Asymmetric Synthesis of Activated α-Amino Esters via Chromium Carbene Complex Photochemistry

Article abstract of DOI:10.1021/jo9709817

Optically active chromium aminocarbene complexes were α-alkylated and then photolyzed in the presence of 2,4,5-trichlorophenol to produce a number of optically active activated α-amino esters. These underwent coupling with amino acid esters or a soluble PEG-supported dipeptide to introduce the chromium-carbene-complex-derived amino acid residue into the peptide.

Full text of DOI:10.1021/jo9709817

7704
J . Org. Chem. 1997, 62, 7704-7710
Asym m etr ic Syn th esis of Activa ted r-Am in o Ester s via Ch r om iu m
Ca r ben e Com p lex P h otoch em istr y
J iawang Zhu, Christopher Deur, and Louis S. Hegedus*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Received J une 2, 1997X
Optically active chromium aminocarbene complexes were R-alkylated and then photolyzed in the
presence of 2,4,5-trichlorophenol to produce a number of optically active activated R-amino esters.
These underwent coupling with amino acid esters or a soluble PEG-supported dipeptide to introduce
the chromium-carbene-complex-derived amino acid residue into the peptide.
In tr od u ction
peptides particularly in the presence of 1-hydroxyben-
zotriazole (HOBT).11 In principle, by using one of these
“activated” alcohols as the nucleophile in eq 1, activated
esters could be prepared and purified to give a single
diastereoisomer, permitting subsequent introduction of
this optically pure amino acid residue into peptides using
conventional peptide-coupling methodology. The results
of studies addressing this problem are presented below.
With the growing importance of peptides, both in
biology1 and in the treatment of disease,2 the synthesis
of amino acids3 and their incorporation into peptides4 has
undergone dramatic recent development. In addition to
the synthesis of natural, unmodified peptides, the intro-
duction of unnatural amino acid residues into peptides
to increase biostability and bioavailability has become
important.5 Recently a novel procedure for the introduc-
tion of natural, unnatural, or isotopically labeled amino
acid residues into peptides in solution,6 on soluble poly-
(ethylene glycol) (PEG) supports,7 or on typical Merrifield
resin supports8 has been reported from these laboratories.
The process involves the photolysis of optically active
chromium aminocarbene complexes in the presence of
amino acid esters, and although it is highly diastereose-
lective (88-97% de), it is not stereospecific, and small
amounts of the undesired diastereoisomeric peptide are
produced.
Early studies9 had shown that photolysis of optically
active chromium aminocarbene complexes in the pres-
ence of alcohols produced R-amino esters in good yield
and with high diastereoselectivity (eq 1). A class of
R-amino ester of use in peptide synthesis is the “acti-
vated” ester10sesters having particularly good leaving
groups as the alcohol component, such as p-nitrophenol,
pentafluorophenol, trichlorophenol, or N-hydroxysuccin-
imide. Many of these esters can be purified and stored
for long periods, yet couple efficiently to amino acids or
Resu lts a n d Discu ssion
To provide a range of amino acid activated ester
precursors, it was necessary to synthesize appropriate
optically active chromium aminocarbene complexes hav-
ing a variety of different R groups. This was best
accomplished by utilizing the acidity12 of the R-protons
of the parent methyl aminocarbene complex 1. Treat-
ment of carbene complex 1 with n-butyllithium followed
by an electrophile produced alkylated chromium carbene
complexes 2 (Table 1) in fair to good yield in most cases
(eq 2).13
Ta ble 1. P r ep a r a tion of Op tica lly Active Am in oca r ben e
Com p lexes 2
X Abstract published in Advance ACS Abstracts, October 1, 1997.
(1) Xo¨nig, W. Peptides and Protein Hormones - Structure, Regulation,
Activity - A Reference Manual; VCH Publishers: New York, 1993.
(2) (a) Liskamp, R. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 305.
(b) Polypeptide and Protein Drugs; Hider, R. C., Barlow, D., Eds.,
Horwood: London, 1991. (c) Peptide Pharmaceuticals; Ward, D. J ., Ed.,
Open University Press: Milton Keynes, U.K., 1991.
(3) (a) Duthaler, R. Tetrahedron 1994, 50, 1539. (b) Williams, R.
M. Synthesis of Optically Active Amino Acids; Baldwin, J . E., Magnus,
P. D., Eds.; Pergamon, Oxford, U.K., 1989.
(4) The field of peptide synthesis is reviewed annually: Elmore, D.
T. In Amino Acids and Peptides: Specialists Periodical Report; Davies,
J . S., senior reporter, The Royal Society of Chemistry, Cambridge, U.K.
(5) Bain, J . D.; Wacker, D. A.; Kuo, E. E.; Chamberlin, A. R.
Tetrahedron 1991, 47, 2389.
(6) (a) Miller, J . R.; Pulley, S. R.; Hegedus, L. S.; Delombaert, S. J .
Am. Chem. Soc. 1992, 114, 5602. (b) Dubuisson, C.; Fukumoto, Y.;
Hegedus, L. S. J . Am. Chem. Soc. 1995, 117, 3697. (c) For a review,
see Hegedus, L. S. Acc. Chem. Res. 1995, 28, 299.
(7) Zhu, J .; Hegedus, L. S. J . Org. Chem. 1995, 60, 5831.
(8) Pulley, S. R.; Hegedus, L. S. J . Am. Chem. Soc. 1993, 115, 9037.
(9) Hegedus, L. S.; Schwindt, M. A.; DeLombaert, S.; Imwinkelried,
R. J . Am. Chem. Soc. 1990, 112, 2264.
(10) Bodansky, M. Activated Esters in Peptide Synthesis. In The
Peptides; Gross, E., Meinenhoffer, J ., Eds., Academic Press: New York,
1979; Vol. 1.
(11) Ko¨nig, W.; Geiger, R. Chem. Ber. 1973, 186, 3626.
(12) (a) Casey, C. P.; Boggs, R. A.; Anderson, R. L. J . Am. Chem.
Soc. 1972, 94, 8947. (b) Gandler, J . R.; Bernasconi, C. F. Organome-
tallics 1989, 8, 2282.
S0022-3263(97)00981-X CCC: $14.00 © 1997 American Chemical Society

Asymmetric Synthesis of Activated R-Amino Esters
J . Org. Chem., Vol. 62, No. 22, 1997 7705
Ta ble 3. F or m a tion of Activa ted Ester s
Active ester formation studies were initiated using the
parent carbene complex 1 to screen a range of activated
phenols as trapping agents (eq 3). Photolysis of complex
a
Reported yields are for isolated, pure single diastereoisomers
b
and are based on starting carbene complex. Estimated from
integration of relevant peaks in the 1H NMR spectrum of crude
material. c A two-fold excess of complex was used. Diastereoiso-
d
meric excess could not be determined.
an examination of the crude reaction mixtures indicated
the diastereoselectivity of the process was good. Some
problems were encountered. Carbene complex 2d was
relatively unstable and decomposed both on standing and
on handling. It had to be freshly prepared and used in
excess in the photoreaction to get reasonable amounts
of activated esters. Complexes 2g and 2h underwent
photoreaction, but only unidentified decomposition prod-
ucts were obtained. The phenyl analog of complex 1, a
potential source of aryl glycine activated esters, could not
be synthesized, probably because of increased steric
hindrance about the carbene carbon due to R-branching.
Its less-hindered analog 5 was available and was sub-
jected to photolysis in the presence of the trichlorophenol
(eq 5). Unfortunately, the reaction suffered from low
diastereoselectivity, and the diastereoisomers were in-
separable, making this process of no use in peptide
synthesis.
1 in the presence of 20% excess of the alcohol at room
temperature under 50 psi of carbon monoxide led to the
activated alanine esters in fair yield and good diastereo-
selectivity (Table 2). Of the alcohols screened, 2,4,5-
trichlorophenol gave the best yields, and the best dia-
stereoselectivity. In addition, this compound was
relatively inexpensive and was easy to handle, leading
to its choice for subsequent studies.
Ta ble 2. F or m a tion of Activa ted Ester s
a
Reported yields are for isolated, pure single diastereoisomers.
Estimated from integration of relevant peaks in the 1H-NMR
b
spectra of crude material. c Diastereoisomeric excess could not be
determined because of interferring by-products.
Photolysis of chromium carbene complexes 2a -g in the
presence of a slight excess of 2,4,5-trichlorophenol pro-
duced activated esters 4a -e (eq 4, Table 3). These could
The above methodology permits the synthesis of a
range of both natural and “unnatural” (e.g. unnatural R
group, R-configuration) activated amino acid esters for
use in peptide synthesis. Since 13C-labeled carbene
complex 1 can be made,14 isotopically-labeled activated
amino acid esters should also be available by this process.
With activated esters 3b, 3c, and 4b,d ,e in hand, the
coupling reactions were next examined. Under conven-
tional peptide coupling conditions, using only a slight
excess (1.1 equiv) of activated esters, dipeptides 7 were
formed in fair to good yields (eq 6, Table 4).
In the photolytic coupling of chromium aminocarbene
complexes with proline esters, poor diastereoselectivity
(48%) was observed.6a In contrast, proline methyl ester
coupled to activated ester 3b in good isolated yield to give
a single diastereoisomer (eq 7). Pentafluorophenol acti-
be isolated as pure diastereoisomers in modest yield, and
(13) (a) Xu, Y.-C.; Wulff, W. D. J . Org. Chem. 1987, 52, 3263 and
references therein. (b) Anderson, B. A.; Wulff, W. D.; Rahm, A. J . Am.
Chem. Soc. 1993, 115, 4602.
(14) Lastra, E.; Hegedus, L. S. J . Am. Chem. Soc. 1993, 115, 87.
(15) Fischer, E. O.; Massbo¨l, A. Angew. Chem., Int. Ed. Engl. 1964,
3, 580.

7706 J . Org. Chem., Vol. 62, No. 22, 1997
Zhu et al.
Ta ble 4. P ep tid e Cou p lin g w ith Activa ted Ester s
ester
(config)
peptide 7
(config)
R1
R2
R3
yield, %a
3c (R,S)
3d (S,R)
4b (S,R)
4d (R,S)
4e (R,S)
H
H
Me
iPr
Me
Me
Me
Me
t-Bu
Me
Me
Me
7a (R,S,S)
7b (S,R,S)
7c (S,R,S)
7d (R,S,S)
7e (R,S,S)
86
90
64
63
56
PhCH2
n-C8H17
MeO2CCH2
a
Reported yields are for isolated, pure dipeptide.
vated ester 3d coupled efficiently to soluble poly(ethylene
glycol)-bound dipeptide 9 to give good yields of the free
tripeptide 10. The oxazolidine N-protecting group could
be removed under either oxidative or reductive conditions
(eq 8).8 Finally, activated ester 6 was coupled to alanine
methyl ester to give a 4:1 mixture of diastereoisomers of
the dipeptide (eq 9), confirming the diastereoisomeric
ratio of the activated ester.
In summary, optically active chromium aminocarbene
complexes could be R-alkylated and photolyzed in the
presence of trichlorophenol to give a modest range of
unnatural activated R-amino acid esters. These coupled
to amino acid esters or a dipeptide to introduce these
unnatural amino acid residues into peptides in reason-
able yield.
Exp er im en ta l Section
Gen er a l Meth od s. Optical rotations were obtained on a
Perkin-Elmer 24 polarimeter at a wavelength of 589 nm
(sodium D line) in a 1.0-dm cell with a total volume of 1.00
mL. Specific rotation, [R]D, was reported in degrees per
decimeter at the specified temperature, and the concentration
(c) was given in grams per 100 mL in the specified solvent.
The photoreactions were carried out in oven-dried pressure
tubes that were charged with the carbene complex, the phenol,
and glass beads while under a stream of argon. The tube was
fitted with a pressure head and saturated with CO (three
cycles). The reaction mixture was placed under 70-80 psi CO,
and then photolysis was begun. Irradiation of the reaction
mixtures was carried out in 20-mL Pyrex pressure tubes placed
at a distance of 10 cm from a Conrad-Hanovia 7825 medium-
pressure mercury lamp operating at 450 W, which was placed
in a water-cooled immersion well. A Conrad-Hanovia 7830-C
power supply was used. When the photoreaction was com-
plete, the solvent was removed by rotary evaporator, and the
crude reaction mixture was separated from any Cr(CO)6 using
the chosen chromatography solvent (hexane/ethyl acetate).
Purification via standard chromatography techniques gave the
desired product. 1H NMR spectra were run at 300 MHz in
CDCl3 and referenced to Me4Si at 0 ppm. 13C NMR spectra
were run at 75 MHz in CDCl3 and referenced to the solvent
peak at δ 77.00 ppm unless otherwise noted. Column chro-
matography was performed with ICN 32-63 nm, 60 Å silica
gel using flash column techniques.
filtered through Celite directly into an airless flask under
argon. The bright yellow solution was cooled to -78 °C, and
freshly distilled acetyl bromide (2.96 g, 24.1 mmol) was added
dropwise via syringe. The reaction mixture was stirred at -78
°C for 30 min to give a dark red slurry. After 30 min, a
solution of 2,2-dimethyl-5-phenyl-1,3-oxazolidine (7.49 g, 43.8
mmol) in 10 mL of CH2Cl2 was added with a syringe. After
an additional 5 min, N-methylmorpholine (2.6 ml, 24.1 mmol)
was added, and the deep purple suspension was gradually
warmed until the suspension turned bright yellow. The
mixture was filtered through Celite, and the red/orange
solution was absorbed onto silica gel and dried under reduced
pressure. The resulting bright yellow solid was transferred
to a silica gel column and eluted with 1:1 CH2Cl2/hexanes
under argon to give a bright yellow solid 5.49 g (65%) after
the solvent was removed under reduced pressure. The mate-
rial was identical in all respects to previously reported results.9
1H NMR δ 7.41-7.14 (m, 5H), 5.84 (d, J ) 5.9 Hz, 1H), 4.53
(dd, J ) 10.5, 6.1 Hz, 1H), 4.25 (m, 1H), 3.14 (s, 3H), 1.82 (s,
3H), 1.74 (s, 3H).
Gen er a l P r oced u r e for th e r-Alk yla tion of Ca r ben e
Com p lex 1 (Ta ble 1). P r ep a r a tion of Com p lexes 2a -h .
A THF solution (10-20 mL/mmol) of complex 1 was placed in
a flame-dried airless flask containing a magnetic stir bar,
degassed by evacuation/back-fill with argon (3×) and cooled
to -78 °C. To this yellow solution, n-butyllithium (1.6 M in
hexane, 1.2-1.5 equiv) was added dropwise from a syringe.
The resulting orange-red solution was stirred for 0.25-0.5 h
at this temperature, and the electrophile (1.2-1.5 equiv) was
then added to the stirred solution dropwise through a syringe.
P r ep a r a tion of P en ta ca r bon yl [((5S) or (5R)-1-Aza -2,2-
d im eth yl-3-oxa -5-p h en ylcyclop en tyl)(m eth yl)ca r ben e]-
ch r om iu m (0), (R)- a n d (S)-1. The tetramethylammonium
salt of pentacarbonyl[(oxy)(methyl)carbene]chromium (6.8 g,
21.9 mmol) was dissolved in 100 mL of degassed CH2Cl2 and

Asymmetric Synthesis of Activated R-Amino Esters
J . Org. Chem., Vol. 62, No. 22, 1997 7707
The reaction was stirred at -78 °C for an additional 0.5 h and
then allowed to gradually warm to rt and stirred at rt for 2 h.
The crude reaction mixture was adsorbed onto silica gel and
subjected to flash column chromatography, by using argon as
a source of pressure. Removal of the elution solvent under
reduced pressure gave the pure carbene complex as a yellow
solid. Although many of these could be stored for long periods
without apparent decomposition, acceptable elemental analysis
were difficult to obtain, and the complexes were often synthe-
sized and utilized directly.
2a . From 0.50 g (1.27 mol) of (R)-complex 1, 0.69 mL (1.38
mmol) of 2.0 M n-butyllithium in hexane, and 0.17 g (1.39
mmol) of allyl bromide in 3 mL of THF, (R)-complex 2a (0.45
g, 1.03 mmol, 82%) was obtained as a yellow crystalline solid
after chromatography (silica gel, 1:1 hexane/CH2Cl2). 1H NMR
δ 1.79 (s, 3H), 1.84 (s, 3H), 2.20 (m, 1H), 2.92 (m, 1H), 3.30
(dt, J ) 6.6, 11.4 Hz, 1H), 3.44 (dt, J ) 6.6, 11.4 Hz, 1H), 4.15
(d, J ) 9.0 Hz, 1H), 4.60 (dd, J ) 5.7, 9.3 Hz, 1H), 5.12 (d, J
) 10.4, 1H), 5.21 (d, J ) 14.1 Hz, 1H), 5.87 (d, J ) 5.7 Hz,
1H), 5.92 (m, 1H), 7.2-7.5 (m, 5H). 13C NMR δ 278.4, 221.9,
217.8, 139.5, 135.8, 128.8, 127.9, 126.3, 116.3, 101.3, 74.1, 70.1,
52.6, 34.2, 28.1, 27.0. Mass spectrum C21H21O6NCr M+ 435
(parent).
2b. From 0.50 g (1.27 mmol) of (S) complex 1, 0.92 mL of
2.06 M n-butyllithium in hexane (1.90 mmol), and 0.23 mL
(1.90 mmol) of benzyl bromide was obtained complex (S)-2b
(0.52 g, 1.07 mmol, 85%) as a yellow crystalline solid after
column chromatography (silica gel 1:1 CH2Cl2/hexane). 1H
NMR δ 1.78 (s, 3H), 1.93 (s, 3H), 2.69 (m, 1H), 3.4-3.8 (m,
3H), 4.20 (dd, J ) 1.7, 9.5 Hz, 1H), 4.60 (dd, J ) 5.6, 9.3 Hz,
1H), 5.93 (d, J ) 5.2 Hz, 1H), 7.2-7.5 (m, 5H). Mass spectrum
C25H23NO6Cr M+ 487 (parent).
2c. From 0.70 g (1.77 mmol) of (S) complex 1, 1.26 mL of
1.55 M n-butyllithium in hexane (1.95 mmol), and 0.42 g (2.12
mmol) of tert-butyl bromoacetate in 6 mL of THF was obtained
(S)-complex 2c (0.65 g, 72%) as a yellow solid after column
chromatography (1:1 CH2Cl2/hexane). 1H NMR δ 1.49 (s, 9H),
1.82 (s, 3H), 1.88 (s, 3H), 2.49 (m, 1H), 3.03 (m, 1H), 3.54 (t, J
) 9.0 Hz, 2H), 4.20 (d, J ) 9.6 Hz, 1H), 4.57 (m, 1H), 5.89 (d,
J ) 5.4 Hz, 1H), 7.2-7.5 (m, 5H). 13C NMR δ 279.2, 222.3,
217.1, 170.7, 139.4, 128.8, 127.9, 126.2, 125.6, 101.8, 81.6, 74.1,
70.1, 48.1, 35.3, 28.1, 27.8, 26.9. Mass spectrum C24H27NOCr
M+ 509 (parent).
75.2, 69.7, 58.8, 48.6, 41.4, 41.0, 31.2, 29.7, 26.8, 24.5. Mass
spectrum C24H25O7N1Cr: M+ 491.
2g. From 0.30 g (0.76 mmol) of (R) complex 1, 0.52 mL (0.84
mmol) of 1.6 M n-butyllithium in hexane, and 0.14 mL (0.91
mmol) of benzyl chloroformate in 2 mL of THF was obtained
(R)-complex 2g (0.18 g, 45%) as a yellow solid after column
chromatography (silica gel 1:1 hexane/CH2Cl2). 1H NMR δ
1.73 (s, 3H), 1.79 (s, 3H), 4.25 (d, J ) 8.7 Hz, 1H), 4.40 (d, J
) 15.9 Hz, 1H), 4.55 (m, 1H), 4.58 (d, J ) 15.7 Hz, 1H), 5.20
(d, J ) 12.7 Hz, 1H), 5.28 (d, J ) 10.5 Hz, 1H), 5.89 (d, J )
5.1 Hz, 1H), 7.40-7.25 (m, 10H). 13C NMR δ 278.4, 223.4,
216.8, 169.3, 134.8, 129.0, 128.7, 126.0, 101.8, 74.2, 70.2, 67.8,
56.8, 28.1, 27.1. Mass spectrum C26H23O8N1Cr: M+ 529.
2h . From 0.15 g (0.38 mmol) of (R) complex 1, 0.24 mL (0.39
mmol) of 1.6 n-butyllithium in hexane, and 47 µL (0.42 mmol)
of ethyl acrylate in 1 mL of THF was obtained (R)-complex
2h (38 mg, 20%) after column chromatography (silica gel 3:1
hexane/CH2Cl2). 1H NMR δ 1.30 (t, J ) 71 Hz, 3H), 1.81 (s,
3H), 1.93 (m, 1H), 1.97 (s, 3H), 2.38 (m, 1H), 2.61 (m, 2H),
3.30 (m, 1H), 3.44 (m, 1H), 4.17 (m, 3H), 4.57 (dd, J ) 9.3, 5.9
Hz, 1H), 5.89 (d, J ) 5.0 Hz, 1H), 7.42-7.15 (m, 5H). 13C NMR
δ 278.2, 222.9, 217.8, 172.9, 139.4, 128.8, 127.9, 126.2, 101.7,
74.2, 70.1, 60.7, 52.6, 34.1, 27.7, 27.0, 25.1, 14.2. Mass
spectrum C23H25O8N1Cr: M+ 495.
Gen er a l P r oced u r e for th e Asym m etr ic Syn th esis of
Activa ted Ester s (Ta ble 2). Syn th esis of Ester s 3a-e. The
optically active chromium aminocarbene complex 1 was dis-
solved in THF (or CH2Cl2) and was filtered through Celite into
a Pyrex (Ace Glass) pressure tube that contained the nucleo-
phile. A pressure head was attached, and the solution was
degassed twice (freeze-thawing and back-filled with argon),
followed by saturation of the solution with CO to about 50-
60 psi. The Pyrex pressure tube was irradiated (stirred at rt,
30 h) with the apparatus described above. Partial recovery of
the Cr(CO)6 was achieved by filtering this solution. Oxidation
of reaction mixture to remove remaining chromium residues
was carried out by saturating a 1:1 hexane/EtOAc solution of
crude product with air and exposing the resulting solution to
light, in a box equipped with six 20 W Vitalite fluorescent
lamps, until most of the chromium residues turned brown and
precipitated. The activated esters were purified by flash
chromatography over silica gel, followed by removal of the
solvent.
2d . From 0.94 g (2.38 mmol) of (S) complex 1, 1.64 mL (2.62
mmol) of 1.6 M n-butyllithium in hexane, and 0.75 g (2.85
mmol) of n-octyl triflate in 50 mL of THF was obtained (S)-
complex 2d (0.91 g, 75%) as a dark red, unstable oil after
purification by column chromatography (silica gel 9:1 hexane/
EtOAc). 1H NMR δ 0.89 (m, 5H), 1.30 (m, 11H), 1.80 (s, 3H),
1.84 (s, 3H), 2.13 (m, 1H), 3.31 (m, 2H), 4.16 (d, J ) 9.3 Hz,
1H), 4.57 (dd, J ) 5.7, 9.3 Hz, 1H), 5.88 (d, J ) 5.1 Hz, 1H),
7.30 (m, 5H). This material was used without further puri-
fication since it decomposed on standing.
2e. From 0.76 g (1.92 mmol) of (R) complex 1, 1.41 mL (2.04
mmol) of 1.45 M n-butyllithium in hexane, and 0.32 g (2.11
mmol) of methyl bromoacetate in 40 mL of THF was obtained
(R)-complex 2e (0.29 g, 32%) as a yellow solid, along with 0.24
g of recovered complex 1, after column chromatography (silica
gel 4:1 hexane/EtOAc). 1H NMR δ 1.76 (s, 3H), 1.87 (s, 3H),
2.62 (m, 1H), 3.14 (m, 1H), 3.75 (s, 3H), 4.18 (d, J ) 8.7 Hz,
1H), 4.57 (dd, J ) 5.2, 9.4 Hz, 1H), 5.85 (d, J ) 5.2 Hz, 1H),
7.2-7.5 (m, 5H). This material was used without further
purification because of its relative instability.
Ester 3a . From 50 mg (0.13 mmol) of (S) complex 1 and
15 mg (0.16 mmol) of phenol in 1 mL of CH2Cl2 after
irradiation and separation by flash chromatography (silica gel,
4:1:1 hexane/CH2Cl2/EtOAc) was obtained 23 mg (55%) of a
single diastereoisomer ester 3a as a pale yellow oil [R]20
)
D
+69.5° (c 0.4, CH2Cl2). The crude reaction mixture consisted
of a 93:7 ratio of two diastereomers (86%, de), determined by
integration of the geminal methyl singlets (1.58 ppm major,
1.81 ppm minor) from the H NMR spectrum. 1H NMR δ 1.40
1
(d, J ) 7.1 Hz, 3H), 1.42 (s, 3H), 1.58 (s, 3H), 3.70 (t, J ) 7.5
Hz, 1H), 3.80 (q, J ) 7.1 Hz, 1H), 4.21 (dd, J ) 8.0, 7.1 Hz,
1H), 4.52 (t, J ) 7.2 Hz, 1H), 6.85 (d, J ) 7.7 Hz, 2H), 7.33-
7.14 (m, 6H), 7.44 (d, J ) 8.1 Hz, 2H). 13C NMR δ 172.1, 150.6,
141.2, 129.0, 128.3, 127.5, 127.4, 125.3, 121.0, 95.9, 72.1, 61.7,
53.2, 28.7, 23.0, 15.1. IR (film) ν 1755 cm-1. Anal. Calcd for
C20H23O3N: C, 73.82; H, 7.12. Found: C, 73.89; H, 7.16.
Ester 3b a n d 3c. From 0.10 g (0.25 mmol) of (S) complex
1 and 0.06 g (0.30 mmol) of 2,4,5-trichlorophenol in 1 mL of
CH2Cl2, after irradiation and purification by flash chromatog-
raphy (silica gel 4:1:1 hexane/CH2Cl2/EtOAc) was obtained 56
mg (52%) of a single diastereoisomer of ester 3b as a pale
2f. From 0.30 g (0.38 mmol) of (R) complex 1, 0.52 mL (0.84
mmol) of 1.6 M n-butyllithium in hexane, and 93 µL (0.91
mmol) of cyclohexanone in 2 mL of THF was obtained (R)-
complex 2f (0.28 g, 75%) as a yellow solid after column
chromatography (silica gel 1:1 hexane/CH2Cl2 followed by 1:1
hexane/EtOAc). The crude product mixture consisted of a 20:1
ratio of the diastereomers, determined by integration of the
yellow oil after removal of the solvent: [R]23 ) +5.2° (c 1.3,
D
CH2Cl2). The crude reaction mixture consisted of a 95:5 ratio
of two diastereomers (90%, de), determined by integration of
the methine peaks (δ 4.41 ppm major, 4.69 ppm minor) from
the 1H NMR spectrum. 1H NMR δ 1.35 (s, 1H), 1.39 (d, J )
7.1 Hz, 3H), 1.53 (s, 3H), 3.69 (dd, J ) 8.2, 6.8 Hz, 1H), 3.83
(q, J ) 7.1 Hz, 1H), 4.18 (dd, J ) 8.1, 7.2 Hz, 1H), 4.41 (t, J )
7.0 Hz, 1H), 6.63 (s, 1H), 7.41-7.19 (m, 6H). 13C NMR δ 170.6,
145.5, 141.3, 131.2, 130.9, 130.5, 129.8, 128.4, 127.7, 125.3,
124.8, 96.1, 72.7, 60.8, 52.9, 28.4, 23.0, 14.5. IR (film) ν 1769
1
methine peaks (4.35 ppm major, 4.00 ppm minor) from the H
NMR. 1H NMR δ 1.41 (m, 1H), 1.78 (s, 3H), 1.84 (m, 1H), 1.90
(s, 3H), 2.13 (m, 4H), 2.42 (m, 1H), 2.70 (m, 1H), 3.10 (m, 1H),
3.18 (m, 2H), 4.34 (dd, J ) 15.3, 8.8 Hz, 1H), 4.57 (dd, J )
13.3, 4.6 Hz, 1H), 6.08 (t, J ) 6.0 Hz, 1H), 7.43 (m, 5H). 13C
NMR δ 270.2, 217.5, 167.4, 138.7, 128.6, 128.0, 125.9, 101.8,
cm-1
Found: C, 55.95; H, 4.76.
. Anal. Calcd for C20H20O3NCl3: C, 56.03; H, 4.70.

7708 J . Org. Chem., Vol. 62, No. 22, 1997
Zhu et al.
Identical results were obtained starting with the (R) carbene
complex to give 3c.
by integration of the methylene peaks (δ 4.26 ppm major, 3.90
ppm minor) from the 1H NMR spectrum. 1H NMR δ 1.43 (s,
9H), 1.46 (s, 3H), 1.61 (s, 3H), 2.03 (m, 1H), 2.32 (m, 3H), 3.77
(m, 2H), 4.26 (dd, J ) 8.2, 7.2 Hz, 1H), 4.52 (t, J ) 6.6 Hz,
1H), 6.71 (s, 1H), 7.43-7.26 (m, 5H), 7.48 (s, 1H). 13C NMR δ
172.0, 169.4, 141.7, 131.2, 130.7, 128.6, 127.9, 127.8, 125.5,
125.0, 96.3, 80.7, 72.3, 61.1, 56.8, 32.2, 28.6, 28.1, 24.1, 23.3.
IR (film) ν 1770, 1726. Mass (HR FAB) Calcd for C26H30O5-
NCl3: M + H ) 542.1268. Found: 542.1197 ( 0.0011 (δ ) 13
ppm).
Ester 3d . From 0.15 g (0.38 mmol) of (S) carbene complex
1 and 0.10 g (0.53 mmol) pentafluorophenol in 2 mL of CH2-
Cl2, after irradiation and separation by flash chromatography
(silica gel 6:1 hexane/EtOAc), was obtained 56 mg (38%) of a
single diastereoisomer of ester 3d as a pale yellow oil. [R]24
D
) +28.1° (c 1.1, CH2Cl2). The crude reaction mixture consisted
of a 90:10 ratio of two diastereomers (80%, de), determined
by integration of the methyl doublets (δ 1.49 ppm major, 1.23
ppm minor) from the 1H NMR spectrum. 1H NMR δ 1.43 (s,
3H), 1.44 (d, J ) 7.0 Hz, 3H), 1.56 (s, 3H), 3.71 (t, J ) 8.0 Hz,
1H), 3.95 (q, J ) 7.2 Hz, 1H), 4.21 (dd, J ) 8.1, 7.0 Hz, 1H),
4.49 (t, J ) 7.5 Hz, 1H), 7.45-7.24 (m, 5H). 13C NMR δ 169.9,
140.5, 128.5, 127.8, 127.5, 126.6, 96.0, 72.2, 62.4, 53.3, 28.8,
22.9, 15.7. IR (film) ν 1786 cm-1. Mass (HR FAB): Calcd for
C20H18O3NF5: M + H ) 416.1285. Found: 416.1282 ( 0.0008
(∆ ) 0.4 ppm).
Ester 4d . From 0.46 g (0.90 mmol) of (R) carbene complex
2d and 0.09 g (0.45 mmol) of 2,4,5-trichlorophenol in 6 mL of
THF, after irradiation and purification by flash chromatog-
raphy (silica gel 14:1 hexane/EtOAc), was obtained 0.19 g
(40%) of ester 4d as a single diastereoisomer. The diastereo-
selectivity of the reaction could not be assessed from the NMR
spectrum of the crude material. 1H NMR δ 0.88 (t, J ) 6.3
Hz, 3H), 1.25 (bs, 14H), 1.42 (s, 3H), 1.60 (s, 3H), 1.63 (m, 1H),
2.11 (m, 1H), 3.64 (dd, J ) 4.5, 9.9 Hz, 1H), 3.77 (dd, J ) 6.0,
8.4 Hz, 1H), 4.25 (t, J ) 7.5 Hz, 1H), 4.50 (t, J ) 6.6 Hz, 1H),
6.67 (s, 1H), 7.39 (m, 6H). 13C NMR δ 169.8, 145.8, 142.2,
131.1, 130.7, 130.0, 127.8, 127.7, 125.6, 125.0, 96.4, 72.3, 61.2,
58.3, 31.8, 29.4, 29.2, 29.0, 28.7, 27.0, 23.3, 22.6, 14.1. IR (film)
Ester 3e. From 0.15 g (0.38 mmol) of (R) complex 1 and
0.06 g (0.50 mmol) of N-hydroxysuccinimide in 2 mL of CH2-
Cl2, after irradiation and separation of flash chromatography
(silica gel 3:1 hexane/EtOAc), was obtained 25 mg (19%) of a
single diastereiosimer of 3e as a colorless oil. The crude
reaction mixture consisted of other unidentified side products,
so the diastereoselectivity was not available from the 1H NMR
spectrum. 1H NMR δ 1.30 (d, J ) 7.9 Hz, 3H), 1.34 (s, 3H),
1.47 (s, 3H), 2.68 (s, 2H), 2.74 (s, 2H), 3.57 (t, J ) 8.5 Hz, 1H),
3.88 (q, J ) 8.2 Hz, 1H), 4.08 (dd, J ) 8.2, 7.3 Hz, 1H), 4.37 (t,
J ) 7.9 Hz, 1H), 7.45-7.12 (m, 5H). 13C NMR δ 169.2, 169.0,
168.9, 140.1, 128.6, 127.7, 126.8, 96.0, 72.1, 62.9, 52.0, 28.9,
25.6, 23.1, 20.4, 16.0. IR (film) ν 1783, 1739 cm-1. Mass (HR
FAB) Calcd for C18H22O5N2: M + H ) 347.1607. Found:
347.1603 ( 0.0014 (∆ ) 0.4 ppm).
ν 1770 cm-1
. This material was carried on to dipeptide 7d
without further characterization.
Ester 4e. From 0.12 g (0.26 mmol) of (R) complex 2e and
60 mg (0.35 mmol) of 2,4,5-trichlorophenol in 3 mL of THF,
after irradiation and purification by flash chromatography
(silica gel, 9:1 hexane/EtOAc), was obtained 38 mg (29%) of
ester 4e as a colorless oil. [R]25 -8.80° (c 0.98, CH2Cl2). 1H
D
NMR δ 1.46 (s, 3H), 1.61 (s, 3H), 2.08 (m, 1H), 2.40 (m, 3H),
3.66 (s, 3H), 3.78 (m, 2H), 4.26 (t, J ) 7.5 Hz, 1H), 4.54 (t, J
) 6.6 Hz, 1H), 6.72 (s, 1H), 7.36 (m, 6H). 13C NMR δ 173.1,
169.5, 145.6, 141.4, 131.2, 130.8, 130.2, 128.6, 128.0, 127.8,
125.5, 124.9, 96.3, 72.2, 61.4, 56.8, 51.7, 30.7, 28.7, 24.1, 23.3.
IR (film) ν 1768, 1738 cm-1. This material was carried on to
dipeptide 7e without further characterization.
P r ep a r a tion of Hom ologa ted Activa ted Ester s (Ta ble
3). Ester s 4a -f. Ester 4a . Following the general procedure
above, from 0.40 g (0.92 mmol) of (R) carbene complex 2a and
0.26 g (1.30 mmol) of 2,4,5-trichlorophenol in 2 mL of THF,
after irradiation and flash chromatography (silica gel 7:1
hexane/EtOAc), was obtained 0.26 g (60%) of a single dia-
Ester 4f. From 0.15 g (0.31 mmol) of (R) carbene complex
2f and 78 mg (0.40 mmol) of 2,4,5-trichlorophenol in 2 mL of
THF, after irradiation and purification by flash chromatog-
raphy (silica gel 3:1 hexane/EtOAc), was obtained 59 mg (36%)
stereoisomer of ester 4a as a pale yellow oil. [R]25 ) -20.7°
D
(c 4.5, CH2Cl2). The crude reaction mixture consisted of a 97.5:
2.5 ratio of two diastereomers (95%, de). 1H NMR δ 1.33 (s,
3H), 1.52 (s, 3H), 1.67 (m, 1H), 2.23-1.94 (m, 3H), 3.60 (dd, J
) 9.9, 4.1 Hz, 1H), 3.69 (dd, J ) 8.4, 6.1 Hz, 1H), 4.17 (t, J )
8.0 Hz, 1H), 4.41 (t, J ) 6.6 Hz, 1H), 4.95 (m, 2H), 5.68 (m,
1H), 6.61 (s, 1H), 7.34-7.14 (m, 5H), 7.39 (s, 1H). 13C NMR δ
169.5, 145.7, 141.9, 137.0, 131.1, 130.7, 128.5, 127.9, 127.7,
125.0, 116.1, 96.4, 72.3, 61.2, 57.1, 30.9, 28.7, 28.0, 23.3. IR
(film) ν 1770, 1640 cm-1. Mass (HR FAB) Calcd for C23H24O3-
NCl3: M + H ) 470.0871. Found: 470.0844 ( 0.0017 (∆ )
2.3 ppm).
of a single diastereoisomer of 4f as a colorless oil. [R]25
)
D
-40.1° (c 1.2, CH2Cl2). 1H NMR δ 1.18 (m, 1H), 1.36 (s, 3H),
1.41 (m, 1H), 1.53 (s, 3H), 1.58 (m, 2H), 1.98 (m, 3H), 2.41-
2.05 (m, 4H), 3.70 (m, 2H), 4.18 (t, J ) 8.3 Hz, 1H), 4.49 (dd,
J ) 10.9, 6.3 Hz, 1H), 6.62 (s, 1H), 7.34-7.19 (m, 5H), 7.43 (s,
1H). 13C NMR δ 170.0, 169.3, 145.7, 142.2, 130.8, 129.5, 128.6,
128.1, 127.8, 127.7, 125.5, 124.9, 96.2, 72.1, 61.4, 55.2, 48.1,
41.2, 36.0, 35.5, 30.6, 28.9, 24.9, 23.5. IR (film) ν 1775, 1712
cm-1
. Mass (HR FAB) Calcd for C26H28O4NCl3: M + H )
524.1162. Found: 524.1112 ( 0.0031 (∆ ) 5.0 ppm).
Syn th esis of Activa ted Ester 6. From 0.24 g (0.55 mmol)
of carbene complex 59 and 55 mg (0.28 mmol) of 2,4,5-
trichlorophenol in 4 mL of THF, after irradiation and purifica-
tion by flash chromatography (silica gel 9:1 hexane/EtOAc),
was obtained 0.11 g (42%) of activated ester 6 as an insepa-
rable 2:1 mixture of diastereoisomers. 1H NMR δ 3.76 (m, 1H),
4.31 (m, 2H), 4.42 (d, J ) 5.4 Hz, 1H), 4.55 (d, J ) 5.4 Hz,
1H), 4.62 (d, J ) 4.8 Hz, 1H), 4.83 (s, 1H), 4.85 (d, J ) 4.8 Hz,
1H), 4.87 (s, 1H), 7.32 (m, 12H). This material was carried
on to dipeptide 11 without further purification.
Gen er a l Rea ction P r oced u r e for Syn th esis of Dip ep -
tid es 7a -e a n d 8 (Ta ble 4). In a round bottom flask
containing ∼2 mL of DMF were placed 1.0 equiv of the amino
acid ester hydrochloride, 1.0 equiv of the dry amine base, 1.1
equiv of the activated ester, and 0.2 equiv of HOBT. The
reaction mixture was stirred for 5-10 h unless otherwise
indicated. The solvent was removed under reduced pressure,
and the crude reaction mixture was taken up in EtOAc,
washed twice with H2O, and dried over NaSO4. This mixture
was filtered, and the solvent was removed under reduced
pressure leaving the crude reaction product. This crude
product was purified by column or radial layer chromatography
on silica gel.
Ester 4b. From 0.15 g (0.31 mmol) of (S) carbene complex
2b and 73 mg (0.37 mmol) of 2,4,5-trichlorophenol in 2 mL of
THF, after irradiation and purification by flash chromatog-
raphy (silica gel 7:1 hexane/EtOAc), was obtained 81 mg (51%)
of a single diastereoisomer of ester 4b as a pale yellow oil.
[R]23 ) +2.3° (c 1.8, CH2Cl2). The crude reaction mixture
D
consisted of a 96:4 ratio of two diastereomers (92%, de),
determined by integration of the geminal methyl singlets (δ
1
1.23 ppm major, 1.38 ppm minor) from the H NMR spectrum.
1H NMR δ 1.23 (s, 3H), 1.54 (s, 3H), 1.94 (m, 1H), 2.54 (m,
2H), 2.75 (m, 1H), 3.65 (dd, J ) 9.5, 3.5 Hz, 1H), 3.76 (dd, J )
8.4, 6.1 Hz, 1H), 4.22 (dd, J ) 8.2, 7.2 Hz, 1H), 4.47 (t, J ) 6.8
Hz, 1H), 6.64 (s, 1H), 7.41-7.12 (m, 10H), 7.50 (s, 1H). 13C
NMR δ 169.4, 145.6, 141.8, 140.7, 131.0, 130.6, 129.9, 128.4,
128.1, 127.7, 127.6, 126.1, 125.4, 124.8, 96.3, 72.1, 60.9, 57.0,
32.9, 30.7, 28.4, 22.8. IR (film) ν 3086, 3026, 2975, 1769 cm-1
.
Anal. Calcd for C27H26O3NCl3: C, 62.50; H, 5.05. Found: C,
62.36; H, 5.18.
Ester 4c. From 0.15 g (0.29 mmol) of (S) complex 2c and
82 mg (0.42 mmol) of 2,4,5-trichlorophenol in 2 mL of THF,
after irradiation and purification by flash chromatography
(silica gel 4:1 hexane/EtOAc), was obtained 77 mg (48%) of a
single diastereoisomer of ester 4c as a pale yellow oil. [R]24
D
) +14.3° (c 0.7, CHCl3). The crude reaction mixture consisted
Dip ep tid e 7a . From 13 mg (0.10 mmol) of (S) alanine
methyl ester hydrochloride, 13 µL (0.10 mmol) of triethyl-
of a 92.5:7.5 ratio of two diastereomers (85%, de), determined

Asymmetric Synthesis of Activated R-Amino Esters
J . Org. Chem., Vol. 62, No. 22, 1997 7709
amine, and 45 mg (0.11 mmol) of activated ester 3c was
obtained 28 mg (83%) of dipeptide 7a after flash chromatog-
raphy (2:1 hexane/EtOAc) as a pale yellow oil. 1H NMR δ 0.89
(d, J ) 7.2 Hz, 3H), 1.33 (s, 3H), 1.43 (d, J ) 7.2 Hz, 3H), 1.52
(s, 3H), 3.43 (q, J ) 7.2 Hz, 1H), 3.72 (s, 3H), 3.94 (m, 1H),
4.30 (m, 3H), 7.37 (m, 5H). 13C NMR δ 14.2, 17.8, 21.0, 27.6,
47.5, 52.2, 54.9, 59.9, 72.4, 97.1, 127.6, 127.9, 128.8, 143.0,
173.2, 173.4. IR (film) 3377, 1744, 1671 cm-1. Anal. Calcd
for C18H26N2O4: C, 64.65; H, 7.84; N, 8.38. Found: 64.53; H,
7.73; N, 8.27.
P r ep a r a tion of P EG-Su p p or ted Tr ip ep tid e 9. To the
PEG-supported H-Leu-Gly-O-PEG7 (508 mg, 0.175 mmol)
dissolved in 2.5 mL of dry CH2Cl2 was added the pentafluo-
rophenyl (R)-alanine ester 3d (218 mg, 0.525 mmol), and the
solution was stirred at rt for 8 h. The resulting PEG-supported
tripeptide 9 (485 mg) as a white powder was purified by
precipitation with Et2O, filtration, redissolution in MeOH/CH2-
Cl2 1.5:1, and reprecipitation 3× at 0 °C to give 425 mg. 1H
NMR δ 0.75 (d, J ) 5.4 Hz, 3H), 0.84 (d, J ) 5.26 Hz, 3H),
1.23 (s, 3H), 1.36 (6H), 1.47 (m, 3H), 4.03-3.30 (m), 4.35 (m,
2H), 5.78 (1H), 7.34-7.15 (m, 5H), 7.71 (1H). 13C NMR δ 175.0,
171.8, 169.5, 143.0, 129.4, 128.3, 97.5, 73.0, 70.8, 68.5, 65.0,
64.5, 60.0, 55.0, 52.4, 41.2, 40.2, 28.2, 25.2, 23.8, 21.8, 21.2,
Dip ep tid e 7b. From 45 mg (0.21 mmol) of (S)-valine tert-
butyl ester hydrochloride, 0.11 g (0.26 mmol) activated ester
3d , N,N-dimethylaminopyridine (30 mg, 0.26 mmol), and
HOBT (6 mg, 0.04 mmol) in 2 mL of DMF was obtained 78
mg (90%) of dipeptide 7b as a pale yellow oil after flash
chromatography (5:1:1 hexane/CH2Cl2/EtOAc). 1H NMR δ 0.85
(d, J ) 6.9 Hz, 3H), 0.90 (d, J ) 6.9 Hz, 3H), 1.29 (d, J ) 7.2
Hz, 3H), 1.32 (s, 3H), 1.48 (s, 9H), 1.52 (s, 3H), 2.09 (m, 1H),
3.40 (q, J ) 7.2 Hz, 1H), 3.80 (t, J ) 7.9 Hz, 1H), 4.10 (dd, J
) 7.9, 4.2 Hz, 1H), 4.18 (t, J ) 7.5 Hz, 1H), 4.31 (t, J ) 7.7
Hz, 1H), 7.42-7.14 (m, 5H), 7.73 (d, J ) 7.7 Hz, 1H). 13C NMR
δ 174.2, 170.8, 140.0, 128.8, 128.1, 97.0, 82.0, 72.9, 61.6, 57.8,
55.4, 32.1, 28.6, 28.5, 21.8, 18.9, 18.5, 13.9. IR (film) ν 1720,
14.9. IR (film) ν 1753, 1726, 1665 cm-1
.
Clea va ge of P EG-Su p p or ted Tr ip ep tid e 9 To Give
Tr ip ep tid e 10. The PEG-supported tripeptide 9 (425 mg)
dissolved in a mixture of MeOH (1.5 mL) and DMF (1.5 mL)
was added to a flask containing 20 mg of KCN catalyst for
the cleavage. The solution was stirred at 50 °C for 36 h. The
solvent was removed under reduced pressure, and most of the
polymer was isolated by precipitation with Et2O for 4× at 0
°C (the solvent for the mixture was 1:2 of MeOH/CH2Cl2, at
40 °C), followed by filtration, and the filtrate was collected.
Traces of PEG were removed from the cleaved tripeptide by
flash chromatography on silica gel (MeOH), and the pure
tripeptide 10 (51 mg, 81% overall from 9) as a pale yellow oil
was obtained by further flash chromatography on silica gel
(10:9:1 hexane/EtOAc/MeOH). 1H NMR δ 0.79 (d, J ) 6.2 Hz,
3H), 0.88 (d, J ) 6.3 Hz, 3H), 1.27 (s, 3H), 1.41 (s, 3H), 1.43
(d, J ) 7.3 Hz, 3H), 1.50-1.63 (m, 3H), 3.41 (q, J ) 7.2 Hz,
1H), 3.59 (dd, J ) 18.1, 5.0 Hz, 1H), 3.67 (s, 3H), 3.79 (dd, J
) 18.2, 5.0 Hz, 1H), 3.79 (dd, J ) 18.2, 5.0 Hz, 1H), 3.85 (dd,
J ) 12.5, 9.2 Hz, 1H), 4.02 (m, 1H), 4.25 (m, 2H), 5.75 (t, J )
5.3 Hz, 1H), 7.13-7.35 (m, 6H). 13C NMR δ 174.7, 171.4, 169.8,
142.3, 128.8, 127.8, 127.6, 96.8, 72.3, 59.7, 54.8, 51.9, 51.8, 40.7,
39.9, 27.4, 24.6, 22.8, 21.3, 20.6, 14.0. IR (film) ν 3500-3200,
1755, 1660 cm-1. Mass (HR FAB) Calcd for C23H35O5N3: M
+ H ) 434.2655. Found: 434.2678 ( 0.0023 (∆ ) -5.6 ppm).
Oxid a t ive R em ova l of t h e Oxa zolid in e P r ot ect in g
Gr ou p . Syn th esis of Tr ip ep tid e 11. The fully protected
tripeptide 10 (0.10 g, 0.23 mmol) dissolved in 5 mL of 1:4 HCl
(1 N aqueous solution)/MeOH was stirred at rt for 2.5 h. The
solvents were removed in vacuo, and the residue was dissolved
in an aqueous sodium bicarbonate solution (5 mL of 10%
NaHCO3) followed by extraction with EtOAc (3 × 6 mL) to
give the crude product. The crude product dissolved in MeOH
was cooled to 0 °C in an ice bath, followed by addition of Pb-
(OAc)4 (0.13 g, 95%, 0.30 mmol), and the mixture was allowed
to warm to rt and stirred for 6 h. The reaction mixture was
filtered through Celite, and the Celite was washed with a 1:1
mixture solvent of MeOH/CH2Cl2, followed by removal of the
solvent under reduced pressure. The resulting intermediate
was again dissolved in a 1:4 of HCl (1 N aqueous solution)/
MeOH, and the solution was stirred at room temperature for
5 h, followed by removal of the solvent in vacuo. The residue
was dissolved in an aqueous sodium bicarbonate solution (5
mL of 10% NaHCO3), followed by extraction with EtOAc (3 ×
6 mL) to give the crude tripeptide product, which was purified
by flash chromatography on silica gel (10% MeOH/CH2Cl2) to
give 11 (48 mg, 76%) as a pale yellow oil. 1H NMR δ 0.90 (d,
J ) 5.3 Hz, 3H), 0.94 (d, J ) 5.2 Hz, 3H), 1.37 (d, J ) 6.0 Hz,
3H), 1.65 (m, 3H), 3.18 (bs, 2H), 3.69 (s, 3H), 3.72 (m, 1H),
3.98 (m, 2H), 4.49 (q, J ) 5.9 Hz, 1H), 7.51 (bs, 1H), 8.01 (bs,
1H). 13C NMR δ 175.0, 172.7, 170.4, 52.2, 51.5, 50.2, 41.0, 40.3,
1685 cm-1
. Anal. Calcd for C23H36O4N2: C, 68.29; H, 8.97.
Found: C, 68.21; H, 8.76.
Dip ep tid e 7c. From 15 mg (0.11 mmol) of (S) alanine
methyl ester hydrochloride, 61 mg (0.12 mmol) activated ester
4b, 15 µL (0.11 mmol) triethylamine, and 3 mg (0.02 mmol) of
HOBT in 2 mL of DMF was obtained 29 mg (64%) of dipeptide
7c as a clear oil after flash chromatography (2:1 hexane/
EtOAc). 1H NMR δ 0.85 (d, J ) 7.2 Hz, 3H), 1.22 (s, 3H), 1.50
(s, 3H), 1.92 (m, 1H), 2.38 (m, 1H), 2.76 (m, 1H), 2.93 (m, 1H),
3.32 (dd, J ) 4.8, 8.1 Hz, 1H), 3.72 (s, 3H), 3.93 (m, 1H), 4.26
(m, 3H), 7.28 (m, 10H). 13C NMR δ 17.7, 21.6, 27.9, 32.3, 34.8,
47.4, 52.2, 58.7, 60.2, 72.2, 96.9, 126.0, 127.6, 127.7, 128.4,
128.5, 128.9, 141.6, 143.6, 173.2, 173.3. IR (film) ν 1744, 1671
cm-1. Anal. Calcd for C25H32N2O4: C, 70.73; H, 7.60; N, 6.60.
Found: C, 70.28; H, 6.91; N, 6.54.
Dip ep tid e 7d . From 8 mg (0.06 mmol) of (S) alanine
methyl ester hydrochloride, 34 mg (0.06 mmol) of activated
ester 4d , 8 µL (0.06 mmol) triethylamine, and 2 mg (0.01
mmol) HOBT in 2 mL of DMF was obtained 15 mg (56%) of
dipeptide 7d as a clear oil after radial layer chromatography
(6:1 hexane/EtOAc). 1H NMR δ 0.87 (d, J ) 7.2 Hz, 3H), 1.38
(s, 3H), 1.53 (s, 3H), 1.65 (s, 1H), 2.12 (m, 2H), 2.58 (m, 1H),
2.71 (m, 1H), 3.32 (dd, J ) 4.5, 9.0 Hz, 1H), 3.68 (s, 3H), 3.71
(s, 3H), 3.94 (m, 1H), 4.20 (m, 1H), 4.31 (m, 2H), 7.31 (m, 5H).
13C NMR δ 17.7, 21.6, 25.4, 27.9, 32.4, 47.5, 51.6, 52.2, 58.2,
60.2, 72.2, 97.0, 127.7, 127.7, 128.9, 143.3, 172.9, 173.2, 173.6.
IR (film) ν 1740, 1670 cm-1. HRMS Calcd for C21H30N2O6: [M
+ H] ) 407.2177. Found: [M + H] ) 407.2182.
Dip ep tid e 7e. From 7 mg (0.05 mmol) of (S) alanine
methyl ester hydrochloride, 19 mg (0.04 mmol) of activated
ester 4e, 7 µL of triethylamine, and 1.1 mg of HOBT was
obtained 10 mg (63%) of dipeptide 7e as a clear oil after radial
layer chromatography (3:1 hexane/EtOAc). 1H NMR δ 0.87
(m, 6H), 1.27 (bs, 14H), 1.34 (s, 3H), 1.52 (s, 3H), 1.66 (m, 2H),
1.89 (m, 1H), 3.25 (t, J ) 6.8 Hz, 1H), 3.71 (s, 3H), 3.93 (dd, J
) 3.2, 8.0 Hz, 1H), 4.27 (m, 3H), 7.32 (m, 5H). 13C NMR δ
14.1, 17.8, 21.9, 22.6, 28.0, 28.6, 29.3, 29.4, 29.6, 29.8, 30.4,
31.8, 47.4, 52.1, 60.3, 72.2, 96.8, 127.5, 127.6, 128.9, 144.0,
173.2, 173.8. IR (film) ν 1745, 1673 cm-1
. Anal. Calcd for
C26H42N2O4: C, 69.92; H, 9.48; N, 6.27. Found: C, 70.16; H,
9.21; N, 6.35.
24.8, 22.8, 21.8, 20.7. IR (film) ν 1740, 1768 cm-1
.
Dip ep tid e 8. From 15 mg (0.09 mmol) of (S) proline methyl
ester hydrochloride, 42 mg (0.10 mol) of activated ester 3b,
15 µL of triethylamine, and 3 mg of HOBT was obtained 27
mg (74%) of dipeptide 8 as a yellow oil after flash chromatog-
raphy (1:1 hexane/EtOAc). 1H NMR δ 1.31 (d, J ) 6.0 Hz,
3H), 1.47 (s, 6H), 1.77-1.97 (m, 4H), 3.56 (m, 2H), 3.60 (s, 3H),
3.73 (dd, J ) 8.4, 5.2 Hz, 1H), 3.84 (m, 2H), 4.24 (t, J ) 8.2
Red u ctive Rem ova l of th e Oxa zolid in e Ch ir a l Au xil-
ia r y fr om th e F u lly P r otected Tr ip ep tid e 10 a n d P r ep a -
r a tion of 11. The fully protected tripeptide 10 (0.10 g, 0.23
mmol) dissolved in 5 mL of 1:4 HCl (1 N aqueous solution)/
MeOH was stirred at rt for 2.5 h, the solvents were removed
in vacuo, and the residue was dissolved in an aqueous sodium
bicarbonate solution (5 mL of 10% NaHCO3) followed by
extraction with EtOAc (3 × 6 mL) to give the crude product.
The crude product dissolved in 5 mL of MeOH was transferred
to a pressure tube containing the Pearlman’s catalyst (98 mg,
20% Pd(OH)2/C, 0.069 mmol). A pressure head was attached,
Hz, 1H), 4.39 (dd, J ) 7.9, 5.2 Hz, 1H), 7.17-7.29 (m, 5H). 13
C
NMR δ 173.0, 170.2, 144.0, 127.8, 126.8, 96.8, 71.2, 60.1, 58.8,
53.6, 51.9, 46.2, 28.7, 28.1, 24.9, 24.5, 13.3. IR (film) ν 1749,
1644 cm-1
.
Anal. Calcd for C20H28O4N2: C, 66.65; H, 7.83.
Found: C, 66.78; H, 7.59.

7710 J . Org. Chem., Vol. 62, No. 22, 1997
Zhu et al.
and the reaction mixture was flushed with argon (3×), pres-
surized to 50 psi of H2, heated to 50 °C in an oil bath, and
kept stirring at that temperature for 2 h. After hydrogenation
was completed, the black slurry was stirred for an additional
1 h under 50 psi of CO at rt, followed by filtration of the Pd-
(OH)2/C through Celite, and the filter cake was washed with
3 mL of MeOH/CH2Cl2 (1:1 v/v). The filtrate was concentrated
in vacuo. The tripeptide with a free amino group at the
N-terminus 11 (53 mg, 84%) was obtained after flash chro-
matography on silica gel (10% MeOH/CH2Cl2) as a pale yellow
oil. For characterization this material was N-acylated.
Tr ip ep tid e 11 (35 mg, 0.127 mmol) was dissolved in a 2
mL of Ac2O/pyridine (1:1 v/v) and was stirred at rt under an
argon atmosphere for 2.5 h. After the reaction was complete,
the solvent was removed under reduced pressure, and the
acetyl tripeptide (36 mg, 91%) was obtained after flash
chromatography over silica gel (4:1:1 hex/EtOAc/MeOH) as a
pale yellow oil. 1H NMR (in MeOD-d4 and CDCl3) δ 0.92 (d, J
) 6.1 Hz, 3H), 0.97 (d, J ) 6.2 Hz, 3H), 1.35 (d, J ) 7.1 Hz,
3H), 1.56-1.78 (m, 3H), 2.01 (s, 3H), 3.73 (s, 3H), 3.89 (d, J )
17.5 Hz, 1H), 4.04 (d, J ) 17.6 Hz, 1H), 4.27 (q, J ) 7.1 Hz,
1H), 4.43 (m, 1H). 13C NMR (in MeOD-d4 and CDCl3) δ 174.3,
174.0, 172.4, 170.9, 52.1, 51.9, 50.0, 41.2, 40.5, 25.2, 23.1, 22.1,
21.2, 17.1. IR (film) ν 3285, 1755, 1653 cm-1. Mass (HR FAB)
Calcd for C14H25O5N3: M + H ) 316.1872. Found: 316.1901
( 0.0009 (∆ ) -8.2 ppm).
h. The general procedure for the workup was followed, and
the crude was purified by radial layer chromatography (3:1
hexane/EtOAc) giving 7 mg (35%) 12 as a clear oil and a 4:1
mixture of diastereoisomers: 1H NMR δ 0.82 (d, J ) 7.2 Hz,
3H), 1.35 (d, J ) 6.9 Hz, 3H), 3.59 (m, 1H), 3.60 (s, 3H), 3.72
(m, 1H), 3.78 (s, 3H), 4.08 (m, 1H), 4.23 (m, 1H), 4.43 (s, 1H),
4.66 (m, 4H), 7.26 (m, 10H). 13C NMR δ 17.4, 18.1, 47.3, 47.5,
52.5, 63.6, 67.1, 72.7, 73.3, 86.4, 126.3, 126.8, 127.2, 128.5,
128.5, 128.5, 128.6, 128.7, 128.8, 129.1, 136.6, 141.7, 171.0,
173.4. IR (film) ν 1743, 1674 cm-1
.
HRMS Calcd for
C21H24N2O4: [M + H] ) 369.1822. Found: [M + H] )
369.1814.
Ack n ow led gm en t. Support for this research under
Grant No. GM26178 from the National Institutes of
General Medical Sciences (Public Health Service) is
gratefully acknowledged. Mass spectra were obtained
on instruments supported by the National Institutes of
Health shared instrumentation grant GM49631.
Su p p or tin g In for m a tion Ava ila ble: 1H-NMR spectra for
compounds 2a , 2b, 2c, 2d , 2e, 2f, 2g, 2h , 3d , 3e, 4a , 4c, 4d ,
4e, 4f, 6, 7d , 10, 11, 12 and 13C-NMR spectra for compounds
2a , 2c, 2f, 2g, 2h , 3d , 3e, 4a , 4c, 4d , 4e, 4f, 7d , 10, 11, 12 (38
pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
P r ep a r a tion of Dip ep tid e 12. The general procedure
described above was followed using 8 mg (0.05 mmol) of (S)-
alanine methyl ester hydrochloride, 28 mg (0.06 mmol) of
activated ester 6 (a 4:1 mixture of diastereoisomers), and 8
µL (0.06 mmol) of Et3N. The reaction mixture was stirred 15
J O9709817