Probes for narcotic receptor mediated phenomena. 39. Enantiomeric n-substituted benzofuro[2,3-c]pyridin-6-ols: synthesis and topological relationship to oxide-bridged phenylmorphans

Article abstract of DOI:10.1021/jm9004225

Enantiomers of N-substituted benzofuro[2,3-c]pyridin-6-ols have been synthesized, and the subnanomolar affinity and potent agonist activity of the known racemicN-phenethyl substituted benzofuro[2,3-c]pyridin-6-ol can now be ascribed to the 4aS,9aRenantiomer. The energy-minimized structures suggest that the active enantiomer bears a greater three-dimensional resemblance to morphine than to an ostensibly structurally similar oxide-bridged phenylmorphan. Structural features of the conformers of N-substituted benzofuro[2,3-c]pyridin- 6-ols were compared to provide the rationale for their binding affinity.

Full text of DOI:10.1021/jm9004225

7570 J. Med. Chem. 2009, 52, 7570–7579
DOI: 10.1021/jm9004225
Probes for Narcotic Receptor Mediated Phenomena. 39.1 Enantiomeric N-Substituted
Benzofuro[2,3-c]pyridin-6-ols: Synthesis and Topological Relationship to Oxide-Bridged
Phenylmorphans2,†
Yi Zhang,‡ Yong Sok Lee,¥ Richard B. Rothman,z Christina M. Dersch,z Jeffrey R. Deschamps,§ Arthur E. Jacobson,‡ and
Kenner C. Rice*,‡
‡Drug Design and Synthesis Section, Chemical Biology Research Branch, National Institute on Drug Abuse and the National Institute on Alcohol
Abuse and Alcoholism, National Institutes of Health, Department of Health and Human Services, 5625 Fishers Lane, Room 4N03, Bethesda,
Maryland 20892-9415, zClinical Psychopharmacology Section, Chemical Biology Research Branch, National Institute on Drug Abuse, Addiction
Research Center, National Institutes of Health, Department of Health and Human Services, Baltimore, Maryland 21224, §Laboratory for the
Structure of Matter, Naval Research Laboratory, Washington, D.C. 20375, and ¥Center for Molecular Modeling, Division of Computational
Bioscience, CIT, National Institutes of Health, DHHS, Bethesda, Maryland 20892
Received April 1, 2009
Enantiomers of N-substituted benzofuro[2,3-c]pyridin-6-ols have been synthesized, and the subnano-
molar affinity and potent agonist activity of the known racemic N-phenethyl substituted benzofuro[2,3-
c]pyridin-6-ol can now be ascribed to the 4aS,9aR enantiomer. The energy-minimized structures suggest
that the active enantiomer bears a greater three-dimensional resemblance to morphine than to an
ostensibly structurally similar oxide-bridged phenylmorphan. Structural features of the conformers of
N-substituted benzofuro[2,3-c]pyridin-6-ols were compared to provide the rationale for their binding
affinity.
Introduction
In order to gain additional insight into the ligand-receptor
interaction, we thought it would prove useful to find new rigid
ligands with relatively few conformational shapes that could
interact with opioid receptors as agonists and/or antagonists.
We decided to explore the benzofuro[2,3-c]pyridin-6-ols3
because they appeared to be fairly rigid, partial structures
of the oxide-bridged phenylmorphans. We synthesized the
enantiomers ((þ)-15 and (-)-22, Scheme 3) of the most potent
racemic compound (rac-cis-2-phenethyl-4a-ethyl-1,2,3,4,4a,
9a-hexahydrobenzofuro[2,3-c]pyridin-6-ol) on a larger scale
than previously reported for the racemate and determined
which of the enantiomers was responsible for the high opioid
receptor affinity. New N-substituted compounds with chiral
side chains (16, 23, 17, and 24) were also prepared and
evaluated to see if chiral N-hydroxyphenethyl moieties affect
binding to opioid receptors. We then examined whether the
active enantiomer topologically resembled any of our rigid
oxide-bridged phenylmorphans utilizing quantum chemistry
and a superposition study.
In 1989, Hutchison, et al.3 published the synthesis of
racemic N-substituted benzofuro[2,3-c]pyridin-6-ols and
noted that one of them, the N-phenethyl derivative of the
cis-benzofuropyridin-6-ol had high μ-opioid affinity (Ki = 0.9
nM) and potent antinociceptive activity. These compounds
appeared to have marked structural resemblance to the oxide-
bridged phenylmorphans. We published our initial work on
the synthesis of the 12 racemic topologically rigid N-methyl-
substituted a-through-f-oxide-bridged phenylmorphans
(Figure 1) in 19844,5 and have recently reported the prepara-
tion of the final pair of oxide-bridged phenylmorphans.4-13
Some of the N-phenethyl analogues have been synthesized
and their pharmacological activity determined.12,13 A few of
the enantiomers11 of the racemates have also been examined.
The topology of the ortho-b, -d,a, and -f isomers has been
reported,13 and comparisons have previously been made to a
highly potent azabicyclo[3.3.1]nonane μ-opioid agonist.14 We
believed that we could gain information about the three-
dimensional shape of a molecule required for it to interact
with a specific opioid receptor if any of the 12 oxide-bridged
racemic phenylmorphans was found to have agonist or an-
tagonist activity. There was no way then, or now, to determine
theagonist orantagonistconformation of aligand actingatan
opioid receptor.
Chemistry
Synthesis and Resolution. The starting material, 2-hydro-
xy-5-methoxypropiophenone (1, Scheme 1), was isolated as a
byproduct by Shulgin and Dyer,15 and their procedure for
the preparation of 1 was used by Hutchison et al.3 We
modified the procedure15 to obtain 1 in high yield on a large
scale from the reaction between propionyl chloride and 1,4-
dimethoxybenzene, by using aluminum chloride with nitro-
methane as the solvent. Selective O-demethylation of the
intermediate product, 2,5-dimethoxypropriophenone, was
required in this reaction to obtain the desired monomethoxy
compound 1, and preliminary experiments with BBr3 gave
†Dedicated to the 100th anniversary of the Division of Medicinal
Chemistry.
*To whom correspondence should be addressed. Phone: 301-496-
1856. Fax: 301-402-0589. E-mail: kr21f@nih.gov.
a Abbreviations: para-d-oxide-bridged phenylmorphan, (3R*,6aS*11aR*)-
1,3,4,5,6,11a-hexahydro-2H-3,6a-methanobenzofuro[2,3-c]azocin-8-ol; EI,
electron impact; HRMS, high-resolution mass spectra; TLC, thin layer
chromatography.
pubs.acs.org/jmc
Published on Web 07/24/2009
This article not subject to U.S. Copyright. Published 2009 by the American Chemical Society

Article
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23 7571
Figure 1. Twelve racemic N-methyl substituted oxide-bridged phenylmorphan isomers.
Scheme 1a
a Reagents and conditions: (a) propionyl chloride, AlCl3, CH3NO2, 60 ꢀC; (b) BrCH2CO2Et, K2CO3, acetone, reflux, 30 h; (c) (EtO)2POCH2CN,
NaH, THF, 0 ꢀC, 2 h; (d) NaH, EtOH, reflux, 15 min; (e) H2, 5% Pt/C, acetic acid, 50 psi, room temperature; (f) 60% NaH in mineral oil, EtOH, reflux,
8.5 h; (g) NaBH4, (CH3O)2SO2, THF, reflux 48 h, purified through oxalate salt (oxalic acid, acetone), followed by KOH (aqueous) and distillation of the
free base at 183 ꢀC, in vacuo (68%). The asterisk (/) indicates that the yield of (()-7 after the combination steps of e and f was about 60%.
unsatisfactory results. The use of anhydrous aluminum
chloride in dry nitromethane at 60 ꢀC gave, in a rapid
reaction, the desired product 1 in 84% yield, after workup.
At a lower temperature (10 ꢀC) the reaction resulted in the
formation of only the 2,5-dimethoxypropriophenone inter-
mediate and little of the desired O-demethylated product 1.
The reaction sequence to the known racemic secondary
amine 8 (Scheme 1) was similar to that used by Hutchison
et al.3 but modified for the larger scale synthesis. The yield in
the conversion of 1 to the propiophenone (2) was improved
from 65%3 to 90% by running the reaction in acetone,
monitoring its completion by TLC and NMR, and by
crystallization using hexane and isopropanol. Borane reduc-
tion16 of the amide 7 gave the racemic cis-6-methoxyamine 8.
Compound 8 was treated with (S)-(þ)-mandelic acid to give
the salt with the (þ)-amine base 9 (Scheme 2). From the

7572 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23
Zhang et al.
Figure 2. X-ray crystallographic structure of the R-(-)-mandelic acid salt of 4aS,9aR-(-)-cis-4a-ethyl-6-methoxy-1,2,3,4,4a,9a-
hexahydrobenzofuro[2,3-c]pyridine ((-)-10). Displacement ellipsoids are shown at the 50% level.
Scheme 2a
Scheme 3a
a Reagents and conditions: (a) (S)-(þ)mandelic acid, EtOAc/Et2O
(recrystallized from acetonitrile/Et2O (1:1) (1ꢀ), EtOAc/Et2O (1:1)
(1ꢀ), and acetonitrile/Et2O (1:1) (1ꢀ)), free-based with NaOH (2 N),
extracted by CHCl3, solvent removed, high vacuum distilled at 130 ꢀC;
(b) (R)-(-)mandelic acid, EtOAc/Et2O (recrystallized from EtOAc/
Et2O (1:1) (1ꢀ), acetonitrile/Et2O (1:1) (2ꢀ)), 68% recovery. Free-based
with NaOH (2 N), extracted with CHCl3, solvent removed, high vacuum
distilled at 125 ꢀC.
a Reagents and conditions: (a) 10% Pd/C, H2, 37% formic acid,
MeOH, room temperature, 4 h; (b) (2-bromoethyl)benzene, K2CO3,
DMF, 50 ꢀC, 20 h; (c) 48% HBr, reflux, 1 h, then NH4OH, CH2Cl2; (d)
48% HBr, reflux, 30 min, then NH4OH, CH2Cl2; (e) 48% HBr, HOAc,
reflux, 7.5 h; (f) (S)-(-)-styrene oxide, toluene, reflux, 72 h; (g) (R)-(þ)-
styrene oxide, toluene, reflux, 72 h.
mother liquors of the recrystallization of the salt, material
enriched in the (-)-enantiomer 10 was obtained and con-
verted to the base, and treatment with (R)-(-)-mandelic acid
gave the desired salt with the (-)-amine base 10 (Scheme 2).
The enantiomers (þ)-9 and (-)-10 were obtained from their
mandelate salts on basification. The melting points of these
enantiomers were similar, and their optical rotations were
about equal and opposite.
obtained from the 4aR,9aS-aminophenol 13 and the
4aS,9aR-aminophenol 20, respectively. These phenols
(4aR,9aS-13 and 4aS,9aR-20) were prepared by 48% HBr
cleavage of the aromatic methoxy secondary amine com-
pounds 4aR,9aS-9 and 4aS,9aR-10. The various chiral com-
pounds were generally prepared in moderate to high yields.
Derivatization of (þ)-9 or (-)-10 with R-(þ)-1-phenethyl
isocyanate to form the diastereomeric ureas17 enabled the
determination of the optical purity of these amines. The
optical purity of 9 was established by the disappearance of
the chemical shift at δ 1.453 and that of 10 by the dis-
appearance of the chemical shift at δ 1.408. Their optical
purity was estimated to be ∼99% ee.17
Results and Discussion
Five compounds (20-24) were examined for their efficacy
as μ-agonists in the [35S]GTP-γ-S functional assay (Table 1).
In that assay, the secondary amine 20 was found to be a weak
partial agonist (ED50 = 150 nM) and had weak antagonist
activity (Ke = 95 nM), the N-methyl substituted pyridin-6-ol
21 had good affinity for the μ-receptor (Ki = 16 nM) but had
little efficacy inthe functional assay, 23was noted tobe a weak
partial agonist, compound 24 was a weak agonist, and the
4aS,9aR-N-phenethyl substituted pyridin-6-ol (22) was, as
expected, a potent agonist with very high affinity for the μ-
receptor (Ki = 0.7 nM). As shown in Table 1, the 4aS,9aR
compounds (all levorotatory) werefound to have much higher
affinity than the comparable 4aR,9aS compounds (all
dextrorotatory). Clearly, the activity of the racemic mixtures
The chirality of (-)-10 was determined to be 4aS,9aR by
X-ray crystallographic analysis of the salt (-)-10 R-(-)-
3
mandelate (Figure 2). N-Alkylation of the secondary amine
4aR,9aS-9 or 4aS,9aR-10 (Scheme 3) gave the dextrorotary
N-methyl (4aR,9aS-11) and N-phenylethyl analogue
(4aR,9aS-12) and their levorotatory relatives (4aS,9aR-18
and 4aS,9aR-19). Cleavage of the aromatic methoxy group
with 48% HBr gave the phenols, 4aR,9aS-14 and 4aR,9aS-
15 and 4aS,9aR-21 and 4aS,9aR-22 (Scheme 3). The (þ)-S-
and R-20-hydroxyphenethyl compounds 4aR,9aS-16 and
4aR,9aS-17 and the (-)-S- and R-20-hydroxyphenethyl
compounds 4aS,9aR-23 and 4aS,9aR-24 (Scheme 3) were

Article
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23 7573
Table 1. [3H]Opioid Receptor Binding Dataa for cis-Benzofuro[2,3-c]pyridin-6-ols and Functional Data ([35S]GTP-γ-S)b for Selected Compounds
Ki (nM)
μc
δd
κe
μ-agonism Emax (%)f
ED50 (nM)
4aS,9aR
20g
21
48 ( 2.9
15.9 ( 1
0.70 ( 0.06
9.3 ( 0. 7
80 ( 5
97 ( 4
45 ( 2
84 ( 4
100 ( 3
36 ( 2
150 ( 25
241 ( 47
24 ( 6
560 ( 60
480 ( 90
1350 ( 90
75 ( 8.8
443 ( 32
807 ( 45
450 ( 34
88 ( 8
413 ( 44
1090 ( 60
22
23 (20S)
24 (20R)
4aR,9aS
13
14
3500 ( 417
1330 ( 50
200 ( 9
600 ( 34
1330 ( 80
4270 ( 240
2690 ( 210
2490 ( 230
3100 ( 90
>10000
15
16 (20S)
17 (20R)
2325 ( 160
5870 ( 470
9500 ( 780
a Assays22 were conducted using CHO cells, which were stably transfected and expressthe μ-, δ-, or κ-opiate receptors, respectively. For all results, n =
3. For comparison, the Ki of morphine at μ is 2.55 ( 0.01 nM. b 35S]GTP-γ-S binding was performed using CHO hMOR cells, which express the human
[
μ-opiate receptor. For all values, n = 3. c [3H]DAMGO binding. d [3H]DADLE binding. e [3H]U69,593 binding. f The Emax is the extrapolated maximal
stimulation where 100% is defined as the stimulation produced by 10 μM DAMGO (DAMGO ED50 at μ is 42 ( 4 nM). g Secondary amine 20 is a μ-
antagonist, Ke = 95 ( 16 nM ([35S]GTP-γ-S assay with CHO hMOR cells, n = 5).
examined by Hutchison et al.3 reflected the activity of the
compounds with the 4aS,9aR stereochemistry. The 4aS,9aR-
N-phenethyl compound 22 had subnanomolar affinity for the
μ-opioid receptor and slightly more than 100-fold less affinity
at the δ- and κ-receptors. Interestingly, introduction of a
hydroxy substituent in the N-phenethyl side chain decreased
affinity for the μ-receptor 10-fold with the hydroxy in the S-
configuration (4aS,9aR,2’S-23) and over 100-fold when the
hydroxy was in the R-configuration (4aS,9aR,20R-24).
known to be less than 11 kcal/mol,19,20 22 may well undergo
a rapid interconversion between A and C and thus is likely to
exist as a conformer mixture at room temperature.
Energetics suggest that 22 exists in CHCl3 predominantly in
the form of A and C (>95%) at room temperature. To
identify the conformer most likely to be recognized at the μ-
opioid receptor, the heavy atoms of the piperidine rings of A
and C and the oxide-bridged para-d phenylmorphan isomer
(which was found to be structurally more similar to 22 than
any of the other oxide-bridged phenylmorphans) were fitted
to morphine with the equatorial N-methyl as shown in
Figure 4. Their respective rmsd value of the fitting is 0.13,
Figure 3 illustrates the geometry optimized conformers of
the high affinity N-phenethyl compound (22) in the gaseous
phase in its protonated form; the active form of 22 is likely to
be protonated at physiological pH. For the comparison of the
energetics, the Gibbs free energy of each conformer was
calculated at 298.15 K from its respective fully optimized
geometry in CHCl3. The topology of A, except for the
equatorial N-phenylethyl substituent, is essentially identical
to the X-ray structure of 10 as evidenced by the less than 0.04
˚
0.12, and 0.07 A. The dihydrofuran ring of conformer C
overlaps well with that of the para-d isomer that is known
to bind poorly to the μ-opioid receptor (Ki = 1220 nM).13
This indicates that conformer C, although thermochemically
more stable than A, is not recognized by the receptor, and
thus, the subnanomolar affinity of 22 (at μ, Ki = 0.7 nM) is
likely to arise from conformer A. The benzofuran ring in
conformer A overlaps better with the benzofuran ring of
morphine than that of the para-d isomer (Figure 4). The
affinity of 22 for the μ-receptor is much closer to that of
morphine (Ki = 2.55 ( 0.1 nM) than to the para-d isomer.
This work has enabled us to determine the enantiomer
responsible for the μ-affinity and activity of the racemic N-
substituted benzofuro[2,3-c]pyridin-6-ols.3 In addition, we
have compared the structures of the energy-minimized con-
formers of the most active enantiomer 22 and the various
oxide-bridged phenylmorphans, determining that the activity
and affinity of 22 were not likely to be due to conformer C,
which is structurally similar to the para-d-oxide-bridged
phenylmorphan. Compound 22, however, was found to have
a less stable conformer A, topologically similar to morphine,
that could be responsible for its activity. Also, we synthesized
new N-derivatives with a hydroxyl substituent on the phenyl-
ethyl side chain (23 and 24) and determined that a hydroxyl
moiety on that side chain hindered binding and reduced
efficacy.
˚
A root-mean-square deviation (rmsd) between the heavy
atoms of both the dihydrofuran and the piperidine rings.
Conformer B1 is epimeric to A and was obtained by nitrogen
inversion; it is 2.6 kcal/mol less stable relative to A, a
consequence of the change in the orientation of the N-
phenylethyl from equatorial to axial. This nitrogen inversion
in solution is likely to proceed through deprotonation fol-
lowed by reprotonation. B2 was obtained by ∼180ꢀ rotation
of B1 to point both the hydrogen on the nitrogen atom and the
dihydrofuran ring of B1 out of the plane. A conformational
change of the piperidine ring in B2 or B1 of Figure 3 via the
variation of the dihedral angle of O9-C9a-C1-N2 (φ1) and
C4a-C4-C3-N2 (φ2) leads to conformer C with the equa-
torially oriented N-phenylethyl moiety; conformer C is
slightly (0.26 kcal/mol) more stable than A, and this stability
may arise from the charge interaction between the dihydro-
furan oxygen and the hydrogen on the nitrogen atom, as
˚
evidenced by their distance of 2.4 A. Note that A can undergo
the piperidine conformation change first, followed by the
nitrogen inversion to conformer C. Both the reaction pathway
and the energy barrier between the two conformers A and C
are not knownand are currently being investigated withthe ab
initio replica path method.18 Nonetheless, considering that
both the nitrogen inversion energy barrier and the conforma-
tion energy barrier of the neutral N-methylpiperidine are
Experimental Section
All reactions were performed in oven-dried glassware under
an argon atmosphere. Some of the large scale mixtures were
magnetically stirred. All melting points were determined on a

7574 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23
Zhang et al.
Figure 3. Geometry optimized conformers of the protonated 4aS,9aR-(-)-cis-4a-ethyl-2-(2-phenylethyl)-1,2,3,4,4a,9a-hexahydrobenzofuro-
[2,3-c]pyridin-6-ol (22) in the gaseous phase. Topology of A is essentially identical with the X-ray structure of 10 in Figure 2 (without the N-
phenylethyl moiety). Conformer B1 is epimeric to A with respect to the nitrogen. B2 is another representation of B1 via a 180ꢀ rotation, and C is
formed by a conformational change of the piperidine ring of B2 or B1. φ1 is the dihedral angle of O9-C9a-C1-N2, and φ2 is the dihedral angle
of C4a-C4-C3-N2.
Thomas-Hoover capillary melting point apparatus and are un-
corrected. The optical rotation data were obtained on a Perkin-
Elmer polarimeter model 341 or on a Jasco DIP-370 digital
was added slowly (exothermic) keeping the temperature at
<0 ꢀC with a dry ice-acetone bath. A 50 mL portion of
nitromethane was used to wash in the AlCl3. Propionyl chloride
(201.05 g, 2.17 mol, 1.20equiv) was then slowlyaddedkeeping the
temperature at <0 ꢀC. A mixture of 150 mL of nitromethane and
1,4-dimethoxybenzene (250.0 g, 1.81 mol) was heated to solution.
This solution was added slowly to the reaction mixture beginning
at -20 ꢀC, and the temperature was allowed to rise to 10 ꢀC.
A 50 mL portion of nitromethane was used as wash-in. This is a
very rapid reaction. TLC (petroleum ether/Et2O, 3:1) of an
aliquot quenched with H2O gave essentially a single spot indicat-
ing the intermediate product 2,5-dimethoxypropiophenone, no
starting material, and only a trace of 1. The stirred mixture
was slowly and cautiously heated to reflux with the rapid evolu-
tion of HCl and methyl chloride. At 60 ꢀC TLC showed
∼2:1 mixture of intermediate and the desired 1. After 1.5 h of
reflux, TLC showed the absence of the intermediate product. The
reaction mixture was cooled and slowly quenched with 500 mL of
H2O, and the nitromethane evaporated under water aspirator
vacuum. The mixture was extracted with CHCl3 (600 mL, 2 ꢀ
200 mL) and the combined CHCl3 washed with 250 mL of H2O
and evaporated in vacuo. The dark residue was vacuum
1
polarimeter (589 nm). H NMR (in CDCl3 with TMS at δ 0.0
ppm) spectra were recorded on a Varian XL-300 spectrometer at
300 MHz and on a Varian AS-400 spectrometer at 400 MHz.
Mass spectra (EI) and high-resolution mass spectra (HRMS)
were obtained using a JEOL SX102 instrument. Thin layer
chromatography (TLC) was performed on precoated plates of
silica gel GF (0.25 mm, F254, Alltech) using various gradients of
CHCl3/MeOH containing 1% NH4OH or gradients of ethyl
acetate/n-hexanes. Visualization was accomplished under UV
or by staining in an iodine chamber. Flash chromatography
was conductedusing silicagel(230-400mesh, Merck). Elemental
analyses were performed by Atlantic Microlabs Inc., Norcross,
GA, to determine the purity of the compounds and were within
(0.4% of theoretical values, confirming g95% purity.
2-Hydroxy-5-methoxypropiophenone (1). In a 5 L three-
necked flask fitted with a mechanical stirrer, thermometer, nitro-
gen purge and pressure equalizing funnel was placed
500 mL of dry nitromethane. The stirred solvent was cooled to
-20 ꢀC, and 277.82 g (2.08 mol, 1.15 equiv) of anhydrous AlCl3

Article
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23 7575
rac-2-Carbethoxy-3-(cyanomethyl)-3-ethyl-5-methoxy-2,3-di-
hydrobenzofuran (4). A mixture of compound 3 (130.7 g), EtOH
(anhydrous, 18.5 mL), and sodium hydride (60% in mineral oil,
1.807 g) was refluxed for 15 min. The completion of the reaction
was monitored by NMR. The yield of the 4 was improved by
modifying the workup procedure of Hutchison et al.3 After
the mixture was cooled to room temperature, EtOH was eva-
porated from the reaction mixture, which was then mixed with
ice-water (100 mL) and extracted with CH2Cl2 (100 mL ꢀ 3).
The crude product (130.69 g, 100%) was used to synthesize 5
(cis-(2R*,3S*)-ethyl
3-(2-aminoethyl)-3-ethyl-5-methoxy-2,
3-dihydrobenzofuran-2-carboxylate) and 6 (trans-(2R*,3R*)-
ethyl 3-(2-aminoethyl)-3-ethyl-5-methoxy-2,3-dihydrobenzo-
furan-2-carboxylate).
rac-cis-4a-Ethyl-6-methoxy-1,2,3,4,4a,9a-hexahydrobenzofuro-
[2,3-c]-pyridin-1-one (7). A mixture of 4 (130.69 g, 1 equiv) and Pt/
C (5%Pt/C powder, 63.05%H2O, 71.55 g) in acetic acid (727 mL)
was hydrogenated at 50 psiat room temperaturefor 40 h (reaction
completion monitored by TLC (EtOAc)). The mixture was
filtered, concentrated, and then poured into ice-water. Ammo-
nium hydroxide (28%, 50 mL) was used to adjust the pH to ∼9.
The solution was then extracted with CH2Cl2 (200 mL ꢀ 4), dried
over MgSO4, and concentrated in vacuo to give 7. The residual
material containing 5 (cis-(2R*,3S*)-ethyl 3-(2-aminoethyl)-3-
Figure 4. Overlay of the piperidine ring of conformers A and C of
compound 4aS,9aR-(-)-22, and the para-d-oxide-bridged phenyl-
morphan, onto that of morphine, illustrating the topographical
similarity of conformer A with morphine and conformer C with the
para-d-oxide-bridged phenylmorphan that has little affinity for the
μ-opioid receptor. Hydrogen atoms are not shown.
ethyl-5-methoxy-2,3-dihydrobenzofuran-2-carboxylate) and
6
(trans-(2R*,3R*)-ethyl 3-(2-aminoethyl)-3-ethyl-5-methoxy-2,3-
dihydrobenzofuran-2-carboxylate), but enriched with 5 (104.0
g), was mixed with sodium hydride (60% in mineral oil, 1.66 g)
in EtOH (anhydrous, 78 mL) in order to epimerize 6 to 5. The
mixture of 5 þ 6 was refluxed under argon for 8.5 h (reaction
completion monitored by TLC (EtOAc)), concentrated, and
redissolved in EtOAc (350 mL) and H2O (200 mL). After the
separation of the organic phase, the aqueous phase was extracted
with EtOAc (150 mL ꢀ 3), followed by CH2Cl2 (150 mL). The
combined organic phase was dried with brine (150 mL ꢀ 2) and
concentrated in vacuo to give additional 7 (109.1 g) as a solid. The
crude 7 (105 g) was dissolved in Et2O (300 mL) at room
temperature by mechanical stirring for 2 h. The solution was then
cooled to 0 ꢀC, filtered, and washed with cold Et2O (300 mL). The
solvent was removed in vacuo, and the product was air-dried.
Pure 7 (60.2 g, 57%) was obtained as a beige solid, mp 117.5 ꢀC
(lit.3 mp 122-123 ꢀC). TLC (a 1:1 mixture of CHCl3 and the
mixture CHCl3/MeOH/28% NH4OH, 90:9:1) indicated that the
impurities remained in the ether filtrate. NMR (CDCl3, 300
MHz): δ 0.87 (3H, t, J = 7.6 Hz), 1.74 (2H, m), 2.04 (2H, m),
3.21 (2H, m), 3.77 (3H, s), 4.66 (1H, s), 6.60 (1H, d, J = 2.7 Hz),
6.61 (1H, brs), 6.70 (1H, dd, J = 2.6, 8.6 Hz), 6.79 (1H, d, J = 8.7
Hz). MS m/z 248 [M þ H]þ.
distilled at 120-125 ꢀC/1-2 mm to give 288.84 g of yellow oil.
This was dissolved in CH3OH (722 mL) and treated dropwise
with H2O (577 mL) (CH3OH/H2O, 2.5:2) to give crystalline
material (cooling to a final temperature of 15 ꢀC). This was
filtered and washed with a mixture of CH3OH (360 mL) and H2O
(289 mL) (CH3OH/H2O, 1.25:1) at 15 ꢀC and dried on the filter to
constant weight to give 273.9 g (84%) of 1, mp 45-46 ꢀC (lit.15
47-48 ꢀC). NMR (CDCl3, 300 MHz): δ 1.25 (3 H, t, J = 7.2 Hz),
3.02 (2 H, q, J = 7.2 Hz), 3.80 (3 H, s), 6.93 (1 H, d, J = 9.0 Hz),
7.10 (1 H, dd, J = 3.0, 9.0 Hz), 6.72 (1 H, d, J = 3.0 Hz).
2-(Carbethoxymethoxy)-5-methoxypropiophenone (2). A mix-
ture of 2-hydroxy-5-methoxypropiophenone (114.5 g), ethyl
bromoacetate (74 mL), and powdered anhydrous K2CO3
(131.5 g) in anhydrous acetone (900 mL) was refluxed and
vigorously stirred under argon for 30 h. The completion of the
reaction was initially monitored by TLC (hexane/EtOAc, 60:40)
and after about 20 h by NMR. The reaction mixture was filtered
and concentrated to afford 2 (173.9 g) as a yellow solid.
Compound 2 was dissolved in hexane (690 mL) with heating,
and the solution was cooled to room temperature. Isopropanol
(170 mL) was added to solubilize the oily material that came out
of solution. A crystalline solid precipitated, and the solution was
cooled to 0 ꢀC to complete the crystallization. The solid was
filtered and washed three times with the solvent mixture
(hexane, 690 mL, and isopropanol, 170 mL, 0 ꢀC). The air-dried
product (2, 147.8 g) was obtained as an ivory-colored crystalline
solid in 90% yield, mp 61-62.5 ꢀC (lit.3 mp 58-60 ꢀC). NMR
(CDCl3, 300 MHz): δ 1.18 (3H, t, J = 7.4 Hz), 1.30 (3H, t, J =
7.2 Hz), 3.12 (2H, quartet, J = 7.2 Hz), 4.27 (2H, q, J = 7.2 Hz),
4.66 (2H, s), 6.79 (1H, d, J = 9.0 Hz), 6.97 (1H, dd, J = 3.3, 9.0
Hz), 7.26 (1H, d, J = 3.3 Hz). MS m/z 267 [M þ H]þ.
rac-cis-4a-Ethyl-6-methoxy-1,2,3,4,4a,9a-hexahydrobenzofuro-
[2,3-c]pyridine (8). In a dry, 2 L three-necked round-bottomed
flask equipped with stirrer bar, thermometer, dropping funnel,
˚
and reflux condenser was placed THF (3 A molecular sieve
treated, 490 mL) and lactam 7 (49.2 g, 0.199 mol), under argon.
Sodium borohydride (60.2 g, 1.592 mol) was added to the stirred
mixture, followed by dimethyl sulfate (150.6 mL, 1.592 mol)
˚
dissolved in THF (3 A molecular sieve treated, 160 mL), over a
period of 1 h. Stirring was continued foran additional 30 min with
the temperature maintained at 35-45 ꢀC. The mixture was then
refluxed for 2 days (reaction completion monitored by TLC
(CHCl3/CH3OH/28% NH4OH, 90:9:1)). Methanol (250 mL)
was added slowly to quench the reaction. The reaction mixture
was then concentrated to a foam and redissolved in CH3OH
(700 mL). The methanol solution was saturated with hydrogen
chloride gas, then refluxed overnight. The mixture was then
concentrated and partitioned in H2O (500 mL) and Et2O (400
mL). The aqueousphasewas then washed with another portionof
Et2O (300 mL). The combined etherate wash was extracted again
with H2O (200 mL) to reduce the loss of the product. At the
end, the combined aqueous phase was basified with NH4OH
(120 mL) to pH 9-9.5, extracted with CHCl3 (500 mL ꢀ 4), and
(E,Z)-2-(Carbethoxymethoxy)-5-methoxy-β-ethylcinnamonitrile
(3). The procedure of Hutchison et al.3 was modified for use on a
larger scale. Compound 2 (147.77 g) in THF (350 mL) was added
to a prereacted mixture of sodium hydride (60% in mineral oil,
26.64 g), diethyl(cyanomethyl)phosphonate (96 mL), and anhy-
drous THF (500 mL). The reaction was complete after 2 h at 0 ꢀC
(monitored by TLC (hexane/EtOAc, 60:40)) to give 3 (141.35 g,
88.3%) as a yellow oil that was used without further purification.
MS m/z 290 [M þ H]þ.

7576 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23
Zhang et al.
concentrated to afford the crude racemic amine (8, 56.21 g).
Oxalic acid (21.69 g, 0.24 mol) and crude amine 8 (56.21 g) were
dissolved in acetone (400 mL). The clear-yellow solution was
stirred until the solid oxalate salt precipitated from the solution.
After the mixture was cooled to 0 ꢀC, the oxalate was filtered,
washed with cold acetone (300 mL), and dried in vacuo to afford
R-(-)-mandelic acid (14.98 g, 0.098 mol) were heated to solution
in EtOAc (62 mL) in a 250 mL beaker. The solution was cooled,
and Et2O (86 mL) was added slowly until an oil began to
separate. Scratching was used to induce crystallization. The
solution was cooled with chilled H2O until the crystallization
ended. The crystals were washed as described for enantiomer
(þ)-9 (EtOAc, 60mL, andEt2O,86 mL) togive the(-)-10 R-(-)-
8 oxalate (50.48 g). The oxalate was placed in a separatory funnel
3
3
mandelate (25.58 g) as a white solid. The NMR of the derivatized
free-based amine showed ∼88% of the desired enantiomer.
The solid was recrystallized 3ꢀ from EtOAc or acetonitrile to
give the pure enantiomer (68% yield, mp 145-146 ꢀC), as
determined by the NMR spectra of the derivatized free-based
amine with R-(þ)-1-phenethyl isocyanate. The chirality of (-)-10
and dissolved in H2O (200 mL), and a KOH aqueous solution
(40.2 g in 160 mL H2O) was added. The mixture was shaken to
give 8 as the free base, and CHCl3 (200 mL ꢀ 4) was added to
extract the amine. The first two CHCl3 extracts were combined,
washedwithH2O (200 mL), thenfilteredthroughCelite. The third
extract was washed with that H2O (200 mL) and filtered through
Celite, and the Celitefilter-cakewas washed withCHCl3. This was
repeated again with the fourth CHCl3 extract. Theorganicsolvent
was removed in vacuo to give 8 (42.16 g, 81%). Compound 8 was
distilled under water aspirator vacuum at 183 ꢀC to give the pure
racemic amine 8 (35.2 g, 68%) as a colorless viscous oil. MS m/z
234 [M þ H]þ.
was determined by X-ray diffraction analysisof the (-)-10 R-(-)-
3
mandelate to be 4aS,9aR. The R-(-)-mandelate salt (13.34 g,
34.6 mmol) was dissolved in CHCl3 (40 mL) and free-based using
NaOH (2 N, 35 mL, 69.2 mmol). This was worked up as with the
mandelate salt of (þ)-9 to give the amine (-)-10 (8.21 g) that was
distilled under high vacuum at 130 ꢀC to afford pure (-)-10 (8.01
g) in quantitative yield as a colorless viscous oil that crystallized
4aR,9aS-(þ)-cis-4a-Ethyl-6-methoxy-1,2,3,4,4a,9a-hexahy-
drobenzofuro[2,3-c]pyridine ((þ)-9). Racemic amine 8 (33.58 g,
0.144 mol) and S-(þ)-mandelic acid (22.34 g, 0.147 mol) were
heated to solution in EtOAc (91 mL) in a 250 mL beaker. Ether
(105 mL) was then slowly added to the hot solution. Scratching
or a seed crystal was used to induce crystallization. Another
portion of Et2O (27 mL) was added to the boiling mixture.
Scratching was continued until crystallization ended. The solu-
tion was cooled in a chilled water bath, and another portion of
Et2O (4.5 mL) was added. An oil appeared with this addition,
and a small amount of EtOAc was added to resolubilize the oil.
The solid was filtered and washed with a portion of a mixture of
EtOAc (91 mL) and Et2O (136.5 mL) and was briefly vacuum-
dried (to avoid drying any oil on the crystals). Three additional
washes used the remaining amount of the rinsing solvent
mixture. Ether (25 mL) was used as a final rinse to dry the filter
cake. The white solid (30.55 g) was recrystallized 3ꢀ from
very slowly on long-standing, mp 50-51 ꢀC. [R]20 -79.4ꢀ
D
(c 1.166, CDCl3). NMR (CDCl3, 300 MHz): δ 0.86 (3H, t, J =
7.4 Hz), 1.58-1.84 (4H, m), 2.65 (1H, ddd, J = 4.2, 8.1, 12.6 Hz),
2.79 (1H, ddd, J = 4.5, 6.6, 11.1 Hz), 2.94 (1H, dd, J = 5.2, 13.6
Hz), 3.09 (1H, dd, J = 4.4, 13.6 Hz), 3.77 (3H, s), 4.30 (1H, t, J =
5.0 Hz), 6.68-6.63 (2H, m), 6.72 (1H, m). Anal. (C14H19NO2) C,
H, N.
4aR,9aS-(þ)-cis-4a-Ethyl-6-methoxy-2-methyl-1,2,3,4,4a,9a-
hexahydrobenzofuro[2,3-c]pyridine ((þ)-11). Pd/C (10 wt %,
21.3 mg) was added in an argon atmosphere to (þ)-9 (233 mg)
that was dissolved in MeOH (10 mL). The argon was evacuated
and re-added 3ꢀ. Formaldehyde solution (37% aqueous, 89 μL)
was added and the argon replaced by hydrogen via a hydrogen
balloon connected to the flask through a needle (16 gauge, 11/2).
The reaction was complete after 4 h at room temperature
(monitored by TLC (CHCl3/MeOH/28% NH4OH, 90:9:1)).
The mixture was then filtered through Celite, the filtered catalyst
was rinsed with ethanol, and the solvent was removed in
vacuo. Column chromatography (CHCl3/MeOH/28% NH4OH,
90:9:1) afforded (þ)-11 (247 mg, 100%) as a colorless viscous
EtOAc or acetonitrile to give pure (þ)-9 S-(þ)-mandelate
3
(17.96 g, 64%), mp 144.5-145.5 ꢀC. The mandalate salt was
dissolved in CHCl3 (40 mL), and NaOH (2 N, 45 mL) was used
to convert it to the free base. The aqueous phase was extracted
with CHCl3 (3ꢀ). The combined organic phase was washed with
H2O (30 mL) and the solvent removed in vacuo to give (þ)-9 as
the free base (11.23 g). High vacuum distillation at 120-125 ꢀC
gave a quantitative yield of (þ)-9 (10.88 g) as a colorless viscous
oil. The oil slowly crystallized to a white solid, mp 50-51 ꢀC.
oil that crystallized on standing, mp 50-51 ꢀC. [R]20 þ89.3
D
(c 0.750, CDCl3). NMR (CDCl3, 300 MHz): δ 0.84 (3H, t, J =
7.5 Hz), 1.58 (1H, sextet, J = 7.3 Hz), 1.69 (1H, sextet, J = 7.2
Hz), 1.78 (1H, ddd, J = 4.2, 9.3, 13.8 Hz), 2.02 (1H, ddd, J =
3.4, 5.8, 13.8 Hz), 2.12 (1H, ddd, J = 3.4, 9.4, 14.6 Hz), 2.25 (3H,
s), 2.30 (1H, dd, J = 6.9, 12.3 Hz), 2.42 (1H, m), 2.75 (1 H, ddd,
J = 1.4, 5.4, 12.3 Hz), 3.77 (3H, s), 4.45 (1H, dd, J = 5.3,
6.8 Hz), 6.65 (2H, m), 6.72 (1H, m). MS m/z 248 [M þ H]þ. Anal.
(C15H21NO2) C, H, N.
[R]20 þ79.5ꢀ (c 1.014, CDCl3). NMR (CDCl3, 300 MHz): δ
D
0.86 (3H, t, J = 7.4 Hz), 1.58-1.84 (4H, m), 2.65 (1H, ddd, J =
4.2, 8.1, 12.6 Hz), 2.79 (1H, ddd, J = 4.5, 6.6, 11.1 Hz), 2.94 (1H,
dd, J = 5.2, 13.6 Hz), 3.09 (1H, dd, J = 4.4, 13.6 Hz), 3.77 (3H,
s), 4.30 (1H, t, J = 5.0 Hz), 6.68-6.63 (2H, m), 6.72 (1H, m).
Anal. (C14H19NO2) C, H, N.
4aR,9aS-(þ)-cis-4a-Ethyl-6- methoxy-2-(2-phenylethyl)-1,2,3,
4,4a,9a-hexahydrobenzofuro[2,3-c]pyridine (þ)-12). Compound
(þ)-9 (234 mg) was mixed with 2-bromoethylbenzene (611 mg),
K2CO3 (166 mg) in anhydrous DMF (5 mL). The mixture was
heated at 50 ꢀC for 20 h. After the completion of the reaction
(monitored by TLC, CHCl3/EtOAc, 10:3), the mixture was
quenched with H2O (50 mL) and extracted with EtOAc (50 mL
ꢀ 3). The combined organic phase was washed with H2O (50 mL),
followed by brine (50 mL), dried with Na2SO4, and filtered
through Celite. The solvent was removed in vacuo to give crude
(þ)-12 which was purified by column chromatography (CHCl3/
EtOAc, 5:1). The product (þ)-12 (205 mg, 61%) was obtained as a
A portion of the amine (þ)-9 was derivatized with R-(þ)-1-
phenethylisocyanate17 to give the diastereomeric urea, and the
chemical shifts at δ 1.453 and 1.408 were examined. The optical
purity of (þ)-9 (∼99% ee)17 was determined by the disappear-
ance of the chemical shift at δ 1.453, and that of (-)-10 (∼99%
ee)17 by the disappearance of the chemical shift at δ 1.408. The
chirality of (þ)-9 as 4aR,9aS was established by the X-ray
crystallographic examination of the R-(-)-mandelic acid salt
of enantiomer (-)-10.
4aS,9aR-(-)-cis-4a-Ethyl-6-methoxy-1,2,3,4,4a,9a-hexahy-
drobenzofuro[2,3-c]pyridine ((-)-10). The mother liquors from
the preparation of the 4aR,9aS-(þ) isomer, enriched with the
4aS,9aR-(-)-enantiomer, were combined and free-based with
28% NH4OH (13.5 mL) until the pH was around 9-9.5.
Chloroform (80 mL ꢀ 4) was used to extract the amine and
the solvent removed in vacuo to give an amine mixture (23.0 g).
NMR of the derivatized amine mixture indicated that it was
about a 3:2 mixture of the 4aS,9aR and the 4aR,9aS isomers,
respectively. The amine mixture (23.0 g, 0.098 mol) and
white solid, mp 84-86.5 ꢀC. [R]20 þ113.7ꢀ (c 0.750, CDCl3).
D
NMR (CDCl3, 300 MHz): δ 0.83 (3H, t, J = 7.5 Hz), 1.58 (1H,
sextet, J = 7.2 Hz), 1.70 (1H, sextet, J = 7.2 Hz), 1.80 (1H, ddd,
J = 4.2, 9.9, 14.1 Hz), 2.07 (1H, ddd, J = 3.3, 5.4, 14.1 Hz), 2.21
(1H, dt, J = 3.3, 11.4 Hz), 2.36 (1H, dd, J = 7.2, 12.0 Hz), 2.59
(3H, m), 2.79 (2H, m), 2.94 (1H, ddd, J = 1.5, 6.0, 12.0 Hz), 3.78
(3H, s), 4.48 (1H, dd, J = 5.7, 7.2 Hz), 6.66 (2H, m), 6.73 (1H, m),
7.15-7.29 (5H, m). MS m/z 338 [M þ H]þ. Anal. (C22H27-
NO2 0.1H2O) C, H, N.
3

Article
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23 7577
4aR,9aS-(þ)-cis-4a-Ethyl-1,2,3,4,4a,9a-hexahydrobenzofuro-
7.21-7.34 (5H, m). MS m/z 340 [M þ H]þ. Anal. (C21H25NO3)
C, H, N.
[2,3-c]pyridin-6-ol ((þ)-13). Compound 9 (250 mg) was dissolved
in hydrobromic acid (2.5 mL, 48%). The mixture was refluxed
for about 1 h, and the completion of the reaction was monitored
by TLC (CHCl3/MeOH/28% NH4OH, 90:9:1). The acidic
solution was then basified with 28% NH4OH to pH ∼10,
extracted with CH2Cl2 (15 mL ꢀ 4), and concentrated in vacuo
to afford crude (þ)-13. Column chromatography (CHCl3/
MeOH/28% NH4OH, 90:9:1) purification afforded 4aR,9a-
S-(þ)-13 (220 mg, 95%) as a white solid, mp 165-166 ꢀC.
4aR,9aS,20R-(þ)-cis-2-(20-Hydroxy-20-phenylethyl)-4a-ethyl-
1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ol ((þ)-17).
Compound (þ)-13 (179 mg), (R)-(þ)-styrene oxide (206 mg),
and toluene (anhydrous, 5 mL) were reacted under an argon
atmosphere as in (þ)-16. The product (þ)-17 (182 mg, 66%) was
obtained as a white solid, mp 144.0-146.0 ꢀC. [R]26D þ85.4ꢀ (c
0.785, CDCl3). NMR (CDCl3, 300 MHz): δ 0.84 (3H, t, J = 7.5
Hz), 1.58 (1H, sextet, J = 7.2 Hz), 1.68 (1H, sextet, J = 7.2 Hz),
1.80 (1H, ddd, J = 4.4, 10.2, 14.6 Hz), 2.07 (1H, dt, J = 4.1, 12.3
Hz), 2.21 (1H, td, J = 3.5, 14.2 Hz), 2.41 (1H, dd, J = 10.5, 12.6
Hz), 2.50 (1H, dd, J = 3.9, 12.6 Hz), 2.58 (1H, dd, J = 6.9, 12.0
Hz), 2.85 (2H, m), 4.49 (1H, dd, J = 5.6, 6.8 Hz), 4.72 (1H, dd,
J = 3.4, 10.0 Hz), 6.59 (2H, m), 6.65 (1H, m), 7.22-7.34 (5H, m).
[R]24 þ103.0ꢀ (c 0.930, CDCl3). NMR (CDCl3, 300 MHz): δ
D
0.86 (3H, t, J = 7.4 Hz), 1.64 (1H, sextet, J = 7.3 Hz), 1.66-1.82
(3H, m), 2.66 (1H, ddd, J = 4.8, 8.4, 12.6 Hz), 2.80 (1H, ddd,
J = 4.5, 6.6, 12.6 Hz), 2.96 (1H, dd, J = 5.1, 13.8 Hz), 3.08 (1H,
dd, J = 4.2, 13.8 Hz), 4.30 (1H, t, J = 4.8 Hz), 6.58 (2H, m),
6.66 (1H, m). MS m/z 220 [M þ H]þ. Anal. (C13H17-
MS m/z 340 [M þ H]þ. Anal. (C21H25NO3 0.2H2O) C, H, N.
3
NO2 0.1H2O) C, H, N.
4aS,9aR-(-)-cis-4a-Ethyl-6-methoxy-2-methyl-1,2,3,4,4a,9a-
hexahydrobenzofuro[2,3-c]pyridine ((-)-18). Compound (-)-10
(450 mg) was dissolved in MeOH (19 mL), and the flask was
flushed 3ꢀ with argon. Pd/C (5 wt %, 82.0 mg) was added under
the argon atmosphere, and the flask was again flushed with
argon 3ꢀ. Formaldehyde solution (37% aqueous, 172 μL) was
added and the reaction was carried out as with (þ)-11 except
that the reaction was completed in 4.5 h to give (-)-18 (451 mg,
95%) as a colorless viscous oil that crystallizes on standing, mp
50-51 ꢀC. [R]27D -89.5ꢀ (c 0.810, CDCl3). NMR (CDCl3, 400
MHz): δ 0.84 (3H, t, J = 7.5 Hz), 1.58 (1H, sextet, J = 7.3 Hz),
1.71 (1H, sextet, J = 7.2 Hz), 1.78 (1H, ddd, J = 4.3, 9.2, 13.6
Hz), 2.02 (1H, ddd, J = 3.6, 6.4, 14.0 Hz), 2.12 (1H, ddd, J =
3.4, 9.2, 12.0 Hz), 2.25 (3H, s), 2.32 (1H, dd, J = 7.0, 12.2 Hz),
2.42 (1H, m), 2.75 (1H, ddd, J = 1.4, 5.2, 12.0 Hz), 3.77 (3H, s),
4.45 (1H, dd, J = 5.4, 7.0 Hz), 6.65 (2H, m), 6.72 (1H, m). MS m/
z 248 [M þ H]þ. Anal. (C15H21NO2) C, H, N.
3
4aR,9aS-(þ)-cis-4a-Ethyl-2-methyl-1,2,3,4,4a,9a-hexahydrobenzo-
furo[2,3-c]pyridin-6-ol ((þ)-14). Compound (þ)-11 (212 mg) was
dissolved in hydrobromic acid (48%, 2.5 mL). The mixture was
refluxed for about 30 min. The completion of the reaction was
monitored by TLC (CHCl3/MeOH/28% NH4OH, 90:9:1). The
mixture was concentrated in vacuo to give a viscous brownish
glassy residue. It was dissolved in mixture of H2O (5 mL) and
28% NH4OH (1 mL) and extracted with CH2Cl2 (10 mL ꢀ 3)
and the solvent removed in vacuo. Column chromatography
(CHCl3/MeOH/28% NH4OH, 90:9:1) afforded (þ)-14 (169 mg,
85%) as a white solid, mp 159-162 ꢀC. [R]28 þ92.7 (c 0.780,
D
CDCl3). NMR (CDCl3, 300 MHz): δ 0.84 (3H, t, J = 7.5 Hz),
1.53 (1H, sextet, J = 7.3 Hz), 1.61 (1H, sextet, J = 7.3 Hz), 1.78
(1H, ddd, J = 3.9, 9.3, 13.6 Hz), 2.01 (1H, ddd, J = 3.4, 5.8, 13.8
Hz), 2.13 (1H, ddd, J = 3.3, 9.6, 12.2 Hz), 2.25 (3H, s), 2.32 (1H,
dd, J = 7.2, 12.0 Hz), 2.44 (1H, m), 2.76 (1H, ddd, J = 1.4, 5.6,
12.3 Hz), 4.45 (1H, dd, J = 5.6, 6.8 Hz), 6.58 (2H, m), 6.66
4aS,9aR-(-)-cis-4a-Ethyl-6-methoxy-2-(2-phenylethyl)-1,2,3,4,4a,
9a-hexahydrobenzofuro[2,3-c]pyridine ((-)-19). Compound (-)-10
(234 mg) was reacted as shown in the preparation of (þ)-12, except
that the mixture was quenched with H2O (50 mL), and 28% NH4OH
was used to adjust the pH to ∼10. Ethyl acetate (50 mL ꢀ 3) was used
to extract the product. The combined organic phase was washed with
H2O (50 mL), followed by brine (50 mL), dried with MgSO4, and
filtered through Celite. After rotary evaporation, the crude product
was purified by column chromatography (CHCl3/EtOAc, 5:1). The
product (242 mg, 72%) was obtained as a white solid, mp 92-
93.5 ꢀC. [R]27D -113.5ꢀ (c0.755, CDCl3). NMR(CDCl3, 400MHz):
δ 0.83 (H, t, J = 7.5 Hz), 1.59 (1H, sextet, J = 7.2 Hz), 1.72 (1H,
sextet, J = 7.2 Hz), 1.80 (1H, ddd, J = 4.2, 10.0, 14.2 Hz), 2.07 (1H,
ddd, J = 3.2, 5.6, 14.0 Hz), 2.21 (1H, dt, J = 3.4, 10.6 Hz), 2.36 (1H,
dd, J= 7.2, 12.2 Hz), 2.59 (3H, m), 2.79 (2H, m), 2.94 (1H, ddd, J=
1.5, 6.0, 12.0 Hz), 3.78 (3H, s), 4.48 (1H, dd, J = 5.6, 7.2 Hz), 6.66
(2H, m), 6.73 (1H, m), 7.15-7.29 (5H, m). MS m/z 338 [M þ H]þ.
Anal. (C22H27NO2) C, H, N.
(1H, m). MS m/z 234 [M þ H]þ. Anal. (C14H19NO2 0.1H2O) C,
3
H, N.
4aR,9aS-(þ)-cis-4a-Ethyl-2-phenylethyl-1,2,3,4,4a,9a-hexahy-
drobenzofuro[2,3-c]pyridin-6-ol ((þ)-15). Compound (þ)-12
(362.0 mg) was dissolved in a mixture of hydrobromic acid
(3.7 mL, 48%) and acetic acid (16.5 mL). The mixture was
refluxed for about 7.5 h with the completion of the reaction
monitored by TLC (CHCl3/EOAc, 10:3). The acidic solution was
then concentrated under aspirator vacuum, redissolved in H2O
(15 mL), and basified with 28% NH4OH to pH ∼10, extracted
with CH2Cl2 (20 mL ꢀ 3), and the solvent was removed in vacuo
to get crude (þ)-15. Column chromatography (CHCl3/EtOAc,
10:3) afforded pure 4aR,9aS-(þ)-15 (304 mg, 88%) as a white
solid, mp 171.5-173 ꢀC. [R]24D þ115.8ꢀ (c 0.810, CDCl3). NMR
(CDCl3, 300 MHz): δ 0.83 (3H, t, J = 7.5 Hz), 1.58 (1H, sextet,
J = 7.2 Hz), 1.69(1H, sextet, J = 7.2Hz), 1.80(1H, ddd, J = 4.2,
9.9, 14.1 Hz), 2.05 (1H, ddd, J = 3.4, 5.6, 9.0 Hz), 2.22 (1H, dt,
J = 3.3, 10.5 Hz), 2.37 (1H, dd, J = 7.4, 11.8 Hz), 2.59 (3H, m),
2.79 (2H, m), 2.94 (1H, ddd, J = 1.5, 6.0, 12.0 Hz), 4.48 (1H, dd,
J = 6.0, 7.2 Hz), 6.58 (2H, m), 6.66 (1H, m), 7.15-7.29 (5H, m).
MS m/z 324 [M þ H]þ. Anal. (C21H25NO2) C, H, N.
4aS,9aR-(-)-cis-4a-Ethyl-1,2,3,4,4a,9a-hexahydrobenzofuro-
[2,3-c]pyridin-6-ol ((-)-20). Compound (-)-10 (350 mg) was
dissolved in hydrobromic acid (3.5 mL, 48%) and the reaction
run as shown for (þ)-13 to give (-)-20 (325 mg, 100%) as a beige
solid, mp 169-171 ꢀC. [R]27D -100.6ꢀ (c 0.815, CDCl3). NMR
(CDCl3, 400 MHz): δ 0.86 (3H, t, J = 7.5 Hz), 1.64 (1H, sextet,
J = 7.2 Hz), 1.70 (1H, ddd, J = 5.3, 9.2, 14.4 Hz), 1.74 (1H,
sextet, J = 7.2 Hz), 1.78 (1H, dt, J = 4.3, 14.0 Hz), 2.66 (1H,
ddd, J = 4.3, 7.7, 12.4 Hz), 2.80 (1H, ddd, J = 4.3, 6.1, 10.8 Hz),
2.96 (1H, dd, J = 4.8, 13.6 Hz), 3.09 (1H, dd, J = 4.0, 14.0 Hz),
4.30 (1H, t, J = 4.8 Hz), 6.57 (2 H, m), 6.66 (1H, m). MS m/z 220
[M þ H]þ. Anal. (C14H19NO2) C, H, N.
4aR,9aS,20S-(þ)-cis-2-(20-Hydroxy-20-phenylethyl)-4a-ethyl-
1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ol ((þ)-16).
Compound (þ)-13 (306 mg), (S)-(-)-styrene oxide (352.1 mg),
and toluene (anhydrous, 9 mL) were reacted under an argon
atmosphere. The mixture was refluxed for 72 h. The completion
of the reaction was monitored by TLC (hexane/EtOAc, 1:1).
After completion, the mixture was concentrated in vacuo.
Column chromatography (hexane/EtOAc, 3:2) afforded (þ)-
16 (265 mg, 56%) as a white solid, mp 163.0-166.0 ꢀC. [R]27
4aS,9aR-(-)-cis-4a-Ethyl-2-methyl-1,2,3,4,4a,9a-hexahydro-
benzofuro[2,3-c]pyridin-6-ol ((-)-21). Compound (-)-18 (226.4
mg) was dissolved in hydrobromic acid (48%, 2.7 mL). The
mixture was refluxed for about 30 min. The completion of the
reaction was monitored by TLC as in (þ)-14. The mixture was
then neutralized with 28% NH4OH to pH ∼10 and worked up
as in (þ)-14 to give (-)-21 (198 mg, 93%) as a white solid, mp
D
þ173.0ꢀ (c 0.770, CDCl3). NMR (CDCl3, 300 MHz): δ 0.84 (3H,
t, J = 7.4 Hz), 1.58 (1H, sextet, J = 7.2 Hz), 1.71 (1H, sextet,
J = 7.2 Hz), 1.85 (1H, ddd, J = 6.2, 8.7, 14.4 Hz), 2.04 (1H, dt,
J = 4.4, 14.1 Hz), 2.37 (1H, dd, J = 6.8, 12.2 Hz), 2.49 (4H, m),
3.13 (1H, dd, J = 5.4, 12.0 Hz), 4.46 (1H, dd, J = 5.2, 6.8 Hz),
4.68 (1H, dd, J = 5.6, 8.2 Hz), 6.59 (2H, m), 6.67 (1H, m),

7578 Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23
Zhang et al.
161-163 ꢀC. [R]27D -98.9ꢀ (c 0.825, CDCl3). NMR (CDCl3, 400
MHz): δ 0.84 (3H, t, J = 7.5 Hz), 1.56 (1H, sextet, J = 7.3 Hz),
1.70 (sextet, J = 7.3 Hz), 1.78 (1H, ddd, J = 4.0, 9.2, 13.6 Hz),
2.01 (1H, ddd, J = 3.4, 6.0, 14.4 Hz), 2.13 (1H, ddd, J = 3.2, 9.6,
12.0 Hz), 2.26 (3H, s), 2.32 (1H, dd, J = 7.2, 12.0 Hz), 2.44 (1H,
m), 2.76 (1H, ddd, J = 1.2, 5.2, 12.4 Hz), 4.45 (1H, dd, J = 5.6,
7.0 Hz), 6.58 (2H, m), 6.66 (1H, m). MS m/z 234 [M þ H]þ. Anal.
(C14H19NO2) C, H, N.
4aS,9aR-(-)-cis-4a-Ethyl-2-(2-phenylethyl)-1,2,3,4,4a,9a-hexahy-
drobenzofuro[2,3-c]pyridin-6-ol ((-)-22). Compound (-)-19 (201.0
mg) was dissolved in a hydrobromic acid (2.0 mL, 48%) and acetic
acid (10 mL) mixture. The reaction was run and purified as in (þ)-15
to give (-)-22 (150 mg, 78%) as a white solid, mp 172.0-173.5 ꢀC.
[R]27D -120.4ꢀ (c 0.785, CDCl3). NMR (CDCl3, 400 MHz): δ 0.83
(3H, t, J = 7.5 Hz), 1.58 (1H, sextet, J = 7.2 Hz), 1.71 (1H, sextet,
J = 7.2 Hz), 1.80 (1H, ddd, J = 4.2, 9.9, 14.0 Hz), 2.05 (1H, ddd,
J = 3.2, 5.2, 14.0 Hz), 2.22 (1H, dt, J = 3.3, 10.7 Hz), 2.37 (1H, dd,
J= 7.2, 12.0 Hz), 2.59 (3H, m), 2.79 (2H, m), 2.94 (1H, ddd, J=1.5,
6.0, 12.0 Hz), 4.48 (1H, dd, J = 5.6, 7.2 Hz), 6.58 (2H, m), 6.66 (1H,
m), 7.15-7.29 (5H, m). MS m/z 324 [M þ H]þ. Anal. (C21H25NO2)
C, H, N.
contained a single molecule of (-)-10 and a single molecule of
(-)-mandelic acid.
Quantum Chemisty and Superposition Study. The geometry
optimization for the conformers of compound 22, the para-d-
oxide-bridged phenylmorphan, and morphine, in their proto-
nated forms, was done in the gaseous phase with the density
functional theory at the level of B3LYP/6-31G*.21 The con-
formers of 22 were also fully optimized in CHCl3 with the
polarized continuum model with the UAKS parameters set to
compare their energetics, which include the zero-point correc-
tion as well as the enthalpy and the entropy contribution at
298.15 K. For the superposition study, the optimized structures
in the gaseous phase were overlaid onto morphine with the rigid
fit of Quanta 2008 (Accelrys) using the heavy atoms of the
piperidine ring as a common docking site.
Binding and Efficacy Assays. Cell Culture and Membrane
Preparation. As noted previously,22 the recombinant CHO cells
(hMOR-CHO, hDOR-CHO, and hKOR-CHO) were produced
by stable transfection with the respective human opioid receptor
cDNA and were provided by Dr. Larry Toll (SRI International,
CA). The cells were grown on plastic flasks in DMEM (100%)
(hDOR-CHO and hKOR-CHO) or DMEM/F-12 (50%/50%)
medium (hMOR-CHO) containing 10% FBS, and G-418
(0.10-0.2 mg/mL) under 95% air/5% CO2 at 37 ꢀC. Cell
monolayers were harvested and frozen at -80 ꢀC.
4aS,9aR,20S-(-)-cis-2-(20-Hydroxy-2-phenylethyl)-4a-ethyl-1,2,3,
4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ol ((-)-23). Compound
(-)-20 (300 mg), (S)-(-)-styrene oxide (345.2 mg), and toluene
(anhydrous, 8 mL) were reacted and purified as in (þ)-16 to give
(-)-23 (289 mg, 62.3%) as a beige solid, mp 141.0-143.0 ꢀC. [R]
[35S]GTP-γ-S Binding Assays. On the day of the assay, cells
were thawed on ice for 15 min and homogenized using a
Polytron in 50 mM Tris-HCl, pH 7.4, containing 4 μg/mL
leupeptin, 2 μg/mL chymostatin, 10 μg/mL bestatin, and 100
μg/mL bacitracin. The homogenate was centrifuged at 30000g
for 10 min at 4 ꢀC and the supernatant discarded. The membrane
pellets were resuspended in binding buffer and used for
[35S]GTP-γ-S binding assays. [35S]GTP-γ-S binding was
determined as described previously.23 Briefly, test tubes received
the following additions: 50 μL of buffer A (50 mM Tris-HCl,
pH 7.4, containing 100 mM NaCl, 10 mM MgCl2, 1 mM
EDTA), 50 μL of GDP in buffer A/0.1% BSA (final concentra-
tion = 10 μM), 50 μL of drug in buffer A/0.1% BSA, 50 μL
of [35S]GTP-γ-S in buffer A/0.1% BSA (final concentration =
50 pM), and 300 μL of cell membranes (50 μg of protein)
in buffer B. The final concentrations of reagents in the
[35S]GTP-γ-S binding assays were 50 mM Tris-HCl, pH 7.4,
containing 100 mM NaCl, 10 mM MgCl2, 1 mM EDTA, 1 mM
DTT, 10 μM GDP, and 0.1% BSA. Incubations proceeded for
3 h at 25 ꢀC. Nonspecific binding was determined using GTP-γ-
S (40 μM). Bound and free [35S]GTP-γ-S were separated by
vacuum filtration through GF/B filters. The filters were
punched into 24-well plates to which was added 0.6 mL of
LSC cocktail (Cytoscint). Samples were counted, after an over-
night extraction, in a Trilux liquid scintillation counter at 27%
efficiency.
25
D -88.5ꢀ (c 0.850, CDCl3). NMR (CDCl3, 400 MHz): δ 0.84 (3H,
t, J= 7.4 Hz), 1.59 (1H, sextet, J= 7.3 Hz), 1.70 (1H, sextet, J=7.3
Hz), 1.81 (1H, ddd, J = 4.2, 10.4, 14.6 Hz), 2.08 (1H, dt, J = 4.2,
14.8 Hz), 2.20 (1H, td, J = 3.0, 10.6 Hz), 2.41 (1H, t, J = 10.8 Hz),
2.50 (1H, dd, J = 3.4, 12.6 Hz), 2.57 (1H, dd, J = 7.2, 12.4 Hz), 2.86
(2H, m), 4.49 (1H, dd, J = 5.2, 6.8 Hz), 4.72 (1H, dd, J = 3.4, 10.6
Hz), 6.60 (2H, m), 6.66 (1H, m), 7.21-7.38 (5H, m). MS m/z 340 [M
þ H]þ. Anal. (C21H25NO3) C, H, N.
4aS,9aR,20R-(-)-cis-2-(20-Hydroxy-2-phenylethyl)-4a-ethyl-1,2,3,
4,4a,9a-hexahydrobenzofuro[2,3-c]pyridin-6-ol ((-)-24). Compound
(-)-20 (283.5 mg), (R)-(-)-styrene oxide (326.2 mg), and toluene
(anhydrous, 8 mL) were reacted and purified as shown for (þ)-16 to
give (-)-24 (255 mg, 58%) as a white solid, mp 165.0-168.0 ꢀC.
[R]27D -177.0ꢀ (c 0.775, CDCl3). NMR (CDCl3, 400 MHz): δ 0.84
(3H, t, J = 7.5 Hz), 1.58 (1H, sextet, J = 7.2 Hz), 1.71 (1H, sextet,
J = 7.2 Hz), 1.85 (1H, ddd, J = 5.3, 9.2, 14.4 Hz), 2.04 (1H, dt, J =
4.3, 14.0 Hz), 2.37 (1H, dd, J= 6.8, 12.4 Hz), 2.48 (4H, m), 3.14 (1H,
dd, J = 5.2, 12.4 Hz), 4.46 (1H, dd, J = 5.3, 6.9 Hz), 4.68 (1H, dd,
J = 5.2, 8.8 Hz), 6.59 (2H, m), 6.67 (1H, m), 7.21-7.35 (5H, m).
MS m/z 340 [M þ H]þ. Anal. (C21H25NO3 0.3H2O) C, H, N.
3
X-ray Crystal Structure of 4aS,9aR-(-)-cis-4a-Ethyl-6-methoxy-
1,2,3,4,4a,9a-hexahydrobenzofuro[2,3-c]pyridine ((-)-10) R-(-)-Man-
3
delic Acid. Single-crystal X-ray diffraction data on the (R)-(-)-
mandelic acid salt of the enantiomer 10 were collected using Mo
KR radiation and a Bruker APEX 2 CCD area detector. The
structure was solved by direct methods and refined by full-
matrix least-squares on F2 values using the programs found in
the SHELXTL suite (Bruker, SHELXTL, version 6.10, 2000,
Bruker AXS Inc., Madison, WI). Parameters refined included
atomic coordinates and anisotropic thermal parameters for all
non-hydrogen atoms. Hydrogen atoms on carbons were in-
cluded using a riding model [coordinate shifts of C applied to
Acknowledgment. The research of the Drug Design and
Synthesis Section, CBRB, NIDA and NIAAA, was supported
by the NIH Intramural Research Programs of the National
Institute of Diabetes and Digestive and Kidney Diseases, the
National Institute on Drug Abuse (NIDA), and the National
Institute of Alcohol Abuse and Alcoholism, and NIDA
supported the research of the Clinical Psychopharmacology
Section. We thank Dr. John Lloyd (NIDDK) for the mass
spectral data, Dr. Amy Newman (NIDA) for the use of her
laboratory for some of this work, and NIDA for support of
the X-ray crystallographic studies (NIDA Contract Y1-
DA6002). The quantum chemical study utilized PC/Linux
clusters at the Center for Molecular Modeling of the NIH
(http://cit.nih.gov), and this research was supported by the
NIH Intramural Research Program through the Center for
Information Technology.
˚
H atoms] with C-H distance set at 0.96 A.
A 0.504 mm ꢀ 0.112 mm ꢀ 0.037 mm crystal of (-)-10 R-(-)-
3
mandelic acid was prepared for data collection coating with
high viscosity microscope oil (Paratone-N, Hampton Research).
The oil-coated crystal was mounted on a MicroMesh mount
(MiTeGen, Ithaca, NY) and transferred to the cold stream
(113 K) on the diffractometer. The crystal was monoclinic in
˚
space group P21 with unit cell dimensions a = 11.1970(7) A, b =
˚
˚
6.2958(4) A, c = 13.8939(9) A, and β = 96.147(1)ꢀ. Corrections
were applied for Lorentz, polarization, and absorption effects.
˚
Data were 98.9% complete to 29.28ꢀ θ (approximately 0.73 A)
with an average redundancy of 3.76. The asymmetric unit

Article
Supporting Information Available: Elemental analysis and
crystallographic data. This material is available free of charge
via the Internet at http://pubs.acs.org. Atomic coordinates for
Journal of Medicinal Chemistry, 2009, Vol. 52, No. 23 7579
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3
with the Cambridge Crystallographic Data Centre (deposition
number 723854). Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge,
CB2 1EZ, U.K. [fax, þ44(0)-1223-336033; e-mail, deposit@
ccdc.cam.ac.uk].
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