Novel retinoidal tropolone derivatives. Bioisosteric relationship of tropolone ring with benzoic acid moiety in retinoid structure

Article abstract of DOI:10.1248/cpb.49.501

Several tropolone derivatives (4-7) were designed as novel retinoids on the assumption that the tropolone ring may mimic the benzoic acid moiety in retinoid structures, such as Am80 (2). Among the synthesized compounds, 5-[2-(5,6,7,8-tetrahydro5,5,8,8-tetramethyl-2-naphthyl)ethynyl]tropolone (7a) showed moderate potency as a differentiation-inducer of HL-60 cells. The activities of the tropolones were greatly enhanced in the presence of HX630, an RXR agonist (retinoid synergist).

Full text of DOI:10.1248/cpb.49.501

April 2001
Communications to the Editor
Chem. Pharm. Bull. 49(4) 501—503 (2001)
501
1). Based on this hypothesis, several tropolone derivatives
(4—7, Fig. 2) were synthesized. The synthetic routes are il-
lustrated in Chart 1. Nitrosonation (90%) of 8, followed by
reduction afforded 5-aminotropolone (9, 95%).8) The reaction
of non-protected 9 with 5,6,7,8-tetrahydro-5,5,8,8-tetram-
ethyl-2-naphthoyl chloride (10, 2 eq) gave the diacylated
compound 11 (83%), which was hydrolyzed under basic con-
ditions to afford 4. Since the hydroxyl group of 9 is more re-
active than the amino group, the reaction of 9 with mesyl
chloride (1 eq) gave the O-mesylated compound 12 (52%),
which was reacted with the isocyanate 14 (15%), followed by
hydrolysis to afford 5 (quant). The azo compound 6 was syn-
thesized by reaction of 8 with the diazonium salt, prepared
from the amine 15 (23%). In order to synthesize compound 7
having an acetylene group, 9 was converted to 5-iodo-2-
methoxytropone (17, 31%). 6-Bromo-1,2,3,4-tetrahydro-
1,1,4,4-tetramethylnaphthalene (18) was coupled with
(trimethylsilyl)acetylene by means of the Sonogashira reac-
tion (79%). After removal of the TMS group (94%), 20 was
coupled with 17 by means of the Sonogashira reaction to
give 7b (81%), which was hydrolyzed to afford 7a (quant).
The methoxyl group of 7b was also converted to hydrazine
(7c, 90%),14) and then a hydrogen atom (7d, 7%).
Novel Retinoidal Tropolone Derivatives.
Bioisosteric Relationship of Tropolone
Ring with Benzoic Acid Moiety in
Retinoid Structure
Masayuki EBISAWA,a Kiminori OHTA,a
Emiko KAWACHI,a Hiroshi FUKASAWA,a
Yuichi HASHIMOTO,b and Hiroyuki KAGECHIKA*,a
Graduate School of Pharmaceutical Sciences, The University of
Tokyo,a 7–3–1 Hongo, Bunkyo-ku, Tokyo 113–0033, Japan and
Institute of Molecular and Cellular Biosciences, The University of
Tokyo,b 1–1–1 Yayoi, Bunkyo-ku, Tokyo 113–0032, Japan.
Received January 16, 2001; accepted February 13, 2001
Several tropolone derivatives (4—7) were designed as novel
retinoids on the assumption that the tropolone ring may mimic
the benzoic acid moiety in retinoid structures, such as Am80 (2).
Among the synthesized compounds, 5-[2-(5,6,7,8-tetrahydro-
5,5,8,8-tetramethyl-2-naphthyl)ethynyl]tropolone (7a) showed
moderate potency as a differentiation-inducer of HL-60 cells.
The activities of the tropolones were greatly enhanced in the
presence of HX630, an RXR agonist (retinoid synergist).
The retinoidal activity of the synthesized compounds was
examined in terms of the ability to induce differentiation of
human promyelocytic leukemia cells HL-60.11) Differentiated
cells were determined by means of nitro blue tetrazolium
(NBT) reduction assay.12) Tropolone derivatives, except 5, in-
Key words retinoid; tropolone; bioisoster; cell differentiation
Retinoids, retinoic acid (all-trans, 1, Fig. 1) and its ana-
logues modulate various biological functions, such as cell
differentiation, proliferation, and embryonic development in
vertebrates.1,2) The biological activities of retinoids are medi-
ated by two types of nuclear receptors, retinoic acid receptors
(RARs) and retinoid X receptors (RXRs).3) Retinoids bind to
the RAR site of RXR·RAR heterodimers and the liganded
heterodimers regulate the expression of specific genes. RXR-
specific ligands cannot activate RXR·RAR heterodimers, but
can enhance the potency of RAR ligands.4) Since retinoids
have significant preventive or therapeutic potential in the
fields of dermatology and oncology, a number of synthetic
retinoids have been synthesized.1,5) Most potent retinoids,
such as Am80 (2), have a benzoic acid or other aromatic car-
boxylic acid moiety instead of the unstable polyenecar-
boxylic acid of retinoic acid (1). As shown in the generic
structure (3) of retinoidal benzoic acids, their structures con-
sist of a hydrophobic part, terminal carboxyl group and the
linking group (X) between them. The structure at the hy-
drophobic region or the linking group (X) can be varied with
retention of high activity, while the terminal carboxylic acid
generally can not be replaced by other functional groups
(bioisosters), such as an aminosulfonyl, amidino or tetrazolyl
group, without a significant decrease or loss of activity.6) One
exception is the successful replacement of the carboxylic
acid of RAR and RXR ligands with a thiazolidinedione moi-
ety.7) Here, we describe the design, synthesis, and biological
activities of tropolone derivatives as novel retinoids without a
carboxyl group.
Fig. 1. (a) Structures of Retinoids. (b) Design of Tropolone Derivatives as
Novel Retinoid Candidates
Tropolone (8), 2-hydroxy-2,4,6-cycloheptatrien-1-one, an
isomer of benzoic acid, is a seven-membered, non-benzenoid
aromatic molecule possessing three double bonds conjugated
with a carbonyl group.8,9) Since tropolone is regarded as a
vinylog of a carboxylic acid,10) we considered that the
tropolone ring might be bioisosteric with benzoic acid (Fig.
Fig. 2. Structures of Tropolone Derivatives
To whom correspondence should be addressed. e-mail: kage@mol.f.u-tokyo.ac.jp
© 2001 Pharmaceutical Society of Japan

502
Vol. 49, No. 4
Chart 1
Fig. 3. Differentiation-Inducing Activity of Tropolone Derivatives (a) Alone or (b, c) in the Presence of a Retinoid Synergist, HX630 (1ϫ10Ϫ7 M), in HL-
60 Cells
The vertical scale is the percentage of differentiated cells evaluated from NBT reduction assay, and the horizontal scale is the molar concentration of test compounds. Added
compound: (᭺) Am80, (᭹) 4, (ϫ) 5, (᭛) 6, (᭡) 7a, (᭝) 7b, (᭿) 7c, or (ᮀ) 7d.
duced HL-60 cell differentiation into mature granulocytes in
a similar manner as 2, as shown in Fig 3a. However, their po-
tency is weaker than that of 2, and the maximum responses at
high concentration (1ϫ10Ϫ7 M) are smaller than that of 2.
Even the acetylenic compound 7a, the most active among the
tropolone derivatives, induced at most 60% of the cells to
differentiate. However, the differentiation-inducing activities
of tropolone derivatives were strongly enhanced in the pres-
ence of an RXR agonist, HX630 (Figs. 3b, 4). HX630 does
Fig. 4. Structure of HX630
not exhibit retinoidal activity alone, but enhances the activi-
ties of various RAR agonists in HL-60 assay.13) The EC50 val- tivity of 7a was also enhanced by HX630, and the EC50 value
ues of 2 are 5.5ϫ10Ϫ10 M and 3.6ϫ10Ϫ11 M in the absence and of 7a in the presence of 1ϫ10Ϫ7 M HX630 is 1.3ϫ10Ϫ10 M,
presence of 1ϫ10Ϫ7 M HX630, respectively. Thus, HX630 lower than that of 2 alone, but only about 1/4 as potent as the
enhances the activity of 2 by one order of magnitude. The ac- combination of 2 and HX630. Further, it is significant that

April 2001
503
the maximum response to 7a reached a similar value (90%) ological activity, they may provide further scope for employ-
to that of 2. More remarkable increases in the activity were ing retinoids in clinical applications.
observed for compounds 4 and 6. These compounds are poor
References and Notes
1) Sporn M. B., Roberts A. B., Goodman D. S., (eds.), “The Retinoids,”
2nd ed., Raven Press, New York, 1994.
2) Kastner P., Mark M., Chambon P., Cell, 83, 859—869 (1995).
differentiation-inducers alone, but highly active in combina-
tion with HX630. Their EC50 values (4.4ϫ10Ϫ10 M for 4 and
2.0ϫ10Ϫ10 M for 6) in the presence of 1ϫ10Ϫ7 M HX630 are
3) Chambon P., FASEB J., 10, 940—954 (1996).
similar to that of 7a under the same conditions. Only com-
pound 5 was inactive in this experiment, which suggests that
the ureido bond may be too long a linking group between the
hydrophobic aromatic and tropolone rings. These results in-
dicate that the tropolones 4, 6 and 7a are potent retinoids
(RAR agonists) only in the presence of RXR agonist.
Compounds 7b—d are nearly inactive alone (data not
shown), and exhibited differentiation-inducing activity only
at high concentrations even in the presence of HX630 (Fig.
3c). The hydroxyl group on the tropone ring seems to be im-
portant for potent activity.
In conclusion, we found that the tropolone ring can act as
a bioisoster of the benzoic acid moiety in retinoid structures.
Some tropolone derivatives elicited potent retinoidal activity
in combination with the RXR agonist HX630, although they
showed poor activity alone in HL-60 assay. At present, the
reason for the dramatic change in potency is unknown, but it
is presumably related to some specific affinity to RARs,
RXRs, or their heterodimers, including related cofactors. We
are now investigating the action mechanisms of the tropolone
derivatives using these receptors and cofactors. Since
retinoidal tropolones may exhibit different pharmacological
behaviors from retinoidal benzoic acids, as well as unique bi-
4) Minucci S., Leid M., Toyama R., Saint-Jeannet J.-P., Peterson V. J.,
Horn V., Ishmael J. E., Bhattacharyya N., Dey A., Dawid I. B., Ozato
K., Mol. Cell. Biol., 17, 644—655 (1997).
5) Shudo K., Kagechika H., Adv. Drug Res., 24, 81—119 (1993);
Kagechika H., IDrug 3, 73—83 (2000).
6) Yamakawa T., Kagechika H., Kawachi E., Hashimoto Y., Shudo K., J.
Med. Chem., 33, 1430—1437 (1990).
7) Tashima T., Kagechika H., Tsuji M., Fukasawa H., Kawachi E.,
Hashimoto Y., Shudo K., Chem. Pharm. Bull., 45, 1805—1813
(1997); Ebisawa M., Kawachi E., Fukasawa H., Hashimoto Y., Itai A.,
Shudo K., Kagechika H., Biol. Pharm. Bull., 21, 547—549 (1998).
8) Nozoe T., Seto S., Ebine S., Ito S., J. Am. Chem. Soc., 73, 1895
(1951); Nozoe T., Seto S., Proc. Jpn Acad., 27, 188—189 (1951);
Nozoe T., Seto S., Ito S., Sato M., Proc. Jpn. Acad., 27, 426—429
(1951); Pauson P. L., Chem. Rev., 55, 9—136 (1955).
9) Nozoe T., Mukai T., Takase K., Sci. Rept. Tohoku Univ. Ser. I, 39,
164—181 (1956); Pietra F., Chem. Rev., 73, 293—364 (1973).
10) Nozoe T., Mukai T., Minegishi J., Sci. Rept. Tohoku Univ. Ser. I, 37,
388—406 (1953).
11) Koeffler H. P., Blood, 62, 709—721 (1983).
12) Collins S. J., Ruscetti F. W., Gallagher R. E., Gallo R. C., J. Exp. Med.,
149, 969—974 (1979).
13) Umemiya H., Fukasawa H., Ebisawa M., Eyrolles L., Kawachi E.,
Eisenmann G., Gronemeyer H., Hashimoto Y., Shudo K., Kagechika
H., J. Med. Chem., 40, 4222—4234 (1997).
14) Seto S., Sci. Rept. Tohoku Univ. Ser. 1, 37, 275—285 (1953).