Novel 3-pyridyl ethers with subnanomolar affinity for central neuronal nicotinic acetylcholine receptors

Article abstract of DOI:10.1021/jm9506884

Recent evidence indicating the therapeutic potential of cholinergic channel modulators for the treatment of central nervous system (CNS) disorders as well as the diversity of brain neuronal nicotinic acetylcholine receptors (nAChRs) have suggested an opportunity to develop subtype-selective nAChR ligands for the treatment of specific CNS disorders with reduced side effect liabilities. We report a novel series of 3-pyridyl ether compounds which possess subnanomolar affinity for brain nAChRs and differentially activate subtypes of neuronal nAChRs. The synthesis and structure-activity relationships for the leading members of the series are described, including A-85380 (4a), which possesses ca. 50 pM affinity for rat brain [³H]-(- )cytisine binding sites and 163% efficacy compared to nicotine to stimulate ion flux at human α4β2 nAChR subtype, and A-84543 (2a), which exhibits 84- fold selectivity to stimulate ion flux at human α4β2 nAChR subtype compared to human ganglionic type nAChRs. Computational studies indicate that a reasonable superposition of a low energy conformer of 4a with (S)-nicotine and (-)-epibatidine can be achieved.

Full text of DOI:10.1021/jm9506884

J . Med. Chem. 1996, 39, 817-825
817
Expedited Articles
Novel 3-P yr id yl Eth er s w ith Su bn a n om ola r Affin ity for Cen tr a l Neu r on a l
Nicotin ic Acetylch olin e Recep tor s
Melwyn A. Abreo, Nan-Horng Lin,* David S. Garvey, David E. Gunn, Ann-Marie Hettinger, J ames T. Wasicak,
Patricia A. Pavlik,† Yvonne C. Martin,† Diana L. Donnelly-Roberts, David J . Anderson, J ames P. Sullivan,
Michael Williams, Stephen P. Arneric, and Mark W. Holladay
Neuroscience Research, D-47W, and Advanced Technology, D-47E, Pharmaceutical Products Division, Abbott Laboratories,
AP-10, 100 Abbott Park Road, Abbott Park, Illinois 60064-3500
Received September 19, 1995X
Recent evidence indicating the therapeutic potential of cholinergic channel modulators for the
treatment of central nervous system (CNS) disorders as well as the diversity of brain neuronal
nicotinic acetylcholine receptors (nAChRs) have suggested an opportunity to develop subtype-
selective nAChR ligands for the treatment of specific CNS disorders with reduced side effect
liabilities. We report a novel series of 3-pyridyl ether compounds which possess subnanomolar
affinity for brain nAChRs and differentially activate subtypes of neuronal nAChRs. The
synthesis and structure-activity relationships for the leading members of the series are
described, including A-85380 (4a ), which possesses ca. 50 pM affinity for rat brain [3H]-(-)-
cytisine binding sites and 163% efficacy compared to nicotine to stimulate ion flux at human
R4â2 nAChR subtype, and A-84543 (2a ), which exhibits 84-fold selectivity to stimulate ion
flux at human R4â2 nAChR subtype compared to human ganglionic type nAChRs. Compu-
tational studies indicate that a reasonable superposition of a low energy conformer of 4a with
(S)-nicotine and (-)-epibatidine can be achieved.
The hypothesis that cholinergic dysfunction contrib-
utes to cognitive impairments in patients with Alzhe-
imer’s disease (AD)1 has prompted considerable explo-
ration of potential therapies designed to replace lost
cholinergic function, including acetylcholine (ACh) pre-
cursor loading, inhibition of ACh catabolism, and choli-
nomimetic treatments.2 ACh receptors can be divided
into two families, i.e., muscarinic and nicotinic. Whereas
considerable effort has been directed toward discovery
F igu r e 1. Generic structure of synthesized analogs.
of muscarinic agents with improved safety profiles and
bioavailability compared to classical agents, targeting
of neuronal nicotinic acetylcholine receptors (nAChRs)
has received less attention. However, the observation
of decreased nAChR density in AD, the ability of
nicotine to upregulate nAChRs, the properties of nico-
tine as a cognitive-enhancing agent in animals and
humans, and the reported neuroprotective effects of
nicotine support the attractiveness of this approach for
the potential treatment of AD.3
search targeting the neuronal nAChRs would be to
achieve selectivity for central versus ganglionic nAChRs.
Recent advances in the search for novel nAChR
ligands include the discovery of ABT-418,13,14 currently
in development for treatment of AD, and epibatidine, a
potent analgesic compound with subnanomolar affinity
for nAChRs.15 In the present report is disclosed a new
series of 3-pyridyl ether compounds 1 (Figure 1), which
generally possess subnanomolar affinity for brain
nAChRs. These compounds are structurally dissimilar
to previously known nAChR ligands and have provided
structural leads to a new series of nAChR ligands.
Neuronal nAChRs are widely distributed throughout
the central and peripheral nervous systems where they
modulate a number of CNS functions including neu-
totransmittor release and the control of cerebral blood
flow.3-8 A major subtype in brain is composed of the
R4â2 subunit combination,9 whereas ganglionic-type
nAChRs are thought to contain R3 in combination with
â2 or â4 and possibly R5.10 Since ganglionic-type
nAChRs are believed to at least partially mediate the
cardiovascular and gastrointestinal liabilities of nico-
tine,11,12 an important objective of pharmaceutical re-
Meth od s
Ch em istr y. Throughout, compounds with the a designa-
tion are of the S stereochemical series, whereas the b designa-
tion signifies R stereochemistry. Compounds 2a -5a and 2b-
5b (Table 1) were prepared and characterized by 1H-NMR, MS,
and elemental analysis prior to biological evaluation. Pre-
parative methods for these compounds are summarized in
Scheme 1. The key ether-forming step for all analogs was
carried out under Mitsunobu conditions starting with an
appropriately protected imino alcohol (7a , 7b, 8a , 8b, 9a , or
10) and 3-pyridinol. N-Protected prolinol derivatives 8a and
8b were prepared, respectively, from (S)- and (R)-proline
according to literature procedures,16,17 for which it is estab-
lished that the transformations occur without racemization.
* Author to whom correspondence should be addressed.
† Advanced Technology.
X Abstract published in Advance ACS Abstracts, February 1, 1996.
0022-2623/96/1839-0817$12.00/0 © 1996 American Chemical Society

818 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4
Abreo et al.
Ta ble 1. Radioligand Binding and Functional Properties of Cholinergic Channel Ligandsa
86
86
Sch em e 1a
was in turn derived from D-methionine according to literature
procedures.18,19 It should be noted that the (R)-azetidine-2-
carboxylic acid obtained from this procedure was not optically
pure. Therefore, the pure (R)-enantiomer was isolated as the
N-Cbz derivative according to a literature resolution protocol.20
The optical purity was established after conversion to 5b,
which was found by chiral HPLC assay to possess 99% ee.21
Following Mitsunobu couplings, N-Cbz- or N-Boc-protected
derivatives were deprotected by standard methods. N-Methyl-
azetidines were prepared by reductive methylation of the
secondary amines with formaldehyde. Final compounds were
isolated and characterized as dihydrochloride or fumarate
salts.
Biologica l Assa ys. The binding affinity of each compound
to central nAChRs was determined by measuring the displace-
ment of [3H]-(-)-cytisine from a preparation of whole rat brain
according to the procedure of Pabreza et al.22 [3H]-(-)-Cytisine
has been shown to label primarily a receptor subtype in rat
brain composed of R4 and â2 subunits.9 Compounds also were
evaluated for their ability to stimulate 86Rb+ efflux from cell
preparations containing different subtypes of nAChRs. Ion
flux at human R4â2 receptors was assessed using a human
cell line (K177) in which these receptors have been stably
expressed.14 86Rb+ efflux from the human neuroblastoma-
derived IMR-32 cell line, which expresses ganglionic-like
nAChRs, was investigated using the procedure described
previously.23
Molecu la r Mod elin g. Molecular modeling was performed
with SYBYL.24 Conformations for (S)-nicotine, (-)-epibatidine,
and compound 4a were generated with DGEOM and mini-
mized with MMP2.25 The missing parameters were estimated
by ab initio calculations as follows: Model compounds were
studied with the torsion angle of interest at 0°, 30°, 60°, and
120°; these structures were optimized at the STO-3G level;
then energies were calculated at the 6-31G* level; and the
results fitted to the needed parameters to reproduce the barrier
heights. The following V1 V2 and V3 torsional parameters
were estimated: 12-2-37-2, 0 10 0; 12-2-37-20, 0 10 0; 6-2-2-
37, 0 15 0. The following in-plane bending parameter was
estimated: 12-2-37, 0.4 116.54.
a
(a) 3-Pyridinol, diethyl azodicarboxylate, triphenylphosphine,
THF; (b) TFA, CH2Cl2; (c) HCl, EtOH; (d) aqueous HCHO,
NaBH3CN; (e) refs 18-20; (f) BH3, THF; (g) H2, Pd/C; (h) (HCHO)n;
H2, Pd/C.
Azetidine 9a was prepared in analogous fashion from (S)-
azetidine-2-carboxylic acid. For the preparation of 10, the
precursor (R)-N-benzyloxycarbonyl-azetidine-2-carboxylic acid
was synthesized from the corresponding free amino acid, which

Novel 3-Pyridyl Ethers
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4 819
For nicotine and epibatidine, 250 conformations were gener-
ated, whereas 500 conformations were generated for compound
4a . Unique conformations were recognized as those which
differed in one or more interatomic distances by more than
0.2 Å. By this criterion, there are eight low-energy conforma-
tions of epibatidine and nicotine, whereas 57 conformations
of 4a were obtained with MM2 energies up to 2.5 kcal above
the minimum. The conformations of epibatidine and nicotine
occur in pairs, of approximately equal energy, that differ by
approximately 180° in the rotation of the pyridyl ring.
Potential superpositions were generated with DISCO26 as
implemented in SYBYL, using (-)-epibatidine as the reference.
Two types of models were investigated. The first required the
inclusion of the pyridyl nitrogen atom, its complementary site
point, and the sp3 nitrogen atom and its complementary site
point, whereas the second omitted the requirement for the site
point complementary to the sp3 nitrogen.
exceptionally high efficacy at the R4â2 subtype, stimu-
lating a maximum response >160% that of nicotine, and
comparable to that of (()-epibatidine. Moreover, com-
pounds 4a and 4b also exhibited exceptionally high
potency at both receptor subtypes. Thus, compound 4a
is, respectively, 7- and 30-fold more potent than (S)-
nicotine to stimulate cation efflux mediated by the
human R4â2 and R3âx receptors. It is noteworthy,
however, that although compounds 4a and 4b are
equipotent to (()-epibatidine to interact with the [3H]-
cytisine binding site on the R4â2 subtype, these com-
pounds are 24-40-fold less potent to activate this
subtype. The reasons for this difference are under
investigation.
Several of the compounds exhibit significant nAChR
subtype selectivity at the functional level with respect
to either efficacy or potency. (S)-N-Me-pyrrolidine 2a
is 84-fold more potent to stimulate the R4â2 subtype
than R3âx, and (S)-N-Me-azetidine 5a was both more
potent and more efficacious at the R4â2 subtype. Also,
the efficacy of (R)-N-Me-azetidine 5b was markedly
lower at the R3âx subtype (39%) than at the R4â2
subtype (103%). The corresponding desmethyl com-
pounds were much less selective; thus, N-methylation
had the effect of improving selectivity for the R4â2
subtype in both the (S)-pyrrolidine and the (S)-azetidine
series.
Resu lts a n d Discu ssion
Bin din g Data. In binding assays, prototypical nAChR
ligands exhibit diverse structure-activity patterns with
respect to N-substitution and stereochemistry. For
example, (S)-nicotine possesses 19-fold higher affinity
than (S)-nornicotine for central neuronal nAChRs. In
contrast, N-Me-(-)-epibatidine is approximately 2-fold
weaker than (-)-epibatidine,27 and N-Me-(+)-anatoxin
is several hundred fold weaker than (+)-anatoxin.28
With respect to stereoselectivity, (S)-nicotine possesses
14-fold higher affinity than (R)-nicotine, and (+)-ana-
toxin is also highly stereoselective, whereas nornico-
tine,13,29 epibatidine,15 and N-Me-epibatidine27 show
little stereoselectivity.
In Table 1, pyrrolidine and azetidine analogs are
varied with respect to stereochemistry and N-substitu-
tion. The binding data indicate that most of the analogs
possess subnanomolar affinities for central nAChRs. Of
particular note are compounds 4a (A-85380, Ki ) 52 pM)
and 4b (Ki ) 50 pM), which possess affinity comparable
to that of epibatidine, the most potent nAChR ligand
reported to date.
The effect of N-methylation for the compounds in
Table 1 is different from that found in the nicotine
series. Thus, the (S)-N-H-pyrrolidine analog 3a has
affinity that is comparable to that of the corresponding
(S)-N-Me analog 2a ; for the corresponding azetidines,
the (S)-N-H compound 4a binds nearly 9-fold more
tightly than the (S)-N-Me congener 5a . This trend with
respect to N-methylation resembles the pattern ob-
served for (-)-epibatidine more closely than that for (S)-
nicotine. With respect to stereoselectivity, the trends
are similar to those found for nicotine and nornicotine.
Thus, the (S)-N-Me compounds have higher affinity
than their (R)-enantiomers (2a vs 2b and 5a vs 5b), and
the desmethylpyrrolidine enantiomers show little ste-
reoselectivity (3a vs 3b and 4a vs 4b). Further phar-
macological characterization of compound 4a , including
binding affinity in other receptor systems, is the subject
of a separate report.30
F u n ction a l Activity. The ability of the analogs to
activate nAChRs was investigated using cell lines that
express the human R4â2 subunit combination (K177
cells) or a human ganglionic-like (R3âx) nAChR (IMR-
32 cells). With the exception of (R)-N-Me-pyrrolidine
2b and (R)-N-Me-azetidine 5b, all of the analogs stimu-
lated cation efflux with high efficacy at both receptor
subtypes, and in several cases the maximum stimula-
tion exceeded that of nicotine. Most notably, both the
(S)- and the (R)-N-H-azetidines (4a and 4b) exhibit
Since (R)-N-Me-pyrrolidine 2b displayed very low
efficacy (<20%) at both subtypes, it was tested for
antagonist properties in both preparations. At the R4â2
subtype, 2b antagonized the stimulatory effect of 100
µM (S)-nicotine (IC50 ) 110 µM). By comparison, the
competitive nicotinic antagonist dihydro-â-erythroidine,
which possesses similar binding affinity to that of 2b,
was nevertheless considerably more potent in inhibiting
the (S)-nicotine response (IC50 ) 1.9 µM).14 At R3âx,
compound 2b antagonized cation efflux mediated by 100
µM (S)-nicotine with an IC50 value of 54 µM. Thus,
compound 2b represents a novel structural class of
nAChR antagonists.
Com p u ta tion a l Stu d ies. An interesting question
regarding this series of compounds is whether they can
interact with the nicotinic receptor similarly to nicotine
and/or epibatidine. The classical Beers and Reich model
proposes that the essential elements of the nicotinic
pharmacophore are a hydrogen bond acceptor (i.e. the
pyridine lone pair of nicotine) and a charged species,
e.g. a protonated or quaternary nitrogen approximately
5.9 Å from the hydrogen bond acceptor.31 Subsequently,
Sheridan performed ensemble distance geometry with
several known ligands, in which the pharmacophoric
elements selected for superposition were those that
correspond to the two nitrogen atoms of nicotine and a
pyridine ring centroid.32 Both the Beers and Reich and
the Sheridan models suggested an internitrogen dis-
tance for nicotine of around 4.8 Å. The subsequent
discovery of (()-epibatidine as an exceptionally high-
affinity ligand for nAChRs prompted the more recent
proposal33 that the optimal internitrogen distance for
high affinity binding may be closer to 5.5 Å. This
hypothesis was supported by the finding that, of the
compounds studied, those having minimum-energy
conformations with internitrogen distances either shorter
or longer than 5.5 Å had substantially weaker affinities
than (()-epibatidine. For example, (()-isonornicotine,

820 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4
Abreo et al.
Ta ble 2. Relative Energies, Root Mean Square (RMS) Deviations, and Overlap Values for Three Models Obtained from DISCO
Analysis of (-)-Epibatidine (as Reference Compound), (S)-Nicotine, and Compound 4a
energy (kcal/mol, MM2)
RMS (Å)a
Overlap (Å3)b
N-N distance in
epibatidine (Å)
model
(-)-epibatidine
(S)-nicotine
4a
(S)-nicotine
4a
(S)-nicotine
4a
1
2
3
4.6
4.6
5.6
0.00
0.00
0.01
0.00
0.02
0.02
1.34
1.31
0.16
0.70
1.34
0.65
0.61
0.72
0.42
84.38
70.38
91.75
100.88
91.50
94.00
a
RMS deviations of the coordinates of the four pharmacophore elements from those of the corresponding features of the reference
b
compound; lower RMS values imply better fit. Extent of overlap with the reference compound.
with an internitrogen distance of 5.74 Å, has 133-fold
lower affinity than (()-epibatidine. We have performed
calculations on compound 4a , which possesses affinity
comparable to that of (()-epibatidine. The minimum-
energy conformer possessed an internitrogen distance
of 6.1 Å, which is somewhat longer than that proposed
by the models discussed above.
compounds, which also possess high affinity for the
nicotinic receptor, are accommodated as well.
When the analysis is conducted superimposing only
three points (anionic site omitted), solutions were found
that correspond closely to those represented in Table 2
and Figure 2, as expected. The question is whether
removal of the additional constraint permitted ad-
ditional unique solutions that should be considered. The
results show that, in the case where the epibatidine
reference conformer corresponds to that of models 1 and
2 above, no conformer of compound 4a with MM2 energy
lower than 1.34 kcal was found. The epibatidine
conformer corresponding to model 3 yielded a solution
set containing conformers of compound 4a with energies
of 0, 0.16, 0.65, and 0.67 kcal and higher. Among the
lower energy members, the RMS distances and overlap
were most favorable for the 0.16 kcal conformer. In
model 3, the 0.02 kcal conformer of nicotine is super-
imposed on epibatidine with RMS distance (0.44 Å) and
overlap (105.25 Å3) that were slightly more favorable
compared to the values shown in Table 2, but the
general features of the superposition are similar to that
shown in Figure 2c. Thus, with the basis set comprised
of epibatidine, nicotine, and compound 4a , the three-
point and four-point analyses yielded comparable re-
sults.
Since compound 4a is a flexible molecule, it is reason-
able to consider other conformations with energies
higher than the minimum. In the following analysis,
we consider whether (S)-nicotine, (-)-epibatidine (natu-
ral isomer), and compound 4a are able to achieve a
reasonable unified superposition of putative pharma-
cophoric elements. A key issue is the selection of
pharmacophoric elements to be superimposed. In the
primary analysis, the points included are the sp3
nitrogen, its complementary anionic site point, the
pyridine nitrogen, and its complementary hydrogen-
bond donor site point. The existence of an anionic
binding site at the nicotinic receptor is controversial,34,35
and in the case of acetylcholinesterase, interaction of
the quaternary ammonium group of acetylcholine with
several aromatic residues, possibly through a cation-π
interaction, appears to be operative as the principal
stabilizing interaction.36,37 Therefore, an additional
analysis was conducted in which consideration of a
specific anionic site point was not included.
Therefore, it is at least plausible that nicotine, epi-
batidine, and 4a interact similarly with residues in the
nAChR binding site. Further evaluation of these models
for their ability to accommodate other known nicotinic
ligands will be necessary to further distinguish between
them and to gain support for the validity of any one of
them. Such studies will be aided by the availability of
functional data, which will help to characterize ligands
which may interact with the receptor in distinct binding
modes (e.g. agonists vs antagonists).
In the four-point analysis, DISCO found 12 pharma-
cophore models at a tolerance of 1.5 Å. Visual inspec-
tion revealed that these models can be represented by
three classes. The first two classes differ from each
other with respect to which of two possible site points
on the sp3 nitrogen of epibatidine was superimposed
with sitepoints from the other ligands, and the third
class differed from the first with respect to an ap-
proximate 180° rotation of the bond connecting the
pyridine ring to the azabicyclic moiety. Table 2 shows
the details of the best model of each class, with pictorial
representations shown in Figure 2. In models 1 and 2,
the pyridine ring of the reference epibatidine conformer
is rotated so that its nitrogen atom is proximal to the
sp3 nitrogen (N-N distance of 4.6 Å). Only a compara-
tively high energy conformation of compound 4a is
accommodated by the first model (MM2 ∆E, 1.34 kcal/
mol; Tripos force field ∆E, 2.60 kcal/mol, Figure 2a).
Model 2 also required a higher energy conformation of
4a . In addition, the superposition of nicotine was such
that its pyridine ring does not overlay the aromatic rings
of the other two compounds (Figure 2b). For model 3,
a lower energy conformation of compound 4a is possible,
while the root-mean-square superposition and overlap
volume of nicotine with epibatidine are comparable to
those of the first model. The N-N distance for 4a in
model 3 is 6.3 Å. Since the four points in this model
are approximately in a plane, the enantiomers of the
Su m m a r y
We have demonstrated 3-pyridyl ether compounds are
potent nAChR ligands which possess subnanomolar
affinity for brain nAChRs and differentially activate
subtypes of neuronal nAChRs. A-85380 (4a ), which
possesses ca. 50 pM affinity for rat brain [3H]-(-)-
cytisine binding sites and 163% efficacy compared to
nicotine to stimulate ion flux at human R4â2 nAChR
subtype and is not stereoselective in the assays exam-
ined. A-84543 (2a ) exhibits 84-fold selectivity to stimu-
late ion flux at human R4â2 nAChR subtype compared
to human ganglionic type nAChRs and is stereoselec-
tive. Computer-modeling studies indicate that a rea-
sonable superposition of a low-energy conformer of 4a
with (S)-nicotine and (-)-epibatidine can be achieved.
SAR studies are now in progress to further develop this
series as novel, highly potent nAChR ligands.

Novel 3-Pyridyl Ethers
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4 821
F igu r e 2. Stereoviews of three different four point models of 4a (thick black) superimposed with (-)-epibatidine (gray) and
(S)-nicotine (thin black).
(S)-1-Methyl-2-pyrrolidinemethanol and (S)-2-azetidinecar-
boxylic acid were purchased from Aldrich Chemical Co. and
used without further purification. (R)-1-Methyl-2-pyrro-
lidinemethanol was purchased from Fluka. (S)-1-(tert-Butoxy-
carbonyl)-2-pyrrolidinemethanol and (R)-1-(tert-butoxycarbonyl)-
2-pyrrolidinemethanol were prepared according to the literature
procedure16,17 starting from commercially available D-proline
and L-proline, which were purchased from Aldrich.
Exp er im en ta l Section
Proton magnetic resonance spectra were obtained on a
Nicolet QE-300 (300 MHz) or a General Electric GN-300 (300
MHz) instrument. Chemical shifts are reported as δ values
(ppm) relative to Me4Si as an internal standard unless
otherwise indicated. Mass spectra were obtained with a
Hewlett-Packard HP5965 spectrometer. The above determi-
nations were performed by the Analytical Research Depart-
ment, Abbott Laboratories, and elemental analyses were
performed by Robertson Microlit Laboratiories, Inc., Madison,
New J ersey.
Thin-layer chromatography (TLC) was carried out using E.
Merck precoated silica gel F-254 plates (thickness 0.25 mm).
Flash chromatography was carried out using Merck silica gel
60, 200-400 mesh.
Melting points are uncorrected and were determined on a
Buchi melting point apparatus. Optical rotation data was
obtained on a Perkin-Elmer Model 241 polarimeter. All
reactions were performed under anhydrous conditions unless
otherwise noted.
The following abbreviations are used in the Experimental
Section: THF for tetrahydrofuran, D2O for deuterium oxide,
CDCl3 for deuteriochloroform, DMSO-d6 for deuteriodimethyl
sulfoxide, BOC for tert-butoxycarbonyl, Cbz for benzyloxycar-
bonyl, CHCl3 for chloroform, EDC for 1-(3-(dimethylamino)-
propyl)-3-ethylcarbodiimide, TFA for trifluoroacetic acid, DEAD
for diethyl azodicarboxylate, HOBt for 1-hydroxybenzotriazole.
3-((1-Met h yl-2(S)-p yr r olid in yl)m et h oxy)p yr id in efu -
m a r a te (2a ). To a solution of triphenylphosphine (1.71 g, 6.5
mmol) in THF (30 mL) at -15 °C was added DEAD (1.02 mL,
6.5 mmol) dropwise with stirring. Then, (S)-1-methyl-2-
pyrrolidinemethanol 7a ([R]19D -49.5° (c 5, MeOH)) (5.0 g, 4.34
mmol) and 3-hydroxypyridine (0.43 g, 4.5 mmol) were added,
and the mixture was kept at -20 °C overnight. After evapora-
tion of the solvent, the residue was dissolved in methylene
chloride and washed with 1 N HCl. The aqueous layer was
adjusted to basic pH with aqueous sodium bicarbonate and
extracted with methylene chloride. The organic extract was
dried (Na2SO4), filtered, and concentrated. The residue was
purified by silica gel column chromatography. Elution with
CHCl3/MeOH (10:1) provided the product (260 mg, 31%): MS
1
(DCI/NH3) m/ e 193 (M + H)+; H NMR (CDCl3, 300 MHz) δ
8.32 (t, J ) 1.5 Hz, 1H), 8.22 (t, J ) 3 Hz, 1H), 7.24-7.2 (m,
2H), 4.14-4.05 (dd, J ) 9, 6 Hz, 1H), 4.00-3.93 (dd, J ) 9, 6
Hz, 1H), 3.24-3.14 (m, 1H), 2.81-2.7 (m, 1H), 2.54 (s, 3H),
2.44-2.31 (m, 1H), 2.14- 2.00 (m, 1H), 1.96-1.71 (m, 3H).

822 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4
Abreo et al.
The compound from above was dissolved in anhydrous
MeOH and brought to 0 °C with stirring. Fumaric acid was
dissolved in MeOH with sonication and added dropwise to the
solution containing the amine. The mixture was warmed to
room temperature with stirring. After 30 min the solvent was
evaporated in vacuo, and the remaining solid was vacuum
filtered. The solid was then recrystallized from MeOH/Et2O
to give the desired product as a white powder (262 mg, 21%):
mp 124-125 °C; [R]25D -3.9° (c 1, MeOH); MS (DCI/NH3) m/ e
193 (M + H)+; 1H NMR (D2O, 300 MHz) δ 8.43 (br s, 1H), 8.33
(d, J ) 4.5 Hz, 1H), 7.88-7.84 (m, 1H), 7.3 (dd, J ) 8.82, 5.15
Hz, 1H), 6.58 (s, 2H), 4.59 (dd, J ) 11, 3 Hz, 1H), 4.42 (dd, J
) 11, 5.88 Hz, 1H), 4.05-3.9 (m, 1H), 3.82-3.74 (m, 1H), 3.34-
3.22 (m, 1H), 3.05 (s, 3H), 2.47-2.37 (m, 1H), 2.30-2.06 (m,
3H). Anal. (C11H16N2O‚C4H4O4) C, H, N.
evaporated in vacuo, and the remaining solid was triturated
with anhydrous diethyl ether. The product was obtained as a
hygroscopic solid (218 mg, 47%): mp 92-95 °C; [R]25 -15.0°
D
1
(c 0.35, MeOH); MS (DCI/NH3) m/ e 179 (M + H)+; H NMR
(D2O, 300 MHz) δ 8.38 (m, 1H), 8.30 (m 1H), 7.76-7.72 (m,
1H), 7.72-7.62 (m, 1H), 6.57 (s, 2H), 4.60-4.50 (m, 1H), 4.35-
4.25 (m, 1H), 4.10-4.08 (m, 1H), 3.42 (t, J ) 7.5 Hz, 2H), 2.37-
2.23 (m, 1H), 2.23-2.06 (m, 2H), 2.06-1.98 (m, 1H). Anal.
(C10H14N2O‚C2H4O2‚0.9 H2O) C, H, N.
3-(2(R)-P yr r olid in ylm et h oxy)p yr id in e Dih yd r och lo-
r id e (3b). To a solution of triphenylphosphine (1.24g, 4.74
mmol) in THF (20 mL) at room temperature was added DEAD
(0.746 mL, 4.74 mmol) dropwise with stirring. After stirring
at room temperature for 30 min, (R)-1-(tert-butoxycarbonyl)-
2-pyrrolidinemethanol 8b16,17 (350 mg, 3.16 mmol) and 3-hy-
droxypyridine (450 mg, 4.74 mmol) were added to the reaction
mixture. The resultant solution was then stirred at room
temperature overnight. After all of the starting material was
consumed, the organic solvent was evaporated in vacuo. The
residue was purified by flash silica gel column chromatogra-
phy. Elution with CHCl3/MeOH (10:1) provided 518 mg of the
product contaminated with diethylhydrazine dicarboxylate:
MS (DCI/NH3) m/ e 279 (M + H)+.
3-((1-Meth yl-2(R)-p yr r olid in yl)m eth oxy)p yr id in e Di-
h yd r och lor id e (2b). To a solution of triphenylphosphine (509
mg, 1.94 mmol) in THF (10 mL) at room temperature was
added DEAD (0.35 mL, 1.94 mmol) dropwise with stirring.
After stirring at room temperature for 30 min, (R)-1-methyl-
2-pyrrolidinemethanol (7b, [R]20D -31 ( 2° (c 1, toluene)) (150
mg, 1.30 mmol) and 3-hydroxypyridine (185 mg, 1.94 mmol)
were added to the reaction mixture. The resultant solution
was then stirred at room temperature overnight. After all of
the starting material was consumed, the organic solvent was
evaporated in vacuo. The residue was purified by silica gel
column chromatography. Elution with CHCl3/MeOH (10:1)
provided the product (76 mg, 21%): MS (DCI/NH3) m/ e 193
To a solution of the compound from above (115 mg, 0.42
mmol) in CH2Cl2 (2 mL) solution was added TFA (2 mL). The
resultant solution was stirred at room temperature for 2.5 h.
Evaporation of both solvent and trifluoroacetic acid gave a
brown oil which was basified with saturated ammonium
hydroxide solution. This oil was purified by flash silica gel
column chromatography. Elution with CHCl3/MeOH (10:1.5)
provided the desired product (67 mg, 96%): MS (DCI/NH3) m/ e
179 (M + H)+; 1H NMR (D2O, 300 MHz) δ 8.33 (br s, 1H), 8.22
(m, 1H), 7.22 (m, 2H), 3.82-4.03 (m, 2H), 3.50-3.61 (m, 1H),
2.92-3.10 (m, 2H), 1.70-2.10 (m, 4H), 1.51-1.66 (m, 1H).
The compound from above (67 mg, 0.4 mmol) was dissolved
in ethanol, and hydrochloric acid in diethyl ether was added
dropwise at ambient temperature. The resultant white pre-
cipitate was then collected by evaporation of solvent followed
by trituration with three portions of diethyl ether yielding a
1
(M + H)+; H NMR (CDCl3, 300 MHz) δ 8.32 (t, J ) 1.5 Hz,
1H), 8.22 (t, J ) 3 Hz, 1H) 7.24-7.2 (m, 2H), 4.14-4.05 (dd,
J ) 9, 6 Hz, 1H), 4.00-3.93 (dd, J ) 9, 6 Hz, 1H), 3.24-3.14
(m, 1H), 2.81-2.7 (m, 1H), 2.54 (s, 3H), 2.44-2.31 (m, 1H),
2.14-2.00 (m, 1H), 1.96-1.71 (m, 3H).
The compound from above (76 mg, 0.4 mmol) was dissolved
in ethanol. Hydrochloric acid in diethyl ether was added
dropwise to a stirring solution of base at ambient temperature.
The resultant white precipitate was then collected by evapora-
tion of solvent and triturated with three portions of diethyl
ether producing (53 mg, 50%) a hygroscopic solid: [R]25D +6.54
° (c 1, MeOH); MS (DCI/NH3) m/ e 193 (M + H)+; 1H NMR
(D2O, 300 MHz) δ 8.43 (br s, 1H), 8.33 (d, J ) 5.6 Hz, 1H),
7.88-7.82 (m, 1H), 7.72 (dd, J ) 8.50, 5.15 Hz, 1H), 4.59 (dd,
J ) 11, 3 Hz, 1H), 4.42 (dd, J ) 11.4, 6.2 Hz, 1H), 4.17-4.25
(m, 1H), 4.05-3.9 (m, 1H), 3.82-3.74 (m, 1H), 3.34-3.22 (m,
1H), 3.04 (s, 3H), 2.42-2.36 (m, 1H), 2.26-2.06 (m, 3H). Anal.
(C11H16N2O‚HCl‚0.2 H2O) C, H, N.
3-(2-(S)-P yr r olid in ylm eth oxy)p yr id in e F u m a r a te (3a ).
To a solution of triphenylphosphine (1.97 g, 7.5 mmol) in THF
(30 mL) at room temperature was added DEAD (1.13 mL, 7.5
mmol) dropwise with stirring. After stirring at room temper-
ature for 30 min, (S)-1-(tert-butoxycarbonyl)-2-pyrrolidinemeth-
anol 8a 16,17 (1 g, 5.0 mmol) and 3-hydroxypyridine (713 mg,
7.5 mmol) were added to the reaction mixture. The resultant
solution was stirred at room temperature for 16 h. After all
the starting material was consumed, the organic solvent was
evaporated in vacuo. The residue was purified by flash silica
gel column chromatography. Elution with CHCl3/MeOH (10:
1) provided 3-((1-(tert-butoxycarbonyl)-2(S)-pyrrolidinyl)methox-
y)pyridine (2 g, contaminated with PPh3O): MS (DCI/NH3)
m/ e 279 (M + H)+.
hygroscopic solid (53 mg, 55%): [R]25 +7.5° (c 1.2, MeOH);
D
MS (DCI/NH3) m/ e 179 (M + H)+; 1H NMR (D2O, 300 MHz) δ
8.43 (br s, 1H), 8.34 (br s 1H), 7.85 (dd, J ) 8.80, 2.90 Hz,
1H), 7.72 (dd, J ) 8.80, 5.15 Hz, 1H), 4.54 (dd, J ) 11, 3.3 Hz,
1H), 4.32 (dd, J ) 10.6, 3.3 Hz, 1H), 4.10-4.19 (m, 1H), 3.42
(t, J ) 7.5 Hz, 1H), 2.27-2.35 (m, 1H), 2.06-2.21 (m, 2H),
1.90-2.02 (m, 1H). Anal. (C10H14N2O‚2.4HCl) C, H, N.
3-(2(S)-Azetid in ylm eth oxy)p yr id in e Dih yd r och lor id e
(4a ). (a ) (S)-1-(ter t-Bu toxyca r bon yl)-2-a zetid in em eth a -
n ol (9a ). To an ice-cooled solution of (S)-2-azetidinecarboxylic
acid ([R]20 -120° (c 3.6, H2O)) (10.15 g, 100.39 mmol) in 1,
D
4-dioxane:water (300 mL, 1:1) was added di-tert-butyl dicar-
bonate (28.48 g, 130.51 mmol) at 0 °C, followed by 4-methyl-
morpholine (11.68 g, 115.45 mmol). The reaction mixture was
stirred for 18 h, with gradual warming to room temperature.
The reaction mixture was then poured into a saturated
solution of sodium bicarbonate (250 mL) at 0 °C and washed
with ethyl acetate (3 × 250 mL). The aqueous layer was then
acidified with potassium hydrogen sulfate to pH ) 1 and
extracted with ethyl acetate (3 × 300 mL). These extracts
were then dried (Na2SO4), filtered, and concentrated in vacuo.
The resulting semisolid (S)-1-(tert-butyloxycarbonyl)azetidine-
2-carboxylic acid38 was carried forward without further
purification: MS (DCI/NH3) m/ e 202 (M + H)+, 219 (M +
NH4)+; 1H NMR (CDCl3, 300 MHz) δ 10.0 (br s, 1H), 4.81-
4.76 (t, J ) 15 Hz, 1H), 3.99-3.83 (m, 2H), 2.62-2.38 (m, 2H),
1.48 (s, 9H).
To a solution of the compound from above in CH2Cl2 (12 mL)
was added TFA (12 mL). The resultant solution was stirred
at room temperature for 3 h. Evaporation of both solvent and
TFA gave a brown oil which was basified with saturated
ammonium hydroxide solution. This oil was purified by flash
silica gel column chromatography. Elution with CHCl3/MeOH/
NH4OH (10:1:0.1) provided the desired product (280 mg,
To a solution of the compound from above (9.39 g, 46.72
mmol) in THF (100 mL) was added borane/THF complex (1
M, 210 mL, 4.5 equiv) at 0 °C under nitrogen. The mixture
was allowed to gradually warm to room temperature and was
stirred for 48 h. A 10% aqueous potassium hydrogen sulfate
solution (60 mL) was added gradually, and the volatile
components were then evaporated in vacuo. The remaining
slurry was extracted with EtOAc (3×). The organic phase was
then washed with a saturated solution of sodium hydrogen
carbonate (3 × 75 mL), dried (MgSO4), filtered, and concen-
1
31%): MS (DCI/NH3) m/ e 179 (M + H)+; H NMR (D2O, 300
MHz) δ 8.33 (br s, 1H), 8.22 (m, 1H), 7.22 (m, 2H), 3.82-4.03
(m, 2H), 3.50-3.61 (m, 1H), 2.92-3.10 (m, 2H), 1.70-2.10 (m,
4H), 1.51-1.66 (m, 1H).
The free base (281 mg, 1.57 mmol) was dissolved in
anhydrous MeOH and brought to 0 °C with stirring. Excess
fumaric acid was dissolved in MeOH with sonication and added
dropwise to the base. The mixture was warmed to room
temperature with stirring. After 30 min the solvent was

Novel 3-Pyridyl Ethers
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4 823
trated in vacuo, yielding 9a (8.4 g, 96% yield) as a colorless
oil. This material was carried forward without further puri-
fication: MS (DCI/NH3) m/ e 188 (M + H)+; 1H NMR (CDCl3,
300 MHz) δ 4.49-4.40 (ddd, J ) 9.0, 9.0, 3.0 Hz, 1H), 3.95-
3.68 (m, 4H), 2.23-2.12 (m, 1H), 1.99-1.87 (m, 1H), 1.46 (s,
9H).
(d, J ) 2.2Hz, 1H), 8.41 (d, J ) 5.5 Hz, 1H), 8.07 (dd, J ) 8.8,
3.0 Hz, 1H), 7.89 (dd, J ) 8.8, 5.4 Hz, 1H), 4.90-4.80 (m, 1H),
4.61 (dd, J ) 11.8, 2.9 Hz, 1H), 4.53 (dd, J ) 11.8, 5.3 Hz,
1H), 4.30 (ddd, J ) 9.9, 9.9, 5.1 Hz, 1H), 4.02 (dd, J ) 19.5,
9.4 Hz, 1H), 3.00 (s, 3H), 2.77-2.60 (m, 2H). Anal. (C10
H14N2O‚2.0HCl‚0.7H2O) C, H, N.
-
3-(2(R)-Azetid in ylm eth oxy)p yr id in e Dih yd r och lor id e
(4b). (a ) (R)-1-(Ben zyloxyca r bon yl)-2-a zetid in em eth a -
n ol (10). 1-(p-Tolylsulfonyl)azetidine-2-carboxylic acid was
prepared from D-methionine by the method of Miyoshi et al.18
Analysis of enantiomeric purity was carried out by conversion
to the R-methylbenzylamide [EDC (1.2 equiv), HOBt (2.3
equiv), and R-methylbenzylamine (1.2 equiv)] and evaluation
by 1H-NMR, which indicated a ca. 4:1 mixture of enantiomers.
This mixture (1.48 g, 5.8 mmol) was slurried in liquid NH3
(25 mL) at -78 °C, and sodium metal was added until a dark
blue color persisted for 30 min. Solid ammonium chloride was
added until the blue color disappeared. The cold bath was
replaced with a water bath as the ammonia was allowed to
evaporate. The remaining white solid was carefully dissolved
in H2O (30 mL) and HOAc to adjust the mixture to pH 7.0.
1,4-Dioxane (30 mL) and N-((benzyloxycarbonyl)oxy)succin-
imide (2.1 g, 8.7 mmol) were added, and the mixture was
stirred for 2 h. The biphasic mixture was partitioned between
(b) Com p ou n d 4a . A solution of compound 9a (2.8 g, 14.97
mmol) in THF (40 mL) was stirred at 0 °C under a nitrogen
atmosphere. To this solution was added DEAD (3.54 mL,
22.46 mmol) followed by triphenylphosphine (4.78 g, 22.46
mmol), and the mixture was stirred for 10 min. 3-Hydroxy-
pyridine (2.14 g, 22.46 mmol) was then added to the reaction
mixture with additional THF (40 mL). After 18 h, additional
3-hydroxypyridine (0.1 g, 1.05 mmol) was added and the
mixture was stirred for 24 h. The reaction mixture was
concentrated in vacuo, and the crude mixture was then
acidified (pH < 2) with a 10% solution of potassium hydrogen
sulfate (80 mL) and washed with ethyl acetate (3 × 75 mL).
The aqueous portion was then basified with a saturated
solution of potassium carbonate (pH ) 10) and extracted with
ethyl acetate (4 × 75 mL). These extracts were dried (MgSO4),
filtered, and concentrated in vacuo to a red-brown oil. The
crude product was purified on flash silica gel column chroma-
tography. Elution with EtOAc/hexane (2:1) provided a light
yellow oil (927 mg, 24% yield): MS (DCI/NH3) m/ e 265 (M +
H)+, 282 (M + NH4)+; 1H NMR (CDCl3) δ 8.36 (dd, J ) 3.7, 0.7
Hz, 1H), 8.23 (dd, J ) 4.0, 1.5 Hz, 1H), 7.25-7.22 (m, 2H),
4.56-4.48 (m, 1H), 4.34 (dd, J ) 10, 4.9 Hz, 1H), 4.15 (dd, J
) 10, 2.9 Hz, 1H), 3.90 (dd, J ) 8.2, 6.8 Hz, 2H), 2.42-2.25
(m, 2H), 1.42 (s, 9H).
saturated K2
The aqueous phase was acidified with concentrated HCl and
Cl2. The organic phase was dried
CO3 and Et2O, and the phases were separated.
then extracted with CH2
(MgSO4) and concentrated. The crude product was purified
on flash silica gel column chromatography. Elution with
CHCl3/MeOH/HOAc (95:5:0.5) provided a colorless oil (955 mg,
70%): MS (CI/NH3) m/ e 236 (M + H)+; 1H NMR (CDCl3, 300
MHz) δ 7.28-7.40 (m, 5H), 5.65 (s, 2H), 4.78-4.87 (m, 1H),
3.98-4.07 (m, 2H), 2.47-2.60 (m, 2H).
The mixture of (R)- and (S)-1-(benzyloxycarbonyl)azetidine-
2-carboxylic acid from above (932 mg, 3.96 mmol) was dissolved
in MeOH (20 mL) and L-tyrosine hydrazide (773 mg, 3.96
mmol) added. The slurry was heated at reflux for 10 min,
allowed to cool to ambient temperature, and then filtered. The
filter cake was dissolved in 6 N HCl and extracted with EtOAc
(2×). The organic fractions were combined, dried (MgSO4),
and concentrated to give the (R)-enantiomer as a colorless oil
(403 mg, 55%): [R]20D +104.7° (c 4.0, CHCl3) (lit.20 [R]20D +98.5°
(c 3.9, CHCl3)).
To a solution of the compound from above (286 mg, 1.08
mmol) in absolute ethanol (4 mL) at 0 °C under nitrogen was
added a saturated solution of hydrogen chloride in ethanol (4
mL). The reaction mixture was stirred for 18 h while gradu-
ally warming to room temperature. The mixture was then
concentrated in vacuo, and the product was dissolved in
absolute ethanol and triturated with diethyl ether. Two
recrystallizations from ethanol and diethyl ether yielded a
white powder (174 mg, 81% yield): mp 135-137 °C; [R]25
)
D
- 5.0° (c 0.4, MeOH); MS (DCI/NH3) m/ e 165 (M + H)+, 182
(M + NH4)+; 1NMR (D2O, 300 MHz) δ 8.59 (d, J ) 2.9 Hz,
1H), 8.47 (d, J ) 5.8 Hz, 1H), 8.23 (ddd, J ) 9.0, 2.6, 1.1 Hz,
1H), 5.05-4.97 (m, 1H), 4.58 (d, J ) 4.0 Hz, 2H), 4.22-4.05
(m, 2H), 2.72 (dd, J ) 16.9, 8.45 Hz, 2H). Anal. (C9H12N2O‚2.
HCl‚0.2H2O) C, H, N.
(R)-1-(Benzyloxycarbonyl)azetidine-2-carboxylic acid (2.0 g,
8.6 mmol) in THF (35 mL) was cooled to 0 °C, and 1.0 M BH3‚
THF (12.9 mL, 12.9 mmol) was added dropwise. The mixture
was allowed to warm to ambient temperature and was stirred
for 2.5 h. A solution of 2 N HCl was carefully added, and the
heterogeneous mixture was allowed to stir for 1 h. The slurry
was extracted with CH2Cl2, and the organic phase was dried
(MgSO4) and concentrated. The crude product was purified
on flash silica gel column chromatography. Elution with
EtOAc/hexane (1:1) provided 10 as a colorless oil (1.46 g,
3-((1-Meth yl-2(S)-a zetid in yl)m eth oxy)p yr id in e Dih y-
d r och lor id e (5a ). To a solution of Boc-protected 4a from
above (550 mg, 1.89 mmol) in methylene chloride (3 mL) at 0
°C under nitrogen was added TFA in methylene chloride (5
mL, 1:1). The mixture was stirred for 18 h, gradually warming
to room temperature. The reaction mixture was then concen-
trated in vacuo and dissolved in absolute ethanol (5 mL).
Formalin (37%, 0.75 mL) was added, and the acidity was
adjusted to pH 5 with the addition of acetic acid and sodium
acetate. The reaction mixture was stirred for 15 min, and
sodium cyanoborohydride (180 mg, 2.86 mmol) was added. A
small amount of bromocresol green was added to the reaction
mixture as indicator. The mixture was allowed to stir for 18
h, after which time the reaction was complete as monitored
by TLC. The reaction mixture was then acidified (pH ) 1)
with a saturated solution of potassium hydrogen sulfate, and
the volatiles were evaporated in vacuo. The aqueous phase
was then washed with ethyl acetate (3 × 20 mL), basified (pH
) 10) with potassium carbonate, and extracted with ethyl
acetate (4 × 20 mL). The extracts were dried (MgSO4), filtered,
and concentrated in vacuo (274 mg, 81% yield). The crude
product was then purified on flash silica gel column chroma-
tography. Elution with CHCl3/MeOH/NH4OH (800:15:1.5)
provided the product as a colorless oil (147 mg, 44% yield).
This oil was dissolved in absolute ethanol (1.5 mL) and treated
with a saturated solution of hydrogen chloride in diethyl ether.
After one recrystallization from ethanol and diethyl ether, pure
product was obtained as fine hygroscopic needles (47 mg,
12% from ((1-(tert-butoxycarbonyl)-2(S)-azetidinyl)methoxy)-
77%): [R]20 ) +15.5° (c 1.2, CHCl3); MS (CI/NH3) m/ e 222
D
1
(M + H)+; H NMR (CDCl3, 300 MHz) δ 7.30-7.41 (m, 5H),
5.12 (s, 2H), 4.47-4.58 (m, 1H), 3.72-4.01 (m, 4H), 2.18-2.29
(m, 1H), 1.93-2.08 (m, 1H).
(b) Com p ou n d 4b. DEAD (1.2 mL, 7.9 mmol) was added
to a stirred solution of Ph3P (2.1 g, 7.9 mmol) in THF (60 mL)
at 0 °C. After 15 min, compound 10 (1.46 g, 6.6 mmol) in THF
(6.6 mL) was added followed by 3-hydroxypyridine (690 mg,
7.3 mmol). After stirring for 18 h at ambient temperature the
solvent was removed and the residue was dissolved in CH2-
Cl2, washed with saturated K2CO3, dried (MgSO4), and con-
centrated. The crude product was then purified on flash silica
gel column chromatography. Elution with EtOAc/hexane (1:
2) provided a mixture (2.8 g) of (R)-1-(benzyloxycarbonyl)-3-
((2-azetidinylmethyl)oxy)pyridine and Ph3PO: MS (CI/NH3)
m/ e 299 (M + H)+. A sample (1.6 g) of this mixture was
dissolved in EtOH (25 mL) and stirred in the presence of 10%
Pd/C (320 mg) under an atmosphere of H2 (1 atm) for 4 h. The
reaction was filtered and concentrated. The residue was
purified by flash silica gel column chromatography. Elution
with CHCl3/MeOH/NH4OH (90:10:0 to 90:10:0.5) provided an
pyridine): [R]25 -21.4° (c 0.5, MeOH); MS (CI/NH3) m/ e 179
amber oil (465 mg, 75%): [R]20 +5.8° (c 1.6, CHCl3); MS (CI/
D
D
1
1
(M + H)+, 196 (M + NH4)+; H NMR (D2O/300 MHz) δ 8.53
NH3) m/ e 165 (M + H)+; H NMR (CDCl3, 300 MHz) δ 8.33

824 J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4
Abreo et al.
(11) McBride, P. E. The Health Consequences of Smoking. Med. Clin.
North Am. 1992, 76, 333-353.
(12) Stolerman, I. P.; Garcha, H. S.; Mirza, N. R. Dissociations
Between the Locomotor Stimulant and Depressant Effects of
Nicotinic Agonists in Rats. Psychopharmacology (Berlin) 1995,
117, 430-437.
(13) Garvey, D. S.; Wasicak, J . T.; Decker, M. W.; Brioni, J . D.;
Buckley, M. J .; Sullivan, J . P.; Carrera, G. M.; Holladay, M. W.;
Arneric, S. P.; Williams, M. Novel Isoxazoles Which Interact with
Brain Cholinergic Channel Receptors Have Intrinsic Cognitive
Enhancing and Anxiolytic Activities. J . Med. Chem. 1994, 37,
1055-1059.
(14) Gopalakrishnan, M.; Monteggia, L. M.; Anderson, D. J .; Molinari,
E. J .; Piattoni-Kaplan, M.; Donnelly-Roberts, D.; Arneric, S. P.;
Sullivan, J . P. Stable Expression, Pharmacologic Properties and
Regulation of the Human Neuronal Nicotinic R4â2 Receptor. J .
Pharmacol. Exp. Ther. 1996, 276, 289-297.
(15) Badio, B.; Daly, J . W. Epibatidine, a Potent Analgetic and
Nicotinic Agonist. Mol. Pharmacol. 1994, 45, 563-569.
(16) Evans, D. A.; Takacs, J . M.; Hurst, K. M. Phosphonamide
Stabilized Allylic Carbanions. New Homoenolate Anion Equiva-
lents. J . Am. Chem. Soc. 1979, 101, 371-378.
(17) Harris, B. D.; Bhat, K. L.; J oullie, M. M. Synthetic Studies of
Detoxin Complex II: Synthesis of Detoxin B1 and B3. Hetero-
cycles 1986, 24, 1045-1060.
(18) Miyoshi, M.; Sugano, H.; Fujii, T.; Ishihara, T.; Yoneda, N. A.
Novel Synthesis of Optically Active Azetidine-2-carboxylic Acid.
Chem. Lett. 1973, 5.
(19) Sugano, H.; Miyoshi, M. A Convenient Method of Synthesizing
Protected R-Amino-γ-butyrolactones. Bull. Chem. Soc. J pn. 1973,
46, 669-670.
(dd, J ) 1.5, 2.2 Hz, 1H), 8.22 (dd, J ) 2.9, 3.0 Hz, 1H), 7.21-
7.24 (m, 2H), 4.26-4.35 (m, 1H), 4.00-4.10 (m, 2H), 3.73 (dd,
J ) 7.7, 8.5 Hz, 1H), 3.45-3.51 (m, 1H), 2.22-2.46 (m, 2H).
(R)-3-(2-Azetidinylmethoxy)pyridine (450 mg, 2.74 mmol)
was slurried in Et2O (20 mL) and MeOH (∼2 mL), and then
Et2O saturated with HCl gas was added at ambient temper-
ature. The solvent was removed, and the remaining solid was
recrystallized from MeOH/Et2O to afford product as a deli-
quescent white solid (206 mg, 31%): mp 138-140 °C; [R]20
)
D
+9.8° (c 0.5, MeOH); MS (CI/NH3) m/ e 165 (M + H)+; 1H NMR
(D2O, 300 MHz) δ 8.59 (d, J ) 2.8 Hz, 1H), 8.46 (d, J ) 5.7
Hz, 1H), 8.21 (ddd, J ) 1.2, 2.8, 9.0 Hz, 1H), 7.99 (dd, J ) 5.7,
9.0 Hz, 1H), 4.96-5.03 (m, 1H), 4.57 (d, J ) 4.4 Hz, 2H), 4.05-
4.21 (m, 2H), 2.71 (dd, J ) 8.5, 17.3 Hz, 2H). Anal. (C9H11N2O‚
2HCl‚0.2H2O) C, H, N.
3-((1-Meth yl-2(R)-a zetid in yl)m eth oxy)p yr id in e Dih y-
d r och lor id e (5b). (R)-1-(benzyloxycarbonyl)-3-(2-azetidinyl-
methoxy)pyridine from above (1.2 g, contaminated with Ph3-
PO) and paraformaldehyde (375 mg, 12.5 mmol) in EtOH (8
mL) was stirred in the presence of 10% Pd/C (80 mg) under 1
atm of H2 for 6 h. The mixture was filtered and the filtrate
concentrated. The residue was purified on flash silica gel
column chromatography. Elution with CHCl3/MeOH (90:10)
afforded a colorless oil (250 mg, 50% yield from (R)-1-(benzyl-
oxycarbonyl)-2-azetidinemethanol): [R]20D +58° (c 1.5, CHCl3);
1
MS (CI/NH3) m/ e 179 (M + H)+; H NMR (CDCl3, 300 MHz)
δ 8.32 (dd, J ) 1.8, 1.8 Hz, 1H), 8.21 (dd, J ) 2.9, 3.3 Hz, 1H),
7.20-7.22 (m, 2H), 4.02 (d, J ) 5.5 Hz, 2H), 3.35-3.49 (m,
2H), 2.83-2.91 (m, 1H), 2.40 (s, 3H), 2.05-2.13 (m, 2H).
(20) Rodebaugh, R. M.; Cromwell, N. H. Resolution of DL-Azetidine-
2-Carboxylic Acid. J . Heterocycl. Chem. 1969, 6, 993-994.
(21) Assays for chiral purity were conducted using a Chiralcel-OD
column (Daicel Industries). The eluting solvent was i-PrOH/
hexane (5:95) at a flow rate of 1 mL/min, with detection at 254
nM.
(R)-3-((1-Methyl-2-azetidinyl)methoxy)pyridine (220 mg, 1.23
mmol) was slurried in Et2O (10 mL) and treated with Et2O
saturated with HCl gas. The solvent was removed and the
remaining solid triturated with EtOAc (4×) to afford a deli-
(22) Pabreza, L. A.; Dhawan, S.; Kellar, K. J . [3H] Cytisine Binding
to Nicotinic Cholinergic Receptors in Brain. Mol. Pharmacol.
1990, 39, 9-12.
(23) Sullivan, J . P.; Decker, M. W.; Brioni, J . D.; Donnelly-Roberts,
D.; Anderson, D. J .; Bannon, A. W.; Kang, C.-H.; Adams, P.;
Piattoni-Kaplan, M.; Buckley, M.; Gopalakrishnan, M.; Williams,
M.; Arneric, S. P. (()-Epibatidine Elicits a Diversity of In Vitro
and In Vivo Effects Mediated by Nicotinic Acetylcholine Recep-
tors. J . Pharmacol. Exp. Ther. 1994, 271, 624-631.
(24) SYBYL, vesion 6.2, Tripos Inc., 1699 S. Hanley Rd., suite 303,
St. Louis, MO, 1994.
(25) Sprague, J . T.; Tai, J . C.; Yuh, Y.; Allinger, N. L. The MMP2
Calculation Method. J . Comput. Chem. 1987, 8, 581-603.
(26) Martin, Y. C.; Bures, M. G.; Danaher, E. A.; DeLazzer, J .; Lico,
I.; Pavlik, P. A Fast New Approach to Pharmacophore Mapping
and Its Application to Dopaminergic and Benzodiazepine Ago-
nists. J . Comput.-Aided Mol. Design 1993, 7, 83-102.
(27) Badio, B.; Shi, D.; Garrafo, M.; Daly, J . W. Antinociceptive
Effects of the Alkaloid Epibatidine: Further Studies on Involve-
ment of Nicotinis Receptors. Drug Dev. Res. 1995, 36, 46-59.
(28) Macallan, D. R. E.; Lunt, G. G.; Wonnacott, S.; Swanson, K. L.;
Rapoport, H.; Albuquerque, E. X. Methyllycaconitine and (+)-
Anatoxin-a Differentiate Between Nicotinic Receptors in Ver-
tebrate and Invertebrate Nervous Systems. FEBS Lett. 1988,
226, 357-363.
(29) Reavill, C.; J enner, P.; Kumar, R.; Stolerman, I. P. High Affinity
Binding of [3H](-)-Nicotine to Rat Brain Membranes and Its
Inhibition by Analogues of Nicotine. Neuropharmacology 1988,
27, 235-241.
(30) Sullivan, J . P.; Donnelly-Roberts, D.; Briggs, C. A.; Anderson,
D. J .; Gopalakrishnan, M.; Kaplan, M. P.; Campell, J . E.;
Mckenna, D. G.; Molinari, E.; Hettinger, A.-M.; Garvey, D. S.;
Wasicak, J .; Holladay, M. W.; Arneric, S. P. A-85380 [3-2(S)-
Azetidinylmethoxy)pyridine]: in Vitro Pharmacological Proper-
ties of A Novel, 50 pM Affinity Cholinergic Channel Activator.
Neuropharmacology 1996, in press.
(31) Beers, W. H.; Reich, E. Structure and Activity of Acetylcholine.
Nature 1970, 228, 917-922.
(32) Sheridan, R. P.; Nilakantan, R.; Dixon, J . S.; Venkataraghavan,
R. The Ensemble Approach to Distance Geometry: Application
to the Nicotinic Pharmacophore. J . Med. Chem. 1986, 29, 899-
906.
quescent white solid (220 mg, 83%): mp 111-113 °C; [R]25
)
D
+25.1° (c 0.5, MeOH); MS (CI/NH3) m/ e 179 (M + H)+; 1H
NMR (D2O, 300 MHz) δ 8.43 (d, J ) 2.9 Hz, 1H), 8.32 (d, J )
5.0 Hz, 1H), 7.81 (ddd, J ) 1.1, 2.9, 8.6 Hz, 1H), 7.68 (dd, J )
5.0, 8.6 Hz, 1H), 4.79-4.88 (m, 1H), 4.45-4.59 (m, 2H), 4.25-
4.34 (m, 1H), 4.01 (dd, J ) 9.4, 19.7 Hz, 1H), 3.00 (s, 3H), 2.56-
2.77 (m, 2H). Anal. (C10H14N2O‚0.5HCl‚0.6H2O) C, H, N.
Ack n ow led gm en t. The contributions of Mr. Wil-
liam H. Arnold and Dr. Suzanne Lebold to the prepara-
tion of (R)- azetidine 2-carboxylic acid are gratefully
acknowledged.
Refer en ces
(1) Davis, K.; Yamamura, H. Cholinergic Underactivity in Human
Memory Disorders. Life Sci. 1978, 23, 1729-1734.
(2) Cacabelos, R.; Nordberg, A.; Caamano, J .; Franco-Maside, A.;
Fernandez-Novoa, L.; Gomez, M. J .; Alvarez, X. A.; Takeda, M.;
Prous, J ., J r.; Nishimura, T.; Winblad, B. Molecular Strategies
for the First Generations of Antidementia Drugs (I). Tacrine and
Related Compounds. Drugs Today 1994, 30, 295-337.
(3) Arneric, S. P.; Sullivan, J . P.; Williams, M. In Psychopharma-
cology: Fourth generation of progress; Bloom, E. F., Kupfer, D.
J ., Eds.; Raven: New York, 1995; pp 95-110.
(4) Luetje, C. W.; Patrick, J .; Seguela, P. Nicotine Receptors in the
Mammalian Brain. FASEB J . 1990, 4, 2753-2760.
(5) Sargent, P. B. The Diversity of Neuronal Nicotinic Acetylcholine
Receptors. Annu. Rev. Neurosci. 1993, 16, 403-43.
(6) McGehee, D. S.; Role, L. W. Physiological Diversity of Nicotinic
Acetylcholine Receptors Expressed By Vertebrate Neurons.
Annu. Rev. Physiol. 1995, 57, 521-546.
(7) Wonnacott, S.; Drasdo, A.; Sanderson, E.; Rowell, P. In The
biology of nicotine dependence; Block, G., Marsh, J ., Eds.;
Wiley: Chichester, 1990; pp 87-105.
(8) Linville, D. G.; Williams, S.; Raszkiewicz, J . R.; Arneric, S. P.
Nicotinic Agonists Modulate Basal Forebrain Control of Cortical
Cerebral Blood Flow. J . Pharmacol. Exp. Ther. 1993, 267, 440-
448.
(9) Flores, C. M.; Rogers, S. W.; Pabreza, L. A.; Wolfe, B. B.; Kellar,
K. J . A Subtype of Nicotinic Cholinergic Receptor in Rat Brain
is Composed of R4 and â2 Subunits and Is Up-regulated by
Chronic Nicotine Treatment. Mol. Pharmacol. 1991, 41, 31-37.
(10) Lukas, R. J . Expression of Ganglia-Type Nicotinic Acetylcholine
Receptors and Nicotinic Binding Sites by Cells of the IMR-32
Human Neuroblastoma Clonal Line. J . Pharmacol. Exp. Ther.
1993, 265, 294-302.
(33) Glennon, R. A.; Herndon, J . L.; Dukat, M. Epibatidine-aided
Studies Toward Definition of a Nicotine Receptor Pharmaco-
phore. Med. Chem. Res. 1994, 4, 461-473.
(34) Czajkowski, C.; Karlin, A. Structure of The Nicotinic Receptor
Acetylcholine-Binding Site. Identification of Acidic Residues In
the δ Subunit Within 0.9 nm of the R Subunit-Binding Site
Disulfide. J . Biol.Chem. 1995, 270, 3160-3164.

Novel 3-Pyridyl Ethers
J ournal of Medicinal Chemistry, 1996, Vol. 39, No. 4 825
(35) Nowak, M. W.; Kearney, P. C.; Sampson, J . R.; Saks, M. E.;
Labarca, C. G.; Silverman, S. K.; Zhong, W.; Thorson, J .; Abelson,
J . N.; Davidson, N.; Schultz, P. G.; Dougherty, D. A.; Lester, H.
A. Nicotinic Receptor Binding Site Probed with Unnatural
Amino Acid Incorporation in Intact Cells. Science 1995, 268,
439-442.
(36) Harel, M.; Schalk, I.; Ehret-Sabatier, L.; Bouet, F.; Goeldner,
M.; Hirth, C.; Axelsen, P. H.; Silman, I.; Sussman, J . L.
Quaternary Ligand Binding to Aromatic Residues in the Active-
site Gorge of Acetylcholinesterase. Proc. Natl. Acad. Sci. U.S.A.
1993, 90, 9031-9035.
(37) Sussman, J . L.; Harel, M.; Frolow, F.; Oefner, C.; Goldman, A.;
Toker, L.; Silman, I. Atomic Structure of Acetylcholinesterase
from Torpedo californica: A Prototypic Acetylcholine-Binding
Protein. Science 1991, 253, 872-879.
(38) Perseo, P.; Piani, S.; Castiglione, R. D. Preparation and Analyti-
cal Data of Some Unusual t-Butyloxycarbonyl-amino Acids. Int.
J . Pept. Protein Res. 1983, 21, 227-230.
J M9506884