Study of the effect of the nature of the side chain in esters of α-amino acids on the diastereoselectivity of condensation with 5(4H)-oxazolone in the synthesis of dipeptides with N-terminal N-acetylphenylalanine

Article abstract of DOI:10.1023/B:RUCB.0000042296.13831.bd

Conditions for fast racemization of 5(4H)-oxazolones prepared from N-acylphenylalanine were found. Reactions of 4-benzyl-2-methyl-5(4H)-oxazolone with amino acid esters proceed diastereoselectively to give predominantly dipeptides comprising R-phenylalanine. The diastereoselectivity increases with complication of the structure of the side chain in the amino acid esters.

Full text of DOI:10.1023/B:RUCB.0000042296.13831.bd

Russian Chemical Bulletin, International Edition, Vol. 53, No. 6, pp. 1331—1334, June, 2004
1331
Study of the effect of the nature of the side chain in esters
of αꢀamino acids on the diastereoselectivity of condensation
with 5(4H )ꢀoxazolone in the synthesis of dipeptides
with Nꢀterminal Nꢀacetylphenylalanine ✾
V. P. Krasnov, E. A. Zhdanova,ꢀ N. Z. Solieva, L. Sh. Sadretdinova, I. M. Bukrina,
A. M. Demin, G. L. Levit, M. A. Ezhikova, and M. I. Kodess
I. Ya. Postovsky Institute of Organic Synthesis,
Ural Branch of the Russian Academy of Sciences,
20 ul. S. Kovalevskoi, 620219 Ekaterinburg, Russian Federation,
Fax: +7 (343) 374 1189. Eꢀmail: ca@ios.uran.ru
Conditions for fast racemization of 5(4H )ꢀoxazolones prepared from Nꢀacylphenylalanine
were found. Reactions of 4ꢀbenzylꢀ2ꢀmethylꢀ5(4H )ꢀoxazolone with amino acid esters proceed
diastereoselectively to give predominantly dipeptides comprising Rꢀphenylalanine. The
diastereoselectivity increases with complication of the structure of the side chain in the amino
acid esters.
Key words: dynamic kinetic resolution, dipeptides, amino acids, racemization, oxazoꢀ
lone, NMR.
Derivatives of Nꢀacyl amino acids are of considerable
interest as optically labile intermediates suitable for the
syntheses involving dynamic kinetic resolution.1—4 Parꢀ
ticular attention is attracted by 5(4H )ꢀoxazolones, which
are readily formed upon dehydration of acylamino acids.5
The synthesis of peptides by the "mixed anhydride"
approach using the classical twoꢀstep procedure, which
includes activation of the acid component with alkyl
chloroformate in the presence of a tertiary base (preparaꢀ
tion of a mixed anhydride) and its subsequent condensaꢀ
tion with the amino component, is known to be accomꢀ
panied by substantial racemization.6 Activated Nꢀacylꢀ
amino acids easily cyclize under the action of bases to
give 5(4H )ꢀoxazolones, which react with amino acid esꢀ
ters to give mixtures of diastereomeric peptides. If racemꢀ
ization of 5(4H )ꢀoxazolone is fast, the stereochemical
outcome of the reaction is determined by the relative
rates of the reaction of 5(4H )ꢀoxazolone stereoisomers
with the amino component. Thus, the dipeptide formaꢀ
tion represents a dynamic kinetic resolution.3,4 The search
for conditions that favor racemization and enhance the
difference between the rates of interaction of oxazolone
isomers with the amino component can open up the way
to stereoselective synthesis of the diastereomers of dipepꢀ
tides containing (R)ꢀamino acids that are difficult to
obtain.
the nature of the base, the solvent, etc. In this work,
5(4H )ꢀoxazolones prepared from Nꢀacylphenylalanine
were used as models for studying the factors that deterꢀ
mine the stereochemical outcome of their reactions with
ethyl esters of amino acids differing in the structure of the
side chain.
Results and Discussion
It is known that the use of an excess of a base used in
the synthesis of peptides by the "mixed anhydride" method
favors racemization. We used Et3N (1.2 equiv) as the base
in the preparation of dipeptides by this method starting
from Nꢀacetylphenylalanine 1 and ethyl esters of amino
acids 4—7 (Scheme 1).
The fact that the process carried out under these conꢀ
ditions is accompanied by fast and complete racemization
was confirmed by the formation of a diastereomer mixture
of the same composition from both (S)ꢀ and (RS)ꢀNꢀaceꢀ
tylphenylalanine. With a higher excess of Et3N, the yield
of dipeptides decreased. The resulting diastereomer mixꢀ
tures were analyzed by HPLC (Table 1). For assignment of
diastereomer mixtures in HPLC analysis, pure (S,S)ꢀdiaꢀ
stereomers 8a—11a prepared by a known procedure were
used.6 In this case, Nꢀmethylmorpholine was taken as the
base, the activation time was 2 min, and all components
introduced in the reaction were taken in equimolar ratios.
It should be noted that, despite all precautions underꢀ
The process diastereoselectivity depends on many
factors including the structure of the amino component,
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1278—1281, June, 2004.
1066ꢀ5285/04/5306ꢀ1331 © 2004 Plenum Publishing Corporation

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Russ.Chem.Bull., Int.Ed., Vol. 53, No. 6, June, 2004
Krasnov et al.
Scheme 1
R = Me (4, 8a,b), CH2Ph (5, 9a,b), Pri (6, 10a,b), (CH2)2COOEt (7, 11a,b)
Table 1. Some physicochemical properties of the synthesized
dipeptides
series Ala < Phe < Val < Glu. Thus, as in the case of
4ꢀisopropylꢀ2ꢀphenylꢀ5ꢀoxazolone formed from Nꢀbenꢀ
zoylvaline,5 the (R)ꢀisomer of 5(4H)ꢀoxazolone (3b) reꢀ
acts faster with (S)ꢀamino esters, resulting in the preꢀ
dominant formation of the (R,S)ꢀdiastereomer of the
dipeptide. (S)ꢀOxazolone (3a) reacts more slowly, and
during the reaction it is converted into the (R)ꢀisomer (3b).
As a result of this dynamic kinetic resolution, the dipepꢀ
tide diastereomer containing the Rꢀphenylalanine preꢀ
dominates in the mixture (see Scheme 1).
Comꢀ
pound
M.p./°C
τ */min
Mobile phase
for HPLC
R
8a
8b
9a
9b
10a
10b
11a
11b
153—155
154—155
119—121
135—137
5.67
6.17
4.20
5.71
3.97
4.83
5.46
6.06
Hexane—PriOH—MeOH
15 : 1 : 0.2
Hexane—PriOH—MeOH
10 : 0.5 : 0.2
Hexane—PriOH—MeOH
10 : 0.5 : 0.2
1H NMR and 13C NMR spectroscopic studies
gave direct evidence for the intermediate formation of
5(4H )ꢀoxazolone. Following the addition of an equimoꢀ
lar amount of ethyl chloroformate to a solution of
Nꢀacetylꢀ(S)ꢀphenylalanine and Et3N in CDCl3 at
room temperature, a signal at δ 4.45 for the C(4)H proꢀ
ton of oxazolone as a doublet of doublets of quartets
(3JC(4)H,CH = 6.7, 3JC(4)H,CH = 4.7, 5JC(4)H,CH3 = 2.0 Hz)
Hexane—CHCl3—
PriOH—MeCN
40 : 3 : 3 : 3
* HPLC retention times of the diastereomers.
taken to suppress racemization, the dipeptides contained
from 2 to 9% of the (R,S)ꢀdiastereomer. Recrystallization
from acetone gave individual (S,S)ꢀdiastereomers. The
structures of the compounds were proved by the data from
B
A
was observed in the 1H NMR spectrum, together with the
signal with δ 4.73 corresponding to αꢀCH in the starting
compound 1. The doublet for the oxazolone methyl group
is recorded at δ 2.09 and has a spin coupling constant of
2.0 Hz, whereas in the spectrum of the starting comꢀ
pound 1, this signal occurs as a singlet at δ 1.96. The
presence of longꢀrange spin—spin interaction through five
bonds between the C(4)H proton and the protons of the
methyl group at C(2) was proved using a COSYꢀLR 2D
experiment optimized for detecting longꢀrange proꢀ
ton—proton coupling constants. This homoallylic interꢀ
1
elemental analysis and H NMR spectroscopy (Table 2).
The elemental analysis data are consistent with the strucꢀ
tures of the products.
In all cases, the reaction gave diastereomer mixtures
in which the (R,S)ꢀdiastereomer 8b—11b of the correꢀ
sponding dipeptide predominated (Table 3). The process
diastereoselectivity depends on the nature of the side chain
in the amino component. The amount of the (R,S)ꢀdiaꢀ
stereomer tends to increase following complication of the
structure of the side chain in the amino component in the
1
action is a typical feature of the H NMR spectra of

Diastereoselectivity in dipeptide synthesis
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 6, June, 2004
1333
1
Table 2. Characteristic signals in the H NMR spectra of dipeptides (δ, J/Hz)
Comꢀ
pound
Phe
Amino component
αꢀCH
βꢀCHA
βꢀCHB
αꢀCH
βꢀCHn
γꢀCHn
8a
8b
9a
4.72 (dt, J = 8.0,
J = 6.9)
4.70 (td, J = 8.2,
J = 6.2)
4.65 (dt, J = 7.5,
J = 7.0)
3.06 (d, J = 6.9)
4.44 (dq,
J = J = 7.1)
4.43 (dq,
J = J = 7.2)
4.72 (ddd,
J = 7.4, J =
6.3, J = 6.0)
4.76 (dt, J =
8.1, J = 6.0)
4.40 (dd, J =
8.5, J = 5.1)
4.38 (dd, J =
8.5, J = 4.7)
4.49 (td, J =
7.6, J = 5.3)
4.46 (td, J =
7.8, J = 4.9)
1.34 (d, J = 7.1)
1.21 (d, J = 7.2)
—
3.10 (dd, J =
13.6, J = 6.2)
2.99 (dd, J =
13.6, J = 8.2)
—
3.02 (d, J = 7.0)
2.98 (dd, J = 13.8,
J = 6.3); 3.07 (dd,
J = 13.8, J = 6.0)
2.90, 2.99 (both dd,
J = 13.9, J = 6.0)
2.10 (sept.d, J =
6.9, J = 5.1)
9b
4.69 (ddd, J = 7.9,
J = 7.6, J = 7.0)
4.75 (dt, J = 7.6,
J = 7.1)
4.78 (ddd, J = 8.1,
J = 8.0, J = 6.4)
4.71 (td, J = 7.0,
J = 6.5)
*
*
10a
10b
11a
11b
3.06 (d, J = 7.1)
0.84, 0.88
(both d, J = 6.9)
0.75, 0.76
(both d, J = 6.9)
2.30 (m)
3.11 (dd, J =
13.8, J = 6.4)
3.08 (dd, J =
13.9, J = 6.5)
3.09 (dd, J =
13.7, J = 6.5)
3.04 (dd, J =
13.8, J = 8.1)
3.05 (dd, J =
13.9, J = 7.0)
3.03 (dd, J =
13.7, J = 8.0)
2.01 (sept.d, J =
6.9, J = 4.7)
2.15, 1.95
(both m)
4.75 (td, J = 8.0,
J = 6.5)
*
*
* The signals overlap with the corresponding signals of the S,Sꢀisomer.
Table 3. Effect of the structure of the side chain of the amino
Scheme 2
component on the reaction diastereoselectivity
b
Compound
Contenta
S,S R,S
α
Yield
(%)
8a,b
9a,b
AcꢀPheꢀ(S)ꢀAlaOEt
AcꢀPheꢀ(S)ꢀPheOEt
39.1 60.9 1.09 89.0
31.3 68.7 1.36 71.7
25.1 74.9 1.22 73.3
10a,b AcꢀPheꢀ(S)ꢀValOEt
11a,b AcꢀPheꢀ(S)ꢀGlu(OEt)2 10.3 89.7 1.10 70.4
a The content of diastereomers in the final product according to
HPLC data (%).
b Resolution coefficient.
5(4H )ꢀoxazolones and has been described previously7 for
2,4ꢀdimethylꢀ5ꢀoxazolone.
In the 13C NMR spectrum of oxazolone 3, the signal
for C(4) is shifted downfield (δ 65.98) with respect to that
for C(2) in the starting compound 1 (δ 54.31), whereas
the signal for the methyl group is shifted upfield (δ 13.96
vs. δ 23.21 for the starting compound). The assignment of
signals of protonated carbon atoms was confirmed by a
HETCOR 2D experiment.
The reaction gave diastereomeric oxazolones (S,S)ꢀ13
and (S,R)ꢀ13, which can be distinguished in the 1H NMR
spectra owing to the presence of two asymmetric centers.
The triplets of doublets for C(4)H of the oxazolone fragꢀ
ment are characteristic signals: δ 4.49 (3JC(4)H,CH = 5.2,
5JC(4)H,CHꢀNap = 1.0 Hz) and δ 4.51 (3JC(4)H,CH 2 = 5.2,
5JC(4)H,CHꢀNap = 2.1 Hz) for (S,S)ꢀ13 and (S,R)2ꢀ13, reꢀ
spectively.
For quantitative estimation of the relative rate of forꢀ
mation and racemization of oxazolone during the reacꢀ
tion, Nꢀ[(S)ꢀ2ꢀ(6ꢀmethoxyꢀ2ꢀnaphthyl)propionyl]ꢀ(S)ꢀ
phenylalanine8 (12) was taken as the model compound,
which was converted into oxazolone (13) in the presence
of Et3N in CDCl3 at room temperature (Scheme 2). The
1
structure of compound 13 was confirmed by H NMR
When the reaction was carried out with a stoichiometꢀ
ric amount of Et3N, the total content of oxazolones deꢀ
spectroscopy.

1334
Russ.Chem.Bull., Int.Ed., Vol. 53, No. 6, June, 2004
Krasnov et al.
0.05 mmol) were added to a solution of Nꢀacetylꢀ(S)ꢀphenylalaꢀ
nine or compound 12 (0.05 mmol) in CDCl3 (1.0 mL). The
1H NMR spectra were recorded at specified intervals, and the
yields of oxazolones 3a,b (13) were determined based on the
ratio of the integral intensities of the proton signals of the C(2)H
fragment of phenylalanine and the C(4)H fragment of oxazoꢀ
lones.
Oxazolones 13. The proton signal of the C(2)H fragment of
the starting compound 12 was observed at δ 4.65 (ddd), the
proton signal of the C(4)H fragment of oxazolones 13 occurred
at δ 4.49 and 4.51 (both td). The diastereomer ratio in a mixture
of (S,S)ꢀ13 and (R,S)ꢀ13 was determined from the integral inꢀ
tensities of the signals of the C(4)H proton.
termined 10 min after mixing the reactants was 40% and
the (S,S)ꢀ13 to (S,R)ꢀ13 ratio was 83 : 17. After 8 h, the
total content of oxazolones increased to 60% and the
(S,S)ꢀ13 to (S,R)ꢀ13 ratio was 60 : 40. After 30 h, comꢀ
plete racemization was observed. In the reaction carried
out with a 20% molar excess of Et3N, both the formation
and racemization rates of oxazolone substantially inꢀ
creased. As soon as 10 min after mixing the reactants, the
total content of oxazolone 13 was 98%. This was accomꢀ
panied by complete racemization.
The conditions found can be used for stereodirected
synthesis of stereoisomers of the dipeptides containing
difficultꢀtoꢀprepare R amino acids.
This work was financially supported by the Russian
Foundation for Basic Research (Projects No. 04ꢀ03ꢀ32344
and No. 04ꢀ03ꢀ96006ural) and President of the Russian
Federation (State Program for the Support of Leading
Scientific Schools, grant NSh 1766.2003.3).
Experimental
1
H and 13C NMR spectra were recorded on a Bruker DRX
400 spectrometer (400 and 100 MHz, respectively). HPLC of
the compounds synthesized was carried out on a Milikhrom
4ꢀUF chromatograph using Silasorbꢀ60 as the sorbent, a
64×2 mm column, detection at 230 nm, and the elution rate of
200 µL min–1. The melting points were measured on a Boetius
hot stage and were not corrected.
Nꢀ[(S)ꢀ2ꢀ(6ꢀMethoxyꢀ2ꢀnaphthyl)propionyl]ꢀ(S)ꢀphenylꢀ
alanine8 and Nꢀacetylꢀ(S)ꢀphenylalanine9 were synthesized by
previously described procedures. Esterification of amino acids
with ethanol was carried out by a known procedure.10
References
1. M. Calmes, C. Glot, T. Michel, M. Rolland, and J. Martinez,
Tetrahedron: Asymmetry, 2000, 11, 737.
2. I. Tomida, Sh. Senada, T. Kuwabara, and K. Katayma,
Agric. Biol. Chem., 1976, 40, 2033.
3. R. S. Ward, Tetrahedron: Asymmetry, 1995, 6, 1475.
4. H. Pellissier, Tetrahedron, 2003, 59, 8291.
5. F. Weygand, W. Steglich, and X. Barocio de la Lama, Tetraꢀ
hedron, 1966, 8, 9.
6. The Peptides. Analysis, Synthesis, Biology. V. 1. Major Methods
of Peptide Bond Formation, Eds E. Gross and J. Meienhofer,
Academic Press, New York—San Francisco—London, 1979.
7. F. Weygand, W. Steglich, D. Mayer, and W. von Philipsborn,
Chem. Ber., 1964, 97, 2023.
8. G. L. Levit, L. V. Anikina, Yu. B. Vikharev, A. M. Demin,
V. A. Safin, T. V. Matveeva, and V. P. Krasnov, Khim.ꢀ
Farm. Zhurn., 2002, 36, No. 5, 12 [Pharm. Chem. J., 2002,
36 (Engl. Transl.)].
9. B. Weinstein and A. E. Pritchard, J. Chem. Soc., Perkin
Trans. 2, 1972, 8, 1017.
Synthesis of a mixture of compounds 8a,b—11a,b (general
procedure). Triethylamine (0.61 mL, 4.34 mmol) and BuiOCOCl
(0.48 mL, 3.62 mmol) were added dropwise with stirring to a
solution of Nꢀacetylꢀ(S)ꢀphenylalanine (0.75 g, 3.62 mmol) in
THF (17 mL) cooled to –12 °C. The reaction mixture was stirred
for 30 min at –13 °C, and a mixture prepared from an (S)ꢀamino
acid ethyl ester hydrochloride (3.62 mmol) and Et3N (0.51 mL,
3.62 mmol) in THF (9 mL) cooled to –10 °C was added. The
reaction mixture was stirred for 1 h at –12 °C, for 1 h at 0 °C,
and for 3 h at ~20 °C, and kept without stirring for 17 h. The
precipitate was filtered off and the filtrate was concentrated
in vacuo. The residue was dissolved in chloroform, washed with
a 5% solution of NaHCO3, water, 5% HCl, and water, and dried
with Na2SO4. The chloroform solution was concentrated in vacuo
10. W. I. Humphlett and C. V. Wilson, J. Org. Chem., 1961,
26, 2507.
1
to dryness. The residue was analyzed by H NMR spectroscopy
and HPLC.
Detection of oxazolones 3a,b and 13 (general procedure).
Triethylamine (0.05 or 0.06 mmol) and EtOCOCl (0.005 mL,
Received December 28, 2003;
in revised form April 30, 2004