Stereoselective synthesis of pseudotripeptides incorporating uncommon bis-α-aminoacid derivatives and X-ray analysis. Part 3

Article abstract of DOI:10.1016/j.tetasy.2004.11.018

Stereoselective synthesis of pseudotripeptides 4, 5, 6, 8, 9, 13 and 14, incorporating an uncommon bis(α-aminoacid) derivative, has been accomplished starting from the L-valine derived chiral synthon 1. The configuration of the introduced stereogenic cent

Full text of DOI:10.1016/j.tetasy.2004.11.018

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 3929–3937
Stereoselective synthesis of pseudotripeptides
incorporating uncommon bis-a-aminoacid derivatives
and X-ray analysis. Part 3q
D. Balducci, A. Grandi, G. Porzi,* P. Sabatino and S. Sandri*
`
Dipartimento di Chimica ‘G.Ciamician’, Universita di Bologna, Via Selmi 2, 40126 Bologna, Italy
Received 28 September 2004; accepted 10 November 2004
Abstract—Stereoselective synthesis of pseudotripeptides 4, 5, 6, 8, 9, 13 and 14, incorporating an uncommon bis(a-aminoacid) deriv-
ative, has been accomplished starting from the L-valine derived chiral synthon 1. The configuration of the introduced stereogenic
centres has been assigned on the basis of H NMR spectroscopic data. The geometry of the tripeptides, deduced on the basis of
1
1H NMR parameters and IR spectra, was confirmed by X-ray crystal structure analysis of 6.
Ó 2004 Elsevier Ltd. All rights reserved.
1. Introduction
chemical yield >90% and with a practically total double
1,4-trans-induction (de P96%) with respect to the iso-
propyl group. In fact, as previously observed for similar
substrates,1–3 the 1H NMR spectrum exclusively showed
the trans-isomer. Diastereomer 2, which has a C2-axis of
symmetry, was obtained pure by silica gel chromatogra-
phy and its stereochemistry was established on the basis
In continuation of our program directed towards pro-
ducing uncommon tripeptides, which are C-terminal at
both ends of the chain,1–3 we undertook the stereocon-
trolled synthesis of new pseudotripeptides 4, 5, 6, 8, 9,
13 and 14 containing (S,S)-ortho-phenylene-bis-alanine,
which can be considered as a cysteine isoster with L-va-
line units at the ends of the chain. Our interest in these
unusual pseudopeptides arises from their potential bio-
logical activity as antibacterial4 and/or herbicide
agents.5 In addition, short peptides can mimic some
important aspect of protein structure or function. The
strategy followed to accomplish the synthesis of these
unusual tripeptides is based on the experience previously
acquired on the stereoselective approach to similar sub-
strates.1–3
1
of the H NMR spectra, as already reported.1,3 The
intermediate 2 was converted into 3 and then into the
aminoester 4 by means of the Birch reaction and subse-
quent acid hydrolysis under mild conditions. The acetyl-
ation of 4 furnished the pseudotripeptide 5, C-terminal
at both ends of the chain, incorporating a bis(a-amino-
acid) derivative cysteine isoster6 (Scheme 1). The more
complex pseudopeptide 6 was then obtained by treating
4 with the t-Boc-(L)-valine pentafluorophenylester
(prepared from t-Boc-L-valine and pentafluorophenyl-
trifluoroacetate), as reported in Scheme 1.
2. Synthesis and stereochemical assignments
The intermediate 2, alkylated at both equivalent posi-
tions C-3, gave 7 in good yield and with a total trans-
stereoselection induced by the isopropyl group, as
already observed3 (Scheme 2).
The stereoselective synthesis followed makes use of the
chiral synthon 1, a mono-lactim ether easily synthesized
starting from L-valine, as reported previously.1,3 Depro-
tonation of 1 with LHMDS followed by the alkylation
with 0.5equiv of a,a0-dibromo-o-xylene afforded 2 in
1
The trans-configuration of 7 was followed from the H
NMR and 13C NMR spectra, which showed the exis-
tence of a symmetry element, that is, a C2-axis. In fact,
the signals of both methyl and isopropyl groups overlap
as do the signals of the benzyl protons indicating their
magnetic equivalence.1–3 The stereochemistry was also
q Refs. 1 and 2 are considered to be Parts 1–2.
*
Corresponding authors. E-mail: gianni.porzi@unibo.it
0957-4166/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2004.11.018

3930
D. Balducci et al. / Tetrahedron: Asymmetry 15 (2004) 3929–3937
OEt
OEt
OEt
N
N
6
i, ii
iii
N
6
3
N
N
3
N
PhH2C
PhH2C
CH2Ph
O
O
O
2
1
OEt
OEt
N
N
iv, v
HN
NH
O
O
3
COOEt
EtOOC
3
NH2
3
NH2
1
NH
1
5
NH
5
6
6
4
O
O
vi
vii
2'
t-BOCHN
t-BOCHN
2'
COOEt
EtOOC
CO
COOEt
2
EtOOC
OC
2
NH Ac
NH Ac
2
3
NH
2
3
NH
1
1
NH
NH
1
NH
1
NH
6
6
O
O
O
O
6
5
Scheme 1. Reagents and conditions: (i) 1M LHMDS/THF; (ii) o,o-C6H4–(CH2Br)2; (iii) Li/NH3; (iv) 0.5M HCl at rt; (v) 1M K2CO3; (vi) CH3COCl/
Et3N, CH2Cl2; (vii) t-Boc-L-valine pentafluorophenylester, DMF at 50°C.
OEt
OEt
N
N
ii, iii, iv
i
2
N
N
CH2Ph
CH2Ph
R
R
O
O
7a R = CH3
7b R = CH2Ph
COOEt
NH2
3
COOEt
3
COOEt
COOEt
NH2Ac
NH2
NH2Ac
v
6
NH1
6
NH1
NH1
NH1
CH3
CH3
H3C
H3C
O
O
O
O
8
9
Scheme 2. Reagents and conditions: (i) 1M LHMDS/THF, then R–Br; (ii) Li/NH3; (iii) 0.5M HCl at rt; (iv) 1M K2CO3; (v) CH3COCl/Et3N in
CH2Cl2.
evident from the shielding effect observed in 7b induced
by the phenyl ring of (C-3)–CH2Ph group on the (C-6)–
H (see Experimental), the absolute configuration of ste-
reocentre C-6 being known.1–3 The intermediate 7a was

D. Balducci et al. / Tetrahedron: Asymmetry 15 (2004) 3929–3937
3931
1
3. H NMR and IR studies
then converted into 9 (Scheme 2) by following the same
procedure employed to obtain 5 (Scheme 1).
The meaningful spectroscopic data of the unusual
pseudopeptides investigated 4, 5, 6, 8, 9, 13 and 14 are
listed in Table 1 and their amide protons are labelled
as H1 and H2 for clarity (see Schemes 1–3). As men-
tioned above, these compounds possess a C2-axis of
symmetry and then all the 1H NMR and 13C NMR
signals overlap indicating their magnetic equivalence,
as previously observed for similar derivatives.1–3
The synthesis of diastereomeric derivative 14, with the
opposite configuration at the C-6 stereocentres with re-
spect to 9, was accomplished on the basis of the stereo-
controlled trans-induction well tested in the alkylation
reaction. To this end, the strategy reported in Scheme
3 was followed, starting from diastereomer 10 obtained
by the alkylation of the chiral synthon 1 with CH3I. The
reaction occurred in good chemical yield and with a
1
trans/cis ratio ffi 7:3 (measured by H NMR or HPLC),
The spectroscopic studies of substrates with a free amine
group, that is, 4, 8 and 13 (Table 1), suggest that these
nonclassical peptides form structures organized through
the diastereomer 10 being easily separable by silica gel
chromatography.
OEt
OEt
N
N
i, ii
1
+
N
N
CH2Ph
CH3
CH2Ph
CH3
O
O
10
10'
i, ii
OEt
OEt
OEt
OEt
N
N
N
N
iii
NH
N
NH
N
CH2Ph
CH2Ph
CH3
CH3
CH3
CH3
O
O
O
O
11
12
iv, v
COOEt
3
COOEt
NH2
COOEt
3
NH2Ac
COOEt
NH2Ac
NH2
vi
NH1
6
NH1
6
NH1
5
NH1
5
CH3
CH3
CH3
CH3
O
O
O
O
14
13
Scheme 3. Reagents and conditions: (i) 1M LHMDS/THF; (ii) CH3I; (iii) Li/NH3; (iv) 0.5M HCl at rt; (v) 1M K2CO3; (vi) CH3COCl/Et3N,
CH2Cl2.
1
Table 1. Significant H NMR and IR data of pseudopeptides 4, 5, 6, 8, 9, 13 and 14
dNH (ppm)
(in 2mM CDCl3)
dNH (ppm)
(in 2mM DMSO)
d
NH/DT (ppb/°C)
IR (cmꢀ1) (2mM CHCl3)
(in 2mM CDCl3)
H1
H2
H1
H2
H1
H2
4
5
7.7
8.4
—
6.6
8.1a
8.5
—
8.15
ꢀ2.3
ꢀ2.4
—
ꢀ0.8
3368 (broad)
3411 (sharp)
3307 (broad)
3386 (broad)
3308 (broad)
3350 (broad)
3419 (sharp)
3370 (broad)
3361 (broad)
3426 (sharp)
3381(broad)
6
8.2–8.6
7.0
8.18.2
ꢀ4.4b
ꢀ4.0b
8
9
8.05
7.3
—
6.4
8.05
7.65
—
7.7
ꢀ2.0
ꢀ8.5
ꢀ
ꢀ2.4
13
14
8.1—
6.8
8—.1
6.17.65
ꢀ1.7
ꢀ0.4
—
ꢀ0.2
7.75
a dNH = 7.85ppm in CDCl3/DMSO = 80:20.
b Measured in DMSO.

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D. Balducci et al. / Tetrahedron: Asymmetry 15 (2004) 3929–3937
intramolecular hydrogen bond between H1 and (C-
5)@O, giving rise to cyclic 11-membered conforma-
tions.7,8 In fact, in all substrates the H1 chemical shift
is >7, the temperature coefficient DdNH/DT is <2.6ppb
and the IR spectra show only one broad band in the
range 3350–3368cmꢀ1, ascribable to hydrogen bonded
NH amides. Most probably, in the case of 4, the intra-
molecular hydrogen bond is not as strong as in 8 and
13, because the H1 signal displays a moderate downshift
of 0.15ppm by adding 20% DMSO. These findings lead
us to deduce that the presence of the methyl group at
both C-6 positions regardless of their absolute configu-
rations, increases the tendency to assume conformations
organized by a strong intramolecular hydrogen bond,
which is resistant to a strongly competitive solvent in
the formation of hydrogen bonds like DMSO.
the proton, which is furthermore sterically less accessible
to the solvent. Besides, the IR spectrum in diluted
CHCl3 solution shows two broad bands of comparable
intensity at 3308 and 3386cmꢀ1 ascribable to hydrogen
bonded NHs. All these findings suggest that most
probably in solution H1 forms a stable intramolecular
hydrogen bond, while H2 is involved in a weak one
and t-Boc-NH is in a nonhydrogen bonded state.
Thus, it can be inferred that in acetamide derivatives 5, 9
and 14 the introduction of the methyl group at both C-6
stereocentres, regardless of their absolute configuration,
considerably decreases the tendency to form intramole-
cular hydrogen bond between H1 and (C-5)@O or
CH3C@O so much that in 14 the hydrogen bond can
be considered barely present.
1
In conclusion, from the H NMR and IR data it is rea-
From the spectroscopic data of 5 (Table 1) it is possible
once again to deduce the presence of a strong intramo-
lecular hydrogen bond between H1 and (C-5)@O or
CH3C@O, inducing the formation of a cyclic 11- or
12-membered structure, respectively, while the acetami-
dic –NH2 does not give rise to a hydrogen bond, as
shown by its large downshift (1.55ppm) registered on
going from CDCl3 to DMSO and the sharp band at
3411cmꢀ1, ascribable to free acetamide –NH2. Analo-
gously to 5, also in compounds 9 and 14 the
acetamide –NH2 does not form a hydrogen bond, while
the amidic H1 shows a different behaviour. In fact, in the
pseudopeptide 9, the chemical shift value (7.3ppm),
the consistent coefficient temperature (ꢀ8.5ppb/°C),
the downshift of 0.35ppm observed on going from
CDCl3 to DMSO and the broad band at 3370cmꢀ1 lead
us to argue that the H1 probably exists in a dynamic
equilibrium between the hydrogen-bonded and nonhy-
drogen-bonded states. Conversely, in the substrate 14
the H1 shows very little tendency to form a hydrogen
bond, as shown by the chemical shift value (6.8ppm),
the downshift of 0.85ppm observed on going from
CDCl3 to DMSO and the presence of a broad band at
3381cmꢀ1, largely weaker than the sharp one at
3426cmꢀ1 due to free acetamidic NH.
sonable to deduce that all the nonclassical peptides
investigated, except 14, are aggregated through the for-
mation of intramolecular hydrogen bonds, between the
carbonyl oxygens and the amide protons, to form a re-
verse-turn conformation, that is a ÔU shaped structureÕ.
However, such an intramolecularly compact conforma-
tion was firmly established by single crystal X-ray stud-
ies performed on pseudopeptide 6 (see below).
4. X-ray analysis
The solid state molecular structure of 6 is shown in
Figure 1 together with the crystallographic numbering
and the relevant bond parameters are listed in Table 2.
Two independent molecules are present in the asymmet-
ric unit, which correspond to different conformers of the
same enantiomer. The absolute configuration at the car-
bon atoms was assigned as R for C(8) and C(28) and S
for C(10), C(20), C(30) and C(40). The stereogeometry
of 6, if the aromatic moiety is not considered, conforms
The 1H NMR spectrum of the pseudopeptide 6 in
CDCl3 does not appear well resolved and in particular
the signal ascribable to H1, centred at 8.4ppm, is
severely broadened. However, after the addition of
increasing quantities of DMSO the spectrum resolution
is improved and just in the presence of 12% DMSO the
signal at 8.1ppm becomes a doublet and in 50% DMSO/
CDCl3 it suffers practically no further shift. The H2,
which resonates at 7ppm in CDCl3, is shifted to
8.2ppm in DMSO and also the t-Boc-NH suffers a
downshift of 1.2ppm on going from CDCl3 (a broad
doublet centred at 5.3ppm) to DMSO (a sharp doublet
at 6.5ppm). The temperature coefficient values in
DMSO for H1 and H2 are borderline between the pres-
ence and the absence of an intramolecular hydrogen
bonded state. Besides, the three amide protons show a
different exchange rate in CD3OD: after 60min, H2
was totally exchanged, while H1 was about 70% and
t-Boc-NH was about 30%. The slow exchange rate of
the t-Boc-NH could be ascribed to the low acidity of
Figure 1. Molecular structure of 6 showing the atomic numbering. H-
bonds are drawn in purple.

D. Balducci et al. / Tetrahedron: Asymmetry 15 (2004) 3929–3937
3933
a
˚
Table 2. Relevant bond parameters (A) for 6
C(1)–C(7)
C(7)–C(8)
1.530(5)
1.541(5)
1.451(5)
1.383(6)
1.209(5)
1.527(6)
1.532(7)
1.534(8)
1.530(8)
1.457(6)
1.395(6)
1.197(6)
1.327(6)
1.472(6)
1.528(8)
1.533(8)
1.540(8)
1.526(6)
1.209(5)
1.374(6)
1.465(6)
1.508(7)
1.183(6)
1.314(7)
1.510(8)
1.518(9)
1.541(7)
1.529(7)
1.528(9)
C(2)–C(27)
C(27)–C(28)
C(28)–N(4)
N(4)–C(29)
C(29)–O(7)
C(29)–C(30)
C(30)–C(31)
C(31)–C(32)
C(31)–C(33)
C(30)–N(5)
N(5)–C(34)
C(34)–O(8)
C(34)–O(9)
O(9)–C(35)
C(35)–C(36)
C(35)–C(37)
C(35)–C(38)
C(28)–C(39)
C(39)–O(10)
C(39)–N(6)
N(6)–C(40)
C(40)–C(41)
C(41)–O(11)
C(41)–O(12)
O(12)–C(42)
C(42)–C(43)
C(40)–C(44)
C(44)–C(45)
C(44)–C(46)
1.531(6)
1.534(6)
1.459(5)
1.370(6)
1.212(5)
1.527(6)
1.543(6)
1.519(7)
1.515(7)
1.445(5)
1.390(6)
1.194(6)
1.335(5)
1.475(6)
1.546(7)
1.521(7)
1.534(7)
1.530(6)
1.209(5)
1.384(6)
1.464(6)
1.510(7)
1.192(6)
1.326(6)
1.504(8)
1.488(9)
1.528(9)
1.520(8)
1.535(7)
C(8)–N(1)
N(1)–C(9)
C(9)–O(1)
C(9)–C(10)
C(10)–C(11)
C(11)–C(12)
C(11)–C(13)
C(10)–N(2)
N(2)–C(14)
C(14)–O(2)
C(14)–O(3)
O(3)–C(15)
C(15)–C(16)
C(15)–C(17)
C(15)–C(18)
C(8)–C(19)
C(19)–O(4)
C(19)–N(3)
N(3)–C(20)
C(20)–C(21)
C(21)–O(5)
C(21)–O(6)
O(6)–C(22)
C(22)–C(23)
C(20)–C(24)
C(24)–C(25)
C(24)–C(26)
Figure 2. Intramolecular H-bonding interactions shown in detail. The
L-valine side chains, t-Boc- and EtO-moieties have been omitted for
clarity.
because of their outward orientation with respect to the
anti-parallel chains (Fig. 2).
5. Experimental
5.1. General information
a Average on the two molecules.
1H and 13C NMR spectra were recorded on a Gemini
spectrometer at 300MHz using CDCl3 as the solvent,
unless otherwise stated. Chemical shifts are reported in
ppm relative to CDCl3 and the coupling constants (J)
are in Hz. IR spectra were recorded on a Nicolet 210
spectrometer. Optical rotation values were measured at
25°C on a Perkin–Elmer 343 polarimeter. Melting
points are uncorrected. Chromatographic separations
were performed with silica gel 60 (230–400 mesh). Dry
THF was distilled from sodium benzophenone ketyl.
to an approximate C2-symmetry. The configurations of
the stereogenic centres allows the ortho-substituents to
orient their two chains in an antiparallel fashion, with
the t-Boc moieties facing the COOEt ones of the oppo-
site substituent and the valine side chains facing each
other. As shown in Figure 1, the overall shape of the
molecule is that of a reverse-turn or a ÔU shaped struc-
tureÕ stabilized by intramolecular N–Hꢁ ꢁ ꢁO interactions.
5.2. (3R,6S)-ortho-Bis-(1-benzyl-3,6-dihydro-5-ethoxy-6-
isopropyl-pirazin-2-one-3-methylene)benzene 2
Two intramolecular hydrogen bonds between the amidic
hydrogen and the carbonyl oxygen atom of the L-valine
moieties from the opposite chain each form a 12-mem-
bered cycle, including the hydrogen atom, with the
ortho-aromatic carbon atoms (Fig. 2), but they are quite
asymmetrical.
To a solution of 1 (5.5g, 20mmol) in dry THF (50mL)
and cooled at ꢀ78°C, a solution of 1M LHMDS in
THF (21mL, 21mmol) was dropped under stirring.
After about 1h, a,a0-dibromo-o-xylene (1.35mL,
10mmol) was added and the reaction monitored by
TLC. When the reaction was complete, the mixture
was allowed to warm up to room temperature under
stirring. Water and ethyl acetate were added and after
separation the organic solution was evaporated in va-
cuo. The residue was submitted to purification by silica
gel chromatography eluting with hexane/ethyl acetate.
The pure product was recovered as an oil in 90% yield.
1H NMR d: 0.89 (d, 6H, J = 6.8); 1(d, 6H, J = 7); 1.18
(t, 6H, J = 6.8); 2.19 (m, 2H); 3.33 (dd, 2H, J = 7, 13.6);
3.55 (dd, 2H, J = 1.4, 3.6); 3.93 (d, 2H, J = 15); 3.97–4.2
(m, 6H); 4.38 (m, 2H); 5.5 (d, 2H, J = 15); 7–7.4 (m,
14ArH).13C NMR d: 14, 17.2, 19.8, 31.2, 35.7, 47,
59.6, 60.9, 61.4, 125.5, 127.2, 127.6, 128.4, 130.5,
135.9, 138.6, 158.3, 170. [a]D = +64.2 (c 0.5, CHCl3).
Anal. Calcd for C40H50N4O4: C, 73.82; H, 7.74; N,
8.61. Found: C, 74.15; H, 7.77; N, 8.64.
One of them, N(3)–H(3)ꢁ ꢁ ꢁO(7) with H(3)ꢁ ꢁ ꢁO(7)
˚
2.00(2), N(3)ꢁ ꢁ ꢁO(7) 2.862(6)A is in fact very linear,
176.2°, while the second one, N(6)–H(6)ꢁ ꢁ ꢁO(1), with
˚
H(6)ꢁ ꢁ ꢁO(1) 2.10(2) and N(6)ꢁ ꢁ ꢁO(1) 2.894(6)A, is
152.3°, probably distorted by the proximity of the
phenyl ring. Moreover, two additional, although
weaker, hydrogen bonding interactions involve the
t-Boc-NH with N(2)–H(2)ꢁ ꢁ ꢁO(11), H(2)ꢁ ꢁ ꢁO(11)
˚
2.28(6), N(2)ꢁ ꢁ ꢁO(11) 3.133(7)A and N–Hꢁ ꢁ ꢁO angle
of 172.1°; N(5)–H(5)ꢁ ꢁ ꢁ(5), H(5)ꢁ ꢁ ꢁO(5) 2.29(3),
˚
N(5)ꢁ ꢁ ꢁO(5) 3.141(8)A and N–Hꢁ ꢁ ꢁO angle of 168.6°.
Of the three amide protons present in each chain, only
the ones labelled H2 in solution (Scheme 1), that is,
N(1)–H(1) and N(4)–H(4) in the solid state, are not in-
volved in intramolecular hydrogen bonding interactions

3934
D. Balducci et al. / Tetrahedron: Asymmetry 15 (2004) 3929–3937
5.3. (3R,6S)-ortho-Bis-(3,6-dihydro-5-ethoxy-6-isoprop-
yl-pirazin-2-one-3-methylene)benzene 3
pletely evaporated under vacuum. The residue was then
submitted to purification by silica gel chromatography
eluting with hexane/ethyl acetate. The pure product
was recovered as a solid (mp 205.5–207°C) in 80% yield.
1H NMR d: 0.88 (d, 6H, J = 6.6); 0.91(d, 6H, J = 6.6);
1.28 (t, 6H, J = 7.2); 1.85 (s, 6H); 2.15 (m, 2H); 3.12 (m,
4H); 4.2 (q, 4H, J = 7.2); 4.3 (m, 2H); 5.28 (m, 2H); 6.64
(d, 2H, J = 7.6); 7 (m, 4ArH); 8.4 (br s, 2H). 13C NMR
d: 14.2, 17.7, 19, 22.6, 30.1, 38.3, 54.2, 58.3, 60.8, 126.6,
131.5, 135.5, 169.4, 171.2, 171.7. [a]D = + 33 (c 1,
CHCl3). Anal. Calcd for C30H46N4O8: C, 61.0; H,
7.85; N, 9.48. Found: C, 60.78; H, 7.82; N, 9.45.
A solution of 2 (3.25g, 5mmol) in 50mL of dry THF/t-
butanol 9:1was added to about 100mL of liquid ammo-
nia cooled at ꢀ50°C and Li (0.07g, 10mmol) was then
added to the substrate under stirring. The addition of
Li, in small pieces, was controlled by the monitoring
of the TLC in the presence of substrate (starting mate-
rial) and was stopped as soon as the reaction mixture be-
came blue. The reaction was then quenched with NH4Cl
and the cooling bath removed allowing the complete re-
moval of NH3. After addition of water, the aqueous
solution was extracted with ethyl acetate and the organic
solution evaporated to dryness under vacuum. The pure
product was recovered as an oil in 85% yield after silica
gel chromatography eluting with hexane/ethyl acetate.
1H NMR d: 0.79 (d, 6H, J = 7); 0.89 (d, 6H, J = 7);
1.23 (t, 6H, J = 6.8); 2.13 (m, 2H); 3.18 (dd, 2H,
J = 5.8, 13.6); 3.28 (m, 2H); 3.7 (dd, 2H, J = 4.8, 13.6);
4.09 (m, 4H); 4.35 (m, 2H); 5.94 (br s, 2H); 7.08 (m,
4ArH). 13C NMR d: 14.2, 15.9, 18.1, 31.2, 36.1, 57.8,
59.8, 61.1, 126.1, 130.4, 137.5, 158.3, 171.8.
[a]D = +43.2 (c 1.5, CHCl3). Anal. Calcd for
C26H38N4O4: C, 66.36; H, 8.14; N, 11.91. Found: C,
66.13; H, 8.15; N, 11.9.
5.6. ortho-Bis-[(6R)-((20S)-t-butoxycarbonylamino-30-
methyl)butyrylamino-4-aza-(3S)-ethoxycarbonyl-2-
methyl-5-oxa-hept-7-yl]benzene 6
It was obtained by treating 4 with the activated pentaflu-
orophenylester of N-Boc-L-valine, prepared by stirring
at 0°C for about 2h the N-Boc-L-valine (1.1g, 5mmol),
dissolved in DMF (10mL), with pentafluorophenyltri-
fluoroacetate (1.3mL, 7.5mmol) in the presence of pyr-
idine (1.7mL, 20mmol). Water was then added, the ester
extracted with ethyl acetate and after complete elimina-
tion of the organic solvent in vacuo, the residue was
purified by silica gel chromatography eluting with hex-
ane/ethyl acetate. The crude intermediate 4 (2g, 4mmol)
was dissolved in DMF (10mL) and stirred with penta-
fluorophenyl ester of N-Boc-(S)-valine (3.8g, 10mmol)
at 50°C. After 24h, water was added and the reaction
product extracted with ethyl acetate. After complete
elimination of the organic solvent in vacuo, the residue
was purified by silica gel chromatography eluting with
hexane/ethyl acetate. The pure product was recovered
5.4. (3S,6R)-ortho-Bis-(6-amino-4-aza-3-ethoxycarbonyl-
2-methyl-5-oxa-hept-7-yl)benzene 4
To a solution of 3 (2.35g, 5mmol) in ethanol (50mL),
2M HCl (10mL) was added and the reaction mixture,
1
monitored by TLC and/or H NMR, stirred at room
temperature for about 12h. The acid solution was evap-
orated in vacuo and the intermediate hydrochloride, iso-
lated as a solid in practically quantitative yield, was
dissolved in water (20mL). The solution, after addition
of K2CO3 (1.38g, 10mmol) was stirred for about 1h,
then the reaction product extracted with ethyl acetate
and the organic solution washed with water. The organ-
ic solvent was completely removed in vacuo and the
pure product was isolated as a wax in 90% yield after sil-
ica gel chromatography eluting with hexane/ethyl ace-
1
as a solid (mp 208.5–209.5°C) in 90% yield. H NMR
(DMSO) d: 0.6 (d,6H, J = 6.6); 0.66 (d, 6H, J = 7);
0.78 (d, 12H, J = 6); 1.18 (t, 6H, J = 6.2); 1.38 (s,
18H); 1.75 (m, 2H); 1.93 (m, 2H); 2.95 (m, 4H); 3.86
(dd, 2H, J = 6.2, 8.4); 4.12 (m, 8H); 4.95 (m, 2H); 6.55
(d, 2H, J = 8.6); 7.06 (m, 4ArH); 8.1(d, 2H, J = 8);
8.2 (d, 2H, J = 8.8). 13C NMR d: 14.2, 17.2, 18.2, 19,
19.3, 28.3, 31.1, 38.3, 54.3, 58, 59.2, 60.3, 61, 79.5,
127.1, 131.9, 135, 155.7, 170.9, 171.2. [a]D = ꢀ49 (c
1.1, CHCl3). Anal. Calcd for C46H76N6O12: C,
61.04; H, 8.46; N, 9.28. Found: C, 61.3; H, 8.48; N,
9.25.
tate. 1H NMR: 0.91(d, 21H,
J = 7); 1.3 (t, 6H,
J = 7.2); 2.08 (m, 2H); 2.78 (dd, 2H, J = 8.8, 14.2);
3.42 (dd, 2H, J = 4.6, 14.2); 3.7 (dd, 2H, J = 4.8, 8.8);
4.2 (q, 4H, J = 7.2); 4.49 (dd, 2H, J = 4.6, 8.8); 7.2 (s,
4ArH); 7.71(d, 2H, J = 8.8). 13C NMR d: 13.8, 17.4,
18.5, 30.5, 37.7, 55.4, 56.6, 60.5, 126.5, 130.1, 136.4,
171.3, 174.1. [a]D = +25.7 (c 1, CHCl3). Anal. Calcd
for C26H42N4O6: C, 61.64; H, 8.36; N, 11.06. Found:
C, 61.93; H, 8.4; N, 11.09.
5.7. (3R,6S)-ortho-Bis-(1-benzyl-3,6-dihydro-5-ethoxy-6-
isopropyl-3-methyl-pirazin-2-one-3-methylene)benzene 7a
It was obtained by alkylating 21 with CH3I and follow-
ing the procedure already reported for an analogous
derivative.3 After gel chromatography eluting with hex-
1
5.5. (3S,6R)-ortho-Bis-(6-acetylamino-4-aza-3-ethoxy-
carbonyl-2-methyl-5-oxa-pent-7-yl)benzene 5
ane/ethyl acetate it was obtained pure in 85% yield H
NMR d: 0.03 (d, 6H, J = 7); 0.83 (d, 6H, J = 6.8); 1.23
(t, 6H, J = 7); 1.5 (s, 6H); 1.65 (m, 2H); 3.28 (d, 2H,
J = 13.6); 3.56 (d, 2H, J = 2.4); 3.78 (d, 2H, J = 13.4);
3.92 (d, 2H, J = 15); 4.11 (m, 4H); 5.41 (d, 2H,
J = 15); 7.2 (m, 14ArH). 13C NMR d: 14.2, 15.1, 20.7,
29.9, 32.3, 42.6, 47.1, 60.2, 61.1, 63.5, 126.2, 127.3,
127.9, 128.5, 131.6, 136.6, 138.0, 155.1, 172.0.
[a]D = ꢀ34.4 (c 0.2, CHCl3). Anal. Calcd for
To a solution of 4 (2.53g, 5mmol) and triethylamine
(2.1mL, 15mmol) in CH 2Cl2 (30mL) cooled at 0–5°C
was dropped acetyl chloride (1.07mL, 15mmol) under
stirring. After about 2h, 2M HCl (3mL) was added
and the reaction product extracted by ethyl acetate.
The organic solution was dried and the solvent was com-

D. Balducci et al. / Tetrahedron: Asymmetry 15 (2004) 3929–3937
3935
C42H54N4O4: C, 74.3; H, 8.02; N, 8.25. Found: C, 74.43;
H, 8.05; N, 8.22.
was complete, the mixture was allowed to warm up to
room temperature under stirring. Water and ethyl
acetate were added and after separation the organic
solution was evaporated in vacuo. The residue was
submitted to silica gel chromatography eluting with hex-
ane/ethyl acetate in order to separate the diastereomers
10 and 100.
5.8. (3R,6S)-ortho-Bis-(1,3-dibenzyl-3,6-dihydro-5-eth-
oxy-6-isopropyl-3-pirazin-2-one-3-methylene)benzene 7b
It was obtained by alkylating 21 with PhCH2Br follow-
ing the procedure already reported for an analogous
derivative3 and it was obtained pure as an oil in 80%
yield after gel chromatography eluting with hexane/ethyl
5.12. (3R,6S)-1-Benzyl-3,6-dihydro-5-ethoxy-6-isopropyl-
3-methyl-pirazin-2-one 10
1
acetate. H NMR d: ꢀ0.2 (d, 6H, J = 6.9); 0.68 (d, 6H,
J = 6.9); 1.33 (t, 6H, J = 7.2); 2.92 (d, 2H, J = 2.4); 3.04
(d, 2H, J = 12.3); 3.46 (d, 2H, J = 12.3); 3.5 (d, 2H,
J = 12.9); 4 (d, 2H, J = 12.9); 4.13 (d, 2H, J = 15.3);
4.29 (m, 4H); 4.71(d, 2H, J = 15.3); 6.7–7.36 (m,
24ArH). 13C NMR d: 14, 14.5, 20.8, 29.2, 42.9, 48.1,
49.5, 60.4, 61.4, 68.3, 126.4, 126.9, 127.8, 128.2, 130.8,
It was recovered as an oil in 65% yield. 1H NMR d: 0.95
(d, 3H, J = 7); 1.06 (d, 3H, J = 7); 1.26 (t, 3H, J = 7.2);
1.57 (d, 3H, J = 7); 2.22 (m, 1H); 3.7 (dd, 1H, J = 1.4, 4);
3.95 (d, 1H, J = 15); 4.1 (m, 3H); 5.47 (d, 1H, J = 15);
7.17 (m, 5ArH). 13C NMR d: 14.2, 17.7, 20.1, 20.7,
31.6, 47.5, 53.9, 61.1, 62.4, 127.4, 127.7, 128.6, 136.3,
159.2, 171.3. [a]D = +24.2 (c 1.7, CHCl3). Anal. Calcd
for C17H24N2O2: C, 70.8; H, 8.39; N, 9.71. Found: C,
71.1; H, 8.41; N, 9.65.
131.9, 136.1, 137.3, 137.9. [a]D = ꢀ97 (c 1 , CHCl).
3
Anal. Calcd for C54H62N4O4: C, 78.04; H, 7.52; N,
6.74. Found: C, 78.68; H, 7.5; N, 6.75.
5.9. (3S,6S)-ortho-Bis-(6-amino-4-aza-3-ethoxycarbonyl-
2,6-dimethyl-5-oxa-hept-7-yl)benzene 8
5.13. (3S,6S)-1-Benzyl-3,6-dihydro-5-ethoxy-6-isopropyl-
3-methyl-pirazin-2-one 100
It was prepared starting from 7a following the proce-
dure reported to convert the derivative 2 into 4 and it
was obtained pure as a wax in 70% overall yield after
gel chromatography eluting with hexane/ethyl acetate.
1H NMR d: 0.83 (d, 6H, J = 6.8); 0.86 (d, 6H,
J = 6.8); 1.28 (t, 6H, J = 7); 1.39 (s, 6H); 1.46 (br s,
4H); 2.15 (m, 2H); 3.05 (d, 2H, J = 13.8); 3.41 (d, 2H,
J = 13.8); 4.2 (m, 4H); 4.42 (dd, 2H, J = 4.8, 8.8); 7.1–
7.27 (m, 4ArH); 8.08 (d, 2H, J = 8.8).13C NMR d:
14.1, 17.7, 18.8, 30, 30.7, 41.7, 57.1, 59.2, 60.9, 126.9,
130.9, 136.2, 171.7, 176.5. [a]D = ꢀ75.9 (c 0.6, CHCl3).
Anal. Calcd for C28H46N4O6: C, 62.9; H, 8.67; N,
10.48. Found: C, 62.78; H, 7.55; N, 10.45.
It was recovered as an oil in 25% yield. 1H NMR d: 0.94
(d, 3H, J = 7); 1.09 (d, 3H, J = 7.2); 1.26 (t, 3H, J = 7);
2.18 (m, 1H); 3.73 (dd, 1H, J = 1.8, 3.4); 3.94 (d, 1H,
J = 15); 4.02–4.29 (m, 2H); 5.49 (d, 1H, J = 15); 7.23–
7.37 (m, 5ArH). 13C NMR d : 13.5, 17.1, 19.8, 20.8,
29.5, 46.2, 54.6, 60.3, 60.6, 127, 127.5, 128.1, 135.5,
156.7, 169.9. [a]D = ꢀ12.1 (c 2.2, CHCl3). Anal. Calcd
for C17H24N2O2: C, 70.8; H, 8.39; N, 9.71. Found: C,
71.0; H, 8.43; N, 9.69.
5.14. (3R,6S)-ortho-Bis-(1-benzyl-3,6-dihydro-5-ethoxy-
6-isopropyl-3-methyl-pirazin-2-one-3-methylene)benzene
11
5.10. (3S,6S)-ortho-Bis-(6-acetylamino-4-aza-3-ethoxy-
carbonyl-2,6-dimethyl-5-oxa-hept-7-yl)benzene 9
It was obtained by alkylating 10 with a,a0-dibromo-o-
xylene and following the procedure used for 2. The pure
product was recovered as an oil in 85% yield after puri-
fication by silica gel chromatography eluting with hex-
ane/ethyl acetate. 1H NMR d: 0.81(d, 6H, J = 6.6);
0.89 (d, 6H, J = 6.8); 1.2 (t, 6H, J = 7); 1.6 (s, 6H);
2.02 (m, 2H); 3.28 (d, 2H, J = 2.6); 3.48 (d, 2H,
J = 13.6); 3.6 (d, 2H, J = 13.6); 3.84 (d, 2H, J = 15);
3.8–4.3 (m, 4H); 5.4 (d, 2H, J = 15); 6.8–7.25 (m,
14ArH). 13C NMR d: 14.1, 16.7, 20.1, 28.8, 29.9, 44.5,
46.1, 59.9, 60.2, 62.7, 125.6, 127.1, 128.1, 128.3, 130.6,
135.1, 137.7, 155, 171.7. [a]D = ꢀ10.7 (c 1, CHCl3).
Anal. Calcd. for C42H54N4O4: C, 74.3; H, 8.02; N,
8.25. Found: C, 74.57; H, 8.03; N, 8.29.
It was obtained starting from 8 following the procedure
reported for derivative 5 and it was obtained as a solid
(mp 137–138.5°C) in 80% yield after gel chromatogra-
1
phy eluting with hexane/ethyl acetate H NMR d: 0.94
(d, 6H, J = 6.8); 0.97 (d, 6H, J = 6.8); 1.34 (t, 6H,
J = 7.4); 1.73 (s, 6H); 1.94 (s, 6H); 2.25 (m, 2H); 3.37
(qAB, 4H, J = 14.2); 4.26 (m, 4H); 4.7 (dd, 2H, J = 4.4,
9.2); 6.36 (s, 2H); 7.12 (s, 4ArH); 7.35 (d, 2H, J = 9.2).
13C NMR d: 14.1, 17.4, 19, 23.5, 24.4, 31.7, 37.4, 57.3,
61.1, 61.5, 126.3, 131.4, 135.3, 170.1, 172.2, 173.6.
[a]D = +1 3.9 (c 0.8, CHCl3). Anal. Calcd for
C32H50N4O8: C, 62.11; H, 8.14; N, 9.05. Found: C,
61.98; H, 8.15; N, 9.05.
5.15. (3R,6S)-ortho-Bis-(3,6-dihydro-5-ethoxy-6-isoprop-
yl-3-methyl-pirazin-2-one-3-methylene)benzene 12
5.11. Conversion of 1 into 10 and 100
It was obtained submitting the intermediate 11 to the
Birch reaction and following the procedure used for 3.
The pure product was recovered as an oil in 80% yield
after purification by silica gel chromatography eluting
with hexane/ethyl acetate.1H NMR d : 0.72 (d, 6H,
J = 7); 0.8 (d, 6H, J = 7.2); 1.23 (t, 6H, J = 7.4); 1.6 (s,
To a solution of 1 (2.25g, 10mmol) in dry THF (30mL)
and cooled at ꢀ78°C, a solution of 1M LHMDS in
THF (10.5mL, 10.5mmol) was dropped under stirring.
After about 1h, CH3I (0.6mL, 10mmol) was added
and the reaction monitored by TLC. When the reaction

3936
D. Balducci et al. / Tetrahedron: Asymmetry 15 (2004) 3929–3937
6H); 2.08 (m, 2H); 2.89 (dd, 2H, J = 1.2, 3); 3.11 (d, 2H,
J = 13.2); 3.49 (d, 2H, J = 13.2); 3.9–4.3 (m, 4H); 5.55
(br s, 2H); 7.1(m, 4ArH). 13C NMR d: 14.2, 15.5, 18,
29.1, 30, 43.9, 57.4, 60.8, 62.8, 126.3, 130.2, 137, 155.8,
173.8. [a]D = ꢀ30.3 (c 1.1, CHCl3). Anal. Calcd for
C28H42N4O4: C, 67.44; H, 8.49; N, 11.24. Found: C,
67.67; H, 8.53; N, 11.29.
higher thermal parameters. An attempt of a low temper-
ature data collection was prevented from cracking of the
crystal. The absolute configuration was assigned by
using the Flack parameter (ꢀ0.05 for the correct abso-
lute structure).13 H atoms were geometrically positioned
˚
[C–H 0.93 and 0.97A for aromatic and aliphatic
˚
distances, N–H 0.86A] and refined ÔridingÕ on their
corresponding carbon or nitrogen atoms. Molecular
5.16. (3S,6R)-ortho-Bis-(6-amino-4-aza-3-ethoxycar-
bonyl-2,6-dimethyl-5-oxa-hept-7-yl)benzene 13
graphics were prepared using SCHAKAL 97.14
5.19. Crystallographic data
It was obtained from 12 and following the procedure
used to convert 3 into 4. The pure product was recov-
ered as an oil in 80% yield after purification by silica
gel chromatography eluting with hexane/ethyl acetate.
1H NMR d: 0.92 (d, 6H, J = 7); 0.96 (d, 6H, J = 7);
1.27 (t, 6H, J = 7); 1.37 (s, 6H); 2.2 (m, 2H); 3.22
(qAB, 4H, J = 14); 4.18 (q, 4H, J = 7); 4.47 (dd, 2H,
J = 5.2, 9); 7.2 (m, 4ArH); 8.11 (d, 2H, J = 9). 13C
NMR d: 14.2, 17.7, 19, 28.2, 31.2, 41.5, 57, 59, 60.9,
126.9, 130.8, 136.3, 171.7, 176.8. [a]D = + 56 (c 1.2,
CHCl3). Anal. Calcd for C28H46N4O6: C, 62.9; H,
8.67; N, 10.48. Found: C, 63.11; H, 8.63; N, 10.49.
C46H76N6O12, triclinic, P1(No. 1), a = 13.033(1), b =
˚
13.307(1), c = 16.624(1)A, a = 94.76(1), b = 106.32(ꢀ11),
3
˚
c = 94.79(1), V = 2740(1)A , Z = 2, l = 0.079mm
.
26872 reflections were collected, 15414 unique, 8424 ob-
served for I>2r(I), which were used in all calculations.
Final R factors: R1 = 0.0624, wR2 = 0.177.
Crystallographic data (excluding structure factors) for
the structure of 6 have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary
publication number CCDC 249923. Copies of the data
can be obtained free of charge on application to CCDC,
12 Union Road, Cambridge, CB2 1EZ, UK [fax:
+44(0)1223336033 or e-mail: deposit@ccdc.cam.ac.uk].
5.17. (3S,6R)-ortho-Bis(6-acetylamino-4-aza-3-ethoxy-
carbonyl-2,6-dimethyl-5-oxa-hept-7-yl)benzene 14
It was obtained from 13 and following the procedure
used to convert 4 into 5. The pure product was recov-
ered as a solid (mp 180–1°C) in 80% yield, after purifi-
cation by silica gel chromatography eluting with
Acknowledgements
Thanks are due to the University of Bologna for finan-
cial support (ÔRicerca fondamentale orientataÕ).
1
hexane/ethyl acetate. H NMR d: 0.87 (d, 6H, J = 6.8);
0.90 (d, 6H, J = 6.8); 1.28 (t, 6H, J = 6.8); 1.51 (s, 6H);
2.0 (s, 6H); 2.12 (m, 2H); 3.45 (qAB, 4H, J = 14.8); 4.2
(m, 4H); 4.45 (dd, 2H, J = 4.6, 8); 6.2 (s, 2H); 6.87 (d,
2H, J = 8); 7.15 (m, 4ArH). 13C NMR d: 14, 17.7,
18.6, 23.4, 23.7, 31, 37, 57.5, 60.8, 61, 126.5, 130.8,
135.6, 170.3, 171.6, 173.5. [a]D = +69.7 (c 0.9, CHCl3).
Anal. Calcd for C32H50N4O8: C, 62.11; H, 8.14; N,
9.05. Found: C, 62.1; H, 8.15; N, 9.07.
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˚
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