Lipophilic 4-isoxazolyl-1,4-dihydropyridines: Synthesis and structure- activity relationships

Article abstract of DOI:10.1021/jm980439q

A series of 4-isoxazolyl-1,4-dihydropyridines bearing lipophilic side chains at the C-5 position of the isoxazole ring have been prepared. The calcium channel antagonistic activity of these compounds has been evaluated. A hypothetical model for binding of these compounds in the calcium channel is proposed, and the validity of this model is evaluated based on the SAR of this series of calcium binding, especially for the two most active derivatives, 1a,g. The solid-state structure for the most active compound, 1a, has also been determined, and its important features are reported.

Full text of DOI:10.1021/jm980439q

J . Med. Chem. 1999, 42, 3087-3093
3087
Lip op h ilic 4-Isoxa zolyl-1,4-d ih yd r op yr id in es: Syn th esis a n d Str u ctu r e-Activity
Rela tion sh ip s
Nicholas R. Natale,*,† Mark E. Rogers,† Richard Staples,† David J . Triggle,§ and Aleta Rutledge§
School of Pharmacy, State University of New York at Buffalo, C126 Cooke-Hochstetter Complex, Buffalo, New York 14260,
and Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343
Received J uly 27, 1998
A series of 4-isoxazolyl-1,4-dihydropyridines bearing lipophilic side chains at the C-5 position
of the isoxazole ring have been prepared. The calcium channel antagonistic activity of these
compounds has been evaluated. A hypothetical model for binding of these compounds in the
calcium channel is proposed, and the validity of this model is evaluated based on the SAR of
this series of calcium binding, especially for the two most active derivatives, 1a ,g. The solid-
state structure for the most active compound, 1a , has also been determined, and its important
features are reported.
In tr od u ction
mental work indicate that the binding regions are
within the channel and are closer to the extracellular
membrane as opposed to the intracellular or cytoplasmic
membrane. Kalasz et al. have also used photoaffinity
labeling and localized the DHP binding site to S5-S6
linker peptides of repeats I, III, and IV.7 Other recent
studies by Tang and co-workers3d focused on using
chimeric calcium channels created using the cardiac R-1
subunit as the parent protein and replacing specific
segments with segments from the B1-2 calcium channel,
which is insensitive to DHPs, to examine essential
domains for DHP binding. Their results indicate that
the S5-S6 linker in motif IV is critical for DHP action.
Furthermore, their studies also indicate that several of
the regions for binding determined by Catterall’s photo-
affinity labeling are not involved in the functional effects
of DHPs, a conclusion speculated by Catterall’s group.3a
However, all of these studies indicate that there are
DHP binding sites in the extracellular S5-S6 regions
of motifs III and IV. Since different DHPs were used in
each of these studies, it is conceivable that the struc-
tural differences in the calcium antagonists give rise to
differential binding sites.
4-Aryl-1,4-dihydropyridines (DHPs) constitute an im-
portant class of calcium channel antagonists.1 Several
1,4-dihydropyridine derivatives are widely used as
therapeutic agents for the treatment of hypertension,
angina, and other cardiovascular diseases. Synthetic
interest in this class of compounds is an active area of
research as new, more pharmacologically active, and
tissue-selective agents are prepared.2 The DHPs have
also proven to be powerful probes of the molecular basis
of antagonism and the site of action in the calcium
channel.3 We have found that bioisosteric replacement
of the 4-aryl moiety with a 4-isoxazolyl group yields
analogous 4-isoxazolyl-1,4-dihydropyridines (IDHPs)
which retain potent calcium antagonist activity.4
A
major goal of our research is to prepare IDHPs with
pharmacological activity equal to or greater than that
of currently used therapeutic agents, such as nifedipine.
Ideally, new drug developments arise from the detailed
study of drug-receptor complexes both in the solid state
(X-ray diffractometry) and in the solution phase (NMR
spectroscopy). In the absence of such detailed informa-
tion, drug development must proceed based on lead
compounds and information on the nature of drug-
receptor interactions gathered from the best-available
methodology.
Photoaffinity labeling studies by Catterall and co-
workers3 using the 1,4-dihydropyridines [3H]PN200-110,
[3H]azidopine, and diazepine have identified several
binding domains in the R-1 subunit of L-type calcium
channels near the extracellular end of the transmem-
brane segment 6 of the third and fourth membrane-
spanning domains. Initially, these results conflicted
with modeling studies carried out by Langs, Strong, and
Triggle5 and with the theoretical studies of Holtje and
Marrer.6 However, a more recent report from Triggle
describing the use of permanently charged 1,4-DHPs
and whole-cell patch clamp procedures to assay channel
inhibition has seemingly reconciled some of these
inconsistencies.3c The combined results of this experi-
Herein we propose a model for DHP binding which
attempts to reconcile some of the differences in the
binding models discussed above. Catterall’s photoaffin-
ity study using PN200-110 with the photolabile group
attached to the 4-aryl moiety shows the major binding
site to be IIIS6. When the covalent affinity label was
placed on the ester group, the major binding site was
on the loop connecting IIIS5 and IIIS6. In the Tanabe
primary sequence of the R-1 subunit, there is a tryp-
tophan residue (Tryp 1016) located midway on this
loop.8 In the theoretical study by Holtje and Marrer, a
strong interaction was proposed between a tryptophan
and the port side ester of DHPs.6 Solution-phase and
solid-state studies have shown the 1,4-dihydropyridine
ring to be in a flattened boat conformation with the
4-aryl group in the pseudoaxial position. The common
nomenclature adopted for these compounds places the
NH at the stern of the boat and the 4-aryl moiety at
the bow.
† University of Idaho.
§ State University of New York at Buffalo.
10.1021/jm980439q CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/16/1999

3088 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 16
Natale et al.
Sch em e 1. Edmundson Helical Wheel Representation of the Binding Model, Based on Covalent Affinity Labeling
Data, of the Hypothetical Role of the IVS6 Portion of the Calcium Channel in DHP Bindinga
a
The IVS6 is positioned to interact with the starboard ester functional group of the DHP.
Con str u ction of a Wor k in g Bin d in g Mod el
Langs, Strong, and Triggle carried out modeling
studies which indicated that there may be important
H-bond interactions between the ester groups and
arginine residues which are separated by two amino
acids. This sequence (Arg-X-X-Arg) is repeated 10 times
in the S4 strands.5 However, Catterall’s studies do not
show binding on the S4 strand. The IIIS6 strand does
have the tryptophan residue and a glutamic acid residue
(1040) positioned in such a way that the dihydropyridine
NH can hydrogen bond to the glutamic acid and the port
side ester to the tryptophan. This positions the DHP
ring down with respect to the transmembrane strand.
A correlation between the flatness of the DHP boat
conformation in the solid state and the binding affinity
of the DHP derivatives was recognized by Triggle.9 It
has been postulated that this correlation relates to the
forward presentation of the 4-aryl moiety. The current
model provides an explanation for this observation: The
binding site on the IIIS6 has two phenylalanines (1044
and 1045) which form a pocket between the roof of the
binding site at Glu 1040 and the floor of the binding
site at Tyr 1048. Thus, the flatter the DHP ring the
more it presents the 4-aryl group forward making it
easier for it to stretch between the two hydrogen-
bonding receptor groups. There is also the potential for
donor-acceptor type interactions between the electron-
rich phenylalanines and the electron-poor isoxazole
moiety. A schematic representation of the hypothetical
binding of IDHPs within the calcium channel is shown
in Scheme 1.
The interaction of the IVS6 is envisioned as a hydro-
gen-bonding interaction with the starboard ester, since
the amino acid sequence of the IVS6 places three
hydrogen-bonding residues (Tyr 1359, Tyr 1360, and Ser
1363) at approximately the right transmembrane height
for interaction with an adjacent IIIS6-bound IDHP
shown in Figure 1. Our model illustrates how the
interaction of multiple functional groups is critical for
drug-receptor binding. In our current model, the C-5
position of the isoxazole is postulated to be pointed
“down” or O-exo when binding to the IIIS6 strand. The
amino acid sequence of IIIS6 continues from residue Tyr
1048 as an R-helix until residue 1061, with the sequence
F igu r e 1. Hypothetical binding interactions of a 4-isoxazolyl-
1,4-dihydropyridine to the IIIS6 membrane-bound strand of
the calcium channel.
I-I-L-I-A-F-F-M-M-N-I-F-V. The helix should turn four
amino acid residues later, and the residues placed on
the same side of the R-helix would be I-A-F-F, a very
lipophilic pocket. Examination of molecular models
indicates that lipophilic aromatic substituents attached
to the C-5 position of the isoxazole ring could be properly
oriented to interact with the lipophilic pocket of our
hypothesized channel. Furthermore, the pocket contains
two electron-rich phenylalanine residues which may be
able to undergo donor-acceptor type interactions with
an electron-poor substituent such as a halo-substituted
aryl ring. A final consideration is how the size of the
lipophilic group will affect binding.
At the time this work was performed no crystal
structure for the calcium channel was yet reported.
After the completion of our work the X-ray structure of

Synthesis, SAR of 4-Isoxazolyl-1,4-dihydropyridines
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 16 3089
Ta ble 1. Lipophilic IDHPs
Ta ble 2. Displacement of [3H](+)-PN200-110 Binding by
IDHPs in Rat Cardiac Membranes
compd
Ki (M), mean ( SEM
nH, mean ( SEM
1a
1b
1c
1d
1e
1f
2.16 ( 0.43 × 10-9
1.07 ( 0.16 × 10-8
8.69 ( 1.15 × 10-9
4.55 ( 0.39 × 10-8
2.49 ( 0.32 × 10-8
5.53 ( 0.91 × 10-8
5.63 ( 0.68 × 10-9
3.04 ( 0.43 × 10-8
1.09 ( 0.03
0.93 ( 0.07
0.81 ( 0.03
0.84 ( 0.05
1.1 ( 0.06
0.83 ( 0.04
0.90 ( 0.03
1.13 ( 0.08
1g
1h
Ta ble 3. Displacement of [3H](+)-PN200-110 Binding by
entry
R1
Me
Me
Me
Me
Me
phenyl
Me
Me
R2, aryl group
IDHPs in Guinea Pig Cardiac Membranes
1a
1b
1c
1d
1e
1f
1-naphthyl
2-naphthyl
4-biphenylyl
2-biphenylyl
phenyl
phenyl
m-Br-phenyl
m-MeO-phenyl
compd
Ki (M), mean ( SEM
nH, mean ( SEM
1a
1b
1c
1d
1e
1f
4.11 × 10-9
1.13
1.06
1.10
0.95
1.06 ( 0.06
0.90 ( 0.03
1.02
2.02 × 10-8
1.82 × 10-8
9.11 × 10-8
2.29 ( 0.13 × 10-8
7.29 ( 0.16 × 10-8
6.89 × 10-9
1g
1h
1g
1h
3.95 × 10-8
1.36
Sch em e 2. Synthesis of Isoxazolyldihydropyridines
Bearing Lipophilic Aryl Substituents
synthesis conditions in an aerosol dispersion tube at
elevated pressure and temperature to give the IDHPs
1a -h , in low to modest yields, Scheme 2. The low yields
of IDHPs may be due to the large steric bulk and
rotational mobility of the lipophilic substituents that
hinder approach to the aldehyde group by the ethyl
acetoacetate anion.
Resu lts a n d Discu ssion
The results of biological evaluation of these com-
pounds are given in Tables 2 and 3. The radioligand
binding assays were carried out as previously
described.11-13 A brief general description is given in
the Supporting Information. Several important features
of the biological activity stand out. All of the lipophilic
compounds exhibit high binding affinity, the 1-naphthyl
derivative 1a being the most potent IDHP prepared and
evaluated to date. Another interesting feature is that
the m-bromophenyl derivative 1g is approximately 5
times more active than the derivative bearing the
electron-donating methoxy substituent, 1h . This indi-
cates that the electron-rich character of the lipophilic
pocket containing the two phenylalanine groups may
indeed increase the binding affinity for the electron-poor
derivative over the electron-rich compound.
the potassium channel was reported, which is analogous
in many respects to the calcium channel.16 A prominent
feature of the potassium channel’s ion pore is that (1)
the extracellular end of the pore contains many hydrogen-
bonding residues and (2) the intracellular end of the
pore is highly lipophilic. In addition, Catterall has
recently reported site-directed mutagenesis studies.17 In
his study, Y1048F and Y1048A substitutions resulted
in the conclusion that Y1048 is essential to DHP
binding, in accordance with our current hypothesis.
Thus, the information to date is consistent with our own
hypothesis.
The crystal structure for the most active derivative,
1a , is shown in Figure 2, showing the thermal ellipsoids
and proper numbering scheme. Important torsion angles
for 1a are listed in Table 4; crystallographic data, bond
distances and angles, atomic coordinates, and anisotro-
pic thermal parameters can be found in the Supporting
Information. The crystal structure shows the isoxazole
ring to be in the O-endo conformation with the largest
group positioned over the DHP ring. We attribute this
conformation to crystal packing forces. It has long been
accepted that the larger substituent on the 4-aryl ring
points away from the dihydropyridine nitrogen in the
efficacious conformer. We have previously reported that
the barrier to rotation about the dihydropyridine-
isoxazole bond is on the order of 20-40 kJ , and thus
ready conformational interconversion is expected in the
presence of a receptor.15 The value for Σ|F| is 71.5° and
Ch em istr y
To test our hypothesis, we have prepared a series of
IDHPs bearing lipophilic substituents at the C-5, or
lateral, position of the isoxazole ring (1a -f, Table 1).
We also prepared two compounds to examine whether
an electron-rich or electron-poor aryl ring will affect
binding affinity (1g,h , Table 1). The general synthetic
route for these compounds involves lateral metalation
followed by electrophilic quenching of the oxazoline-
protected isoxazole 2, Scheme 2, using methodology
extensively employed in our laboratories.10 Deprotection
is smoothly accomplished by quaternization of the
oxazoline nitrogen, followed by NaBH4 reduction and
mild acid hydrolysis to give the aldehydes 4, Scheme 2.
The aldehydes are then subjected to Hantzsch pyridine

3090 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 16
Natale et al.
nel. Finally, the site-directed mutagenesis work of
Catterall supports the central focus of our working
hypothesis, an essential role for Y1048 in DHP bind-
ing.17 Furthermore, most of the amino acid residue
perturbations found to affect DHP binding are in
proximity to our proposed binding site. Whether these
exert their effect on DHP binding by direct interaction
with the DHP or by their effect on channel conformation,
is a question which may be answered by the study of
new synthetic analogues. Efforts are underway in our
laboratory to further test our hypothesis and to obtain
a better understanding of what factors are most impor-
tant for isoxazolyldihydropyridine activity.
F igu r e 2. Single-crystal X-ray diffractometry structure for
IDHP 1a .
Exp er im en ta l Section
All reagents used were purchased from Aldrich Chemical
Co. and used without purification unless otherwise indicated,
or prepared as described below. Solvents were reagent grade
and dried just prior to use by standard methods. Melting points
were determined in open capillary tubes on a Melt-Temp
apparatus and are uncorrected. Radial chromatography was
performed on a Harrison Associates chromatotron. Flash
chromatography was performed using Baker flash gel using
in house compressed air. All chromatography solvents were
distilled prior to use. NMR spectra were recorded on a Bruker
Ta ble 4. Important Torsion Angles for 1a
angle
deg
N1C1C2C3
C1C2C3C4
C2C3C4C5
C3C4C5N1
C4C5N1C1
C5N1C1C2
C1C2C3C14
C5C4C3C14
4.3 (3)
-18.6 (2)
20.3 (2)
-7.6 (3)
-9.5 (3)
11.2 (3)
106.2 (2)
-106.0 (2)
1
AC200 (200 MHz H, 50 MHz 13C) or an IBM Bruker AF300
1
(300 MHz H, 75 MHz 13C) spectrometer at ambient temper-
ature in CDCl3. Chemical shifts are reported as ppm (δ)
downfield from an internal tetramethylsilane (TMS) standard.
Mass spectra were obtained on a VG Micromass 70/70 HS
mass spectrometer using EI or CI as indicated. Combustion
analyses were performed by Desert Analytics, Tucson, AZ.
4-(4′,5′-Dihydro-4′,4′-dimethyl-∆2-oxazolin-2-yl)-3,5-dimeth-
ylisoxazole, 2,19 and 4-(4′,5′-dihydro-4′,4′-dimethyl-∆2-oxazolin-
2-yl)-5-methyl-3-phenylisoxazole, 3f,19 were prepared according
to literature procedures.
1-(Ch lor om eth yl)n a p h th a len e. A 250-mL flask with stir
bar and 75 mL of concentrated HCl was cooled to 0 °C, and
1-naphthalenemethanol (1.10 g, 6.95 mmol) was added slowly
with stirring. The mixture was allowed to stir uncovered and
slowly warm to room temperature for 10 h, by which time the
solid had been replaced by an oil suspended in the HCl. The
mixture was extracted with CH2Cl2 (3 × 20 mL). The combined
CH2Cl2 layers were washed with water until the aqueous
layers were neutral by Hydrion paper (3 × 25 mL) and then
brine (1 × 25 mL), dried over anhydrous Na2SO4, filtered, and
concentrated under vacuum to give 1.18 g as a light-yellow
oil (97% crude yield): 1H NMR (200 MHz, CDCl3) δ 5.07 (s,
2H, CH2Cl2) 7.42-7.68 (m, 4H, Ar-H) 7.90 (t, 2H, Ar-H) 8.20
(d, 1H, ipso Ar H).
2-(Ch lor om eth yl)n a p h th a len e. A solution of 2-naphtha-
lenemethanol in 25 mL of CH2Cl2 was added with stirring to
100 mL of cold (0 °C) concentrated HCl. The mixture was
allowed to warm to room temperature over the course of 1 h.
The layers were separated, and the HCl was extracted with
CH2Cl2 (2 × 25 mL). Combined CH2Cl2 layers were washed
with water until the aqueous extracts were neutral by Hydrion
paper (3 × 25 mL) and brine (1 × 25 mL), dried over anhydrous
Na2SO4, filtered, and concentrated under vacuum to give a
light-yellow solid which was Kugelrohr distilled (0.05 mmHg,
160-180 °C) to give 2.247 g as a white powder (80%): 1H NMR
(200 MHz) δ 4.76 (s, 2H, CH2) 7.45-7.53 (m, 3H, Ar H) 7.83-
7.87 (m, 2H, Ar H).
Gen er a l P r oced u r e for P r ep a r a t ion of Com p ou n d s
3a -h . 4-(4′,5′-Dih yd r o-4′,4′-d im eth yl-∆2-oxa zolin -2-yl)-5-
(1-n a p h th yleth yl)-3-m eth ylisoxa zole (3a ). To a stirred
solution of 4-(4′,5′-dihydro-4′,4′-dimethyl-∆2-oxazolin-2-yl)-3,5-
dimethylisoxazole, 2 (2.17 g, 11.1 mmol), in 100 mL of dry THF
cooled to -78 °C under N2 was added 1.05 equiv of 1.5 M nBuLi
(7.9 mL). The yellow solution was stirred at -78 °C for 2 h.
Then a solution of 1.05 equiv of 1-(chloromethyl)naphthalene
(2.08 g, 11.8 mmol) in dry THF (15 mL) was added dropwise
for Σ|τ|, 106.1°. The former value is indicative of the
planarity of the DHP ring which influences the binding
affinity.9 It is defined as the absolute value of the six
intra-ring torsion angles. This value can be compared
to the corresponding Σ|F| value for nifedipine of 72.1°.
The value Σ|τ| is a measure of how close the two rings
are to perfectly bisecting one another. Finally, the solid-
state structure shows both of the esters to be in the
synperiplaner orientation with respect to the dihydro-
pyridine C-C double bond.
Con clu sion
The biological activity for this series of compounds is
consistent with our hypothesis about the site and nature
of IDHP binding in the calcium channel. Our rationale
for the observation that the 1-naphthyl derivative is the
most potent in this series, based on examination of
molecular models, is that this derivative can slip into
the hypothetical lipophilic pocket more readily than the
2-naphthyl or either biphenyl derivative. In the case of
the biphenyl derivatives, it appears that their increased
size may prevent them from fitting into the pocket while
still allowing for the other binding interactions such as
H-bonding to be optimum. The greater activity of the
m-bromophenyl compound 1g over that of the m-
methoxyphenyl 1h and the two unsubstituted phenyl
derivatives 1e,f may indicate that the electronic nature
of the aryl ring is very influential on binding affinity.
This result also supports our hypothesis that the lipo-
philic pocket plays a role in the binding of IDHPs of this
type since it contains electron-rich phenylalanine resi-
dues which would tend to interact more strongly with
an electron-poor aryl group. That lipophilicity can
contribute to enhanced calcium channel binding is
dramatically illustrated by the three-dimensional shape
of the analogous potassium channel.16 An important
aspect of this structure is the overall cone shape of the
channel, resulting in part from the interactions in the
hydrophobic lining of the intracellular end of the chan-

Synthesis, SAR of 4-Isoxazolyl-1,4-dihydropyridines
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 16 3091
and the mixture stirred at -78 °C for 30 min. The mixture
was allowed to warm to room temperature and the solvent
removed under vacuum to give a brown oil. The crude product
was dissolved in CH2Cl2 (30 mL), washed with water (2 × 20
mL) brine (1 × 20 mL) and dried over anhydrous Na2SO4,
filtered, and concentrated under vacuum. The crude product
was Kugelrohr distilled (0.05 mmHg, 150-170 °C) to give 3.15
the combined organic layers dried over anhydrous Na2SO4,
filtered, and concentrated to give the crude aldehyde as a
yellow oil. The aldehyde was purified by flash chromatography
(silica gel, 80% hexane, 15% ETOAc, 5% CH2Cl2) to give 1.01
g as a pale-yellow oil (44%): 1H NMR (200 MHz, CDCl3) δ 2.40
(s, 3H, isoxazole Me) 3.30-3.45 (m, 4H, CH2CH2) 7.13-7.96
(m, 7H, Ar-H) 9.52 (s, 1H, aldehyde H).
1
g of a light-yellow solid (85%): mp 74-76 °C; H NMR (200
The aldehyde 1.01 g (3.82 mmol) was dissolved in ethanol
(10 mL) and transferred to an aerosol dispersion tube to which
ethyl acetoacetate (995 mg, 7.65 mmol) and aqueous ammonia
(6 mL, 28.7%) were added. The mixture was heated to 100-
110 °C for 48 h, the pressure rising to 30-40 psi. After cooling
the solvents were removed under vacuum to give a brown oil.
The crude product was purified by flash chromatography (silica
gel, 4:2:1 hexane:EtOAc:CH2Cl2) and crystallization by slow
diffusion of hexane into a benzene-ethanol solution of the
product. This gave 50 mg of pale-yellow crystals (8%): mp
MHz, CDCl3) δ 1.29 (s, 2, 6H, oxazoline Me) 2.44 (s, 3H,
isoxazole Me) 3.45 (s, 4H, CH2CH2) 3.85 (s, 2H, oxazoline CH2)
7.31-7.54 (m, 4H, Ar-H) 7.70-7.83 (m, 2H, Ar-H) 8.25 (d,
1H, ipso Ar-H). Anal. (C21H22N2O2) C, H, N.
4-(4′,5′-Dih yd r o-4′,4′-d im et h yl-∆2-oxa zolin -2-yl)-5-(2-
n a p h th yleth yl)-3-m eth ylisoxa zole (3b): 80% yield; mp 86-
88 °C; 1H NMR (200 MHz, CDCl3) δ 1.26 (s, 6H, oxazoline Me)
2.42 (s, 3H, isoxazole Me) 3.12-3.20 (m, 2H, CH2CH2) 3.37-
3.45 (m, 2H, CH2CH2) 3.89 (s, 2H, oxazoline CH2) 7.30-7.46
(m, 3H, Ar-H) 7.62 (br s, 1H, Ar-H) 7.73-7.81 (m, 3H, Ar-
H). Anal. (C21H22N2O2) C, H, N.
1
156-158 °C; H NMR (300 MHz, CDCl3) δ 1.16 (t, 6H, CH3-
CH2O-, J ) 7.1 Hz) 2.14 (s, 6H, DHP-Me) 2.29 (s, 3H, isoxazole
Me) 3.06-3.11 (m, 2H, CH2CH2) 3.36 (m, 2H, CH2CH2) 4.01-
4.12 (m, 4H, CH3CH2O-) 4.92 (s, 1H, CH) 5.8 (br s, 1H, NH)
7.30-7.51 (m, 4H, Ar-H) 7.7 (d, 1H, Ar-H, J ) 7.9 Hz) 7.83
(d, 1H, Ar-H, J ) 7.2 Hz) 8.13 (d, 1H, ipso Ar-H, J ) 8.2
Hz); 13C NMR (75 MHz, CDCl3) δ 10.2, 14.4, 19.4, 26.9, 29.1,
30.3, 59.9, 101.7, 120.2, 123.7, 125.5, 125.6, 126.9, 128.8, 131.8,
133.9, 137.3, 143.2, 159.8, 167.3, 168.8; MS (EI) 488. Anal.
Calcd for C29H32N2O5: C, 71.29; H, 6.60; N, 5.73. Found: C,
71.38; H, 6.46; N, 5.39.
4-(4′,5′-Dih yd r o-4′,4′-d im eth yl-∆2-oxa zolin -2-yl)-5-(4-bi-
p h en yleth yl)-3-m eth ylisoxa zole (3c): 84% yield; mp 84-
86 °C; 1H NMR (200 MHz, CDCl3) δ 1.3 (s, 6H, oxazoline Me)
2.49 (s, 3H, isoxazole Me) 3.09 (t, 2H, J ) 6 Hz) 3.4 (t, 2H, J
) 6 Hz) 3.96 (s, 2H, oxazoline CH2) 7.27-7.62 (m, 9H, Ar H).
Anal. (C23H24N2O2) C, H, N.
4-(4′,5′-Dih yd r o-4′,4′-d im eth yl-∆2-oxa zolin -2-yl)-5-(2-bi-
p h en ylyleth yl)-3-m eth ylisoxa zole (3d ): 83% yield; mp 68-
70 °C; 1H NMR (200 MHz, CDCl3) δ 1.27 (s, 6H, oxazoline Me)
2.40 (s, 3H, isoxazole Me) 2.99-3.04 (m, 2H, CH2CH2) 3.12-
3.18 (m, 2H, CH2CH2) 3.85 (s, 2H, oxazoline CH2) 7.20-7.43
(m, 9H, Ar H). Anal. (C23H24N2O2) C, H, N.
4-(4′,5′-Dih ydr o-4′,4′-dim eth yl-∆2-oxazolin -2-yl)-5-(ph en -
yleth yl)-3-m eth ylisoxa zole (3e): 92% yield; mp 34-38 °C;
1H NMR (200 MHz, CDCl3) δ 1.30 (s, 6H, oxazoline Me) 2.43
(s, 3H, isoxazole Me) 3.00 (t, 2H, J ) 6 Hz, CH2CH2) 3.32 (t,
2H, J ) 6 Hz, CH2CH2) 3.93 (s, 2H, oxazoline CH2) 7.14-7.31
(m, 5H, Ar H).
4-(4′,5′-Dih yd r o-4′,4′-d im et h yl-∆2-oxa zolin -2-yl)-5-(m -
br om op h en yleth yl)-3-m eth ylisoxa zole (3g): 82% yield as
a pale-yellow liquid after Kugelrohr distillation (140-150 °C,
0.05 mmHg); 1H NMR (200 MHz, CDCl3) δ 1.27 (s, 6H,
oxazoline Me) 2.38 (s, 3H, isoxazole Me) 2.92 (t, 2H, J ) 7.5
Hz, CH2CH2) 3.27 (t, 2H, J ) 7.5 Hz, CH2CH2) 3.91 (s, 2H,
oxazoline CH2) 7.04 (br s, 2H, Ar H) 7.26-7.32 (m, 2H, Ar H).
4-(4′,5′-Dih yd r o-4′,4′-d im et h yl-∆2-oxa zolin -2-yl)-5-(m -
m eth oxyp h en yleth yl)-3-m eth ylisoxa zole (3h ): 85% yield
as a pale-yellow viscous liquid after Kugelrohr distillation
(140-150 °C, 0.02 mmHg); 1H NMR (200 MHz, CDCl3) δ 1.29
(s, 6H, oxazoline Me) 2.41 (s, 3H, isoxazole Me) 2.92-3.0 (m,
2H, CH2CH2) 3.26-3.34 (m, 2H, CH2CH2) 3.75 (s, 3H, CH3O-
Ar) 3.92 (s 2H, oxazoline CH2) 6.71-6.78 (m, 3H, Ar H) 7.13-
7.23 (m, 1H, Ar H).
Gen er a l P r oced u r e for P r ep a r a t ion of Com p ou n d s
1a-h . Dieth yl 2,6-Dim eth yl-4-(5-[1-n aph th yleth yl]-3-m eth -
ylisoxazol-4-yl)-1,4-dih ydr opyr idin e-3,5-dicar boxylate (1a).
To a stirred solution of 3a (2.91 g, 8.7 mmol) in 75 mL of dry
CH2Cl2 was added 1.6 mL (2.32 g, 14.0 mmol) of CF3SO3CH3,
and the mixture stirred under N2 until TLC (silica, 80%
hexane, 20% ETOAc) showed only baseline material. The
mixture was cooled to 0 °C, and a solution of 605 mg (16 mmol)
of NaBH4 in 20 mL of 4:1 THF:MeOH was added in one
portion. This mixture was stirred at 0 °C for 30 min. Then 5
mL of saturated NH4Cl was added and the mixture allowed
to warm to room temperature. Ether, 50 mL, was added, and
the layers were separated. The ether layer was washed with
saturated NaCl (1 × 25 mL). The combined aqueous layers
were extracted with CH2Cl2 (1 × 25 mL). The combined organic
layers were dried over anhydrous Na2SO4, filtered, and
concentrated to give an orange oil. The crude product was
hydrolyzed with 15 mL of 1 M aqueous HCl in 20 mL of 4:1
THF:H2O. The reaction mixture was poured into 50 mL of
ether, and the layers were separated. The ether layer was
washed with saturated NaHCO3 (3 × 25 mL). Combined
aqueous layers were extracted with CH2Cl2 (1 × 25 mL) and
Dieth yl 2,6-Dim eth yl-4-(5-[2-n a p h th yleth yl]-3-m eth yl-
isoxazol-4-yl)-1,4-dih ydr opyr idin e-3,5-dicar boxylate (1b).
From 3.15 g (9.4 mmol) of 3b, 340 mg (13.7%) of purified
aldehyde was obtained: 1H NMR (200 MHz, CDCl3) δ 2.04 (s,
3H, isoxazole Me) 3.14-3.20 (m, 2H, CH2CH2) 3.31-3.36 (m,
2H, CH2CH2) 7.24-7.78 (m, 7H, Ar-H) 9.68 (s, 1H, aldehyde-
H). The aldehyde (340 mg, 1.28 mmol) was dissolved in 4 mL
of ethanol and transferred to an aerosol dispersion tube to
which ethyl acetoacetate (334 mg, 2.56 mmol) and aqueous
ammonia (3 mL, 28.7%) were added. The mixture was heated
to 100-110 °C (30-40 psi) for 48 h. The solvent was removed
and the crude brown oil purified as described for 1a . This gave
52.1 mg as pale-yellow crystals (9%): mp 174-175 °C; 1H NMR
(300 MHz, CDCl3) δ 1.18 (t, 6H, CH3CH2O-, J ) 7.1 Hz) 2.22
(s, 6H, DHP-Me) 2.30 (s, 3H, isoxazole Me) 3.04-3.08 (m, 4H,
CH2CH2) 4.02-4.13 (m, 4H, CH3CH2O-) 4.92 (s, 1H, CH) 5.87
(br s, 1H, NH) 7.26-7.81 (m, 7H, Ar-H); 13C NMR (75 MHz,
CDCl3) δ 10.2, 14.4, 19.4, 27.6, 29.1, 33.5, 59.9, 101.8, 120.4,
125.3, 126.2, 126.9, 127.5, 127.6, 128.0, 132.1, 133.6, 138.7,
143.2, 159.8, 167.3, 168.7; MS (EI) 488. Anal. Calcd for
C
29H32N2O5: C, 71.29; H, 6.60; N, 5.73. Found: C, 71.03; H,
6.41; N, 5.47.
Dieth yl 2,6-Dim eth yl-4-(5-[4-bip h en ylyleth yl]-3-m eth -
ylisoxazol-4-yl)-1,4-dih ydr opyr idin e-3,5-dicar boxylate (1c).
From 1.23 g (3.4 mmol) of 3c, 260 mg (26%) of purified
aldehyde was obtained: 1H NMR (300 MHz, CDCl3) δ 2.43 (s,
3H, Me) 3.05-3.10 (m, 2H, CH2CH2) 3.29-3.34 (m, 2H, CH2-
CH2) 7.19-7.56 (m, 9H, Ar-H) 9.71 (s, 1H, aldehyde-H). The
aldehyde (260 mg, 0.89 mmol) was dissolved in ethanol (5 mL)
and transferred to an aerosol dispersion tube to which ethyl
acetoacetate (234.8 mg, 1.8 mmol) and aqueous ammonia (4
mL, 28.7%) were added. The mixture was heated to 100-110
°C (30-40 psi) for 48 h. The solvents were removed, and the
crude product was purified as described above. This gave 19
mg as white crystals (4%): mp 132-134 °C; 1H NMR (300
MHz, CDCl3) δ 1.20 (t, 6H, CH3CH2O-, J ) 7.1 Hz) 2.26 (s,
6H, DHP-Me) 2.30 (s, 3H, isoxazole Me) 2.99 (br s, 4H, CH2-
CH2) 4.07-4.12 (m, 4H, CH3CH2O-) 4.93 (s, 1H, CH) 5.7 (br s,
1H, NH) 7.26-7.6 (m, 9H, Ar-H); 13C NMR (75 MHz, CDCl3)
δ 10.2, 14.5, 19.6, 27.6, 29.2, 32.9, 59.9, 101.9, 120.3, 127.1,
127.2, 128.6, 128.7, 139.1, 140.3, 141.0, 143.1, 159.8, 167.3,
168.7; MS (EI) 514. Anal. Calcd for C31H34N2O5: C, 72.35; H,
6.66; N, 5.44. Found: C, 72.28; H, 6.79; N, 5.22.
Dieth yl 2,6-Dim eth yl-4-(5-[2-bip h en ylyleth yl]-3-m eth -
ylisoxa zol-4-yl)-1,4-d ih yd r op yr id in e-3,5-d ica r b oxyla t e
(1d ). From 4.89 g of 3d , was obtained 0.70 g of purified

3092 J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 16
Natale et al.
aldehyde (18%): 1H NMR (200 MHz, CDCl3) δ 2.35 (s, 3H,
isoxazole Me) 3.00-3.05 (m, 4H, CH2CH2) 7.15-7.39 (m, 9H,
Ar-H) 9.30 (s, 1H, aldehyde-H). The aldehyde (700 mg, 2.40
mmol) was dissolved in 5 mL of ethanol and transferred to an
aerosol dispersion tube to which ethyl acetoacetate (625 mg,
4.80 mmol) and aqueous ammonia (5 mL, 28.7%) were added.
The mixture was heated to 100-110 °C (30-40 psi). The
solvents were removed, and the product was purified as
described above. This gave 148.2 mg as white crystals (12%):
mp 158-160 °C; 1H NMR (300 MHz, CDCl3) δ 1.73 (t, 6H, J )
7.1 Hz, CH3CH2O) 2.21 (s, 6H, DHP-Me) 2.24 (s, 3H, isoxazole
Me) 2.83-2.95 (m, 4H, CH2CH2) 3.99-4.11 (m, 4H, CH3CH2O)
4.88 (s, 1H, CH) 5.82 (s, 1H, NH) 7.23 (m, 9H, Ar-H); 13C NMR
(75 MHz, CDCl3) δ 10.2, 14.4, 19.4, 26.9, 29.1, 30.3, 59.8, 101.7,
119.9, 126.1, 126.9, 127.5, 128.2, 128.6, 129.1, 130.2, 138.5,
141.5, 141.9, 143.1, 159.6, 167.2, 168.7; MS (EI) 514. Anal.
Calcd for C31H34N2O5: C, 72.35; H, 6.66; N, 5.44. Found: C,
72.08; H, 6.46; N, 5.39.
7.2 Hz CH3CH2O) 2.26 (s, 6H, DHP-Me) 2.29 (s, 3H, isoxazole
Me) 2.90-2.96 (m, 4H, CH2CH2) 4.08-4.12 (m, 4H, CH3CH2O)
4.91 (s, 1H, CH) 5.59 (br s, 1H, NH) 7.13-7.15 (m, 2H, Ar-H)
7.32-7.37 (m, 2H, Ar-H); 13C NMR (75 MHz, CDCl3) δ 10.1,
14.4, 19.2, 27.4, 29.3, 32.8, 59.8, 101.57, 120.6, 122.5, 126.9,
128.3, 129.3, 130, 131.3, 143.4, 159.8, 167.3, 168.2; MS (EI)
516 m.+, 518 m + 2. Anal. Calcd for C25H29BrN2O5: C, 58.03;
H, 5.65; N, 5.41. Found: C, 58.34; H, 5.69; N, 5.39.
Dieth yl 2,6-Dim eth yl-4-(5-[m -m eth oxyp h en yleth yl]-3-
m eth ylisoxa zol-4-yl)-1,4-d ih yd r op yr id in e-3,5-d ica r boxy-
la te (1h ). From 3.50 g (11.1 mmol) of 3h was obtained 962
mg of purified aldehyde (35%): 1H NMR (200 MHz, CDCl3) δ
2.42 (s, 3H, isoxazole Me) 3.01 (t, 2H, J ) 8 Hz, CH2CH2) 3.29
(t, 2H, J ) 8 Hz, CH2CH2) 3.70 (s, 3H, CH3O) 6.65-6.76 (m,
3H, Ar-H) 7.13-7.24 (m, 1H, Ar-H) 9.65 (s, 1H, aldehyde-
H). The aldehyde (962 mg, 3.9 mmol) was dissolved in 5 mL
of ethanol and transferred to a 25-mL round-bottom flask, to
which ethyl acetoacetate (1.01 g, 7.8 mmol) and aqueous
ammonia (5 mL, 28.7%) were added, and the mixture refluxed
for 48 h. The solvents were removed under vacuum and the
crude product crystallized by slow diffusion of hexane into a
bezene solution. This gave 1.04 g of light-yellow crystals
Dieth yl 2,6-Dim eth yl-4-(5-[p h en yleth yl]-3-m eth ylisox-
azol-4-yl)-1,4-dih ydr opyr idin e-3,5-dicar boxylate (1e). From
2.36 g (8.32 mmol) of 3e was obtained 805.5 mg of purified
aldehyde (45%): 1H NMR (200 MHz, CDCl3) δ 2.42 (s, 3H,
isoxazole Me) 2.89-3.10 (m, 4H, CH2CH2) 7.13-7.25 (m, 5H,
Ar-H) 9.62 (s, 1H, aldehyde-H). The aldehyde (801 mg, 3.7
mmol) was dissolved in 6 mL of ethanol and transferred to a
20-mL round-bottom flask, to which ethyl acetoacetate (963
mg, 7.4 mmol) and aqueous ammonia (3 mL, 28.7%) were
added. The mixture was refluxed for 48 h. The solvents were
removed under vacuum, and the crude product was crystallized
by slow diffusion of hexane into a 10:1 benzene:MeOH solution.
This gave 243 mg as white crystals (15%): mp 180-182 °C;
1H NMR (300 MHz, CDCl3) δ 1.20 (t, 6H, J ) 7.1 Hz, CH3-
CH2O) 2.49 (s, 6H, DHP-Me) 2.30 (s, 3H, isoxazole Me) 2.92
(br s, 4H, CH2CH2) 4.03-4.16 (m, 4H, CH3CH2O) 4.91 (s, 1H,
CH) 5.74 (br s, 1H, NH) 7.18-7.31 (m, 5H, Ar-H); 13C NMR
(75 MHz, CDCl3) d 10.1, 14.4, 19.3, 27.6, 29.1, 33.2, 59.8, 101.6,
120.4, 126.1, 128.1, 141.1, 143.4, 159.8, 167.3, 168.8; MS (EI)
438. Anal. Calcd for C25H30N2O5: C, 68.47; H, 6.89; N, 6.38.
Found: C, 68.33; H, 6.86; N, 6.46.
1
(57%): mp 98-100 °C; H NMR (300 MHz, CDCl3) δ 1.21 (t,
3H, J ) 7 Hz, CH3CH2O) 2.25 (s, 6H, DHP-Me) 2.30 (s, 3H,
isoxazole Me) 2.93 (br s, 4H, CH2CH2) 3.78 (s, 3H, CH3O) 4.0-
4.2 (m, 4H, CH3CH2O) 4.91 (s, 1H, CH) 5.74 (br s, 1H, NH)
7.70-6.82 (m, 3H, Ar-H) 7.17-7.20 (m, 1H, Ar-H); 13C NMR
(75 MHz, CDCl3) δ 10.2, 14.4, 19.2, 27.4, 29.1, 33.4, 55.1, 59.9,
101.9, 111.5, 113.9, 120.33, 120.4, 129.4, 142.8, 143.1, 159.7,
167.2, 168.7; MS (EI) 468. Anal. Calcd for C26H32N2O6: C, 66.65;
H, 6.88; N, 5.98. Found: C, 66.54; H, 6.77; N, 6.19.
Radioligan d Bin din g. Microsomal membranes from guinea
pig heart ventricles were prepared as previously described.12,13
Binding of the 1,4-dihydropyridine (+)-[3H]PN200-110 (iso-
propyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-5-(methoxycar-
bonyl)-2,6-dimethyl-3-pyridinecarboxylate) and its competition
by the IDHPs were carried out as previously described. Briefly,
membrane protein (40-120 µg) was incubated in 5 mL of 50
mM tri[(hydroxymethyl)amino]methane (Tris) buffer at pH 7.2
for 90 min at 25 °C with 5 × 10-11 (+)-[3H]PN200-110 and
varying concentrations of competing compounds. Duplicate
tubes were filtered and washed rapidly with two 5-mL portions
of ice-cold Tris buffer in a Brandel cell harvester (model M-24-
R, Brandel Instruments Ltd., Gaithesburg, MD). Trapped
radioactivity was counted by liquid scintillation spectrometry
at an efficiency of 40-45%. Competing compounds were
prepared in 100% ethanol as 10-3 stock solutions. Subsequent
dilutions were made in 50% ethanol (10-4 M) or distilled water
(10-5 onward). All dilutions were conducted on the day of the
experiment. Concentrations of ethanol to 0.2% (v:v) did not
affect specific binding. Binding data were analyzed by iterative
curve-fitting programs (BDATA, CDATA, EMF Software,
Knoxville, TN). (+)-[3H]PN200-110 with a specific activity of
70 Ci/mol (1 Ci ) 3.7 × 10 10 bq) was purchased from DuPont-
New England Nuclear (Boston, MA).
Dieth yl 2,6-Dim eth yl-4-(5-[p h en yleth yl]-3-p h en ylisox-
azol-4-yl)-1,4-dih ydr opyr idin e-3,5-dicar boxylate (1f). From
(3.9 g, 11.2 mmol) 3f was obtained 1.24 g of purified aldehyde
(40%): 1H NMR (200 MHz, CDCl3) δ 3.32-3.50 (m, 4H, CH2-
CH2) 7.20-7.69 (m, 10H, Ar-H) 9.83 (s, 1H, aldehyde-H). The
aldehyde (1.20 g, 4.3 mmol) was dissolved in 7 mL of isopropyl
alcohol and transferred to a 25-mL round-bottom flask to
which ethyl acetoacetate (1.12 g, 8.6 mmol) and aqueous
ammonia (5 mL, 28.7%) were added. The mixture was refluxed
for 36 h, after which time the solvent was removed. The crude
product was crystallized by slow diffusion of petroleum ether
into a bezene solution. This gave 172 mg of white crystals
1
(8%): mp 127-129 °C; H NMR (300 MHz, CDCl3) δ 1.12 (t,
6H, J ) 7.1 Hz, CH3CH2O) 1.95 (s, 6H, DHP-Me) 3.06-3.20
(m, 4H, CH2CH2) 3.88-4.14 (m, 4H, CH3CH2O) 5.11 (s, 1H,
CH) 5.15 (s, 1H, NH) 7.20-7.45 (m, 10H, Ar-H); 13C NMR
(300 MHz, CDCl3) δ 14.4, 19.2, 29.4, 33.159.7, 101.2, 119.2,
126.1, 127.6, 128.1, 128.2, 128.4, 129.5, 131.0, 141.3, 143.6,
163.5, 167.3, 168.4; MS (EI) 500. Anal. Calcd for C30H32N2O5:
C, 71.98; H, 6.44; N, 5.59. Found: C, 72.25; H, 6.37; N, 5.64.
Ack n ow led gm en t. The establishment of the Single-
Crystal X-ray Diffraction Laboratory at the University
of Idaho was supported by the NSF-Idaho EPSCoR
Program, the National Science Foundation, and the M.
J . Murdock Charitable Trust of Vancouver, Washington.
Diet h yl 2,6-Dim et h yl-4-(5-[m -b r om op h en ylet h yl]-3-
m eth ylisoxa zol-4-yl)-1,4-d ih yd r op yr id in e-3,5-d ica r boxy-
la te (1g). From 6.05 g (16.6 mmol) of 3g was obtained 556.7
mg of pure aldehyde (11%): 1H NMR (200 MHz, CDCl3) δ 2.38
(s, 3H, isoxazole Me) 2.88-2.96 (m, 2H, CH2CH2) 3.23-3.31
(m, 2H, CH2CH2) 7.04-7.14 (m, 2H, Ar-H) 7.25-7.34 (m, 2H,
Ar-H). The aldehyde (556.7 mg, 1.9 mmol) was dissolved in 4
mL of ethanol and transferred to an aerosol dispersion tube
to which ethyl acetoacetate (495 mg, 3.8 mmol) and aqueous
ammonia (3 mL, 28.7%) were added. The mixture was heated
to 100-110 °C, 35-40 psi, for 48 h after which time the
solvents were removed under vacuum. The crude product was
crystallized by slow diffusion of hexane into a 8:1 benzene:
ethanol solution. This gave 118 mg of white crystals (12%):
mp 136-137 °C; 1H NMR (300 MHz, CDCl3) δ 1.21 (t, 6H, J )
Su p p or tin g In for m a tion Ava ila ble: Brief description of
radioligand binding assays and tables of crystallographic data.
This information is available free of charge via the Internet
at http://pubs.acs.org.
Refer en ces
(1) Triggle, D. J .; Langs, D. A.; J anis, R. A. Ca2+ Channel Ligands:
Structure-Function-Relationships of the 1,4-Dihydropyridines.
Med. Res. Rev. 1989, 9, 123-180.
(2) (a) Iqbal, N.; Akula, M. R.; Vo, D.; Wandikayi, C. M.; McEwen,
C.-A.; Wolowyk, M. W.; Knaus, E. E. J . Med. Chem. 1998, 41,
1827-1837. (b) Sun, J .; Triggle, D. J . Calcium Channel Antago-
nists: Cardioselectivity of Action. J . Pharmacol. Exp. Thera-
peutics. 1995, 274, 419-426.

Synthesis, SAR of 4-Isoxazolyl-1,4-dihydropyridines
J ournal of Medicinal Chemistry, 1999, Vol. 42, No. 16 3093
(3) (a) Striessnig, J .; Murphy, B. J .; Catterall, W. A. Dihydropyridine
Receptor of L-Type Calcium Channels: Identification of Binding
Domains for 3H-PN-200-110 and 3H-azidopine Within the Alpha
1 Subunit. Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 10769-10773.
(b) Nakayama, H.; Taki, M.; Striessnig, J .; Glossmann, H.;
Catterall, W. A.; Kanaoke, Y. Identification of 1,4-Dihydropyri-
dine Binding Regions Within the Alpha 1 Subunit of Skeletal
Muscle Ca2+ Channels by Photoaffinity Labeling with Diazepine.
Proc. Natl. Acad. Sci. U.S.A. 1991, 88, 9203-9207. (c) Bangalore,
R.; Baindur, N.; Rutledge, A.; Triggle, D. J .; Kass, R. S. L-Type
Calcium Channels: Asymmetrical Intramembrane Binding Do-
main Revealed byVariable Length, Permanently Charged 1,4-
Dihydropyridines. Mol. Pharmacol. 1994, 46, 660-666. (d) Tang,
S.; Yatani, A.; Bahinski, A.; Mori, Y.; Schwartz, A. Molecular
Localization of Regions in the L-Type Calcium Channel Critical
for Dihydropyridine Action. Neuron 1993, 11, 1013-1021.
(4) (a) Mirzaei, Y. R.; Simpson, B. M.; Triggle, D. J .; Natale, N. R.
Diastereoselectivity in the Metalation and Electrophilic Quench-
ing of Isoxazolyloxazolines. J . Org. Chem. 1992, 57, 6271-6279.
(b) Natale, N. R.; Triggle, D. J .; Palmer, R. B.; Lefler, B. J .;
Edwards, W. D. 4-Isoxazolyl-1,4-dihydropyridines: Biological,
Theoretical, and Structural Studies. J . Med. Chem. 1990, 33,
2255-2259. (c) McKenna, J . I.; Schlicksupp, L.; Natale, N. R.;
Maryanoff, B. E.; Flain, S. F.; Willet, R. D. Cardioactivity and
Solid State Structure of Two 4-isoxazolyl-1,4-dihydropyridines
Related to the 4-aryldihydropyridine Calcium Channel Blockers.
J . Med. Chem. 1988, 31, 473-476.
S. Primary Structure of the Receptor for Calcium Channel
Blockers from Skeletal Muscle. Nature 1987, 328, 313-318.
(9) Fossheim, R.; Svarteng, K.; Mostad, A.; Romming, C.; Shefter,
E.; Triggle, D. J . Crystal Structures and Pharmacological
Activity of Calcium Channel Antagonists: 2,6-Dimethyl-3,5-
dicarbomethoxy, 4-(unsubstituted, 3-methyl-, 4-methyl-, 3-nitro-,
4-nitro-, and 2,4-dinitrophenyl)-1,4-dihydropyridine. J . Med.
Chem. 1982, 25, 126-131.
(10) Natale, N. R.; Mirzaei, Y. R. The Lateral Metalation of Isox-
azoles. A Review. Org. Prepr. Proceed Int. 1993, 25, 515-556.
(11) Hubler, T. L.; Meikrantz, S. B.; Bitterwolf, T. E.; Natale, N. R.;
Triggle, D. J .; Kwon, Y. W. Tricarbonylchromium Complexes of
Hantzsch Esters Possess Robust Calcium Antagonist Activity.
J . Med. Chem. 1992, 35, 1165.
(12) Bolger, G. T.; Gengo, P.; Klockowski, R.; Luchowski, E.; Siegel,
H.; J anis, R. A.; Triggle, A. M.; Triggle, D. J . Characterization
of Binding of the Ca2+ Channel Antagonist [3H]Nitrendipine to
Guinea Pig Ileal Smooth Muscle. J . Pharmacol. Exp. Ther. 1983,
225, 191-309.
(13) J anis, R. A.; Sarmiento, J . G.; Maurer, S. C.; Bolger, G. T.;
Triggle, D. J . Characteristics of the Binding of [3H]Nitrendipine
to the Rabbit Ventricular Membranes: Modification of the Ca2+
Channel Antagonist Bay K 8644. J . Pharmacol. Exp. Ther. 1984,
231, 291.
(14) Natale, N. R.; McKenna, J . I.; Niou, C.-S.; Borth, M. Metalation
of Isoxazolyloxazolines, a Facile Route to Functionally Complex
Isoxazoles: Utility, Scope, and Comparison to Dianion Method-
ology. J . Org. Chem. 1985, 50, 5560-5666.
(15) Palmer, R. B.; Andro, T. M.; Natale, N. R.; Anderson, N. H.
Conformational Preferences and Dynamics of 4-Isoxazolyl 1,4-
dihydropyridine Calcium Channel Antagonists as Determined
by Variable Temperature NMR and NOE Experiments. J . Magn.
Reson. 1996, 34, 495-504.
(16) Doyle, D. A.; Cabral, J . M.; Pfuetzner, R. A.; Kuo, A.; Gulbis, J .
M.; Coben, S. L.; Chait, B. T.; MacKinnon, R. The Structure of
the Potassium Channel: Molecular Basis of K+ Conduction and
Selectivity. Science 1998, 280, 69-77.
(17) Hocherman, G. H.; Peterson, B. Z.; J ohnson, B. D.; Catterall,
W. A. Molecular Determinants of Drug Binding and Action on
C-Type Calcium Channels. Annu. Rev. Pharmacol. Toxicol. 1997,
37, 361-396.
(5) (a) Langs, D. A.; Kwon, Y. W.; Strong, P. D.; Triggle, D. A.
Molecular level model for theagonist/antagonist selectivity of the
1,4-dehydropyridine calcium channel receptor. J . Comput.-Aided
Mol. Des. 1991, 5, 95-106. (b) Langs, D. A.; Strong, P. D.;
Triggle, D. J . Receptor Model for the Molecular Basis of Tissue
Selectivity of 1,4-Dihydropyridine Calcium Channel Drugs. J .
Comput.-Aided Mol. Des. 1990, 4, 215-230.
(6) Hoeltje, H.-D.; Marrer, S. A Molecular graphics study on
structure-action relationships of calcium antagonistic 1,4-dihy-
dropyridines. J . Comput.-Aided Mol. Des. 1987, 1, 23-30.
(7) Kalasz, H.; Watanabe, T.; Yabana, H.; Itagaki, K.; Naito, K.;
Nakayama, H.; Schwartz, A.; Vaghy, P. L. Identification of 1-4-
Dihydropyridine Binding Domains Within the Primary Structure
of the Alpha 1 Subunit of the Skeletal Muscle L-Type Calcium
Channel. FEBS Lett. 1993, 331, 77-181.
(8) Tanabe, T.; Takeshima, H.; Mikami, A.; Flockerzi, V.; Takahashi,
H.; Kanagawa, K.; Kajima, M.; Matsuo, H.; Hirase, T.; Numa,
J M980439Q