Synthesis and CYP24 inhibitory activity of 2-substituted-3,4-dihydro-2H- naphthalen-1-one (tetralone) derivatives

Article abstract of DOI:10.1016/j.bmcl.2004.08.040

The synthesis of novel 2-benzyl- and 2-benzylidene-3,4-dihydro-2H- naphthalen-1-one derivatives and their inhibitory activity versus kidney mitochondrial 25-hydroxyvitamin D₃ 24-hydroxylase (CYP24) is described. The synthesis of novel 2-benzyl-

Full text of DOI:10.1016/j.bmcl.2004.08.040

Bioorganic & Medicinal Chemistry Letters 14 (2004) 5651–5654
Synthesis and CYP24 inhibitory activity of
2-substituted-3,4-dihydro-2H-naphthalen-1-one
(tetralone) derivatives
Sook Wah Yee and Claire Simons*
Welsh School of Pharmacy, Cardiff University, King Edward VII Avenue, Cardiff CF10 3XF, UK
Received 6 July 2004; revised 17 August 2004; accepted 18 August 2004
Available online 17 September 2004
Abstract—The synthesis of novel 2-benzyl- and 2-benzylidene-3,4-dihydro-2H-naphthalen-1-one (tetralone) derivatives and their
inhibitory activity versus kidney mitochondrial 25-hydroxyvitamin D3 24-hydroxylase (CYP24) is described. The 2-benzylidenetetr-
alone derivatives were found to be very weak inhibitors (IC50 20 >100lM), whereas the 2-benzyltetralone derivatives showed prom-
isinginhibitory activity (IC 0.9lM for the most active derivative) compared with ketoconazole (IC50 20lM).
50
Ó 2004 Elsevier Ltd. All rights reserved.
Prostate cancer, beingthe second leadingcause of can-
cer death in human males, is a major disease for thera-
peutic intervention.1 Androgens play an important role
in the development, growth and progression of prostate
cancer,2 therefore androgen ablation therapy by gona-
dotropin suppression and androgen receptor blockade
are current methods of treatment.3 Although most pa-
tients respond well to this therapy, eventually many tu-
mours recur as a result of transition of the cancer cells to
androgen-independent growth.4 Owingto the lack of
effective treatments for androgen-independent meta-
static prostate cancer, alternative strategies, such as Ôdif-
ferentiation therapyÕ may be useful.5 Pro-differentiating
agents of interest include retinoic acid (vitamin A) and
vitamin D3 (and their analogues).
of such analogues is uncertain owing to additional phar-
macological effects such as transcaltachia (elevation of
intracellular calcium activation of intestinal calcium up-
take).7 In addition the natural substrate calcitriol, and
derivative VDR agonists, are rapidly metabolised into
less active metabolites by the 24-hydroxylase (CYP24)
resultingin a very limited use for either calcitriol or its
8
derivatives as differentiatingaegnts.
Therefore, com-
pounds capable of inhibitingCYP24, the cytochrome
P450 enzyme that initiates calcitriol metabolism, would
have the effect of increasingendogenous levels of calcit-
riol so enhancingits differentiatingcapabilities.
Known inhibitors of CYP24 include (i) the CYP26 and
CYP17 (P450 17, 17,20-lyase) inhibitor liarozole9 and
(ii) ketoconazole, the nonspecific competitive inhibitor
of cytochrome P450-catalysed reactions, which inhibit
both CYP24 and 1a-hydroxylase.10 More selective
CYP24 inhibitors have been described such as SDZ
89-443 and VID400.11 All these inhibitors have a char-
acteristic nitrogen heterocyclic moiety capable of coor-
dinatingto the Fe 3+-haem component of the P450
active site.
1a,25(OH)2D3 (calcitriol) is the hormonally active
metabolite of vitamin D3, which functions as an antipro-
liferative and pro-differentiatingagent, especially in epi-
thelial and hematopoietic cells.6 The use of vitamin D
analogues (VDR agonists) as differentiating agents has
been studied,6 however the overall therapeutic activity
We have recently demonstrated the CYP26 inhibitory
activity of a series of 2-(4-aminophenylmethyl)-tetralone
derivatives,12 therefore tetralones with varyingsubstitu-
ents in both naphthalene and 2-aryl moieties were syn-
thesised and evaluated for CYP24 inhibitory activity.
Keywords: 2-Substituted-3,4-dihydro-2H-naphthalen-1-one (tetralone)
derivatives; Enzyme inhibition; 24-Hydroxylase (CYP24); Differenti-
atingtherapy.
*
Correspondingauthor. Tel.: +44 02920 876307; fax: +44 02920
874149; e-mail: simonsc@cardiff.ac.uk
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.08.040

5652
S. W. Yee, C. Simons / Bioorg. Med. Chem. Lett. 14 (2004) 5651–5654
The method employed for the preparation of the 6-
methoxy-2-(phenylmethylidene)-3,4-dihydro-2H-naph-
thalen-1-one derivatives (2a–d) involved direct conden-
sation of the commercially available tetralone (1) with
the appropriate benzaldehyde in ethanolic KOH solu-
tion.13 This method was successfully used in the absence
of a hydroxyl group on the tetralone or benzaldehyde
(Scheme 1).
the presence of Pd(PPh3)4 catalyst14 (Scheme 3). The
reduced 2-(benzyl)-3,4-dihydro-2H-naphthalen-1-one
derivatives (3a–b, 6, 9, 12a–b and 15) were readily ob-
tained by hydrogenation with 10% Pd/C catalyst for
1h, at approximately 30psi (Parr hydrogenator). When
the hydrogenation reaction was allowed to proceed
for 2h, deoxygenation at C1 was found to occur (7,
Scheme 2).
The synthesis of hydroxyl derivatives (5, 8 and 14)
required initial protection of one or both benzaldehyde
and tetralone hydroxyl groups (see Schemes 2–4, respec-
tively) with the tetrahydropyran (THP) protecting
group, which was stable under the basic ethanolic
KOH condensation conditions.
The dihydroxy derivative 15 was prepared from the cor-
respondingTHP protected tetralone and benzaldehyde
(Scheme 4).
The nine 2-(benzylidene)-(2a–d, 5, 8, 11a, 11b and 14)
and eight 2-(benzyl)-tetralone (3a–b, 6, 7, 9, 12a–b and
15) derivatives were evaluated for their inhibitory activity
versus CYP24 usingrat kidney mitochondria. The assay
performed was based on a modification of the general
The 6-biphenyl substituted derivatives (11a–b) were pre-
pared by Suzuki couplingwith phenylboronic acid in
R1
O
R1
O
O
i
ii
R2
H3CO
R2
H3CO
H3CO
R3
R3
1
2a-d
3a-b
Scheme 1. Reagents and conditions: (i) R–C6H4CHO, 4% KOH/EtOH, rt, 1–72h; (ii) 10% Pd/C, H2, MeOH, rt, 1h [a, R1 = R3 = CF3, R2 = H; b,
R1 = CH3, R2 = R3 = H; c, R2 = NMe2, R1 = R2 = H; d, R1 = Br, R2 = R3 = H].
O
O
O
ii
i
H3CO
OH
H3CO
H3CO
OTHP
iii
1
5
4
iv
O
H3CO
OH
H3CO
OH
6
7
Scheme 2. Reagents and conditions: (i) THPO–C6H4CHO, 4% KOH/EtOH, rt, 12h; (ii) 2M HCl (aq), EtOAc/2-butanone (1:1 v/v), reflux, 1h;
(iii) 10% Pd/C, H2, MeOH, rt, 1h; (iv) 10% Pd/C, H2, MeOH, rt, 2h.
O
O
O
iii
i
ii
HO
NMe2
HO
NMe2
THPO
8
4
9
iv
O
R1
O
O
iii
v
Ph
Ph
THPO
R2
ii
HO
HO
10a, R1 = Br, R2 = H
10b, R1 = H, R2 = Br
11a, 2-Ph
11b, 4-Ph
12a, 2-Ph
12b, 4-Ph
Scheme 3. Reagents and conditions: (i) Me2N–C6H4CHO, 4% KOH/EtOH, rt, 72h; (ii) 2M HCl (aq), EtOAc/2-butanone (1:1 v/v), reflux, 1h;
(iii) 10% Pd/C, H2, MeOH, rt, 1h; (iv) Br–C6H4CHO, 4% KOH/EtOH, rt, 1–4h; (v) phenylboronic acid, Pd(PPh3)4, toluene, 100°C, 5h.

S. W. Yee, C. Simons / Bioorg. Med. Chem. Lett. 14 (2004) 5651–5654
5653
O
THPO
O
4
O
iii
i
+
O
ii
HO
OH
HO
OH
H
15
14
OTHP
13
Scheme 4. Reagents and conditions: (i) 4% KOH/EtOH, rt, 1h; (ii) 2M HCl (aq), EtOAc/2-butanone (1:1 v/v), reflux, 1h; (iii)10% Pd/C, H2, MeOH,
rt, 1h.
procedure previously described for CYP26,15 using
[26,27-methyl-3H]-25-hydroxyvitamin D3 (from a stock
mixture containing100 lL of [26,27-methyl-3H]-25-
hydroxyvitamin D3 and 1mL of unlabelled 25-hydroxy-
vitamin D3 (25lM), with a total of 5lCi of radioactivity
in the 1mL of stock mixture), NADPH, inhibitor (vary-
ingconcentrations usingacetonitrile as solvent) and
phosphate buffer (pH7.4). After incubation in a shaking
water bath for 30min at 37°C, the vitamin D metabolites
were obtained by extraction with ethyl acetate. After
evaporation of the organic solvent, the residue was ana-
lysed by a HPLC system connected to a b-RAM online
scintillation detector, connected to a Compaq PC run-
ningLaura data acquisition and analysis software (Lab-
Logic Ltd). The separated [3H]-metabolites were
quantitatively calculated from the areas under the curves.
Usinga control with acetonitrile instead of inhibitor,
these results were expressed as Ôpercentage inhibition rel-
ative to controlÕ = 100[metabolites (control)Àmetabo-
lites (inhibitor)/(metabolites control)]%. Ketoconazole
was used as a standard for comparison (Table 1).
alkyl or aryl substituent at the 2-benzyl position, for
example, 2-(2-methylbenzyl)- and 2-(2-biphenyl)-deriva-
tives 3b and 12a (IC50 0.9 and 2.1lM, respectively), re-
sulted in good activity, whereas introduction of an aryl
substituent at the 4-benzyl position, for example, 2-(4-
biphenyl)-derivative 12b (IC50 >20lM) substantially re-
duced activity. Introduction of a hydroxyl group at the
4-benzyl position was tolerated, for example 6 and 7
(IC50 3.5 and 2.6lM, respectively), however introduc-
tion of a more bulky moiety, for example, N,N-dimethyl
in compound 9 (IC50 18lM) was not tolerated. These
preliminary results suggest that the compounds may be
orientated in the active site with a hydrophobic region
or large pocket above the 2-position of the benzyl ring,
and a small pocket containingan amino-acid residue
capable of forminga hydrogen bond at the 4-benzyl
position.
There would appear to be a slight preference for a 6-
methoxy rather than a 6-hydroxy substitutent in the
naphthalene ring(cf. 6 and 15, IC50 3.5 and 8.9lM,
respectively). However, no significant difference in inhib-
itory activity was noted for the tetralone and 1,2,3,4-
tetrahydro-naphthalene structures (cf. 6 and 7, IC50
3.5 and 2.6lM, respectively). Compared with ketocon-
azole, potent inhibitory activity was observed for the
2-(2-methylbenzyl)tetralone derivatives 3b, which may
The benzylidene derivatives were all poor inhibitors of
CYP24, perhaps indicatingthe requirement for flexibil-
ity at the C2 position for optimal structure conforma-
tion with respect to interaction at the enzyme active
site. In the 2-benzyltetralone series, introduction of an
Table 1. IC50 data for the novel benzylidene and benzyl tetralone derivatives
Benzylidene tetralones
Benzyl tetralones
O
O
X
X
RO
OH
RO
RO
Compound
IC50 (lM)
Compound
IC50 (lM)
2a
2b
2c
2d
5
>100
>20
3a
3b
4.5
0.9
3.5
2.6
18
>100
100
>20
6
7
9
12a
8
>100
>100
>100
>20
2.1
>20
8.9
20
11a
11b
14
12b
15
Ketoconazole
IC50 values are the mean of two experiments.

5654
S. W. Yee, C. Simons / Bioorg. Med. Chem. Lett. 14 (2004) 5651–5654
7. Baran, D. T. Nongenomic Rapid Effects of Vitamin D. In
Vitamin D: Molecular Biology, Physiology, and Clinical
Applications; Horlick, M. F., Ed.; Humana: Totowa, NJ,
1999; pp 195–205.
be a useful lead compound for further development of
potent CYP24 inhibitors.
8. Jones, G.; Strugnell, A. S.; DeLuca, H. F. Physiol. Rev.
1998, 78, 1193–1231.
Acknowledgements
9. Zhao, J.; Tan, B. K.; Marcelis, S.; Verstuyf, A.; Bouillon,
R. J. Steroid Biochem. 1996, 57, 197–202.
10. Reinhardt, T. A.; Horst, R. L. Arch. Biochem. Biophys.
1989, 272, 459–465.
Miss Sook Wah Yee would like to acknowledge
the ORS Awards Scheme for a United Kingdom
Scholarship.
11. Schuster, I.; Egger, H.; Bikle, D.; Herzig, G.; Reddy, G.
S.; Stuetz, A.; Stuetz, P.; Vorisek, G. Steroids 2001, 66,
409–422.
References and notes
12. Kirby, A. J.; LeLain, R.; Mason, P.; Maharbouie, F.;
Nicholls, P. J.; Smith, H. J.; Simons, C. J. Enzym. Inhib.
Med. Chem. 2003, 18, 27–33.
1. Greenlee, R. T.; Murray, T.; Bolden, S.; Wingo, P. A.
CA-Cancer J. Clin. 2000, 50, 7–33.
2. Jarman, M.; Smith, H. J.; Nicholls, P. J.; Simons, C. Nat.
Prod. Rev. 1998, 15, 495–512.
13. Rapson, W. S.; Shuttleworth, R. G. J. Chem. Soc. 1940,
636–641.
3. Smith, H. J.; Nicholls, P. J.; Simons, C.; LeLain, R. Exp.
Opin. Ther. Pat. 2001, 11, 789–824.
14. Weskamp, T.; Bohm, V. P. W.; Herrmann, W. A.
J. Organomet. Chem. 1999, 585, 348–352.
4. Isaacs, J. T. Urol. Clin. N. Am. 1999, 26, 263–273.
5. Zhao, X.-Y.; Feldman, D. Steroids 2001, 66, 293–300.
6. Bikle, D. D. Endocrine Rev. 1992, 13, 765–784.
15. Kirby, A. J.; LeLain, R.; Mason, P.; Maharbouie, F.;
Nicholls, P. J.; Smith, H. J.; Simons, C. J. Enzym. Inhib.
Med. Chem. 2002, 17, 321–327.