Estrogen receptor ligands. Part 5: The SAR of dihydrobenzoxathiins containing modified basic side chains

Article abstract of DOI:10.1016/j.bmcl.2004.04.100

Dihydrobenzoxathiin analogs (1-11) with modifications on the basic side chain region were prepared and evaluated for estrogen/anti-estrogen activity in both in vitro and in vivo models. The compounds generally maintained a high degree of selectivity for ERα over ERβ, similar to the original lead compound I. Many of the compounds also maintained high potency in the inhibition of human carcinoma MCF-7 cell growth. However, all were less potent in the inhibition of estradiol-triggered uterine growth. This work demonstrates the sensitive nature of modification to the antagonist basic side chain region.

Full text of DOI:10.1016/j.bmcl.2004.04.100

Bioorganic & Medicinal Chemistry Letters 14 (2004) 3747–3751
Estrogen receptor ligands. Part 5: The SAR of
dihydrobenzoxathiins containing modified basic side chains
Qiang Tan,a,* Elizabeth T. Birzin,b Wanda Chan,b Yi Tien Yang,b Lee-Yuh Pai,b
Edward C. Hayes,b Carolyn A. DaSilva,b Frank DiNinno,a Susan P. Rohrer,b
James M. Schaefferb and Milton L. Hammonda
aDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, 800B-107, Rahway, NJ 07065, USA
bDepartment of Atherosclerosis and Endocrinology, Merck Research Laboratories, PO Box 2000, 800B-107, Rahway, NJ 07065, USA
Received 18 March 2004; revised 29 April 2004; accepted 30 April 2004
Available online 2 June 2004
Abstract—Dihydrobenzoxathiin analogs (1–11) with modifications on the basic side chain region were prepared and evaluated for
estrogen/anti-estrogen activity in both in vitro and in vivo models. The compounds generally maintained a high degree of selectivity
for ERa over ERb, similar to the original lead compound I. Many of the compounds also maintained high potency in the inhibition
of human carcinoma MCF-7 cell growth. However, all were less potent in the inhibition of estradiol-triggered uterine growth. This
work demonstrates the sensitive nature of modification to the antagonist basic side chain region.
Ó 2004 Elsevier Ltd. All rights reserved.
Selective estrogen receptor modulators (SERMs) are
unnatural ligands of the estrogen receptor and
show tissue-selectivity in regulating the receptor activi-
ties, and thus have the potential to effectively treat
Figure 1. Representative SERMs.
Keywords: Dihyrobenzoxathiin; Estrogen receptor; SERM; SERAM.
* Corresponding author. Tel.: +1-732-594-1276; fax: +1-732-594-9556; e-mail: qiang_tan@merck.com
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.04.100

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Q. Tan et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3747–3751
Figure 2. Modified dihydrobenzoxathiins.
estrogen-related disorders while avoiding the adverse
side-effects of the natural hormones.1 SERMs have
generated a tremendous amount of interest in pharma-
ceutical research, in which several entities, a–e, are
currently marketed or in late stage clinical development
for the treatment and prevention of osteoporosis or
estrogen-sensitive cancers (Fig. 1).2 All of these com-
pounds exhibit less than 10-fold selectivity in the bind-
ing to ERa and ERb. Our own effort in this area focused
on finding SERMs with significant ER binding selec-
tivity, since the ER subtypes are expected to have dis-
tinctly different functions. Recently, we identified a new
class of dihydrobenzoxathiin-based SERMs, such as I,
which exhibited greater than 50-fold selectivity favoring
ERa, and thus were characterized as SERAMs (Selec-
tive Estrogen Receptor Alpha Modulators).3
interaction between the basic amine side-chain moiety
and the acidic amino acid residues in the ER provide a
major force in stabilizing the receptor in the inactive
state, the precise positioning as well as the electrophys-
ical character of the amine moiety is critical for SERM
function. Therefore, it should be valuable to examine
various side chain modifications in order to have a clear
understanding of the SAR. It is from this perspective
that we herein disclose our recent SAR work on the side
chain region of the benzoxathiin platform, as studied in
compound classes I–III, wherein compounds 1–11 were
synthesized (Fig. 2). (For more information on the
preparation of relevant structures, see Refs. 3 and 5.)
Scheme 1 depicts the syntheses of compounds 1, 2 and 4.
Alkylation of thiol 124 followed by O-acylation set the
stage for an oxygen to carbon acyl transfer process.
Under the influence of base, upon warming of the
reaction mixture from )78 °C to ice-water temperatures
and aging overnight, the acyl group in 14 was efficiently
migrated to the adjacent benzylic carbon next to sulfur.
Most second and third generation SERMs, including
our benzoxathiin platform, share a common feature in
the side chain region: an aminoethyl linkage tethered to
a phenolic oxygen atom.1 Since it is postulated that the
Scheme 1. Synthesis of 1, 2 and 4. Reagents and conditions: (a) 4-benzyloxybenzyl chloride, Hunig’s base, CH2Cl2, 0 °C, 83%. (b) 4-Iodobenzoyl
chloride, Hunig’s base, DMAP, CH2Cl2. (c) 3 equiv LHMDS, THF, )78 °C 1 h, then 0 °C 14 h, two steps 68% for 15 and 31% for 21. (d) TFA,
Et3SiH, CH2Cl2, 86%. (e) Propargyl alcohol, PdCl2(PPh3)2, CuI, K2CO3, THF, 40%; (f) CBr4, PPh3, CH2Cl2. (g) Piperidine, 81% two steps for 19,
41% from 21 toward 1. (h) H2, Pd black, ammonium formate, EtOH/EtOAc/H2O ¼ 7:2:1, 46% for 2, 44% for 1, 35% for 4. (i) Methyl 4-chloro-
carbonylbenzoate, Hunig’s base, CH2Cl2, DMAP. (j) (i) CH2Cl2, TFA, Et3SiH; (ii) DIBAL, CH2Cl2. (k) 1-Piperidineethanol, NaH, THF, Bu4NI, ca.
23% from 21.

Q. Tan et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3747–3751
3749
The ketone 15, thus formed, was treated with TFA–
Et3SiH (TES) to furnish the dehydrative reductive
cyclization product 16, in a stereo-controlled fashion, as
previously reported by Kim et al.5 Sonogashira coupling
yielded propargyl alcohol 17, which in turn was con-
verted to bromide 18, and subsequently displaced by
piperidine to yield 19. Finally, deprotection of the ben-
zyl groups under transfer-hydrogenation conditions
provided target 2. Similarly, 1 and 4 were prepared.
Removal of the two TIPS protecting groups yielded
compound 3.
As shown in Scheme 3, the synthesis of 5–11 was
achieved using the aryl triflate 37 as a versatile synthon
for the installation of various side chains. Thus, the
piperazine and homopiperazine side chains 7–11 were
readily installed via Buchwald coupling reaction,8 and
the piperidine side chains 5 and 6 via the Stille coupling
reaction with 43.9
The synthesis of 3 is shown in Scheme 2. A copper(I)
mediated coupling reaction6 between BrZn(CH2)3CN
and 4-methoxy phenylacetyl chloride gave rise to ketone
25. Following a protecting group change from methyl to
TIPS, the a-position of ketone 27 was brominated and
subsequent displacement by mercaptan 12 furnished 29.7
Dehydrative, reductive cyclization as before produced
30. After the benzyl protecting group was replaced by
TIPS in a two-step sequence, the nitrile moiety was
reduced to aldehyde 31, and subsequent Wittig olefin-
ation gave rise to 32. The side chain construction was
completed by hydrogenation of the double bond, de-
benzylation to expose the terminal hydroxyl, conversion
to bromide 35, and finally displacement by piperidine.
The compounds were evaluated for ER binding affinity,
the ability to antagonize the effect of estradiol on the
growth of the MCF-7 cancer cell line, and the effect on
the stimulation of an immature rat uterus in the pres-
ence and absence of estradiol. It is clear from the data in
Table 1 that compared to the parent compound I, all of
the new compounds except 1 and 10 lead to significantly
increased agonism in immature rat uterine weight assay,
despite the similarity in receptor binding and estradiol
antagonism in MCF-7 cells. Compounds 1–4 were pre-
pared to evaluate the effect of the linker length. Thus the
slightly shorter linker 1 gave rise to an inactive com-
pound, whereas 2 with a slightly longer linker exhibited
Scheme 2. Synthesis of 3. Reagents and conditions: (a) BrZn(CH2)3CN, CuCN, LiBr, THF. (b) Pyridinium hydrochloride, 190 °C. (c) TIPSCl,
Hunig’s base, DMF, 52% three steps. (d) PTAB, 0 °C, CH2Cl2. (e) 12, Et3N, CH2Cl2, 55% two steps. (f) TFA, Et3SiH, CH2Cl2, 59%. (g) (i) Pd black,
formic acid, EtOAc; (ii) TIPSCl, Hunig’s base, DMF; (iii) DIBAL, toluene, CH2Cl2. (h) BnO(CH2)3PþPh3Brꢀ, BuLi, THF, 77% from 30. (i) Pd,
EtOH–EtOAc, H2. (j) Pd black, EtOH–EtOAc, formic acid. (k) CBr4, CH2Cl2, PPh3. (l) (i) Piperidine, THF; (ii) TBAF, THF; 89% from 32.
Scheme 3. Synthesis of 5–11. Reagents and conditions: (a) (i) NaH, Tf2NPh, THF; (ii) Pd black, formic acid, EtOAc, EtOH; (iii) TIPSCl, Hunig’s
base, DMF; 93% overall based on recovered SM; (b) (i) 41 or 42, Pd2dba3, 44, K3PO4, toluene, 80 °C; (ii) TBAF, THF, 34–90% overall. (c) 43,
Pd2dba3, PdCl2(PPh3)2, K3PO4, NMP, LiCl, 100 °C, 50%. (d) EtOAc, Pd, H2. (e) Pd black, formic acid, EtOAc, 48%. (f) TBAF, THF, >90% for 5,
67% from 38 to 6.

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Q. Tan et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3747–3751
Table 1. Assayf data
piperazines and homopiperazines 7–11. Although these
Compound Binding affinity,a
IC50 (nM)
MCF-7
Uterine weightc
modifications slightly improved the antagonism/agon-
ism profile, as exemplified by 7 and 9, the uterine weight
results for 8 and 10–11 once again demonstrated the
extreme sensitivity of the receptor in response to subtle
changes around the basic amine region.
Inhibitionb % inhibition/
IC50 (nM) % control
(antagonism/
ERa/ERb (fold
selective for ERa)
agonism)
1
155/4372 (28)
1.4/186 (133)
0.7/27 (39)
––
6.7
0/3
2
29/73
41/48
52/46
54/45
54/40
60/38
47/40
66/37
43/0
In conclusion, we have shown that modifications to the
basic side chain region of the dihydrobenzoxathiin
scaffold had a relatively small influence on the ERa
selectivity, but dramatically altered the in vivo antago-
nism/agonism activity profile of the lead I. These mod-
ifications included those factors, which may contribute
to the optimal antagonist side chain; such as length, the
nature of the heteroatom in the chain, and the presence
of an s-cis-conformation formed between the hetero-
atom and the cyclic amine. Further work along these
lines will be the subject of future publications from our
laboratories.
3
1.2
14.6
0.9
1.9
2.8
3.4
1.4
28
4
2.3/259 (115)
1.4/31 (22)
5
6
1.7/115 (62)
4.4/186 (43)
1.0/59 (59)
7
8
9
10
1.0/27 (27)
3.6/161 (45)
3.6/365 (101)
0.8/45 (56)d
1.3/1.1 (1)e
11
I
Estradiol
29
44/32
92/0.4
––/100
3.0
––
a The single IC50 values were generated in an estrogen receptor ligand
binding assay. This scintillation proximity assay was conducted in
NEN basic flashplates using tritiated estradiol and full length
recombinant human ERa and ERb proteins, with an incubation time
of 3 h. In our experience, this assay provides IC50 values that are
reproducible to within a factor of 2–3.
b The estrogen depleted MCF-7 cells were plated into 96-well cell
culture plates at a density of 1000 cells/well in a volume of 180 lL/
well. The test compounds and 3 pmol estradiol were applied to the
cells on days 1, 4, and 7. The assay was terminated between days 8
and 10.
c 20-Day old intact female Sprague–Dawley rats were treated (sc) with
test compounds for 3 days at 1 mpk. The uteri wet weights were
determined on day 4 and dry weights were determined after air-drying
the tissue samples for 3 days. The anti-estrogenic (antagonism)
activity of compounds was determined by co-administration of the
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a subcutaneous injection of 17-b-estradiol at
0.004 mpk and reported as % inhibition of uterine growth induced by
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d Average of 36 measurements.
e Average of 130 measurements.
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Q. Tan et al. / Bioorg. Med. Chem. Lett. 14 (2004) 3747–3751
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