Stereoselective synthesis of a new chiral synthon: a cyclic pseudodipeptide containing an aspartic acid derivative and l-valine

Article abstract of DOI:10.1016/j.tetasy.2006.05.021

A simple stereoselective synthesis of cyclic synthons 4a,b and 10a,b, derived from an l-valine unit and an unnatural derivative of the aspartic acid, has been accomplished starting from the chiral synthon 1 and following the procedure already reported by

Full text of DOI:10.1016/j.tetasy.2006.05.021

Tetrahedron: Asymmetry 17 (2006) 1521–1528
Stereoselective synthesis of a new chiral synthon: a cyclic
pseudodipeptide containing an aspartic acid derivative and L-valine
Daniele Balducci, Alessandro Grandi, Gianni Porzi* and Sergio Sandri*
`
Dipartimento di Chimica ‘G.Ciamician’, Universita di Bologna, Via Selmi 2, 40126 Bologna, Italy
Received 29 March 2006; accepted 22 May 2006
Abstract—A simple stereoselective synthesis of cyclic synthons 4a,b and 10a,b, derived from an L-valine unit and an unnatural derivative
of the aspartic acid, has been accomplished starting from the chiral synthon 1 and following the procedure already reported by us for the
synthesis of similar compounds. These cyclic synthons are interesting substrates because they can behave as both electrophiles and nucleo-
philes and could be useful starting materials for the preparation of higher peptides such as, for instance, the unnatural tetrapeptides 15
and 18, through the condensation of 4a with 6a or 13 with 17, respectively.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
uation of our studies in this area, we herein report a simple
stereoselective synthesis of cyclic synthons 4 (Scheme 1)
and 10 (Scheme 2) derived from an L-valine unit and unnat-
ural a-aminoacids derivatives of aspartic acid. In our
opinion, these diastereomeric synthons, which can be
Previously, we reported the stereocontrolled synthesis of
unnatural di- and tripeptides containing an L-valine unit
and a cyclic nonproteinogenic a-aminoacid.1,2 As a contin-
OEt
OEt
OEt
N
N
i
N
iii
ii
5
2
N
CH2CO2CH2Ph
N
N
R
R
Ph
Ph
Ph
O
O
O
1
OEt
3
2
OEt
COOEt
NH2.HCl
N
N
v
iv
5
CH2CO2CH2Ph
CH2COOH
CH2CO2CH2Ph
HN
HN
HN
R
R
R
O
O
O
4
6
5
R = a) CH3; b) CH2-Ph
Scheme 1. Reagents and conditions: (i) LHMDS/THF, R–X; (ii) LHMDS/THF, BrCH2CO2CH2Ph; (iii) Li/NH3; (iv) Br–CH2Ph, Et3N in acetone; (v)
0.5 M HCl.
*
Corresponding authors. Fax: +39 051 2099512; e-mail: gianni.porzi@unibo.it
0957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.05.021

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D. Balducci et al. / Tetrahedron: Asymmetry 17 (2006) 1521–1528
OEt
OEt
OEt
N
N
N
5
i
iii
ii
2
CH2COOEt
N
N
N
CH2COOEt
R
Ph
Ph
Ph
O
O
O
1
8
7
OEt
OEt
OEt
N
N
N
v
iv
CH2COOH
CH2CO2CH2Ph
CH2COOH
N
HN
HN
R
R
R
Ph
O
O
O
vi
9
10
11
COOEt
NH2.HCl
5
CH2CO2CH2Ph
HN
R = a) CH3; b) CH2-Ph
R
O
12
Scheme 2. Reagents and conditions: (i) LHMDS/THF, BrCH2CO2Et; (ii) LHMDS/THF, R–X; (iii) NaOH/EtOH; (iv) Li/NH3; (v) Br–CH2Ph, Et3N in
acetone; (vi) 0.5 M HCl.
considered as masked unnatural dipeptides, are interesting
because they can act as both electrophiles and nucleophiles
and thus be useful as starting materials for preparing higher
peptides. When synthon 4 or 10 is converted into the corre-
sponding activated ester (for instance, a pentafluorophenyl-
ester), it can be used as an electrophile to accomplish a
coupling reaction with a C-protected peptide. As shown,
we synthesized pseudotetrapeptide 15 through the conden-
sation of synthon 4a with pseudodipeptide 6a (Scheme 3).
Analogously, we synthesized the masked pseudotetrapep-
tide 18 by using the unnatural dipeptide 17, easily obtained
from the intermediate 2a, as a nucleophile (Scheme 4).
Alternatively, by converting the diastereomeric synthon 4
or 10 into 6 or 12, respectively, we obtained unnatural
dipeptides, which can be used as nucleophiles by reaction
with an N-protected a-aminoacid or peptide.
can be easily alkylated twice at the C-2 position with suit-
able electrophiles. Deprotonation of 1 with LHMDS at
ꢀ78 °C, followed by alkylation with the appropriate halo-
derivative (iodomethane or benzylbromide), afforded dia-
stereomer 2a,b in good chemical yield and with a largely
prevalent (de 40% for 2a and 80% for 2b) 1,4-trans stereo-
selection (ascertained by NOE measurements) with respect
to the isopropyl group, as already observed1–6 in analogous
cases (Scheme 1).
Monoalkyl intermediate 2a,b, was submitted to a second
alkylation by using benzylbromoacetate as electrophile,
to give derivative 3a,b. The reaction occurred in good yield
and with practically total regio- and 1,4-trans-selection
(de P96%), which was ascertained by NOE experiments.
After removal of the benzyl groups by the Birch reaction,
intermediate 3a,b was converted into the chiral synthon
4a,b, which afforded the corresponding benzyl ester 5a,b
after esterification with benzylbromide. Subsequent acid
hydrolysis under mild conditions allowed us to obtain the
pseudodipeptide 6a,b as the hydrochloride salt.
Thus, we believe that such new synthons, 4 and 10, can be
considered as masked unnatural dipeptides, useful starting
materials for the synthesis of more complex structures.
The strategy followed to synthesize the chiral synthons 4
and 10 is based on the experience previously acquired in
analogous stereoselective approaches,3–6 that is starting
from the chiral mono-lactim ether 1 easily obtained from
L-valine. Herein we report a simple stereoselctive route
occurring in good chemical yield and stereochemical
control.
Diastereomeric pseudodipeptides 12a,b, with an opposite
configuration at the C-5 stereocentre with respect to 6a,b,
were obtained by inverting the alkylation sequence
described above. In the strategy reported in Scheme 2, we
employed ethylbromoacetate because in this case the
alkylation of 1 with benzylbromoacetate occurred in an
inexplicably poor yield. Then, ester 7, which was obtained
in good yield and 70% de, was alkylated with either iodo-
methane or benzylbromide affording 8a,b with a de >96%.
Intermediate ester 8a,b was hydrolyzed in an alkaline med-
ium to 9a,b and the benzyl group then removed by a Birch
reaction. Acid derivative 10a,b was converted into the
2. Results and discussion
To synthesize chiral synthons 4 and 10, we followed the
strategy based on the use of the mono-lactim ether 1, which

D. Balducci et al. / Tetrahedron: Asymmetry 17 (2006) 1521–1528
1523
OEt
N
i
4a
+ 6a
CH2CO2Pfp
HN
ii
O
OEt
13
CH2CO2CH2Ph
O
N
H
N
N
HN
H
O
iii
O
EtO2C
14
4a
+
6a
iv
CO2Et
CH2CO2CH2Ph
O
HCl.H2N
red =
L-Valine ethyl ester
H
N
N
HN
blue = (2R)-methyl Asprticacid
H
O
O
EtO2C
15
Scheme 3. Reagents and conditions: (i) pentafluorophenyl trifluoroacetate (CF3CO2Pfp) in CH2Cl2, pyridine; (ii) 6a in refluxing THF; (iii) HBTU in
CH3CN, Et3N at rt; (iv) 0.5 M HCl in EtOH at rt.
OEt
COOEt
N
NH2.HCl
ii
5
i
2
NH
2a
HN
O
O
16
17
OEt
O
N
red = L-Valine ethyl ester
H
N
HN
H
iii
N
+
blue = (2R)-methyl Asprtic acid
purple = -Alanine
13
17
O
D
O
EtO2C
18
Scheme 4. Reagents and conditions: (i) Li/NH3; (ii) 0.5 M HCl, EtOH at rt; (iii) dry THF, Et3N at rt.
benzylester 11a,b and then the pseudodipeptide 12a,b was
obtained as a hydrochloride salt after acid hydrolysis of
the lactim ether function under mild conditions.
pling reaction between 4a and the C protected pseudo-
dipeptide 6a. For this purpose, we converted 4a into the
corresponding activated ester 13 by treatment with penta-
fluorophenyl trifluoroacetate in the presence of pyridine.
However, the subsequent coupling reaction occurred very
slowly also in refluxing THF, and the product was recov-
ered in an unsatisfactory yield, in addition to other
In order to show that synthon 4 (a masked unnatural
dipeptide) can be considered a useful starting material to
synthesize more complex structures, we performed a cou-

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D. Balducci et al. / Tetrahedron: Asymmetry 17 (2006) 1521–1528
by-products being generated. Conversely, intermediate 14
was obtained in a satisfactory yield by carrying out the
reaction directly on 4a and by employing the activating
reagent HBTU in CH3CN at room temperature.7 After
acid hydrolysis of lactim derivative 14 under mild conditions,
pseudotetrapeptide 15, containing two L-valine and two
(2R)-methyl-aspartic acid units, was then recovered
(Scheme 3).
sized by alkylating 1 with benzylbromide and following the
procedure used for 2a. After purification by silica gel chro-
matography, eluting with hexane/ethyl acetate, the product
1
was recovered as an oil in 85% yield. H NMR d: 0.92 (d,
3H, J = 7); 1.02 (d, 3H, J = 7.9); 1.26 (t, 3H, J = 7); 2.20
(m, 1H); 3.37 (d, 1H, J = 5.1); 3.51 (dd, 1H, J = 1.8, 3.3);
3.82 (d, 1H, J = 15); 4.16 (m, 2H); 4.40 (m, 1H); 5.51 (d,
1H, J = 15); 6.82 (m, 2H); 7.30 (m, 8ArH). 13C NMR d:
13.9, 17.0, 19.6, 31.1, 39.3, 46.4, 59.0, 60.7, 61.0, 125.7,
127.0, 127.3, 127.4, 128.3, 130.4, 135.6, 138.1, 158.3,
169.1. HPLC–MS: 365.2 [M+H]+, 387.2 [M+Na]+, 751.4
[2M+Na]+. IR (CHCl3): 1648.3 cmꢀ1 (mC@O), 1693 cmꢀ1
(mC@N). The product was not isolated in sufficiently pure
form for elemental analysis or to measure the specific
rotation.
It is interesting to note that using reaction of the activated
ester 13 with the nucleophile 17 (obtained from 2a through
the Birch reaction and successive acid hydrolysis), the
masked pseudotetrapeptide 18 [containing two L-valine
units, one (2R)-methyl-aspartic acid and one D-alanine]
was obtained in good yield (Scheme 4).
In our opinion, the different reactivity observed for the
nucleophiles 6a and 17 towards the activated ester 13 can
be ascribed to steric factors: in fact, the two nucleophiles
show a considerable difference in the steric hindrance near
the amine group.
3.1.3. (2R,5S)-(4-Benzyl-6-ethoxy-5-isopropyl-2-methyl-3-
oxo-2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid benzylester
3a. To a solution of 2a (14.35 g, 49.8 mmol) in dry THF
(150 mL) and cooled at ꢀ78 °C, a solution of 1 M LHMDS
in THF (50 mL, 50 mmol) was added dropwise under
stirring. After about 1 h, benzylbromoacetate (9 mL,
56.8 mmol) was added and the reaction mixture kept under
stirring at ꢀ78 °C until the reaction was practically com-
plete (about 3 h). Then, water and ethyl acetate were added
and after separation the organic solution was evaporated in
vacuo. The residue was submitted to purification by silica
gel chromatography, eluting with hexane/ethyl acetate
and the pure product was recovered as an oil in 88% yield.
1H NMR d: 0.89 (d, 3H, J = 7); 1.05 (d, 3H, J = 7); 1.18 (t,
3H, J = 7); 1.53 (s, 3H); 2.2 (m, 1H); 2.71 (d, 1H, J = 15.8);
3.4 (d, 1H, J = 15.8); 3.73 (d, 1H, J = 2.6); 3.79–4.07 (m,
2H); 4.18 (d, 1H, J = 15); 5.05 (qAB, 2H, J = 12.4); 5.25
(d, 1H, J = 15); 7.33 (m, 10ArH). 13C NMR d: 13.9, 17.0,
20.5, 29.0, 29.3, 46.7, 47.0, 58.6, 60.7, 61.7, 65.7, 127.3,
127.9, 128.2, 128.3, 128.4, 128.5, 135.9, 136.0, 156.9,
170.4, 171.6. [a]D = ꢀ6.8 (c 1.4, CHCl3). HPLC–MS:
437.5 [M+H]+, 459.5 [M+Na]+, 475.5 [M+K]+, 896
[2M+Na]+. IR (CHCl3): 1647.5 cmꢀ1 (mC@O), 1694 cmꢀ1
(mC@N), 1738 cmꢀ1 (mC@O). Anal. Calcd for C26H32N2O4:
C, 71.53; H, 7.39; N, 6.42. Found: C, 71.85; H, 7.37;
N, 6.4.
The pseudotetrapeptide 15 thus obtained is unusual
because it is C-terminal at both the ends of the chain,
while the two (2R)-methyl-aspartic acid (an unnatural
a-aminoacid) units are linked to each other.
Finally, we believe that our simple protocol, utilized to syn-
thesize small peptidomimetic structures, provides interest-
ing access to compounds, which could exhibit therapeutic
effects similar to natural peptides with the advantage of
metabolic stability.8
3. Experimental
3.1. General information
1H and 13C NMR spectra were recorded on a Gemini spec-
trometer at 300 MHz using CDCl3 as the solvent. Chemical
shifts are reported in parts per million relative to CDCl3
and the coupling constants (J) are in hertz. IR spectra were
recorded on a Nicolet 210 spectrometer. Optical rotation
values were measured at 25 °C on a Perkin–Elmer 343
polarimeter. Dry THF was distilled from sodium benzo-
phenone ketyl. Chromatographic separations were per-
formed with silica gel 60 (230–400 mesh). Mass spectra
were obtained with HPLC Agilent technologies HP1100
series equipped with diode array detector. MS analysis
was obtained by using a Gemini 3U C18 110A column,
H2O/CH3CN as solvent at 25 °C in a linear gradient from
70/30 to 20/80, respectively, and a flow rate of 0.4 mL/min.
The mass revelator was a Hewlett Packard 1100 MSD ser-
ies equipped with API-ES interface and a single quadruple
detector.
3.1.4.
(2R,5S)-(2,4-Dibenzyl-6-ethoxy-5-isopropyl-3-oxo-
2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid benzylester
3b. Compound 3b was obtained starting from 2b, follow-
ing the procedure used for 3a. The product was recovered
pure as a wax in 90% yield with elution by silica gel chro-
matography with hexane/ethyl acetate. 1H NMR d: 0.33 (d,
3H, J = 7); 0.95 (d, 3H, J = 7.0); 1.22 (t, 3H, J = 7); 1.97
(m, 1H); 2.68 (d, 1H, J = 16.2); 3.06 (d, 1H, J = 12.8);
3.28 (d, 1H, J = 12.8); 3.35 (d, 1H, J = 16.2); 3.68 (d,
1H, J = 2.2); 4.0 (m, 2H); 4.37 (d, 1H, J = 15); 4.86 (d,
1H, J = 15); 5.01 (qAB 2H, J = 12.6); 7.30 (m, 15 ArH).
13C NMR d: 13.8, 15.0, 20.5, 29.0, 45.3, 46.8, 48.0, 60.4,
62.2, 62.4, 65.6, 126.4, 126.8, 127.5, 127.8, 127.9, 128.0,
128.1, 131.0, 135.6, 135.9, 136.3, 157.6, 170.1, 170.5.
[a]D = ꢀ26.3 (c 1, CHCl3). HPLC–MS: 513.5 [M+H]+,
535.5 [M+Na]+, 1048 [2M+Na]+. IR (CHCl3):
1647.5 cmꢀ1 (mC@O), 1694 cmꢀ1 (mC@N), 1738 cmꢀ1 (mC@O).
Anal. Calcd for C32H36N2O4: C, 74.97; H, 7.08; N, 5.46.
Found: C, 75.21; H, 7.06; N, 5.45.
3.1.1. (2R,5S)-4-Benzyl-6-ethoxy-5-isopropyl-2-methyl-3-
oxo-2,3,4,5-tetrahydro-pyrazine 2a. Synthesis and spec-
troscopic data are reported in Ref. 6.
3.1.2.
(2R,5S)-2,4-Dibenzyl-6-ethoxy-5-isopropyl-3-oxo-
2,3,4,5-tetrahydro-pyrazine 2b. Compound 2b was synthe-

D. Balducci et al. / Tetrahedron: Asymmetry 17 (2006) 1521–1528
1525
3.1.5.
(2R,5S)-(6-Ethoxy-5-isopropyl-2-methyl-3-oxo-
(mC@N), 1738.5 cmꢀ1 (mC@O), 3405 cmꢀ1 (mNH). Anal. Calcd
for C19H26N2O4: C, 65.87; H, 7.56; N, 8.09. Found: C,
65.74; H, 7.59; N, 8.11.
2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid 4a. A solution
of 3a (5.28 g, 12.11 mmol) in 30 mL of dry THF/tert-buta-
nol 9:1 was added to about 100 mL of liquid ammonia
cooled at ꢀ50 °C. Then, Li (0.34 g, 48.44 mmol) was added
in small pieces and the reaction monitored by TLC. The
addition of Li was stopped as soon as the reaction mixture
became blue, the starting material having disappeared. The
reaction was then quenched with NH4Cl and the cooling
bath removed to allow the complete evaporation of NH3.
After the addition of water and CH2Cl2, the aqueous solu-
tion was acidified to pH = 4 with dilute HCl and the organ-
ic solution evaporated to dryness under vacuum. The pure
3.1.8. (2R,5S)-(2-Benzyl-6-ethoxy-5-isopropyl-3-oxo-2,3,4,5-
tetrahydro-pyrazin-2-yl)-acetic acid benzyl ester 5b. Com-
pound 5b was obtained starting from 4c, and following the
procedure used for 5a. The product was recovered pure as
an oil in 70% yield. 1H NMR d: 0.05 (d, 3H, J = 7); 0.68 (d,
3H, J = 7); 1.24 (t, 3H, J = 7); 1.95 (m, 1H); 2.74 (d, 1H,
J = 16.2); 2.87 (d, 1H, J = 12.8); 3.33 (d, 1H, J = 12.8);
3.34 (d, 1H, J = 16.2); 3.68 (m, 1H); 4,08 (m, 2H); 5.07
(s, 2H); 5.69 (br s, 1H); 7.25 (m, 10ArH). 13C NMR d:
14.2, 14.6, 18.1, 29.5, 45.5, 46.6, 58.3, 61.1, 62.9, 66.2,
126.6, 127.3, 128.3, 128.5, 131.0, 135.7, 136.0, 158.2,
170.6, 171.9. [a]D = +2.7 (c 0.7, CHCl3). HPLC–MS:
423.5 [M+H]+, 445.5 [M+Na]+, 868 [2M+Na]+. IR
(CHCl3): 1645 cmꢀ1 (mC@O), 1694.4 cmꢀ1 (mC@N),
1738 cmꢀ1 (mC@O), 3404 cmꢀ1 (mNH). Anal. Calcd for
C25H30N2O4: C, 71.07; H, 7.16; N, 6.63. Found: C, 71.35;
H, 7.13; N, 6.61.
1
product was recovered as a wax in 84% yield. H NMR d:
0.89 (d, 3H, J = 6.9); 1.02 (d, 3H, J = 6.9); 1.29 (t, 3H,
J = 7.2); 1.47 (s, 3H); 2.30 (m, 1H); 2.30 (m, 1H); 2.73
(d, 1H, J = 16); 3.05 (d, 1H, J = 16.5); 4.0 (m, 1H); 4.12
(m, 2H); 7.37 (s, 1H). 13C NMR d: 14.0, 16.1, 18.5, 28.6,
30.5, 30.9, 45.3, 57.8, 58.7, 61.5, 157.7, 174.3, 174.9.
[a]D = ꢀ9.2 (c 0.6, CHCl3). HPLC–MS: 257.5 [M+H]+,
536 [2M+Na]+. IR (CHCl3): 1650–1750 cmꢀ1 (mC@O and
m
C@N), 2400–3300 cmꢀ1 (broad, mOH), 3403 cmꢀ1 (mNH).
Anal. Calcd for C12H20N2O4: C, 56.23; H, 7.87; N, 10.93.
Found: C, 55.98; H, 7.89; N, 10.95.
3.1.9. Dipeptide [(EtO)Val-(2R)-methyl-Asp(OBn)]ÆHCl
6a. To a solution of 5a (0.84 g, 2.42 mmol) in ethanol
(10 mL), 0.5 M HCl (10 mL, 5 mmol) was added and the
reaction mixture stirred at room temperature for 20 h.
The acid solution was evaporated in vacuo and the inter-
mediate hydrochloride was isolated as a wax in quantita-
3.1.6. (2R,5S)-(2-Benzyl-6-ethoxy-5-isopropyl-3-oxo-2,3,4,5-
tetrahydro-pyrazin-2-yl)-acetic acid 4b. Compound 4b
was obtained starting from 3b and following the procedure
used for 4a. The product was recovered pure as a wax in
1
tive yield. H NMR d: 0.92 (m, 6H); 1.24 (t, 3H, J = 7);
1
85% yield. H NMR d: 0.01 (d, 3H, J = 7); 0.70 (d, 3H,
1.88 (s, 3H); 2.22 (m, 1H); 3.35 (br s, 2H); 4.18 (m, 2H);
4.48 (dd, 1H, J = 5.1, 7.8); 5.14 (s, 2H); 7.37 (m, 5ArH);
8.13 (d, 1H, J = 7.8); 8.99 (br s, 3H). 13C NMR d: 14.0,
18.0, 18.8, 22.8, 30.7, 39.8, 58.2, 58.9, 61.3, 67.2, 128.2,
128.3, 128.4, 134.8, 169.5, 171.2, 171.3. [a]D = ꢀ9.6
(c 0.7, CHCl3). HPLC–MS: 365.4 [MꢀHCl+H]+, 387.4
[MꢀHCl+Na]+, 729.8 [2MꢀHCl+H]+. IR (CHCl3):
J = 7); 1.25 (t, 3H, J = 7); 1.95 (m, 1H); 2.72 (d, 1H,
J = 16.8); 2.86 (d, 1H, J = 12.4); 3.19 (d, 1H, J = 16.8);
3.31 (d, 1H, J = 12.4); 3.80 (br s, 1H); 4.13 (q, 2H,
J = 7); 7.18 (m, 5ArH); 7.53 (br s, 1H). 13C NMR d:
14.2, 14.5, 18.4, 29.3, 44.8, 46.2, 58.2, 61.1, 62.4, 126.6,
127.8, 131.0, 135.9, 158.3, 174.2, 175.0. [a]D = +3.2 (c
1.4, CHCl3). HPLC–MS: 333.3 [M+H]+, 455.3 [M+Na]+,
687.6 [2M+Na]+. IR (CHCl3): 1650–1750 cmꢀ1 (mC@O
and mC@N), 2400–3300 cmꢀ1 (broad, mOH), 3400 cmꢀ1
(mNH). Anal. Calcd for C18H24N2O4: C, 65.04; H, 7.28; N,
8.43. Found: C, 65.25; H, 7.26; N, 8.41.
1682.5 cmꢀ1 (mC@O), 1734 cmꢀ1 (mC@O), 3000 cmꢀ1 ðmNH Þ,
þ
3
3420 cmꢀ1 (mNH). Anal. Calcd for C19H29ClN2O5: C,
56.92; H, 7.29; N, 6.99. Found: C, 57.11; H, 7.28; N, 6.97.
3.1.10. Dipeptide [(EtO)Val-(2R)-benzyl-Asp(OBn)]ÆHCl
6b. Compound 6b was obtained starting from 5b, follow-
ing the procedure used for 6a. The product was recovered
3.1.7. (2R,5S)-(6-Ethoxy-5-isopropyl-2-methyl-3-oxo-2,3,4,5-
tetrahydro-pyrazin-2-yl)-acetic acid benzyl ester 5a. To a
solution of 4a (2.83 g, 11.05 mmol) in acetone (30 mL)
and Et3N (1.32 mL, 11.05 mmol), benzylbromide
(1.44 mL, 12.16 mmol) was added and the reaction mixture
stirred at room temperature for about 30 h. The reaction
mixture was evaporated to dryness under vacuum and to
the residue was added water and ethylacetate. The organic
solution was evaporated in vacuo and the residue was sub-
mitted to silica gel chromatography, eluting with hexane/
ethyl acetate. The pure product was recovered as an oil
1
pure as a wax in quantitative yield. H NMR d: 0.92 (m,
6H); 1.34 (t, 3H, J = 7); 2.17 (m, 1H); 3.04 (d, 1H,
J = 18.2); 3.30 (s, 2H); 3.64 (d, 1H, J = 18.2); 4.30 (m,
3H); 5.22 (s, 2H); 7.40 (m, 10ArH). 13C NMR d: 14.9,
19.2, 19.8, 31.9, 39.7, 43.0, 60.5, 62.7, 62.9, 68.9, 129.7,
130.0, 130.1, 130.4, 132.0, 133.6, 136.9, 170.8, 171.3,
173.0.
HPLC–MS:
441.5
[MꢀHCl+H]+,
463.5
[MꢀHCl+Na]+, 882 [2MꢀHCl+H]+. IR (CHCl3):
1680.2 cmꢀ1 (mC@O), 1738 cmꢀ1 (mC@O), 2980 cmꢀ1 ðmNH Þ,
þ
3
3425 cmꢀ1 (mNH). The product was not isolated in suffi-
1
in 70% yield. H NMR d: 0.85 (d, 3H, J = 7); 0.94 (d,
ciently pure form for the elemental analysis or to measure
3H, J = 7); 1.23 (t, 3H, J = 7); 1.45 (s, 3H); 2.25 (m, 1H);
2.67 (d, 1H, J = 16.2); 3.28 (d, 1H, J = 16.2); 3.74 (m,
1H); 4.03 (m, 2H); 5.08 (qAB, 2H, J = 12.2); 5.58 (br s,
1H); 7.32 (m, 5ArH). 13C NMR d: 13.5, 15.8, 17.8, 28.6,
30.1, 45.3, 57.7, 58.1, 60.3, 65.3, 127.5, 127.7, 127.8,
135.5, 157.2, 169.8, 173.5. [a]D = ꢀ12.8 (c 0.6, CHCl3).
HPLC–MS: 347.5 [M+H]+, 369.5 [M+Na]+, 716
[2M+Na]+. IR (CHCl3): 1645.5 cmꢀ1 (mC@O), 1694.7 cmꢀ1
the specific rotation.
3.1.11. (2R,5S)-(4-Benzyl-6-ethoxy-5-isopropyl-3-oxo-2,3,4,5-
tetrahydro-pyrazin-2-yl)-acetic acid ethyl ester 7. To a
solution of 1 (7.4 g, 27 mmol) in dry THF (100 mL) and
cooled at ꢀ78 °C, a solution of 1 M LHMDS in THF
(27 mL, 27 mmol) was added dropwise under stirring.
After about 1 h, ethylbromoacetate (3 mL, 27 mmol) was

1526
D. Balducci et al. / Tetrahedron: Asymmetry 17 (2006) 1521–1528
added, then the reaction mixture was allowed to warm up
to room temperature and kept under stirring, until the
reaction was practically complete (overnight). After the
addition of water and ethyl acetate, the organic solution
was separated and then evaporated in vacuo. The residue
was submitted to purification by silica gel chromatography,
eluting with hexane/ethyl acetate. The pure product was
recovered as an oil in 80% yield. 1H NMR d: 0.98 (d,
3H, J = 7); 1.08 (d, 3H, J = 7); 1.24 (t, 3H, J = 7); 1.28
(t, 3H, J = 7); 2.26 (m, 1H); 2.95 (dd, 1H, J = 6.3, 15.9);
3.08 (dd, 1H, J = 5.7, 15.9); 3.72 (dd, 1H, J = 1.8, 4.5);
3.97 (d, 1H, J = 15); 4.02–4.30 (m, 4H); 4.48 (m, 1H);
5.51 (d, 1H, J = 15); 7.31 (m, 5ArH). 13C NMR d: 13.7,
13.8, 17.3, 19.6, 31.2, 38.7, 47.0, 55.1, 59.7, 60.9, 61.9,
127.2, 127.5, 128.3, 135.8, 159.5, 169.1, 171.1.
[a]D = +31.2 (c 0.6, CHCl3). HPLC–MS: 361.5 [M+H]+,
383.5 [M+Na]+, 744 [2M+Na]+. IR (CHCl3):
1646.5 cmꢀ1 (mC@O), 1695.5 cmꢀ1 (mC@N), 1740 cmꢀ1
(mC@O). Anal. Calcd for C20H28N2O4: C, 66.64; H, 7.83;
N, 7.77. Found: C, 66.45; H, 7.86; N, 7.79.
centrated under vacuum and the residue was acidified to
pH = 4 with diluted HCl in the presence of CH2Cl2. The
organic solution was separated and then evaporated in
vacuo. The product was recovered as a wax in quantitative
1
yield. H NMR d: 0.89 (d, 3H, J = 7); 1.10 (d, 3H, J = 7);
1.31 (t, 3H, J = 7); 1.60 (s, 3H); 2.29 (m, 1H); 2.76 (d, 1H,
J = 15.6); 3.14 (d, 1H, J = 15.6); 3.84 (m, 1H); 3.94 (d, 1H,
J = 15); 4.13 (m, 2H); 5.52 (d, 1H, J = 15); 7.30 (m, 5ArH).
13C NMR d: 13.6, 16.3, 20.1, 29.3, 29.5, 45.1, 46.6, 58.1,
61.0, 61.3, 127.6, 127.8, 128.6, 135.3, 156.9, 171.3, 173.2.
[a]D = ꢀ1.0 (c 0.8, CHCl3). HPLC–MS: 347.4 [M+H]+,
369.4 [M+Na]+, 385.4 [M+K]+, 715.8 [2M+Na]+. IR
(CHCl3): 1650–1750 cmꢀ1 (mC@O and
mC@N), 2400–
3300 cmꢀ1 (broad, mOH). Anal. Calcd for C19H26N2O4: C,
65.87; H, 7.56; N, 8.09. Found: C, 65.63; H, 7.59; N, 8.11.
3.1.15. (2S,5S)-(2,4-Dibenzyl-6-ethoxy-5-isopropyl-3-oxo-
2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid 9b. Com-
pound 9b was obtained starting from 8b, following the pro-
cedure used for 9a. The product was recovered pure as a
wax in quantitative yield. 1H NMR d: 0.82 (d, 3H,
J = 7); 0.85 (d, 3H, J = 7 Hz); 1.27 (t, 3H, J = 7); 2.01
(m, 1H); 2.97 (d, 1H, J = 15.4); 3.12 (m, 2H); 3.48 (d,
1H, J = 15.4); 3.97 (d, 1H, J = 15); 4.23 (m, 2H); 5.03 (d,
1H, J = 15); 6.70 (m, 2ArH); 7.25 (m, 8ArH) 12.18 (br s,
1H). 13C NMR d: 14.0, 16.1, 20.1, 29.0, 46.1, 46.4, 46.9,
60.4, 61.6, 63.0, 126.7, 127.4, 128.0, 128.4, 130.6, 134.4,
135.8, 158.4, 169.4, 172.9. [a]D = ꢀ52.6 (c 0.4, CHCl3).
HPLC–MS: 423.5 [M+H]+, 445.5 [M+Na]+, 461.5
[M+K]+, 868 [2M+Na]+. IR (CHCl3): 1650–1750 cmꢀ1
(mC@O and mC@N), 2400–3300 cmꢀ1 (broad, mOH). Anal.
Calcd for C25H30N2O4: C, 71.07; H, 7.16; N, 6.63. Found:
C, 71.4; H, 7.13; N, 6.61.
3.1.12. (2S,5S)-(4-Benzyl-6-ethoxy-5-isopropyl-2-methyl-3-
oxo-2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid ethyl ester
8a. Compound 8a was obtained by the alkylation of 7
with CH3I and following the procedure used to obtain 7.
The product was recovered pure as an oil in 85% yield, after
elution by silica gel chromatography with hexane/ethyl ace-
1
tate. H NMR d: 0.95 (d, 3H, J = 7); 1.08 (d, 3H, J = 7);
1.25 (m, 6H); 1.58 (s, 3H); 2.23 (m, 1H); 2.82 (br s, 2H);
3.75 (d, 1H, J = 2.2); 4.01 (d, 1H, J = 15); 4.16 (m, 4H);
5.46 (d, 1H, J = 15); 7.30 (m, 5ArH). 13C NMR d: 13.5,
13.7, 16.2, 20.1, 29.3, 29.4, 45.1, 46.3, 58.6, 59.5, 60.3,
61.0, 127.0, 127.4, 128.2, 135.9, 155.7, 169.7, 171.0. [a]D =
ꢀ3.6 (c 0.5, CHCl3). HPLC–MS: 375.5 [M+H]+, 772
[2M+Na]+. IR (CHCl3): 1647 cmꢀ1 (mC@O), 1693.8 cmꢀ1
(mC@N), 1739.2 cmꢀ1 (mC@O). Anal. Calcd for C21H30N2O4:
C, 67.35; H, 8.07; N, 7.48. Found: C, 67.56; H, 8.06; N, 7.45.
3.1.16.
(2S,5S)-(6-Ethoxy-5-isopropyl-2-methyl-3-oxo-
2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid 10a. Com-
pound 10a was obtained starting from 9a, following the
procedure used for 4a. The product was recovered as a
1
3.1.13. (2S,5S)-(2,4-Dibenzyl-6-ethoxy-5-isopropyl-3-oxo-
2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid ethyl ester
8b. Compound 8b was obtained by the alkylation of 7
with benzylbromide and following the procedure used to
obtain 7. The product was recovered as an oil in 80% yield
after elution by silica gel chromatography with hexane/
ethyl acetate. 1H NMR d: 0.83 (d, 3H, J = 7); 0.90
(d, 3H, J = 7); 1.28 (t, 3H, J = 7); 1.30 (t, 3H, J = 7);
1.98 (m, 1H); 3.03 (m, 3H); 3.23 (d, 1H, J = 12.4); 3.44
(d, 1H, J = 12.4); 4.20 (m, 5H), 4.83 (d, 1H, J = 15.4);
6.75 (m, 2ArH); 7.20 (m, 8ArH). 13C NMR d: 13.9, 14.0,
16.0, 20.3, 29.0, 45.9, 46.6, 47.2, 60.0, 60.6, 61.0, 63.3,
126.3, 126.8, 127.5, 127.6, 128.1, 130.6, 135.4, 136.5,
157.3, 169.3, 170.0. HPLC–MS: 451.5 [M+H]+, 473.5
[M+Na]+, 489.5 [M+K]+. IR (CHCl3): 1650 cmꢀ1 (mC@O),
1694.8 cmꢀ1 (mC@N), 1738.5 cmꢀ1 (mC@O). The product was
not isolated in sufficiently pure form for elemental analysis
or to measure the specific rotation.
wax in 90% yield. H NMR d: 0.87 (d, 3H, J = 7); 1.05
(d, 3H, J = 7); 1.32 (t, 3H, J = 7); 1.48 (s, 3H); 2.38 (m,
1H); 2.87 (qAB, 2H, J = 15.9); 4.05 (m, 1H); 4.16 (q, 2H,
J = 7); 7.15 (br s, 1H). 13C NMR d: 13.8, 15.9, 18.1, 27.9,
30.3, 44.6, 57.9, 61.7, 157.7, 174.1, 174.7. [a]D = ꢀ4.3 (c
0.3, CHCl3). HPLC–MS: 257.3 [M+H]+, 535.6
[2M+Na]+. IR (CHCl3): 1650–1750 cmꢀ1 (mC@O and
m
C@N), 2400–3300 cmꢀ1 (broad, mOH), 3401.7 cmꢀ1 (mNH).
Anal. Calcd for C12H20N2O4: C, 56.23; H, 7.87; N, 10.93.
Found: C, 56.43; H, 7.84; N, 10.91.
3.1.17.
(2S,5S)-(2-Benzyl-6-ethoxy-5-isopropyl-3-oxo-
2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid 10b. Com-
pound 10b was obtained starting from 9b, following the
procedure used for 4a. The product was recovered pure
1
as a wax in 90% yield. H NMR d: 0.69 (d, 3H, J = 7);
0.79 (d, 3H, J = 7); 1.29 (t, 3H, J = 7); 2.02 (m, 1H); 2.52
(d, 1H, J = 2.2); 2.84 (d, 1H, J = 15.8); 2.85 (d, 1H,
J = 12.6); 3.20 (d, 1H, J = 12.6); 3.24 (d, 1H, J = 15.8);
4.20 (m, 2H); 7.20 (m, 5ArH); 7.60 (br s, 1H); 11.92 (br
s, 1H). 13C NMR d: 14.0, 15.9, 17.9, 29.7, 44.5, 46.4,
57.4, 61.6, 62.7, 126.9, 127.8, 130.5, 135.0, 159.0, 173.1,
174.6. [a]D = ꢀ69 (c 0.2, CHCl3). HPLC–MS: 333.5
[M+H]+, 688 [2M+Na]+. IR (CHCl3): 1650–1750 cmꢀ1
3.1.14. (2S,5S)-(4-Benzyl-6-ethoxy-5-isopropyl-2-methyl-3-
oxo-2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid 9a. To 8a
(2.61 g, 6.98 mmol) dissolved in ethanol (10 mL) was added
2 M NaOH (10.5 mL, 21 mmol) and the solution stirred for
24 h at room temperature. The reaction mixture was con-

D. Balducci et al. / Tetrahedron: Asymmetry 17 (2006) 1521–1528
1527
(mC@O and
m
C@N), 2400–3300 cmꢀ1 (broad,
m
OH),
ered as a wax in quantitative yield. 1H NMR d: 1.05
(d, 3H, J = 7); 1.08 (d, 3H, J = 7); 1.32 (t, 3H, J = 7);
2.28 (m, 1H); 3.06 (d, 1H, J = 18); 3.65 (d, 1H, J = 18);
4.23 (m, 2H), 4.39 (m, 1H); 5.23 (s, 2H); 7.38 (m,
10ArH). 13C NMR d: 14.9, 19.5, 19.9, 24.5, 31.9, 39.3,
43.0, 60.7, 62.6, 68.8, 129.9, 130.5, 131.9, 133.7, 137.0,
171.3, 172.8, 172.9. HPLC–MS: 441.5 [MꢀHCl+H]+,
3400.8 cmꢀ1 (mNH). Anal. Calcd for C18H24N2O4: C,
65.04; H, 7.28; N, 8.43. Found: C, 65.23; H, 7.26; N, 8.41.
3.1.18. (2S,5S)-(6-Ethoxy-5-isopropyl-2-methyl-3-oxo-2,3,4,5-
tetrahydro-pyrazin-2-yl)-acetic acid benzyl ester 11a. Com-
pound 11a was obtained starting from 10a, following the
procedure used for 5a. The product was recovered pure
as a wax in 70% yield, after elution by silica gel chromato-
graphy with hexane/ethyl acetate. 1H NMR d: 0.90 (d, 3H,
J = 7); 1.02 (d, 3H, J = 7); 1.22 (t, 3H, J = 7); 1.62 (s,
3H); 2.34 (m, 1H); 2.73 (d, 1H, J = 15); 3.16 (d, 1H,
J = 15); 4.04 (m, 3H); 5.09 (qAB, 2H, J = 12.4); 5.74 (br
s, 1H); 7.30 (m, 5ArH). 13C NMR d: 13.9, 16.0, 18.2,
28.3, 30.3, 44.6, 58.1, 58.5, 61.1, 65.6, 65.9, 127.8, 127.9,
128.2, 136.1, 157.2, 170.1, 173.7. [a]D = ꢀ5.2 (c 0.6,
CHCl3). HPLC–MS: 347.4 [M+H]+, 348.4 [M+2H]+,
385.4 [M+K]+, 715.8 [2M+Na]+. IR (CHCl3):
1644.7 cmꢀ1 (mC@O), 1695.2 cmꢀ1 (mC@N), 1737.3 cmꢀ1
(mC@O), 3404 cmꢀ1 (mNH). Anal. Calcd for C19H26N2O4:
C, 65.87; H, 7.56; N, 8.09. Found: C, 66.1; H, 7.54; N, 8.07.
463.5 [MꢀHCl+Na]+, 904 [2MꢀHCl+Na]+. IR (CHCl3):
ꢀ1
1685 cm (mC@O), 1735.5 cmꢀ1 (mC@O), 2990 cmꢀ1 ðmNH Þ,
þ
3
3430 cmꢀ1 (mNH). The product was not isolated in suffi-
ciently pure form for elemental analysis or to measure
the specific rotation.
3.1.22.
(2R,5S)-(6-Ethoxy-5-isopropyl-2-methyl-3-oxo-
2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid pentafluoro-
phenyl ester 13. To a stirred solution of 4a (1.28 g,
5 mmol) in dry CH2Cl2 (20 mL) at 0 °C, pyridine
(0.4 mL, 5 mmol) was added followed by pentafluorophen-
yl trifluoroacetate (1.3 mL, 7.5 mmol). The cooling bath
was then removed, the reaction stirred at rt for about 6 h
and monitored by TLC. When the reaction was complete,
CH2Cl2 (10 mL) was added, the organic solution washed
with water, dried over CaCl2 and then evaporated to dry-
ness in vacuo. The crude reaction product was purified
by silica gel chromatography, eluting with hexane/ethyl
acetate and the pure product was obtained as a wax in
3.1.19.
(2S,5S)-(2-Benzyl-6-ethoxy-5-isopropyl-3-oxo-
2,3,4,5-tetrahydro-pyrazin-2-yl)-acetic acid benzyl ester
11b. Compound 11b was obtained starting from 10c, fol-
lowing the procedure used for 5a. The product was recov-
ered as a wax in 70% yield, after elution by silica gel
1
90% yield. H NMR d: 0.91 (d, 3H, J = 7); 1.02 (d, 3H,
1
chromatography with hexane/ethyl acetate. H NMR d:
J = 7); 1.28 (t, 3H, J = 7.4); 1.55 (s, 3H); 2.32 (m, 1H);
2.97 (d, 1H, J = 16.2); 3.56 (d, 1H, J = 16.2); 4.03 (m,
1H); 4.13 (m, 2H); 5.93 (s, 1H). 13C NMR d: 13.9, 16.3,
18.4, 29.1, 30.8, 44.8, 55.4, 59.0, 61.4, 133.0–148.0, 134–
148 (m, C–F), 158.3, 166.9, 173.2. [a]D = ꢀ0.2 (c 0.6,
CHCl3). HPLC–MS: 423.1 [M+H]+, 424.1 [M+2H]+,
445.1 [M+Na]+, 446.2 [M+H+Na]+, 462.1 [M+H+K]+,
867.2 [2M+Na]+, 868.2 [2M+H+Na]+. IR (CHCl3):
1668.7 cmꢀ1 (mC@O), 1793.6 cmꢀ1 (mC@N), 1783 cmꢀ1
(mC@O), 3400 cmꢀ1 (mNH). Anal. Calcd for C18H19F5N2O4:
C, 51.59; H, 4.53; N, 6.63. Found: C, 51.73; H, 4.51; N,
6.61.
0.77 (d, 3H, J = 7); 0.78 (d, 3H, J = 7); 1.29 (t, 3H,
J = 7); 2.05 (m, 1H); 2.54 (d, 1H, J = 2.7); 2.90 (d, 1H,
J = 15.6); 2.91 (d, 1H, J = 12.9); 3.24 (d, 1H, J = 12.9);
3.42 (d, 1H, J = 15.6); 4.17 (m, 2H); 5.12 (qAB, 2H,
J = 12.3); 5.43 (br s, 1H); 7.27 (m, 10ArH). 13C NMR d:
14.0, 15.8, 17.9, 29.6, 44.7, 46.7, 57.6, 61.2, 63.2, 65.8,
126.8, 127.7, 127.8, 128.0, 128.3, 130.6, 135.5, 136.1,
158.5, 170.0, 171.6. HPLC–MS: 423.5 [M+H]+, 868
[2M+Na]+.
IR
(CHCl3):
1645.3 cmꢀ1
(mC@O),
1694.7 cmꢀ1 (mC@N), 1737.8 cmꢀ1 (mC@O), 3402.5 cmꢀ1
(mNH). The product was not isolated in sufficiently pure
form for elemental analysis or to measure the specific
rotation.
3.1.23. (2R,5S,20S,50R)-(6-Ethoxy-5-isopropyl-2-methyl-3-
oxo-2,3,4,5-tetrahydro-pyrazin-2-yl)-80-[30,60-diaza-40,70-
dioxo-2-isopropyl-50-methyl-50-(acetic acid benzylester)]-
ethyloctanoate 14. Under an inert atmosphere, a solution
of 6a (1.72 g, 4.8 mmol) and triethylamine (1.2 mL,
14.4 mmol) was added dropwise into a stirred solution of
4a (1.24 g, 4.8 mmol) and HBTU (2.06 g, 5.4 mmol) in
CH3CN (100 mL). The reaction mixture was stirred at rt
and after about 24 h, CH2Cl2 and then 0.1 M HCl
(50 mL) were added. The organic extract was dried over
CaCl2 and evaporated in vacuo. The crude reaction prod-
uct was then purified by silica gel chromatography, eluting
with hexane/ethyl acetate. The pure product was obtained
as an oil in 80% yield. 1H NMR d: 0.90 (m, 12H); 1.25 (m,
6H); 1.44 (s, 3H); 1.61 (s, 3H); 2.20 (m, 2H); 2.51 (d, 1H,
J = 14.6); 2.86 (d, 1H, J = 14.6); 2.98 (d, 1H, J = 16.6);
3.25 (d, 1H, J = 16.6); 4.15 (m, 5H); 4.43 (dd, 1H,
J = 5.1, 7.8); 5.10 (s, 2H); 6.34 (br s, 1H); 7.16 (br s, 1H);
7.35 (d, 1H, J = 7.8); 7.37 (m, 5ArH). 13C NMR d: 14.1,
14.2, 16.2, 17.8, 18.4, 18.9, 21.0, 23.4, 28.7, 30.7, 31.0,
40.0, 48.1, 57.7, 58.3, 58.7, 60.3, 61.0, 66.4, 128.1, 128.2,
128.5, 135.5, 151.0, 170.4, 170.9, 171.9, 172.9, 173.5.
3.1.20. Dipeptide [(OEt)Val-(2S)-methyl-Asp(OBn)]ÆHCl
12a. Compound 12a was obtained starting from 11a,
and following the procedure used for 6a. The product
was recovered pure as a wax in quantitative yield. 1H
NMR d: 1.00 (d, 3H, J = 7); 1.01 (d, 3H, J = 7); 1.29 (t,
3H, J = 7); 168 (s, 3H); 2.21 (m, 1H); 3.10 (d, 1H,
J = 18.2); 3.48 (d, 1H, J = 18.2); 4.18 (q, 2H; J = 7.2);
4.39 (d, 1H, J = 6.8); 5.23 (br s, 2H); 7.40 (br s, 5ArH).
13C NMR d: 14.1, 18.2, 18.9, 22.6, 30.3, 40.0, 58.6, 58.8,
61.2, 67.3, 128.2, 128.3, 128.5, 134.8, 170.0, 171.1, 171.2.
[a]D = ꢀ4.5 (c 0.4, CHCl3). HPLC–MS: 365.4
[MꢀHCl+H]+, 387.4 [MꢀHCl+Na]+, 729.8 [2MꢀHCl+H]+.
IR (CHCl3): 1683 cmꢀ1 (mC@O), 1734.8 cmꢀ1 (mC@O),
3000 cmꢀ1 ðmNH Þ, 3425 cm
(mNH). Anal. Calcd for
ꢀ1
þ
3
C19H29ClN2O5: C, 56.92; H, 7.29; N, 6.99. Found: C,
56.73; H, 7.32; N, 7.01.
3.1.21. Dipeptide [(OEt)Val-(2S)-benzyl-Asp(OBn)]ÆHCl
12b. Compound 12b was obtained starting from 11c, fol-
lowing the procedure used for 6a. The product was recov-

1528
D. Balducci et al. / Tetrahedron: Asymmetry 17 (2006) 1521–1528
[a]D = ꢀ4.3 (c 0.8, CHCl3). HPLC–MS: 603.3 [M+H]+,
625.3 [M+Na]+, 641.3 [M+K]+, 1227.6 [2M+Na]+. IR
(CHCl3): 1650 cmꢀ1 (mC@O), 1695 cmꢀ1 (mC@N), 1734 cmꢀ1
(mC@O), 3405 cmꢀ1 (mNH). Anal. Calcd for C31H46N4O8:
C, 61.78; H, 7.69; N, 9.3. Found: C, 61.89; H, 7.66; N,
9.27.
47.52; H, 8.37; N, 11.08. Found: C, 47.33; H, 8.39; N,
11.13.
3.1.27. (2R,5R,20S,50R)-(6-Ethoxy-5-isopropyl-2-methyl-3-
oxo-2,3,4,5-tetrahydro-pyrazin-2-yl)-80-(30,60-diaza-40,70-
dioxo-2-isopropyl-50-methyl)-ethyloctanoate 18. Under an
inert atmosphere, activated ester 13 (0.312 g, 0.74 mmol)
in dry THF (10 mL) was added to a stirred solution of
17 (0.16 g, 0.74 mmol), dissolved in THF (5 mL) and trieth-
ylamine (0.1 mL, 0.74 mmol) and the reaction monitored
by TLC. After about 12 h at rt, CH2Cl2 (15 mL) was added
and the organic solution washed with diluted HCl and then
with water. The organic extract was dried over CaCl2 and
evaporated to dryness in vacuo. The crude reaction prod-
uct was purified by the silica gel chromatography eluting,
with hexane/ethyl acetate. The pure product was recovered
as a white waxy solid in 85% yield. 1H NMR d: 0.90 (d, 3H,
J = 7); 0.92 (d, 3H, J = 7) 0.96 (d, 3H, J = 7); 1.03 (d, 3H,
J = 7); 1.28 (t, 3H, J = 7); 1.30 (t, 3H, J = 7); 1.39 (d, 3H,
J = 7.2); 1.49 (s, 3H); 2.18 (m, 1H); 2.30 (m, 1H); 2.77 (d,
1H, J = 14.7); 2.93 (d, 1H, J = 14.7); 4.17 (m, 5H); 4.56 (m,
2H); 6.20 (br s, 1H); 6.80 (d, J = 8.7); 7.01 (d, 1H, J = 7.2).
13C NMR d: 13.9, 14.0, 16.3, 17.6, 18.2, 18.3, 18.8, 28.0,
30.8, 30.9, 46.8, 48.4, 57.0, 58.3, 58.5, 60.9, 61.4, 157.4,
170.1, 171.8, 172.4, 173.4. [a]D = +32.2 (c 0.9, CHCl3).
HPLC–MS: 455.4 [M+H]+, 477.4 [M+Na]+, 931.8
[2M+Na]+. IR (CHCl3): 16450.2 cmꢀ1 (mC@O), 1694 cmꢀ1
(mC@N), 1735.5 (mC@O), 3402 cmꢀ1 (mNH). Anal. Calcd for
C22H38N4O6: C, 58.13; H, 8.43; N, 12.33. Found: C, 58.4;
H, 8.41; N, 12.29.
3.1.24. Tetrapeptide [(OEt)Val-(2R)-methyl-Asp-(2R)-
methyl-Asp(OBn)-Val(OEt)]ÆHCl 15. A solution of 14
(0.468 g, 0.8 mmol) in 6.2 mL of ethanol and 3.1 mL of
0.5 M HCl was stirred at rt for about 5 h. The acid solution
was evaporated in vacuo and the product as hydrochloride
was isolated as a wax in quantitative yield. 1H NMR d: 0.93
(d, 3H, J = 7); 0.94 (d, 3H, J = 7); 0.98 (d, 3H, J = 7); 0.99
(d, 3H, J = 7); 1.28 (t, 6H, J = 7); 1.62 (s, 3H); 1.65 (s, 3H);
2.20 (m, 2H); 2.98 (qAB, 2H, J = 16.4); 3.22 (qAB, 2H,
J = 17.8); 4.22 (m, 6H); 5.16 (s, 2H); 7.37 (m, 5ArH). 13C
NMR d: 14.8, 19.1, 19.4, 19.8, 22.6, 24.4, 31.7, 32.2, 40.4,
41.9, 59.8, 59.9, 60.0, 60.5, 62.6, 62.7, 67.9, 129.6, 129.7,
129.9, 137.6, 170.7, 172.1, 172.2, 173.0, 173.6, 175.1.
HPLC–MS: 621.7 [MꢀHCl+H]+, 622.7 [MꢀHCl+2H]+,
643.7 [MꢀHCl+Na]+, 1242.4 [2MꢀHCl+H]+. IR (CHCl3):
1682.5 cmꢀ1 (mC@O), 1734 cmꢀ1 (mC@O), 2980 cmꢀ1 ðmNH Þ,
þ
3
3200–3400 cmꢀ1 (mNH). The product was not isolated in suf-
ficiently pure form for elemental analysis or to measure the
specific rotation.
3.1.25. (2R,5S)-(6-ethoxy-5-isopropyl-2-methyl-3-oxo)-2,3,4,5-
tetrahydropyrazine 16. Compound 16 was obtained start-
ing from 2a, following the procedure used for 4a. After
quenching with NH4Cl, and evaporation of NH3, water
and CH2Cl2 were added and the organic extract evaporated
to dryness under vacuum. The pure product was recovered
Acknowledgement
1
as an oil in quantitative yield. H NMR d: 0.89 (d, 3H,
J = 7); 1.01 (d, 3H, J = 7); 1.3 (t, 3H, J = 7); 1.49 (d, 3H,
J = 7); 2.25 (m, 1H); 3.5 (q, 1H, J = 7); 3.9 (m, 1H); 4.15
(m, 2H); 5.8–6.1 (br s, 1H). 13C NMR d: 13.8, 16.0, 17.9,
20.4, 32.0, 53.2, 58.2, 60.7, 158.2, 173.4. [a]D = +101.2 (c
0.8, CHCl3). HPLC–MS: 199.3 [M+H]+, 221.3 [M+Na]+,
419.6 [2M+Na]+. IR (CHCl3): 1648.5 cmꢀ1 (mC@O),
1694 cmꢀ1 (mC@N), 3402 cmꢀ1 (mNH). Anal. Calcd for
C10H18N2O2: C, 60.58; H, 9.15; N, 14.13. Found: C,
60.71; H, 9.12; N, 14.09.
Thanks are due to the University of Bologna for the finan-
cial support (‘Ricerca Fondamentale Orientata’, ex 60%).
References
1. Balducci, D.; Crupi, S.; Galeazzi, R.; Piccinelli, F.; Porzi, G.;
Sandri, S. Tetrahedron: Asymmetry 2005, 16, 1103.
2. Balducci, D.; Grandi, A.; Porzi, G.; Sandri, S. Tetrahedron:
Asymmetry 2005, 16, 1453.
3.1.26. Dipeptide [(OEt)Val-(2R)-Ala]ÆHCl 17. Com-
pound 17 was obtained starting from 16, following the pro-
cedure used for 15. The product was recovered pure as a
3. Balducci, D.; Emer, E.; Piccinelli, F.; Porzi, G.; Recanatini,
M.; Sandri, S. Tetrahedron: Asymmetry 2005, 16, 3785.
4. Galeazzi, R.; Garavelli, M.; Grandi, A.; Monari, M.; Porzi,
G.; Sandri, S. Tetrahedron: Asymmetry 2003, 14, 2639.
5. Balducci, D.; Porzi, G.; Sandri, S. Tetrahedron: Asymmetry
2004, 15, 1085.
6. Balducci, D.; Grandi, A.; Porzi, G.; Sabatino, P.; Sandri, S.
Tetrahedron: Asymmetry 2004, 15, 3929.
7. Montalbetti, C. A. G. N.; Falque, V. Tetrahedron 2005, 61,
10827, and references cited therein.
1
wax in quantitative yield. H NMR d: 0.98 (m, 6H); 1.30
(t, 3H, J = 7); 1.56 (d, 3H, J = 7); 2.23 (m, 1H); 4.09 (q,
1H, J = 7); 4.22 (q, 2H, J = 7); 4.40 (dd, 1H, J = 5.1,
8.4); 8.62 (d, 1H, J = 8.4). 13C NMR d: 14.8, 18.5, 18.7,
19.8, 32.1, 50.5, 59.5, 62.6, 171.8, 173.0. [a]D = ꢀ9.4 (c
1.5, CHCl3). HPLC–MS: 217.3 [MꢀHCl+H]+, 239.3
[MꢀHCl+Na]+, 455.6 [2MꢀHCl+Na]+. IR (CHCl3):
8. Kahn, M. Synlett 1993, 821; Genin, M. J.; Johnson, L. J. Am.
Chem. Soc. 1992, 114, 8778; Smith, J. A.; Pease, L. G. CRC
Crit. Rev. Biochem. 1980, 8, 315.
1680 cmꢀ1 (mC@O), 1739 cmꢀ1 (mC@O), 2980 cmꢀ1 ðmNH Þ,
þ
3
3400 cmꢀ1 (mNH). Anal. Calcd for C10H21ClN2O3: C,