A new stereocontrolled synthesis of uncommon tripeptides derived from 2,6-diaminopimelic acid (2,6-DAP)

Article abstract of DOI:10.1016/S0957-4166(02)00002-2

The stereocontrolled synthesis of uncommon tripeptides 8 and 11a-c, structural variants of 2,6-diaminopimelic acid, was carried out starting from the mono-lactim ether 1 easily obtained from L-valine. The configurations of the introduced stereogenic centers were assigned on the basis of ¹H NMR spectroscopic data.

Full text of DOI:10.1016/S0957-4166(02)00002-2

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 3319–3324
A new stereocontrolled synthesis of uncommon tripeptides
derived from 2,6-diaminopimelic acid (2,6-DAP)
Francesca Paradisi,* Gianni Porzi† and Sergio Sandri*
Dipartimento di Chimica ‘G.Ciamician’ Universita` di Bologna, Via Selmi 2, 40126 Bologna, Italy
Received 29 November 2001; accepted 20 December 2001
Abstract—The stereocontrolled synthesis of uncommon tripeptides 8 and 11a–c, structural variants of 2,6-diaminopimelic acid,
was carried out starting from the mono-lactim ether 1 easily obtained from
L-valine. The configurations of the introduced
stereogenic centers were assigned on the basis of H NMR spectroscopic data. © 2002 Elsevier Science Ltd. All rights reserved.
1
1. Introduction
2. Results and discussion
Owing to our interest in new stereoselective approaches
for the synthesis of a-amino acids1 and peptides,2,3 we
have recently focused our attention towards new struc-
tural variants of 2,6-diaminopimelic acid (2,6-DAP) for
their potential antibacterial activity.4 In continuation of
our program we have directed our efforts towards
producing new tripeptides, such as 8 and 11a–c, con-
taining the 2,6-DAP acid and valine units at the ends of
the chain, i.e. Val-(2,6-DAP)-Val, the latter being
monoalkyl derivatives of 8. Such tripeptides are
unusual, because they are C-terminal at both the
ends of the chain. The strategy employed is based on
experience previously acquired on the asymmetric syn-
thesis of (+)- and (−)-2,6-DAP,5 (+)- and (−)-2,7-
diaminosuberic acid,6 (+)-o-phenylene-bis-alanine6 and
on the stereoselective synthesis of sterically constrained
dipeptides.3
In the present study we have followed a strategy which
makes use of the mono-lactim ether 4, a chiral synthon
easily synthesized starting from L-valine (Scheme 1).
When the lithium enolate of synthon 4 was treated with
0.5 equiv. of 1,3-diiodopropane in THF at −78°C a
practically total conversion into the diastereomers 5
and 6 is obtained in (Scheme 2).
The prevalence of the diastereomer 5 is controlled by
the stereochemistry of the iso-propyl group at C(6): in
fact, as already observed for similar mono-2 and bis-lac-
tim ethers,7 the trans-addition of electrophile is gener-
ally preferred because the si face of the lithium enolate
of 4 is shielded by the sterically demanding iso-propyl
group. Nevertheless, the formation of the minor
diastereomer 6 is, most probably, because the stereo-
OEt
O
OMe
OMe
OMe
N
NH
iii
iv
i
O
ii
O
O
N
N
NH
N
>98%
75%
85%
96%
Cl
NH2
Ph
O
Ph
O
Ph
Ph
O
4
2
3
1
Scheme 1. (i) PhCH2Br/pyridine in CH2Cl2; (ii) ClCH2COCl/TEA in CH2Cl2; (iii) 10 M NH3 in EtOH; (iv) Et3OBF4 in CH2Cl2.
* Corresponding authors.
†
E-mail: porzi@ciam.unibo.it
0957-4166/01/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(02)00002-2

3320
F. Paradisi et al. / Tetrahedron: Asymmetry 12 (2001) 3319–3324
OEt
5'
OEt
5''
OEt
OEt
NH3Cl
NH3Cl
O
6'
N
H
N
N
N
ii
N
N
H
N
6''
iii
2
HOOC
COOH
N
N
N
95%
H
H
3'
3''
>98%
O
Ph
O
O
Ph
O
O
i
5 (78%)
7
8
4
OEt
OEt
N
N
N
N
Ph
O
O
Ph
6 (12%)
OEt
OEt
OEt
OEt
NH3Cl
O
NH3Cl
R
iv
H
N
N
N
H
N
ii
iii
N
N
HOOC
COOH
5
N
N
N
N
85-95%
90-95%
>98%
H
H
R
O
R
Ph
O
O
Ph
O
O
9(a-e)
11(a-c)
10(a-c)
R= (a) CH2CHCH2, (b) CH2COOCH2Ph,
(c) CH2Cl, (d) CH3, (e) CH2Ph
R=(a) CH2CHCH2, (b) CH2COOH, (c) CH3
R=(a) CH2CHCH2, (b) CH2COOH, (c) CH3
Scheme 2. (i) 1 M LHMDS/THF, I(CH2)3I; (ii) Na/NH3; (iii) 1 M HCl at 60°C; (iv) 1 M LHMDS/THF, RX (see Table 1).
The second alkylation, i.e. the conversion of 5 into
9a–e, again occurs with trans stereoselectivity with
respect to the iso-propyl group, as already observed for
similar substrates.3,7 Nevertheless, the configuration of
the introduced stereogenic centre of monoalkylated 9a–
e was ascertained on the basis of the nOe measure-
ments, as already observed for similar substrates.2 On
this basis, the nOe observed for C(6%)H (at 3.74 ppm) of
derivative 9d upon irradiation of the C(3%) methyl pro-
tons (at 1.42 ppm) is consistent with a trans relationship
between the CH3 and iso-propyl groups. Confirmation
of the trans stereochemistry has been obtained from the
derivative 9e that shows a higher field resonance of the
C(6%)H (3.21 ppm) with respect to C(6%%)H (3.72 ppm).
In fact, the upfield shift is induced by the shielding
effect of the phenyl ring of the C(3%) benzyl group,
which lies in the axial-like position adopting the prefer-
ential ‘aryl inside’ arrangement. As previously observed
in analogous substrates,3 this shielding (0.5 ppm) is
clear evidence that the benzyl group lies trans with
respect to the iso-propyl group.
selectivity in the first alkylation of 4 is generally vari-
able, giving low to moderate cis-induction.2,7
As reported in Scheme 2, the monoalkylated intermedi-
ates 9a–e were obtained by adding one equivalent of
alkylating reagent (allyl bromide, benzyl bromoacetate,
chloroiodomethane, iodomethane or benzyl bromide)
to the lithium enolate of the pure isomer 5 in THF at
−78°C. The conversion of 5 into 9a–e occurs in good to
very good yields and with high trans stereocontrol as
can be seen from the data collected in Table 1. The
alkylation reactions with allyl bromide and chloro- or
iodomethane furnished a small quantity of dialkylated
product, which is easily separable by silica gel
chromatography.
The stereochemistry of the intermediates 5 and 6 was
1
established on the basis of the H NMR spectra consid-
ering that the former stereoisomer has a C2 axis. In
fact, in the product 5 the signals for both iso-propyl
groups (at 0.94 and 1.06 ppm) overlap as do the signals
for the benzylic protons (at 3.88 and 5.57 ppm), indicat-
ing their magnetic equivalence. It is also worth pointing
out the overlapping signals for the C(3%) and C(3%%)
protons at 3.69 ppm and those for the C(5%)- and
C(5%%)-OC2H5 methyl protons at 1.17 ppm.
The intermediates 9a–d were then converted into the
corresponding tripeptides 11a–c through a Birch reac-
tion (Na/NH3), which furnishes derivatives 10a–c, fol-
lowed by heterocyclic ring cleavage by hydrolysis with
1N aqueous HCl (Scheme 2).
Table 1. Diastereoselective alkylation of 5
Conversely, in the stereoisomer 6, the above-mentioned
protons being magnetically inequivalent, have different
chemical shifts, i.e. the overlap observed for 5 does not
occur (see Section 4). Thus, it can be inferred that the
stereoisomer 5 possesses trans–trans relative configura-
tion, while 6 has trans–cis configuration. Nevertheless,
the absolute configuration was unequivocally assigned
by converting 5 into the corresponding (2R,6R)-DAP
through acid hydrolysis by refluxing with 57% HI.5,6
Analogously, the stereoisomer 6 was converted into the
optically inactive (2R,6S)-meso-DAP.
Alkylating reagent
Yield (%)a of 9a–e
R
X
a- CH2CHCH2
b- CH2CO2CH2Ph
c- CH2Cl
Br
Br
I
90
95
90
85
90
d- CH3
I
e- CH2Ph
Br
a Determined by 1H NMR.

F. Paradisi et al. / Tetrahedron: Asymmetry 12 (2001) 3319–3324
3321
3. Conclusion
51.5, 62.2, 125.4, 127.3, 127.5, 127.9, 128.6, 136,
167.7, 170. [h]D −78.8 (c 1.06, CHCl3).
The reported strategy provides a new approach to the
asymmetric synthesis of unusual tripeptides (C-termi-
nal at both ends of the chain) starting from a chiral
4.4. (2S)-1-N-Benzyl-2-isopropyl-pyperazine-3,6-dione 3
synthon easily synthesized from
L
-valine. This syn-
The intermediate 2 (23 g, 77.3 mmol) was dissolved
in ca. 10 M ethanolic ammonia solution (100 mL).
The flask was sealed and the reaction stirred
overnight at rt. After evaporation of NH3, the solu-
tion was concentrated in vacuo and a mixture of
water/ethyl acetate was added to the residue. The
organic extract was concentrated under vacuum and
the residue submitted to silica gel chromatography
eluting with hexane/ethyl acetate. The pure product
was obtained as a solid in practically quantitative
thetic strategy is a versatile approach because it
allows to produce a wide variety of tripeptides, as
well those with potential biological activity.8,9 In fact,
tripeptides containing various bis(a-amino acid) units
and different terminal amino acids could be prepared
by reacting a chiral synthon obtained from a suitable
a-amino acid and an appropriate dihalo derivative.
1
yield (mp 107–109°C). H NMR l: 1.06 (d, 3H, J=
4. Experimental
7); 1.13 (d, 3H, J=7); 2.20–2.36 (m, 1H); 3.68 (d,
1H, J=4.8); 3.93 (d, 1H, J=15); 3.98 (dd, 1H, J=3,
17.6); 4.16 (d, 1H, J=17.6); 5.45 (d, 1H, J=15);
6.42–6.62 (bs, 1H); 7.31 (m, 5ArH). 13C NMR l:
17.6, 19.7, 31.8, 45.1, 48.2, 64.5, 127.8, 128.7, 135.3,
164.6, 168. [h]D +10.8 (c 1.012, CHCl3).
4.1. General information
1H and 13C NMR spectra were recorded on a Gemini
spectrometer at 300 MHz using CDCl3 as solvent,
unless otherwise stated. Chemical shifts are reported
in ppm relative to CDCl3 or to 1,4-dioxane if D2O is
used. The coupling constants (J) are in Hz. Optical
rotation values were measured on a Perkin–Elmer 343
polarimeter. Dry THF was distilled from sodium ben-
zophenone ketyl. Chromatographic separations were
performed with silica gel 60 (230–400 mesh).
4.5. (6S)-1-Benzyl-5-ethoxy-3,6-dihydro-6-isopropyl-
pyrazine-2-one 4
A solution of 3 (19 g, 77.2 mmol) in dry CH2Cl2 (100
mL) was added to triethyloxonium tetrafluoroborate
prepared from BF3/etherate (23.5 mL) and epichloro-
hydrin (11 mL) as reported in the literature.10 The
reaction mixture was stirred overnight at rt under
inert atmosphere, then it was added to phosphate
buffer solution at pH 7. The organic layer was sepa-
rated, evaporated in vacuo and the residue submitted
to chromatographic separation by silica gel eluting
with hexane/ethyl acetate. The reaction product was
recovered pure as an oil in 75% yield. 1H NMR l:
0.95 (d, 3H, J=7.2); 1.05 (d, 3H, J=7.2); 1.26 (t,
3H, J=7.4); 2.15–2.31 (m, 1H); 3.65 (m, 1H); 3.93 (d,
1H, J=15); 3.98–4.23 (m, 4H); 5.48 (d, 1H, J=15);
7.3 (m, 5ArH): 13C NMR l: 14.1, 17.4, 19.9, 31.8,
47.1, 50.8, 61.3, 127.5, 127.9, 128.6, 135.8, 160.2,
168.0. [h]D +68.0 (c 1.1, CHCl3). Anal. calcd for
C16H22N2O2: C, 70.04; H, 8.08; N, 10.21. Found: C,
70.3; H, 8.1; N, 10.18%.
4.2. (S)-N-Benzylvaline methyl ester 1
To a solution of (S)-valine methyl ester (20.1 g, 153.4
mmol) in CH2Cl2 (100 mL) and pyridine (13.5 mL,
168.8 mmol) was added benzyl bromide (20.1 mL,
168.77 mmol) at 0°C and the reaction stirred
overnight at room temperature. After addition of
water (100 mL) the organic solution was separated
and the solvent removed under vacuum. The crude
product was purified by silica gel chromatography
with hexane/ethyl acetate to afford pure product in
1
85% yield. H NMR l: 0.96 (d, 3H, J=7); 0.98 (d,
3H, J=7); 1.86–2.03 (m, 1H); 3.05 (d, 1H, J=6.2);
3.61 (d, 1H, J=13.2); 3.74 (s, 3H); 3.86 (d, 1H, J=
13.2); 7.3 (m, 5ArH). 13C NMR l: 18.5, 19.2, 31.6,
51.1, 52.4, 66.3, 126.7, 128, 139.8, 175.4. [h]D −53.9 (c
1.03, CHCl3).
4.3. (S)-N-Benzyl-N-chloroacetylvaline methyl ester 2
4.6. Conversion of 4 into 5 and 6
A solution of 1 (17.6 g, 79.6 mmol) in CH2Cl2 (100
mL) was treated with TEA (13.3 mL, 95.4 mmol) and
the solution was cooled to 0°C. Chloroacetyl chloride
(7.6 mL, 95.4 mmol) was added dropwise and the
reaction monitored by TLC. Water (100 mL) was
added, the organic layer was collected and concen-
trated in vacuo. After purification by silica gel chro-
matography with hexane/ethyl acetate, the pure
To a solution of 4 (5.4 g, 19.6 mmol), in dry THF
(100 mL) and cooled at −78°C,
a solution of
LHMDS in THF (1 M, 21.6 mL) was added drop-
wise under stirring. After about 1 h, 1,3-diiodo-
propane (1.13 mL, 9.8 mmol) was added and the
reaction was monitored by TLC. When the reaction
was complete, the mixture was allowed to warm up
to room temperature under stirring. Water and ethyl
acetate are added and after separation the organic
solution was evaporated in vacuo. The residue was
submitted to silica gel chromatography eluting with
hexane/ethyl acetate and the diastereomers 5 and 6
are separated.
1
product was obtained in 96% yield. H NMR l: 0.97
(d, 3H, J=6.8); 1.01 (d, 3H, J=6.8); 2.28–2.45 (m,
1H); 3.44 (s, 3H); 3.97 (d, 1H, J=2.2); 4.38 (d, 1H,
J=10.4); 4.75 (s, 2H); 4.90 (d, 1H, J=10.4); 7.1–7.4
(m, 5ArH). 13C NMR l: 18.2, 19.7, 27.9, 41.4, 48.2,

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F. Paradisi et al. / Tetrahedron: Asymmetry 12 (2001) 3319–3324
4.6.1. 1-[(3%R,6%S)-1%-Benzyl-5%-ethoxy-3%,6%-dihydro-6%-
isopropylpyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-1%%-benzyl-5%%-
ethoxy-3%%,6%%-dihydro-6%%-isopropylpyrazin-3%%-yl-2%%-one]
propane 5. Compound 5 was isolated pure as a solid
(mp 149–51°C) in 78% yield. 1H NMR l: 0.94 (d,
6H, J=7); 1.06 (d, 6H, J=7); 1.17 (t, 6H, J=7.2);
1.4–1.6 (m, 2H); 1.62–1.75 (m, 2H); 1.9–2.32 (m, 4H);
3.69 (dd, 2H, J=1.7, 2.2); 3.88 (d, 2H, J=15); 3.95–
4.2 (m, 6H); 5.57 (d, 2H, J=15); 7.15 (m, 10ArH);
13C NMR l: 14.1, 17.6, 19.9, 20.6, 31.5, 33.7, 47,
57.8, 61, 61.5, 127.3, 127.6, 128.5, 136.2, 158.6, 170.5.
[h]D +41.6 (c 1.0, CHCl3). Anal. calcd for
C35H48N4O4: C, 71.4; H, 8.22; N, 9.52. Found: C,
71.65; H, 8.25; N, 9.48%.
155.7, 158.7, 170.6, 171.5. [h]D −10.7 (c 1, CHCl3).
Anal. calcd for C38H52N4O4: C, 72.58; H, 8.34; N,
8.91. Found: C, 72.7; H, 8.36; N, 8.88%.
4.7.2. 1-[(3%R,6%S)-1%-Benzyl-3%-benzyloxyacetil-5%-ethoxy-
6%-hydro-6%-isopropylpyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-
1%%-benzyl-5%%-ethoxy-3%%,6%%-dihydro-6%%-isopropylpyrazin-
3%%-yl-2%%-one] propane 9b. Benzyl bromoacetate was
1
used as alkylating reagent. H NMR l: 0.85 (d, 3H,
J=6.8); 0.92 (d, 3H, J=6.8); 1.04 (d, 6H, J=7);
1.12–1.23 (m, 6H); 1.5–2.3 (m, 8H); 2.81 (d, 1H, J=
16); 3.32 (d, 1H, J=16); 3.7 (m, 2H); 3.8–4.2 (m,
6H); 4.28 (d, 1H, J=15); 5.04 (qAB, 2H, J=12); 5.1
(d, 1H, J=15); 5.47 (d, 1H, J=15); 7.3 (m, 15ArH).
13C NMR l: 14, 17, 17.5, 19.4, 19.9, 20.8, 26.8, 29.5,
31.5, 33.9, 41.7, 44, 47.1, 47.5, 57.4, 60.6, 60.9, 61.3,
61.7, 62, 65.7, 127.1, 127.3, 127.6, 127.9, 128.2, 128.5,
135.8, 136.1, 136.3, 156.8, 158.7, 170.2, 170.7, 171.6.
[h]D −3.1 (c 1.014, CHCl3). Anal. calcd for
C44H56N4O6: C, 74.97; H, 8.01; N, 7.95. Found: C,
75.2; H, 8.03; N, 7.93%.
4.6.2. 1-[(3%R,6%S)-1%-Benzyl-5%-ethoxy-3%,6%-dihydro-6%-
isopropylpyrazin-3%-yl-2%-one]-3-[(3%%S,6%%S)-1%%-benzyl-5%%-
ethoxy-3%%,6%%-dihydro-6%%-isopropylpyrazin-3%%-yl-2%%-one]
propane 6. Compound 6 was isolated pure, as an oil,
in 12% yield. 1H NMR l: 0.89 (d, 3H, J=6.9); 0.9
(d, 3H, J=6.9); 1.02 (d, 3H, J=6.9); 1.06 (d, 3H,
J=6.9); 1.19 (t, 3H, J=7.4); 1.22 (t, 3H, J=7.4);
1.5–1.8 (m, 2H); 1.9 (m, 2H); 2.07–2.27 (m, 4H); 3.65
(dd, 1H, J=1.5, 3.9); 3.68 (dd, 1H, J=2.1, 3.3); 3.90
(d, 1H, J=15); 3.92 (d, 1H, J=15.3); 4.1 (m, 6H);
5.45 (d, 1H, J=15.3); 5.47 (d, 1H, J=15); 7.25 (m,
10ArH). 13C NMR l: 14.2, 14.3, 17.6, 18, 20.1, 20.6,
23, 30.5, 31.6, 33.8, 36.5, 47, 47.2, 57.7, 60, 60.9, 61,
61.1, 61.8, 127.4, 127.5, 127.8, 128.1, 128.6, 136.1,
136.3, 157.1, 158.8, 170.5. [h]D −20.0 (c 1.014,
CHCl3).
4.7.3. 1-[(3%R,6%S)-1%-Benzyl-3%-chloromethyl-5%-ethoxy-
6%-hydro-6%-isopropylpyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-
1%%-benzyl-5%%-ethoxy-3%%,6%%-dihydro-6%-isopropylpyrazin-
3%%-yl-2%%-one] propane 9c. Chloroiodomethane was used
1
as alkylating reagent. H NMR l: 0.87 (d, 3H, J=7);
0.93 (d, 3H, J=7); 1.06 (d, 6H, J=7.4); 1.24 (t, 3H,
J=7.4); 1.26 (t, 3H, J=7.4); 1.5–1.78 (m, 2H); 1.8–
2.15 (m, 4H); 2.18–2.32 (m, 2H); 3.65 (d, 1H, J=9.6);
3.69 (m, 1H); 3.77 (d, 1H, J=2.4); 3.91 (d, 1H, J=
15); 3.97 (d, 1H, J=15); 4.05–4.18 (m, 5H); 4.18 (d,
1H, J=9.6); 5.49 (d, 1H, J=15); 5.62 (d, 1H, J=15);
7.29 (m, 10ArH). 13C NMR l: 14.2, 17, 17.6, 19.9,
20.1, 20.6, 29.5, 31.6, 34.1, 39.4, 46.6, 47.3, 53.6, 57.6,
60.7, 61.1, 61.8, 64.7, 127.5, 128.5, 128.6, 135.5,
136.3, 157.8, 158.9, 169.5, 170.4. [h]D −5.3 (c 1,
CHCl3). Anal. calcd for C36H49ClN4O4: C, 67.85; H,
8.79; Cl, 5.56. Found: C, 68.1; H, 8.81; Cl, 5.55%.
4.7. Alkylation of 5
To a solution of 5 (0.588 g, 1 mmol) in dry THF (30
mL) and cooled at −78°C was added a solution of
LHMDS in THF (1 M, 1.1 mL) under stirring. After
about 1 h, the appropriate alkylating reagent (1
mmol) was added and the reaction was then moni-
tored by TLC. When the reaction was practically
complete, the mixture was allowed to warm to room
temperature under stirring. Water and ethyl acetate
are added and after separation the organic solution
was evaporated in vacuo. The residue was submitted
to silica gel chromatography eluting with hexane/ethyl
acetate.
From the reaction mixture the following sub-products
were isolated.
cis–trans derivative (3S%,6S%,3%%R,6%%S): 1H NMR l:
0.92 (d, 6H, J=7); 1.05–1.11 (m, 6H); 1.23–1.31 (m,
6H); 1.6–2.28 (m, 8H); 3.68 (m, 2H); 3.78 (d, 1H,
J=2.2); 3.9–4.2 (m, 7H); 5.47 (d, 1H, J=15); 5.63 (d,
1H, J=15); 7.28 (m, 10ArH). 13C NMR l: 14.3, 17,
17.9, 20.6, 20.7, 21.7, 29.4, 30.4, 36.7, 39.5, 46.6, 47,
53.7, 59.6, 60.6, 60.9, 61.1, 64.7, 127.5, 127.9, 128.1,
128.6, 128.7, 135.4, 136, 157.2, 158, 169.5, 170.1.
4.7.1. 1-[(3%R,6%S)-3%-Allyl-1%-benzyl-5%-ethoxy-6%-hydro-
6%-isopropylpyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-1%%-benzyl-
5%%-ethoxy-3%%,6%%-dihydro-6%%-isopropylpyrazin-3%%-yl-2%%-
one] propane 9a. Allyl bromide was used as alkylating
reagent. 1H NMR l: 0.82 (d, 3H, J=6.8); 0.89 (d,
3H, J=6.8); 1.0–1.3 (m, 6H); 1.2 (m, 6H); 1.5–1.72
(m, 2H); 1.75–2.1 (m, 4H); 2.15–2.3 (m, 2H); 2.42
(dd, 1H, J=7.2, 13.2); 2.7 (dd, 1H, J=7.5, 13.2); 3.65
(m, 2H); 3.87 (d, 1H, J=15); 3.89 (d, 1H, J=15);
3.9–4.2 (m, 5H); 4.93–5.1 (m, 2H); 5.47 (d, 2H, J=
15); 5.42–5.61 (m, 1H); 7.25 (m, 10ArH). 13C NMR
l: 14.2, 17.1, 17.6, 19.8, 20, 20.7, 29.6, 31.5, 34.3, 41,
44.9, 46.6, 47.2, 57.7, 60.6, 60.8, 61, 61.7, 63.7, 118,
127.4, 127.9, 128.4, 128.5, 128.6, 134.2, 136, 136.4,
Dialkylated product (3%R,6%S,3%%R,6%%S)-3%,3%%dichloro-
methyl derivative: 1H NMR l: 0.91 (d, 6H, J=7);
1.07 (d, 6H, J=7); 1.28 (t, 6H, J=7.2); 1.5–1.7 (m,
2H); 1.8–2.0 (m, 4H); 2.18–2.35 (m, 2H); 3.61 (d, 2H,
J=10); 3.79 (d, 2H, J=2.4); 4.01 (d, 2H, J=15); 4.15
(m, 6H); 5.60 (d, 2H, J=15); 7.34 (m, 10ArH). 13C
NMR l: 14.2, 16.9, 18.5, 20.6, 29.3, 39.8, 46.6, 53.6,
60.7, 61, 64.5, 127.6, 128.5, 135.3, 158, 169.3. [h]D
−59.7 (c 0.63, CHCl3).

F. Paradisi et al. / Tetrahedron: Asymmetry 12 (2001) 3319–3324
3323
1
4.7.4. 1-[(3%S,6%S)-1%-Benzyl-5%-ethoxy-6%-hydro-6%-isopro-
pyl-3%-methyl-pyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-1%%-ben-
zyl-5%%-ethoxy-3%%,6%%-dihydro-6%%-isopropylpyrazin-3%%-yl-2%%-
one] propane 9d. Iodomethane was used as alkylating
isolated in 90% yield. H NMR l: 0.81 (d, 3H, J=
6.9); 0.82 (d, 3H, J=6.9); 0.96 (d, 3H, J=7.2); 0.98
(d, 3H, J=7.2); 1.27 (m, 6H); 1.5–1.7 (m, 1H); 1.75–
1.9 (m, 2H); 1.98 (m, 2H); 2.15–2.35 (m, 4H); 2.62
(dd, 1H, J=7.2, 12.9); 3.82 (dd, 1H, J=2.1, 4.8); 3.91
(dd, 1H, J=1.2, 2.7); 4.0–4.21 (m, 5H); 5.05 (m, 2H);
5.58–5.72 (m, 1H); 6.4–6.8 (bs, 2H). 13C NMR l: 14.3,
15.9, 16.7, 18.3, 19.7, 30.7, 32.2, 34.2, 39.8, 46, 57.5,
58.1, 60.8, 60.9, 63.8, 118.3, 133.3, 157.1, 158.2, 172.4,
173.3. [h]D +94.3 (c 1, CHCl3).
1
reagent. H NMR l: 0.88 (d, 3H, J=7); 0.93 (d, 3H,
J=7); 1.05 (d, 3H, J=7); 1.07 (d, 3H, J=7); 1.21 (t,
3H, J=7.4); 1.25 (t, 3H, J=7.4); 1.44 (s, 3H); 1.5–2.1
(m, 6H); 2.15–2.3 (m, 2H); 3.67 (dd, 1H, J=1.4, 3.6);
3.73 (d, 1H, J=2.6); 3.85–4.2 (m, 7H); 5.51 (d, 2H,
J=15); 7.3 (m, 10ArH). 13C NMR l: 14.3, 17.4, 17.5,
17.6, 19.8, 20.1, 20.8, 28.8, 30.1, 31.6, 34.3, 41.2, 46.7,
47.2, 57.7, 60.6, 61, 61.2, 61.7, 127.4, 127.8, 127.9,
128.6, 136.4, 136.5, 154.9, 158.7, 170.6, 173.3. [h]D
+9.1 (c 0.316, CHCl3). Anal. calcd for C36H50N4O4: C,
71.73; H, 8.36; N, 9.29. Found: C, 71.95; H, 8.38; N,
9.25%.
4.8.3. 1 - [(3%R,6%S) - 3% - Carboxymethylen - 5% - ethoxy - 6%-
hydro - 6% - isopropyl - 1%H - pyrazin - 3% - yl - 2% - one] - 3-
[(3%%R,6%%S)-5%%-ethoxy-3%%,6%%-dihydro-6%%-isopropyl-1%%H-
pyrazin-3%%-yl-2%%-one] propane 10b. Starting from 9b,
1
the pure product was isolated in 90% yield. H NMR
l: 0.66 (d, 3H, J=6.6); 0.68 (d, 3H, J=6.6); 0.83 (t,
6H, J=7.2); 0.9–1.05 (m, 2H); 1.15 (m, 6H); 1.25–1.45
(m, 2H); 1.5–1.75 (m, 2H); 1.8–2.1 (m, 2H); 2.27 (d,
1H, J=16); 2.73 (d, 1H, J=15.8); 3.76 (t, 1H, J=2.6);
3.87–3.99 (m, 4H); 4.0–4.15 (m, 2H). 13C NMR l:
14.7, 17.3, 17.8, 18.4, 18.7, 20.6, 24.4, 31.4, 34, 34.4,
41.6, 49.7, 49.8, 58.3, 59.4, 62.1, 62.3, 63.7, 159.7,
160.3, 173.6, 177.1, 178.6.
4.7.5. 1-[(3%R,6%S)-1%,3%-Dibenzyl-5%-ethoxy-6%-hydro-6%-
isopropylpyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-1%%-benzyl-5%%-
ethoxy-3%%,6%%-dihydro-6%%-isopropylpyrazin-3%%-yl-2%%-one]
propane 9e. Benzyl bromide was used as the alkylating
1
reagent. H NMR l: 0.8 (d, 3H, J=6.9); 0.91 (d, 3H,
J=6.9); 0.96 (d, 3H, J=6.9); 1.08 (d, 3H, J=6.9);
1.28 (t, 6H, J=6.9); 1.65–2.3 (m, 8H); 3.03 (d, 1H,
J=12.5); 3.21 (d, 1H, J=2.4); 3.34 (d, 1H, J=12.5);
3.72 (dd, 1H, J=1.8, 3.9); 3.88 (d, 1H, J=15.6); 3.95
(d, 1H, J=15); 4.25 (m, 5H); 5.25 (d, 1H, J=15); 5.53
(d, 1H, J=15.6); 7 (m, 15ArH). 13C NMR l: 14.2,
14.3, 16.8, 17.6, 19.9, 20, 20.5, 29.2, 31.6, 34.3, 41.9,
46.1, 46.4, 47.3, 57.7, 60.2, 60.5, 61, 61.8, 65.2, 126.1,
127.1, 127.5, 127.8, 127.9, 128, 128.4, 128.7, 130.8,
135.5, 136.5, 137.9, 156, 158.9, 170.7, 171.
4.8.4. 1-[(3%S,6%S)-5%-Ethoxy-6%-hydro-6%-isopropyl-3%-
methyl-1%H-pyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-5%%-ethoxy-
3%%,6%%-dihydro-6%%-isopropyl-1%%H-pyrazin-3%%-yl-2%%-one]
propane 10c. Starting from 9c, the pure product was
1
isolated in 95% yield. H NMR l: 0.79 (d, 3H, J=7);
0.83 (d, 3H, J=7); 0.95 (d, 3H, J=7.2); 1.03 (d, 3H,
J=7.2); 1.25 (t, 3H, J=7); 1.26 (t, 3H, J=7); 1.33 (s,
3H); 1.4–2.37 (m, 8H); 3.78 (dd, 1H, J=2.2, 4.8); 3.94
(dd, 1H, J=1, 2.8); 3.4–4.16 (m, 5H); 7.77 (bs, 1H);
7.93 (bs, 1H). 13C NMR l: 14.2, 15.9, 16.1, 16.6, 18.2,
18.4, 20.1, 29, 30.8, 32, 34.2, 40.6, 57.4, 58.1, 58.3,
60.7, 60.9, 61, 156, 158.1, 172.5, 174.7. [h]D +58.3 (c
1.01, CH3OH).
4.8. General procedure for the Birch reaction on
intermediates 5 and 9a–d
To a stirred solution of Na (0.435 g, 18.9 mmol)
dissolved in liquid ammonia (60 mL), cooled at –50°C
under an inert atmosphere, was added a solution of
intermediate 5 or 9a–d (1.35 mmol) in THF/t-butanol
9:1 (10 mL). After 5 minutes the reaction was
quenched with NH4Cl (1.07 g) and the cooling bath
was removed allowing the complete removal of NH3.
After addition of water, the aqueous solution was
extracted with ethyl acetate and the organic solution
evaporated to dryness under vacuum. The product was
recovered in very high yield.
4.9. General procedure for the hydrolysis of 7 and
10a–c to tripeptides
The hydrolysis was performed by heating 7 or 10a–c
(0.7 mmol) at 60°C in aqueous HCl (1 M, 5 mL). The
reaction was monitored by TLC and by 1H NMR.
After about 40 h the acid solution was evaporated in
vacuo and the residue submitted to NMR analysis.
The pure product was isolated in practically quantita-
tive yield.
4.8.1. 1-[(3%R,6%S)-5%-Ethoxy-3%,6%-dihydro 6%-isopropyl-
1%H-pyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-5%%-ethoxy-3%%,6%%-
dihydro-6%%-isopropyl-1%%H-pyrazin-3%%-yl-2%%-one] propane
7. Starting from 5, the pure product was isolated in
1
95% yield. H NMR l: 0.87 (d, 6H, J=7); 1 (d, 6H,
4.9.1. Tripeptide [(HO)Val - (2R,6R) - DAP - Val(OH)]·
2HCl 8. Compound 8 was obtained as a solid
(decomposes at 148°C) starting from 7. 1H NMR
l (D2O): 0.80 (d, 12H, J=6.9); 1.2–1.35 (m, 2H);
1.7–1.9 (m, 4H); 2.0–2.12 (m, 2H); 3.97 (t, 2H, J=
6.3); 4.1 (d, 2H, J=5.4). 13C NMR l (D2O): 18.1,
19.3, 20.3, 30.3, 31.2, 53.4, 59.3, 170.1, 175.4. [h]D
−56.7 (c 1.014, H2O). Anal. calcd for C17H34Cl2N4O6:
C, 44.26; H, 7.43; Cl, 15.37. Found: C, 44.42; H, 7.45;
Cl, 15.32%.
J=7); 1.29 (t, 6H, J=7); 1.4–1.65 (m, 2H); 1.9 (m,
4H); 2.15–2.35 (m, 2H); 3.9 (dd, 2H, J=2.2, 5.2); 4.03
(m, 2H); 4.09–4.21 (m, 4H), 6.07 (bs, 2H). 13C NMR
l: 14.2, 16.2, 18.2, 21, 32, 34.1, 57.6, 58.4, 61.1, 158,
172.4. [h]D +108 (c 1, CHCl3).
4.8.2. 1-[(3%R,6%S)-3%-Allyl-5%-ethoxy-6%-hydro-6%-isopro-
pyl-1%H-pyrazin-3%-yl-2%-one]-3-[(3%%R,6%%S)-5%%-ethoxy-
3%%,6%%-dihydro-6%%-isopropyl-1%%H-pyrazin-3%%-yl-2%%-one]
propane 10a. Starting from 9a, the pure product was

3324
F. Paradisi et al. / Tetrahedron: Asymmetry 12 (2001) 3319–3324
4.9.2. Tripeptide [(HO)Val-(2R,6R)-2-allyl-2,6-DAP-
Val(OH)]·2HCl 11a. 11a was obtained as a solid
Acknowledgements
1
(decomposes at 138°C) starting from 10a. H NMR l
(D2O): 0.8 (m, 12H); 1.2–1.5 (m, 2H); 1.7–2.2 (m, 6H);
2.58 (dd, 1H, J=7.4, 14.4); 2.74 (dd, 1H, J=6.6, 14.4);
4 (m, 1H); 4.15 (m, 2H), 5.18–5.25 (m, 2H); 5.50–5.67
(m, 1H). 13C NMR l (D2O): 18, 18.6, 18.9, 19.2, 30.4,
31.5, 35.6, 40.8, 53.4, 59.2, 59.9, 64, 66.2, 66.6, 123.7,
129, 170.2, 171.1, 175.3, 175.4. [h]D –26.1 (c 0.6, 1N
HCl). Anal. calcd for C20H38Cl2N4O6: C, 47.9; H, 7.64;
Cl, 14.14. Found: C, 48.1; H, 7.66; Cl, 14.1%.
Thanks are due to MURST (COFIN 2000) and to the
University of Bologna for financial support.
References
1. (a) Porzi, G.; Sandri, S. Tetrahedron: Asymmetry 1994, 5,
453; (b) D%Arrigo, M. C.; Porzi, G.; Sandri, S. J. Chem.
Res. (S) 1995, 430; (c) Porzi, G.; Sandri, S. Tetrahedron:
Asymmetry 1996, 7, 189; (d) Porzi, G.; Sandri, S. Tetra-
hedron: Asymmetry 1998, 9, 3411; (e) Di Felice, P.;
Maestri, M.; Paradisi, F.; Porzi, G.; Sandri, S. Tetra-
hedron: Asymmetry 1999, 10, 4709.
2. (a) Favero, V.; Porzi, G.; Sandri, S. Tetrahedron: Asym-
metry 1997, 8, 599; (b) Di Felice, P.; Porzi, G.; Sandri, S.
Tetrahedron: Asymmetry 1999, 10, 2191.
3. Porzi, G.; Sandri, S.; Verrocchio, P. Tetrahedron: Asym-
metry 1998, 9, 119 and references cited therein.
4. Cox, R. J. Nat. Prod. Rep. 1996, 13, 29.
5. Paradisi, F.; Porzi, G.; Rinaldi, S.; Sandri, S. Tetra-
hedron: Asymmetry 2000, 11, 1259–1262.
6. Paradisi, F.; Porzi, G.; Rinaldi, S.; Sandri, S. Tetra-
hedron: Asymmetry 2000, 11, 4617–4622.
7. Lange, M.; Undheim, K. Tetrahedron 1998, 54, 5337 and
references cited therein.
8. Bouchaudon, J.; Dutruc-Rosset, G.; Farge, D.; James, C.
J. Chem. Soc., Perkin Trans. 1 1989, 695.
9. Fischer, P. M.; Solbakken, M.; Undheim, K. Tetrahedron
1994, 50, 2277.
4.9.3. Tripeptide [(HO)Val-(2R,6R)-2-carboxymethylene-
2,6-DAP-Val(OH)]·2HCl 11b. 11b was obtained as an
1
oil starting from 10b. H NMR l (D2O): 0.78–0.84 (m,
12H); 1.1–1.5 (m, 2H); 1.72 (m, 2H); 1.95 (m, 2H); 2.13
(m, 2H); 3.04 (d, 1H, J=18.6); 3.33 (d, 1H, J=18.6);
4.02 (t, 1H, J=6.3); 4.2 (d, 1H, J=5.7); 4.22 (d, 1H,
J=5.1). 13C NMR l (D2O): 17.9, 18.1, 18.4, 19.1, 30.3,
31.2, 36, 39.5, 53.4, 59.2, 59.7, 61.5, 170.4, 170.8, 173.2,
175.3, 175.5. [h]D −41 (c 1.16, 1N HCl). Anal. calcd for
C19H36Cl2N4O8: C, 43.94; H, 6.99; Cl, 13.65. Found: C,
43.1; H, 7.12; Cl, 13.6%.
4.9.4. Tripeptide [(HO)Val-(2S,6R)-2-methyl-2,6-DAP-
Val(OH)]·2HCl 11c. 11c was obtained as a solid
1
(decomposes at 131°C) starting from 10c. H NMR l
(D2O): 0.84 (m, 12H); 1.1–1.4 (m, 12H); 1.56 (s, 3H);
1.75–1.94 (m, 4H); 2.05–2.15 (m, 2H); 4.0 (t, 1H, J=
6.2); 4.18 (m, 2H). 13C NMR l (D2O): 18, 18.1, 18.5,
19, 19.2, 19.3, 22.6, 30.4, 31.4, 37.1, 53.4, 59.1, 59.6, 61,
170.2, 172.2, 175.2, 175.3. [h]D −29.5 (c 0.51, 1N HCl).
Anal. calcd for C18H36Cl2N4O6: C, 45.48; H, 7.63; Cl,
14.91. Found: C, 45.58; H, 7.65; Cl, 14.85%.
10. Meerwein, H. Org. Synth. 1966, 46, 113.