Estrogen receptor ligands. Part 15: Synthesis of benzothiopyran-based selective estrogen receptor alpha modulators (SERAM)

Article abstract of DOI:10.3987/COM-05-S(T)7

Benzothiopyran (2) was prepared and the bioactive (+)-2 was found to exhibit a reduced affinity toward the estrogen receptors (ERα/β) when compared to the corresponding dihydrobenzoxathiin (+)-1.

Full text of DOI:10.3987/COM-05-S(T)7

HETEROCYCLES, Vol. 67, No. 1, 2006
79
HETEROCYCLES, Vol. 67, No. 1, 2006, pp. 79 - 83. © The Japan Institute of Heterocyclic Chemistry
Received, 14th March, 2005, Accepted, 18th May, 2005, Published online, 20th May, 2005. COM-05-S(T)7
ESTROGEN RECEPTOR LIGANDS. PART 15: SYNTHESIS OF
BENZOTHIOPYRAN-BASED SELECTIVE ESTROGEN RECEPTOR
ALPHA MODULATORS (SERAM)
Seongkon Kim,*,‡ Frank DiNinno,‡ Elizabeth T. Birzin,† Wanda Chan,† Yi Tien
Yang,† and Milton L. Hammond‡
Department of Medicinal Chemistry,‡ Atherosclerosis and Endocrinology,†
Merck Research Laboratories, P. O. Box 2000 (800B-109), Rahway, NJ 07065,
U.S.A. E-mail: Seongkon_kim@merck.com
Abstract – Benzothiopyran (2) was prepared and the bioactive (+)-2 was found
to exhibit a reduced affinity toward the estrogen receptors (ERα/β) when
compared to the corresponding dihydrobenzoxathiin (+)-1.
In a previous communication,1 we identified ERα subtype selective ligands or Selective Estrogen
Receptor Alpha Modulators (SERAMs) that centered on the dihydrobenzoxathiin core structure.2 This
compound, as exemplified by 1, exhibited low nanomolar binding affinity and sub-nanomolar functional
activity, as well as in vivo efficacy for the suppression of estradiol-driven uterine proliferation, with
minimal uterotropic activity. Subsequent expanded structure-activity relationship3 eventually led to a
potential developmental candidate.4
OH
OH
HO
S
HO
S
O
N
N
O
O
1
2
Figure 1
Although dihydrobenzoxathiin (1) had excellent potency and selectivity for ERα, it was judged to be
unacceptably prone to oxidative metabolism, with subsequent formation of covalent protein adducts.5
We initially identified that an iminium ion resulting from oxidation of the piperidine residue present in
the side chain of 1 was a significant contributor to the formation of covalent adducts.6 Meanwhile,

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HETEROCYCLES, Vol. 67, No. 1, 2006
Covalent Protein Adducts
OH
O
S
OH
[O]
1
.
[Red]
OH
HO
O
S
O
3
1-I
HO
N
O
Figure 2: Possible Metabolic Pathway leading to Protein Binding Adducts
formation of a quinone intermediate (1-I), generated from a phenolic-radical fragmentation process
embedded in the dihydrobenzoxathiin core, had also been postulated as being responsible for the covalent
adducts with biological proteins.5 Intuitively, the isolation of hydroquinone (3) from the in vitro
incubation of 1 with human liver microsomes supported the plausibility of the above metabolic pathway
(Figure 2).7 Therefore, it seemed prudent to replace the oxygen in the core with a methylene unit, thus
eliminating the possible formation of the quinone intermediate and thereby ameliorating any potential for
cytochrome P-450 mediated activation. Herein, we report the synthesis, estrogen/anti-estrogen activity,
and metabolic stability of target (2).
OMOM
O
12
X
S
BnO
Cl
OTIPS
BnO
S
a
OMOM
b
BnO
CHO
CHO
4
5
6: X=O
OTIPS
7: X=OH/H
OMOM
OMOM
OTIPS
HO
S
c
d
BnO
S
OTIPS
8
(+)-9
Scheme 1 Reagents and Conditions: (a) 1.5 equiv. t-Butylthiol, 1.8 equiv. NaH, Diglyme,
95oC, 15 h, 72% ; (b) Piperidine, AcOH, 110oC, 2 days, 50%; (c) i. LiAlH4, 0oC ii. 5% TFA,
CH2Cl2, -5oC, 10 min, 90% for two steps; (d) 1 equiv. Pd black, H2, EtOH, rt, 2 days, 75%
The synthesis began with the known compound (4), which was easily synthesized from m-chlorophenol
by a Reimer-Tiemann reaction in 60-70% yield.8 Replacement of the chlorine atom with a t-butylthio
group was initially problematic due, in part, to the lability of the benzyl group under the basic conditions.9
However, the best conversion was realized by using a minimum amount of the reagents at lower
temperature. The resulting t-butylthiobenzaldehyde (5) was condensed with a properly protected ketone
(12) in the presence of piperidine/HOAc to give a mixture of (E/Z)-α,β-unsaturated ketones (6).10 This

HETEROCYCLES, Vol. 67, No. 1, 2006
81
S
OMOM
OTIPS
+
BnO
S+
OMOM
BnO
7-I
7-II
OTIPS
Figure 3
Knoevenagel reaction was very sluggish and reaction times of 2 days were necessary to ensure total
consumption of the starting material. Selective 1,2-reduction of α,β-unsaturated ketones (6) with 1
equiv. LiAlH4 in THF gave allylic alcohols (7), thus setting the stage for subsequent cyclization to the
benzothiopyran.
OMOM
OMOM
MOMO
S
HO
S
a
b
(+)-9
(+)-10
OH
OTIPS
OMOM
OH
O
c
MOMO
S
HO
S
N
N
11
(+)-2
O
Scheme 2 Reagents and Conditions: (a) i. 3 equiv. NaH, MOMCl, DMF, rt ii. TBAF, HOAc,
THF, 80% for two steps; (b) 2-Chloroethylpiperidine, Cs2CO3, acetone, reflux; (c) 10% HCl,
MeOH, reflux, 1 h, 81%
The allylic alchohols (7), without further purification, were then cyclized with TFA in dichloromethane to
yield the 2,3-disubstituted 2H-1-benzothiopyran (8) in 90% yield.11 An acid-mediated cyclization of the
allylic carbocation 7-I to 7-II followed by loss of isobutylene was postulated as the reaction mechanism
(Figure 3).11 Hydrogenation of compound (8) with Pd black provided exclusively the racemic,
syn-compound (9).12 The syn relationship of the two phenyl groups was unambiguously established by
the observed small coupling constant (J = 3.5 Hz, δ = 4.2 ppm, see ref. 2), between H2-H3. Subsequent
chiral resolution of the racemic benzothiopyran (9) was realized by HPLC, using a Chiracel AD column
and 30% IPA/hexane as the eluant, to provide the dextrorotatory enantiomer ((+)-9).13
With (+)-9 now readily available, protection of the hydroxyl group in the A-ring, followed by
deprotection of TIPS group produced (+)-10 in quantitative yield (Scheme 2). The phenol ((+)-10) was
next converted to the respective ether (11) by reaction with 2-piperidinoethyl chloride in the presence of
cesium carbonate in acetone. Removal of the two MOM groups with dilute acid resulted in the final

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compound ((+)-2) in 81% yield.14 Similarly, in the racemic series, (9) was also converted to (2) by
application of the same procedure.
It is clear from the data in Table 1 that the benzothiopyran (2) retained the ERα potency and the
magnitude of receptor subtype selectivity (ERβ/ERα ratio) exhibited by the dihydrobenzoxathiin (1) in an
in vitro ER binding assay. However, the replacement of the oxygen atom with a methylene unit altered
the size of the ring, which presumably contributed to the reduction of the binding affinity (ERα for (+)-1=
0.8 nM and (+)-2= 3.1 nM).15 This weaker potency paralleled the weaker estradiol antagonism observed
in the immature rat uterine model (92% inhibition vs 76% inhibition).
Table 1. Binding affinitiesa and in vivo data
Uterine Assay (sc)c
Binding affinity
Compd
(+)-1b
(+)-1b
(+)-2d
% inhib @ 1mpk / % control @ 1 mpk
ERα
0.8
ERβ (α selectivity)
45 (56)
92/0.4
77/5.0
76/4.7
3.0
143 (48)
3.1
5.8
130 (43)
(+)-2
590 (101)
N.A.
a
b
Utilizing full length recombinant human ERα/β proteins; IC50 (nM), n=>48-56, see ref 1. Absolute stereochemistry
c
d
was determined by X-Ray crystallography, see ref 1 See ref 1 & 3; The absolute stereochemistry of (+)-2 was assigned
based on analogy with (+)-1, see ref 13 and 14
As with the dihydrobenzoxathiins, only the single enantiomer ((+)-2) reproduced the activity exhibited by
the racemate (2). In addition, the metabolism of the tritium-labelled ((+)-2) was investigated and, in
spite of the modification, was found to have a similar level of covalent protein adducts as (+)-1 in
incubations with rat liver microsomes. Further results in this area will be reported in future publications
from this laboratory.
ACKNOWLEDGEMENTS
We gratefully acknowledge Professor Barry M. Trost for helpful discussions during his scientific
consultation. This paper is dedicated with our best wishes to Professor Barry M. Trost on the occasion
of his 65th birthday.
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1
14. (+)-2: [α]D +193.2o (c 0.49, MeOH); H NMR (500 MHz, Acetone-d6) δ (ppm) 7.00 (d, J=8.3 Hz,
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