Synthesis of tetrahydroisoquinoline-based pseudopeptides and their characterization as suitable reverse turn mimetics

Article abstract of DOI:10.1016/j.tet.2007.04.024

New peptidomimetics containing the Tic moiety were synthesized in enantiomerically pure form and their conformational features were studied by NMR, IR, and molecular modeling techniques. The presence of a reverse turn conformation was observed in all the

Full text of DOI:10.1016/j.tet.2007.04.024

Tetrahedron 63 (2007) 5567–5578
Synthesis of tetrahydroisoquinoline-based pseudopeptides and
their characterization as suitable reverse turn mimetics
*
Giordano Lesma, Elisa Meschini, Teresa Recca, Alessandro Sacchetti and Alessandra Silvani
*
Dipartimento di Chimica Organica e Industriale e Centro Interdisciplinare Studi biomolecolari e applicazioni Industriali (CISI),
Universitaꢀ degli Studi di Milano, via G. Venezian 21, 20133 Milano, Italy
Received 17 January 2007; revised 20 March 2007; accepted 5 April 2007
Available online 14 April 2007
Abstract—New peptidomimetics containing the Tic moiety were synthesized in enantiomerically pure form and their conformational fea-
tures were studied by NMR, IR, and molecular modeling techniques. The presence of a reverse turn conformation was observed in all the
structures, suggesting the key role of the scaffold as reverse turn inducer. In particular, all the analyses led to the conclusion that a b-turn
conformation is mostly stabilized in tetrapeptide mimetic 4b and in hexapeptide mimetics 5a,b. In the case of 5a,b, the C1 stereochemistry
plays a central role in determining stable conformations, supporting the formation of a b-hairpin arrangement with a 14-membered intramo-
lecular hydrogen bond ring only in 5b.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
functionalities, suitable for the attachment of potential phar-
macophoric groups. During the past decade, a large array of
highly functionalized nitrogen heterocycles, mainly fused
bicyclic lactams, have been synthesized and employed as
reverse turn mimics, thereby providing a great variety of
peptidomimetics as a tool for medicinal chemistry.4
The design, synthesis, and application of peptidomimetic
compounds have been for many years a focal point of a re-
search aimed to develop new therapeutics with peptide-
like activity and enhanced resistance toward proteases.1
The structural transition from a bioactive peptide to a non-
peptidic compound involves the identification of the crucial
amino acid side chains, the establishment of their spatial re-
lationship, and the selection of an organic template, which is
able to reproduce the geometry of the pharmacophoric
model. Among the different strategies adopted to connect
the peptide with the ‘non-peptide world’, of particular inter-
est and success has been the replacement of a dipeptide sub-
structure in a natural substrate with a constrained rigid
analogue, capable to induce a folding in the peptide chain.2
Reverse turns are structural motifs commonly found in bio-
active peptides that, besides being fundamental in protein
folding, play a central role as molecular recognition ele-
ments.3 Therefore, they are very important as starting points
in the design of reverse turn mimics, that is, to say scaffolds
that restrict the conformational freedom of the peptide chain,
thus providing structural stabilization when incorporated
in oligopeptides. In addition, these pseudopeptide scaffolds
may have on the molecular framework additional
In search of new small peptidomimetics that combine the
structural rigidity with the ease of synthetic access, we fo-
cused on the 1,2,3,4-tetrahydroisoquinoline-3-carboxylic
acid (Tic) moiety, a core structure, which is known to exhibit
affinity toward a wide range of receptors. In fact, the strategy
of replacing individual amino acids such as tyrosine or
phenylalanine with Tic derived frameworks has been used
extensively to prepare conformationally constrained ana-
logues of bioactive peptides. For instance, based on this
approach, a number of d- and k-selective opioid receptor
agonists and antagonists have been prepared,5 by systematic
modification of endogenous ligands, such as enkephalin, en-
domorphin, and dynorphin, thus allowing a better under-
standing of how opioid receptors function at molecular
level. Constrained analogues of tyrosine have also been em-
ployed in the total synthesis of mimics of didemnin B,6 the
lead member of a class of marine natural peptides, and in
the preparation of the first non-peptidic selective antagonist
of the neuropeptide orexin.7
Compounds 1a,b (Scheme 1) were recently reported by
Grieco et al.8 as new potential b-turn dipeptide mimics, aris-
ing from a key Pictet–Spengler reaction of the precursor 2,
derived from L-Dopa, with the aldehyde 3, which can in
Keywords: Tetrahydroisoquinoline; Peptidomimetics; Reverse turn; Confor-
mational analysis; Molecular modeling.
* Corresponding authors. Tel.: +39 2 50314080; fax: +39 2 50314078;
e-mail: alessandra.silvani@unimi.it
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.04.024

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G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
turn be prepared by reduction of L-Ala. Actually, 1a,b can be
envisaged as Tyr-Ala dipeptide mimics rigidified through the
formation of the Tic core. Comparison of the structure of
Tyr-Ala dipeptide with that of compounds 1a,b, indicates
an equal number of linkages between the terminal C and N
atoms. On the other hand, the isosteric substitution occurs
at the level of the peptide bond, with the oxo group of Ala
being now replaced by the sp3-hybridized stereogenic car-
bon atom C1.
Herein we report the synthesis of enantiopure tetrapeptide
mimics 4a,b (Ac–AETIC–NHMe, AETIC¼1-(1-amino-
ethyl)-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline-3-car-
boxylic acid) and hexapeptide mimics 5a,b (Ac–(S)-Ala–
AETIC–(S)-Ala–NHMe), and their conformational analysis
by molecular modeling calculations,9 1H NMR10 and
FTIR11 (Fig. 1). This study indicated that both the diastereo-
meric Ac–AETIC–NHMe 4a,b and Ac–(S)-Ala–AETIC–
(S)-Ala–NHMe 5a,b have a good propensity to adopt a
reverse turn-like conformation. In particular, the structure
5b shows a strong preference to form a b-hairpin.
O
3
MeO
OH
NMe
H
NH2
2. Results and discussion
2.1. Synthesis
MeO
1
O
9
MeO
MeO
CHO
NH2
OH
1a
+
NH2
O
Scheme 2 outlines the synthesis of 4a,b starting from the L-
Dopa derivative 612 and N-Cbz-L-alaninal.13 We decided to
employ the temporary benzyloxycarbonyl (Cbz) protecting
group in the aldehyde component, since we have encoun-
tered many difficulties in handling N-acetyl-L-alaninal,
because of its sensitivity toward racemization.14
2
3
MeO
MeO
OH
NMe
H
NH2
1b
Scheme 1. Retrosynthetic analysis for dipeptide mimics 1a,b.
Conversion (MeNH2, rt) of methyl ester 6 into methylamide
7 occurred almost quantitatively. N-Boc removal (30% TFA,
CH2Cl2) was followed by Pictet–Spengler condensation (N-
Cbz-L-alaninal, 5% TFA, CH2Cl2), thus affording cleanly
tetrahydroisoquinolines 8, as an isomeric mixture (1:3, 8a
and 8b, after chromatographic separation) at C1. Since in
the preliminary investigation of these new peptidomimetic
scaffolds, both diastereoisomers at the newly created C1
stereogenic center are in principle necessary, no attempt
was made to control the stereogenic outcome of the Pic-
tet–Spengler condensation. Detailed inspection of 1D and
2D NMR spectra permitted the chemical shift complete as-
signment for all protons and carbons in both the diastereoiso-
mers. The determination of the configuration at the new
stereogenic center C1 was deduced by 2D 1H NOESY
NMR, on the basis of the NOE interaction between H-1
and H-3, which was present in compound 8a and absent in
O
O
5
8
4
9
5
8
4
9
3
6
7
MeO
MeO
6
3
MeO
MeO
NHMe
NHMe
NMe
H
NHAc
NMe
H
NHAc
7
1
3
1
3
4a
4b
O
O
5
8
4
5
8
11
4
11
6
MeO
MeO
6
MeO
MeO
N
CONHMe
N
CONHMe
H
H
NMe
NMe
7
7
1
1
O
H
H
O
NHAc
NHAc
9
9
N
H
N
H
10
10
5a
5b
Figure 1. Atom labeling as used in the NMR interpretation.
O
3
MeO
NHMe
NH
H
MeO
8a (1R,3S,9S)
MeO
MeO
O
1
MeO
MeO
MeO
MeO
CO2Me
NHBoc
9
NHCbz
NHMe
a
b,c,d
NHBoc
O
6
7
3
NHMe
NH
H
1
9
O
NHCbz
O
MeO
MeO
MeO
MeO
8b (1S,3S,9S)
e
NHMe
NHMe
f, g
NMe
H
NMe
H
CHO
NHAc
NHCbz
NHCbz
N-Cbz-L-Alaninal
9a (1R,3S,9S)
9b (1S,3S,9S)
4a (1R,3S,9S)
4b (1S, 3S,9S)
Scheme 2. Reagents and conditions: (a) MeNH2, methanol (98%). (b) TFA 30% in dichloromethane. (c) N-Cbz-L-alaninal, TFA 5% in dichloromethane. (d)
Chromatographic separation (8a: 20%; 8b: 60%, from step b). (e) CH2O, NaCNBH3, 1 M AcOH in methanol (9a: 91%; 9b: 93%). (f) H2, Pd/C, methanol. (g)
Ac2O, pyridine (4a: 70%; 4b: 68%, from step f).

G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
5569
O
the case of compound 8b. Thus, the absolute configuration at
Cα
i + 3
C1 was assigned as R in 8a and S in 8b. In order to avoid
undesirable interactions of the tetrahydroisoquinoline ring
N–H group, as a possible competitor on intramolecular hy-
drogen bond formation with carbonyl oxygen atoms, amines
8a and 8b were converted to the corresponding N-methyl
derivatives 9a and 9b, via reductive amination with form-
aldehyde (CH2O, 1 M AcOH in MeOH). Finally, removal
of the Cbz protecting group (H2, Pd/C 5%) and subsequent
acetylation of the primary amine (Ac2O, Py) afforded the
targets Ac–AETIC–NHMe 4a and 4b.
MeO
MeO
C
i + 2N
Cα
N
i + 3
H
dα = d
NMe
Cαi - Cαi + 3
β (C-Cαi + 1-Cαi + 2-N
)
O
i
i + 3
Cα
i + 1
C
N
H
C
i
Cα
i
Figure 2. Definition of parameters to characterize b-turn propensity of
AETIC systems.
structures was evaluated by computing and analyzing differ-
ent geometric parameters3 (Fig. 2).
The synthesis of hexapeptide mimics 5a,b is reported in
Scheme 3, starting from the same L-Dopa derivative 6. Hy-
drolysis of methyl ester 6 (LiOH, THF/H2O) was followed
by coupling with (S)-Ala-methyl ester (IBCF, 4-methylmor-
pholine) to give 10. Conversion of methyl ester into methyl-
amide (MeNH2, rt), provided tripeptide 11 in almost
quantitative yield. Boc removal (30% TFA, CH2Cl2) and
subsequent Pictet–Spengler condensation (N-Cbz-L-alani-
nal, 5% TFA, CH2Cl2) afforded cleanly tetrahydroisoquino-
lines 12. As for 8a,b, the isomeric mixture (2:3, 12a and
12b) at C1 was separated by flash chromatography and the
distinct diastereoisomers were fully characterized by means
Evaluation of the Cai–Cai+3 interatomic distance (da),
which should be less than 7 A to mimic a reverse turn, is
˚
a well established procedure.3a The virtual torsion angle b,
defined by Ci–Cai+1–Cai+2–Ni+3, is another element, which
has to be evaluated.3,16 A measure of jbj<30ꢀ is associated
with a tight reverse turn, while jbj<60ꢀ is usually referred
to an open reverse turn.17 An important feature of b-turns
is the presence of a 10-membered ring hydrogen bond be-
tween the carbonyl oxygen at Ci and the hydrogen on Ni+3
.
This was evaluated by means of the ‘hydrogen bonds’ func-
tion implemented in the software, according to which hydro-
gen bonds are defined as non-bonded contacts between
a nitrogen or oxygen and an hydrogen attached to nitrogen
1
of 1D and 2D NMR spectra. By 2D H NOESY NMR, the
configuration at the new stereogenic center C1 was deduced
to be R in 12a and S in 12b. Methylation of the tetrahydro-
isoquinoline ring N–H group on both the diastereoisomers
12a and 12b furnished 13a,b quantitatively. The final prod-
ucts 5a and 5b were obtained by hydrogenolysis of the Cbz
protecting group, followed by coupling (IBCF, 4-methyl-
morpholine) with N-acetyl-L-alanine.
˚
or oxygen, separated by a distance ranging from 1.6 A to
ꢀ
˚
2.1 A and making an X–H/Y (X,Y¼N,O) angle >120 .
The computational procedure consisted of an unconstrained
Monte Carlo/energy minimization conformational search
using the molecular mechanics MMFF94 force field18 in va-
cuo. For compounds 4a,b and 5a,b, respectively, 441 and
1089 conformers were generated. Only conformations
within 6 kcal/mol of the global minimum were kept. Results
are reported in Table 1 as percentage of conformers, which
meet the cited requirements for a reverse turn.
2.2. Conformational analysis
In order to evaluate the ability of the AETIC scaffolds to in-
duce reverse turn conformations, computational studies
were performed on 4a,b and 5a,b using the Spartan’06 soft-
ware package.15 The reverse turn mimicry of the different
Compounds 4a and 4b show moderate percentage of con-
formers satisfying the b-turn requirements. Compound 4b,
O
O
MeO
MeO
a, b
N
H
COOMe
c
N
H
CONHMe
CONHMe
6
NHBoc
NHBoc
MeO
MeO
10
11
O
O
3
3
MeO
MeO
MeO
MeO
N
CONHMe
N
d,e, f
H
H
NH
H
NH
H
1
1
9
9
NHCbz
NHCbz
12a (1R,3S,9S)
12b (1S,3S,9S)
O
O
MeO
MeO
MeO
MeO
N
H
CONHMe
NHAc
N
CONHMe
g
h, i
H
NMe
H
N
H
NMe
H
O
NHCbz
13a (1R,3S,9S)
13b (1S,3S,9S)
5a (1R,3S,9S)
5b (1S,3S,9S)
Scheme 3. Reagents and conditions: (a) LiOH, THF/water 1:1. (b) L-Alanine-OMe, isobutyl chloroformate, 4-methylmorpholine, DMF (80%, from step a). (c)
MeNH2, methanol (95%). (d) TFA 30% in dichloromethane. (e) N-Cbz-L-alaninal, TFA 5% in dichloromethane. (f) Chromatographic separation (12a: 26%;
12b: 39%, from step d). (g) CH2O, NaCNBH3, 1 M AcOH in methanol (13a: 88%; 13b: 86%). (h) H2, Pd/C, methanol; (i) N-acetyl-L-alanine, isobutyl chloro-
formate, 4-methylmorpholine, DMF (5a: 72%; 5b: 68%, from step h).

5570
G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
Table 1. MC/EM conformational analysis for AETIC structures
a
b
b
% jbj<30ꢀ
% jbj<60ꢀ
% Hydrogen bond
% Hydrogen bond
˚
% da<7 A
Compound No. of
conf.<6 kcal/mol
(10-membered ring)c (14-membered ring b-hairpin conformation)d
4a
4b
5a
5b
64
45 (29)
62 (36)
89 (42)
98 (54)
20 (13)
60 (35)
25.5 (12)
67 (37)
53 (34)
62 (36)
74 (35)
83 (46)
17 (11)
28 (16)
34 (16)
20 (11)
—
—
32 (15)
43 (24)
58
47
55
a
˚
Percentage of conformers for which the distance between Cai and Cai+3 is <7 A. The occurrence numbers are given in parenthesis.
Percentage of conformers in which the virtual torsion angle b (absolute value) is <30ꢀ (or 60ꢀ). The occurrence numbers are given in parenthesis.
Percentage of conformers for which a 10-membered ring hydrogen bond between the carbonyl oxygen at Ci and the hydrogen on Ni+3 is present. The occur-
rence numbers are given in parenthesis. Hydrogen bonds are evaluated by mean of the software ‘hydrogen bonds’ function.
Percentage of conformers for which a 14-membered ring hydrogen bond between the carbonyl oxygen at C4 and the hydrogen on Niꢁ1 is present. This pa-
rameter is associated to a b-hairpin 2:2.19 The occurrence numbers are given in parenthesis. Hydrogen bonds are evaluated by mean of the software ‘hydrogen
bonds’ function.
b
c
d
bearing a 1,3-trans relative configuration, seems to be
slightly favored in assuming a reverse turn conforꢀmation,
angle greater than 90ꢀ is considered as a favorable condition.
Non-planarity about the N–C]O/H torsion angle is ener-
getically unfavorable.
˚
with 62% of conformers having da<7 A and jbj<60 . Char-
acteristic intramolecular hydrogen bond is present in 28% of
conformers for 4b and in 17% of conformers for 4a. On the
contrary, structures 5a and 5b are predicted to easily assume
a b-turn conformation (Fig. 3). In particular 5b, with a 1,3-
trans relative configuration, proꢀduced 98% of conformers
The lowest energy conformation of 4a is not a b-turn mimic.
The first b-turn inducing conformer for 4a is calculated to lie
0.47 kcal/mol above the minimum, suggesting that 4a can
easily adopt the b-turn conformation. Results from the
same analyses for structures 4b, 5a, and 5b, clearly show
that in these cases a b-turn conformation is stabilized.
ꢀ
˚
with da<7 A, 83% with jbj<60 (67% with jbj<30 ), and
20% forming a 10-membered intramolecular hydrogen
bond ring. Structures 5a,b can also exhibit a further hydro-
gen bond involving carbonyl oxygen at C4 and the hydrogen
on Niꢁ1, thus leading to the formation of a 14-membered
ring. The existence of these conformations demonstrates
that the AETIC scaffolds 5a,b have the proper requirements
to induce an antiparallel alignment of the two peptide chains
in a 2:2 type b-hairpin19 arrangement.
2.3. Spectroscopic NMR and IR analyses
To validate the predictions from modeling, we investigated
some structural features of the tetrapeptide mimics Ac–
AETIC–NHMe 4a,b and of the hexapeptide mimics Ac–
(S)-Ala–AETIC–(S)-Ala–NHMe 5a,b. In particular, the
presence of an intramolecular hydrogen bond between the
1
Geometric parameters for the lowest energy conformations
of 4a,b and 5a,b are reported in Table 2, where da and
b values are reported together with data characterizing the
Ci]O/HNi+3 hydrogen bond. In geometrical definition10
of hydrogen bond, it is assumed that the optimum bond
termini of the turn region was investigated by H NMR10
and FTIR11 spectroscopies.
1H NMR spectra were recorded in a relatively nonpolar sol-
vent (i.e., CDCl3)20,21 that doesn’t provide strong hydrogen
bonding competition. As NH chemical shift values d proved
to be independent of concentration below 4.0 mM at 295 K,
˚
distance for O/H is approximately 1.9 A. The related
N–H/O angle should be around 160ꢀ, while a C]O/H
Figure 3. 3D representation of the lowest-energy conformers.

G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
5571
Table 2. Geometric parameters for lowest energy conformers
4a
4b
4.992
1.87
1.819
172.55
117.54
3.02
5a
5b
5.252
ꢁ21.65
˚
da [A]
8.359
4.931
ꢁ20.48
b[ꢀ]
d(C]O–HN) [A]
ꢁ86.22
5.572
148.92
66.73
4.62
1.821a
161.58a
125.80a
6.96a
1.815b
167.24b
141.36b
3.47b
2.008a
160.13a
102.62a
5.22a
1.768b
170.99b
134.83b
2.10b
˚
NHO angle [ꢀ]
C]O–H angle [ꢀ]
N–C]O–H dihedral angle [ꢀ]
Energy [kcal/mol]
85.17
0.47 (85.64)
80.44
0
90.97
96.40
DE of first b-turn [kcal/mol]c
0
0
a
Geometric measurements for 10-membered ring hydrogen bond.
Geometric measurements for 14-membered ring hydrogen bond.
DE is referred to the lowest energy conformer. Absolute energy value is given in parenthesis.
b
c
all analyses were performed on 3.0 mM CDCl3 solutions,
that is, in the absence of a significant aggregation. To ascer-
tain which amide protons are involved in intramolecular
hydrogen bonds, the 1H NMR chemical shifts of the amide
protons and their temperature dependence (Dd/DT) have
been evaluated. In general, chemical shift values of the carb-
oxamide hydrogens higher than 7 ppm in CDCl3, and a low
temperature coefficient (ꢁ2.6 ppb/K or smaller in absolute
value) are indicative of NH involved in strong hydrogen
bonds. A low chemical shift value (d<7.0) and a low temper-
ature coefficient (ꢁ2.6 ppb/K or smaller in absolute value)
are significant of non-hydrogen-bonded amide protons. A
temperature dependence larger than 2.6–3.0 ppb/K in abso-
lute value implies an equilibrium between a hydrogen-
bonded and a non-hydrogen-bonded state, apart from the
chemical shift value.22
tendency seems to be more pronounced in Ac–AETIC–
NHMe 4b.
The IR spectroscopy permits the observation of distinct N–H
stretching absorptions for hydrogen-bonded and non-hydro-
gen-bonded states. The IR spectrum of a 3 mM solution of
4a exhibited two bands of almost equal intensity in the
region of NH stretch, at 3342 cmꢁ1 (hydrogen-bonded state)
and at 3428 cmꢁ1 (non-hydrogen-bonded state). On the
other hand, the IR spectrum of a 3 mM solution of 4b
showed an extensive absorption band at 3326 cmꢁ1 for an
hydrogen-bonded NH stretch and a narrower band at
3432 cmꢁ1 for a non-hydrogen-bonded NH stretch, thus at-
testing the presence of a more stable secondary structure
for 4b.
Similar conformational studies by NMR and IR were also
performed on hexapeptide mimics (Ac–(S)-Ala–AETIC–
(S)-Ala–NHMe) 5a,b. In this case, investigation of both se-
quential and long-range NOEs in NOESY spectra would
provide evidence of preferred conformations and give in-
sight into stable b-hairpin-like conformations (Fig. 4).
Compounds 4a,b possess the minimal structural elements to
form the characteristic intramolecular hydrogen bond that is
a good indication to prove the formation of a reverse turn. In
CDCl3 solution, the NHMe resonates at 7.35 ppm in 4a and
at 7.75 ppm in 4b that is at lower fields with respect to
NHAc, appearing at 5.40 ppm in 4a and 6.32 ppm in 4b
(Table 3).
From NOESY spectrum of 5a, amide NHA was found to be
located internally to the potential reverse turn, in accordance
with the cross-strand NOE cross-peak between NHA and
Me9 that confirms that a tight reverse turn may occur. In ad-
dition, the cross-strand NOEs between NHDAc and H11 and
between NHDAc and NHBMe suggest the formation of
a minimal hairpin.
Temperature-dependent chemical shift changes suggest that
NHMe is probably involved in an equilibrium between a hy-
drogen-bonded and a non-hydrogen-bonded state, both in 4a
and 4b. Indeed, Dd (NHMe)/DT values of ꢁ5.1 ppb/K (for
4a) and ꢁ4.0 ppb/K (for 4b) were observed. As expected,
the non-hydrogen-bonded NHAc showed low temperature
coefficients, both in 4a and 4b.
1
In diastereoisomer 5a the H NMR data show the predo-
minance of a hydrogen-bonded state for NHA (d (CDCl3)
8.21 ppb/K, (Dd/DT) ꢁ3.3 ppb/K), thus confirming the exis-
tence of the classical b-turn hydrogen-bonded 10-membered
ring. The value of chemical shift for amide protons NHBMe
and NHDAc indicated that these H atoms essentially don’t
participate to stable hydrogen bonds. On the other hand,
the NHC chemical shift over 7 ppm, together with the high
value of Dd/DT, are consistent with this atom being involved
in an equilibrium between non-hydrogen-bonded and hydro-
gen-bonded states, at room temperature. This is in accor-
dance with what was predicted from conformational
analysis, that is, to say the presence of stabilized g-turn con-
formers involving NHC and the Ac terminal group.
These data are in good agreement with modeling predic-
tions, according to which both the Ac–AETIC–NHMe 4a
and 4b show a good propensity to adopt a b-turn conforma-
tion, in which the NHMe would be involved in a 10-mem-
bered intramolecular hydrogen bonding. In particular, this
Table 3. Spectroscopic data of tetrapeptide mimics Ac–AETIC–NHMe
4a,b
d (CDCl3) (Dd/DT)a IR absorption
bands
Ac–AETIC–NHMe 4a NHMe 7.35
NHAc 5.40
Ac–AETIC–NHMe 4b NHMe 7.75
NHAc 6.32
ꢁ5.1
ꢁ1.1
ꢁ4.0
ꢁ1.2
3342, 3428
3326, 3432
The diastereoisomer 5b, having a 1,3-trans relative configu-
ration, seems to have a more organized structure. The
NOESY spectrum showed sequential NOEs indicating that
a
ppb/K.

5572
G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
O
Me11
H11
Me
N
O
Me11
H11
Me
N
H3
H3
MeO
MeO
MeO
MeO
N
HA
N
HA
HB
HB
H1
H1
NMe
O
NMe
O
O
O
Me9
Me9
HD
N
H8
H8
H9
Me10
NHD
Me
H9
O
N
HC
N
HC
5a
Me10 H10
Me
O
H11
HB
N
O
H3
MeO
MeO
N
HA
Me
H1
Me11
NMe
O
O
HD
Me9
Me10
H8
Me
N
H9
N
H10
HC
O
5b
Figure 4. Schematic representation of the proposed structures for (Ac–(S)-Ala–AETIC–(S)-Ala–NHMe) 5a,b. The expected hydrogen bonds are shown by
dashed lines. Selected observed NOEs are highlighted by double-edged arrows.
Table 4. Spectroscopic data of hexapeptide mimics (Ac–(S)-Ala–AETIC–(S)-Ala–NHMe) 5a,b
d (CDCl3)
(Dd/DT)
IR absorption bands
3332, 3438
(Ac–(S)-Ala–AETIC–(S)-Ala–NHMe) 5a
(Ac–(S)-Ala–AETIC–(S)-Ala–NHMe) 5b
NHA
NHBMe
NHC
NHDAc
NHA
8.21
6.45
7.07
6.56
8.20
6.34
6.64
8.09
ꢁ3.3
ꢁ2.7
ꢁ13.9
ꢁ5.0
ꢁ2.0
ꢁ2.4
ꢁ1.3
ꢁ5.0
3308, 3441
NHBMe
NHC
NHDAc
the two arms are organized into b-strands. The presence of
cross-strand NOEs between Me10 and Me11 and between
the NMe and Ac groups fully supports the formation of
a b-hairpin conformation. The hydrogen bonding studies
showed the small magnitude of the temperature coefficient
for NHA (d (CDCl3) 8.20 ppb/K, (Dd/DT) ꢁ2.0 ppb/K),
clearly indicating participation of this atom in a strong intra-
molecular 10-membered ring hydrogen-bonding. The high
chemical shift of NHDAc, together with the largetemperature
dependence (d (CDCl3)8.09 ppb/K, (Dd/DT) ꢁ5.0 ppb/K), is
in accord with the presence of a weakly hydrogen-bonded
state for this proton. It participates with a 14-membered intra-
molecular hydrogen bond ring, while NHBMe and NHC are
essentially non-hydrogen-bonded (Table 4).
is a better turn inducer than 1a. In fact, in the case of the tet-
rapeptide mimics Ac–AETIC–NHMe, a b-turn conformation
can be induced more easily in 4b than in 4a. In hexapeptide
mimics (Ac–(S)-Ala–AETIC–(S)-Ala–NHMe), the propen-
sity to adopt a reverse turn conformation is greatly enhanced,
regardless at the stereochemistry at C1. In fact, the 1,3-cis
and 1,3-trans relative configurations, in 5a and 5b, both allow
the formation of a classical b-turn hydrogen-bonded 10-
membered ring. In addition, a b-hairpin-like conformation
can be postulated for 5b in its lowest energy state, based on
an additional hydrogen bond involving a 14-membered intra-
molecular ring.
In order to incorporate these AETIC scaffolds into selected
linear or cyclic peptides, we are currently accomplishing
their synthesis in a suitably protected form. Choosing appro-
priate protecting groups may also enable the introduction of
these templates into SPPS, giving rise to larger peptides
bearing the Tic constraint.
Further evidence of the presence of a stable secondary struc-
ture for 5b was given by the IR spectrum, which exhibited a
broad absorption band at 3308 cmꢁ1 for a hydrogen-bonded
NH stretch and a weak band at 3441 cmꢁ1 for a non-
hydrogen-bonded NH stretch. In the IR spectrum of 5a, we
assigned the hydrogen-bonded NH stretching vibrations at
3332 cmꢁ1 while the free NH appears at 3438 cmꢁ1
(Table 4).
4. Experimental section
4.1. General
3. Conclusion
All solvents were distilled and properly dried (CaH2), when
necessary, prior to use. All chemicals were purchased from
commercial sources and used directly, unless indicated oth-
erwise. All reactions were monitored by thin layer chroma-
tography (TLC) on precoated silica gel 60 F254 (Merck);
spots were visualized with UV light or by treatment with
In this paper, we have described the synthesis of new enantio-
merically pure tetrapeptide and hexapeptide mimics. Both
MM calculations and spectroscopic NMR and IR investiga-
tions support the conclusion that the TIC-based scaffold 1b

G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
5573
1% aq KMnO4 solution. Products were purified by flash
chromatography on Merck silica gel 60 (230–400 mesh).
1H and 13C NMR spectra were recorded with Bruker AC
300 (1H, 300 MHz; 13C, 75.4 MHz) and 400 MHz Avance
(1H, 400 MHz; 13C, 100 MHz) NMR spectrometers.
NOESY spectra were recorded with a mixing time of
700 ms. Chemical shifts are reported in parts per million
downfield from SiMe4 (d¼0.0). The temperature study was
performed on 3 mM CDCl3 solutions, with a temperature in-
crement of 5 K between 297 K and 327 K. For the atom
labeling in the NMR spectra: see Figure 1. HR-EI mass
spectra were measured on VG 70–70 EQ–HF instrument
equipped with its standard sources. Optical rotations were
measured with a Perkin–Elmer 241 polarimeter. IR spectra
were recorded on a Jasco FTIR 300-E spectrometer.
stirring for 3 h at 0 ꢀC, the solvent was evaporated and the
residue oil was treated with saturated NaHCO3 aq solution,
until pH 8 (10 mL). The aqueous phase was extracted with
dichloromethane (3ꢂ15 mL). The combined organic layers
were dried over Na2SO4 and the solvent was removed under
reduced pressure, thus affording the N-Boc deprotected
product (S)-2-amino-3-(3,4-dimethoxyphenyl)-N-methyl-
propionamide as an oil (335 mg). Rf¼0.10 (methanol/ethyl
acetate, 1:9). 1H NMR (CDCl3, 300 MHz): d 6.81 (d,
J¼8.0 Hz, 1H, 8a-H), 6.77 (s, 1H, 5-H), 6.75 (d,
J¼8.0 Hz, 1H, 8-H), 5.31 (s, 1H, NHMe), 3.87 (s, 3H,
ArOCH3), 3.84 (s, 3H, ArOCH3), 3.59 (dd, J¼8.8, 4.0 Hz,
1H, 3-H), 3.19 (dd, J¼14.0, 4.0 Hz, 1H, 4-H), 2.83 (d,
J¼4.8 Hz, 3H, NHCH3), 2.67 (dd, J¼14.0, 8.8 Hz, 1H,
4-H), 2.51 (br s, 2H, NH2).
Molecular modeling: The calculations were carried out us-
ing Spartan ’06 for Windows.14 The MMFF9417 force field
was used in vacuo for the energy minimization of 4a,b and
5a,b. The conformational analyses were carried out with un-
constrained Monte Carlo method. The algorithm was a stan-
dard simulated annealing algorithm. Conformations were
weighted via the normalized Boltzmann criteria. For com-
pounds 4a,b and 5a,b, respectively, 441 and 1089 con-
formers were generated. Only conformations within 6 kcal/
mol of the global minimum were kept. Structures were
minimized using a gradient convergence criterion of
4.5ꢂ10ꢁ4 hartrees/bohr.
Under nitrogen atmosphere, (S)-2-amino-3-(3,4-dimethoxy-
phenyl)-N-methyl-propionamide (300 mg, 1.26 mmol) was
dissolved in 5% TFA/CH2Cl2 solution (10 mL) and cooled
to 0 ꢀC with an ice bath. A solution of N-Cbz-L-alaninal
(260 mg, 1.26 mmol) in 5% TFA/CH2Cl2 solution (5 mL)
was slowly added to the amine solution. After the addition,
the ice bath was removed and the reaction mixture was
stirred at room temperature for 12 h. The solvent was re-
moved under reduced pressure, the residue oil was rinsed
with saturated NaHCO3 aq solution until pH 8 and the aque-
ous phase was extracted with dichloromethane (3ꢂ15 mL).
The organic phase was then dried over Na2SO4 and the
solvent was removed under reduced pressure. The residue
was purified by chromatography over silica gel (ethyl ace-
tate/methanol, 98:2) affording 126 mg (20% yield, from 7)
of 8a as an oil and 377 mg (60% yield, from 7) of 8b as
an oil.
4.1.1. (S)-2-tert-Butoxycarbonylamino-3-(3,4-dimeth-
oxyphenyl)propionic acid methyl ester (6). Compound 6
was prepared according to Ref. 12.
4.1.2. [(S)-2-(3,4-Dimethoxyphenyl)-1-methylcarbamoyl-
ethyl]carbamic acid tert-butyl ester (7). To a solution of
6 (1 g, 2.95 mmol) in dry methanol (10 mL) under nitrogen
atmosphere, methylamine 40% aq solution (10 mL) was
added dropwise. The mixture was stirred for 4 h at room
temperature then the solvent was removed under reduced
pressure affording pure 7 as a foam (977 mg, 98% yield).
Rf¼0.27 (hexane/ethyl acetate, 1:1). [a]2D5 +27.4 (c 1,
CHCl3). 1H NMR (CDCl3, 300 MHz): d 6.78 (d, J¼
6.8 Hz, 1H, 8a-H), 6.70 (d, J¼6.8 Hz, 1H, 8-H), 6.68 (s,
1H, 5-H), 5.78 (br s, 1H, NHMe), 5.06 (br s, 1H, NHBoc),
4.22 (br m, 1H, 3-H), 3.85 (s, 3H, ArOCH3), 3.81 (s, 3H, Ar-
OCH3), 3.04 (dd, J¼13.1, 6.1 Hz, 1H, 4-H), 2.91 (dd, J¼
13.1, 6.7 Hz, 1H, 4-H), 2.70 (d, J¼5.9 Hz, 3H, NHCH3),
1.44 (s, 9H, Boc). 13C NMR (CDCl3, 75 MHz): d 174.2,
154.6, 148.3, 147.5, 128.1, 120.9, 111.8, 110.7, 79.2, 55.3,
55.2, 54.1, 37.2, 27.8, 26.2. HRMS-EI m/z calcd 338.1842,
found 338.1849. Anal. Calcd for C17H26N2O5: C, 60.34%;
H, 7.74%; N, 8.28%; O, 23.64%. Found: C, 60.69%; H,
7.51%; N, 8.12%.
Compound 8a: Rf¼0.40 (methanol/ethyl acetate, 1:9). [a]D25
ꢁ64.5 (c 1, CHCl3). 1H NMR (CDCl3, 400 MHz): d 7.36 (m,
5H, Cbz aromatics), 6.96 (br m, 1H, NHMe), 6.67 (s, 1H, 8-
H), 6.62 (s, 1H, 5-H), 5.34 (d, J¼6.8 Hz, 1H, NHCbz), 5.17
(s, 2H, Cbz-CH2), 4.35 (m, 1H, 9-H), 4.28 (m, 1H, 1-H), 3.86
(s, 3H, ArOCH3), 3.81 (s, 3H, ArOCH3), 3.49 (dd, J¼8.2,
3.6 Hz, 1H, 3-H), 3.07 (dd, J¼15.0, 3.6 Hz, 1H, 4-H), 2.87
(d, J¼5.2 Hz, 3H, NHCH3), 2.68 (dd, J¼15.0, 8.2, 1H,
4-H), 2.24 (br s, 1H, NH), 0.99 (d, J¼6.4 Hz, 3H, 9-CH3).
13C NMR (CDCl3, 100 MHz): d 173.2, 156.2, 148.5,
148.1, 136.8, 128.7–127.8 (5C), 126.5, 126.1, 112.4,
109.1, 67.6, 59.7, 57.4, 55.5, 51.0, 33.0, 26.2, 18.1, 14.8.
HRMS-EI m/z calcd 427.2107, found 427.2101. Anal. Calcd
for C23H29N3O5: C, 64.62%; H, 6.84%; N, 9.83%; O,
18.71%. Found: C, 64.69%; H, 6.79%; N, 9.76%.
Compound 8b: Rf¼0.27 (methanol/ethyl acetate, 1:9). [a]D25
ꢁ81.8 (c 1, CHCl3). 1H NMR (CDCl3, 400 MHz): d 7.32 (m,
5H, Cbz aromatics), 6.79 (br m, 1H, NHMe), 6.71 (s, 1H, 8-
H), 6.56 (s, 1H, 5-H), 5.18 (d, J¼6.7 Hz, 1H, NHCbz), 5.00
(s, 2H, Cbz-CH2), 4.30 (m, 1H, 9-H), 4.08 (m, 1H, 1-H), 3.85
(s, 3H, ArOCH3), 3.77 (s, 3H, ArOCH3), 3.56 (dd, J¼11.2,
3.6 Hz, 1H, 3-H), 2.93 (dd, J¼15.2, 3.6 Hz, 1H, 4-H), 2.86
(d, J¼5.6 Hz, 3H, NHCH3), 2.73 (dd, J¼15.2, 11.2, 1H,
4-H), 2.18 (br s, 1H, NH), 1.30 (d, J¼6.8 Hz, 3H, 9-CH3).
13C NMR (CDCl3, 100 MHz): d 174.0, 156.0, 147.9,
147.7, 136.6, 128.6–127.7 (5C), 126.8, 126.6, 111.7,
109.0, 66.5, 59.7, 57.2, 55.9, 50.4, 33.1, 25.9, 18.6, 14.2.
HRMS-EI m/z calcd 427.2107, found 427.2116. Anal. Calcd
N-Cbz-L-Alaninal was prepared according to Ref. 13.
4.1.3. [(S)-1-((1R,3S)-6,7-Dimethoxy-3-methylcarb-
amoyl-1,2,3,4-tetrahydroisoquinolin-1-yl)ethyl]carbamic
acid benzyl ester (8a) and [(S)-1-((1S,3S)-6,7-dimethoxy-
3-methylcarbamoyl-1,2,3,4-tetrahydroisoquinolin-1-
yl)ethyl]carbamic acid benzyl ester (8b). Product 7
(500 mg, 1.48 mmol) was dissolved in 30% TFA/CH2Cl2
solution (15 mL) under nitrogen atmosphere, at 0 ꢀC. After

5574
G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
for C23H29N3O5: C, 64.62%; H, 6.84%; N, 9.83%; O,
18.71%. Found: C, 64.53%; H, 6.77%; N, 9.88%.
product
(1R,3S)-1-((S)-1-aminoethyl)-6,7-dimethoxy-2-
methyl-1,2,3,4-tetrahydro-isoquinoline-3-carboxylic acid
methylamide as an oil (100 mg). Rf¼0.10 (methanol/ethyl
acetate/NH3, 18:80:2). 1H NMR (CDCl3, 300 MHz):
d 8.02 (d, J¼7.2 Hz, 1H, CONHMe), 6.68 (s, 1H, 8-H),
6.59 (s, 1H, 5-H), 4.05 (m, 1H, 1-H), 3.83 (s, 3H, ArOCH3),
3.80 (s, 3H, ArOCH3), 3.79 (m, 1H, 3-H), 3.06 (m, 2H, 9-H,
4-H), 2.88 (d, J¼4.9 Hz, 3H, CONHCH3), 2.65 (dd, J¼14.8,
3.6 Hz, 1H, 4-H), 2.48 (br s, 2H, NH2), 2.44 (s, 3H, NCH3),
1.18 (d, J¼7.0 Hz, 3H, 9-CH3).
4.1.4. [(S)-1-((1R,3S)-6,7-Dimethoxy-2-methyl-3-methyl-
carbamoyl-1,2,3,4-tetrahydroisoquinolin-1-yl)ethyl]car-
bamic acid benzyl ester (9a). Under nitrogen atmosphere,
8a (200 mg, 0.47 mmol) was dissolved in 1 M AcOH in
MeOH (8 mL), then a 40% formaldehyde aq solution
(65 mL, 0.94 mmol) was added. After 30 min, NaCNBH3
(58 mg, 0.94 mmol) was added in portions and the reaction
was stirred at room temperature for 3 h. Then, a saturated
NaHCO3 aq solution (10 mL) was added and the mixture
was stirred for 30 min. The reaction was extracted with di-
chloromethane (3ꢂ15 mL) and the combined organic layers
were dried over Na2SO4. Evaporation of the solvent afforded
pure product 9a as a pale yellow oil (188 mg, 91% yield).
Rf¼0.21 (ethyl acetate). [a]2D5 ꢁ124.9 (c 1, CHCl3). 1H
NMR (CDCl3, 400 MHz): d 7.36 (m, 5H, Cbz aromatics),
7.30 (br m, 1H, NHMe), 6.65 (s, 1H, 8-H), 6.55 (s, 1H, 5-
H), 5.09 (d, J¼12.1 Hz, 1H, CH2Cbz), 5.03 (d, J¼12.1 Hz,
1H, CH2Cbz), 4.49 (d, J¼8.6 Hz, 1H, NHCbz), 4.17 (m,
1H, 9-H), 3.87 (s, 3H, ArOCH3), 3.85 (s, 3H, ArOCH3),
3.64 (d, J¼3.8 Hz, 1H, 1-H), 2.99 (dd, J¼12.5, 3.6 Hz,
1H, 3-H), 2.90 (dd, J¼15.0, 3.6 Hz, 1H, 4-H), 2.85 (d,
J¼5.2 Hz, 3H, CONHCH3), 2.64 (dd, J¼15.0, 12.5, 1H,
4-H), 2.50 (s, 3H, NCH3), 1.17 (d, J¼6.7 Hz, 3H, 9-CH3).
13C NMR (CDCl3, 100 MHz): d 174.98, 156.37, 148.09,
147.25, 136.29, 128.63–127.97 (5C), 127.54, 124.85,
111.42, 110.79, 68.98, 66.78, 66.28, 56.08, 55.96, 52.96,
45.48, 34.14, 25.93, 18.15. HRMS-EI m/z calcd 441.2264,
found 441.2269. Anal. Calcd for C24H31N3O5: C, 65.29%;
H, 7.08%; N, 9.52%; O, 18.12%. Found: C, 65.18%; H,
6.99%; N, 9.61%.
Under nitrogen atmosphere, (1R,3S)-1-((S)-1-aminoethyl)-
6,7-dimethoxy-2-methyl-1,2,3,4-tetrahydroisoquinoline-3-
carboxylic acid methylamide (90 mg, 0.29 mmol) and
4-(dimethylamino)pyridine (35 mg, 0.29 mmol) were dis-
solved in dry pyridine (4 mL). The solution was cooled to
0 ꢀC with an ice bath and acetic anhydride (55 mL, 0.58
mmol) was added dropwise. The ice bath was removed and
the reaction mixture was stirred overnight at room tempera-
ture. HCl 6 N was added slowly until pH 5 and then the solu-
tion was extracted with dichloromethane (3ꢂ10 mL). The
organic phase was dried over Na2SO4 and the solvent was
removed under reduced pressure. The residue was purified
by chromatography over silica gel (ethyl acetate/methanol,
93:7) affording 4a as an oil (82 mg, 70% yield, from 9a).
Compound 4a: Rf¼0.27 (ethyl acetate). [a]2D5 ꢁ105.1 (c 1,
1
CHCl3). H NMR (CDCl3, 400 MHz): d 7.35 (br m, 1H,
NHMe), 6.69 (s, 1H, 8-H), 6.61 (s, 1H, 5-H), 5.40 (d,
J¼6.9 Hz, 1H, NHAc), 4.27 (m, 1H, 9-H), 4.11 (m, 1H, 1-
H), 3.86 (s, 3H, ArOCH3), 3.84 (s, 3H, ArOCH3), 3.49
(dd, J¼7.6, 3.0 Hz, 1H, 3-H), 3.07 (dd, J¼15.0, 3.0 Hz,
1H, 4-H), 2.87 (d, J¼5.2 Hz, 3H, NHCH3), 2.68 (dd,
J¼15.0, 7.6, 1H, 4-H), 2.40 (s, 3H, NCH3), 2.05 (s, 3H,
NCOCH3), 1.11 (d, J¼6.4 Hz, 3H, 9-CH3). 13C NMR
(CDCl3, 100 MHz): d 172.2, 170.4, 147.5, 147.1, 127.5,
123.8, 111.3, 109.4, 67.6, 66.9, 57.6, 56.8, 48.2, 44.3,
34.0, 26.4, 23.5, 18.1. IR (3 mM CHCl3 solution): 3428,
3342, 3002, 1662, 1517, 1245, 1055, 755 cmꢁ1. HRMS-EI
m/z calcd 349.2002, found 349.2007. Anal. Calcd for
C18H27N3O4: C, 61.87%; H, 7.79%; N, 12.03%; O,
18.32%. Found: C, 61.94%; H, 7.83%; N, 12.08%.
4.1.5. [(S)-1-((1S,3S)-6,7-Dimethoxy-2-methyl-3-methyl-
carbamoyl-1,2,3,4-tetrahydroisoquinolin-1-yl)ethyl]car-
bamic acid benzyl ester (9b). The same procedure for the
preparation of 9a was followed (pale yellow oil, 93% yield).
Rf¼0.42 (ethyl acetate/methanol, 9:1). [a]2D5 ꢁ78.1 (c 1,
1
CHCl3). H NMR (CDCl3, 400 MHz): d 7.79 (br s, 1H,
NHMe), 7.37 (m, 5H, Cbz aromatics), 6.69 (s, 1H, 8-H),
6.56 (s, 1H, 5-H), 5.27 (d, J¼12.3 Hz, 1H, CH2Cbz), 5.11
(d, J¼12.3 Hz, 1H, CH2Cbz), 4.75 (d, J¼9.5 Hz, 1H,
NHCbz), 3.88 (s, 3H, ArOCH3), 3.84 (s, 3H, ArOCH3),
3.79 (m, 1H, 9-H), 3.31 (d, J¼8.4 Hz, 1H, 1-H), 2.92 (dd,
J¼13.8, 2.6 Hz, 1H, 4-H), 2.86 (m, 2H, 4-H, 3-H), 2.75 (d,
J¼4.5 Hz, 3H, CONHCH3), 2.44 (s, 3H, NCH3), 0.96 (d, J¼
6.8 Hz, 3H, 9-CH3). 13C NMR (CDCl3, 100 MHz): d 174.1,
156.3, 147.9, 147.5, 136.3, 128.6–127.8 (5C), 126.7, 126.5,
111.5, 109.8, 67.0, 66.5, 65.9, 56.7, 55.9, 51.4, 46.8, 33.5,
25.9, 18.6. HRMS-EI m/z calcd 441.2264, found 441.2260.
Anal. Calcd for C24H31N3O5: C, 65.29%; H, 7.08%; N,
9.52%; O, 18.12%. Found: C, 65.34%; H, 7.16%; N, 9.60%.
4.1.7. (1S,3S)-1-((S)-1-Acetylaminoethyl)-6,7-dimethoxy-
2-methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic
acid methylamide (4b). The same procedure for the prepa-
ration of 4a was followed (oil, 68% yield, from 9b).
4.1.8. (1S,3S)-1-((S)-1-Aminoethyl)-6,7-dimethoxy-2-
methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid
methylamide. Pale yellow oil, Rf¼0.15 (methanol/ethyl
acetate/NH3, 18:80:2). 1H NMR (CDCl3, 300 MHz):
d 7.96 (d, J¼7.4 Hz, 1H, CONHMe), 6.75 (s, 1H, 8-H),
6.58 (s, 1H, 5-H), 3.83 (s, 3H, ArOCH3), 3.80 (s, 3H, Ar-
OCH3), 3.58 (d, J¼9.2 Hz, 1H, 1-H), 3.11 (m, 2H, 4-H),
2.87 (d, J¼5.2 Hz, 3H, CONHCH3), 3.81 (m, 1H, 3-H),
2.65 (m, 1H, 9-H), 2.47 (s, 3H, NCH3), 2.01 (br s, 2H,
NH2), 1.23 (d, J¼7.6 Hz, 3H, 9-CH3).
4.1.6. (1R,3S)-1-((S)-1-Acetylaminoethyl)-6,7-di-
methoxy-2-methyl-1,2,3,4-tetrahydroisoquinoline-3-car-
boxylic acid methylamide (4a). Product 9a (150 mg,
0.34 mmol) was dissolved in methanol (10 mL) and 10%
Pd/C (15 mg, 10% w/w) was added. The reaction mixture
was then placed under a hydrogen atmosphere (1 bar) and
stirred at room temperature overnight. The mixture was
filtered through Celite and the solvent was evaporated
under reduced pressure to afford the N-Cbz deprotected
Compound 4b: Pale yellow oil, Rf¼0.18 (ethyl acetate).
1
[a]2D5 ꢁ57.5 (c 1, CHCl3). H NMR (CDCl3, 400 MHz):
d 7.75 (br d, J¼4.6 Hz, 1H, CONHMe), 6.73 (s, 1H, 8-H),
6.66 (s, 1H, 5-H), 6.32 (d, J¼8.9 Hz, 1H, NHAc), 5.38 (d,

G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
5575
J¼10.6 Hz, 1H, 1-H), 4.27 (m, 1H, 9-H), 4.13 (m, 1H, 3-H),
3.89 (s, 3H, ArOCH3), 3.86 (s, 3H, ArOCH3), 3.21 (dd,
J¼14.2, 5.3 Hz, 1H, 4-H), 3.10 (dd, J¼14.2, 8.2 Hz, 1H,
4-H), 2.87 (d, J¼4.6 Hz, 3H, CONHCH3), 2.18 (s, 3H,
NCH3), 2.05 (s, 3H, NCOCH3), 1.05 (d, J¼6.4 Hz, 3H, 9-
CH3). 13C NMR (CDCl3, 100 MHz): d 173.4, 170.0, 148.1,
147.9, 127.0, 124.2, 112.1, 110.0, 67.0, 65.7, 57.5, 56.5,
51.3, 42.8, 34.4, 26.4, 23.5, 18.1. IR (3 mM CHCl3 solution):
dry methanol (20 mL) under nitrogen atmosphere, a methyl-
amine 40% aq solution (30 mL) was added dropwise. The
mixture was stirred for 6 h at room temperature, and then
the solvent was removed under reduced pressure affording
pure 11 as an oil (1.42 g, 95% yield). Rf¼0.27 (ethyl ace-
tate/hexane, 1:1). [a]2D5 ꢁ50.1 (c 1, CHCl3). 1H NMR
(CDCl3, 300 MHz): d 6.82 (d, J¼8.0 Hz, 1H, 8-H), 6.70
(dd, J¼8.0, 2.0 Hz, 1H, 8a-H), 6.68 (d, J¼2.0 Hz, 1H, 5-
H), 6.58 (d, J¼7.0 Hz, 1H, HA), 6.40 (d, J¼6.7 Hz, 1H,
HB), 5.05 (d, J¼7.0 Hz, 1H, NHBoc), 4.44 (m, 1H, 3-H),
4.25 (br q, J¼7.1 Hz, 1H, 11-H), 3.85 (s, 3H, ArOCH3),
3.83 (s, 3H, ArOCH3), 3.00 (m, 2H, 4-H), 2.68 (d, J¼6.7,
3H, CONHCH3), 1.44 (s, 9H, Boc), 1.31 (d, J¼7.1 Hz,
3H, 11-CH3). 13C NMR (CDCl3, 75 MHz): d 173.8, 170.2,
156.4, 149.1, 147.9, 129.8, 121.5, 111.8, 110.5, 81.0 56.8,
56.4, 53.8, 52.5, 48.6, 36.4, 25.8, 18.0. HRMS-EI m/z calcd
409.2213, found 409.2219. Anal. Calcd for C20H31N3O6: C,
58.66%; H, 7.63%; N, 10.26%; O, 23.44%. Found: C,
58.60%; H, 7.58%; N, 10.31%.
3432, 3326, 2998, 2949, 1657, 1511, 1254, 805 cmꢁ1
.
HRMS-EI m/z calcd 349.2002, found 349.1998. Anal. Calcd
for C18H27N3O4: C, 61.87%; H, 7.79%; N, 12.03%; O,
18.32%. Found: C, 61.79%; H, 7.84%; N, 12.09%.
4.1.9. (S)-2-[(S)-2-tert-Butoxycarbonylamino-3-(3,4-di-
methoxyphenyl)propionylamino]propionic acid methyl
ester (10). To a THF/water 1:1 solution (50 mL) of 6 (2 g,
5.90 mmol), LiOH (354 mg, 14.75 mmol) was added. After
1 h, the solvent was concentrated in vacuo and 5% H3PO4 aq
solution was added until the solution was at pH 3. The aque-
ous phase was extracted with ethyl acetate (2ꢂ30 mL). The
organic layer was dried over Na2SO4 and the solvent was re-
moved under reduced pressure affording pure (S)-2-tert-but-
oxycarbonylamino-3-(3,4-dimethoxyphenyl)propionic acid
as a foam (1.86 g). Rf¼0.36 (ethyl acetate/methanol, 7:3).
1H NMR (CDCl3, 300 MHz): d 8.11 (br s, 1H, COOH),
7.25 (d, J¼2.6 Hz, 1H, 5-H), 6.83 (d, J¼8.4 Hz, 1H, 8-H),
6.65 (dd, J¼8.4, 2.6 Hz, 1H, 8a-H), 5.06 (d, J¼5.8 Hz,
1H, NHBoc), 4.52 (br m, 1H, 3-H), 3.89 (s, 3H, ArOCH3),
3.81 (s, 3H, ArOCH3), 3.06 (m, 2H, 4-H), 1.44 (s, 9H, Boc).
4.1.11. {(S)-1-[(1R,3S)-6,7-Dimethoxy-3-((S)-1-methyl-
carbamoylethylcarbamoyl)-1,2,3,4-tetrahydroisoquino-
lin-1-yl]ethyl}carbamic acid benzyl ester (12a) and {(S)-
1-[(1S,3S)-6,7-dimethoxy-3-((S)-1-methylcarbamoyl-
ethylcarbamoyl)-1,2,3,4-tetrahydroisoquinolin-1-yl]-
ethyl}carbamic acid benzyl ester (12b). Product11(1.25 g,
3.06 mmol) was dissolved in 30% TFA/CH2Cl2 solution
(15 mL) under nitrogen atmosphere, at 0 ꢀC. The reaction
mixture was stirred for 5 h at 0 ꢀC, then the solvent was
evaporated and the residue oil was treated with saturated
NaHCO3 aq solution (20 mL) until pH 8. The aqueous phase
was extracted with dichloromethane (3ꢂ20 mL). The com-
bined organic layers were dried over Na2SO4 and the solvent
was removed under reduced pressure. The N-Boc depro-
tected product (S)-2-amino-3-(3,4-dimethoxyphenyl)-N-
((S)-1-methylcarbamoylethyl)propionamide (908 mg) was
obtained as an oil without further purification. Rf¼0.08
(methanol/ethyl acetate, 1:9). 1H NMR (CDCl3, 300 MHz):
d 7.66 (d, J¼7.2 Hz, 1H, HA), 6.78 (d, J¼8.1 Hz, 1H, 8-H),
6.72 (dd, J¼8.2, 2.4 Hz, 1H, 8a-H), 6.70 (d, J¼2.4 Hz, 1H,
5-H), 6.45 (d, J¼6.2 Hz, 1H, HB), 4.40 (m, 1H, 11-H), 3.85
(s, 3H, ArOCH3), 3.83 (s, 3H, ArOCH3), 3.58 (dd, J¼9.8,
3.4 Hz, 1H, 3-H), 3.15 (dd, J¼11.4, 3.4 Hz, 1H, 4-H), 2.68
(d, J¼6.2 Hz, 3H, CONHCH3), 2.63 (dd, J¼11.4, 9.8 Hz,
1H, 4-H), 1.56 (s, 2H, NH2), 1.34 (d, J¼7.1 Hz, 3H, 11-CH3).
Under nitrogen atmosphere, (S)-2-tert-butoxycarbonyl-
amino-3-(3,4-dimethoxyphenyl)propionic acid (1.70 g,
5.23 mmol) was dissolved in dry DMF (12 mL). The solu-
tion was cooled to ꢁ20 ꢀC and 4-methylmorpholine
(1.15 mL, 10.46 mmol) and isobutyl chloroformate
(681 mL, 5.23 mmol) were added. After 30 min, a solution
of L-alanine-OMe hydrochloride (730 mg, 5.23 mmol) in
dry DMF (5 mL) was added dropwise. The mixture was
stirred at room temperature overnight. A 5% NaHCO3 aq so-
lution (25 mL) was added and the mixture was extracted
with dichloromethane (3ꢂ15 mL). The organic phase was
washed with brine, then with 20 mL of H3PO4 5% aq solu-
tion, dried over Na2SO4, and concentrated under reduced
pressure to give pure 10 as an oil (1.93 g, 80% yield).
Rf¼0.27 (ethyl acetate/hexane, 1:1). [a]2D5 ꢁ50.1 (c 1,
CHCl3). 1H NMR (CDCl3, 400 MHz):
d 6.81 (d,
J¼7.9 Hz, 1H, 8-H), 6.77 (dd, J¼7.9, 1.5 Hz, 1H, 8a-H),
6.75 (d, J¼1.5 Hz, 1H, 5-H), 6.43 (d, J¼7.1 Hz, 1H, HA),
5.03 (br d, 1H, NHBoc), 4.52 (m, 1H, 3-H), 4.34 (br q,
J¼7.1 Hz, 1H, 11-H), 3.87 (s, 3H, ArOCH3), 3.87 (s, 3H, Ar-
OCH3), 3.73 (s, 3H, COOCH3), 3.07 (dd, J¼13.8, 6.3 Hz,
1H, 4-H), 2.99 (dd, J¼13.8, 6.8 Hz, 1H, 4-H), 1.44 (s, 9H,
Boc), 1.36 (d, J¼7.1 Hz, 3H, 11-CH3). 13C NMR (CDCl3,
100 MHz): d 173.5, 171.5, 156.0, 149.9, 148.9, 129.8,
122.2, 113.4, 112.3, 80.9, 56.7, 56.6, 54.1, 53.1, 48.9,
38.7, 29.0, 18.4. HRMS-EI m/z calcd 410.2053, found
410.2044. Anal. Calcd for C20H30N2O7: C, 58.52%; H,
7.37%; N, 6.82%; O, 27.29%. Found: C, 58.42%; H,
7.45%; N, 6.75%.
Under nitrogen atmosphere, (S)-2-amino-3-(3,4-dimeth-
oxyphenyl)-N-((S)-1-methylcarbamoylethyl)propionamide
(900 mg, 2.91 mmol) was dissolved in 5% TFA/CH2Cl2 solu-
tion (20 mL) and cooled to 0 ꢀC with an ice bath. A solution
of N-Cbz-L-alaninal (602 mg, 2.91 mmol) in 5% TFA/
CH2Cl2 solution (8 mL) was slowly added to the amine solu-
tion. After the addition, the ice bath was removed and the re-
action mixture was stirred at room temperature for 12 h. The
solvent was removed under reduced pressure and the residue
oil was rinsed with saturated NaHCO3 aq solution (20 mL)
until pH 8 and the aqueous phase was extracted with di-
chloromethane (3ꢂ20 mL). The organic phase was then
dried over Na2SO4 and the solvent was removed under re-
duced pressure. The residue was purified by chromatography
over silica gel (ethyl acetate/methanol, 98:2) affording
337 mg (26% yield) of 12a as an oil and 505 mg (39% yield)
of 12b as an oil.
4.1.10. [(S)-2-(3,4-Dimethoxyphenyl)-1-((S)-1-methyl-
carbamoylethylcarbamoyl)ethyl]carbamic acid tert-bu-
tyl ester (11). To a solution of 10 (1.5 g, 3.66 mmol) in

5576
G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
Compound 12a: Rf¼0.18 (methanol/ethyl acetate, 1:9). [a]D25
ꢁ84.5 (c 1, CHCl3). 1H NMR (CDCl3, 400 MHz): d 7.55 (d,
J¼7.7 Hz, 1H, HA), 7.38 (m, 5H, Cbz aromatics), 6.69 (s,
1H, 8-H), 6.61 (s, 1H, 5-H), 6.51 (d, J¼4.8 Hz, 1H, HB),
5.63 (d, J¼8.3 Hz, 1H, NHCbz), 5.16 (d, J¼12.0 Hz, 1H,
Cbz-CH2), 5.11 (d, J¼12.0 Hz, 1H, Cbz-CH2), 4.49 (m,
1H, 11-H), 4.30 (m, 2H, 1-H+9-H), 3.87 (s, 3H, ArOCH3),
3.84 (s, 3H, ArOCH3), 3.53 (dd, J¼11.6, 3.7 Hz, 1H, 3-H),
3.00 (dd, J¼15.5, 3.7 Hz, 1H, 4-H), 2.82 (d, J¼4.8 Hz,
3H, CONHCH3), 2.69 (dd, J¼15.5, 11.6 Hz, 1H, 4-H),
2.38 (s, 1H, NH), 1.41 (d, J¼7.0 Hz, 3H, 11-CH3), 0.99 (d,
J¼6.6 Hz, 3H, 9-CH3). 13C NMR (CDCl3, 100 MHz):
d 174.2, 173.5, 156.6, 148.7, 148.6, 137.2, 129.3–128.7
(5C), 127.6, 127.5, 112.5, 109.6, 67.5, 59.8, 57.4, 56.9,
56.6, 52.0, 49.2, 33.8, 27.0, 18.6, 15.0. HRMS-EI m/z calcd
498.2478, found 498.2477. Anal. Calcd for C26H34N4O6: C,
62.63%; H, 6.87%; N, 11.24%; O, 19.25%. Found: C,
62.68%; H, 6.91%; N, 11.27%.
148.9, 148.4, 137.2, 129.2–128.7 (5C), 127.6, 126.9,
112.2, 111.6, 69.6, 68.2, 67.4, 56.8, 56.7, 54.2, 49.3, 47.0,
34.1, 26.9, 18.6, 17.9. HRMS-EI m/z calcd 512.2635, found
512.2639. Anal. Calcd for C27H36N4O6: C, 63.26; H, 7.08%;
N, 10.93%; O, 18.73%. Found: C, 63.21; H, 7.11%; N,
10.88%.
4.1.13. {(S)-1-[(1S,3S)-6,7-Dimethoxy-2-methyl-3-((S)-1-
methylcarbamoylethylcarbamoyl)-1,2,3,4-tetrahydro-
isoquinolin-1-yl]ethyl}carbamic acid benzyl ester (13b).
The same procedure for the preparation of 13a as an oil
was followed (86% yield). Rf¼0.22 (methanol/ethyl acetate,
1:9). [a]2D5 ꢁ58.1 (c 1, CHCl3). 1H NMR (CDCl3, 400 MHz):
d 8.31 (d, J¼7.7 Hz, 1H, HA), 7.39 (m, 5H, Cbz aromatics),
6.68 (s, 1H, 8-H), 6.56 (s, 1H, 5-H), 6.43 (br q, J¼5.2 Hz,
1H, HB), 5.24 (d, J¼12.0 Hz, 1H, Cbz-CH2), 5.13 (d,
J¼12.0 Hz, 1H, Cbz-CH2), 4.89 (d, J¼8.0 Hz, 1H, NHCbz),
4.50 (m, 1H, 11-H), 3.88 (s, 3H, ArOCH3), 3.84 (br m, 1H,
9-H), 3.82 (s, 3H, ArOCH3), 3.71 (br d, J¼8.4 Hz, 1H, 1-H),
2.90 (dd, J¼12.0, 3.2 Hz, 1H, 3-H), 2.83 (m, 2H, 4-H), 2.81
(d, J¼5.2 Hz, 3H, CONHCH3), 2.49 (s, 3H, NCH3), 1.50 (d,
Compound 12b: Rf¼0.12 (methanol/ethyl acetate, 1:9).
1
[a]2D5 ꢁ101.8 (c 1, CHCl3). H NMR (CDCl3, 400 MHz):
d 7.40 (q, J¼2.9 Hz, 1H, HA), 7.31 (m, 5H, Cbz aromatics),
6.75 (s, 1H, 8-H), 6.59 (s, 1H, 5-H), 6.32 (d, J¼4.7 Hz, 1H,
HB), 5.24 (br m, 1H, NHCbz), 5.02 (s, 2H, Cbz-CH2), 4.48
(m, 1H, 11-Ala), 4.35 (br m, 1H, 9-H), 4.09 (m, 1H, 1-H),
3.87 (s, 3H, ArOCH3), 3.82 (s, 3H, ArOCH3), 3.60 (dd,
J¼10.6, 3.8 Hz, 1H, 3-H), 2.94 (dd, J¼15.1, 3.8 Hz, 1H,
4-H), 2.83 (d, J¼4.7 Hz, 3H. NHCH3), 2.77 (dd, J¼15.1,
10.6 Hz, 1H, 4-H), 2.23 (s, 1H, NH), 1.39 (d, J¼7.0 Hz,
3H, 11-CH3), 1.32 (d, J¼7.0 Hz, 3H, 9-CH3). 13C NMR
(CDCl3, 100 MHz): d 174.0, 173.3, 156.8, 148.7, 148.6,
137.4, 129.3–128.5 (5C), 127.5, 127.3, 112.1, 110.0, 67.2,
60.3, 57.6, 56.7, 56.6, 50.9, 49.3, 33.6, 27.0, 19.4, 18.7.
HRMS-EI m/z calcd 498.2478, found 498.2480. Anal. Calcd
for C26H34N4O6: C, 62.63%; H, 6.87%; N, 11.24%; O,
19.25%. Found: C, 62.69%; H, 6.81%; N, 11.30%.
J¼6.8 Hz, 3H, 11-CH3), 0.96 (d, J¼6.8 Hz, 3H, 9-CH3). 13
C
NMR (CDCl3, 100 MHz): d 175.0, 173.1, 156.7, 148.2,
147.3, 136.4, 129.2–128.7 (5C), 127.4, 126.5, 112.0,
110.6, 70.8, 67.9, 67.0, 56.0, 55.9, 53.35, 48.5, 46.1, 33.3,
26.3, 18.3, 16.8. HRMS-EI m/z calcd 512.2635, found
512.2637. Anal. Calcd for C27H36N4O6: C, 63.26; H,
7.08%; N, 10.93%; O, 18.73%. Found: C, 63.28; H,
7.10%; N, 10.96%.
4.1.14. (1R,3S)-1-[(S)-1-((S)-2-Acetylaminopropionyl-
amino)ethyl]-6,7-dimethoxy-2-methyl-1,2,3,4-tetrahy-
droisoquinoline-3-carboxylic acid ((S)-1-methyl-
carbamoylethyl)amide (5a). Product 13a (400 mg,
0.78 mmol) was dissolved in methanol (10 mL) and 10%
Pd/C (40 mg, 10% w/w) was added. The reaction mixture
was then placed under a hydrogen atmosphere (1 bar) and
stirred at room temperature overnight. The mixture was
filtered through Celite and the solvent was evaporated
under reduced pressure to afford the N-Cbz deprotected
4.1.12. {(S)-1-[(1R,3S)-6,7-Dimethoxy-2-methyl-3-((S)-1-
methylcarbamoylethylcarbamoyl)-1,2,3,4-tetrahydro-
isoquinolin-1-yl]ethyl}carbamic acid benzyl ester (13a).
Under nitrogen atmosphere, 12a (500 mg, 1.00 mmol) was
dissolved in 1 M AcOH in MeOH (10 mL), then a 40%
formaldehyde aq solution (138 mL, 2.00 mmol) was added.
After 30 min, NaCNBH3 (126 mg, 2.00 mmol) was added
in portions and the reaction was stirred at room temperature
for 4 h. Saturated NaHCO3 aq solution (10 mL) was added
and the mixture was stirred for 30 min. The reaction was ex-
tracted with dichloromethane (3ꢂ15 mL) and the combined
organic layers were dried over Na2SO4. Evaporation of the
solvent afforded pure product 13a as an oil (451 mg, 88%
yield). Rf¼0.24 (methanol/ethyl acetate, 1:9). [a]2D5 ꢁ104.9
(c 1, CHCl3). 1H NMR (CDCl3, 400 MHz): d 7.96 (d,
J¼7.8 Hz, 1H, HA), 7.32 (m, 5H, Cbz aromatics), 6.64 (s,
1H, 8-H), 6.61 (s, 1H, 5-H), 6.54 (br q, J¼4.7 Hz, 1H,
HB), 5.18 (d, J¼8.2 Hz, 1H, NHCbz), 5.12 (d, J¼12.0 Hz,
1H, Cbz-CH2), 5.09 (d, J¼12.0 Hz, 1H, Cbz-CH2), 4.46
(m, 1H, 11-H), 4.08 (br m, 1H, 9-H), 3.86 (s, 3H, ArOCH3),
3.83 (s, 3H, ArOCH3), 3.71 (br m, 1H, 1-H), 2.97 (dd,
J¼12.3, 3.5 Hz, 1H, 3-H), 2.90 (dd, J¼15.0, 3.5 Hz, 1H,
4-H), 2.82 (d, J¼4.7 Hz, 3H, CONHCH3), 2.67 (dd,
J¼15.0, 12.3 Hz, 1H, 4-H), 2.49 (s, 3H, NCH3), 1.44 (d,
J¼7.0 Hz, 3H, 11-CH3), 1.17 (d, J¼6.8 Hz, 3H, 9-CH3).
13C NMR (CDCl3, 100 MHz): d 175.2, 173.6, 156.6,
product
methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic
(1R,3S)-1-((S)-1-aminoethyl)-6,7-dimethoxy-2-
acid
((S)-1-methylcarbamoylethyl)amide as an oil (283 mg).
1
Rf¼0.14 (methanol/ethyl acetate/NH3, 18:80:2). H NMR
(CDCl3, 300 MHz): d 8.11 (d, J¼7.8 Hz, 1H, HA), 6.68 (br
q, J¼5.0 Hz, 1H, HB), 6.57 (s, 1H, 8-H), 6.41 (s, 1H, 5-H),
4.48 (m, 1H, 11-H), 4.08 (br m, 1H, 1-H), 3.81 (s, 3H, Ar-
OCH3), 3.78 (s, 3H, ArOCH3), 3.61 (m, 1H, 9-H), 3.45
(m, 1H, 3-H), 3.12 (dd, J¼14.8, 3.8 Hz, 1H, 4-H), 2.92 (m,
1H, 4-H), 2.81 (d, J¼5.0 Hz, 3H, CONHCH3), 2.51 (s, 3H,
NCH3), 2.46 (br s, 2H, NH2), 1.51 (d, J¼7.0 Hz, 3H, 11-
CH3), 1.05 (d, J¼7.0 Hz, 3H, 9-CH3).
Under nitrogen atmosphere, L-alanine-OMe hydrochloride
(91 mg, 0.66 mmol) was dissolved in dry DMF (4 mL).
The solution was cooled to ꢁ20 ꢀC and 4-methylmorpholine
(144 mL, 1.32 mmol) and isobutyl chloroformate (86 mL,
0.66 mmol) were added. After 30 min, a solution of (1R,3S)-
1-((S)-1-aminoethyl)-6,7-dimethoxy-2-methyl-1,2,3,4-tetra-
hydroisoquinoline-3-carboxylic acid ((S)-1-methylcarb-
amoylethyl)amide (250 mg, 0.66 mmol) in dry DMF (2 mL)
was added dropwise. The mixture was stirred at room tem-
perature overnight. Then 5% NaHCO3 aq solution (12 mL)

G. Lesma et al. / Tetrahedron 63 (2007) 5567–5578
5577
was added and the mixture was extracted with dichlorome-
thane (3ꢂ10 mL). The organic phase was washed with brine
(2ꢂ20 mL), then with 20 mL of H3PO4 5% aq solution,
dried over Na2SO4 and concentrated under reduced pressure.
The residue was purified by chromatography over silica gel
(dichloromethane/methanol, 97:3) affording 5a (276 mg,
72%, from 13a), as a foam. Rf¼0.24 (methanol/ethyl acetate,
1:9). [a]2D5 ꢁ121.9 (c 1, CHCl3). 1H NMR (CDCl3,
400 MHz): d 8.21 (d, J¼7.6 Hz, 1H, HA), 7.07 (d,
J¼8.6 Hz, 1H, HC), 6.63 (s, 1H, 5-H), 6.56 (d, J¼7.4 Hz,
1H, HD), 6.49 (s, 1H, 8-H), 6.45 (br q, J¼8.7 Hz, 1H, HB),
4.51 (dq, J¼7.4, 3.5 Hz, 1H, 10-H), 4.47 (br q, J¼7.1 Hz,
1H, 11-H), 4.17 (dq, J¼6.7, 4.5 Hz, 1H, 9-H), 3.88 (s, 3H,
ArOCH3), 3.84 (s, 3H, ArOCH3), 3.64 (m, 2H, 3-H+1-H),
3.22 (dd, J¼17.1, 6.8 Hz, 1H, 4-H), 2.87 (m, 4H, 4-
H+NHCH3), 2.52 (s, 3H, NCH3), 2.00 (s, 3H, COCH3),
1.39 (d, J¼7.0 Hz, 3H, 11-CH3), 1.34 (d, J¼7.0 Hz, 3H,
10-CH3), 1.08 (d, J¼4.5 Hz, 3H, 9-CH3). 13C NMR
(CDCl3, 100 MHz): d 174.1, 173.2, 172.4, 171.7, 148.7,
146.9, 125.4, 121.3, 112.8, 111.4, 60.1, 58.6, 56.1, 55.9,
51.3, 49.2, 48.3, 39.9, 27.3, 26.4, 23.1, 18.9, 18.3, 18.2. IR
(3 mM CHCl3 solution): 3332, 3438, 2995, 1665, 1518,
calcd 491.2744, found 491.2741. Anal. Calcd for
C24H37N5O6: C, 58.64%; H, 7.59%; N, 14.25%; O,
19.53%. Found: C, 58.60%; H, 7.55%; N, 14.22%.
References and notes
1. For reviews, see: (a) Gentilucci, L.; Tolomelli, A.; Squassabia,
F. Curr. Med. Chem. 2006, 13, 2449–2466; (b) Perez, J. J.;
Corcho, F.; Llorens, O. Curr. Med. Chem. 2002, 9, 2209–
2229; (c) Bursavich, M. G.; Rich, D. H. J. Med. Chem. 2002,
45, 541–558; (d) Haubner, R.; Finsinger, D.; Kessler, H.
Angew. Chem., Int. Ed. 1997, 36, 1374–1389.
2. For reviews, see: (a) Ohfune, Y.; Shinada, T. Eur. J. Org. Chem.
2005, 24, 5127–5143; (b) Cheng, R. P.; Gellman, S. H.;
DeGrado, W. F. Chem. Rev. 2001, 101, 3219–3232; (c)
Gibson, S. E.; Guillo, N.; Tozer, M. J. Tetrahedron 1999, 55,
585–615; (d) Hanessian, S.; McNaughton-Smith, G.;
Lombart, H.-G.; Lubell, W. D. Tetrahedron 1997, 53, 12789–
12854.
3. (a) Ball, J. B.; Hughes, R. A.; Alewood, P. F.; Andrews, P. R.
Tetrahedron 1993, 49, 3467–3478; (b) Grison, C.; Coutrot, P.;
1450, 1256, 910, 704 cmꢁ1
.
HRMS-EI m/z calcd
Geneve, S.; Didierjean, C.; Marraud, M. J. Org. Chem. 2005,
ꢀ
491.2744, found 491.2749. Anal. Calcd for C24H37N5O6:
C, 58.64; H, 7.59%; N, 14.25%; O, 19.53%. Found: C,
58.69; H, 7.63%; N, 14.20%.
70, 10753–10764; (c) Trabocchi, A.; Occhiato, E. G.;
Potenza, D.; Guarna, A. J. Org. Chem. 2002, 67, 7483–7492;
(d) Chakraborty, T. K.; Srinivasu, P.; Vengal Rao, R.; Kiran
Kumar, S.; Kunwar, A. C. J. Org. Chem. 2004, 69, 7399–7402.
4. (a) Perdih, A.; Kikelj, D. Curr. Med. Chem. 2006, 13, 1525–
1556 and references cited therein; (b) Freidinger, R. M.
J. Med. Chem. 2003, 46, 5553–5566; (c) Cordero, F. M.;
Pisaneschi, F.; Meschini Batista, K.; Valenza, S.; Machetti,
F.; Brandi, A. J. Org. Chem. 2005, 70, 856–867; (d)
Bittermann, H.; Gmeiner, P. J. Org. Chem. 2006, 71, 97–102;
(e) Andreu, D.; Ruiz, S.; Carren˜o, C.; Alsina, J.; Albericio,
4.1.15. (1S,3S)-1-[(S)-1-((S)-2-Acetylaminopropionyl-
amino)ethyl]-6,7-dimethoxy-2-methyl-1,2,3,4-tetrahy-
droisoquinoline-3-carboxylic acid ((S)-1-methyl-
carbamoylethyl)amide (5b). The same procedure for the
preparation of 5a was followed (foam, 68% yield, from 13b).
4.1.16. (1R,3S)-1-((S)-1-Amino-ethyl)-6,7-dimethoxy-2-
methyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid
((S)-1-methylcarbamoylethyl)amide. Oil, Rf¼0.11 (meth-
anol/ethyl acetate/NH3, 18:80:2). 1H NMR (CDCl3,
300 MHz): d 8.01 (d, J¼7.4 Hz, 1H, HA), 6.81 (br q,
J¼5.2 Hz, 1H, HB), 6.55 (s, 1H, 8-H), 6.48 (s, 1H, 5-H),
4.39 (m, 1H, 11-H), 3.81 (s, 3H, ArOCH3), 3.79 (s, 3H,
ArOCH3), 3.65 (br m, 1H, 1-H), 3.28 (m, 1H, 3-H), 3.24
(m, 1H, 9-H), 3.04 (dd, J¼13.5, 4.2 Hz, 1H, 4-H), 2.80
(d, J¼5.2 Hz, 3H, CONHCH3), 2.71 (m, 1H, 4-H), 2.45
(s, 3H, NCH3), 2.15 (br s, 2H, NH2), 1.50 (d, J¼6.7 Hz,
3H, 11-CH3), 1.15 (d, J¼6.8 Hz, 3H, 9-CH3).
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1
[a]2D5 ꢁ85.1 (c 1, CHCl3). H NMR (CDCl3, 400 MHz):
d 8.20 (d, J¼9.1 Hz, 1H, HA), 8.09 (d, J¼8.8 Hz, 1H, HD),
6.69 (s, 1H, 5-H), 6.64 (d, J¼9.7 Hz, 1H, HC), 6.57 (s, 1H,
8-H), 6.34 (q, J¼4.8 Hz, 1H, HB), 4.70 (m, 1H, 10-H),
4.59 (m, 1H, 11-H), 3.98 (m, 1H, 9-H), 3.88 (s, 3H, Ar-
OCH3), 3.87 (s, 3H, ArOCH3), 3.14 (d, J¼10.2 Hz, 1H, 1-
H), 2.91 (dd, J¼12.4, 3.2 Hz, 1H, 4-H), 2.81 (d, J¼4.8 Hz,
3H, NHCH3), 2.79 (dd, J¼3.2, 2.2 Hz, 1H, 3-H), 2.74 (dd,
J¼12.4, 2.2 Hz, 1H, 4-H), 2.25 (s, 3H, NCH3), 2.02 (s,
3H, COCH3), 1.55 (d, J¼7.1 Hz, 3H, 11-CH3), 1.44 (d,
J¼6.9 Hz, 3H, 10-CH3), 0.97 (d, J¼6.7 Hz, 3H, 9-CH3).
13C NMR (CDCl3, 100 MHz): d 175.0, 173.9, 172.7,
171.2, 149.5, 147.8, 128.1, 127.1, 112.5, 110.7, 71.5, 69.3,
56.2, 56.0, 51.2, 48.2, 48.1, 45.8, 33.2, 26.2, 23.0, 18.8,
16.8, 15.8. IR (3 mM CHCl3 solution): 3441, 3308, 2994,
1660, 1514, 1455, 1246, 794, 775 cmꢁ1. HRMS-EI m/z
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