Novel selective PDE4 inhibitors. 1. Synthesis, structure-activity relationships, and molecular modeling of 4-(3,4-dimethoxyphenyl)-2H-phthalazin-1-ones and analogues

Article abstract of DOI:10.1021/jm010837k

A number of 6-(3,4-dimethoxyphenyl)-4,5-dihydro-2H-pyridazin-3-ones and a novel series of 4-(3,4-dimethoxyphenyl)-2H-phthalazin-1-ones were prepared and tested on the cGMP-inhibited phosphodiesterase (PDE3) and cAMP-specific phosphodiesterase (PDE4) enzymes. All tested compounds were found to specifically inhibit PDE4 except for pyridazinone 3b, which showed moderate PDE4 (pIC₅₀ = 6.5) as well as PDE3 (pIC₅₀ = 6.6) inhibitory activity. In both the pyridazinone and phthlazinone series it was found that N-substitution is beneficial for PDE4 inhibition, whereas in the pyridazinone series it also accounts for PDE4 selectivity. In the phthalazinone series, the cis-4a,5,6,7,8,8a-hexahydrophthalazinones and their corresponding 4a,5,8,8a-tetrahydro analogues showed potent PDE4 inhibitory potency (10/11c,d: pIC₅₀ = 7.6-8.4). A molecular modeling study revealed that the cis-fused cyclohexa(e)ne rings occupy a region in space different from that occupied by the other fused (un)saturated hydrocarbon rings applied; we therefore assume that the steric interactions of these rings with the binding site play an important role in enzyme inhibition.

Full text of DOI:10.1021/jm010837k

J . Med. Chem. 2001, 44, 2511-2522
2511
Articles
Novel Selective P DE4 In h ibitor s. 1. Syn th esis, Str u ctu r e-Activity
Rela tion sh ip s, a n d Molecu la r Mod elin g of
4-(3,4-Dim eth oxyp h en yl)-2H-p h th a la zin -1-on es a n d An a logu es
Margaretha Van der Mey,*,†,§ Armin Hatzelmann,‡ Ivonne J . Van der Laan,§ Geert J . Sterk,§
Ulrich Thibaut,‡ and Hendrik Timmerman†
Leiden/ Amsterdam Center for Drug Research, Division of Medicinal Chemistry, Department of Pharmacochemistry, Vrije
Universiteit, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands; Byk Nederland, Zwanenburg, The Netherlands; and
Byk Gulden, Konstanz, Germany
Received February 7, 2001
A number of 6-(3,4-dimethoxyphenyl)-4,5-dihydro-2H-pyridazin-3-ones and a novel series of
4-(3,4-dimethoxyphenyl)-2H-phthalazin-1-ones were prepared and tested on the cGMP-inhibited
phosphodiesterase (PDE3) and cAMP-specific phosphodiesterase (PDE4) enzymes. All tested
compounds were found to specifically inhibit PDE4 except for pyridazinone 3b, which showed
moderate PDE4 (pIC50 ) 6.5) as well as PDE3 (pIC50 ) 6.6) inhibitory activity. In both the
pyridazinone and phthlazinone series it was found that N-substitution is beneficial for PDE4
inhibition, whereas in the pyridazinone series it also accounts for PDE4 selectivity. In the
phthalazinone series, the cis-4a,5,6,7,8,8a-hexahydrophthalazinones and their corresponding
4a,5,8,8a-tetrahydro analogues showed potent PDE4 inhibitory potency (10/11c,d : pIC50
)
7.6-8.4). A molecular modeling study revealed that the cis-fused cyclohexa(e)ne rings occupy
a region in space different from that occupied by the other fused (un)saturated hydrocarbon
rings applied; we therefore assume that the steric interactions of these rings with the binding
site play an important role in enzyme inhibition.
In tr od u ction
xanthines (e.g., theophylline),7 which appear to possess
bronchodilatory and anti-inflammatory as well as im-
munomodulatory properties. Newer drugs include leu-
kotriene antagonists such as montelukast.8
To date, much research has been directed toward the
discovery of new antiasthmatic agents with high selec-
tivity and efficacy and a reduced side effect profile.
Phosphodiesterases (PDEs),9 enzymes involved in the
intracellular degradation of cAMP and cGMP to their
corresponding 5′-monophosphate counterparts, have
received a considerable amount of attention as molec-
ular targets for the treatment of asthma.
Currently, a number of PDE isoenzyme families have
been identified, which have been classified according to
their substrate affinities and specificities (cAMP versus
cGMP), kinetic characteristics (Km and Vmax), subcellular
distributions (soluble versus membrane-bound), and
regulatory properties (e.g., activation by calcium/calm-
odulin (CaM)).10-16 During the past few years molecular
biological methods have revealed an even higher com-
plexity by the demonstration that most of these PDE
families include more than one, and as many as four,
distinct gene products (subtypes) as well as multiple
splice variants within one gene family.17 Currently,
major interest centers on selective inhibitors of
PDE4,18-20 mainly due to the fact that PDE4 is the
prominent isoenzyme present in inflammatory cells
thought to be important in asthma21-23 and because
inhibitors of PDE4 are effective in attenuating the
activity of these inflammatory cells.24-26 PDE4 is also
Asthma is one of the most common chronic diseases,1
worldwide and antiasthma medications are widely
prescribed. In fact, asthma treatments account for 1-2%
of the total health budget in industrialized countries.2
Despite advancements in treatment, the incidences of
asthma, asthma-related deaths, and hospitalizations for
asthma have increased significantly during the past
decade.
Asthma is characterized by a reversible airway ob-
struction, ongoing cellular inflammation, and nonspe-
cific hyper-responsiveness to a variety of challenges.
Both acute and long-term manifestations of asthma are
believed to be a consequence of various inflammatory
mediators released by activated inflammatory and im-
mune cells.3-6 Therefore, the ability to suppress activa-
tion of these cells is an essential activity for a compound
to have a therapeutic effect on asthma.
Four major classes of compounds are currently used
in the treatment of asthma, including bronchodilators
(particularly â-adrenoceptor agonists), immunosuppres-
sive agents (corticosteroids), antiallergic agents for
prophylactic use (e.g., disodium cromoglycate), and
* Address correspondence to this author at the Radionuclidencen-
trum, Vrije Universiteit, De Boelelaan 1085c, 1081 HV Amsterdam,
The Netherlands (telephone +31 20 4449720; fax +31 20 4449121;
e-mail mmeij@rnc.vu.nl).
† Vrije Universiteit.
§ Byk Nederland.
‡ Byk Gulden.
10.1021/jm010837k CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/03/2001

2512 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16
Van der Mey et al.
Sch em e 1a
a
Reagents: (i) H2NNH2, EtOH, reflux; (ii) 1, NaH, DMF; 2,
BnCl; (iii) PhNHNH2, toluene, reflux.
Sch em e 2a
F igu r e 1.
involved in the regulation of airway smooth muscle tone,
together with PDE3, which is the major PDE present
in airway smooth muscle. Consistent with the presence
of large amounts of PDE3 and PDE4, studies with
human isolated bronchi have demonstrated that selec-
tive inhibitors of either isoenzyme partially reverse
spontaneous tone and elicit bronchorelaxation,27,28 PDE3
inhibitors being somewhat more effective.28 Interest-
ingly, either a combination of PDE3 and PDE4 inhibi-
tors or dual PDE3/4 inhibitors produce a much larger
bronchorelaxant effect than individual isoenzyme-selec-
tive agents alone,29,30 suggesting that these isoenzymes
act in an synergistic manner in human airway smooth
muscle.
a
Reagents: method A (2a); 1,2-dimethoxybenzene, AlCl3, CH2Cl2,
reflux; method B (2c, 2d ); 3,4-dimethoxyphenylmagnesium bro-
mide (1), THF.
Condensation of phenylhydrazine with γ-keto acids I
in toluene at reflux temperature provided the N-phenyl
pyridazinones and phthalazinones III in variable yields.
The target compounds were purified by crystalliza-
tion. If (re)crystallization was not sufficient to give pure
products, the compounds were purified by flash column
chromatography, followed by crystallization. The overall
yields of the N-substituted derivatives III were strongly
influenced by the crystallization procedures.
Syn th esis of th e 4-Oxobu tyr ic Acid s a n d 4,5-
Dih yd r o-2H-p yr id a zin -3-on es. The γ-keto acid 2a 36,37
was prepared by a Friedel-Crafts reaction between 1,2-
dimethoxybenzene and succinic anhydride in the pres-
ence of aluminum chloride (Scheme 2).
The Friedel-Crafts reaction using methylsuccinic
anhydride usually leads to a preponderance of the
R-methylketo acids.38 For this reason, the â-methylketo
acid 2b39 was synthesized analogously to literature
procedures40 using an alternate pathway (Scheme 3).
The Friedel-Crafts acylation of 1,2-dimethoxybenzene
with 2-chloropropionyl chloride in the presence of
aluminum chloride led to the R-chloroketone 6. Dis-
placement of chloride with tert-butyl sodiomalonate in
DMF gave the required γ-keto diester 7. After workup,
the crude diester was hydrolyzed by treatment with
TFA, followed by decarboxylation of the resulting diacid
in refluxing acetic acid to give the easily isolable γ-keto
acid 2b.
In our search for new lead structures for selective
PDE4 or dual PDE3/4 inhibitors (Figure 1) we combined
structural elements of the pyridazinone zardaverine,31
which has been found to be active on PDE3 (pIC50
)
6.24; human platelet) and PDE4 (pIC50 ) 6.80; canine
trachea), and a pyridopyridazinone analogue,32 a potent
and selective inhibitor of PDE4 (pIC50 ) 8.30; human
lymphocyte). The target compounds contain the py-
ridazinone nucleus present in both zardaverine and the
pyridopyridazinone compound and a catechol ether
moiety, which was found to be important for inhibition
in many PDE4 inhibitors.33
This paper describes the synthesis, structure-activity
relationships (SAR), and molecular modeling studies for
a series of pyridazinones and phthalazinones34,35 among
which several potent PDE4 inhibitors are present.
Ch em istr y
Gen er a l P r oced u r es for Syn th esis of th e Ta r get
Com p ou n d s. The target compounds were synthesized
according to the procedures depicted in Scheme 1. The
γ-keto acids I readily underwent cyclization upon treat-
ment with hydrazine hydrate in EtOH to give the
corresponding pyridazinones or phthalazinones II, which
afforded the desired N-benzylated analogues III in high
yields upon treatment with NaH followed by benzyl
chloride.
To synthesize R-methylketo acid 2c,41 methylsuccinic
anhydride was treated with 3,4-dimethoxyphenylmag-
nesium bromide (1) in a Grignard reaction. Workup and
subsequent crystallization from diethyl ether gave an
inseparable mixture of two regioisomers (ratio 2c/2b )
6:4), the minor component being identical to â-meth-

Novel Selective PDE4 Inhibitors. 1
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16 2513
Sch em e 3a
Sch em e 4a
a
Reagents: method C; (i) CH3ClCHC(O)Cl, AlCl3, ClCH2CH2Cl,
a
reflux; (ii) sodiomalonic acid tert-butyl ester, DMF; (iii) 1, TFA,
CH2Cl2, reflux; 2, AcOH, reflux.
Reagents: method A (8a ,c-e); 1,2-dimethoxybenzene, AlCl3,
CH2Cl2, reflux; method B (8b-d ); 1, THF.
Ta ble 1. 6-(3,4-Dimethoxyphenyl)-4,5-dihydro-2H-pyridazin-3-
ones and Their PDE3 and PDE4 Inhibitory Activities
pound was predominantly formed in a ratio of 1:7 (4d /
4e). Although analogue 4e was pure according to TLC
and NMR, it could not be obtained in crystalline form.
The stereoisomeric mixture of γ-keto acids 2d was
also treated with hydrazine monohydrate to yield the
4,5-dimethylpyridazinones 3d (cis) and 3e (trans) in a
ratio of 1:8. The two product ratios obtained agree with
the findings that in the dimethylsuccinic series, the
open-chain compounds (γ-keto acids) are stable in the
meso-forms and the corresponding heterocyclic deriva-
tives (pyridazinones) in the DL(trans)-forms.45
The configuration of the 4,5-dimethylpyridazinones
was determined by NOE (nuclear Overhauser enhance-
ment) measurements. Upon saturation the H4 proton
of analogue 4d , the NOE difference spectrum showed
NOEs on the 4-methyl group and the H5 proton (weak).
The 4-methyl group was then saturated, and interaction
with the H4 proton and 5-methyl group was observed.
From these experiments it was concluded that this
compound possesses a cis configuration. In the case of
derivative 4e the H4 proton was saturated and strong
NOEs were observed on the 4- and 5-methyl groups as
well as the H5 proton. In a second experiment the NOE
spectrum showed, upon saturation of the H5 proton,
nearly the same strong NOE on the H4 proton, the 4-
and 5-methyl groups, and both ortho-hydrogen atoms
of the 6-phenyl ring. On the basis of these results a trans
configuration was assigned to analogue 4e.
conf
C4-C5
PDE3
pIC50
PDE4
pIC50
a
a
compd
R1
R2
R3
3a
3b
3d
3e
4a
4b
4c
4d
5a
H
H
H
Me
Me
H
H
H
H
H
Ph
Ph
Ph
Ph
Bn
5.6
6.6
<5.0
5.4
<5.0
<5.0
<5.0
<5.0
5.3
5.9
6.5
6.7
6.2
6.4
6.8
6.9
7.3
7.1
6.8
Me
Me
Me
H
Me
H
cis
trans
H
Me
Me
H
Me
H
cis
Zardaverine
6.2
a
pIC50 ) -log IC50. Inhibition of PDE4 was investigated in the
cytosol of human neutrophils, and activity on PDE3 was deter-
mined in homogenates of human blood platelets. The data are
means of two independent determinations in triplicate.
ylketo acid 2b. The major acylation product 2c was
assigned the R-methyl configuration on the basis of 1H
NMR spectroscopy. The R-methylketo acids can be
differentiated from the corresponding â-methyl deriva-
1
tives due to the position of the H NMR signal of the
methine proton.42 In the â-methyl series this proton is
further downfield and separated from the methylene
protons in contrast to the methine proton in the
R-methyl analogues. After condensation of the mixture
of γ-keto acids 2b and 2c with phenylhydrazine, the
pyridazinones 4b and 4c (Table 1) were isolated in
yields of 46 and 7%, respectively.
A mixture of D,L- and meso-2,3-dimethylsuccinic acid
was, according to literature procedures, refluxed in
acetic anhydride to give a mixture of cis- and trans-2,3-
dimethylsuccinic anhydride.43 Synthesis of the 2,3-
dimethyl-4-oxobutyric acids 2d 44 (up to four stereoiso-
mers are possible) was effected by a Grignard reaction
using the cis-trans mixture of 2,3-dimethylsuccinic
anhydride and Grignard reagent 1 (Scheme 2). The
mixture of stereoisomeric γ-keto acids 2d was then
treated with phenylhydrazine, and subsequent purifica-
tion of the reaction products using flash column chro-
matography resolved the mixture of N-phenyl-4,5-
dimethylpyridazinones 4d (cis) and 4e (trans, Table 1).
Comparison of the yields indicated that the trans com-
Syn th esis of th e Cyclic γ-Keto Acid s a n d 4-(3,4-
Dim eth oxyph en yl)-2H-ph th alazin -1-on es. γ-Keto ac-
ids 8a 46 and 8c-e (Scheme 4) were prepared in mod-
erate yields (30-40%) from the corresponding phthalic
anhydrides by Friedel-Crafts acylations with 1,2-
dimethoxybenzene as described above for 4-oxobutyric
acid 2a . Nucleophilic addition of Grignard reagent 1 to
cis-1,2,3,6-tetrahydrophthalic and cis-1,2-cyclohexanedi-
carboxylic anhydride also afforded compounds 8c and
8d , respectively, but in somewhat higher yields (40
versus 30%). Derivative 8b was synthesized via a
Grignard reaction because Friedel-Crafts acylation of
1,2-dimethoxybenzene with 3,4,5,6-tetrahydrophthalic
anhydride was unsuccessful.
In the tetra- and hexahydrophthalic series, γ-keto
acids having a cis configuration (e.g., 8c and 8d ) are
less conformationally stable than the corresponding
trans diastereomers.42,47 Therefore, γ-keto acid 8c was
subjected to aqueous base-mediated epimerization to
give the trans analogue 8f as a single diastereomer in
90% yield (Scheme 5).

2514 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16
Van der Mey et al.
Sch em e 5a
methine signal at 3.20 ppm caused enhancement of the
signals of the ortho-hydrogen atoms of the 4-phenyl ring,
thus indicating their vicinity to the methine hydrogen
atom, which could then be assigned as H4a. The strong
NOE between the two annealed hydrogen atoms proved
their cis position.
The NMR data for the hexahydrophthalazinones 10d
and 10e differ significantly. A cis to trans change in the
structure causes significantly weaker shielding on the
cyclohexane carbon atoms. The signals of the annealed
hydrogen atoms of analogue 10e were therefore ap-
preciably upfield-shifted, whereas the overall shift for
the six cyclohexane carbon atoms was 16 ppm more than
for 10d , which was regarded as evidence for the trans
annealation. The absence of an NOE between the two
annealed hydrogen atoms once again confirmed their
trans arrangement.
a
Reagents: method D; (i) 2 N KOH.
Ta ble 2. 4-(3,4-Dimethoxyphenyl)-2H-phthalazin-1-ones and
Their PDE4 Inhibitory Activitiesa
conf
PDE4
pIC50
b
compd
ring X
C4a-C8a
R
The configurations of phthalazinones 9c-e, 10c,f, and
11c,d were determined in a similar way. The results
indicated that in this series of compounds, the cis and
trans configurations of the γ-keto acids were stable
under the conditions used during condensation and
alkylation reactions.
9a
9b
9c
9d
A
B
C
D
D
A
B
C
C
D
D
A
B
C
D
H
H
H
H
5.8
5.2
7.0
6.4
5.3
5.8
5.3
7.9
6.1
7.6
5.1
6.5
5.8
8.4
8.1
7.0
cis
cis
trans
9e
H
10a
10b
10c
10f
10d
10e
11a
11b
11c
11d
Ariflo
Ph
Ph
Ph
Ph
Ph
Ph
Bn
Bn
Bn
Bn
1
Finally, the H NMR spectra of phthalazinones 9-11
cis
trans
cis
showed that H8′ is strongly deshielded because of its
coplanarity with the carbonyl bond, and the anisotropic
effect of the latter causes downfield shifts of their
signals.
trans
cis
cis
P h a r m a cology
The pyridazinone and phthalazinone analogues (Tables
1 and 2) were tested for their PDE3 and PDE4 inhibi-
tory activities. PDE3 inhibition was determined in
homogenates of human blood platelets.31 PDE4 inhibi-
tion was investigated in the cytosol of human neutro-
phils.48 The tests were carried out according to the
method of Bauer and Schwabe.49 In this assay, the PDE
a
The compounds show no substantial PDE3 inhibitory activity
b
(pIC50 < 5.4); the activities are therefore not listed. See footnote
a of Table 1.
The target phthalazinones 9-11 (Table 2) were
prepared analogously to the procedures depicted in
Scheme 1.
3
reaction with H-labeled cAMP in the absence or pres-
Con figu r a tion a l An a lysis of th e 4a ,5,8,8a -Tetr a -
a n d 4a ,5,6,7,8,8a -Hexa h yd r op h th a la zin on es. For
the 4a,5,8,8a-tetra- and 4a,5,6,7,8,8a-hexahydrophthalazi-
nones described in this paper, several (up to four)
stereoisomers are possible. As the cis or trans fusion of
the cyclohex(a)ene and heterocycle (pyridazinone ring)
depends on the reaction conditions during syntheses,
the configurations of the starting materials (γ-keto
acids) and the products (phthalazinones) can differ. This
knowledge emphasizes the importance of determination
of the stereostructures of the reaction products. Infor-
mation about the configurations of the phthalazinones
9c-e, 10c-f, and 11c,d (Table 2) was obtained by the
application of different NMR techniques. The configu-
rational assignments of the investigated compounds are
based on their homo- and heteronuclear correlation
spectra (HH- and CH-COSY, respectively) as well as
their attached proton test (APT) and NOE difference
spectra.
For hexahydrophthalazinone 10d , the signals of the
two annealed carbon atoms (i.e., C4a and C8a) at 36.11
and 37.12 ppm were identified by APT, and the 1H NMR
signals corresponding to these resonances, 3.20 and 2.93
ppm, respectively, were located in the CH-COSY spec-
trum. NOE measurements were used to determine the
position of the methine protons and the configuration
of the compound under investigation. Saturation of the
ence of test compound (different concentrations) is
carried out in the first step. In a second step, the
resultant 5′-monophosphate is then further hydrolyzed
to the uncharged nucleoside by a snake venom 5′-
nucleotidase from Crotalus atrox. In the third step, the
nucleoside is separated from the remaining charged
substrate on ion-exchange columns. The radioactivity
of the elute is measured and corrected by the corre-
sponding blank values. IC50 values, which are the drug
concentrations at which the enzyme activity is reduced
to 50%, are calculated from the drug concentration-
inhibition curves by nonlinear regression analysis. The
-logIC50 () pIC50) values determined for the compounds
are summarized in Tables 1 and 2.
P DE In h ibition a n d SAR
In h ibition of P DE3. Among the pyridazinones with
variations at the 2-, 4-, and 5-positions of the hetero-
cyclic ring (Table 1), only 5-methylpyridazinone 3b
shows reasonable PDE3 inhibitory activity. Comparison
of 3b with 3a ,d and 3e reveals that removal of the
5-methyl group or addition of a methyl substituent at
the 4-position of the pyridazinone subunit leads to a
substantial decrease in activity. Also, the introduction
of a phenyl ring at the nitrogen of the pyridazinone
moiety (compare 3b with 4b) diminishes the potency

Novel Selective PDE4 Inhibitors. 1
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16 2515
dramatically. These results agree with findings of other
research groups.50,51 The phthalazinones (Table 2) show
no substantial PDE3 inhibitory activity (pIC50 < 5.4);
the activities are therefore not listed.
In h ibition of P DE4. In the pyridazinone series, a
general trend was observed regarding inhibition of
PDE4. An increase in the number of methyl substitu-
ents (3a vs 3b vs 3d and 4a vs 4b,c vs 4d ) leads to more
potent analogues. This tendency is most obvious in the
N-phenyl series 4.
The diastereomers 3d (cis) and 3e (trans) possess the
same lipophilic and electronic features; nevertheless, the
cis isomer is more active than the trans analogue;
therefore, steric interactions of the methyl groups with
the binding site presumably play a more important role
than lipophilicity.
The introduction of a phenyl group at the pyridazi-
none nitrogen leads to a 2-4-fold increase in PDE4
inhibitory activity (3a vs 4a , 3b vs 4b and 3d vs 4d ).
Moreover, the N-benzyl-substituted analogue 5a is even
14 times more potent than 3a . Thus, lipophilic and
aromatic moieties at the 2-position of the pyridazinone
subunit enhance enzyme inhibition.
The PDE4 inhibitory activities of phthalazinones
9-11 with variations in ring X are shown in Table 2.
Comparison of compounds 9-11 reveals that analogues
in which ring X is an aromatic ring (9a , 10a , and 11a ),
a cyclohex-1-ene ring (9b, 10b, and 11b), a trans-
cyclohexane ring (9e and 10e), or a trans-cyclohex-3-
ene ring (10f) are only weakly to moderately active
regardless of the N-substituents. However, cyclohex(a)-
ene derivatives 9c,d , 10c,d , and 11c,d , having a cis
configuration, exhibit high activities. In fact, analogues
10c and 10d are up to 350-fold more potent than the
corresponding trans diastereomers 10f and 10e. This
may be due to unfavorable steric interactions of the
trans compounds with the binding site. The latter
rationale probably also holds true for 9a ,b, 10a ,b, and
11a ,b. Finally, phthalazinones with a cis-cyclohex-3-ene
ring are optimal for PDE4 inhibition (9c, 10c, and 11c
vs 9d , 10d , and 11d , respectively).
F igu r e 2. Phthalazinone inter-ring dihedral angles Φ (bold).
6.3.252 on a Silicon Graphics Indigo2 (R10.000 under
IRIX 6.2) workstation and after a preliminary minimi-
zation with the TRIPOS force field using MAXIMIN II
submitted to full-energy minimization with MOPAC
6.0.53 A systematic conformational search was per-
formed on the rotable bonds using an increment of 10°
or 15° followed by full-geometry optimization using
MOPAC 6.0 to identify low-energy conformations.
Con for m a tion a l Asp ects a n d Selection of Sta r t-
in g Geom etr ies. The rotational barrier of the two
phenyl-phthalazinone inter-ring bonds (Figure 2) is very
low with respect to all molecules studied. Therefore, the
origin of the difference in PDE4 inhibition of phthalazi-
nones 10a -f should lie in the phthalazinone part of the
molecules. In the minimum-energy conformations, the
cyclohex(a)ene rings adapt the chair conformation, in
which the carbonyl groups are at the axial position as
reported previously for analogous compounds.54
Analogue 10c was chosen as the template because of
its high affinity. The minimum-energy conformer having
a catechol-heterocycle inter-ring dihedral angle of ∼15°
(Figure 2) was selected as the template for the molecular
overlay of derivatives 10a ,b,d -f. The molecules were
aligned via root-mean-square (RMS) fit of (a) the cat-
echol oxygen atoms, (b) the nitrogen atom in position
2, and (c) the two annealed carbon atoms. Low-energy
conformers of the molecules were chosen for optimal
overlap with one another, under the assumption that
the catechol and pyridazinone rings always occupy the
same cavity in the enzyme.
Resu lts a n d Discu ssion
The activities of the cis-annealed phthalazinones
generally parallel those of the pyridazinone series (Table
1) with respect to substitutions at the 2-position of the
heterocycle. The N-phenyl-substituted compounds 10c
and 10d exhibit activities up to an order of 16 greater
than those of the nonsubstituted analogues 9c and 9d ,
respectively. Again, the introduction of a benzyl group
at N2 increases potency even more (11c vs 9c and 11d
vs 9d ), the best activity residing in the N-benzyl
analogue 11c.
The final superposition of phthalazinones 10a -f is
displayed in Figure 3a,b. These pictures show that the
annealed aromatic ring (10a ), cyclohex-1-ene ring (10b),
trans-cyclohex-3-ene ring (10f, both enantiomers), and
trans-cyclohexane ring (10e, both enantiomers) occupy
the same region in space; that is, the mean plane of
ring X is nearly coplanar with the heterocyclic ring
(Figure 3a). However, the cis-cyclohex(a)ene rings of
analogues 10c and 10d are not coplanar with the
pyridazinone ring; in fact, they fill a completely different
area in space (Figure 3b). Furthermore, Figure 3b
clearly shows that the A-rings of the (4aS,8aR)- and
(4aR,8aS)-enantiomers of these structures point to
opposite directions.
As the electronic features of the active cis and
corresponding inactive trans analogues are most likely
the same (10c vs 10f and 10d vs 10e) and because no
significant correlation was found between the PDE4
inhibitory activity and the lipophilicity of ring X, as
mentioned earlier, variance in activity is mainly caused
by steric interactions. With regard to compounds 10a,b,e
and 10f this means a steric unfavorable interaction of
ring X with the binding site, explaining the strong
Molecu la r Mod elin g
Molecular modeling studies were carried out espe-
cially to examine the marked difference in PDE4 inhibi-
tory activity upon changes in the structure of the fused
hydrocarbon ring of the phthalazinones under investi-
gation (Table 2). The N-phenyl-substituted series (ana-
logues 10a -f) was used for this purpose. Both the cis
and trans enantiomers of compounds 10c-f were taken
into account because only the racemates have been
synthesized and tested.
Meth od s of Geom etr y Op tim iza tion of 3D Str u c-
tu r es. All structures under study were built in SYBYL

2516 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16
Van der Mey et al.
In both the pyridazinone and phthlazinone series
N-substitution is beneficial for PDE4 inhibition. This
effect is in agreement with the SAR developed for well-
known PDE4 inhibitors such as nitraquazone.55,56 Par-
ticularly the cis-4a,5,6,7,8,8a-hexa- and cis-4a,5,8,8a-
tetrahydrophthalazinones show high PDE4 inhibitory
activities. Molecular modeling studies revealed that
these cis-fused cyclohexa(e)ne rings occupy a region in
space different from that occupied by the fused rings of
the other phthalazinones studied. Thus, exciting new
areas of space have been identified for further explora-
tion in enantiomerically pure cis-fused series.
With 11c and 11d we have discovered new lead
structures for the development of selective PDE4 inhibi-
tors. The resolution of some cis-fused hexa- and tet-
rahydrophthalazinone racemates and optimization of
the 4-aryl moiety and N-substituent will be the aim of
further research.
Exp er im en ta l Section
I. Ch em istr y. Gen er a l Meth od s a n d Ma ter ia ls. THF
was freshly distilled from LiAlH4 before use. DMF was stored
over 4 Å molecular sieves. All other solvents were used as
received. Starting materials were commercially available.
Reactions were performed under anhydrous conditions unless
noted otherwise. Grignard reactions and Friedel-Crafts acy-
lations were performed under an N2 atmosphere. Reactions
were followed by TLC analysis on Merck TLC aluminum
sheets Silicagel 60 F254. Flash column chromatography was
performed on silica gel, 30-60 µm (J . T. Baker). Melting points
were measured on
a Mettler FP-5 + FP-052 apparatus
equipped with a microscope and are uncorrected. 1H NMR and
13C NMR spectra were recorded on a Bruker AC 200 (1H NMR,
δ 200.1 MHz; 13C NMR, δ 50.29 MHz), unless stated otherwise.
1H NMR (400.1 MHz) and 13C NMR (100.6 MHz) were recorded
on a Bruker 400 MSL spectrometer. The 1H NMR chemical
shifts (δ) are expressed in parts per million values relative to
CDCl3 (δ ) 7.26 ppm) or DMSO-d6 (δ ) 2.50 ppm). 13C NMR
chemical shifts (δ) are reported in parts per million values
relative to CDCl3 (δ ) 77.0 ppm). Abbreviations used in
description of NMR spectra are as follows: s ) singlet, d )
doublet, t ) triplet, q ) quartet, dd ) double doublet, dt )
double triplet, m ) multiplet, and bs ) broad singulet. 2D
NMR (H-H and C-H) COSY techniques were frequently used
to support interpretation of 1D spectra. All phthalazinones had
an elemental analysis (C, H, and N) within (0.4% of the
theoretical value.
F igu r e 3. (a, top) Superimposition of phthalazinones 10a -f
(Table 2). The heterocycle in the center is more or less coplanar
with ring X of compounds 10a ,b,e,f. (b, bottom) Superimposi-
tion of phthalazinones 10a -f as shown in (a), however,
displayed from a different angle. The cis-fused hydrocarbon
rings of derivatives 10c and 10d are obviously not coplanar
with the heterocycle, which is almost perpendicular to the
plane of the paper. The fused hydrocarbon rings of the separate
cis-enantiomers point to opposite directions. Hydrogen atoms
have been omitted for clarity.
Meth od A. Gen er a l P r oced u r e for F r ied el)Cr a fts
Acyla tion of 1,2-Dim eth oxyben zen e. 1,2-Dimethoxyben-
zene (0.10-1.0 mol) was added slowly to an ice-cooled suspen-
sion of aluminum chloride (1 equiv) in CH2Cl2 (0.10-1.0 L).
After complete addition, the desired cyclic anhydride (1 equiv)
was added to the reaction mixture. The resulting solution was
refluxed until TLC showed completion of the reaction (4-8 h).
The mixture was poured into ice-water (0.10-1.0 L), the
organic layer was dried over MgSO4, and the solvent was
removed in vacuo. The residue was dissolved in CH2Cl2 and
filtered over silica gel to remove the dicarboxylic acid formed
during workup. The product was crystallized from diethyl
ether. Experimental data for the separate compounds are listed
below.
decrease of PDE inhibition. Further research will
concentrate on the synthesis of both cis-enantiomers,
which may lead to two new areas of space to explore.
Con clu sion
We describe a number of pyridazinones and a novel
series of phthalazinones with potent PDE4 inhibitory
activities. All compounds prepared are selective PDE4
inhibitors except for 5-methylpyridazinone 3b, which in
addition shows PDE3 inhibitory activity as does zard-
averine. SAR studies indicate the importance of the
presence of a methyl group at the 5-position of the
pyridazinone subunit and a nonsubstituted amide moi-
ety (-NHCO-) for potent PDE3 inhibition, in ac-
cordance with literature examples.
4-(3,4-Dim eth oxyp h en yl)-4-oxobu tyr ic a cid 36,37 (2a ) was
prepared according to method A using succinic anhydride:
3
yield 53%; mp 159-161 °C; 1H NMR (CDCl3) δ 2.81 (t, 2H, J
3
) 6.6 Hz, CH2CO2H), 3.29 (t, 2H, J ) 6.6 Hz, CH2C(O)), 3.94
(s, 3H, OCH3), 3.96 (s, 3H, OCH3), 6.90 (d, 1H, 3J ) 8.4 Hz,
4
H5-arom), 7.54 (d, 1H, J ) 2.0 Hz, H2-arom), 7.62 (dd, 1H,
3
4J ) 2.0 Hz, J ) 8.4 Hz, H6-arom).
2-(3,4-Dim eth oxyben zoyl)ben zoic a cid 46 (8a ) was pre-
pared according to method A using phthalic anhydride: yield

Novel Selective PDE4 Inhibitors. 1
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16 2517
38%; mp 233-235 °C; 1H NMR (CDCl3) δ 3.69 (s, 6H, OCH3),
6.61 (d, 1H, J ) 8.4 Hz, H-arom), 6.90 (dd, 1H, J ) 1.9 Hz,
3J ) 8.4 Hz, H-arom), 7.08-7.19 (dd, 1H H-arom), 7.28-7.50
(m, 3H, H-arom), 7.81-7.92 (dd, 1H, H-arom).
cis-6-(3,4-Dim eth oxyben zoyl)cycloh ex-3-en eca r boxyl-
ic a cid (8c) was prepared according to method B using cis-
1,2,3,6-tetrahydrophthalic anhydride: yield 39%; mp 110-112
3
4
1
°C; H NMR (CDCl3) δ 2.32-2.68 (m, 3H, CH2, CHH′), 2.72-
3.10 (m, 2H, CHH′, H1), 3.75-4.06 (m, 7H, OCH3, H6), 5.52-
5.85 (m, 2H, CHdCH), 6.88 (d, 1H, 3J ) 8.4 Hz, H5-arom),
7.42-7.57 (m, 2H, H2-arom, H6-arom).
tr a n s-2-(3,4-Dim et h oxyb en zoyl)cycloh exa n eca r b ox-
ylic a cid (8e) was prepared according to method A using
trans-1,2-cyclohexanedicarboxylic anhydride: yield 40%; mp
202-205 °C; 1H NMR (DMSO-d6) δ 1.15-1.61 (m, 4H, H-
cyclohexane), 1.70-2.30 (m, 4H, H-cyclohexane), 2.81-3.01 (m,
1H, H1), 3.40-3.60 (m, 1H, H2), 3.93 (s, 3H, OCH3), 3.96 (s,
3H, OCH3), 6.88 (d, 1H, 3J ) 8.4 Hz, H5-arom), 7.50 (d, 1H, 4J
cis-2-(3,4-Dim et h oxyb en zoyl)cycloh exa n eca r b oxylic
a cid (8d ) was prepared according to method B using cis-1,2-
cyclohexanedicarboxylic anhydride: yield 38%; mp 171-175
1
°C; H NMR (CDCl3) δ 1.18-2.36 (m, 8H, CH2), 2.68 (dt, 1H,
4
3
3J ) 4.5 Hz, H1), 3.81-4.06 (m, 7H, OCH3, H2), 6.87 (d, 1H,
) 1.9 Hz, H2-arom), 7.61 (dd, 1H, J ) 1.9 Hz, J ) 8.4 Hz,
3J ) 8.2 Hz, H5-arom), 7.42-7.58 (m, 2H, H2-arom, H6-arom).
H6-arom).
3,4-Dim eth oxyp h en ylm a gn esiu m Br om id e (1, 0.29 M
in THF ). A solution of 1-bromo-3,4-dimethoxybenzene (8.5 mL,
59 mmol) in THF (50 mL) was added dropwise to a suspension
of magnesium powder (1.43 g, 58.8 mmol) in THF (150 mL) at
such a rate as to maintain a gentle reflux. After the addition
was complete, the resulting red mixture was stirred for an
additional 5 h at 60 °C.
Meth od C. P r ep a r a tion of 2b. 2-Ch lor o-1-(3,4-d im eth -
oxyp h en yl)p r op a n -1-on e (6). Aluminum chloride (31.0 g,
232 mmol) was added to a solution of 1,2-dimethoxybenzene
(30.0 mL, 235 mmol) and 2-chloropropionyl chloride (23.0 mL,
237 mmol) in dichloroethane (250 mL). The resulting mixture
was refluxed for 28 h. After cooling, the reaction mixture was
poured into ice-water, and the product was extracted with
CH2Cl2. The combined organic extract was washed with water
and dried over MgSO4. Evaporation of the solvent in vacuo
yielded the crude product, which was purified by flash column
chromatography using ethyl acetate/petroleum ether (60-80)
4:7 to obtain 32.9 g (61%) of 6 as a yellow oil: 1H NMR (CDCl3)
Meth od B. Gen er a l P r oced u r e for Gr ign a r d Ad d ition
of 1 to Cyclic An h yd r id es. A solution of 3,4-dimethoxyphe-
nylmagnesium bromide (1) (20-100 mmol, 0.29 M) in THF
was added dropwise to an ice-cooled solution of the desired
cyclic anhydride (1 equiv) in THF (50-250 mL). After the
addition was complete, the resulting mixture was stirred for
another 30 min at 0 °C. The reaction mixture was allowed to
warm to room temperature overnight. Next, the reaction was
quenched by the addition of a saturated NH4Cl solution, and
the product was extracted with diethyl ether. The organic layer
was washed with water and subsequently extracted with 1 N
NaOH. The combined aqueous extract was acidified with
concentrated HCl and extracted with ethyl acetate. The
combined extracts were dried over MgSO4, filtered, and
concentrated under reduced pressure. The remainder was
dissolved in CH2Cl2 and filtered over silica gel to remove the
dicarboxylic acid formed during workup. The product was
crystallized from diethyl ether.
3
δ 1.74 (d, 3H, J ) 6.7 Hz, CH3), 3.95 (s, 3H, OCH3), 3.97 (s,
3
3
3H, OCH3), 5.25 (q, 1H, J ) 6.7 Hz, CHCl), 6.92 (d, 1H, J )
8.4 Hz, H5-arom), 7.58 (d, 1H, 4J ) 2.0 Hz, H2-arom), 7.66
(dd, 1H, J ) 2.0 Hz, J ) 8.4 Hz, H6-arom).
4
3
2-[2-(3,4-Dim et h oxyp h en yl)-1-m et h yl-2-oxoet h yl]m a -
lon ic Acid Di(ter t-bu tyl) Ester (7). Malonic acid di(tert-
butyl) ester (30.0 g, 139 mmol) was added dropwise to a
suspension of sodium hydride (60% dispersion in oil, 5.60 g,
140 mmol) in DMF (300 mL), and the mixture was stirred at
room temperature overnight. Under ice cooling, a solution of
R-chloroketone 6 (32.9 g, 144 mmol) in DMF (100 mL) was
added dropwise to the reaction mixture. The resulting mixture
was stirred at room temperature for 1 h and then heated to
50 °C for 4 h. The reaction mixture was poured into ice-water
and the product was extracted with CH2Cl2. After the com-
bined organic extract was washed with water and dried over
MgSO4, the solvent was removed in vacuo to obtain the desired
crude product as an oily material: yield 35.0 g (81%); 1H NMR
(CDCl3) δ 1.18 (d, 3H, 3J ) 7.1 Hz, CH3), 1.36 (s, 6H, CH3-
tert-butyl), 1.47 (s, 6H, CH3-tert-butyl), 1.52 (s, 6H, CH3-tert-
butyl), 3.82 (d, 1H, 3J ) 10.9 Hz, C(O)CHC(O)), 3.88-4.10 (m,
1H, CHCH3), 3.94 (s, 3H, OCH3), 3.95 (s, 3H, OCH3), 6.92 (d,
1H, 3J ) 8.4 Hz, H5-arom), 7.55 (d, 1H, 4J ) 1.9 Hz, H2-arom),
4-(3,4-Dim eth oxyp h en yl)-2-m eth yl-4-oxobu tyr ic Acid
(Mixtu r e of 2b a n d 2c). 2-Methylsuccinic anhydride (2.9 g,
25 mmol) was treated with Grignard reagent 1 (0.62 M, 25
mmol) as described above to give an inseparable mixture of
the regioisomers 2b and 2c (ratio 4:6). The mixture was
crystallized from ethyl acetate/petroleum ether (60-80): yield
1
3
1.9 g (31%); mp 94-95 °C; H NMR (CDCl3) δ 1.25 (d, 3H, J
) 7.2 Hz, CH3-2c), 1.31 (d, 3H, 3J ) 6.9 Hz, CH3-2d ), 2.50
(dd, 1H, 3J ) 5.6 Hz, 2J ) 17.0 Hz, H2-2c), 2.92-3.26 (m, 3H,
3
2
H2′-2c, H2-2d , H3-2d ), 3.43 (dd, 1H, J ) 7.1 Hz, J ) 16.7
Hz, H3′-2d ), 3.80-4.03 (m, 13H, H3-2c, OCH3-2c, OCH3-2d ),
6.89 (d, 1H, J ) 8.4 Hz, H5-arom-2d ), 6.91 (d, 1H, J ) 8.4
Hz, H5-arom-2c), 7.50-7.69 (m, 4H, H2-arom-2c,d , H6-arom-
2c,d ).
4
3
7.70 (dd, 1H, J ) 1.9 Hz, J ) 8.4 Hz, H6-arom).
3
3
4-(3,4-Dim eth oxyp h en yl)-3-m eth yl-4-oxobu tyr ic Acid
(2b). TFA (80 mL) was added to a solution of malonic acid
di(tert-butyl) ester 7 (35.0 g, 112 mmol) in CH2Cl2 (500 mL).
The reaction mixture was stirred at room temperature for 1.5
h and then refluxed for 3 h. After removal of the solvent in
vacuo, acetic acid (400 mL) was added to the residue and the
mixture was refluxed for 3 h. The acetic acid was removed
under reduced pressure, and the product was obtained by
crystallization from diethyl ether: yield 19.4 g (69%); mp 118-
123 °C; 1H NMR (CDCl3) δ 1.25 (d, 3H, 3J ) 7.2 Hz, CH3),
4-(3,4-Dim et h oxyp h en yl)-2,3-d im et h yl-4-oxob u t yr ic
Acid (2d ). A mixture of 2,3-dimethylsuccinic acid (D,L and
meso, 5.0 g, 34 mmol) and acetic anhydride (6.4 mL, 68 mmol)
was refluxed for 4 h. The reaction mixture was then concen-
trated in vacuo and coevaporated with toluene to give the crude
product, which crystallized upon cooling to room temperature.
The crystals were washed carefully with dry diethyl ether,
filtered off, and dried in vacuo to yield 2.8 g (22 mmol) of the
anhydride. The product thus obtained was used in a Grignard
reaction with compound 1 (22 mmol), following the general
procedure: yield 2.6 g (45%); mp 163-166 °C; 1H NMR (CDCl3)
3
2
3
2.50 (dd, 1H, J ) 5.6 Hz, J ) 17.0 Hz, H2), 2.99 (dd, 1H, J
2
) 8.4 Hz, J ) 17.0 Hz, H2′), 3.79-4.02 (m, 7H, OCH3, H3),
3
4
6.91 (d, 1H, J ) 8.4 Hz, H5-arom), 7.54 (d, 1H, J ) 1.9 Hz,
4
3
H2-arom), 7.63 (dd, 1H, J ) 1.9 Hz, J ) 8.4 Hz, H6-arom).
3
3
δ 1.20 (d, 3H, J ) 7.2 Hz, 3-CH3), 1.26 (d, 3H, J ) 7.3 Hz,
Meth od D. Ep im er iza tion of γ-Keto Acid 8c. tr a n s-6-
(3,4-Dim et h oxyb en zoyl)cycloh ex-3-en eca r b oxylic Acid
(8f). A solution of compound 8c (10.0 g, 34.5 mmol) in 2 N
KOH (200 mL) was stirred at room temperature for 3 days.
The reaction mixture was acidified with concentrated HCl, and
the precipitate was filtered off and washed carefully with water
to give the pure trans isomer: yield 8.01 g (80%); mp 226-
3
3
2-CH3), 2.99 (dq, 1H, J ) 7.3 Hz, H2), 3.72 (dq, 1H, J ) 7.3
Hz, H3), 3.93 (s, 3H, OCH3), 3.95 (s, 3H, OCH3), 6.90 (d, 1H,
3J ) 8.4 Hz, H5-arom), 7.53 (d, 1H, 4J ) 1.9 Hz, H2-arom),
4
3
7.26 (dd, 1H, J ) 1.9 Hz, J ) 8.4 Hz, H6-arom).
2-(3,4-Dim eth oxyben zoyl)cycloh ex-1-en ecar boxylic acid
(8b) was prepared according to method B using 3,4,5,6-
tetrahydrophthalic anhydride: yield 56%; mp 180-181 °C; 1H
NMR (CDCl3) δ 1.52-2.55 (m, 8H, CH2), 3.89 (s, 3H, OCH3),
1
230 °C; H NMR (CDCl3) δ 1.96-2.72 (m, 4H, H2, H5), 3.10
3
3
3
(dt, 1H, J ) 5.6 Hz, J ) 11.1 Hz, H1), 3.81 (dt, 1H, J ) 5.3
Hz, 3J ) 11.1 Hz, H6), 3.93 (s, 3H, OCH3), 3.95 (s, 3H, OCH3),
5.68-5.89 (m, 2H, H3, H4), 6.92 (d, 1H, 3J ) 8.4 Hz, H5-arom),
3
3.90 (s, 3H, OCH3), 6.86 (d, 1H, J ) 8.5 Hz, H5-arom), 6.92-
7.22 (m, 2H, H2-arom, H6-arom).

2518 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16
Van der Mey et al.
4
4
7.54 (d, 1H, J ) 1.9 Hz, H2-arom), 7.68 (dd, 1H, J ) 1.9 Hz,
arom), 119.06 (C6-arom), 127.88 (C1-arom), 149.24, 150.75 (C3-
arom, C4-arom), 152.91 (CdN), 170.34 (CdO). Anal. Calcd
(C14H18N2O3): C, H, N.
3J ) 8.4 Hz, H6-arom).
Gen er a l P r oced u r e for Con d en sa tion of γ-Keto Acid s
w ith Hyd r a zin e. A mixture of the desired γ-keto acid (2 or
8, 10-100 mmol) and hydrazine monohydrate (2 equiv) in
EtOH (50-500 mL) was refluxed for 4 h. If, upon cooling to
room temperature, the product crystallized from the reaction
mixture, the crude product was filtered off and washed with
water and EtOH to yield the pure product. Otherwise, the bulk
of EtOH was removed in vacuo and the remainder was
dissolved in ethyl acetate. The solution was washed with
water, dilute NaHCO3, and 1 N HCl and dried over MgSO4.
Evaporation of the solvent yielded the crude product, which
was purified by crystallization or flash column chromatography
and subsequent crystallization. Experimental data for the
separate compounds are listed below.
4-(3,4-Dim eth oxyp h en yl)-2H-p h th a la zin -1-on e (9a ) was
prepared from γ-keto acid 8a according to the general proce-
dure and crystallized from EtOH: yield 81%; mp 244-249 °C
(mp 251-252 °C58); 1H NMR (CDCl3) δ 3.94 (s, 3H, OCH3),
3
3.98 (s, 3H, OCH3), 7.02 (d, 1H, J ) 8.0 Hz, H5-arom), 7.08-
7.22 (m, 2H, H2-arom, H6-arom), 7.71-7.87 (m, 3H, H5-phth,
H6-phth, H7-phth), 8.48-8.60 (m, 1H, H8-phth), 10.54 (bs, 1H,
NH); 13C NMR (CDCl3) δ 55.89 (OCH3), 110.79 (C5-arom),
112.19 (C2-arom), 122.01 (C6-arom), 126.79, 126.98 (CH-phth),
127.30, 128.21, 129.74 (C1-arom, C4a, C8a), 131.40, 133.28
(CH-phth), 147.93, 148.96, 149.74 (C3-arom, C4-arom, CdN),
159.96 (CdO). Anal. Calcd (C16H14N2O3): C, H, N.
4-(3,4-Dim eth oxyp h en yl)-5,6,7,8-tetr a h ydr o-2H-p h th a l-
a zin -1-on e (9b) was prepared from γ-keto acid 8b according
to the general procedure and crystallized from MeOH: yield
6-(3,4-Dim eth oxyp h en yl)-4,5-d ih yd r o-2H-p yr id a zin -3-
on e (3a ) was prepared from γ-keto acid 2a according to the
general procedure and crystallized from ethyl acetate: yield
1
82%; mp 214-215 °C; H NMR (CDCl3) δ 1.64-1.96 (m, 4H,
63%; mp 164-167 °C (mp 168-170 °C57); H NMR (CDCl3) δ
1
H6, H7), 2.40-2.58 (m, 2H, H5), 2.62-2.70 (m, 2H, H8), 3.91
(s, 3H, OCH3), 3.93 (s, 3H, OCH3), 6.86-7.03 (m, 3H, H-arom),
11.70 (bs, 1H, NH); 13C NMR (CDCl3) δ 20.70 (C7), 21.34 (C6),
22.95 (C8), 27.64 (C5), 55.73 (OCH3), 110.60 (C5-arom), 111.79
(C2-arom), 121.25 (C6-arom), 127.94 (C1-arom), 137.84, 140.76
(C4a, C8a), 148.51, 148.91, 149.23 (C3-arom, C4-arom, CdN),
161.67 (CdO). Anal. Calcd (C16H18N2O3): C, H, N.
2.53-2.68 (m, 2H, H4), 2.92-3.06 (m, 2H, H5), 3.93 (s, 3H,
OCH3), 3.94 (s, 3H, OCH3), 6.87 (d, 1H, 3J ) 8.4 Hz, H5-arom),
4
3
7.19 (dd, 1H, J ) 2.1 Hz, J ) 8.4 Hz, H6-arom), 7.42 (d, 1H,
4J ) 2.0 Hz, H2-arom), 8.77 (bs, 1H, NH); 13C NMR (CDCl3) δ
22.19 (5-CH2), 26.22 (4-CH2), 55.74, 55.78 (OCH3), 107.92 (C2-
arom), 110.17 (C5-arom), 119.07 (C6-arom), 128.08 (C1-arom),
148.99, 150.15, 150.56 (C3-arom, C4-arom, CdN), 167.80 (Cd
O). Anal. Calcd (C12H14N2O3): C, H, N.
cis-4-(3,4-Dim eth oxyp h en yl)-4a ,5,8,8a -tetr a h yd r o-2H-
p h th a la zin -1-on e (9c) was prepared from γ-keto acid 8c
according to the general procedure and crystallized from
6-(3,4-Dim eth oxyp h en yl)-5-m eth yl-4,5-d ih yd r o-2H-p y-
r id a zin -3-on e (3b) was prepared from γ-keto acid 2b accord-
ing to the general procedure and crystallized from EtOH: yield
80%; mp 188-191 °C; 1H NMR (CDCl3) δ 1.26 (d, 3H, 3J ) 7.4
1
EtOH: yield 86%; mp 173-174 °C; H NMR (CDCl3) δ 2.08-
2.37 (m, 3H, H5, H8), 2.85 (t, 1H, 3J ) 6.0 Hz, H8a), 2.91-
3.11 (m, 1H, H8′), 3.42 (dt, 1H, 3J ) 8.7 Hz, 3J ) 5.5 Hz, H4a),
3.93 (s, 3H, OCH3), 3.95 (s, 3H, OCH3), 5.63-5.88 (m, 2H, H6,
H7), 6.88 (d, 1H, 3J ) 8.4 Hz, H5-arom), 7.24 (dd, 1H, 4J )
3
2
Hz, CH3), 2.47 (dd, 1H, J ) 1.4 Hz, J ) 16.9 Hz, H4), 2.72
3
2
3
(dd, 1H, J ) 6.7 Hz, J ) 16.9 Hz, H4′), 3.36 (dq, 1H, J ) 7
Hz, H5), 3.93 (s, 3H, OCH3), 3.94 (s, 3H, OCH3), 6.88 (d, 1H,
3J ) 8.4 Hz, H5-arom), 7.22 (dd, 1H, 4J ) 2.1 Hz, 3J ) 8.4 Hz,
H6-arom), 7.46 (d, 1H, 4J ) 2.1 Hz, H2-arom), 8.78 (bs, 1H,
NH); 13C NMR (CDCl3) δ 16.25 (CH3), 27.69 (C5), 33.70 (C4),
55.73, 55.77 (OCH3), 108.11 (C2-arom), 110.20 (C5-arom),
118.91 (C6-arom), 127.06 (C1-arom), 149.11, 150.58 (C3-arom,
C4-arom), 153.60 (CdN), 166.82 (CdO). Anal. Cacld
(C13H16N2O3): C, H, N.
3
4
2.0 Hz, J ) 8.4 Hz, H6-arom), 7.48 (d, 1H, J ) 2.0 Hz, H2-
arom), 8.77 (bs, 1H, NH); 13C NMR (CDCl3) δ 21.35 (C8), 23.05
(C5), 31.09 (C4a), 33.90 (C8a), 55.75, 55.77 (OCH3), 107.85
(C2-arom), 110.18 (C5-arom), 119.01 (C6-arom), 124.06 (C6),
125.42 (C7), 127.23 (C1-arom), 149.13, 150.59 (C3-arom, C4-
arom), 154.31 (CdN), 169.69 (CdO). Anal. Calcd (C16H18
-
N2O3): C, H, N.
cis-4-(3,4-Dim eth oxyp h en yl)-4a ,5,6,7,8,8a -h exa h yd r o-
2H-p h th a la zin -1-on e (9d ) was prepared from γ-keto acid 8d
according to the general procedure and crystallized from
cis-6-(3,4-Dim eth oxyp h en yl)-4,5-d im eth yl-4,5-d ih yd r o-
2H-p yr id a zin -3-on e (3d ). γ-Keto acid 2d was treated with
hydrazine monohydrate according to the general procedure to
give two products (Rf 0.51 and 0.66) as indicated by TLC
analysis (ethyl acetate). Upon concentration of the organic
layer after workup, the two diastereomers crystallized. The
white solids were filtered off and purified by flash column
chromatography using ethyl acetate/petroleum ether (60-80)
6:4 to afford the minor compound 3d (cis), which was crystal-
1
EtOH: yield 75%; mp 170 °C; H NMR (CDCl3) δ 1.25-1.97
(m, 7H, H5, H6, H7, H8), 2.46-2.67 (m, 1H, H8′), 2.70-2.85
(m, 1H, H8a), 3.06-3.26 (m, 1H, H4a), 3.93 (s, 3H, OCH3),
3
3.94 (s, 3H, OCH3), 6.87 (d, 1H, J ) 8.4 Hz, H5-arom), 7.20
(dd, 1H, 4J ) 2.0 Hz, 3J ) 8.4 Hz, H6-arom), 7.45 (d, 1H, 4J )
2.0 Hz, H2-arom), 8.60 (bs, 1H, NH); 13C NMR (CDCl3) δ 21.87,
23.20, 24.37, 25.46 (C5, C6, C7, C8), 35.66 (C4a), 36.04 (C8a),
55.76 (OCH3), 108.02 (C2-arom), 110.13 (C5-arom), 118.91
(C6-arom), 127.26 (C1-arom), 149.10, 150.48 (C3-arom, C4-
1
lized from diethyl ether: yield 3%; mp 154-155 °C; H NMR
(CDCl3) δ 1.09 (d, 3H, J ) 7.3 Hz, 5-CH3), 1.28 (d, 3H, J )
3
3
3
3
arom), 153.55 (CdN), 169.72 (CdO). Anal. Calcd (C16H20
-
7.1 Hz, 4-CH3), 2.73 (dq, 1H, J ) 7 Hz, H4), 3.15 (dq, 1H, J
) 7 Hz, H5), 3.90 (s, 3H, OCH3), 3.91 (s, 3H, OCH3), 6.85 (d,
1H, 3J ) 8.4 Hz, H5-arom), 7.18 (dd, 1H, 4J ) 2.0 Hz, 3J ) 8.4
Hz, H6-arom), 7.42 (d, 1H, 4J ) 2.0 Hz, H2-arom), 8.47 (bs,
1H, NH); 13C NMR (CDCl3) δ 9.41 (5-CH3), 9.80 (4-CH3), 32.73
(C5), 34.59 (C4), 55.47, 55.53 (OCH3), 108.11 (C2-arom), 110.20
(C5-arom), 119.06 (C6-arom), 126.90 (C1-arom), 148.91, 150.41
(C3-arom, C4-arom), 155.66 (CdN), 170.83 (CdO). Anal. Calcd
(C14H18N2O3): C, H, N.
N2O3): C, H, N.
tr a n s-4-(3,4-Dim eth oxyph en yl)-4a,5,6,7,8,8a-h exah ydr o-
2H-p h th a la zin -1-on e (9e) was prepared from γ-keto acid 8e
according to the general procedure and crystallized from
1
EtOH: yield 70%; mp 143-146 °C; H NMR (CDCl3) δ 0.97-
1.48 (m, 4H, H-cyclohexane), 1.70-1.96 (m, 2H, H-cyclohex-
ane), 2.03-2.22 (m, 2H, H-cyclohexane), 2.42-2.75 (m, 2H,
H-cyclohexane), 3.92 (s, 6H, OCH3), 6.88 (m, 3H, H2-arom, H5-
arom, H6-arom), 8.72 (bs, 1H, NH); 13C NMR (CDCl3) δ 24.90,
25.27 (C6, C7), 25.59 (C8), 29.89 (C5), 38.62 (C4a), 40.14 (C8a),
55.76, 55.80 (OCH3), 110.33 (C5-arom), 110.75 (C2-arom),
120.29 (C6-arom), 128.48 (C1-arom), 148.50, 149.41 (C3-arom,
tr a n s-6-(3,4-Dim eth oxyp h en yl)-4,5-d im eth yl-4,5-d ih y-
d r o-2H-p yr id a zin -3-on e (3e). Further elution of the column
material (see 3d ) with ethyl acetate/petroleum ether (60-80)
6:4 gave the trans isomer 3e. This compound was crystallized
from diethyl ether: yield 25%; mp 178-180 °C; 1H NMR
(CDCl3) δ 1.19a, 1.23b (overlapping d, 3H each, 3J ) 7.6 Hz,
C4-arom), 157.50 (CdN), 169.70 (CdO). Anal. Calcd (C16H20
-
N2O3): C, H, N.
a4-CH3, 5-CH3), 2.51 (q, 1H, J ) 7.5 Hz, H4), 3.06 (q, 1H, J
) 7.4 Hz, H5), 3.93 (s, 3H, OCH3), 3.95 (s, 3H, OCH3), 6.88 (d,
1H, 3J ) 8.4 Hz, H5-arom), 7.21 (dd, 1H, 4J ) 2.0 Hz, 3J ) 8.4
Hz, H6-arom), 7.47 (d, 1H, 4J ) 2.0 Hz, H2-arom), 8.64 (bs,
1H, NH); 13C NMR (CDCl3) δ 16.34, 16.41 (CH3), 35.40 (C5),
39.35 (C4), 55.85, 55.92 (OCH3), 108.16 (C2-arom), 110.31 (C5-
Gen er a l P r oced u r e for Con d en sa tion of γ-Keto Acid s
w ith P h en ylh yd r a zin e. A mixture of γ-keto acid 2 or 8 (5.0
mmol) and phenylhydrazine (2 equiv) in toluene (50 mL) was
refluxed until TLC showed completion of the reaction (8-16
h). After the mixture had cooled to room temperature, the bulk
of toluene was removed in vacuo and the residue was dissolved
b
3
3

Novel Selective PDE4 Inhibitors. 1
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16 2519
in ethyl acetate. The solution was washed with water, aqueous
HCl (1 N), and dilute NaHCO3 and dried over MgSO4.
Evaporation of the solvent yielded the crude product, which
was purified by crystallization or flash column chromatography
and subsequent crystallization. The experimental data for the
respective compounds are listed below.
4J ) 2.0 Hz, H2-arom), 7.55-7.61 (m, 2H, H-Ph); 13C NMR
(CDCl3, 100.6 MHz) δ 10.25 (5-CH3), 11.01 (4-CH3), 33.34 (C5),
36.16 (C4), 55.80 (OCH3), 108.42 (C2-arom), 110.33 (C5),
119.30 (C6), 124.51 (CH-Ph), 126.08 (CH-Ph), 127.06 (C1-
arom), 128.26 (CH-Ph), 141.22 (C1-Ph), 149.09, 150.73 (C3-
arom, C4-arom), 156.09 (CdN), 168.27 (CdO). Anal. Calcd
(C20H22N2O3‚0.5H2O): C, H, N.
6-(3,4-Dim eth oxyp h en yl)-2-p h en yl-4,5-d ih yd r o-2H-p y-
r id a zin -3-on e (4a ) was prepared from γ-keto acid 2a accord-
ing to the general procedure and crystallized from MeOH:
tr a n s-6-(3,4-Dim eth oxyp h en yl)-4,5-d im eth yl-2-p h en yl-
4,5-d ih yd r o-2H-p yr id a zin -3-on e (4e). Further elution of the
column material (see 4d ) with ethyl acetate/petroleum ether
(60-80) 6:4 yielded trans isomer 4e. The pure compound was
obtained as an oil and could not be crystallized: yield 40%;
1H NMR (CDCl3, 400 MHz) δ 1.26 (d, 3H, 3J ) 7.4 Hz, 4-CH3),
1
yield 56%; mp 111 °C; H NMR (CDCl3) δ 2.71-2.85 (m, 2H,
H4), 3.02-3.17 (m, 2H, H5), 3.92 (s, 3H, OCH3), 3.93 (s, 3H,
OCH3), 6.89 (d, 1H, 3J ) 8.4 Hz, H5-arom), 7.22-7.36 (m, 2H,
H6-arom, H-Ph), 7.38-7.50 (m, 3H, H2-arom, H-Ph), 7.55-
7.68 (m, 2H, H-Ph); 13C NMR (CDCl3) δ 22.61 (C5), 27.95 (C4),
55.82 (OCH3), 108.34 (C2-arom), 110.27 (C5-arom), 119.39
(C6-arom), 124.80 (CH-Ph), 126.37 (CH-Ph), 128.07 (C1-
arom), 128.36 (CH-Ph), 141.15 (C1-Ph), 148.96, 150.72,
151.22 (C3-arom, C4-arom, CdN), 165.23 (CdO). Anal. Calcd
(C18H18N2O3): C, H, N.
3
3
3
1.30 (d, 3H, J ) 7.4 Hz, 5-CH3), 2.68 (dq, 1H, J ) 7.4 Hz, J
) 1.4 Hz, H4), 3.11 (dq, 1H, 3J ) 7.4 Hz, 3J ) 1.4 Hz, H5),
3.91 (s, 3H, OCH3), 3.92 (s, 3H, OCH3), 6.89 (d, 1H, J ) 8.4
3
Hz, H5-arom), 7.25-7.31 (m, 2H, H6-arom, H-Ph), 7.38-7.46
(m, 2H, H-Ph), 7.51 (d, 1H, 4J ) 2.0 Hz, H2-arom), 7.56-7.60
(m, 2H, H-Ph); 13C NMR (CDCl3, 100.6 MHz) δ 16.07, 16.54
(CH3), 35.44 (C5), 40.55 (C4), 55.77 (OCH3), 108.47 (C2-arom),
110.39 (C5), 119.36 (C6), 124.71 (CH-Ph), 126.32 (CH-Ph),
127.74 (C1-arom), 128.34 (CH-Ph), 141.20 (C1-Ph), 149.06,
150.79 (C3-arom, C4-arom), 153.65 (CdN), 168.18 (CdO).
6-(3,4-Dim eth oxyp h en yl)-5-m eth yl-2-p h en yl-4,5-d ih y-
d r o-2H-p yr id a zin -3-on e (4b) was prepared from γ-keto acid
2b according to the general procedure, purified by flash column
chromatography using ethyl acetate/petroleum ether (60-80)
6:4, and crystallized from ethyl acetate/petroleum ether: yield
4-(3,4-Dim et h oxyp h en yl)-2-p h en yl-2H -p h t h a la zin -1-
on e (10a ) was prepared from γ-keto acid 8a according to the
general procedure and crystallized from MeOH: yield 60%;
1
3
57%; mp 71-72 °C; H NMR (CDCl3) δ 1.33 (d, 3H, J ) 7.3
3
2
Hz, CH3), 2.65 (dd, 1H, J ) 1.3 Hz, J ) 16.9 Hz, H4), 2.90
(dd, 1H, 3J ) 6.4 Hz, 2J ) 16.9 Hz, H4′), 3.45 (dq, 1H, H5),
mp 220-221 °C; H NMR (CDCl3) δ 3.93 (s, 3H, OCH3), 3.96
1
3
3.92 (s, 3H, OCH3), 3.94 (s, 3H, OCH3), 6.89 (d, 1H, J ) 8.5
(s, 3H, OCH3), 7.01 (d, 1H, 3J ) 8.2 Hz, H5-arom), 7.16 (d,
1H, 4J ) 1.9 Hz, H2-arom), 7.21 (dd, 1H, 4J ) 1.9 Hz, 3J ) 8.2
Hz, H6-arom), 7.31-7.58 (m, 3H, H-Ph), 7.67-7.78 (m, 2H,
H-Ph), 7.79-7.93 (m, 3H, H5-phth, H6-phth, H7-phth), 8.55-
8.68 (m, 1H, H8-phth); 13C NMR (CDCl3) δ 55.89, 55.93
(OCH3), 110.88 (C5-arom), 112.45 (C2-arom), 122.20 (C6-arom),
125.70 (CH-Ph), 126.77 (CH-phth), 127.42 (Cq), 127.47 (CH-
Ph), 127.51 (C8-phth), 128.55 (CH-Ph), 128.73 (Cq), 129.10
(Cq), 131.46, 133.00 (CH-phth), 141.84 (C1-Ph), 147.26,
148.89, 149.76 (C3-arom, C4-arom, CdN), 158.71 (CdO). Anal.
Calcd (C22H18N2O3): C, H, N.
Hz, H5-arom), 7.23-7.38 (m, 2H, H6-arom, H-Ph), 7.40-7.50
(m, 2H, H-Ph), 7.51 (d, 1H, 4J ) 2.0 Hz, H2-arom), 7.58-
7.68 (m, 2H, H-Ph); 13C NMR (CDCl3) δ 16.34 (CH3), 28.07
(C5), 35.31 (C4), 55.80 (OCH3), 108.49 (C2-arom), 110.35 (C5-
arom), 119.33 (C6-arom), 124.67 (CH-Ph), 126.34 (CH-Ph),
126.99 (C1-arom), 128.37 (CH-Ph), 141.04 (C1-Ph), 149.08,
150.77 (C3-arom, C4-arom), 154.82 (CdN), 164.57 (CdO).
Anal. Calcd (C19H20N2O3‚0.4H2O): C, H, N.
6-(3,4-Dim eth oxyp h en yl)-4-m eth yl-2-p h en yl-4,5-d ih y-
d r o-2H-p yr id a zin -3-on e (4c). The mixture of 2b and 2c (0.74
g, 2.9 mmol) was treated with phenylhydrazine as described
above, resulting in two products (Rf 0.33 and 0.49) as indicated
by TLC analysis [ethyl acetate/petroleum ether (60-80) 1:1].
The remaining oil after workup was purified by flash column
chromatography using ethyl acetate/petroleum ether (60-80)
6:4 to give the major compound 4c, which was crystallized from
MeOH: yield 0.44 g (46%); mp 112-113 °C; 1H NMR (CDCl3)
4-(3,4-Dim eth oxyp h en yl)-2-p h en yl-5,6,7,8-tetr a h yd r o-
2H-p h th a la zin -1-on e (10b) was prepared from γ-keto acid
8b according to the general procedure, subjected to flash
column chromatography using ethyl acetate/petroleum ether
(60-80) 1:5 > ethyl acetate, and crystallized from diethyl
1
ether: yield 18%; mp 155-156 °C; H NMR (CDCl3) δ 1.67-
1.94 (m, 4H, H6, H7), 2.41-2.56 (m, 2H, H5), 2.66-2.80 (m,
2H, H8), 3.91 (s, 3H, OCH3), 3.92 (s, 3H, OCH3), 6.92 (d, 1H,
3J ) 8.2 Hz, H5-arom), 6.95 (d, 1H, 4J ) 1.9 Hz, H2-arom),
3
δ 1.38 (d, 3H, J ) 6.4 Hz, CH3), 2.69-2.91 (m, 2H, H4, H5),
3.06-3.30 (m, 1H, H5′), 3.92 (s, 3H, OCH3), 3.93 (s, 3H, OCH3),
6.88 (d, 1H, 3J ) 8.4 Hz, H5-arom), 7.22-7.33 (m, 2H, H6-
arom, H-Ph), 7.38-7.50 (m, 3H, H2-arom, H-Ph), 7.55-7.65
(m, 2H, H-Ph); 13C NMR (CDCl3) δ 15.32 (CH3), 30.21 (C5),
32.18 (C4), 55.82 (OCH3), 108.28 (C2-arom), 110.28 (C5-arom),
119.38 (C6-arom), 124.81 (CH-Ph), 126.26 (CH-Ph), 128.30
(CH-Ph), 128.39 (C1-arom), 141.37 (C1-Ph), 148.96, 150.67,
150.93 (C3-arom, C4-arom, CdN), 168.67 (CdO). Anal. Calcd
(C19H20N2O3): C, H, N.
4
3
7.01 (dd, 1H, J ) 1.9 Hz, J ) 8.2 Hz, H6-arom), 7.30-7.53
(m, 3H, H-Ph), 7.61-7.72 (m, 2H, H-Ph); 13C NMR (CDCl3)
δ 21.06 (C7), 21.48 (C6), 23.81 (C8), 27.64 (C5), 55.82, 55.85
(OCH3), 110.68 (C5-arom), 111.93 (C2-arom), 121.40 (C6-arom),
125.49 (CH-Ph), 127.62 (CH-Ph), 128.23 (C1-arom), 128.46
(CH-Ph), 138.64, 139.03 (C4a, C8a), 141.72 (C1-Ph), 147.94,
148.60, 149.33 (C3-arom, C4-arom, CdN), 159.74 (CdO). Anal.
Calcd (C22H22N2O3): C, H, N.
Further elution with ethyl acetate/petroleum ether (60-80)
6:4 gave 4b. This compound was crystallized from ethyl
acetate/petroleum ether: yield 70 mg (7%).
cis-4-(3,4-Dim eth oxyph en yl)-2-ph en yl-4a,5,8,8a-tetr ah y-
d r o-2H-p h th a la zin -1-on e (10c) was prepared from γ-keto
acid 8c according to the general procedure and crystallized
from diethyl ether: yield 50%; mp 134-135 °C; 1H NMR
(CDCl3) δ 2.18-2.40 (m, 3H, H5, H8), 2.93-3.17 (m, 2H, H8′,
H8a), 3.33-3.55 (m, 1H, H4a), 3.92 (s, 3H, OCH3), 3.93 (s, 3H,
cis-6-(3,4-Dim et h oxyp h en yl)-4,5-d im et h yl-2-p h en yl-
4,5-d ih yd r o-2H-p yr id a zin -3-on e (4d ). The γ-keto acid 2d
(0.80 g, 3.0 mmol) was treated with phenylhydrazine according
to the general procedure to give two products (Rf 0.49 and 0.66)
as indicated by TLC analysis [ethyl acetate/petroleum ether
(60-80) 1:1]. The oil obtained after workup was purified by
flash column chromatography using ethyl acetate/petroleum
ether (60-80) 6:4 to afford the minor compound 4d (cis), which
crystallized after concentration of the elute. The white solid
was recrystallized from diethyl ether and dried in vacuo: yield
60 mg (6%); mp 144-145 °C; 1H NMR (CDCl3, 400 MHz) δ
1.18 (d, 3H, 3J ) 7.3 Hz, 5-CH3), 1.35 (d, 3H, 3J ) 7.0 Hz,
3
OCH3), 5.52-5.88 (m, 2H, H6, H7), 6.89 (d, 1H, J ) 8.5 Hz,
H5-arom), 7.18-7.50 (m, 4H, H6-arom, H-Ph), 7.52 (d, 1H,
4J ) 2.0 Hz, H2-arom), 7.57-7.65 (m, 2H, H-Ph); 13C NMR
(CDCl3) δ 22.27 (C8), 23.61 (C5), 31.55 (C4a), 35.18 (C8a), 55.84
(OCH3), 108.22 (C2-arom), 110.31 (C5-arom), 119.36 (C6-arom),
123.61 (C6), 124.63 (CH-Ph), 125.80 (C7), 126.16 (CH-Ph),
127.22 (C1-arom), 128.26 (CH-Ph), 141.34 (C1-Ph), 149.14,
150.82 (C3-arom, C4-arom), 155.22 (CdN), 167.06 (CdO).
Anal. Calcd (C22H22N2O3): C, H, N.
tr a n s-4-(3,4-Dim eth oxyp h en yl)-2-p h en yl-4a ,5,8,8a -tet-
r a h yd r o-2H-p h th a la zin -1-on e (10f) was prepared from
γ-keto acid 8f according to the general procedure and subjected
to flash column chromatography using ethyl acetate/petroleum
3
3
4-CH3), 2.92 (dq, 1H, J ) 7.0 Hz, J ) 5.5 Hz, H4), 3.22 (dq,
3
3
1H, J ) 7.3 Hz, J ) 5.6 Hz, H5), 3.91 (s, 3H, OCH3), 3.92 (s,
3H, OCH3), 6.88 (d, 1H, 3J ) 8.4 Hz, H5-arom), 7.22-7.33 (m,
2H, H6-arom, H-Ph), 7.37-7.45 (m, 2H, H-Ph), 7.50 (d, 1H,

2520 J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16
Van der Mey et al.
ether (60-80) 1:5 > 2:3 > 1:1 to give the product as a colorless
oil, which was crystallized from diethyl ether: yield 34%; mp
170-171 °C; 1H NMR (CDCl3) δ 1.85-2.10 (m, 1H, H5), 2.23-
2.91 (m, 4H, H5′, H8, H8a), 3.04-3.28 (m, 1H, H4a), 3.91 (s,
6H, OCH3), 5.61-5.92 (m, 2H, H6, H7), 5.61-5.75 (m, 1H, H7),
5.77-5.92 (m, 1H, H6), 6.89 (d, 1H, 3J ) 8.9 Hz, H5-arom),
6.95-7.07 (m, 2H, H2-arom, H6-arom), 7.20-7.31 (m, 1H,
H-Ph), 7.32-7.45 (m, 2H, H-Ph), 7.50-7.62 (m, 2H, H-Ph);
13C NMR (CDCl3) δ 26.32 (C8), 30.55 (C5), 34.89 (C4a), 37.77
(C8a), 55.82, 55.87 (OCH3), 110.43 (C5), 110.64 (C2), 120.19
(C6), 124.68 (CH-Ph), 124.82 (C7), 125.77 (C6), 126.31 (CH-
Ph), 128.32 (CH-Ph), 128.75 (C1-arom), 140.97 (C1-Ph),
148.57, 149.64 (C3-arom, C4-arom), 156.44 (CdN), 166.87 (Cd
O). Anal. Calcd (C22H22N2O3) C, H, N.
2-Ben zyl-4-(3,4-d im et h oxyp h en yl)-2H -p h t h a la zin -1-
on e (11a ) was prepared from phthalazinone 9a according to
the general procedure and crystallized from ethyl acetate at 0
°C: yield 61%; mp 151-154 °C; 1H NMR (CDCl3) δ 3.90 (s,
3H, OCH3), 3.94 (s, 3H, OCH3), 5.45 (s, 2H, CH2-Bn), 6.98 (d,
1H, 3J ) 8.2 Hz, H5-arom), 7.07 (d, 1H, 4J ) 1.9 Hz, H2-arom),
7.13 (dd, 1H, J ) 1.9 Hz, J ) 8.2 Hz, H6-arom), 7.20-7.37
(m, 3H, CH-Bn), 7.42-7.57 (m, 2H, CH-Bn), 7.64-7.83 (m,
3H, H5-phth, H6-phth, H7-phth), 8.45-8.54 (m, 1H, H8-phth);
13C NMR (CDCl3) δ 54.77 (CH2-Bn), 55.88 (OCH3), 110.85 (C5-
arom), 112.47 (C2-arom), 122.10 (C6-arom), 126.55 (CH-phth),
127.14 (C8-phth), 127.50 (CH-Bn), 127.61, 128.23 (Cq-phth,
Cq-phth/C1-arom), 128.31 (CH-Bn), 128.62 (CH-Bn), 129.19
(Cq-phth/C1-arom), 131.16, 132.63 (CH-phth), 136.85 (C1-Bn),
146.71, 148.84, 149.69 (C3-arom, C4-arom, CdN), 158.82 (Cd
O). Anal. Calcd (C23H20N2O3): C, H, N.
2-Ben zyl-4-(3,4-d im eth oxyp h en yl)-5,6,7,8-tetr a h yd r o-
2H-p h th a la zin -1-on e (11b) was prepared from phthalazi-
none 9b according to the general procedure, subjected to flash
column chromatography using ethyl acetate/petroleum ether
(60-80) 1:3, and crystallized from diethyl ether: yield 57%;
mp 137-139 °C; 1H NMR (CDCl3) δ 1.53-1.87 (m, 4H, H6,
H7), 2.35-2.50 (m, 2H, H5), 2.57-2.73 (m, 2H, H8), 3.90 (s,
3H, OCH3), 3.92 (s, 3H, OCH3), 5.36 (s, 2H, CH2-Bn), 6.85-
7.01 (m, 3H, H2-arom, H5-arom, H6-arom), 7.21-7.40 (m, 3H,
CH-Bn), 7.43-7.58 (m, 2H, CH-Bn); 13C NMR (CDCl3) δ
21.01 (C7), 21.44 (C6), 23.70 (C8), 27.51 (C5), 55.12 (CH2-
Bn), 55.79 (OCH3), 110.66 (C5-arom), 111.99 (C2-arom), 121.35
(C6-arom), 127.52 (CH-Bn), 128.26 (CH-Bn), 128.40 (C1-
arom), 125.73 (CH-Bn), 136.52, 137.71, 138.83 (C4a, C8a, C1-
Bn), 147.33, 148.54, 149.26 (C3-arom, C4-arom, CdN), 159.83
(CdO). Anal. Calcd (C23H24N2O3): C, H, N.
cis-2-Ben zyl-4-(3,4-dim eth oxyph en yl)-4a,5,8,8a-tetr ah y-
dr o-2H-ph th alazin -1-on e (11c) was prepared from phthalazi-
none 9c according to the general procedure, subjected to flash
column chromatography using ethyl acetate/petroleum ether
(60-80) 1:3, and crystallized from diethyl ether: yield 61%;
mp 133-135 °C; 1H NMR (CDCl3) δ 1.81-2.35 (m, 3H, H5,
H8), 2.82 (t, 1H, 3J ) 5.8 Hz, H8a), 2.93-3.11 (m, 1H, H8′),
3.34 (dt, 1H, 3J ) 11.5 Hz, 3J ) 5.9 Hz, H4a), 3.91 (s, 3H,
OCH3), 3.92 (s, 3H, OCH3), 5.04 (AB, 2H, CH2-Bn), 5.55-5.87
(m, 2H, H6, H7), 6.85 (d, 1H, 3J ) 8.4 Hz, H5-arom), 7.14-
7.48 (m, 7H, H2-arom, H6-arom, H-Bn); 13C NMR (CDCl3) δ
22.08 (C8), 23.20 (C5), 31.37 (C4a), 34.34 (C8a), 52.41 (CH2-
Bn), 55.70, 55.78 (OCH3), 108.10 (C2-arom), 110.29 (C5-arom),
119.06 (C6-arom), 123.81 (C6), 125.68 (C7), 127.13 (CH-Bn),
127.33 (C1-arom), 128.20, 128.30 (CH-Bn), 137.72 (C1-Bn),
149.05, 150.59 (C3-arom, C4-arom), 153.90 (CdN), 167.13 (Cd
O). Anal. Calcd (C23H24N2O3): C, H, N.
4
3
cis-4-(3,4-Dim eth oxyph en yl)-2-ph en yl-4a,5,6,7,8,8a-h ex-
a h yd r o-2H-p h th a la zin -1-on e (10d ) was prepared from γ-ke-
to acid 8d according to the general procedure and crystallized
from ethyl acetate/petroleum ether (60-80): yield 47%; mp
1
122-124 °C; H NMR (CDCl3) δ 1.33-1.98 (m, 7H, H5, H6,
H7, H8), 2.53-2.69 (m, 1H, H8′), 2.85-3.00 (m, 1H, H8a),
3.11-3.28 (m, 1H, H4a), 3.91 (s, 3H, OCH3), 3.93 (s, 3H,
OCH3), 6.88 (d, 1H, 3J ) 8.4 Hz, H5-arom), 7.19-7.33 (m, 2H,
H6-arom, H-Ph), 7.37-7.49 (m, 2H, H-Ph), 7.51 (d, 1H, 4J
) 2.0 Hz, H2-arom), 7.55-7.68 (m, 2H, H-Ph); 13C NMR
a
(CDCl3) δ 21.73, 23.91a, 24.98, 25.61 (C5, C6, C7, C8), 36.11
(C4a), 37.12 (C8a), 55.82 (OCH3), 108.41 (C2-arom), 110.25
(C5-arom), 119.25 (C6-arom), 124.76 (CH-Ph), 126.14 (CH-
Ph), 127.27 (C1-arom), 128.26 (CH-Ph), 141.42 (C1-Ph),
149.08, 150.67 (C3-arom, C4-arom), 154.14 (CdN), 167.25 (Cd
O). Anal. Calcd (C22H24N2O3): C, H, N.
tr a n s-4-(3,4-Dim eth oxyp h en yl)-2-p h en yl-4a ,5,6,7,8,8a -
h exa h yd r o-2H-p h th a la zin -1-on e (10e) was prepared from
γ-keto acid 8e according to the general procedure and crystal-
lized from diethyl ether: yield 41%; mp 139-140 °C; 1H NMR
(CDCl3) δ 1.03-1.52 (m, 4H, H5, H6, H7, H8), 1.73-2.01 (m,
2H, H6′, H7′), 2.11-2.40 (m, 2H, H5′, H8a), 2.43-2.61 (m, 1H,
H8′), 2.65-2.86 (m, 1H, H4a), 3.90 (m, 3H, OCH3), 3.91 (m,
3H, OCH3), 6.82-6.99 (m, 3H, H2-arom, H5-arom, H6-arom),
7.16-7.29 (m, 1H, H-Ph), 7.30-7.43 (m, 2H, H-Ph), 7.50-
7.61 (m, 2H, H-Ph); 13C NMR (CDCl3) δ 24.84, 25.13 (C6, C7),
26.13 (C8), 30.08 (C5), 38.50 (C4a), 41.13 (C8a), 55.80, 55.88
(OCH3), 110.40 (C5-arom), 111.00 (C2-arom), 120.52 (C6-arom),
124.67 (CH-Ph), 126.20 (CH-Ph), 128.30 (CH-Ph), 128.57
(C1-arom), 140.99 (C1-Ph), 148.45, 149.48 (C3-arom, C4-arom),
158.31 (CdN), 167.70 (CdO). Anal. Calcd (C22H24N2O3): C, H,
N.
Gen er a l P r oced u r e for Ben zyla tion of P yr id a zin on es
a n d P h th a la zin on es. Sodium hydride (60% dispersion in
mineral oil, 1.1 equiv) was added to a suspension of the desired
pyridazinone 3 or phthalazinone 9 (3.0-5.0 mmol) in DMF
(30-50 mL). After the reaction mixture had been stirrred for
3 h, benzyl chloride (1.1 equiv) was added and the mixture
was stirred for another 4 h. The reaction mixture was poured
into water, and the product was extracted with ethyl acetate.
The combined organic extract was washed with water and
brine and dried over MgSO4. After removal of the solvent,
crystallization or flash column chromatography followed by
crystallization yielded the pure products. The experimental
data for the separate compounds are listed below.
2-Ben zyl-6-(3,4-d im eth oxyp h en yl)-4,5-d ih yd r o-2H-p y-
r id a zin -3-on e (5a ) was prepared from pyridazinone 3a ac-
cording to the general procedure, subjected to flash column
chromatography using ethyl acetate/petroleum ether (60-80)
1:1, and crystallized from diethyl ether: yield 49%; mp 128-
130 °C; 1H NMR (CDCl3) δ 2.55-2.69 (m, 2H, H4), 2.88-3.00
(m, 2H, H5), 3.91 (s, 6H, OCH3), 5.03 (s, 2H, CH2-Bn), 6.85
(d, 1H, 3J ) 8.4 Hz, H5-arom), 7.19 (dd, 1H, 4J ) 2.1 Hz, 3J )
8.4 Hz, H6-arom), 7.24-7.48 (m, 6H, H2-arom, H-Bn); 13C
NMR (CDCl3) δ 22.58 (C5), 27.03 (C4), 52.04 (CH2-Bn), 55.69,
55.79 (OCH3), 108.20 (C2-arom), 110.20 (C5-arom), 119.08 (C6-
arom), 127.17 (CH-Bn), 127.87 (C1-arom), 128.21, 128.29
(CH-Bn), 137.53 (C1-Bn), 148.87, 150.05, 150.51 (C3-arom,
C4-arom, CdN), 165.09 (CdO). Anal. Calcd (C19H20N2O3): C,
H, N.
cis-2-Ben zyl-4-(3,4-d im eth oxyp h en yl)-4a ,5,6,7,8,8a -h ex-
a h yd r o-2H -p h t h a la zin -1-on e (11d ) was prepared from
phthalazinone 9d according to the general procedure, subjected
to flash column chromatography using ethyl acetate/petroleum
ether (60-80) 2:3, and crystallized from diethyl ether/
petroleum ether (60-80): yield 53%; mp 117-118 °C; 1H NMR
(CDCl3) δ 1.19-1.93 (m, 7H, H5, H6, H7, H8), 2.50-2.68 (m,
1H, H8′), 2.69-2.79 (m, 1H, H8a), 3.00-3.18 (m, 1H, H4a),
3
3.91 (s, 6H, OCH3), 5.05 (AB, 2H, CH2-Bn), 6.84 (d, 1H, J )
8.3 Hz, H5-arom), 7.13-7.48 (m, 7H, H2-arom, H6-arom,
H-Bn); 13C NMR (CDCl3) δ 21.89, 23.77a, 24.58, 25.55 (C5,
C6, C7, aC8), 35.91 (C4a), 36.37 (C8a), 52.23 (CH2-Bn), 55.67,
55.75 (OCH3), 108.26 (C2-arom), 110.21 (C5-arom), 118.92 (C6-
arom), 127.05 (CH-Bn), 127.39 (C1-arom), 128.17, 128.24
(CH-Bn), 137.88 (C1-Bn), 148.99, 150.44 (C3-arom, C4-
arom), 152.83 (CdN), 169.72 (CdO). Anal. Calcd (C23H26
-
N2O3): C, H, N.
II. P h a r m a cology. Met h od s. In Vit r o Assa y of P DE
In h ibition . The PDE activity was determined according to
the method of Thompson et al.,59 with some modifications.49
The assay mixture contained 50 mM Tris-HCl (pH 7.4), 5 mM
MgCl2, 0.5 µM cAMP, [3H]cAMP (∼30000 cpm/assay), the
indicated concentration of the inhibitor, and an aliquot of
the enzyme solution (see further) at a final assay volume of
200 µL.

Novel Selective PDE4 Inhibitors. 1
J ournal of Medicinal Chemistry, 2001, Vol. 44, No. 16 2521
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Stock solutions of 2 mM were prepared in DMSO and
diluted 1:100 (v/v) in the Tris-HCl buffer mentioned above;
appropriate dilutions were prepared in 1% (v/v) DMSO/Tris-
HCl, which were diluted 1:2 (v/v) in the assays to obtain the
desired final concentrations of the inhibitors at a DMSO
concentration of 0.5% (v/v). None of the PDE activities were
affected by DMSO itself.
After preincubation for 5 min at 37 °C, the reaction was
started by the addition of substrate (cAMP), and the assays
were incubated for a further 15 min at 37 °C. The reaction
was terminated by the addition of 50 µL of 0.2 N HCl, and the
assays were left on ice for ∼10 min. Following incubation with
25 µg of 5′-nucleotidase (snake venom from Crotalus atrox) for
10 min at 37 °C, the assays were loaded on QAE Sephadex
A-25 columns (Econo Columns, Bio-Rad, 1 mL bed volume).
The columns were eluted with 2 mL of 30 mM ammonium
formiate (pH 6.0). The radioactivity of the elute was measured
and corrected for the corresponding blank values (measured
in the presence of denatured protein), which were below 5%
of total radioactivity. The amount of cAMP hydrolyzed did not
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PDE3 was investigated in homogenates from human blood
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PDE4 was investigated in the cytosol of human polymor-
phonuclear leukocytes (PMNL) (isolated from leukocyte con-
centrates);48 the PDE3-specific inhibitor motapizone (1 µM)
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St a t ist ics. The IC50 values were calculated from the
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