Deprotonation of the peptide NH groups and diastereoselective hydrogenation of N-acetyl-α,β-dehydrodipeptides

Article abstract of DOI:10.1023/A:1011397803696

The peptide protons in N-acetyl-α,β-dehydrodipeptides (DHDP) dissociate in aqueous methanol in the presence of magnesium salts upon the addition of alkali, which favors the diastereoselectivities of their hydrogenation over Pd/C. By contrast, the N-H bond

Full text of DOI:10.1023/A:1011397803696

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38
Russian Chemical Bulletin, International Edition, Vol. 50, No. 4, pp. 738—739, April, 2001
Deprotonation of the peptide NH groups and
diastereoselective hydrogenation of N-acetyl-α,β-dehydrodipeptides
«
I. N. Lisichkina, A. G. Ivanova, A. S. Peregudov, and V. M. Belikov†
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul Vavilova, 117813 Moscow, Russian Federation.
Fax: +7 (095) 135 5085. E-mail: inlis@ineos.ac.ru; vmbel@ineos.ac.ru
The peptide protons in N-acetyl-α,β-dehydrodipeptides (DHDP) dissociate in aqueous
methanol in the presence of magnesium salts upon the addition of alkali, which favors the
diastereoselectivities of their hydrogenation over Pd/C. By contrast, the N—H bonds of the
N-acetyl groups in DHDP seem not to dissociate under these conditions. The dissociation of
the peptide N—H bonds in strongly alkaline solutions was studied by 19F NMR spectroscopy
for model compounds containing the 4-FC H fragment.
6
4
Key words: N-acetyl-α,β-dehydrodipeptides, deprotonation of the peptide NH groups,
1
9
diastereoselective hydrogenation, F NMR spectroscopy.
Dissociation of the peptide NH protons is a decisive
pH
3
factor in complexation of peptides with transition met-
als. These protons dissociate in aqueous media in the
presence of a metal cation even at pH ∼ 5, but they
cannot be titrated without the metal cation at pH < 12.1
In the present work, it was shown that the deprotona-
tion of the peptide NH groups of N-acetyl-α,β-de-
hydrodipeptide (DHDP) in alcoholic media at pH > 12
in the presence of Mg salts (non-transition metal) favors
complexation. According to a general rule, the forma-
tion of cyclic complexes strongly limits conformational
mobility and hence increases asymmetric induction in
chemical reactions. We studied the diastereoselective
hydrogenation of complexes of N-acetyl-α,β-dehydro-
phenylalanyl-(S)-valine (1), N-acetyl-α,β-dehydro-
phenylalanyl-(S)-leucine (2), and N-acetyl-α,β-dehydro-
phenylalanyl-(S)-proline (3) with Mg2+ over Pd/C in
alkaline media to the corresponding saturated N-acetyl-
dipeptides.
1
1
1
9
7
1
2
n
Fig. 1. Potentiometric titration of complex 1•MgCl2 in 95%
MeOH (n is the number of equivalents of NaOH).
The potentiometric titration of 1 in the presence of
MgCl2 in 95% ÌåÎÍ shows that only the COOH
protons dissociate, while the peptide protons are not
involved in complexation at ðÍ < 12 (Fig. 1). Above
pH 12, these cannot be seen in the titration curve since
the range of dissociation is beyond the scope of the
method. Nevertheless, it turned out that the amount of
NHAc
NH
NHAc
NaOH used to prepare DHDP complexes with Mg2+
Ph
R
Ph
N
markedly influences the diastereomeric excess in their
hydrogenation, as can be seen in Table 1.
O
COOH
O
COOH
We attributed this effect to the formation of more
rigid Mg chelates upon deprotonation of the peptide NH
groups. To prove this hypothesis, we examined methan-
olic solutions of N-acetyl-α,β-dehydro-p-fluorophenyl-
alanine (4), N-acetyl-α,β-dehydro-p-fluorophenylalanyl-
1
, 2
3
1
: R = CHMe2
: R = CH CHMe
2
2
2
(
S)-valine (5), and p-FC H COOH all containing an
6 4
Peptides 1 and 2 contain three protons that can
dissociate, namely, a COOH proton, a peptide proton,
and an amide proton. Peptide 3 possesses no peptide
1
9
indicator F atom by
presence of different amounts of alkali. Earlier,
F NMR spectroscopy in the
3
,4
we
showed that this method provides valuable information
on the state of electron density in relevant complexes.
The results obtained are presented in Table 2.
It can be seen from the data for compound 5 that
successive addition of one and two equivalents of NaOH
shifts the signal for the 19F atom upfield by 0.34 and
proton.
Previously,2 we have shown that DHDP reacts with
Ñà or Mg salts in 95% MeOH, the acidity of the carboxy
groups increasing by two orders of magnitude.
†
Deceased.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 708—709, April, 2001.
066-5285/01/5004-738 $25.00 © 2001 Plenum Publishing Corporation
1

Deprotonation of the peptide NH groups
Russ.Chem.Bull., Int.Ed., Vol. 50, No. 4, April, 2001
739
Table 1. Effect of the amount of NaOH on the diastereoselec-
tivity of the hydrogenation of DHDP complexes with Mg2+
The ionization of only COOH and peptide NH
protons was indirectly confirmed by the hydrogenation
of complexes 3 with MgCl , which were formed in the
2
Complex
NaOH (equiv.)
de (SS)* (%)
presence of one or two equivalents of NaOH (see
Table 1). As this peptide does not contain the peptide
proton, only the COOH group is ionized, the second
equivalent of alkali producing no effect on the
stereoselectivity of the reaction.
In conclusion, it should be emphasized that 19F NMR
spectroscopy makes it possible to reveal the deprotonation
of the peptide NH groups in strongly alkaline media and
thus supplement the data from potentiostatic titration
for the range where the latter fails.
1
•MgSO4
—
1
2
3
—
1
2
—
1
10
35 (22**)
60 (33**)
52
7
2
3
•MgCl2
19
34
30
58
56
•MgCl2
2
*
*
de (RS) for complex 3•MgCl2.
* In the absence of MgSO4.
Experimental
Table 2. Effect of the amount of NaOH on the 19F chemical
shifts in complexes with 4, 5, and p-FC H COOH
1
H NMR spectra were recorded on a Bruker WP-200SY
spectrometer in CD OD. 19F NMR spectra were recorded as
6
4
3
described in Ref. 3. Potentiostatic titration was carried out
under the conditions reported in Ref. 5.
Compound
NaOH (equiv.)
–δF
All DHDP were prepared by the azlactone method accord-
4
—
1
2
—
1
2
3
—
1
2
3
2.73
0.14
0.01
1.97
1.63
0.97
1.01
6.51
1.77
1.78
1.80
ing to the known procedure.4,5
N-Acetyl-α,β-dehydro-p-fluorophenylalanine (4) was syn-
thesized by alkaline hydrolysis of 2-methyl-4-(p-fluoroben-
zylidene)oxazol-5-one, m.p. 217—218 °Ñ (from EtOH) (cf.
Ref. 6: 214—216 °Ñ).
The procedures described in Ref. 3 were used to hydroge-
nate the complexes and determine the diastereomeric ratios of
the dipeptides.
5
p-FC H COOH
6
4
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 98-03-
33034).
The 19F shift was determined with respect to PhF according to
*
Taft. More negative values indicate that the 19F signal is shifted
downfield.
References
1
.0 ppm, respectively, compared to the signal for the
1
2
. C. R. Hartzell and F. R. N. Gurd, J. Biol. Chem., 1969, 244,
47; M. R. McDonald, W. M. Scheper, H. D. Lee, and
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nonionized molecule. This suggests that the F atom is
shielded by negative charges appearing in succession in
the molecule of 5 upon dissociation of both the ÑÎÎÍ
proton of the substrate (the first equivalent) and the
peptide or amide NH proton (the second equivalent).
1
4
5, 1862 (Engl. Transl.)].
The addition of a third equivalent of alkali does not
virtually change the position of the signal for the 19F
3. I. N. Lisichkina, T. A. Larina, A. S. Peregudov, T. Yu.
Vasil´eva, and V. M. Belikov, Izv. Akad. Nauk, Ser. Khim.,
atom, indicating the ionization of only one NH proton.
1
999, 904 [Russ. Chem. Bull., 1999, 48, 895 (Engl. Transl.)].
The 19F NMR spectrum of p-FC H COOH remains
6
4
4
. I. N. Lisichkina, O. M. Ushakova, M. O. Alekseeva, A. S.
unchanged upon the addition of the second and third
equivalents of alkali, i.e., the magnetic environment of
the dipole in alkaline medium does not affect the 19F
chemical shift. In the case of compound 4, the addition
of one equivalent of alkali gives the carboxylate anion,
and a signal for the F atom is strongly shifted upfield (by
Peregudov, and V. M. Belikov, Izv. Akad. Nauk, Ser. Khim.,
1
999, 1704 [Russ. Chem. Bull., 1999, 48, 1682 (Engl. Transl.)].
5. I. N. Lisichkina, A. I. Vinogradova, N. B. Sukhorukova,
E. V. Tselyapina, M. B. Saporovskaya, and V. M. Belikov,
Izv. Akad. Nauk, Ser. Khim., 1993, 601 [Russ. Chem. Bull.,
1
993, 42, 569 (Engl. Transl.)].
. E. L. Bennett and C. Niemann, J. Am. Chem. Soc., 1950,
2, 1803.
6
2
.59 ppm). The second equivalent of alkali only slightly
7
changes its position (an additional shift of 0.13 ppm),
and therefore the amide proton seems to remain
nonionized.
Received June 19, 2000;
in revised form November 19, 2000