Synthesis of a vicinal tricarbonyl amide derivative of L-phenylalanine

Full text of DOI:10.1021/jo951855a

1872
J . Org. Chem. 1996, 61, 1872-1874
Sch em e 1
Syn th esis of a Vicin a l Tr ica r bon yl Am id e
Der iva tive of L-P h en yla la n in e
J ack H. Lai, Ha Pham, and David G. Hangauer*
Department of Medicinal Chemistry, School of Pharmacy,
State University of New York at Buffalo,
Buffalo, New York 14260
Received October 16, 1995
Unnatural amino acids have found wide utility for the
modification of the physical properties and biological
activities of peptides, peptide mimetics, and, more re-
cently, proteins.1 Due to their expanding utility, syn-
thetic routes to novel unnatural amino acids, particularly
those with unusual chemical properties, broaden the
scope of what can be done in this area. Wasserman and
co-workers have made significant contributions to the
synthesis, structural characterization, and synthetic util-
ity of the highly electrophilic vicinal 1,2,3-tricarbonyl
moiety.2 They have also utilized its reactivity to develop
inhibitors of serine proteases by replacing the terminal
carboxyl group in peptides with the vicinal tricarbonyl
moiety so that the nucleophilic serine in the active site
could form a covalent bond with the central carbonyl
carbon.3 We expect that the reactivity of the vicinal 1,2,3-
tricarbonyl moiety might also find utility when incorpo-
rated into the side chains of amino acids. The resulting
unnatural amino acids could be incorporated into pro-
teins1 and peptides to modify their physical and biological
properties as well as to introduce a new reactive func-
tionality which could form a covalent bond intramolecu-
larly (e.g. with a serine OH reacting at the central
carbonyl carbon) or intermolecularly (e.g. with a nucleo-
phile present in the active site of an enzyme to which
the modified peptide or protein binds). For this purpose,
we have developed the first synthesis of the novel
protected L-phenylalanine analog 5 containing a vicinal
tricarbonyl amide moiety at the para position of the
phenyl ring as shown in Scheme 1. In the course of
developing this synthesis we also extended the scope of
a new oxidation procedure to enamines such as 4 thereby
providing more convenient access to vicinal tricarbonyl
compounds via enamine precursors.
Commercially available p-nitro-L-phenylalanine hy-
drochloride 1 was protected as the N-Boc, 2-(trimethyl-
silyl)ethyl (Tmse) ester4 under the indicated standard
conditions. Subsequent hydrogenation of the p-nitro
group gave 2 in 78% overall yield. Acylation of 2 with
ethyl malonyl chloride gave 3 in 52% yield after flash
chromatography. Conversion of malonate 3 to the
enamine derivative 4 was accomplished in 94% yield
using N,N-dimethylformamide dimethylacetal following
the procedure of Wasserman and Han.5 Our initial
attempts to cleave oxidatively the enamine double bond
in 4 to obtain 5 with the singlet oxygen or ozone methods
used by Wasserman and Han5 resulted in no reaction or
only a 25% yield, respectively. Consequently, we were
attracted to the recent report of Vetelino and Coe6
wherein they achieved the oxidative cleavage of aryl
enamines to aryl aldehydes with sodium periodate in
aqueous THF at room temperature without added os-
mium tetroxide. We investigated the applicability of this
method to the synthesis of vicinal 1,2,3-tricarbonyl func-
tionalities using our present substrate 4 as an example.
The difference between our substrate and the series of
substrates investigated by Vetelino and Coe is that the
enamine functionality to be oxidized in 4 is embedded
immediately between two preexisting carbonyl groups
whereas those previously explored were flanked only by
an aromatic ring and therefore resulted in aromatic
aldehyde products. Reaction of 4 with 3 equiv of NaIO4
in aqueous THF for 18 h at room temperature resulted
in a 79% combined yield of the vicinal tricarbonyl product
5 and the corresponding hydrate 6 after purification and
separation (obtained in a ca. 1/1 ratio) by flash chroma-
tography over silica gel. The isolation of the hydrate 6
was expected since the electrophilic vicinal 1,2,3-tricar-
bonyl moiety is known to hydrate readily.2
The vicinal tricarbonyl product 5 and its hydrate 6
could be readily distinguished by the 13C NMR chemical
shifts of the central carbon atom of the vicinal tricarbonyl
moiety. For the nonhydrated tricarbonyl carbon in 5 the
chemical shift was 178 ppm whereas after hydration to
6 the chemical shift moved upfield to 92 ppm in agree-
ment with earlier 13C NMR characterizations of other
vicinal tricarbonyl compounds and their hydrates.7
(1) Steele, F. R. J . NIH Res. 1995, 7, 43.
(2) Henke, S. L.; Hadad, C. M.; Morgan, K. M.; Wiberg, K. B.;
Wasserman, H. H. J . Org. Chem. 1993, 58, 2830.
(3) Wasserman, H. H.; Ennis, D. S.; Power, P. L.; Ross, M. J . J . Org.
Chem. 1993, 58, 4785.
(4) Seiber, P. Helv. Chim. Acta 1977, 60, 2711.
(5) Wasserman, H. H.; Han, W. T. Tetrahedron Lett. 1984, 25, 3743.
(6) Vetelino, M. G.; Coe, J . W. Tetrahedron Lett. 1994, 35, 219.
(7) J ones, T. K.; Reamer, R. A.; Desmond, R.; Mills, S. G. J . Am.
Chem. Soc. 1990, 112, 2998.
0022-3263/96/1961-1872$12.00/0 © 1996 American Chemical Society

Notes
J . Org. Chem., Vol. 61, No. 5, 1996 1873
EtOAc/hexane) to give 4.36 g (52%) of 3: TLC, Rf ) 0.23 (1/2
Exp er im en ta l Section
1
EtOAc/hexane); H NMR (CDCl3) δ 0.02 (s, 9H), 0.90-1.00 (m,
All reagents were purchased from Aldrich Chemical Co.,
Milwaukee, WI, including p-nitro-L-phenylalanine monohydrate
(1). All reactions were stirred under an atmosphere of dry N2
and at rt unless noted otherwise. Solutions were concentrated
in vacuo on a rotary evaporator with a water bath temperature
of 30 °C unless otherwise noted. Analytical thin-layer chroma-
tography (TLC) was performed with precoated silica gel glass
plates (0.25 mm plates Type GF, Analtech, Newark, DE).
Visualization of TLC slides was by either iodine, UV, or
ninhydrin (for 2). Flash chromatography was carried out with
silica gel 60 (230-400 mesh) from E. Merck. 1H and 13C NMR
spectra are reported in ppm (δ) downfield from TMS. Elemental
analyses were performed by Atlantic Microchemical Lab, Inc.
Norcross, GA.
N-r-Boc-â-(p-Am in op h en yl)-L-a la n in e 2-(Tr im eth ylsilyl)-
eth yl Ester (2). To a solution of p-nitro-L-phenylalanine
monohydrate (1) (5.34 g, 23.4 mmol) in a mixture of dioxane (50
mL), water (25 mL), and 1 N NaOH (25 mL) precooled in an
ice-water bath was added di-tert-butyl dicarbonate (7.55 g, 34.6
mmol). The reaction was allowed to warm to rt and then
continued for 3 h longer. The reaction mixture was concentrated
to about 30 mL, cooled in an ice-water bath, covered with a
layer of EtOAc (80 mL), and acidified with a 1 N KHSO4 to pH
2-3. The aqueous phase was extracted with EtOAc (3 × 350
mL), and then the extracts were combined, washed with water
(350 mL), dried over MgSO4, and concentrated. The residue was
crystallized from EtOAc/hexane to give 6.98 g (96%) of N-Boc-
p-nitro-L-phenylalanine: TLC, Rf ) 0.77 (3/1 EtOAc/MeOH); 1H
NMR (DMSO-d6) δ 1.29 (s, 9H), 2.90-3.00 (m, 1H), 3.15-3.20
(m, 1H), 4.17 (m, 1H), 7.23 (d, J ) 8.5 Hz, 1H), 7.53 (d, J ) 8.6
Hz, 2H), 8.16 (d, J ) 8.6 Hz, 2H), 12.74 (s, 1H).
To a solution of N-Boc-p-nitro-L-phenylalanine (3.10 g, 10.0
mmol) in acetonitrile (10 mL) were added pyridine (7.2 mL) and
2-(trimethylsilyl)ethanol (1.7 mL, 12 mmol). After the reaction
mixture had been cooled in an ice/water bath, 1,3-dicyclohexyl-
carbodiimide (2.3 g, 11 mmol) was added, and the mixture was
stirred in the ice/water bath for 1 h and then kept in the
refrigerator overnight. A solution of oxalic acid (189 mg) in DMF
(0.4 mL) was added. About 0.5 h later, the precipitated
dicyclohexylurea was removed by filtration and washed with
EtOAc. The combined filtrate and washings were extracted with
1 N HCl (3 × 350 mL) followed by 10% aqueous NaHCO3 (350
mL), dried over MgSO4, and concentrated. The residue was
purified by flash chromatography using increasing concentra-
tions of EtOAc in hexane as the eluent to give 4.0 g (97%) of
N-Boc-p-nitro-L-phenylalanine 2-(trimethylsilyl)ethyl ester: TLC,
Rf ) 0.76 (1/2 EtOAc/hexane); 1H NMR (DMSO-d6) δ 0.0 (s, 9H),
0.80-0.90 (m, 2H), 1.30 (s, 9H), 2.90-3.05 (m, 1H), 3.05-3.15
(m, 1H), 4.10 (t, J ) 8.3 Hz, 2H), 4.15-4.25 (m, 1H), 7.35 (d, J
) 8.5 Hz, 1H), 7.51 (d, J ) 8.6 Hz, 2H), 8.13 (d, J ) 8.6 Hz, 2H);
13C NMR (CDCl3) -1.0, 18.0, 28.8, 39.0, 54.8, 64.8, 80.7, 124.1,
130.9, 144.8, 147.7, 155.5, 171.8.
2H), 1.25-1.33 (m, 3H), 1.42 (s, 9H), 2.96-3.14 (m, 2H), 3.46
(s, 2H), 4.13-4.25 (m, 4H), 4.40-4.55 (m, 1H), 4.95-5.05 (m,-
1H), 7.00-7.10 (m, 2H), 7.40-7.50 (m, 2H), 9.20 (s, 1H); 13C
NMR (CDCl3) -1.0, 14.6, 17.9, 28.8, 38.2, 42.4, 55.1, 62.4, 64.3,
80.3, 120.6, 130.4, 132.8, 137.1, 155.7, 163.6, 170.2, 172.4. Anal.
Calcd for C24H38N2O7Si: C, 58.28; H, 7.74; N, 5.66. Found: C,
58.05; H, 7.76; N, 5.57.
N-r-Boc-â-[p -[[E t h oxy-2-[(d im et h yla m in o)m et h ylen e]-
m a lon yl]a m in o]p h en yl]-L-a la n in e 2-(Tr im eth ylsilyl)eth yl
Ester (4). To a 250 mL reaction vessel was added 4.36 g (8.81
mmol) of 3, 1.75 mL (13.2 mmol) of DMF dimethylacetal, and
20 mL of dry CH2Cl2. The reaction vessel was sealed with a
rubber septum under argon and immersed in an oil bath heated
to 55-60 °C for 40 h (Caution: the boiling point of methylene
chloride is 40 °C which will result in a pressure buildup inside
the reaction vessel. Shielding and a reaction vessel designed for
running reactions under pressure should be used). The reaction
was then concentrated followed by further drying under high
vacuum to give 4.5 g (94%) of 4: TLC, Rf ) 0.05 (1/2 EtOAc/
hexane), 0.21 (1/1 EtOAc/hexane); 1H NMR (CDCl3) δ 0.04 (s,
9H), 0.94-1.00 (m, 2H), 1.32 (t, J ) 6.9 Hz, 3H), 1.41 (s, 9H),
2.93-3.08 (m, 2H), 3.15 (br s, 6H), 4.15-4.22 (m, 4H), 4.48 (m,
1H), 4.94 (d, J ) 8.1 Hz, 1H), 7.05 (d, J ) 8.4 Hz, 2H), 7.52 (d,
J ) 8.4 Hz, 2H), 8.01 (br s, 1H), 10.13 (br s, 1H); FABMS (+)
m/ z ) 550 (M + 1).
At t em p t ed Oxid a t ion of N-r-Boc-â-[p -[[E t h oxy-2-[(d i-
m et h yla m in o)m et h ylen e]m a lon yl]a m in o]p h en yl]-L-a la -
n in e 2-(Tr im eth ylsilyl)eth yl Ester (4) w ith Sin glet Oxygen .
A solution of 0.94 g (1.7 mmol) of enamine 4 and 5 mg (0.0038
mmol) of rose bengal bis(triethylammonium) salt (Aldrich) was
prepared in anhydrous CH2Cl2 (10 mL). The solution was cooled
to -78 °C and then a constant supply of oxygen was introduced
by bubbling O2 through the reaction via a needle. The reaction
was then irradiated with a 500-W quartz halogen lamp. The
reaction was irradiated for 45 min, but a TLC (1/1 EtOAc/
hexane) analysis showed no reaction had occurred.
Low Yield Oxid a t ion of N-r-Boc-â-[p -[[E t h oxy-2-[(d i-
m et h yla m in o)m et h ylen e]m a lon yl]a m in o]p h en yl]-L-a la -
n in e 2-(Tr im eth ylsilyl)eth yl Ester (4) w ith Ozon e.
A
solution of 0.94 g (1.7 mmol) of enamine 4 in 20 mL of dry CH2-
Cl2 was prepared and cooled to -78 °C. Ozone was then
continuously passed through the reaction by bubbling through
a submerged needle for 45 min during which time the reaction
became a light yellow-green. Dimethyl sulfide (3 mL) was then
added to prevent further oxidation while the temperature was
gradually increased to 25 °C. Stirring was continued for another
40 min. The solvent and dimethyl sulfide were removed under
vacuum, and the residue was purified by flash chromatography
(2/1 EtOAc/hexane) to give only 70 mg (8%) of the tricarbonyl
compound 5 and 150 mg (16.7%) of its hydrated derivative 6.
N-r-Boc-â-[p -[(E t h oxy-2-oxom a lon yl)a m in o]p h en yl]-L-
a la n in e 2-(Tr im eth ylsilyl)eth yl Ester (5) a n d Its Hyd r a te
(6). To a solution of enamine 4 (290 mg, 0.52 mmol) in 50%
aqueous THF (10 mL) was added NaIO4 (350 mg, 1.63 mmol).
After 18 h at rt the starting material 4 was consumed as judged
by TLC. The reaction was concentrated, the residue was diluted
with EtOAc (5 mL), and the insoluble white precipitate was
removed by filtration. The filtrate was concentrated and purified
by flash chromatography (1/2 EtOAc/hexane containing 1% acetic
acid) to give 5 (100 mg) and 6 (110 mg) in a 79% combined yield.
Compound 5: TLC, Rf ) 0.65 (1/2 EtOAc/hexane containing 1%
acetic acid); 1H NMR(CDCl3) 0.02 (s, 9H), 0.90-1.00 (m, 2H),
1.30-1.42 (m, 12H), 2.90-3.20 (m, 2H), 4.10-4.22 (m, 2H),
4.23-4.30 (m, 2H), 4.40-4.60 (m, 1H), 4.90-5.00 (m, 1H), 7.05-
7.15 (m, 2H), 7.45-7.50 (m, 2H), 8.50-8.70 (m, 1H). 13C NMR
(CDCl3) -1.0, 14.8, 17.9, 28.9, 38.4, 55.1, 61.4, 64.3, 80.4, 121.49
& 121.62 (assigned as conformational isomers), 130.6, 133.5,
To a solution of 7.8 g (19 mmol) of N-Boc-p-nitro-L-phenyl-
alanine 2-(trimethylsilyl)ethyl ester in methanol (20 mL) was
added 10% Pd-C (810 mg), and then the mixture was hydro-
genated on a Parr apparatus at 50 psi H2 and rt. The reaction
was complete within 3 h as determined by TLC. The catalyst
was removed by filtration through Celite, and the pad was rinsed
with additional methanol. The combined filtrate was concen-
trated to give 6.6 g (84%) of 2: TLC, Rf ) 0.35 (1/2 EtOAc/
1
hexane); H NMR (DMSO-d6) δ 0.0 (s, 9H), 0.80-0.90 (m, 2H),
1.31 (s, 9H), 2.59-2.75 (m, 2H), 3.90-4.00 (m, 1H), 4.06 (t, J )
7.8 Hz, 2H), 4.86 (s, 2H), 6.42 (d, J ) 8.4 Hz, 2H), 6.82 (d, J )
8.1 Hz, 2H), 7.07 (d, J ) 7.8 Hz, 1H); 2 tends to air oxidize
readily and was therefore used the same day for the preparation
of 3.
N -r-Boc-â-[[p -(E t h oxym a lon yl)a m in o]p h e n yl]-L -a la -
n in e 2-(Tr im et h ylsilyl)et h yl E st er (3). To a solution of 2
(6.55 g, 17.0 mmol) and pyridine (57 mmol, 4.6 mL) in dry CH2-
Cl2 (20 mL) cooled to 0 °C under an Ar atmosphere was added
dropwise a solution of ethyl malonyl chloride 3.0 mL (23 mmol)
in dry dichloromethane (3.0 mL) over 10 min. After stirring at
0 °C for 15 min and then room temperature for 36 h the mixture
was diluted with EtOAc (500 mL) and washed with 1 N KHSO4
(3 × 350 mL), brine (350 mL), dried over MgSO4, and concen-
trated. The residue was purified by flash chromatography (1/2
136.1, 158.9, 167.9, 169.5, 172.4, 178.2. Anal. Calcd for C24H36
-
N2O8Si: C, 56.67; H, 7.13; N, 5.51. Found: C, 56.81; H, 7.14;
N, 5.42. Compound 5 showed limited stability upon extended
storage at rt. Compound 6: TLC, Rf ) 0.28 (1/2 EtOAc/hexane
containing 1% acetic acid); 1H NMR (CDCl3) δ 0.02 (m, 9H),
0.95-1.00 (m, 2H), 1.30 (t, J ) 6.8 Hz, 3H), 1.42 (s, 9H), 2.90-
3.16 (m, 2H), 4.15-4.44 (m, 4H), 4.45-4.53 (m, 3H), 5.01 (d, J
) 6.5 Hz, 1H), 7.11-7.18 (m, 2H), 7.47-7.50 & 7.56-7.61 (two
m, 2H, assigned as conformational isomers), 8.30 & 8.55 (two s,

1874 J . Org. Chem., Vol. 61, No. 5, 1996
Notes
1H, assigned as conformational isomers). 13C NMR (CDCl3)
-1.0, 14.49 & 14.56 (assigned as conformational isomers), 18.0,
28.9, 38.3, 55.1, 63.8, 64.24 & 64.45 (assigned as conformational
isomers), 80.5, 91.7, 120.47 & 120.54 (assigned as conformational
isomers), 130.66 & 130.80 (assigned as conformational isomers),
133.8, 134.70 & 136.05 (assigned as conformational isomers),
155.6, 166.2, 170.1, 172.35 & 172.41 (assigned as conformational
isomers). Anal. Calcd for C24H38N2O9Si: C, 54.73; H, 7.27; N,
5.32. Found: C, 54.79; H, 7.24; N, 5.22.
Ack n ow led gm en t. This work was supported by
grants from the National Cancer Institute (RO1 CA52800
to D.G.H.), Lederle Laboratories, and the Center for
Biotechnology at Stony Brook, funded by the New York
State Science and Technology Foundation (to D.G.H.).
J O951855A