Syntheses and structure-activity relationships of 5,6,7,8-tetrahydro- 5,5,8,8-tetramethyl-2-quinoxaline derivatives with retinoic acid receptor α agonistic activity

Article abstract of DOI:10.1021/jm990063w

In the course of our studies on retinoic acid receptor (RAR) agonists, we have designed and synthesized a series of quinoxaline derivatives. One of them, 4-[5-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-quinoxalinyl)-1H-2- pyrrolyl]benzoic acid (3a), which

Full text of DOI:10.1021/jm990063w

J . Med. Chem. 2000, 43, 409-419
409
Syn th eses a n d Str u ctu r e-Activity Rela tion sh ip s of
5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa lin e Der iva tives w ith Retin oic
Acid Recep tor r Agon istic Activity
Kouichi Kikuchi, Shigeki Hibi, Hiroyuki Yoshimura, Naoki Tokuhara, Kenji Tai, Takayuki Hida,
Toshihiko Yamauchi, and Mitsuo Nagai*
Tsukuba Basic Research Laboratories for Drug Discovery, Eisai Co., Ltd., 1-3, Tokodai 5-chome, Tsukuba-shi,
Ibaraki, 300-2635, J apan
Received February 5, 1999
In the course of our studies on retinoic acid receptor (RAR) agonists, we have designed and
synthesized a series of quinoxaline derivatives. One of them, 4-[5-(5,6,7,8-tetrahydro-5,5,8,8-
tetramethyl-2-quinoxalinyl)-1H-2-pyrrolyl]benzoic acid (3a ), which possesses a 2,5-disubstituted
pyrrole moiety, showed selectivity for the RARR receptor and exerted highly potent cell-
differentiating activity on HL-60 cells.
Ch a r t 1
In tr od u ction
Retinoids have a variety of potent biological activities,
including induction of cellular proliferation, differentia-
tion, and death, as well as developmental changes.1 The
biological effects of retinoids are mediated by activation
of retinoic acid receptors (RARs), which are ligand-
dependent gene transcription factors. There are three
distinct receptor subtypes (RARR, -â, and -γ), which
possess considerable homology in their ligand binding
domains. RARs elicit their gene transcriptional activity
in the form of heterodimers with retinoid X receptors
(RXRs).2
Although retinoids are thought to have great thera-
peutic potential, the clinical use of retinoids is so far
limited mainly to dermatological diseases3 and some
cancers, for which retinoids may have both chemothera-
peutic and chemopreventive applications.4 The main
reason for this is the wide range of toxic effects of
retinoids, including mucocutaneous irritation, elevation
of plasma triglycerides, headache, bone toxicity, and
teratogenicity.5 The diverse range of actions of retinoids,
both desirable and undesirable, reflects the existence
of multiple retinoid receptor subtypes.
Recent research has focused on the synthesis and
development of subtype-selective retinoids.6 This is
because severe toxicity is thought to be due to nonspe-
cific activation of nuclear retinoid receptor subtypes, so
subtype-specific retinoids might have limited biological
activities through activating only subsets of retinoid-
responsive genes. Benbrook et al. recently reported7 that
heteroarotinoids were less toxic than arotinoid. On the
basis of their results, we considered that the introduc-
tion of a heteroatom into the hydrophobic part of
retinoids might similarly reduce the toxicity. If so,
subtype-specific retinoids should show much better
therapeutic indices than their nonselective counterparts
and might have potential value as medical drugs.
Only a limited number of RARR agonists have been
reported so far. These include Am80, Am580,6a and
AGN193836.6b Am80 is more potent than ATRA (all-
trans retinoic acid) as an in vitro differentiation inducer,
and clinically Am80 was effective in leukemia patients
who had relapsed from ATRA-induced complete remis-
sion.8 Treatment of psoriasis patients with Am80 was
comparable in efficacy to steroid therapy.9 Moreover, a
group at Shionogi Co. recently reported that an RARR-
selective agonist (Am80) inhibited rat CII arthritis
concomitantly with a decrease in the anti-CII Ab level
in vivo.10 Therefore, RARR agonists seemed to be
promising compounds for the treatment of cancer,
dermatological diseases, and immunological disorders.
The structure of retinoid analogues can be divided
into three parts, i.e., a hydrophobic part, a linker, and
a carboxylic part. All reported RARR-selective agonists,
such as Am80, Am580, and AGN 193836, have an amide
in the linker (Chart 1). In the course of our studies
aimed at synthesizing novel retinoids,11 we introduced
nitrogen into the hydrophobic part to synthesize more
polar novel retinoid analogues which might possess
improved pharmacokinetic characteristics and show less
toxicity. We hoped that such retinoid derivatives might
be selective RARR agonists. Initially, we synthesized a
quinoxaline-amide derivative (1; ER-33635),12 which
possesses a tetrahydroquinoxaline moiety at the hydro-
* To whom correspondence should be addressed. Tel: 81-298-47-
5868. Fax: 81-298-47-2037. E-mail: m2-nagai@hhc.eisai.co.jp.
10.1021/jm990063w CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/25/2000

410 J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3
Kikuchi et al.
Ch a r t 2
Sch em e 1a
a
Reagents: (a) conc H2SO4, EtOH; (b) Na, xylene; (c) CrO3, AcOH, H2O; (d) (i) DL-R,â-diaminopropionic acid monohydrobromide, NaOH,
MeOH, (ii) conc H2SO4, MeOH; (e) aq NaOH, EtOH; (f) (EtO)2P(O)Cl, Et3N, ethyl 4-aminobenzoate, THF.
phobic part (Chart 2). Compound 1 showed moderate
selectivity for RARR and had potent retinoid activity.
Subsequently, we focused on an acidic proton in the
amide structure of the linker. We found that the 2,5-
substituted pyrrole derivative (2),13 a BASF-patented
compound which has an acidic proton, showed RARR
selectivity. On the basis of these results, we designed
and synthesized quinoxaline-pyrrole derivatives with
the aim of obtaining novel RARR-selective retinoids.
Here we discuss the synthesis and structure-activity
relationships (SAR) of retinoids which possess hydro-
quinoxaline and various heterocycles as the hydrophobic
part and linker, respectively.
tion with sodium to afford the keto alcohol (6). Com-
pound (6) was oxidized with CrO3 to give the diketone
(7).14 Condensation11a of the ketone (7) with DL-R,â-
diaminopropionic acid monohydrobromide afforded the
quinoxaline methyl ester (8), which was hydrolyzed to
give the carboxylic acid (9). The ethyl ester (10),
obtained by condensation of the carboxylic acid (9) with
ethyl 4-aminobenzoate, was hydrolyzed with NaOH to
afford the free acid (1; ER-33635).
2,5-Disubstituted pyrrole derivatives were prepared
as shown in Scheme 2. Synthesis of the 1,4-diketone (14)
was achieved by means of a thiazolium salt-catalyzed
benzoin condensation type reaction. A commercially
available benzaldehyde derivative (11) was treated with
vinyl Grignard reagent followed by PDC oxidation to
give the enone derivative (12). The counterpart aldehyde
(13) was synthesized as follows. The methyl ester (8)
described above was reduced with diisobutylaluminum
Ch em istr y
The synthesis of 1 (ER-33635) is shown in Scheme 1.
The diester (5), derived from a commercially available
carboxylic acid (4), was subjected to acyloin condensa-

Synthesis of RARR Agonist
J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3 411
Sch em e 2a
a
Reagents: (a) vinylmagnesium bromide, THF; (b) PDC, MS 3A, CH2Cl2; (c) (i) DIBAL-H, CH2Cl2, (ii) (COCl)2, DMSO, Et3N, CH2Cl2;
(d) 12, 3-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium chloride, Et3N, DMF; (e-a) AcONH4, MeOH; (e-b) conc H2SO4; (e-c) P2S5, xylene;
(f) NaH, MeI, DMF; (g) aq NaOH, EtOH.
Sch em e 3a
a
Reagents: (a) (i) MeMgBr, Et2O, (ii) (COCl)2, DMSO, Et3N, CH2Cl2; (b) methyl 4-formylbenzoate, NaOH, MeOH; (c) (i) MeNO2, Triton
B, THF, (ii) NaOMe, MeOH-THF-CH2Cl2, (iii) conc H2SO4, MeOH; (d-a) AcONH4, AcOH; (d-b) conc H2SO4; (d-c) P2S5, xylene; (e) aq
NaOH, EtOH.
hydride (DIBAL-H), followed by Swern oxidation to give
the aldehyde (13). Condensation of the aldehyde (13)
with the enone (12) in the presence of 3-benzyl-5-(2-
hydroxyethyl)-4-methylthiazolium chloride and triethy-
lamine gave the 1,4-diketone (14).15 This was treated
with ammonium acetate to afford the pyrrole derivative
(15a ), which was hydrolyzed to the quinoxaline deriva-
tive (3a ). Treatment of the 1,4-diketone (14) with
concentrated sulfuric acid yielded the furan derivative
(15b), which was then hydrolyzed to the acid (3b),
whereas treatment with P2S5 in xylene at reflux tem-
perature gave the thiophene derivative (15c) and hy-
drolysis of this yielded 3c. N-Alkylation of the 2,5-
substituted pyrrole derivative (15a ) was carried out with
sodium hydride and methyl iodide, followed by hydroly-
sis to give the N-methyl pyrrole derivative (3d ) (Scheme
2).
Preparation of the 2,4-disubstituted heterocyclic de-
rivatives (20) is shown in Scheme 3. Grignard reaction
of the aldehyde (13) followed by Swern oxidation gave
the methyl ketone (16), which was condensed with
methyl 4-formylbenzoate in the presence of NaOH to
afford the enone (17). Conjugate addition of nitromethane
anion to compound 17 followed by reaction with NaOMe
gave the aldehyde, which was acetalized to give com-
pound 18. Treatment of the acetal (18) with ammonium
acetate gave the 2,4-substituted pyrrole derivative
(19a ), which was hydrolyzed with NaOH to give the acid
(20a ). Transformation of the acetal (18) into the furan
(20b) and the thiophene (20c) were accomplished with
H2SO4 and P2S5, respectively (Scheme 3).
Preparation of the pyrazole derivatives is shown in
Scheme 4. The pyrazole moiety was constructed in a
usual manner, by condensation of hydrazine with the

412 J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3
Kikuchi et al.
Sch em e 4a
a
Reagents: (a) LDA, THF; (b) hydrazine, AcOH; (c) aq NaOH, EtOH.
Sch em e 5a
a
Reagents: (a) CuBr2, AcOEt, CHCl3; (b-a) methyl 4-amizinobenzoate acetate, K2CO3, DMF; (b-b) 4-carbomethoxybenzthioamide,
pyridine, IPA; (c) aq NaOH, EtOH.
1,3-diketone derivative (22), which was prepared from
the anion of the ketone (16) and terephthalic acid
monomethyl ester chloride (21), followed by hydrolysis
to afford the pyrazole derivative (24).
Preparation of the imidazole derivative was accom-
plished as follows. The R-bromo ketone (25), obtained
by bromination of the ketone (16) with CuBr2, was
condensed with methyl 4-amizinobenzoate followed by
hydrolysis to give the imidazole derivative (27a ). Con-
densation of the R-bromo ketone with 4-carbomethoxy-
benzthioamide in the presence of pyridine followed by
hydrolysis gave the thiazole (27b) (Scheme 5).
Am80 and Am580 are retinoid analogues in which the
linker moiety possesses an amide structure. Since usual
nonselective retinoids such as ATRA and arotinoid
(TTNPB)17 do not have such an amide structure, we
hypothesized that the RARR receptor possesses a strong
binding site for the acidic proton of the linker; in other
words, ligands which have such an acidic proton would
activate RARR-mediated transactivation. On the basis
of this hypothesis, we introduced various heterocycles,
such as 2,4-substituted pyrrole, 2,5-substituted pyrrole,
pyrazole, and imidazole, as the linker moiety.
In the cotransfection assay, the 2,5-substituted pyr-
role derivative (3a ) showed more potent activity than
ATRA at RARR. However, this compound (3a ) showed
weak affinity for RARâ and RARγ and was less potent
than ATRA at these receptors (Figure 1). It was 8-fold
more potent than ATRA in terms of HL-60 differentia-
tion-inducing activity. Compound 2, which has tetram-
ethyl-tetrahydronaphthalene and 2,5-disubstituted pyr-
role moieties, also showed potent RARR agonistic activity.
However, compound 3a was less potent than 2 in terms
of RARγ transcription activity and binding affinity. In
contrast, other heterocyclic derivatives (20a , 24, and
27a ) with an acidic proton were less potent than
compound 3a and less selective for RARR.
To test the effect of the acidic proton of the 2,5-
substituted pyrrole (3a ), the pyrrole linker was replaced
with other heterocycles, which do not possess an acidic
proton. When the 2,5-substituted pyrrole moiety of
compound 3a was replaced with furan (3b), the activi-
ties at RARR, -â, and -γ were comparable to those of
ATRA. The thiophene derivative (3c) showed weak
potency at RARR whereas this compound was compa-
rable to ATRA at RARâ and -γ. Thus, compounds 3b
Resu lts a n d Discu ssion
The above retinoids were synthesized and assayed in
vitro for the ability to bind to the individual RARs and
to induce gene transcription in the cotransfection assay.
Cotransfection assays were performed as described in
the Experimental Section, and relative ED30 values were
calculated (see footnotes to Tables 1 and 2). Binding
assays for RAR receptor isoforms were performed in a
manner similar to that described by Boehm et al.16 using
[3H]ATRA. The results are summarized in Tables 1 and
2. RXRR transactivation was also studied, but none of
these compounds activated RXR (data not shown).
We initially introduced a nitrogen atom into the
hydrophobic part of the retinoid structure to obtain more
polar retinoids which might possess improved pharma-
cokinetic characteristics. Moreover, we hoped to find
retinoids with potent and selective retinoid activity
among the quinoxaline derivatives. Although ER-33635
(1) had low RARR selectivity, this compound is more
potent than Am580 and Am80 at RARR, -â, and -γ.

Synthesis of RARR Agonist
J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3 413
Ta ble 1. Competitive Binding, Transactivation, and Induction of Differentiation
a
Specific binding affinity was defined as the total binding minus the nonspecific binding, and the 50% inhibitory dose (IC50) values
were obtained from logarithmic plots. The selectivity of test compounds for each receptor is indicated as relative IC50, where the IC50
b
value for each receptor was divided by that of the natural ligand (ATRA). Mean of IC50/ATRA IC50 ( SEM. c nd: not detectable (relative
d
IC50 > 1000). Mean of ATRA IC50 (nM) ( SEM. e EC30 values were determined from full dose-response curves ranging from 0.1 nM to
3 µM. Retinoid activity is expressed in terms of relative EC30, which is the concentration of retinoid required to produce 30% of the
g
maximal observed response, normalized relative to that of ATRA. f Mean of EC30/ATRA EC30 ( SEM. Mean of ATRA EC30 (nM) ( SEM.
used for column chromatography and silica gel (Kieselgel 60
F254, Merck) for analytical thin-layer chromatography (TLC).
Compounds were detected on TLC plates by exposure to UV
and 3c did not show any selectivity for RARR. The 2,4-
substituted pyrrole moiety of compound 20a was also
replaced with furan (20b) and thiophene (20c). Com-
pounds 20b,c both possess comparable activity to ATRA
at RARR, -â, and -γ.
In conclusion, we have described the syntheses and
light (254 nm). Melting points were measured on a Yanagimoto
micro-melting point apparatus without correction. 1H NMR
spectra were recorded on a Varian Unity 400 spectrometer,
and chemical shifts are expressed in ppm downfield from
SAR of a new series of quinoxaline derivatives. We
found out that the quinoxaline derivatives have retinoid
activities. One of them, 4-[5-(5,6,7,8-tetrahydro-5,5,8,8,-
tetramethyl-2-quinoxalinyl)-1H-2-pyrrolyl]benzoic acid
(3a ; ER-34617) has selectivity for the RARR receptor
and shows highly potent cell differentiation-inducing
activity. Thus, ER-34617 should be useful as a tool to
study the physiological role of RARR. It may also be
useful as a lead compound for more highly RARR
selective retinoids, which might have fewer toxic effects
and better therapeutic indices than nonselective retinoid
drugs.
tetramethylsilane (TMS) as an internal reference. Mass spec-
tra (MS) were obtained on a J EOL J MS-HX100 mass spec-
trometer. All organic extracts were dried over MgSO4, and
solvents were removed with a rotary evaporator under reduced
pressure.
Dieth yl 2,2,5,5-Tetr a m eth ylh exa n ed ioa te (5). To a solu-
tion of 2,2,5,5-tetramethylhexanedioic acid (25 g, 123 mmol)
in ethanol (300 mL) was added concentrated sulfuric acid (25
mL) at room temperature, and the mixture was stirred under
reflux for 16 h. Ethanol was removed under reduced pressure,
and residue was poured into saturated aqueous sodium
hydrogen carbonate solution. The mixture was extracted with
ethyl acetate, and the organic layer was washed with water
and brine, then dried, and evaporated. The crude residue was
purified by flash column chromatography on silica gel (sol-
vent: 5% ethyl acetate-n-hexane) to afford 5 (31.7 g, 123
mmol, 99%) as a colorless oil: 1H NMR (400 MHz, CDCl3) δ
1.15 (s, 12H), 1.24 (t, J ) 7.2 Hz, 6H), 1.44 (s, 4H), 4.11 (q, J
) 7.2 Hz, 4H).
Exp er im en ta l Section
Ch em istr y. Reagents and solvents were purchased from
usual commercial sources. Silica gel (Kieselgel 60, Merck) was

414 J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3
Kikuchi et al.
Ta ble 2. Competitive Binding, Transactivation, and Induction of Differentiation
a
Specific binding affinity was defined as the total binding minus the nonspecific binding, and the 50% inhibitory dose (IC50) values
were obtained from logarithmic plots. The selectivity of test compounds for each receptor is indicated as relative IC50, where the IC50
b
value for each receptor was divided by that of the natural ligand (ATRA). Mean of IC50/ATRA IC50 ( SEM. c nd: not detectable (relative
d
IC50 > 1000). Mean of ATRA IC50 (nM) ( SEM. e EC30 values were determined from full dose-response curves ranging from 0.1 nM to
3 µM. Retinoid activity is expressed in terms of relative EC30, which is the concentration of retinoid required to produce 30% of the
g
maximal observed response, normalized relative to that of ATRA. f Mean of EC30/ATRA EC30 ( SEM. Mean of ATRA EC30 (nM) ( SEM.
6-Hyd r oxy-2,2,5,5-tetr a m eth yl-1-cycloh exa n on e (6). To
a suspension of Na (40% dispersion in xylene, 44.6 g, 776
mmol) in xylene (500 mL) was added dropwise over 30 min a
solution of 5 (50 g, 194 mmol) in xylene (100 mL) under a
nitrogen atmosphere at 100 °C. The mixture was stirred for 2
h at the same temperature, then cooled in an ice bath, and a
solution of sulfuric acid (50 mL) in water (50 mL) was added
dropwise to it. The mixture was extracted with ethyl acetate,
and the organic layer was washed with water and brine, then
dried, and evaporated. The crude residue was purified by flash
column chromatography on silica gel (solvent: 5% ethyl
acetate-n-hexane) to afford 6 (28 g, 164 mmol, 85%) as a
colorless oil: 1H NMR (400 MHz, CDCl3) δ 0.67 (s, 3H), 1.09
(s, 3H), 1.13 (s, 3H), 1.20 (s, 3H), 1.43 (ddd, J ) 2.8, 4.0, 14.0
Hz, 1H), 1.62 (ddd, J ) 2.8, 4.8, 14.0 Hz, 1H), 1.70 (ddd, J )
4.0, 14.0, 14.0 Hz, 1H), 1.87 (ddd, J ) 4.8, 14.0, 14.0 Hz, 1H),
3.58 (d, J ) 4.6 Hz, 1H), 4.13 (d, J ) 4.0 Hz, 1H).
3,3,6,6-Tetr a m eth yl-1,2-cycloh exa n ed ion e (7). To a so-
lution of 6 (28 g, 164 mmol) in acetic acid (70 mL) was added
dropwise a solution of chromium(VI) oxide (18 g, 180 mmol)
in acetic acid (70 mL)-water (9 mL) at 15 °C, and the mixture
was stirred for 3 h at the same temperature. It was poured
into water, and the resulting precipitate was collected by
filtration, washed with water, and dried under reduced pres-
sure to afford 7 (19.5 g, 116 mmol, 70%) as a pale yellow
solid: 1H NMR (400 MHz, CDCl3) δ 1.15 (s, 12H), 1.86 (s, 4H).
Meth yl 5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in -
oxa lin eca r boxyla te (8). To a solution of 7 (2.5 g, 14.9 mmol)
and DL-R,â-diaminopropionic acid monohydrobromide (2.76 g,
14.9 mmol) in methanol (100 mL) was added sodium hydroxide
(2.4 g, 59.6 mmol) at room temperature, and then the mixture
was stirred under reflux for 48 h. It was cooled in an ice bath,
concentrated sulfuric acid (12 mL) was added dropwise, and
the whole was stirred under reflux for 6 h. Methanol was
removed under reduced pressure, and the residue was poured
into saturated aqueous sodium hydrogen carbonate solution.
The mixture was extracted with ethyl acetate, and the organic
layer was washed with water and brine, then dried, and
evaporated. The crude residue was purified by flash column
chromatography on silica gel (solvent: 5% ethyl acetate-n-
hexane) to afford 8 (2.65 g, 10.7 mmol, 72%) as a pale yellow
oil: 1H NMR (400 MHz, CDCl3) δ 1.34 (s, 6H), 1.36 (s, 6H),
1.82 (s, 4H), 3.98 (s, 3H), 9.00 (s, 1H).
5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa lin e-
ca r boxylic Acid (9). A solution of 8 (35 mg, 0.141 mmol) in
ethanol (2 mL) was treated with 5 N aqueous sodium hydrox-
ide solution (0.2 mL) at room temperature and stirred for 1 h
at the same temperature. The reaction mixture was diluted
with water, and the pH was adjusted to 5 with 2 N aqueous
hydrochloric acid. The mixture was extracted with ethyl
acetate, and the organic layer was washed with water and
brine, then dried, and evaporated to afford 9 (32 mg, 0.137
mmol, 97%) as a colorless solid: 1H NMR (400 MHz, CDCl3) δ
1.36 (s, 12H), 1.85 (s, 4H), 9.18 (s, 1H).
Eth yl 4-{[(5,6,7,8-Tetr ah ydr o-5,5,8,8-tetr am eth yl-2-qu in -
oxa lin yl)ca r boyl]a m in o}ben zoa te (10). To a solution of 9
(40 mg, 0.171 mmol) in tetrahydrofuran (2 mL) was added
diethylchlorophosphate (30 µL, 0.205 mmol), triethylamine (29
µL, 0.205 mmol), and a solution of ethyl 4-aminobenzoate (28.2
mg, 0.171 mmol) in tetrahydrofuran (1 mL) under a nitrogen
atmosphere at 0 °C. The mixture was stirred for 12 h at room
temperature, then water was added, and the whole was
extracted with ethyl acetate. The organic layer was washed
with water and brine, then dried, and evaporated. The crude
residue was purified by flash column chromatography on silica
gel (solvent: 5% ethyl acetate-n-hexane) to afford 10 (45 mg,
0.118 mmol, 69%) as a colorless solid: 1H NMR (400 MHz,
CDCl3) δ 1.37 (s, 6H), 1.41 (s, 6H), 1.41 (t, J ) 7.2 Hz, 3H),

Synthesis of RARR Agonist
J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3 415
to afford methyl 4-(1-hydroxyallyl)benzoate (11.6 g, 60.4 mmol,
73%) as a pale yellow oil: 1H NMR (400 MHz, CDCl3) δ 2.02
(br s, 1H), 3.91 (s, 3H), 5.23 (d, J ) 10.8 Hz, 1H), 5.25-5.29
(m, 1H), 5.37 (d, J ) 17.6 Hz, 1H), 6.02 (ddd, J ) 6.8, 10.8,
17.6 Hz, 1H), 7.45 (d, J ) 8.4 Hz, 2H), 8.03 (d, J ) 8.4 Hz,
2H).
To a solution of methyl 4-(1-hydroxyallyl)benzoate (11.6 g,
60.4 mmol) in dichloromethane (600 mL) were added pyri-
dinium dichromate (27 g, 71.8 mmol) and molecular sieve 3A
at room temperature, and the mixture was stirred for 4 h at
the same temperature. It was filtered on Celite, and the filtrate
was evaporated. The crude residue was purified by flash
column chromatography on silica gel (solvent: 5% ethyl
acetate-n-hexane) to afford 12 (5.5 g, 28.9 mmol, 48%) as a
colorless solid: 1H NMR (400 MHz, CDCl3) δ 3.96 (s, 3H), 6.00
(d, J ) 10.4 Hz, 1H), 6.46 (d, J ) 17.2 Hz, 1H), 7.14 (dd, J )
10.4, 17.2 Hz, 1H), 7.98 (d, J ) 8.4 Hz, 2H), 8.14 (d, J ) 8.4
Hz, 2H).
5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa lin e-
ca r ba ld eh yd e (13). To a solution of 8 (23.2 g, 93.4 mmol) in
dichloromethane (500 mL) was added dropwise a 1 M solution
of DIBAL-H in toluene (280 mL, 280 mmol) under a nitrogen
atmosphere at -78 °C, and the mixture was stirred for 1 h at
the same temperature. It was quenched with saturated aque-
ous ammonium chloride solution, and the resulting precipitate
was filtered off on Celite. The filtrate was concentrated under
reduced pressure, and the crude residue was purified by flash
column chromatography on silica gel (solvent: 20% ethyl
acetate-n-hexane) to afford (5,6,7,8-tetrahydro-5,5,8,8-tet-
ramethyl-2-quinoxalinyl)methanol (12.4 g, 56.3 mmol, 60%) as
a pale yellow oil: 1H NMR (400 MHz, CDCl3) δ 1.32 (s, 6H),
1.33 (s, 6H), 1.80 (s, 4H), 3.62 (t, J ) 5.1 Hz, 1H), 4.74 (d, J )
5.1 Hz, 2H), 8.31 (s, 1H).
To a solution of oxalyl chloride (4.58 mL, 52.5 mmol) in
dichloromethane (250 mL) was added dropwise a solution of
dimethyl sulfoxide (7.45 mL, 105 mmol) in dichloromethane
(17 mL) at -60 °C, and the mixture was stirred for 10 min at
the same temperature. To this solution was added dropwise a
solution of (5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-quinoxa-
linyl)methanol (7.7 g, 35 mmol) in dichloromethane (120 mL)
at a rate sufficient to keep the temperature at -60 °C. After
the reaction mixture had been stirred for 15 min at this
temperature, triethylamine (35.1 mL, 252 mmol) was added
and the whole was stirred for 5 min at -60 °C and for 15 min
at room temperature. Water was added, and the whole was
extracted with dichloromethane. The organic layer was washed
with water and brine, then dried, and evaporated. The crude
residue was purified by flash column chromatography on silica
gel (solvent: 3% ethyl acetate-n-hexane) to afford 13 (6.8 g,
31.1 mmol, 89%) as a pale yellow solid: 1H NMR (400 MHz,
CDCl3) δ 1.35 (s, 6H), 1.38 (s, 6H), 1.84 (s, 4H), 8.91 (s, 1H),
10.10 (s, 1H).
Meth yl 4-[4-Oxo-4-(5,6,7,8-tetr ah ydr o-5,5,8,8-tetr am eth -
yl-2-qu in oxa lin yl)bu ta n oyl]ben zoa te (14). A solution of 13
(300 mg, 1.37 mmol), 12 (260 mg, 1.58 mmol), triethylamine
(0.23 mL, 1.65 mmol), and 3-benzyl-5-(2-hydroxyethyl)-4-meth-
ylthiazolium chloride (74 mg, 0.27 mmol) in N,N-dimethylform-
amide (10 mL) was stirred for 30 min at 100 °C. The mixture
was extracted with ethyl acetate, and the organic layer was
washed with 1 N aqueous hydrochloric acid and brine, then
dried, and evaporated. The crude residue was purified by flash
column chromatography on silica gel (solvent: 7% ethyl
acetate-n-hexane) to afford 14 (420 mg, 1.03 mmol, 75%) as
a colorless solid: 1H NMR (400 MHz, CDCl3) δ 1.35 (s, 6H),
1.38 (s, 6H), 1.84 (s, 4H), 3.46 (t, J ) 6.4 Hz, 2H), 3.68 (t, J )
6.4 Hz, 2H), 3.96 (s, 3H), 8.08 (d, J ) 8.0 Hz, 2H), 8.15 (d, J
) 8.0 Hz, 2H), 8.96 (s, 1H).
F igu r e 1. Dose-response curves for RAR-mediated transac-
tivation activity of 3a and ATRA at each RAR subtype.
1.86 (s, 4H), 4.38 (q, J ) 7.2 Hz, 2H), 7.82 (d, J ) 8.8 Hz, 2H),
8.09 (d, J ) 8.8 Hz, 2H), 9.25 (s, 1H), 9.83 (br s, 1H).
4-{[(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)ca r boyl]a m in o}ben zoic Acid (1). A solution of 10 (250
mg, 0.66 mmol) in ethanol (30 mL) was treated with 5 N
aqueous sodium hydroxide solution (3 mL) at room tempera-
ture, and the mixture was stirred for 6 h at the same
temperature. The reaction mixture was diluted with water,
and the pH was adjusted to 5 with 6 N aqueous hydrochloric
acid. The precipitate was collected by filtration, washed with
water, and dried under reduced pressure to afford 1 (146 mg,
0.41 mmol, 63%) as a colorless solid: mp 257-259 °C (EtOH
1
- water); H NMR (400 MHz, DMSO-d6) δ 1.31 (s, 6H), 1.40
(s, 6H), 1.81 (s, 4H), 7.96 (s, 4H), 9.05 (s, 1H), 10.42 (br s, 1H).
Anal. (C20H23N3O3‚1.2H2O) C, H, N.
Meth yl 4-Acr yloylben zoa te (12). To a solution of methyl
terephthalaldehydate (13.6 g, 82.8 mmol) in tetrahydrofuran
(150 mL) was added a 1 M solution of vinylmagnesium bromide
in tetrahydrofuran (100 mL, 100 mmol) at -78 °C. The mixture
was stirred for 30 min at the same temperature and then
quenched with saturated aqueous ammonium chloride solu-
tion, and the whole was extracted with ethyl acetate. The
organic layer was washed with brine, then dried, and evapo-
rated. The crude residue was purified by flash column chro-
matography on silica gel (solvent: 5% ethyl acetate-n-hexane)
Meth yl 4-[5-(5,6,7,8-tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-1H-2-p yr r olyl]ben zoa te (15a ). A solution of
14 (380 mg, 0.93 mmol) and ammonium acetate (358 mg, 4.65
mmol) in methanol (50 mL) was stirred under reflux for 8 h.
Methanol was removed under reduced pressure, and the
residue was extracted with ethyl acetate. The organic layer
was washed with water and brine, then dried, and evaporated.

416 J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3
Kikuchi et al.
The crude residue was purified by flash column chromatog-
raphy on silica gel (solvent: 5% ethyl acetate-n-hexane) to
afford 15a (260 mg, 0.67 mmol, 72%) as a pale yellow solid:
1H NMR (400 MHz, CDCl3) δ 1.34 (s, 6H), 1.40 (s, 6H), 1.82
(s, 4H), 3.94 (s, 3H), 6.73 (dd, J ) 2.8, 4.0 Hz, 1H), 6.82 (dd, J
)2.4, 4.0 Hz, 1H), 7.61 (d, J ) 8.4 Hz, 2H), 8.08 (d, J ) 8.4
Hz, 2H), 8.66 (s, 1H).
0.7H2O) C, N; H: calcd, 6.32; found, 5.89. HRMS Calcd for
C
4-[1-Meth yl-3-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-
23H25N2O2S (MH+): 393.1637. Found: 393.1620.
2-qu in oxa lin yl)-1H-5-p yr r olyl}ben zoic Acid (3d ). Com-
pound 3d was synthesized from 15d following the represen-
tative procedure described for 1 and obtained as a pale yellow
1
solid in 90% yield: mp 252-254 °C (EtOH-water); H NMR
(400 MHz, CDCl3) δ 1.35 (s, 6H), 1.36 (s, 6H), 1.82 (s, 4H),
3.96 (s, 3H), 6.42 (d, J ) 4.0 Hz, 1H), 6.72 (d, J ) 4.0 Hz, 1H),
7.59 (d, J ) 8.4 Hz, 2H), 8.15 (d, J ) 8.4 Hz, 2H), 8.65 (s, 1H).
Anal. (C24H27N3O2‚0.3H2O) C, H; N: calcd, 10.64; found, 9.97.
HRMS Calcd for C24H28N3O2 (MH+): 390.2181. Found: 390.2190.
Meth yl 4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-2-fu r yl]ben zoa te (15b). A solution of 14 (200
mg, 0.49 mmol) in concentrated sulfuric acid (2 mL) was stirred
for 15 h at room temperature, then poured into saturated
aqueous sodium hydrogen carbonate solution, and the mixture
was extracted with ethyl acetate. The organic layer was
washed with water and brine, then dried, and evaporated. The
crude residue was purified by flash column chromatography
on silica gel (solvent: 5% ethyl acetate-n-hexane) to afford
15b (120 mg, 0.31 mmol, 63%) as a pale yellow solid: 1H NMR
(400 MHz, CDCl3) δ 1.35 (s, 6H), 1.36 (s, 6H), 1.82 (s, 4H),
3.94 (s, 3H), 6.92 (d, J ) 3.6 Hz, 1H), 7.19 (d, J ) 3.6 Hz, 1H),
7.80 (d, J ) 8.0 Hz, 2H), 8.09 (d, J ) 8.0 Hz, 2H), 8.83 (s, 1H).
Meth yl 4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-2-th ien yl]ben zoa te (15c). To a solution of 14
(200 mg, 0.49 mmol) in xylene (10 mL) was added phosphorus
pentasulfide (109 mg, 0.49 mmol) at room temperature, and
the mixture was stirred under reflux for 2 h, then purified by
flash column chromatography on silica gel (solvent: 5% ethyl
acetate-n-hexane) to afford 15c (90 mg, 0.22 mmol, 45%) as
a yellow solid: 1H NMR (400 MHz, CDCl3) δ 1.34 (s, 6H), 1.38
(s, 6H), 1.82 (s, 4H), 3.94 (s, 3H), 7.43 (d, J ) 4.0 Hz, 1H),
7.60 (d, J ) 4.0 Hz, 1H), 7.74 (d, J ) 8.6 Hz, 2H), 8.06 (d, J )
8.6 Hz, 2H), 8.72 (s, 1H).
1-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa li-
n yl)-1-eth a n on e (16). To a solution of 13 (3.00 g, 13.7 mmol)
in diethyl ether (180 mL) was added dropwise a 3 M solution
of methylmagnesium bromide in diethyl ether (5.53 mL, 16.6
mmol) under a nitrogen atmosphere at 0 °C, and the mixture
was stirred for 30 min at the same temperature. The reaction
mixture was quenched with saturated aqueous ammonium
chloride solution and extracted with ethyl acetate. The organic
layer was washed with water and brine, then dried, and
evaporated. The crude residue was purified by flash column
chromatography on silica gel (solvent: 20% ethyl acetate-n-
hexane) to afford 1-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-
quinoxalinyl)-1-ethanol (3.20 g, 13.7 mmol, >99%) as a color-
less oil: 1H NMR (400 MHz, CDCl3) δ 1.32 (s, 12H), 1.52 (d, J
) 6.6 Hz, 3H), 1.80 (s, 4H), 4.16 (d, J ) 4.9 Hz, 1H), 4.85-
4.93 (m, 1H), 8.35 (s, 1H).
To a solution of oxalyl chloride (1.8 mL, 20.7 mmol) in dichlo-
romethane (80 mL) was added dropwise a solution of dimethyl
sulfoxide (2.94 mL, 41.4 mmol) in dichloromethane (3 mL) at
-60 °C, and the mixture was stirred for 5 min at the same
temperature. To this solution was added dropwise a solution
of 1-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-quinoxalinyl)-1-
ethanol (3.20 g, 13.7 mmol) in dichloromethane (40 mL) at a
rate sufficient to keep the temperature at -60 °C. After the
reaction mixture had been stirred for 15 min at this temper-
ature, triethylamine (13.8 mL, 99.4 mmol) was added, and the
whole was stirred for 5 min at -60 °C and for 15 min at room
temperature. Water was added, and the mixture was extracted
with dichloromethane. The organic layer was washed with
water and brine, then dried, and evaporated. The crude residue
was purified by flash column chromatography on silica gel (sol-
vent: 10% ethyl acetate-n-hexane) to afford 16 (1.90 g, 8.2
mmol, 60%) as a pale yellow solid: 1H NMR (400 MHz, CDCl3)
δ 1.34 (s, 6H), 1.36 (s, 6H), 1.82 (s, 4H), 2.68 (s, 3H), 8.96 (s, 1H).
Meth yl 4-[(E)-3-Oxo-3-(5,6,7,8-tetr a h yd r o-5,5,8,8-tetr a -
m eth yl-2-qu in oxa lin yl)-1-p r op en yl]ben zoa te (17). To a
solution of 16 (1.90 g, 8.2 mmol) and methyl 4-formylbenzoate
(1.34 g, 8.2 mmol) in methanol (50 mL) was added a piece of
solid sodium hydroxide at room temperature. After the mixture
had been stirred for 12 h at room temperature, the resulting
precipitate was collected by filtration and dried under reduced
pressure to afford 17 (2.58 g, 6.9 mmol, 84%) as a pale yellow
solid: 1H NMR (400 MHz, CDCl3) δ 1.37 (s, 6H), 1.43 (s, 6H),
1.86 (s, 4H), 3.95 (s, 3H), 7.74 (d, J ) 8.2 Hz, 2H), 7.95 (d, J
) 16.0 Hz, 1H), 8.10 (d, J ) 8.2 Hz, 2H), 8.26 (d, J ) 16.0 Hz,
1H), 9.11 (s, 1H).
Meth yl 4-[1-(Dim eth oxym eth yl)-3-oxo-3-(5,6,7,8-tetr a -
h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa lin yl)p r op yl]ben zo-
a te (18). To a solution of 17 (2.53 g, 6.7 mmol) in nitromethane
(30 mL) and tetrahydrofuran (9 mL) was added Triton B (1.5
mL) at room temperature. The mixture was stirred for 12 h
at room temperature and extracted with ethyl acetate. The
organic layer was washed with diluted aqueous hydrochloric
acid, water, and brine, then dried, and evaporated. The crude
residue was dissolved in tetrahydrofuran: dichloromethane (1:
1) (70 mL). This solution was added to a solution of sodium
methoxide (3.7 mL, 18.8 mmol) in methanol (23 mL) at -35
°C, and the mixture was stirred for 40 min at the same
temperature. It was then added dropwise to a solution of
concentrated sulfuric acid (9.3 mL) in methanol (46 mL) at
-35 °C, and the whole was stirred at room temperature for
40 min. The mixture was poured into ice and extracted with
Meth yl 4-[1-Meth yl-3-(5,6,7,8-tetr a h yd r o-5,5,8,8-tetr a -
m eth yl-2-qu in oxa lin yl)-1H-5-p yr r olyl}ben zoa te (15d ). To
a solution of 15a (120 mg, 0.31 mmol) in N,N-dimethylforma-
mide (10 mL) was added sodium hydride (60% in mineral oil)
(19 mg, 0.48 mmol) and iodomethane (38 µL, 0.61 mmol) at 0
°C, and the mixture was stirred for 1 h at room temperature.
A saturated aqueous ammonium chloride solution was added,
and the whole was extracted with ethyl acetate. The organic
layer was washed with water and brine, then dried, and
evaporated. The crude residue was purified by flash column
chromatography on silica gel (solvent: 5% ethyl acetate-n-
hexane) to afford 15d (110 mg, 0.27 mmol, 88%) as a pale
yellow solid: 1H NMR (400 MHz, CDCl3) δ 1.35 (s, 6H), 1.36
(s, 6H), 1.82 (s, 4H), 3.94 (s, 3H), 3.95 (s, 3H), 6.39 (d, J ) 3.8
Hz, 1H), 6.70 (d, J ) 3.8 Hz, 1H), 7.55 (d, J ) 8.2 Hz, 2H),
8.09 (d, J ) 8.2 Hz, 2H), 8.64 (s, 1H).
4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-1H-2-p yr r olyl]ben zoic Acid (3a ). Compound 3a was
synthesized from 15a following the representative procedure
described for 1 and obtained as a yellow solid in 93% yield:
mp 280-283 °C (EtOH); 1H NMR (400 MHz, CDCl3) δ 1.34 (s,
6H), 1.40 (s, 6H), 1.82 (s, 4H), 6.76 (dd, J ) 2.8, 3.8 Hz, 1H),
6.84 (dd, J ) 2.4, 3.8 Hz, 1H), 7.65 (d, J ) 8.4 Hz, 2H), 8.14
(d, J ) 8.4 Hz, 2H), 8.68 (s, 1H), 9.66 (br s, 1H). Anal.
(C23H25N3O2‚0.2H2O) C, H, N.
4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-2-fu r yl]ben zoic Acid (3b). Compound 3b was syn-
thesized from 15b following the representative procedure
described for 1 and obtained as a orange solid in 78% yield:
mp 275 °C (decomp.) (EtOH-water); 1H NMR (400 MHz,
CDCl3) δ 1.18 (s, 6H), 1.20 (s, 6H), 1.66 (s, 4H), 6.77 (d, J )
3.6 Hz, 1H), 7.03 (d, J ) 3.6 Hz, 1H), 7.64 (d, J ) 8.4 Hz, 2H),
7.94 (d, J ) 8.4 Hz, 2H), 8.66 (s, 1H). Anal. (C23H24N2O3‚
0.2H2O) C, H, N.
4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-2-th ien yl]ben zoic Acid (3c). Compound 3c was syn-
thesized from 15c following the representative procedure
described for 1 and obtained as a pale brown solid in 77%
yield: mp 292 °C (decomp.) (EtOH-water); 1H NMR (400
MHz, CDCl3) δ 1.20 (s, 6H), 1.24 (s, 6H), 1.68 (s, 4H), 7.31 (d,
J ) 3.6 Hz, 1H), 7.47 (d, J ) 3.6 Hz, 1H), 7.60 (d, J ) 8.4 Hz,
2H), 7.94 (d, J ) 8.4 Hz, 2H), 8.58 (s, 1H). Anal. (C23H24N2O2S‚

Synthesis of RARR Agonist
J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3 417
ethyl acetate. The organic layer was washed with water and
brine, then dried, and evaporated to afford 18 (2.80 g, 6.2
mmol, 90%) as a pale brown solid. This crude solid was used
without further purification.
(1.94 mL, 3.10 mmol) at 0 °C, and the mixture was stirred for
30 min at this temperature. This solution was added to a
solution of 16 (600 mg, 2.58 mmol) in tetrahydrofuran (30 mL)
at -78 °C. After the mixture had been stirred for 30 min at
this temperature, a solution of terephthalic acid monomethyl
ester chloride (564 mg, 2.84 mmol) in tetrahydrofuran (10 mL)
was added, and the whole was stirred for 30 min at -78 °C.
Saturated aqueous ammonium chloride solution was added,
and the mixture was extracted with ethyl acetate. The organic
layer was washed with water and brine, then dried, and
evaporated. The crude residue was purified by flash column
chromatography on silica gel (solvent: 5% ethyl acetate-n-
hexane) to afford 22 (375 mg, 0.951 mmol, 37%) as a pale
yellow solid: 1H NMR (400 MHz, CDCl3) δ 1.37 (s, 6H), 1.41
(s, 6H), 1.85 (s, 4H), 3.96 (s, 3H), 7.54 (s, 1H), 8.06 (d, J ) 8.8
Hz, 2H), 8.16 (d, J ) 8.8 Hz, 2H), 9.10 (s, 1H).
Meth yl 4-[5-(5,6,7,8-tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-1H-3-p yr a zolyl]ben zoa te (23). To a solution
of 22 (200 mg, 0.507 mmol) in acetic acid (2 mL) was added
hydrazine monohydrate (38 mg, 0.760 mmol) at room temper-
ature, and the mixture was stirred under reflux for 2 h, poured
into a saturated aqueous sodium hydrogen carbonate solution,
and extracted with ethyl acetate. The organic layer was
washed with water and brine, then dried, and evaporated. The
crude residue was purified by flash column chromatography
on silica gel (solvent: 5% ethyl acetate-n-hexane) to afford
23 (190 mg, 0.487 mmol, 96%) as a colorless solid: 1H NMR
(400 MHz, CDCl3) δ 1.36 (s, 6H), 1.38 (s, 6H), 1.83 (s, 4H),
3.94 (s, 3H), 7.11 (s, 1H), 7.94 (d, J ) 8.4 Hz, 2H), 8.11 (d, J
) 8.4 Hz, 2H), 8.74 (s, 1H).
4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-1H-3-p yr a zolyl]ben zoic Acid (24). Compound 24 was
synthesized from 23 following the representative procedure
described for 1 and obtained as a pale yellow solid in 86%
yield: mp 295-298 °C (EtOH-water); 1H NMR (400 MHz,
DMSO-d6) δ 1.28 (s, 6H), 1.35 (s, 6H), 1.78 (s, 4H), 7.44 (br s,
1H), 7.92-8.04 (m, 4H), 8.92 (s, 1H). Anal. (C22H24N4O2‚
0.9H2O) C, H, N.
2-Br om o-1-(5,6,7,8-t et r a h yd r o-5,5,8,8-t et r a m et h yl-2-
qu in oxa lin yl)-1-eth a n on e (25). To a solution of copper(II)
bromide (1.08 g, 4.82 mmol) in ethyl acetate (5 mL) was added
a solution of 16 (700 mg, 3.01 mmol) in chloroform (5 mL) at
room temperature, and the mixture was stirred under reflux
for 8 h. Water was added, and the whole was extracted with
ethyl acetate. The organic layer was washed with water and
brine, then dried, and evaporated. The crude residue was puri-
fied by flash column chromatography on silica gel (solvent: 5%
ethyl acetate-n-hexane) to afford 25 (815 mg, 2.62 mmol, 87%)
as a pale yellow oil: 1H NMR (400 MHz, CDCl3) δ 1.35 (s, 6H),
1.36 (s, 6H), 1.84 (s, 4H), 4.75 (s, 2H), 9.01 (s, 1H).
Meth yl 4-[4-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-1H-2-im id a zolyl]ben zoa te (26a ). To a solu-
tion of 25 (1.54 g, 4.95 mmol) in N,N-dimethylformamide (10
mL) was added methyl 4-amizinobenzoate acetate (1.24 g, 5.20
mmol) and potassium carbonate (2.39 g, 17.3 mmol) at room
temperature, and the mixture was stirred under reflux for 2
h. Water was added, and the whole was extracted with ethyl
acetate. The organic layer was washed with water and brine,
then dried, and evaporated. The crude residue was purified
by flash column chromatography on silica gel (solvent: 20%
ethyl acetate-n-hexane) to afford 26a (280 mg, 0.60 mmol,
12%) as a pale yellow oil: 1H NMR (400 MHz, CDCl3) δ 1.35
(s, 6H), 1.39 (s, 6H), 1.83 (s, 4H), 3.95 (s, 3H), 7.71 (s, 1H),
8.00 (d, J ) 8.2 Hz, 2H), 8.15 (d, J ) 8.2 Hz, 2H), 8.72 (s, 1H),
10.24 (br s, 1H).
Meth yl 4-[4-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-1,3-th ia zol-2-yl]ben zoa te (26b). To a solu-
tion of 25 (200 mg, 0.643 mmol) in 2-propanol (10 mL) was
added 4-carbomethoxybenzthioamide (132 mg, 0.675 mmol)
and pyridine (0.1 mL) at room temperature, and the mixture
was stirred under reflux for 2 h. Water was added, and the
whole was extracted with ethyl acetate. The organic layer was
washed with water and brine, then dried, and evaporated. The
crude residue was purified by flash column chromatography
Meth yl 4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-1H-3-p yr r olyl]ben zoa te (19a ). To a solution
of 18 (250 mg, 0.55 mmol) in acetic acid (5 mL) was added
ammonium acetate (220 mg, 2.85 mmol) at room temperature,
and the mixture was stirred under reflux for 1 h. It was poured
onto ice, and extracted with ethyl acetate. The organic layer
was washed with saturated aqueous sodium hydrogen carbon-
ate and brine, then dried, and evaporated. The crude residue
was purified by flash column chromatography on silica gel
(solvent: 10% ethyl acetate-n-hexane) to afford 19a (190 mg,
0.49 mmol, 89%) as a dark green solid: 1H NMR (400 MHz,
CDCl3) δ 1.34 (s, 6H), 1.36 (s, 6H), 1.81 (s, 4H), 3.93 (s, 3H),
7.06 (dd, J ) 1.6, 2.4 Hz, 1H), 7.32 (dd, J ) 1.6, 2.8 Hz, 1H),
7.63 (d, J ) 8.4 Hz, 2H), 8.03 (d, J ) 8.4 Hz, 2H), 8.68 (s, 1H),
9.55 (br s, 1H).
Meth yl 4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-3-fu r yl]ben zoa te (19b). A solution of 18 (2.0
g, 4.40 mmol) in concentrated sulfuric acid (10 mL) was stirred
for 12 h at room temperature then poured into saturated
aqueous sodium hydrogen carbonate solution and extracted
with ethyl acetate. The organic layer was washed with water
and brine, dried, and evaporated. The crude residue was
purified by flash column chromatography on silica gel (sol-
vent: 5% ethyl acetate-n-hexane) to afford 19b (300 mg, 0.77
mmol, 17%) as a pale yellow solid: 1H NMR (400 MHz, CDCl3)
δ 1.35 (s, 6H), 1.38 (s, 6H), 1.82 (s, 4H), 3.94 (s, 3H), 7.41 (s,
1H), 7.64 (d, J ) 8.4 Hz, 2H), 7.90 (s, 1H), 8.07 (d, J ) 8.4 Hz,
2H), 8.75 (s, 1H).
Meth yl 4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-
qu in oxa lin yl)-3-th ien yl]ben zoa te (19c). To a solution of 18
(400 mg, 0.88 mmol) in xylene (10 mL) was added phosphorus
pentasulfide (196 mg, 0.88 mmol) at room temperature, and
the mixture was stirred under reflux for 2 h then purified by
flash column chromatography on silica gel (solvent: 5% ethyl
acetate-n-hexane) to afford 19c (100 mg, 0.25 mmol, 28%) as
a yellow solid: 1H NMR (400 MHz, CDCl3) δ 1.35 (s, 6H), 1.38
(s, 6H), 1.82 (s, 4H), 3.94 (s, 3H), 7.62 (d, J ) 1.2 Hz, 2H),
7.71 (d, J ) 8.2 Hz, 2H), 7.90 (d, 1H, J ) 1.2 Hz, 1H), 8.09 (d,
J ) 8.2 Hz, 2H), 8.74 (s, 1H).
4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-1H-3-p yr r olyl]ben zoic Acid (20a ). Compound 20a
was synthesized from 19a following the representative pro-
cedure described for 1 and obtained as a pale brown solid in
76% yield: mp 245-248 °C (EtOH-water); 1H NMR (400
MHz, DMSO-d6) δ 1.27 (s, 6H), 1.35 (s, 6H), 1.75 (s, 4H), 7.30-
7.34 (m, 1H), 7.54-7.59 (m, 1H), 7.72 (d, J ) 8.4 Hz, 2H), 7.88
(d, J ) 8.4 Hz, 2H), 8.79 (s, 1H). Anal. (C23H25N3O2‚0.4H2O)
C, H, N.
4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-3-fu r yl]ben zoic Acid (20b). Compound 20b was syn-
thesized from 19b following the representative procedure
described for 1 and obtained as a pale brown solid in 52%
yield: mp 267-269 °C (EtOH-water); 1H NMR (400 MHz,
CDCl3) δ 1.35 (s, 6H), 1.38 (s, 6H), 1.83 (s, 4H), 7.42 (d, J )
0.8 Hz, 1H), 7.67 (d, J ) 8.4 Hz, 2H), 7.92 (d, J ) 0.8 Hz, 1H),
8.13 (d, J ) 8.4 Hz, 2H), 8.77 (s, 1H). Anal. (C23H24N2O3‚
4.0H2O) C, N; H: calcd, 7.19; found, 5.46. HRMS Calcd for
C
23H25N2O3 (MH+): 376.1865. Found: 377.1857.
4-[5-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-3-th ien yl}ben zoic Acid (20c). Compound 20c was
synthesized from 19c following the representative procedure
described for 1 and obtained as a pale brown solid in 83%
yield: mp 247-249 °C (EtOH-water); 1H NMR (400 MHz,
DMSO-d6) δ 1.28 (s, 6H), 1.30 (s, 6H), 1.77 (s, 4H), 7.90 (d, J
) 8.4 Hz, 2H), 7.99 (d, J ) 8.4 Hz, 2H), 8.17 (s, 1H), 8.42 (s,
1H), 9.06 (s, 1H). Anal. (C23H24N2O2S‚0.9H2O) C, H, N.
Meth yl 4-[3-Oxo-3-(5,6,7,8-tetr ah ydr o-5,5,8,8-tetr am eth -
yl-2-qu in oxa lin yl) p r op a n oyl]ben zoa te (22). To a solution
of diisopropylamine (542 µL, 3.87 mmol) in tetrahydrofuran
(10 mL) was added n-butyllithium (1.6 M solution in n-hexane)

418 J ournal of Medicinal Chemistry, 2000, Vol. 43, No. 3
Kikuchi et al.
184-194. (c) Nagpal, S.; Chandraratna, R. A. S. Retinoids as
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J . 1996, 10, 1031-1039. (e) Hong, W. K.; Sporn, M. B. Recent
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1073-1077.
on silica gel (solvent: 5% ethyl acetate-n-hexane) to afford
26b (135 mg, 0.331 mmol, 52%) as a colorless solid: 1H NMR
(400 MHz, CDCl3) δ 1.36 (s, 6H), 1.38 (s, 6H), 1.83 (s, 4H),
3.95 (s, 3H), 8.10 (d, J ) 8.8 Hz, 2H), 8.13 (d, J ) 8.8 Hz, 2H),
8.15 (s, 1H), 9.22 (s, 1H).
(5) (a) Amstrong, R. B.; Ashenfelter, K. O.; Eckhoff, C.; Levin, A.
A.; Shapio, S. S. General and Reproductive Toxicology of
Retinoids. The Retinoids: Biology, Chemistry, and Medicine, 2nd
ed.; Raven Press: New York, 1994; pp 545-572. (b) Standeven,
A. M.; Beard, R. L.; J ohnson, A. T. Boehm, M. F. Escobar, M.;
Heyman, R. A.; Chandraratna, R. A. S. Retinoid-Induced Hy-
pertriglyceridemia in Rats Is Mediated by Retinoic Acid Recep-
tors. Fundam. Appl. Toxicol. 1996, 33, 264-271. (c) Soprano,
D. R.; Soprano, K. J . Retinoids as Teratogens [Review]. Annu.
Rev. Nutr. 1995, 15, 111-132. (d) Willhite, C. C.; Dawson, M. I.
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Toxicology IV. Planar Cisoid Conformational Restriction. Toxicol.
Appl. Pharmacol. 1990, 103, 324-344.
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K. Retinobenzoic Acids. 1. Structure-Activity Relationships of
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31, 2182-2192. (b) Teng, M.; Duong, T. T.; Klein, E. S.; Chan-
draratna, R. A. S. Identification of a Retinoic Acid Receptor R
Subtype Specific Agonist. J . Med. Chem. 1996, 39, 3035-3038.
(c) J ohnson, A. T.; Klein, E. S.; Wang, L.; Pino, M. E.; Chan-
draratna, R. A. S. Identification of Retinoic Acid Receptor â
Subtype Specific Agonists. J . Med. Chem. 1996, 39, 5027-5030.
(d) Esgleyes-Ribot, T.; Chandraratna, R. A. S.; Lew-Kaya, D.;
Sefton, J .; Duvic, M. J .; Response of Psoriasis to a New Topical
Retinoid, AGN190168. J . Am. Acad. Dermatol. 1994, 30, 581-
590. (e) Delescluse, C.; Cavey, M. T.; Martin, B.; Bernard, B. A.;
Reichert, U.; Maignan, J .; Darmon, M.; Shroot, B. Selective High
Affinity Retinoic Receptor R or â-γ Ligands. Mol. Pharmacol.
1991, 40, 556-562. (f) Charpentier, B.; Bernardon, J .-M.;
Eustache, J .; Millois, C.; Martin, B.; Michel, S.; Shroot, B.
Synthesis, Structure-Activity Relationships, and Biological Ac-
tivities of Ligands Binding to Retinoic Acid Receptor Subtypes.
J . Med. Chem. 1995, 38, 4993-5006. (g) Yu, K.-L.; Spinazze,
P.; Ostrowski, J .; Currier, S. J .; Pack, E. J .; Hammer, L.;
Roalsvig, T.; Honeyman, J . A.; Tortolani, D. R.; Reczek, P. R.;
Mansuri, M. M.; Starret, J . E., J r. Retinoic Acid Receptor â,
γ-Selective Ligands: Synthesis and Biological Activity of 6-Sub-
stituted 2-Naphthoic Acid Retinoids. J . Med. Chem. 1996, 39,
2411-2421.
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J .; Nelson, E. C.; Subramanian, S.; Weerasekare, G. M.; Gale,
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R.; Naoe, T.; Ohno, R. Treatment With a New Synthetic Retinoid,
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(9) (a) Ishibashi, Y.; Harada, S.; Ohkawara, A.; Tagami, H.; Iizuka,
H.; Toda, K.; Takigawa, M.; Shimizu, M.; Nakashima, M. Clinical
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Effect-. Rinshouiyaku 1995, 11, 721-732. (b) Ishibashi, Y.
Efficacy of Am-80 Ointment on Psoriasis -A Bilateral-paired
Comparative Study with Betamethasone Valerate Ointment
(Phase III Study)-. Rinshouiyaku 1995, 11, 733-759.
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4-[4-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-1H-2-im id a zolyl]ben zoic Acid (27a ). Compound 27a
was synthesized from 26a following the representative pro-
cedure described for 1 and obtained as a yellow solid in 85%
yield: mp >300 °C (EtOH-water); 1H NMR (400 MHz, DMSO-
d6) δ 1.28 (s, 6H), 1.32 (s, 6H), 1.77 (s, 4H), 7.91 (br s, 1H),
8.03 (d, J ) 8.2 Hz, 2H), 8.13 (br d, J ) 8.2 Hz, 2H), 8.91 (s,
1H), 13.15 (br s, 1H). Anal. (C22H24N4O2‚1.2H2O) C, H, N.
4-[4-(5,6,7,8-Tetr a h yd r o-5,5,8,8-tetr a m eth yl-2-qu in oxa -
lin yl)-1,3-th ia zol-2-yl]ben zoic Acid (27b). Compound 27b
was synthesized from 26b according to the procedure described
for 1 and obtained as a yellow solid in 90% yield: mp 262 °C
(decomp.) (EtOH-water); 1H NMR (400 MHz, DMSO-d6) δ 1.30
(s, 6H), 1.34 (s, 6H), 1.79 (s, 4H), 8.07 (d, J ) 8.2 Hz, 2H),
8.16 (d, J ) 8.2 Hz, 2H), 8.45 (s, 1H), 9.12 (s, 1H). Anal.
(C22H23N3O2S) C, H, N.
Biology. Binding assays were performed as previously
reported.11b Compounds were tested in log dilutions from 5.0
× 10-7 to 5.0 × 10-11 M with duplicate determinations at each
concentration. Binding in the presence of a 1000-fold excess
of unlabeled ligand was defined as nonspecific binding. Specific
binding was defined as the total binding minus the nonspecific
binding, and the 50% inhibitory dose (IC50) values were
obtained from logarithmic plots. The selectivity of compounds
for receptors is indicated as relative IC50, obtained by dividing
the IC50 value of a compound for a given receptor by that of
ATRA. Transient transactivation assay for each receptor were
also done by the reported method.11b Compounds were tested
in log dilutions from 3.0 × 10-6 to 1.0 × 10-10 M with duplicate
determinations at each concentration. Receptor and reporter
vectors were transfected into COS-1 cells by means of the
Lipofection method. After 4 h of incubation, the medium was
replaced with DMEM supplemented with 10% FBS and
incubation was continued for an additional 20 h. The cells were
then suspended in DMEM supplemented with 10% FBS and
seeded at 3 × 104 per well in 96-well plates. After 6 h of
incubation, test compounds at various concentrations were
added to duplicate wells. The plates were incubated for a
further 48 h, then the cell supernatants were assayed for PLAP
activity.18 HL60 differentiation-inducing activity was measured
by the method described previously.11a Compounds were tested
in log dilutions from 1.0 × 10-5 to 1.0 × 10-12 M with duplicate
determinations at each concentration. HL60 cells (1 × 105)
were suspended in 1 mL of RPMI1640, plated in 48-well plates,
and exposed to compounds for 5 days. The extent of dif-
ferentiation was assayed by measuring CD11b expression on
the cell surface with a FACS method. The values given in the
table are averages of those from at least two experiments.
Ack n ow led gm en t. We thank Ms. Seiko Higashi
and Dr. Naoki Asai for the technical assistance and for
measuring high-resolution mass spectra.
(11) (a) Yoshimura, H.; Nagai, M.; Hibi, S.; Kikuchi, K.; Abe, S.; Hida,
T.; Higashi, S.; Hishinuma, I.; Yamanaka, T. A Novel Type of
Retinoic Acid Receptor Antagonist: Synthesis and Structure-
Activity Relationships of Heterocyclic Ring-Containing Benzoic
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Syntheses and Structure-Activity Relationships of Novel Ret-
inoid X Receptor Agonists. J . Med. Chem. 1998, 41, 3245-
3252.
(12) This compound was originally synthesized in our laboratory. A
patent application was filed in J une 1996 and opened in J anuary
1997. Kikuchi, K.; Tagami, K.; Yoshimura, H.; Hibi, S.; Nagai,
M.; Abe, S.; Okita, M.; Hida, T.; Higashi, S.; Tokuhara, N.;
Kobayashi, S. Heterocyclic Carboxylic Acid Derivatives and
Drugs Containing the Same. WO 97/02244. Recently Dr. P. J ones
et al. reported a new route for the synthesis of ER-33635 and
described the retinoidal activity of this compound. J ones, P.;
Villeneuve, G. B.; Fei, C.; DeMarte, J .; Haggarty, A. J .; Nwe, K.
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J M990063W