, J ) 11.8, 1H), 4.43 (d, J ) 11.8, 1H),
4.85 (ddd, J ) 8.8, 5.7, 4.4, 1H), 7.00-7.38 (m, 20H), 7.59 (d, J
) 7.5, 1H), 7.67 (d, J ) 7.5, 1H), 8.43 (d, J ) 8.4, 1H); 13C NMR
δ 38.1, 52.1, 52.7, 56.2, 70.1, 72.6, 73.0, 119.9, 120.0, 124.7, 125.7,
125.9, 127.0, 127.2, 127.8, 127.9, 128.0, 128.3, 128.4, 128.5, 129.6,
135.8, 137.9, 139.8, 141.1, 144.1, 147.8, 149.6, 171.6, 172.4. Anal.
Calcd for C39H36N2O4: C, 78.5; H, 6.1; N, 4.7. Found: C, 78.4;
H, 6.3; N, 4.5.
169.2. Anal. Calcd for C25H21NO5
Found: C, 72.0; H, 5.2; N, 3.4.
: C, 72.3; H, 5.1; N, 3.4.
N-P f-L-a sp a r tic a cid â-m eth yl ester NCA (5d ): eluting
solvent, EtOAc/hexane, 6/4; crystallized from EtOAc/hexane;
yield, 75%; mp 193-194 °C; [R]22D +585° (c 1.0, CHCl)3; 1H NMR
δ 1.69 (dd, J ) 18.1, 4.6, 1H), 2.48 (dd, J ) 18.1, 2.5, 1H), 3.23
(s, 3H), 4.49 (dd, J ) 4.6, 2.5, 1H), 7.20-7.95 (m, 13H); 13C NMR
δ 35.8, 52.0, 57.8, 72.1, 120.4, 120.9, 124.6, 125.7, 127.9, 128.0,
128.5, 128.7, 129.1, 129.8, 130.0, 139.7, 139.8, 140.1, 143.7, 146.2,
151.8, 168.3, 168.4. Anal. Calcd for C25H19NO5: C, 72.6; H, 4.6;
N, 3.4. Found: C, 72.8; H, 4.7; N, 3.4.
Gen er a l P r oced u r e for Dip ep tid e F or m a tion fr om N-
Tr ityl- a n d N-P h en ylflu or en yl-NCAs. In all reactions the
concentration of NCA was approximately 0.1 M and the condi-
tions of solvent, temperature, and time were those specified in
Table 1. The stoichiometry of the amino ester component was
120 mol %. Times were those determined for the disappearance
of the NCA as monitored by TLC. When additives were used,
100 mol % was added. Isolation from those reactions conducted
in THF consisted in evaporation of the solvent and chromatog-
raphy of the residue. Isolation from those reactions conducted
in DMF consisted in adding EtOAc (twice the reaction volume),
washing with H2O (3×, each wash equal to the reaction volume),
drying, and evaporating. The residue was then chromatographed
and analyzed by HPLC on 5 µm silica. Numbers in front of
headings refer to entry numbers in Table 1.
6. N-Tr ityl-â-OMe-L-Asp -L-P h e-OMe: eluting solvent, hex-
ane/EtOAc, 7.5/2.5; 94% yield; HPLC (EtOAc/hexane, 1.5/8.5, 2.0
mL/min) tR 18.8 min; mp 59-61 °C; [R]22 -28.2° (c 1.0, CHCl3;
D
1H NMR δ 0.88 (dd, J ) 17.5, 5.5, 1H), 2.64 (dd, J ) 17.5, 3.0,
1H), 3.04 (dd, J ) 13.7, 5.5, 1H), 3.22 (d, J ) 9.7, 1H), 3.32 (d,
J ) 13.7, 4.7, 1H), 3.42 (ddd, J ) 9.7, 5.5, 3.0, 1H), 3.51 (s, 3H),
3.76 (s, 3H), 5.04 (ddd, J ) 9.2, 5.5, 4.7, 1H), 7.16-7.46 (m, 20H),
8.47 (d, J ) 9.2, 1H); 13C NMR δ 34.2, 38.2, 51.4, 52.2, 52.6,
54.1, 71.4, 126.7, 127.1, 128.2, 128.5, 128.8, 129.5, 135.8, 145.8,
171.6, 172.8, 172.9. Anal. Calcd for C34H34N2O5: C, 74.2; H, 6.2;
N, 5.1. Found: C, 73.9; H, 6.5; N, 4.9.
7. N-(9-P h en yl-9-flu or en yl)-â-OMe-L-Asp-L-P h e-OMe: elut-
ing solvent, hexane/EtOAc, 7.5/2.5; 92% yield; HPLC (EtOAc/
hexane, 1.5/8.5, 20 mL/min) tR 21.0 min; mp 53-55 °C; [R]22
D
+171° (c 1.0, CHCl3); 1H NMR δ 1.85(dd, J ) 18.0, 5.8, 1H),
2.76-2.81 (m, 2H), 3.04 (dd, J ) 13.6, 5.6, 1H), 3.24 (dd, J )
13.6, 4.4, 1H), 3.52 (d, J ) 8.7, 1H), 3.61 (s, 3H), 3.73 (s, 3H),
4.87 (ddd, J ) 9.0, 5.6, 4.4, 1H), 7.07-7.43 (m, 16H), 7.62 (d, J
) 7.5, 1H), 7.72 (d, J ) 7.5, 1H), 8.47 (d, J ) 9.2, 1H); 13C NMR
δ 35.1, 38.3, 51.6, 52.1, 52.7, 53.0, 72.6, 120.0, 120.4, 124.6, 125.5,
125.9, 127.1, 127.3, 128.0, 128.2, 128.4, 128.6, 128.7, 128.8, 129.6,
135.9, 140.8, 144.1, 147.8, 150.1, 171.6, 172.6, 172.7. Anal. Calcd
for C34H32N2O5: C, 74.4; H, 5.9; N, 5.1. Found: C, 74.5; H, 6.1;
N, 5.0.
1. N-Tr it yl-L-Ala -L-P h e-OMe: eluting solvent, hexane/
EtOAc, 7/3; 81% yield; HPLC (EtOAc/ hexane, 1/9, 2 mL/min)
1
tR 26.6 min; mp 162-164 °C; [R]22 -22.1° (c 0.75, EtOAc); H
8. N-Tr ityl-L- a n d -D-Ala -L-P h e-OMe: eluting solvent, hex-
ane/EtOAc, 7/3; 87% yield; HPLC (EtOAc/hexane, 1/9, 2 mL/min)
D
NMR δ 0.84 (d, J ) 7.1, 3H), 2.03 (d, J ) 5.2, 1H), 3.10 (d, J )
5.7, 2H), 3.26 (dq, J ) 5.2, 7.1, 1H), 3.72 (s, 3H), 4.56 (dt, J )
5.7, 7.4, 1H), 7.10-7.11 (m, 2H), 7.18-7.36 (m, 18H), 7.69 (d, J
) 7.4, 1H); 13C NMR δ 21.2, 38.0, 52.1, 52.6, 53.8, 71.9, 126.7,
217.0, 127.9, 128.5, 128.7, 129.3, 136.0, 145.5, 171.6, 175.3. Anal.
Calcd for C32H32N2O3: C, 78.0; H, 6.5; N, 5.7. Found: C, 77.7;
H, 6.5; N, 5.5.
1
tR 25.8 min for L,L, tR 32.8 min for D,L; H NMR δ 0.84 (d, J )
7.1), (d, J ) 7.0), 2.03 (d, J ) 5.2), 2.28 (d, J ) 5.8), 2.81 (dd, J
) 13.8, 5.7), 3.02 (dd, J ) 13.8, 6.1), 3.10 (d, J ) 5.7), 3.25-3.28
(m), 3.69 (s), 3.72 (s), 4.50-4.57 (m), 7.05-7.36 (m) 7.69 (d, J )
7.4).
9. N-Tr ityl-L- a n d -D-P h e-L-Ala -OMe: eluting solvent, hex-
ane/EtOAc, 7/3; 73% yield; HPLC EtOAc/hexane, 1/9, 2 mL/min)
2. N-Tr it yl-L-P h e-L-Ala -OMe: eluting solvent, hexane/
EtOAc, 7/3; 72% yield; HPLC (EtOAc/hexane, 1/9, 2 mL/min) tR
1
tR 22.4 min for L,L, tR 19.0 min for D,L; H NMR δ 0.99 (d, J )
22.4 min; mp 75-77 °C; [R]22 +18.3 (c 0.3, EtOAc); 1H NMR δ
7.1), 1.24 (d, J ) 7.2), 2.06 (dd J ) 13.5, 5.5), 2.53 (d, J ) 6.0),
2.57 (dd J ) 13.5, 6.6), 2.71 (dd J ) 13.5, 5.5), 2.80 (d, J ) 5.8),
2.96 (dd, J ) 13.5, 6.2), 3.43 (ddd, J ) 6.6, 6.2, 5.8), 3.50 (ddd,
J ) 6.0, 6.5, 5.5), 3.70 (s), 3.71 (s), 4.02 (dq, J ) 5.8, 7.1), 4.32
(dq, J ) 6.0, 7.2), 6.48 (d, J ) 7.0), 6.98-7.37 (m).
D
1.24 (d, J ) 7.2, 3H), 2.06 (dd, J ) 31.5, 5.5, 1H), 2.53 (d, J )
6.0, 1H), 2.71 (dd, J ) 31.5, 5.5, 1H), 3.50 (d, J ) 6.0, 5.5, 5.5,
1H), 3.71 (s, 3H), 4.23 (dq, J ) 6.0, 7.2, 1H), 6.98-7.00 (m, 2H),
7.16-7.37 (m, 19H); 13C NMR δ 18.6, 39.4, 47.8, 52.3, 58.7, 71.8,
126.7, 126.8, 128.0, 128.5, 128.8, 129.9, 136.6, 145.6, 173.0, 173.7.
Anal. Calcd for C32H32N2O3: C, 78.0; H, 6.6; N, 5.7. Found: C,
77.8; H, 6.8; N, 5.5.
10. N-(9-P h en yl-9-flu or en yl)-L- a n d -D-Ala -L-P h e-OMe:
eluting solvent, hexane/EtOAc, 7.3; 73% yield; HPLC (EtOAc/
hexane, 1/9, 1.5 mL/min) tR 55.4 min for L,L; tR 75.4 min for D,L;
1H NMR δ 1.03 (d, J ) 7.1), 1.05 (d, J ) 6.0), 2.20 (br s, 1H),
2.48-2.53 (m, 1H), 3.04-3.17 (m, 2H), 3.71 (s), 3.76 (s), 4.51-
4.54 (m), 5.25-5.40 (ddd, J ) 7.8, 5.9, 5.1), 6.70-7.43 (m 15H),
7.59-7.72 (m, 2H), 7.96 (d, J ) 7.8, 1H).
11. N-Tr ityl-L- a n d -D-Ala -L-P h e-OMe: eluting solvent,
hexane/EtOAc, 7.3; 86% yield; HPLC (EtOAc/hexane, 1/9, 2 mL/
min) tR 25.8 min for L,L; tR 32.8 min for D,L.
12. O-Ben zyl-N-tr ityl-L- a n d -D-Ser -L-P h e-OMe: eluting
solvent, hexane/EtOAc, 8/2; 86% yield; HPLC (EtOAc/hexane,
1/9, 1.5 mL/min) tR 25.0 min for D,L, tR 29.2 min for L,L; 1H NMR
δ 1.87 (dd, J ) 8.8, 3.9), 2.26 (dd, J ) 9.1, 4.7), 2.90-3.34 (m,
4H), 3.48 (dd, J ) 9.1, 3.2), 3.56 (dd, J ) 8.8, 2.1), 3.66 (s), 3.76
(s), 4.03 (d, J ) 11.6), 4.16 (d, J ) 12.1), 4.22 (d, J ) 11.6), 4.23
(d, J ) 12.1), 4.69-4.74 (m), 4.99 (ddd, J ) 9.0, 5.7, 4.4), 7.11-
7.38 (m, 24H), 7.76 (d, J ) 7.5), 8.34 (d, J ) 8.7).
3. O-Ben zyl-N-tr ityl-L-Ser -L-P h e-OMe: eluting solvent,
hexane/EtOAc, 7.5/2.5; 89% yield; HPLC (EtOAc/hexane, 1/9, 1.5
mL/min) tR 29.0 min; mp 47-50 °C; [R]22 -14.3° (c1.0, CHCl3);
D
1H NMR δ 1.87 (dd, J ) 8.8, 3.9, 1H), 2.91 (d, J ) 7.8, 1H), 3.04
(dd, J ) 31.7, 5.7, 1H), 3.25-3.31 (m, 2H), 3.56 (dd, J ) 8.8,
2.1, 1H), 3.76 (s, 3H), 4.03 (d, J ) 11.6, 1H), 4.22 (d, J ) 11.6,
1H), 4.99 (ddd, J ) 8.7, 5.7, 4.4, 1H), 7.11-7.41 (m, 25H), 8.34
(d, J ) 8.7, 1H); 13C NMR δ 38.1, 52.2, 52.5, 57.7, 69.9, 71.6,
73.2, 126.6, 127.0, 127.6, 127.7, 128.0, 128.3, 128.4, 128.5, 129.5,
135.7, 137.9, 145.8, 171.6, 172.7. Anal. Calcd for C39H38N2O4:
C, 78.2; H, 6.4; N, 4.7. Found: C, 78.2; H, 6.7; N, 4.7.
4. N-(9-P h en yl-9-flu or en yl)-L-Ala -L-P h e-OMe: eluting sol-
vent, hexane/EtOAc, 7/3; 82% yield; HPLC (EtOAc/hexane, 1/9,
1.5 mL/min) tR 56.6 min, mp 139 °C; 1H NMR δ 1.03 (d, J ) 7.1,
3H), 2.21 (br s, 1H), 2,51 (q, J ) 7.1, 1H), 3.08 (dd, J ) 13.7,
5.9, 1H), 3.14 (dd, J ) 13.7, 5.1, 1H), 3.75 (s, 3H), 4.71 (ddd, J
) 7.8, 5.9, 5.1, 1H), 7.02-7.48 (m, 15H), 7.61 (d, J ) 7.5, 1H),
7.71 (d, J ) 7.5, 1H), 7.96 (d, J ) 7.8, 1H); 13C NMR δ 21.4,
13. N-(9-P h en yl-9-flu or en yl)-L-Ala -L-P h e-OMe, prepared
in DMF in the presence of NaN3, was identical with the product
of entry 4.

2536 J . Org. Chem., Vol. 64, No. 7, 1999
Notes
14. O-Ben zyl-N-(9-p h en yl-9-flu or en yl)-L- a n d -D-Ser -L-
P h e-OMe: eluting solvent, hexane/EtOAc, 7.5/2.5; 91% yield;
HPLC (EtOAc/hexane, 1.5/8.5, 1.5 mL/min) tR 17.8 min for d,L;
tR 21.6 min for L,L; 1H NMR δ 2.54 (br s, 1H), 2.88 (dd, J ) 9.1,
4.4), 2.93 (dd, J ) 9.0, 4.3), 3.05-3.22 (m, 3H), 3.60-3.75 (m,
4H), 3.70 (s), 3.74 (s), 4.20 (d, J ) 12.1), 4.25 (d, J ) 11.8), 4.35
(d, J ) 12.1), 4.43 (d, J ) 11.8), 4.61-4.66 (m), 4.86 (ddd, J )
8.8, 5.7, 4.4), 6.76-7.38 (m, 20H), 7.58-7.68 (m, 2H), 8.07 (d, J
) 8.0), 8.43 (d, J ) 8.4).
16. N-(9-P h en yl-9-flu or en yl)-â-OMe-L- a n d -D-Asp -L-P h e-
OMe: eluting solvent, hexane/ EtOAc, 7.5/2.5; 86% yield; HPLC
(EtOAc/hexane, 1.5/8.5, 2.0 mL/min) tR 21.0 min for l,L; tR 31.4
1
min for D,L; H NMR δ 1.85 (dd, J ) 18.0, 5.8, 1H), 2.76-2.81
(m, 2H), 3.04 (dd, J ) 13.6, 5.6, 1H), 3.24 (dd, J ) 13.6, 4.4,
1H), 3.52 (d, J ) 8.7, 1H), 3.61 (s, 3H), 3.73 (s, 3H), 4.87 (ddd,
J ) 9.0, 5.6, 4.4, 1H), 7.07-7.43 (m, 16H), 7.62 (d, J ) 7.5, 1H),
7.72 (d, J ) 7.5, 1H), 8.47 (d, J ) 9.2, 1H).
17. N-Tr ityl-â-OMe-L- a n d -D-Asp -L-P h e-OMe, prepared in
the presence of NaF, were the same as the products of entry 15,
except that the ratio of diastereomers was different.
15. N-Tr ityl-â-OMe-L- a n d -D-Asp -L-P h e-OMe: eluting sol-
vent, hexane/EtOAc, 7.5/2.5; 92% yield; HPLC (EtOAc/hexane,
1
1/5, 8.5, 2.0 mL/min) tR 19.4 min for L,L; tR 27,2 min for D,L; H
NMR δ 0.88 (dd, J ) 17.5, 5.5), 1.10 (dd, J ) 16.9, 3.2), 2.51
(dd, J ) 16.9, 3.2), 2.64 (d, J ) 17.5, 3.0), 3.04 (dd, J ) 13.7,
5.5), 3.18-3.32 (m), 3.30-3.35 (m), 3.39-3.44 (m), 3.51 (s), 3.52
(s), 3.75 (s), 3.76 (s), 4.75-4.80 (m), 5.04 (ddd, J ) 9.2, 5.5, 4.7),
7.17-7.42 (m), 8.02 (d, J ) 7.2), 8.47 (d, J ) 9.2).
Ack n ow led gm en t. T.B.S. thanks the Korea Science
and Engineering Foundation for partial financial support.
J O981860P