Synthesis of stable and potent δ/μ opioid peptides: Analogues of H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-OH by ring-closing metathesis

Article abstract of DOI:10.1021/jm061048b

Ring-closing metathesis has emerged as a powerful tool in organic synthesis for generating cyclic structures via C-C double bond formation. Recently, it has been successfully used in peptide chemistry for obtaining cyclic molecules bridged through an olef

Full text of DOI:10.1021/jm061048b

3138
J. Med. Chem. 2007, 50, 3138-3142
Brief Articles
Synthesis of Stable and Potent δ/µ Opioid Peptides: Analogues of
H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-OH by Ring-Closing Metathesis
Adriano Mollica,† Giovanni Guardiani,‡ Peg Davis,§ Shou-Wu Ma,§ Frank Porreca,§ Josephine Lai,§ Luisa Mannina,|,
Anatoli P. Sobolev, and Victor J. Hruby*,#
UniVersita` degli Studi “G. D’ Annunzio”, Via dei Vestini 31, I-66013 Chieti, Italy, Dipartimento di Studi Farmaceutici, UniVersita` degli Studi
di Roma “La Sapienza”, P.le A. Moro 5, I-00185, Rome, Italy, Departments of Pharmacology and Chemistry, UniVersity of Arizona,
Tucson, Arizona 85721, Dipartimento di S.T.A.A.M, UniVersita` degli Studi del Molise, Via De Sanctis, I-86100, Campobasso, Italy, and
Istituto di Metodologie Chimiche, CNR, Area della Ricerca di Roma, I-00016, Monterotondo Stazione, Rome, Italy
ReceiVed September 1, 2006
Ring-closing metathesis has emerged as a powerful tool in organic synthesis for generating cyclic structures
via C-C double bond formation. Recently, it has been successfully used in peptide chemistry for obtaining
cyclic molecules bridged through an olefin unit in place of the usual disulfide bond. Here, we describe this
approach for obtaining cyclic olefin bridged analogues of H-Tyr-c[D-Cys-Gly-Phe-Cys]-OH. The synthesis
of the new ligands was performed using the second generation Grubbs’ catalyst. The resulting cis-8 (cDADAE)
and trans-9 (tDADAE) were fully characterized and tested at δ, µ, and κ opioid receptors. Also the linear
precursor 13 (lDADAE) and the hydrogenated derivative 11 (rDADAE) also were tested. All the cyclic
products containing a olefinic bond are slightly selective but highly active and potent for the δ and µ opioid
receptors. Activity toward the κ opioid receptors was absent or very low.
Introduction
containing analogue shows an exceptionally good activity on δ
and µ receptors, whereas due to the presence of the gem-
dimethyl groups of the penicillamine residues, DPDPE is highly
selective toward δ receptors and is used in the opioid binding
assays as the radio-labeled δ receptor full agonist.5 Furthermore,
while the cystine analogues possessing a C-terminal carboxa-
mide function are essentially nonselective, the corresponding
free acids show a certain preference for δ over µ receptors.4a
Although widely used with success, the disulfide-based
cyclization approach still maintains some limitations. Due to
its redox properties, the S-S bond is exposed to the enzymatic
attack by reductases and can be cleaved by sulfhydryl groups
or other soft nucleophiles. Moreover, the geometry of the bond
is limited by the stereoelectronic effects exerted by the sulfur
atoms’ lone pairs, which fix the dihedral angle C-S-S-C at
about (90°.6
Cysteine-based disulfide bridges are a common feature in
proteins and natural peptides where they play essential roles in
a variety of structural and biochemical functions. Among the
different functions, the limitation of the conformational flex-
ibility with stabilization of secondary structures and the
consequent lowering of the unfavorable entropy loss upon
binding is extremely important.1,2 Disulfide-bridge cyclization
is also one of the most common tools in the design of synthetic
models starting from their linear counterparts.3 The 14-
membered cyclic enkephalin analogues H-Tyr-c[D-Cys-Gly-Phe-
D-Cys]-OH4 and the related [D-Pen2, D-Pen5] model (DPDPEa)5
are the most relevant cyclic opioid peptides. The side-chain thiol
groups at the 2- and 5-position residues of these molecules are
oxidized to form a disulfide bridge that imposes a global
constraint to the structure. In particular, the cystine bridge
Because the stabilization of well-ordered secondary structures
is a major issue in peptide chemistry, several cyclization
strategies, not based on the disulfide bridge formation, have
been applied to obtain more stable and versatile products.7 Very
attractive in this context appears the replacement of the critical
S-S junction with a noncleavable C-C bond. An early
application performed on a bioactive peptide, reported by Keller
and Rudinger8 concerns the synthesis of the dicarba-analogue
of oxytocin: it is based on the incorporation of a 1,6-R,R′-
diamino suberic acid residue in place of the native cystine
fragment. More recently, Stymiest et al.9 reported that the
replacement of cysteines with allylglycine residues, followed
by ruthenium-catalyzed ring-closing olefin metathesis (RCM),
gives an easy access to cis and trans olefinic analogs of oxytocin
as well as to the previously reported8 corresponding bis-
methylene derivative.
* To whom correspondence should be addressed. Tel.: +1 520 621 6332.
Fax: +1 520 621 8407. E-mail: hruby@u.arizona.edu.
† Universita` degli Studi “G. D’ Annunzio”.
‡ Universita` degli Studi di Roma “La Sapienza”.
§ Department of Pharmacology, University of Arizona.
| Universita` degli Studi del Molise.
Istituto di Metodologie Chimiche, CNR.
# Department of Chemistry, University of Arizona.
a Abbreviations: D-Allylgly, D-Allylglycine; cDADAE, cis-c[2-D-allylg-
lycine, 5-D-allylglycine]enkephalin; 3H-DAMGO, [3H]-[D-Ala(2),N-Me-Phe-
(4),Gly-ol(5)]enkephalin; 3H-DPDPE, [3H]-c[2-D-penicillamine,5-D-peni-
3
cillamine]enkephalin; H-U69593, [3H]-(+)-5R,7R,8â)-N-methyl-N-[7-(1-
pyrrolidinyl)-1-oxaspiro[4,5]dec-8-yl]-benzeneacetamide; DCM, dichlorometh-
ane; DMF, N,N-dimethyl formamide; DMSO, dimethylsulfoxide; EDC,1-
ethyl-(3-dimethylaminopropyl)carbodiimide; GPI/LMMP, guinea pig ileum/
longitudinal muscle myenteric plexus (µ opioid receptors); GTP, guanosine
triphosphate; hMOR, human µ opioid receptor; HOBT, 1-hydroxybenzo-
triazole; KOR, κ opioid receptor; lDADAE, [2-D-allylglycine, 5-D-allyl-
glycine]-enkephalin; MVD, mouse vas deferens (δ opioid receptors);
rDADAE, reduced bond-c[2-D-allylglycine, 5-D-allylglycine]enkephalin;
rDOR, rat δ opioid receptor; tDADAE, trans-c[2-D-allylglycine, 5-D-
allylglycine]enkephalin; TEA, triethylamine; TFA, trifluoroacetic acid.
RCM is an emerging powerful tool in the peptide chemistry
for generating cyclic structures via C-C double bond formation.
10.1021/jm061048b CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/01/2007

Brief Articles
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 13 3139
Results and Discussion
The ability of the synthesized enkephalin analogues to inhibit
electrically evoked contractions of the myenteric plexus of the
longitudinal muscle of the guinea pig ileum (GPI) and the mouse
vas deferens (MVD) proves to be very high: IC50 values for 8,
9, 11, and 13 are 0.59, 0.76, 3.4, and 5.7 nM in the predominant
DOR containing MVD and 5.3, 5.2, 59, and 34 nM in the MOR-
rich GPI (Table 1). Tests performed by using nor-binaltor-
phimine (nor-BNI), an antagonist of κ Kappa opioid receptors
(10nM nor-BNI), show little effect on κ receptor activity (data
not reported).
Figure 1. First (A) and second (B) generation Grubbs’ catalysts.
To test the affinity of 8, 9, 11, and 13 for opioid receptors,
binding tests with radiolabeled standards (3H-DPDPE for hDOR,
3H-DAMGO for MOR, and 3H-U69593 for KOR) were
performed19 (see Table 1). Literature data for DPDPE and
H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-OH are also reported for com-
parison.
The results show that compounds 8, 9, 11, and 13 have a
different binding affinity for δ, µ, and κ receptors: Ki values
for 8 and 9 are 1.35 and 1.30 nM, respectively, for the MOR
and 0.43 and 0.57 nM for the DOR; Ki values for 11 and 13
are 56.9 and 64.5 nM, respectively, for the MOR and 7.9 and
8.1 nM, respectively, for the DOR. All compounds show very
small affinity for KOR (Table 1). Therefore, the binding affinity
of 8 and 9 for the µ receptor is higher than that of 11 and 13
and much higher than that of DPDPE, which is similar to H-Tyr-
c[D-Cys-Gly-Phe-D-Cys]-X (X ) NH2).
Figure 2. Structures of cDADAE (8), tDADAE (9), rDADAE (11),
and lDADAE (13).
The above-reported in vitro bioassays show that both 8 and
9 are highly potent agonists at µ and δ receptors, but only
slightly selective (δ/µ ratio is in the range of about 7-9): they
are 10 times more potent than DPDPE for the δ receptors and
more than 1000 times more potent for µ receptors. These results
suggest that the novel H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-OH
analogues show high potency but a low selectivity as previously
found for the related peptides family of H-Tyr-c[D-Cys-Gly-
Phe-D-Cys]-X (X ) OH or NH2). However, the two analogues
8 and 9, characterized by a different geometrical isomerism,
do not show any significant difference in their biological profile.
It is well-established that the spatial orientation of the Tyr1
and Phe4 aromatic moieties19 is a key conformational feature
for the selectivity of the ligands of opioid receptors. The results
reported here suggest that the olefin bridge, present in the H-Tyr-
c[D-Cys-Gly-Phe-D-Cys]-OH dicarba analogues, do not signifi-
cantly affect the orientation of the aromatic moieties when
compared to the disulfide bond. The lack of selectivity obtained
clearly confirms that the selectivity of DPDPE is principally
attributable to the gem-dimethyl moiety of the penicillamine
residues, which generates an enhancement of the affinity for
the δ receptor. It is important to notice that the different
configuration of the double bond in the two new ligands does
not influence their biological profile, but the increased structural
The reaction is catalyzed by ruthenium complexes, first- or
second-generation Grubbs catalysts (Figure 1), and can directly
be used for the synthesis of a variety of peptide diene systems.10
Thus, by adopting the RCM methodology, a “third” generation
of cyclic opioid analogues, characterized by metabolic stability
and rigid conformation, becomes available. However, although
many studies are dedicated to the application of the RCM
methodology in peptide chemistry11 and the perspective of a
fine control of the conformations, based on the incorporation
of cis or trans olefin junctions into the cyclic backbone is
extremely important and promising, only the synthesis and
properties of carboxamides of the dicarba-analogues of the cyclic
14-membered disulfide enkephalin analogues12 are reported in
a symposium record by Schiller et al.
On the basis of the above observations, here we report the
synthetic protocols and in vitro biological tests of cystine-
containing enkephalin dicarba analogues 8, 9, 11, and 13 (Figure
2). All these derivatives possess a C-terminal free carboxyl
group: this feature, in analogy with the behavior exhibited by
the corresponding disulfide analogues, should improve the δ/µ
selectivity as compared with the previously reported C-terminal
carboxamide derivatives.12
Table 1. Binding Affinity and In Vitro Activity for Compounds 8, 9, 11, and 13 and Reference Compounds
bioassay IC50a (nM)
binding Kia,b (nM)
cmpds
MVD
GPI/LMMP
hDOR
hMOR
KOR
cDADAE (8)
tDADAE (9)
rDADAE (11)
lDADAE (13)
0.59 ( 0.19
0.76 ( 0.070
3.4 ( 0.60
5.7 ( 1.1
0.12
5.3 ( 0.21
5.2 ( 0.61
59 ( 24
0.43 ( 0.16
0.57 ( 0.21
7.9 ( 0.61
8.1 ( 0.22
0.822
1.35 ( 0.45
1.30 ( 1.2
56.9 ( 3.5
64.5 ( 4.1
0.550
576 ( 23
74.8 ( 5.1
3960 ( 210
3870 ( 185
44.9
34 ( 11
H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-Xc
1.48
7300 ( 1700
DPDPEd
4.1 ( 0.46
1.6 ( 0.2
609 ( 70
a Displacement of [3H]DAMGO (µ-selective) and [3H]DPDPE (δ-selective) from rat brain membrane binding sites; displacement of [3H]U69593 (κ-
selective) from guinea pig brain membrane binding sites. b (S.E.M. c MVD and GPI data refers to H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-Xc (X ) OH) according
to ref 4a and hDor, hMor, and Kor data refers to H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-Xc (X ) NH2) according to ref 12a. d Data according to ref 19b.

3140 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 13
Brief Articles
Table 2. GTP Binding Assay
[35S]GTPγS bindinga
Scheme 1. Synthesis of Fully Protected
H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-OH Analogues 5, 6, and 7 and
Linear Deprotected Product 13a
EC50 (nM)
hDOR
EC50 (nM)
hMOR
b
b
Emax(%)
Emax%
cDADAE (8)
tDADAE (9)
0.83 ( 0.05
0.88 ( 0.28
121
128
2.35 ( 0.20
2.65 ( 0.45
65
70
a Reference compound: [35S]GTP-γ-S. b Net total bound/basal binding
× 100 ( SEM for compounds 8 and 9.
flexibility of the ring scaffold obtained by the reduction of the
double bond of compounds 8 and 9 leads to a decreased binding
affinity and biological activity for both the DOR and the MOR.
Therefore, the higher activity of cyclic products 8 and 9
compared to that of linear compound 13 can be attributed to
their rather rigid conformations.
A final consideration concerns the results of the [35S]GTP-
γ-S binding. EC50 values exhibited by 8 and 9 for GPTγ[S35]
binding to δ and µ receptor. The results (Table 2) show that
the ligands have a high ability to activate the transduction of
the µ and δ receptors. Also the Emax% value expresses the high
efficacy of all the tested compounds.
In conclusion, the reported modification of the disulfide bridge
in H-Tyr-c[D-Cys-Gly-Phe-D-Cys]-X (X ) OH or NH2) leads
to analogue products that show a significant improvement of
both chemical and biological stability with respect to their
disulfide analogues. This property is accompanied by a slightly
enhanced selectivity toward the δ opioid receptor and by the
same efficacy and potency of D-cysteine-containing peptides.
The choice to synthesize derivatives with a C-terminal free
carboxylic function, instead of the C-terminal amide groups
aimed at verifying if this feature could improve the δ versus µ
selectivity, as observed in the case of the disulfide counterparts.
The present results show that the δ/µ ratio is not significantly
altered by the carboxy terminus: this observed lack of selectivity
may be useful in clinical trials to evaluate the effects of the
synergistic stimulation of both δ and µ opioid receptors.20
Therefore, the presented results may be considered a further
step in understanding the opioid system and the design of more
stable, potent, and efficacious drugs.
a Reagents and conditions: (a) SOCl2/MeOH 10 min at 0 °C, then 6 h
at rt; (b) Boc-Phe-OH/EDC/HOBT·H2O/DMF/NMM, 16 h at rt; (c) TFA/
CH2Cl2 (1:2) 1 h, rt; (d) Boc-Gly-OH/EDC/HOBT·H2O/NMM/DMF, 16 h
at rt; (e) Boc-D-Allyl-Gly-OH/EDC/HOBT·H2O/NMM/DMF, 16 h at rt; (f)
Boc-Tyr-OH/EDC/HOBT·H2O/ NMM/DMF, 16 h at rt; (g) Grubbs’ catalyst
second generation (20%)/CH2Cl2 at rt; (h) TFA/CH2Cl2 (1:2) 1 h at rt; (i)
NaOH 1 N 6 equiv/MeOH, 4 h at rt.
Peptide structures were determined by NMR spectroscopy and
confirmed by high resolution-mass spectra (HR-MS). For the final
products, 8 (cDADAE), 9 (tDADAE), 11 (rDADAE), and 13
(lDADAE), elemental analyses (within (0.4% of the theoretical
values) were performed (for detailed experimental and analytical
data, refer to the Supporting Information).
Chemistry. The linear intermediate pentapeptide 5 (lDADAE;
Scheme 1) was synthesized in the solution phase by using the
NR-Boc strategy. Amino acid couplings were performed by using
either EDC or BOP-Cl/HOBT‚H2O/NMM in DMF,13 both resulting
in high yields. N-Terminus NR-Boc deprotection of compound 1
was accomplished with SOCl2/MeOH, thus realizing contemporary
C-terminal esterification; in all the other cases, Boc cleavage was
performed by using excess of TFA/CH2Cl2 (1:2) at rt for 1 h. All
the intermediate products, unless otherwise specified, were purified
by RP-HPLC and then characterized by Fab+ MS and NMR
spectroscopy. All new final compounds gave correct elemental
analysis. For the RCM reaction, the second generation of Grubbs’
catalyst (Figure 1B), which is rated to be more stable and reactive
than the first generation (Figure 1A), was used.14 The ring closure
was accomplished by adding a catalytic amount of the catalyst to
a 10 mM solution of 5 in CHCl3 at rt until the reaction was
complete. The cyclization progress was monitored by TLC (EtOAc).
The two isomeric peptides cis-8 (cDADAE) and trans-9 (tDADAE;
Figure 2) were obtained in about 1:3 ratio, respectively (Scheme
1). While the two products were clearly distinguishable by TLC
(EtOAc), they showed very close RP-HPLC retention times in the
condition of the analysis. The cis and trans isomers where then
separated by PLC, charging a maximum of 10 mg on each 20 ×
20 cm/0.25 mm silica gel plate.
Experimental Section
General Information. All solvents, reagents, and starting
materials were obtained from commercial sources unless otherwise
indicated. All reactions were performed under N2 unless otherwise
noted. Intermediate products 2, 3, 4, and 5 were purified by silica
gel chromatography. The fully protected products 6, 7, and 10 were
purified by RP-HPLC using a semipreparative Vydac (C18-bonded,
300 Å) column and an isocratic elution at a flow rate of 10 mL/
min monitored at 254 and 270 nm. The mobile phase used was
30% acetonitrile in 0.1% aqueous TFA over 40 min. Approximately
20 mg of crude peptide was injected each time, and the fractions
containing the purified peptide were collected and lyophylized to
dryness. In the case of products 6 and 7, this method was unable
to separate the two geometric isomers that were collected, and their
mixture was purified by HPLC. Products 8, 9, 11, and 13 used for
the biological assay were further purified by RP-HPLC using a
semipreparative Vydac (C18-bonded, 300 Å) column and a gradient
elution at a flow rate of 10 mL/min. The gradient used was 10-
90% acetonitrile in 0.1% aqueous TFA over 40 min. Approximately
10 mg of crude peptide was injected each time, and the fractions
containing the purified peptide were collected and lyophilised to
dryness. The purity of the final products, determined by NMR
analysis and by analytical RP-HPLC (C18-bonded 4.6 × 150 mm)
at a flow rate of 1 mL/min on a Waters Binary pump 1525 using
a isocratic elution of 20% CH3CN/H2O 0.1% TFA, monitored with
a Waters 2996 Photodiode Array Detector, was found to be >95%.
To synthesize the analogue 11 (rDADAE), containing a
-CH2-CH2- junction, a portion of the RP-HPLC purified mixture,
containing both the geometrical isomers 6 and 7 (1:3 ratio), was
catalytically hydrogenated at atmospheric pressure in MeOH
(Scheme 2; route A). The reaction was monitored every 6 h by
TLC. After about 48 h, all the starting material disappeared, giving

Brief Articles
Journal of Medicinal Chemistry, 2007, Vol. 50, No. 13 3141
Scheme 2. Synthesis of the Bismethylene Analogue 11: Route
A, Catalytic Hydrogenation of (6 + 7) Mixture; Route B,
Catalytic Hydrogenation of cis-8 (cDADAE)a
for 15 min. The tissues were then stretched to optimal length
previously determined to be 1 g tension (0.5 g for MVD) and
allowed to equilibrate for 15 min. The tissues were stimulated
transmurally between platinum plate electrodes at 0.1 Hz for 0.4
ms pulses (2.0 ms pulses for MVD) and supramaximal voltage.
Grugs were added to the baths in 20-60 µL volumes. The agonists
remained in contact with the tissue for 3 min and the baths were
then rinsed several times with fresh Krebs solution. Tissues were
given 8 min to re-equilibrate and regain predrug contraction height.
IC50 values represent the mean of not less than four tissues. IC50
estimates and relative potency estimates were determined by fitting
the mean data to the Hill equation by using a computerized
nonlinear least-squares method. All biological data are summarized
in Table 1.
Radioligand-Labeled Binding Assays. Receptor binding af-
finities to the δ, µ, and κ opioid receptors were performed using
cell membrane preparations from transfected cells that stably express
the respective receptor type and were evaluated as previously
described.17 The ligands used were [3H]DPDPE, [3H]DAMGO, and
[3H]U69593 for δ, µ, and κ opioid receptors, respectively.
GTP Binding and Emax% (Agonist Stimulated [35S]GTPγS
Binding).18 We used [35S]GTP-γ-S binding to examine opioid
agonist efficacy for functional characterization of the ligands at
the δ and µ opioid receptors, which are members of the seven
transmembrane G-protein-coupled receptor super family. Agonist
efficacy can be determined at the level of receptor G-protein
interaction by measuring agonist-stimulated binding with a non-
hydrolyzable GTP analogue. The ability of µ and δ opioid agonists
to activate G-proteins has been demonstrated by studying the
binding of the GTP analogue guanosine-5′-O-(3-[35S]thio)triphos-
phate ([35S]GTP-γ-S). The opioid receptor mediated assay was
performed as previously described.19 Cells expressing hDOR for δ
receptor (or rMOR for µ receptor) were incubated with increasing
concentrations of the test compounds in the presence of 0.1 nM
[35S]GTP-γ-S (1000-1500 Ci/mmol, MEN, Boston, MA) in assay
buffer (total volume of 1 mL, duplicate samples) as a measure of
agonist-mediated G-protein activation. After incubation (90 min,
30 °C), the reaction was terminated by rapid filtration under vacuum
through Whatman GF/B glass fiber filters, followed by four washes
with ice-cold 15 mM Tris/120 mM NaCl, pH 7.4. Filters were
pretreated with assay buffer prior to filtration to reduce nonspecific
binding. Bound reactivity was measured by liquid scintillation
spectrophotometry after an overnight extraction with EcoLite (ICN,
Biomedicals, Costa Mesa, CA) scintillation cocktail. The data was
analyzed using GraphPad Prism Software (San Diego, CA).
a Reagents and conditions: (a) H2/Pt/C 10%/MeOH, 48 h, rt; (b) NaOH
(1 N, 3 equiv), 6 h, rt; (c) TFA/CH2Cl2 1:2, 1 h, rt.
Figure 3. Double bond geometry in 8 (cis conformer) and 9 (trans
conformer).
rise to a single spot. The catalyst was filtered and the solvent was
evaporated under vacuum. After purification by silica gel chroma-
tography, the fully protected methyl ester 10 was obtained in almost
quantitative yield. Hydrolysis followed by Boc deprotection under
standard conditions and purification by RP-HPLC gave the desired
free acid 11. This compound was also obtained in one step starting
from the cis analogue 8 (Scheme 2; route B) by applying the same
catalytic hydrogenation conditions described before.
The configuration of the double bond of compounds 8 and 9
was accurately established by the NMR analysis.15 The value of
the J coupling constants between γ-protons of D-All-Gly2 and d-All-
Gly5 (10.4 Hz for 8 and 15.2 Hz for 9) corresponds to a cis and a
trans configuration of the double bond in 8 and 9, respectively.
This assignment is confirmed by the presence of a correlation peak
between H-â of D-All-Gly5 and H-â of D-All-Gly2 in the ROESY
map of 8. In the case of 9, â-protons of D-All-Gly5 and D-All-Gly2
do not give any observable NOE correlation, confirming the trans
configuration of the double bond (Figure 3).
Acknowledgment. The authors thank Prof. Gino Lucente,
Prof. Mario Paglialunga Paradisi, Prof. Annalaura Segre, and
Dr. Yeon Sun Lee for their helpful support and discussions.
This study was supported in part by the U.S. Public Health
Service, Grant DA06284.
Supporting Information Available: Detailed experimental
procedures and representative scheme of the synthesis of compounds
1-13, elemental analyses for products 6-11 and 13, HPLC and
HR-MS for products 5-11 and 13, NMR analyses of products
1-13, and Roesy spectra of 8 and 9. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
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3142 Journal of Medicinal Chemistry, 2007, Vol. 50, No. 13
Brief Articles
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