Design, synthesis and biological evaluation of novel tetrahydroisoquinoline derivatives as potential PDE4 inhibitors

Article abstract of DOI:10.1016/j.bmcl.2015.08.043

The design, synthesis, and biological evaluation of new phosphodiesterase type 4 (PDE4) inhibitors, which possessed 7-(cyclopentyloxy)-6-methoxy 1,2,3,4-tetrahydroisoquinoline ring, were described. Compound 8 [(7-cyclopentyloxy)-6-methoxy-3,4-dihydroisoqu

Full text of DOI:10.1016/j.bmcl.2015.08.043

Accepted Manuscript
Design, Synthesis and Biological Evaluation of Novel Tetrahydroisoquinoline
Derivatives as Potential PDE4 Inhibitors
Gaopeng Song, Dongsheng Zhao, Dekun Hu, Yasheng Li, Hongwei Jin, Zining
Cui
PII:
S0960-894X(15)00878-1
DOI:
http://dx.doi.org/10.1016/j.bmcl.2015.08.043
BMCL 23037
Reference:
Bioorganic & Medicinal Chemistry Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
6 July 2015
12 August 2015
17 August 2015
Please cite this article as: Song, G., Zhao, D., Hu, D., Li, Y., Jin, H., Cui, Z., Design, Synthesis and Biological
Evaluation of Novel Tetrahydroisoquinoline Derivatives as Potential PDE4 Inhibitors, Bioorganic & Medicinal
Chemistry Letters (2015), doi: http://dx.doi.org/10.1016/j.bmcl.2015.08.043
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Design, Synthesis and Biological Evaluation of Novel
Tetrahydroisoquinoline Derivatives as Potential PDE4 Inhibitors
Gaopeng Songa,b,#
,
c,#, Dekun Hu a,
a, Hongwei Jind, Zining Cui a,*
a
Guangdong Province Key Laboratory of Microbial Signals and Disease Control, South China
Agricultural University, Guangzhou 510642, China;
b College of Materials and Energy, South China Agricultural University, Guangzhou 510642, China;
c Department of pharmacy, Quanzhou Medical College, Quanzhou 362100, China
d
State Key Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking
University, Beijing 100191, China
#Both authors contributed equally to this paper.
*Corresponding author. Tel.: +86-20-85288229; Fax: +86-20-85288229
E-mail address: ziningcui@scau.edu.cn (Z. C.)
1

ABSTRACT:
The design, synthesis, and biological evaluation of new phosphodiesterase type 4 (PDE4) inhibitors,
which possessed 7-(cyclopentyloxy)-6-methoxy 1,2,3,4-tetrahydroisoquinoline ring, were described.
Compound 8 [(7-cyclopentyloxy)-6-methoxy-3,4-dihydroisoquinolin-2(1H)-yl) (4-hydroxy-3-methoxy-
phenyl) methanone] showed the best inhibitory activity and good selectivity against PDE4B. The
docking results showed that the catechol diether moiety of compound 8 played a key role to form
integral hydrogen bonds with PDE4B protein while the rest part of the molecule extended into the
catalytic domain to block the access of cAMP and formed the foundation for inhibition of PDE4.
Compound 8 would be great promise as a hit compound for further study based on the preliminary
structure-activity relationship and molecular modeling studies.
Keywords: 7-(cyclopentyloxy)-6-methoxy-1,2,3,4-tetrahydroisoquinoline derivatives; synthesis; PDE4
inhibitor; SAR; molecular simulation
2

Nowadays asthma and chronic obstructive pulmonary diseases (COPD) known as the kind of chronic
obstructive lung disease have been a major public health burden worldwide. COPD is projected to be
the third leading cause of death near the future. Thus, the development of highly effective drugs is
urgently needed because of the increasing mortality and morbidity caused by asthma and COPD.1,2
It is clear that cyclic nucleotide phosphodiesterases (PDEs) play a key role in the metabolism of the
second messengers cAMP and cGMP.3,4 The PDE4 (exists in four different isomers including PDE4A, B,
C and D) subtype as one of the important members of PDEs is responsible for selective high-affinity
hydrolysis of the second messenger cyclic adenosine monophosphate (cAMP). It is well established that
cyclic adenosine monophosphate (cAMP) is able to regulate the function of airway smooth muscle,
inflammatory cells, and immune cells.5,6 Therefore inhibition of PDE4 enzyme can result in increased
cellular levels of cAMP, which contribute to both the relaxation of airway smooth muscle and the
prevention of proinflammatory cell activation.7,8 Thus, inhibition of this enzyme has well established as
a potential target to treat conditions such as asthma and COPD.
The potential of PDE4 inhibitors as anti-inflammatory agents for the treatment of asthma and COPD
has received considerable attention.9-14 However, no selective PDE4 inhibitor has been released yet
because the pioneer compounds, had dose-limiting side effects such as nausea and emesis, that severely
restricted their therapeutic utility.15
The diether derivative of catechol class as the important type of selective PDE4 inhibitors is one of
the earliest and most extensively studied PDE4 inhibitors including rolipram (1, Fig.1), cilomilast (2,
Fig.1), piclamilast (3, Fig.1) and so on. On the basis of the early report,16 a detailed structure-activity
relationship (SAR) study about the diether derivative of catechol class suggested that the 4-(3,4-
dialkoxyphenyl) moiety (Fig.1) was important for PDE4 inhibition where the catechol ether oxygens
played a crucial role in binding to the enzyme. The substituent at the 4-position of the phenyl ring was
restricted to small lipophilic groups, preferably methoxy group, indicating that this group might occupy
a small lipophilic pocket. In contrast, various alkoxy substituents were well tolerated at the 3-position,
suggesting that 3-substituents either fill a large hydrophibic cavity or were located near the enzyme
3

surface. The superposition of the co-crystal structures of the dialkoxyphenyl family of compounds
structurally related to rolipram revealed a scaffold formed by a catechol, suggesting that
moiety played the important role in binding to the enzyme, of which
two alkoxy groups occupied each different lipophilic pocket and constructed H-bond to the purine-
selective glutamine residue, which is surrounded by the P-clamp.
Figure 1. The designed strategy for the title compounds.
It was proved that the rational conformational constraints, introduced unpalatable steric clashes, or
some combination of these effects was usually effective to gain higher selective activity.17,18 Herein we
describe a fruitful approach to conformational constraint by focusing on the modification of the amide
portion of the bioisosteric surrogates with the scaffold of tetrahydroisoquinoline17-19 while retaining the
promising catechol diether moiety. Based on the above results, in this report, we investigated both
replacing the substituents of the basic nitrogen as well as introducing conformational constraints into the
amide region of the molecules to design and synthesize
tetrahydroisoquinoline derivatives 4-15 (Fig.2). The inhibitory activity of all the title compounds was
evaluated. Preliminary structure-activity relationship and docking studies were carried out.
4

The synthetic route of
tetrahydroisoquinoline derivatives 4-15
was shown in Scheme 1. Treatment of the commercially available vanillin 16 with bromocyclopentane
and K2CO3 afforded the compound 17, followed by reaction with nitromethane in the presence of
ammonium acetate provided the intermediate 18. Then reduction of double bond and nitro group in
compound 18 with lithium aluminum hydride furnished the compound 19. Then the Bischler-
Napieralski reaction was adopted to elaborate the intermediate 21. Treatment of the compound 19 with
formic acid in the presence of EDC and DMAP afforded the compound 20, which was treated with
phosphorus oxychloride, then followed by reduction with sodium borohydride gave the important
intermediate 21. With the compound 21 and different carboxylic acid, their coupling reaction was then
performed in the presence of EDC and DMAP to provide the title compounds 4-8, 11-12. Compound 21
reacted with the carboxylic acids 22 and 23 to form the dehydrated coupling intermediates, of which Ac
or Boc group was removed with MeONa/MeOH or HCl/EtOAc to afford the title compounds 14-15,
respectively. The title compounds 9, 10, 13 were easily prepared by reduction of compound 5, 8, 12
with lithium aluminum hydride, respectively (see the supplementary data for the details).
Figure 2. Structures of title compounds 4-15.
5

Reagents and conditions: (a) bromocyclopentane, K2CO3, DMF, 93%; (b) CH3NO2, NH4OAc, 84%; (c)
LiAlH4, THF, 75% for 19, 94% for 9, 92% for 10, 90% for 13; (d) HCOOH, EDC, HOAt, DIPEA,
CH2Cl2, 84%; (e) i:POCl3, toluene, reflux; ii: NaBH4, MeOH, 63% for two steps; (f) carboxylic acid,
EDC, DMAP, CH2Cl2, 81% for 4, 83% for 5, 81% for 6, 79% for 7, 75% for 8, 76% for 11, 76% for 12;
(g) i:EDC, DMAP, CH2Cl2; ii:MeONa/MeOH, 63% for two steps; (h) i:EDC, DMAP, CH2Cl2;
ii:HCl/EtOAc, 66% for two steps.
Scheme 1. The synthetic route of the title compounds 4-15.
All the title compounds were tested for their PDE4 inhibitory potential (see the supplementary data
for the method). From Table 1, it showed that part of the title compounds exhibited considerable effects
on the PDE4B. Among them, compound 8 gave the best inhibition (IC50 = 2.3 μM), which was close to
6

that of the positive control rolipram (IC50 = 1.3 μM). As shown in Table 1, the different activities of the
compounds 4-8 with multiple substitution effects in ring B (Fig.2) demonstrated that the modification of
ring B was crucial. Considering the inhibition activity of compounds 4-7, in which the structural
variations were made at the ortho-position of the phenyl ring B, it was observed that, in general,
substitutions in the phenyl ring B by the halogen atoms -F (5, IC50 = 23.3 μM) and -Cl (6, IC50 = 15.5
μM) resulted in higher or similar activity in relation to the unsubstituted derivative (4, IC50 = 23.5 μM).
In addition, the attachment of methoxy groups at the ortho position of phenyl ring B led to weak activity
against PDE4B, as can be observed for compound 7 (IC50 = 37.0 μM). However, the attachment of
methoxy groups and hydroxyl groups to the phenyl ring B, substituted at the meta- and para-positions
simultaneously, generated the most active compound 8 (IC50 = 2.3 μM), which suggested that the
electron-donating groups at these positions would be favored to the bioactivity.
Table 1 IC50 values (μM) of title compounds 4-15 against PDE4B
Compd.
IC50 (μM)
23.5
Compd.
IC50 (μM)
37.9
15.7
38.1
>50
4
5
11
23.3
12
15.5
6
13
37.0
7
14
15
2.3
>50
8
43.0
1.3
9
rolipram (1)
26.8
10
A primary structure–activity relationship study showed that carbonyl group had great effects on the
bioactivity, when it was reduced to methylene, compared compound 8 (IC50 = 2.3 μM) with 10 (IC50 =
26.8 μM), 5 (IC50 = 23.3 μM) with 9 (IC50 = 43.0 μM), 12 (IC50 = 15.7 μM) with 13 (IC50 = 38.1 μM), the
activity decreased so much. The aromatic ring was also favored to the bioactivity, when the benzene or
pyridine ring was changed to cyclohexane (compounds 14 and 15), the activity sharply decreased. The
polarity of compounds 14 and 15, which contained three free hydroxyl groups and an amine
7

hydrochloride on the cyclohexane rings respectively, were quite different from the other compounds 4-
13. That could be a significant factor on the decreasing inhibitory activities.
Since inhibition of the subtype PDE4D was associated with the emetic response, therefore we
evaluated compounds 6, 8 and 12 for their PDE4B selectivity over PDE4D. The results displayed that
all the title compounds showed inhibitory effects on PDE4D below 20% (15.3%, 17.4% and 12.1%
respectively) at 20 μM indicating their selectivity towards PDE4B.
Given the inhibitory activity of title compounds, it was of interest to explore the binding to the PDE4
structure. The bioassay results demonstrated that compound 8 showed great promise as a novel lead
compound for further discovery. Docking simulation of compound 8 at PDE4B (PDB ID: 1XMY) was
conducted using Surflex-Dock in Sybyl 8.0 (see the supplementary data for the method),20,21 and the
docking contour maps were shown in Fig. 3. Importantly, this docking orientation revealed that the
catechol diether as the key pharmacophore (Fig. 1) formed integral hydrogen bonds with the conserved
glutamine residue (Gln443) (Fig. 3B and 3D) and the tetrahydroisoquinoline moiety was notablely
positioned between the phenylalanine (Phe446) and isoleucine (Ile410) (Fig. 3D), which formed the
cavity accommodating the hydrophobic moiety of compound 8. The remainder of the molecule was
displayed to extend into the catalytic domain in close to both the Zn2+ and Mg2+ cations (Fig. 3C), which
played important roles in the catalytic mechanism of cAMP hydrolysis. The para-hydroxyl group of
benzoyl imide formed hydrogen bond with aspartic acid (Asp275) and histidine (His234) to enhance the
binding affinity with the enzyme (Fig. 3D). Such orientation and interactions would block the access of
cAMP to the catalytic domain and formed the foundation for inhibition of PDE4.
In summary, we reported the design, synthesis, and biological evaluation of new phosphodiesterase
type 4 inhibitors, which possess 7-(cyclopentyloxy)-6-methoxy1,2,3,4-tetrahydroisoquinoline ring.
Compound 8 showed the best inhibitory activity among all the title compounds. A primary structure–
activity relationship study showed that carbonyl group and aromatic ring in the molecule were favored
to the bioactivity, which was very helpful to develop additional small molecule inhibitors with enhanced
activity. The docking results suggested that compound 8 interacted well with PDE4B protein where the
8

ether linkages at the catechol oxygen atoms played a key role in formation of integral hydrogen bonds
while the rest part of the molecule extended into the catalytic domain to block the access of cAMP and
formed the foundation for inhibition of PDE4. The formation of hydrogen bonds and the hydrophobic
interactions in the ligand-receptor complex were both vital for the binding affinity. Such efforts were of
great importance for the further development of novel and effective PDE4 inhibitors.
B
A
D
C
Figure 3. Model of PDE4 and docking of compound 8
Acknowledgement
We greatly acknowledge the financial supports from the Special Fund for the Research and
Construction of Public Service Ability (2014A020210019), the National Key Project for Basic Research
(973 Program, 2015CB150600), the Pearl River S&T Nova Program of Guangzhou (201506010029),
the National Natural Science Foundation of China (21202047 and 31472035), the Specialty and
Innovation Project from the Department of Education in Guangdong Province (2014KTSCX031).
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Graphical Abstract
Design, Synthesis and Biological Evaluation of Novel
Tetrahydroisoquinoline Derivatives as Potential PDE4 Inhibitors
Gaopeng Songa,b,#
,
c,#, Dekun Hu a,
a, Hongwei Jind, Zining Cui a,*
12