Preparation of Constrained Unnatural Aromatic Amino Acids via Unsaturated Diketopiperazine Intermediate

Article abstract of DOI:10.1002/jhet.2489

Unnatural aromatic amino acids are useful tools in drug discovery, since their insertion in bioactive peptide sequences can change the side chains spatial orientation, the backbone conformation and above all, their bioactivity. In this communication, we propose a straightforward method to synthesize 2′,6′-dimethyl-tyrosine and 2′,6′-dimehylphenyl-alanine derivatives as handling building blocks for peptide synthesis via unsaturated diketopiperazine (DKP) intermediate.

Full text of DOI:10.1002/jhet.2489

Month 2015
Preparation of Constrained Unnatural Aromatic Amino Acids via
Unsaturated Diketopiperazine Intermediate
Adriano Mollica, Roberto Costante, Sako Mirzaie, Simone Carradori, Giorgia Macedonio,
Azzurra Stefanucci,c and Ettore Novellinod
a
a
b
a
a
*
aDipartimento di Farmacia, Università di Chieti-Pescara “G. d’Annunzio”, Via dei Vestini 31, 66100 Chieti, Italy
bDepartment of Biochemistry, Sanandaj Branch, Islamic Azad University, Sanandaj, Iran
cDipartimento di Chimica, Sapienza, Università di Roma, P.le A. Moro 5, 00187 Rome, Italy
dDipartimento di Farmacia, Università di Napoli “Federico II”, Via D. Montesano, 49, 80131 Naples, Italy
*
E-mail: a.mollica@unich.it
Additional Supporting Information may be found in the online version of this article.
Received April 16, 2015
DOI 10.1002/jhet.2489
Published online 00 Month 2015 in Wiley Online Library (wileyonlinelibrary.com).
Unnatural aromatic amino acids are useful tools in drug discovery, since their insertion in bioactive pep-
tide sequences can change the side chains spatial orientation, the backbone conformation and above all, their
bioactivity. In this communication, we propose a straightforward method to synthesize 2′,6′-dimethyl-tyro-
sine and 2′,6′-dimehylphenyl-alanine derivatives as handling building blocks for peptide synthesis via unsat-
urated diketopiperazine (DKP) intermediate.
J. Heterocyclic Chem., 00, 00 (2015).
INTRODUCTION
approach could be the reduction of native residues
conformational freedom, in order to obtain the most
favorable topology for the binding to one preferred
receptor subtype [9]. This approach can be virtually
applied to all bioactive peptidomimetics, but the extremely
high cost of such unnatural amino acids and the complexity
of the previously reported preparations can easily
discourage the researchers to the use of these tools in their
design. In this paper, we report the development of a
simple, general method to obtain unnatural constrained
aromatic amino acids as building blocks suitable for
peptide synthesis as partially reported by us in the patent
titled “Sintesi enantioselettiva di amminoacidi aromatici
non-naturali”, Application Number: RM2015A000091,
date: 27/02/2015.
A crucial step of the synthesis is the control of the
stereochemistry of the final product which is generally
achieved by preparative HPLC (in case of intermediate
diastereoisomers) [10] or by using stereoselective
hydrogenation with [Rh(1,5-COD)-(R,R-DIPAMP)]BF4
as catalyst [11,12]. The latter yielded the unnatural amino
acid 2′,6′-dimethyl-L-tyrosine in high purity on the
kilogram scale, but it required high catalyst concentrations
and prolonged reaction times [13] with respect to our
proposed method.
It is well-known that a small change of the sequence of a
native peptide ligand, such as the incorporation of
unnatural amino acids, may contribute markedly to the
biological profile of the peptide. As an example, it has been
demonstrated that the introduction of 2′,6′-dimethyl-
tyrosine (DMT) in the place of the native Tyrosine1 and
2′,6′-dimethyl-phenylalanine (DMP) in place of Phenylala-
nine3 in the neuropeptide endomorphin-2 yielded very
potent analogs, and in the latter case to an enhancement
of the binding affinity and selectivity to μ opioid receptor
(Table 1) [1,2]. This change in the biological profile may
be explained with the concept of χ space [3–5]. Side chains
of natural amino acids, usually possess a certain degree of
rotational freedom around χ dihedral angles (Fig. 1), giving
to the native peptide ligands the possibility to adapt their
shape to the binding pocket of a wide range of receptor
subtypes, as in the case of enkephalins and endomorphins,
which have flexible structures capable to bind to both μ
and δ receptors with the selectivity ranging from none to
good [6–8]. In nature, selectivity and duration of action
of native peptides are usually controlled by a site-directed
biosynthesis and in situ degradation. In peptidomimetics
design, to achieve a high potency and selectivity, one
© 2015 HeteroCorporation

A. Mollica, R. Costante, S. Mirzaie, S. Carradori, G. Macedonio, A. Stefanucci, and E. Novellino Vol 000
Table 1
DISCUSSION
Biological effect of the insertion of DMT and DMP in the endomorphin-2
sequence [1,2].
Asymmetric synthesis of sterically constrained amino
acid is characterized by a high cost and the use of expen-
sive Rh catalysts and Ni(II)-complexes of chiral Schiff
base of glycine or alanine. The here reported synthetic
strategy for compounds 7–10 is characterized by α,β unsat-
urated 2,5-diketopiperazines (5a,b) as the key intermediate
products (Scheme 1). Reagents and solvents were conve-
niently purchased from the common commercial sources
and the experimental procedures are straightforward and
with high chemical yields. The procedure starts with the
coupling between Boc-Valine (1) to HCl.Glycine methyl
ester and the resulting dipeptide was easily converted to
the DPK (2) in high overall yield. The DKP (2) was further
diacetylated by treatment with acetic anhydride [14]. Reac-
tion of the N′,N-diacetyl derivative (3) with potassium tert-
butoxide in DMF in the presence of an appropriate com-
mercial 2,6-dimethylbenzaldehyde (in the case of DMP)
or newly synthesized (11) aldehyde (in the case of DMT,
as outlined in Scheme 2) yielded the corresponding
Compounds
Kiμ
Kiδ
Kiκ
δ/μ ratio
>3000
EM-2
1.33 0.15 6085 1215 >4000
0.26 0.91
[DMT]1-
EM2
99.2 7.9 489 135 1880
[DMP]3-
EM2
0.044 0.003 1440 94
>4000
>91 000
Figure 1. Definition of χ1 and χ2 dihedral angles.
Scheme 1. Reactions and conditions: a) Boc-Val-OH, EDC·HCl, HOBt, NMM, DMF, r.t., overnight; b) TFA/CH2Cl2 1:1, r.t., 1 h, under N2; c) AcOH/2-
butanol, NMM, 120 °C, 3 h; d) Ac2O, reflux, overnight; e) 11 or 2,6-dimethylbenzaldehyde, tBuOK, DMF, r.t., 4 h; f) NH2–NH2·H2O, MeOH, 2 h; g) H2,
Pd/C 10% 6 atm; h) 6N HCl, reflux, overnight; i) Boc2O, NaHCO3, dioxane/H2O.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2015
Preparation of Constrained Unnatural Aromatic Amino Acids via Unsaturated
Diketopiperazine Intermediate
Scheme 2. Reactions and conditions: a) BnBr, DMF, Na2CO3, 12 h, r.t.
HCl overnight, leading to a homogeneous mixture of 7 or 9
and valine. At this point, the amino acids were N-Boc
protected by reaction with Boc2O in Schotten–Baumann
condition and then separated as Boc derivatives 8 and 10 in
RP-HPLC. The enatiomeric excess was calculated by the op-
tical rotatory power and resulted to be 51% (76:24, L:D) for
Boc-DMP-OH and 55% (77:23, L:D) for Boc-DMT-OH.
The partial loss of enatiomeric purity could be attributed to
the harsh acidic conditions required for the DKP hydrolysis.
mono-acetylated unsaturated DKP with Z-configuration
(4a and 4b). E-isomer is not easily accessible in this condi-
tion, due to its thermodynamic instability [15]. The resid-
ual acetyl group was removed in mild condition by
methanolic solution of hydrazine monohydrate providing
the formation of the corresponding deacetylated unsatu-
rated DKPs 5a and 5b. These intermediates were reduced
under 6–8 atm of H2 over Pd-C at 60 °C using a Parr
4843 apparatus.
As regards the reaction mechanism, the steric course of
the synthesis has been justified by assuming that the DKP
ring was nearly planar and that the side chain of the valine
moiety protruded from the molecular plane, forcing the
DKP to be preferentially adsorbed from the less bulky side
(Re face) of the molecule. Thus the catalytic hydrogenation
resulted in the formation of (S)-amino acid (Fig. 2). The
catalytic hydrogenation of 5a and 5b gave 6a and 6b with
very high chiral induction as reported for other optically
active DKPs demonstrating a very effective asymmetric
hydrogenation and the pivotal role played by the rigid
skeleton of DKP ring. As reported by Morrison [16], there
could be some effects that operate prior to the
stereoselective adsorption (Re or Si face) of the substrate
on the catalyst that make the bulkiness of the side chain
(out of the plane) of the amino acid important for high chi-
ral induction. For example, a C¼O (or NH) group of the
DKP may first be adsorbed preferentially on the catalyst
surface vertically from the less bulky face.
CONCLUSION
Aromatic constrained unnatural residues have been
wisely and widely used in the field of opioid peptides. As
discussed above, those building blocks are important tools
to explore the topographical preferences of bioactive pep-
tides. The only limits to the systematic exploration of the
topographical requirements upon other bioactive peptides
are their excessive cost or synthesis that require expensive
chiral catalysts [11,17]. Our methodology, can be used to
prepare a wide range of this type of residues, using a vari-
ety of commercial or synthetic aldehydes as described in
the text, the synthesis provided DMP and DMT and their
protected derivatives. Potentially any N or C terminal
DMP and DMT derivatives are readily accessible from
products 7 and 9. Last, in this report, Boc protection has
been preferred due to its broad application on both solid
and solution peptide synthesis.
EXPERIMENTAL
General.
All reactions involving air- or moisture-
sensitive compounds were performed under a nitrogen
atmosphere using dried glassware and syringes techniques
to transfer solutions. The identity of the compounds was
1
The asymmetric center of the valine residue is held close
to the catalytic surface and can exert an important influence
in determining which face of the unsaturated moiety will
be preferentially adsorbed. In addition, starting from (R)-
valine, the proposed procedure should also afford the cor-
responding (R)-DMP and DMT.
confirmed by H NMR spectra recorded on a 300-MHz
Varian Inova spectrometer (Varian Inc., Palo Alto, CA,
USA). Chemical shifts are reported in parts per million (δ)
downfield from the internal standard tetramethylsilane
(Me4Si). The mass spectrometry system used consisted of
an LCQ (Thermo Finnigan) ion trap mass spectrometer
(San Jose, CA) equipped with an electrospray ionization
(ESI) source. The capillary temperature was set at 300°C,
and the spray voltage at 4.25kV. The fluid was nebulized
using nitrogen (N2) as both the sheath gas and the
auxiliary gas. Optical rotatory power were recorded on
The resulting 2,5-diketopiperazines c[Val-DMP] (6a)
and c[Val-DMT] (6b) were hydrolyzed in refluxing 6N
a
Perkin-Elmer 241 Polarimeter. Homogeneity of
the intermediates was confirmed by TLC on silica gel
Merck 60 F254 (Merck, Germany). Chromatographic
purifications were performed by high purity grade Merck
60 70–230 mesh silica gel column. Analytical thin-layer
chromatography was carried out on Sigma-Aldrich® silica
gel on TLA aluminum foils with fluorescent indicator
254 nm. Visualization was carried out under ultra-violet
Figure 2. Proposed mechanism for the catalytic hydrogenation of unsat-
urated DKPs.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

A. Mollica, R. Costante, S. Mirzaie, S. Carradori, G. Macedonio, A. Stefanucci, and E. Novellino Vol 000
irradiation (254 nm). Temperatures are reported in °C. All
chemicals used were of the highest purity commercially
available. 2,6-Dimethylbenzaldehyde and 2,6-dimethyl-4-
hydroxybenzaldehyde were purchased from Fluorochem;
all other reagents and solvents were purchased from
Sigma-Aldrich (Italy) and VWR (Italy). Purification of
Boc-protected final compounds was performed by RP-
HPLC using a Waters XBridgeTM Prep BEH130 C18,
5.0 μm, 250 mm× 10 mm column at a flow rate of
5 mL/min on a Waters Binary pump 1525, using H2O
0.1% TFA and acetonitrile 0.1% TFA as eluent.
1H NMR (CDCl3) δ: 5.07–5.14 (1H, d, Gly CH2,
J = 19Hz), 5.01 (1H, d, Val αCH, J =9.9 Hz), 4.06–4.12
(1H, d, Gly CH2, J = 19Hz), 2.60–2.57 (6H, s, 2 ×N-Ac),
2.04 (1H, d, Val βCH, J = 9.9 Hz), 1.11–0.97 (6H, d, Val
γCH3, J = 6.3 Hz). MS calcd.: 240.11 found: 240.5.
c(Δz-DMP-N-acetyl-Val) (4a).
Compound (3) (1 eq.)
was dissolved in DMF, then 2,6-dimethylbenzaldehyde
(1.5 eq.) and tBuOK 0.5 M in tBuOH (1.5 eq.) were
added dropwise at 0°C. The mixture was allowed to
warm to r.t. and stirred for 7 h. Then the reaction was
quenched with aqueous NH4Cl solution and extracted
three times with EtOAc (3 × 50mL). The combined
organic layers were dried on anhydrous Na2SO4, filtered
and evaporated. The crude was purified by silica gel
chromatography using CH2Cl2/EtOAc 95:5 as eluent to
obtain the final product as a colorless oil in 40% yield.
1H NMR (CDCl3) δ: 7.23–7.09 (4H, m, Ar, CH¼C),
6.67 (1H, s, NH), 4.98 (1H, d, Val αCH, J = 6.3 Hz), 2.63
(3H, s, N-Ac), 2.20 (6H, s, Dmp CH3), 2.08 (1H, m, Val
βCH), 1.08–1.01 (6H, d, Val γCH3, J= 6.9 Hz). MS calcd.:
314.16 found: 314.6.
CHEMISTRY
Boc-Val-Gly-OMe (1).
To an ice-cooled mixture
containing Boc-Val-OH (1.1 eq.) in DMF, EDC·HCl (1.1
eq.), HOBt (1.1 eq.), NMM (2.2 eq.) and HCl·H-Gly-
OMe (1.0 eq.) were added. The reaction mixture was
allowed to warm at r.t., stirred overnight and evaporated
under reduced pressure. The residue was then dissolved
in EtOAc and washed with three portions of 5% citric
acid, saturated solution of NaHCO3 and brine. The
organic phases were collected and dried on Na2SO4, and
the solvent evaporated under reduced pressure to give the
desired product in quantitative yield.
c(Δz-DMP-Val) (5a).
Compound (4a) (1 eq.) was
dissolved in MeOH and hydrazine monohydrate (2 eq.)
was added. The product began to precipitate from the
solution. After 2 h the solution was filtered, and the pure
final product was obtained as a white powder in
quantitative yield.
1H NMR (CDCl3) δ: 6.65 (1H, t, Gly NH), 5.12 (1H, d,
NHBoc), 4.32–4.25 (1H, m, Val αCH), 4.08–3.95 (2H, m,
Gly CH2), 3.75 (3H, s, OMe), 2.19 (1H, m, Val βCH), 1.35
(9H, s, Boc), 0.92–1.03 (6H, d, Val γCH3). 13C NMR
(CDCl3) δ 18.0, 19.2, 28.2, 30.8, 41.0, 52.3, 59.7, 80.0,
155.7, 170.1, 171.9. MS calcd.: 288.17 found: 288.40
1H NMR (DMSO-d6) δ: 9.38 (1H, s, NH), 8.52 (1H, s,
NH), 7.07–7.01 (3H, m, Ar), 6.62 (1H, s, CH¼C), 3.79
(1H, m, Val αCH), 2.10 (6H, s, DMP CH3), 2.02 (1H, m,
Val βCH), 0.94–0.84 (6H, d, Val γCH3, J = 6.3Hz). 13C
NMR (DMSO-d6) δ: 17.8, 19.1, 20.3, 34.3, 61.4, 114.5,
128.6, 129.4, 130.0, 134.5, 161.5, 167.4. MS calcd.:
272.15 found: 272.3.
c(Val-Gly) (2). Boc-Val-Gly-OMe (1) was deprotected
by a 50% mixture of TFA in CH2Cl2 at r.t. for 1 h. The
intermediate TFA salt was used for subsequent reactions
without further purification. TFA·dipeptide methyl ester
(1 eq.) was dissolved in 0.1 M AcOH/2-butanol (1.5 eq.),
and NMM (1 eq.) was added. The resulting weakly acidic
solution was refluxed in an oil bath (120°C) overnight.
The solvent was evaporated in vacuo, the crude was
suspended in acetone and the product was collected by
filtration, washed with small amounts of cold H2O and
recrystallized from a mixture of H2O/2-butanol to afford
the pure product in quantitative yield.
c(DMP-Val) (6a).
Compound (5a) was dissolved in
MeOH and Pd/C (20%) was added. Then the mixture
was hydrogenated for 24 h in a Parr 4843 apparatus at
6–8 bar of H2 and 45°C. The mixture was filtered to
remove the catalyst and the solvent was evaporated to
give the reduced product as
quantitative yield.
a white powder in
1H NMR (DMSO-d6) δ: 8.28 (1H, d, NH, J =3 Hz), 7.93
(1H, d, NH, J = 3.3 Hz), 6.99–6.98 (3H, m, Ar), 3.83 (1H,
m, Val αCH), 3.51 (1H, m, DMP αCH), 3.22–2.88 (2H,
m, DMP βCH2, AB part of an ABX system JAB = 14.1,
JAX = 6.3, JBX =8.7 Hz), 2.25 (6H, s, DMP CH3), 2.03
(1H, m, Val βCH), 0.98–0.92 (6H, d, Val γCH3,
J = 6.3 Hz). 13C NMR (DMSO-d6) δ: 16.4, 18.1, 21.6,
31.5, 38.4, 56.0, 60.4, 127.1, 129.0, 131.4, 137.0, 166.2,
166.0. MS calcd.: 274.17 found: 274.5.
1H NMR (DMSO-d6) δ: 7.45 (1H, s, NH), 7.38 (1H, s,
NH), 4.60–4.55 (1H, d, Val αCH), 4.36–4.22 (2H, dd,
Gly CH2), 2.05 (1H, m, Val βCH), 0.95–1.10 (6H, d, Val
γCH3). 13C NMR (D2O) 16.6, 18.8, 33.7, 44.6, 60.7,
169.7, 171.1. MS calcd.: 156.09 found: 156.3.
N,N′-diacetyl-c(Val-Gly) (3). A mixture of DKP (2) in
acetic anhydride was stirred under reflux for 12h. The
solvent was removed by azeotropic distillation with
methanol and toluene under reduced pressure. The
residue was crystallized from EtOAc/Et2O to yield (3) as
a brown oil in 80% yield.
Boc-DMP-OH (8). The DKP was dissolved in 6N HCl
(50 mL for 100 mg), and the solution was stirred at 100°C
for 24 h. Then the solvent was evaporated under reduced
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2015
Preparation of Constrained Unnatural Aromatic Amino Acids via Unsaturated
Diketopiperazine Intermediate
pressure, and the obtained powder was used for the next
step without further purification. The mixture of HCl·H-
DMP-OH and HCl·H-Val-OH was dissolved in dioxane/
H2O 1:1. Successively, NaHCO3 (2 eq.) and Boc2O (1.2
eq.) dissolved in dioxane were added at 0°C. The mixture
was allowed to warm to r.t. and stirred for 18h. The
dioxane was evaporated, pH was adjusted to 12 by
adding 1N NaOH and the aqueous solution was washed
two times with Et2O (2 ×50 mL). Then the aqueous phase
was acidified to pH 2–3 by adding 2N HCl, and three
extractions with EtOAc (3 × 50 mL) were performed. The
combined organic layers were dried on anhydrous
Na2SO4, filtered and evaporated in vacuo. The obtained
crude was purified by semi-preparative HPLC (see
General section) to give pure Boc-DMP-OH as a white
powder. [α]2D0 ꢀ9.2° (c =0.8 MeOH e.e. 51 %).
and the reduced product was obtained as a white powder in
92% yield.
1H NMR (DMSO-d6) δ: 9.01 (1H, s, DMT OH), 8.23
(1H, d, NH, J =3.3 Hz), 7.80 (1H, d, NH, J= 3.6 Hz), 6.39
(2H, s, Ar), 3.76 (1H, m, Val αCH), 3.50 (1H, m, DMT
αCH), 3.12–3.01 (2H, m, DMT βCH2), 2.16 (6H, s, DMT
CH3), 2.04 (1H, m, Val βCH), 0.97–0.91 (6H, dd, Val
γCH3, J= 6.6 and 6.9 Hz). 13C NMR (DMSO-d6) δ: 16.5,
18.5, 23.4, 31.3, 37.4, 55.5, 59.4, 114.9, 126.5, 131.3,
156.3, 166.6, 166.9. MS calcd.: 290.16 found: 290.4.
Boc-DMT-OH (10). Hydrolysis and Boc protection of
compound (6b) were performed following the same
procedure of c(Val-DMP) (6a). After preparative HPLC
purification Boc-DMT-OH was obtained as a white
powder. [α]2D0 ꢀ6.5° (c = 0.8 MeOH; e.e. 55%).
1H NMR (CDCl3) δ: 6.51 (2H, s, Ar), 4.79 (1H, d,
BocNH), 4.96–4.42 (1H, m, αCH), 3.15–2.96 (2H, m,
βCH2, AB part of an ABX system JAB = 14.4, JAX = 5.7,
1H NMR (CDCl3) δ: 7.25–6.99 (3H, m, Ar), 5.01 (1H, d,
BocNH), 4.53 (1H, m, αCH), 3.49–3.062 (2H, m, βCH2, AB
part of an ABX system JAB =14.1, JAX =3.6, JBX =10.8Hz),
2.37 (6H, 2×s, 2×CH3), 1.35 and 1.061 (9H, s, Boc).
J
BX = 9.3 Hz), 2.31 (6H, s, 2 × CH3), 1.45 and 1.37 (9H, s,
Boc). 13C NMR (CDCl3) δ: 21.5, 25.9, 34.8, 55.3, 80.8,
126.4, 127.4, 133.3, 137.2, 155.3, 176.6. MS calcd.:
309.16 found: 309.5.
O-Bn-2,6-dimethylbenzaldehyde (11).
2,6-Dimethyl-4-
hydroxybenzaldehyde (1 eq.) was dissolved in DMF and
Na2CO3 (1.2 eq.) and benzyl bromide (1.2 eq.) were
added. The mixture was stirred at 60°C, and the course of
the reaction was monitored by TLC. When complete, the
solvent was evaporated, and the obtained product (90%
yield) was used for the next reaction without further
purification.
REFERENCES AND NOTES
[1] Li, T.; Shiotani, K.; Miyazaki, A.; Tsuda, Y.; Ambo, A.;
Sasaki, Y.; Jinsmaa, Y.; Marczak, E.; Bryant, S. D.; Lazarus, L. H.;
Okada, Y. J Med Chem 2007, 50, 2753.
[2] Harrison, B. A.; Pasternak, G. W.; Verdine, G. L. J Med Chem
2003, 46, 677.
1H NMR (CDCl3) δ: 10.48 (1H, s, CHO), 7.42–7.40
(7H, m, Ar), 5.10 (2H, s, CH2―Ar), 2.60 (6H, s, 2 × CH3).
[3] Stefanucci, A.; Pinnen, F.; Feliciani, F.; Cacciatore, I.;
Lucente, G.; Mollica, A. Int J Mol Sci 2011, 12, 2853.
[4] Torino, D.; Mollica, A.; Pinnen, F.; Lucente, G.; Feliciani, F.;
Davis, P.; Lai, J.; Ma, S. W.; Porreca, F.; Hruby, V. J. Bioorg Med Chem
Lett 2009, 19, 4115.
[5] Torino, D.; Mollica, A.; Pinnen, F.; Feliciani, F.; Lucente, G.;
Fabrizi, G.; Portalone, G.; Davis, P.; Lai, J.; Ma, S. W.; Porreca, F.;
Hruby, V. J. J Med Chem 2010, 53, 4550.
[6] Hruby, V. J.; Cai, M. Annu Rev Pharmacol Toxicol 2013, 53,
557.
[7] Rizo, J.; Gierasch, L. M. Annu Rev Biochem 1992, 61, 387.
[8] Marshall, G. R. Tetrahedron, 1993, 49, 3547.
[9] Hruby, V. J.; Li, G.; Haskell-Luevano, C.; Shenderovich, M.
Biopolymers 1997, 43, 219.
c(O-Bn-Δz-DMT-N-acetyl-Val) (4b).
Compound (4b)
was synthesized starting from c(Val-Gly) (3) and (11)
following the same procedure of product (4a). After
chromatographic purification the final product was obtained
as a colorless oil.
1H NMR (CDCl3) δ: 7.45–7.33 (5H, m, OBn Ar), 7.12
(1H, s, CH¼C), 7.05(1H, s, NH), 5.06 (2H, s, CH2-Ar),
4.97 (1H, d, Val αCH, apparent J = 6.6 Hz), 2.61 (3H, s,
N-Ac), 2.17 (6H, s, DMT CH3), 2.07 (1H, m, Val βCH),
1.07–1.00 (6H, d, Val γCH3). MS calcd.: 420.20 found: 420.6.
c(O-Bn-Δz-DMT-Val) (5b).
Compound (4b) was
[10] Sasaki, Y.; Ambo, A. Int J Med Chem 2012, Article ID
498901.
[11] Dygos, J. H.; Yonan, E. E.; Scaros, M. G.; Goodmonson,
O. J.; Getman, D. P.; Periana, R. A.; Beck, G. R. Synthesis 1992,
1992, 741.
deacetylated following the same procedure of product
(4a). The precipitated was obtained as a white powder in
quantitative yield.
1H NMR (DMSO-d6) δ: 9.35 (1H, s, NH), 8.61 (1H, s,
NH), 7.56–7.52 (5H, m, OBn Ar), 6.87 (2H, s, Ar), 6.67
(1H, s, CH¼C), 5.26 (2H, s, CH2―Ar), 3.76 (1H, m, Val
αCH), 2.18 (6H, s, DMT CH3), 2.05 (1H, m, Val βCH),
0.95–0.88 (6H, d, Val γCH3). MS calcd.: 378.19 found:
378.7. 13C NMR (DMSO-d6) δ: 16.9, 18.2, 21.6, 33.3,
60.5, 114.0, 125.0, 125.7, 130.6, 158.9, 160.7, 166.3.
[12] Perdih, A.; Dolenc, M. S. Curr Org Chem, 2007, 11, 801.
[13] Li, T.; Tsuda, Y.; Minoura, K.; In, Y.; Ishida, T.; Lazarus,
L. H.; Okada, Y. Chem Pharm Bull 2006, 54, 873.
[14] Mollica, A.; Costante, R.; Fiorito, S.; Genovese, S.;
Stefanucci, A.; Mathieu, V.; Kiss, R.; Epifano, F. Fitoterapia, 2014, 98,
91.
[15] Oba, M.; Terauchi T.; Owari, Y.; Imai, Y.; Motoyamab, I.;
Nishiyama, K. J Chem Soc Perkin Trans 1 1998, 38, 1275.
[16] Harada, K. In Asymmetric Synthesis; Morrison, J. D., Ed.; Ac-
ademic Press, Inc.: San Diego, 1985; Vol 5, pp 347–356.
[17] Balducci, D.; Contaldi, S.; Lazzari, I.; Porzi, G. Tetrahedron
Asymm, 2009, 20, 1398.
c(DMT-Val) (6b).
Hydrogenation of compound (5b)
was performed in the same conditions of compound (5a),
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet