Synthesis of (±)-madindolines and chemical models. Studies of chemical reactivity

Article abstract of DOI:10.1021/ol026015e

(Matrix Presented) The madindolines are believed to inhibit cytokine signaling through the gp130 receptor. Model compounds of madindolines were synthesized and tested for thiol reactivity. The heterocyclic moiety of madindoline was shown to form thiol add

Full text of DOI:10.1021/ol026015e

ORGANIC
LETTERS
2002
Vol. 4, No. 14
2337-2340
Synthesis of (±)-Madindolines and
Chemical Models. Studies of Chemical
Reactivity
Casey C. McComas,† Joe B. Perales, and David L. Van Vranken*
Department of Chemistry, UniVersity of California, IrVine, California 92697-2025
dlVanVra@uci.edu
Received April 15, 2002
ABSTRACT
The madindolines are believed to inhibit cytokine signaling through the gp130 receptor. Model compounds of madindolines were synthesized
and tested for thiol reactivity. The heterocyclic moiety of madindoline was shown to form thiol adducts via the Savige−Fontana reaction. The
enedione moiety was found to be unreactive toward simple thiols unless the quaternary center was removed. Using the powerful Moore
reaction, we have synthesized (±)-madindoline A and B in 11 steps.
In 1996, two metabolites of Streptomyces nitrosporeus K93-
0711 were shown to selectively inhibit the growth of IL-6-
dependent cell lines.1 Madindoline A and B possess the same
tricyclic indoline moiety but differ in the position of the butyl
and methyl groups on the cyclopentene ring. The importance
of the madindolines is their selectivity; they inhibit signaling
through IL-6 and IL-11 but not IL-2, IL-4, IL-8, or LIF. An
elegant series of inhibition experiments points to the extra-
cellular domain of gp130 as the target of madindoline.2
on its target. We set out to create models of madindoline,
study their thiol reactivity, and use the chemistry to
synthesize madindolines A and B.
Streptomyces nitrosporeus K93-0711 has stopped produc-
ing madindolines, so new material must be obtained by total
synthesis.4 Quaternary centers represent a continuing chal-
lenge for synthetic organic chemistry, and the quaternary
center of madindoline proves to be a major obstacle. There
are three elegant enantioselective syntheses of madindoline
A.4,5 We hoped to exploit the powerful Moore ring contrac-
tion of azidoquinones to construct models of the madindo-
lines and ultimately the natural products.
Under acidic conditions, cysteine thiols in peptides and
proteins react with 3a-hydroxypyrrolidino[2,3-b]indolines
derived from tryptophan, leading to irreversible formation
of tryptathionine cross-links (the Savige-Fontana reaction,
Scheme 1).6 Saito has shown that tetrahydrofuro[2,3-b]indol-
The extracellular domain of gp130 has two free cysteines
(Cys279 and Cys469) that are hypothesized to form a
disulfide during cytokine activation.3 The structure of mad-
indoline is reminiscent of two key functional groups that are
known to react with cysteine thiols: maleimides and 3a-
hydroxypyrrolidino-indolines. In light of these circumstantial
facts, it is conceivable that madindoline could act covalently
† Scripps Research Institute.
(1) (a) Hayashi, M.; Kim, Y. P.; Takamatsu, S.; Enomoto, A.; Shinose,
M.; Takahashi, Y.; Tanaka, H.; Komiyama, K.; Omura, S. J. Antibiot. 1996,
49, 1091-5. (b) Takamatsu, S.; Kim, Y. P.; Enomoto, A.; Hayashi, M.;
Tanaka, H.; Komiyama, K.; Omura, S. J. Antibiot. 1997, 50, 1069-72.
(2) Omura, S.; Hayashi, M.; Tomoda, H. Pure Appl. Chem. 1999, 71,
1673-81.
(4) Sunazuka, T.; Hirose, T.; Shirahata, T.; Harigaya, Y.; Hayashi, M.;
Komiyama, K.; Omura, S.; Smith, A. B., III. J. Am. Chem. Soc. 2000, 122,
2122-3.
(5) (a) Hosokawa, S.; Sekiguchi, K.; Hayase, K.; Hirukawa, Y.; Koba-
yashi, S. Tetrahedron Lett. 2000, 41, 6435-9. (c) Hosokawa, S.; Sekiguchi,
K.; Enemoto, M.; Kobayashi, S. Tetrahedron Lett 2000, 41, 6429-33. (c)
Hirose, T.; Sunuzaka, T.; Shirata, T.; Yamamoto, D.; Harigaya, Y.;
Kuwajima, I.; Omura, S. Org. Lett., 2002, 4, 501-3.
(3) Moritz, R. L.; Hall, N. E.; Connolly, L. M.; Simpson, R. J. J. Biol.
Chem. 2001, 276, 8244-53.
10.1021/ol026015e CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/13/2002

3a-ol 1 reacts with tryptophan in the presence of Lewis acids
to form 2,2′-biindole cross-links.7 In theory, the heterocyclic
moiety of madindoline could react with either tryptophan or
cysteine side chains.
covalent bond formation has been proposed to account for
the high potency of tetramethrin toward insect ion channels.10
The alkyl substituents on the cyclopent-4-ene-1,3-dione
moiety of madindoline should slow the rate of Michael
addition, conferring the selectivity observed in the biological
studies.
Billington and co-workers have shown that cyclopent-4-
ene-1,3-diones, close analogues of maleimides, also react
rapidly in high yield with N-acetylcysteine at pH 7.4 (Scheme
1).11 However, based on analogy to studies of maleimides,
it is expected that the methyl and butyl substituents of
madindoline should slow thiol addition.
Scheme 1. Precedent for the Thiophilicity of Madindoline
Subunits
To test the potential thiophilicity of the madindoline
cyclopentene unit, a symmeterical analogue was needed that
would simplify the studies with thiols. The Moore azi-
doquinone ring contraction offers an ideal approach to these
analogues.12 The mechanism of the Moore rearrangement
(Scheme 3) is believed to involve formation of a zwitterionic
acylium-enolate that rapidly cyclizes.
Scheme 3. Mechanism of the Moore Reaction
Under acidic conditions indoline 1 condenses with N-
acetylcysteamine to produce the adduct 2 in 57% yield
(Scheme 2). Water competes with the thiol so the corre-
sponding oxindole was also isolated in 31% yield. The
Savige-Fontana reaction of indoline 1 does not proceed at
an appreciable rate at pH 7.4, consistent with the highly
specific biological profile of madindoline.
The symmetrical dione 613 was constructed using the
Moore azidoquinone ring contraction (Scheme 4). In theory
the Moore rearrangement could proceed in situ during the
azide substitution reaction. However, the addition-elimina-
tion with sodium azide was most effective in alcoholic
solvents, whereas the Moore rearrangement was most ef-
fective in nonpolar, nonnucleophilic solvents. Since the
azidoquinone deflagrates on heating it was carried directly
to the rearrangement without separation from unreacted
starting material.
Scheme 2. Savige-Fontana Reaction of Indoline 1
When madindoline model 6 was treated with 1 equiv of
N-acetylcysteamine, the thiol adduct 8 was formed in 15%
yield (as an inseparable mixture of diastereomers) along with
starting material and other products (Scheme 5). However,
the more complete model 7 failed to form thiol adducts, even
when heated. Thus, all three alkyl substituents on madin-
doline conspire to inhibit nonspecific Michael addition by
thiols.
N-Alkylmaleimides exhibit exceptional reactivity and
selectivity toward protein thiols. Miyadera has compared the
reactivity of N-ethylmaleimide with its 3,4-dimethyl analogue
and shown that the extra methyl groups slow the brisk
reaction rate by over 5 orders of magnitude.8 While the rates
are undoubtedly slowed, 3,4-dialkylmaleimides still form
cysteine9 and glutathione10 adducts under physiological
conditions. Substituents on the 3 and 4 positions of the
maleimide tetramethrin destabilize 1,4-thiol adducts, yet
(9) Tressl, R.; Wondrak, G.; Kersten, E.; Rewicki, D. J. Agric. Food
Chem. 1994, 42, 2692-7.
(10) Smith, I. H.; Wood, E. J.; Casida, J. E. J. Agric. Food Chem. 1982,
30, 598-600.
(11) Billington, S.; Mann, J.; Quazi, P.; Alexander, R.; Eaton, M. A.
W.; Millar, K.; Millican, A. Tetrahedron 1991, 47, 5231-6.
(12) (a) Moore, H. W.; Shelden, H. R. J. Org. Chem. 1968, 33, 4019-
24. (b) Moore, H. W.; Weyler, W., Jr.; Shelden, H. R. Tetrahedron Lett.
1969, 3947-50. (c) Weyler, W., Jr.; Pearce, D. S.; Moore, H. W. J. Am.
Chem. Soc. 1973, 95, 2603-10. (d) Hamdan, A. J.; Moore, H. W.
Heterocycles 1989, 29, 51-6.
(13) Cyclopent-4-ene-1,3-diones have been broadly patented for fragrance
applications; dione 7 lacks fragrant properties. Isaac, B. O.; Chan, C. O.;
Marr, I. M. U.S. Patent 5,407,910, 1995.
(6) (a) Savige, W. E.; Fontana, A. J. Chem. Soc., Chem. Commun. 1976,
600-1. (b) Savige, W. E.; Fontana, A. Int. J. Pept. Protein Res. 1980, 15,
102-12. (c) Wieland, T. B., B.; Zanotti, G. Chem. Pept. Proteins 1989, 4,
95-100.
(7) Saito, I.; Morii, T.; Matsugo, S.; Matsuura, T. J. Chem. Soc., Chem.
Commun. 1982, 977-9.
(8) Miyadera, T.; Kosower, E. M.; Kosower, N. S. J. Med. Chem. 1971,
14.
2338
Org. Lett., Vol. 4, No. 14, 2002

racemic madindolines. Unfortunately, an attempted Mannich
reaction of enedione 6 with indoline and formalin led to rapid
generation of the N,N-aminal and decomposition of the
enedione. Even though the Mannich approach appeared
untenable we hoped to exploit the Moore rearrangement by
retaining the nitrile carbon and using it to form the key
neopentyl linkage of madindoline.
Scheme 4. Synthesis of Models of Madindoline
Cyclopent-4-ene-1,3-dione
For the synthesis of the madindolines, the required n-butyl
substituent was introduced through a statistical Stille reaction
of 2,5-dibromo-3,6-dimethylbenzoquinone (Scheme 7). What
this approach lacked in efficiency it made up for in
directness. The bromoquinone 12 was substituted with azide
and heated to induce rearrangement. Thus, a fully function-
alized cyclopentene core was available in three steps from
the readily available dibromoquinone 11.
Scheme 5. Thiol Addition to Cyclopent-4-ene-1,3-diones
Scheme 7. Application of the Moore Azidoquinone Ring
Contraction to Construction of the Madindoline Core
1,3-Diketones that form stable enols are poor substrates
for Mannich reactions because of a sparing equilibrium and
the extreme facility of retro-Mannich fragmentation.14 Cy-
clopentane-1,3-dione, which exists as the enol tautomer, does
not form stable Mannich products under acidic or basic
conditions. In contrast, cyclopentene-1,3-diones such as 6
exist in the keto form. Gas-phase DFT calculations (SVWN/
DN*) predict that the 4,5-double bond of 1,3-dione 6
destabilizes the enol tautomer by over 17 kcal/mol,15 and
not surprisingly, diketone 6 readily forms a Mannich adduct
with diethylamine under basic conditions (Scheme 6).
However, the Mannich product 9 is unstable under aqueous
conditions. Presumably, retro-Mannich cleavage of madin-
doline is inhibited by the presence of the double bond in the
cyclopentenedione moiety and the attenuated reactivity of
the indoline nitrogen.
The cyclization step sets the stereochemistry of the
cyclopentene moiety. In theory a metal coordinated to the
nitrile could exert enantiocontrol in the cyclization step; the
gold-catalyzed aldol reaction of isonitrile enolates generates
high levels of enantioselection through this kind of coordina-
tion.16 Unfortunately, attempts to induce the Moore rear-
rangement using transition metals such as Rh2(OAc)4 and
Cu(OTf)2 were ineffective at accelerating the Moore reac-
tion.17 Thus, the Moore reaction is promising for the
construction of symmetric cyclopentenediones rather than
asymmetric cyclopentenediones.
One price to be paid for such a direct route comes in the
attachment to the indoline ring system. Sadly, the nitrogen
present in the cyano group could not be incorporated into
the indoline ring system. Thus, the carbonyl groups were
protected by stereoselective Luche reduction and TBS ether
formation. Nitrile 14 was reduced to aldehyde 15 and coupled
to the indoline moiety using the reductive amination approach
developed by Smith and Omura in the first synthesis.4
Scheme 6. Mannich and Retro-Mannich Reactions of
Cyclopent-4-ene-1,3-diones
(14) (a) Hellmann, H.; Opitz, G. R-Aminoalkylierung. Verlag: Weinheim,
1960; p 214. (b) Levchenko, N. K.; Sviridova, A. P.; Segal, G. M.; Torgov,
I. V. J. Chem. Res. Miniprint 1978, 5001-27.
(15) There is no evidence of the enol in the IR spectrum of diketone 6.
(16) Hayashi, T.; Sawamura, M.; Ito, Y. Tetrahedron 1999, 48, 1999-
2012.
(17) Azidoquinones have been shown to participate in metal-catalyzed
intramolecular nitrene-like cycloadditions with tethered dienes. However,
Moore rearrangement products were not reported in these studies. Naruta,
Yoshinori; Nagai, Naoshi; Arita, Yoshihiro; Maruyama, Kazuhiro J. Org.
Chem. 1987, 52, 3956-3967.
A Mannich reaction between an indoline and a cyclopent-
4-ene-1,3-dione could offer a simple approach to synthesize
Org. Lett., Vol. 4, No. 14, 2002
2339

Scheme 8. Synthesis of Key Aldehyde (()-15a
Scheme 9. Synthesis of (()-Madindoline A and
(()-Madinoline B Using the Smith and Omura Routea
a Conditions: (a) NaBH4, CeCl3‚7H2O (89%), (b) TBSOTf, 2,6,-
lutidine, DMF (91%), (c) DIBAL-H (84%).
Aldehyde (+)-15 was used by Smith and co-workers in
the synthesis of (+)-madindoline A. We followed the Smith
route to complete the synthesis of madindolines A and B.
The final oxidative cyclization under Sharpless conditions
proceeds in modest yield with only 38% stereoselection.
Thus, even under ideal conditons, we would not be able to
obtain optically pure material using the Sharpless reaction.
Instead, the more reliable DMDO oxidation was employed
in the final oxidative cyclization. Madindoline A 19, the more
active isomer, was formed in preference to madindoline B
20 in a 80:20 ratio.
a Conditions: (a) TBAF, THF, 3 h, 60 °C (75%), (b) TESCl,
Et3N, CH2Cl2, rt (66%), (c) MnO2, CH2Cl2, 1 N HCl (75%).
The level of stereoselection is somewhat surprising when
one considers that the asymmetry in the starting material is
solely due to the methyl and butyl substituents, far removed
from the indole ring.
covalent bonds with its target and such reactions would be
facilitated by preorganization, but confirmation of this
hypothesis awaits further studies.
Cysteines have been implicated in the mechanism of gp130
signaling, so two substructural models of madindoline were
prepared and their reactivity toward thiols was studied. The
furanoindoline core of the madindolines was shown to cross-
link with thiols via a Savige-Fontana reaction. We have
used the Moore azidoquinone ring contraction to synthesize
(()-madindoline A, (()-madindoline B, and model com-
pounds. The Moore reaction allowed rapid construction of
the cyclopent-3-ene-1,4-dione core of madindoline but did
not allow control of absolute stereochemistry. The alkyl
substituents of madindoline were found to inhibit Michael
addition by thiols. Madindoline has the potential to form
Acknowledgment. We gratefully acknowledge the sup-
port of the UC Cancer Research Coordinating Committee,
the NIH (GM54523), and GlaxoSmithKine for a graduate
fellowship (J.B.P.).
Supporting Information Available: Complete experi-
mental procedures and characterization data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL026015E
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Org. Lett., Vol. 4, No. 14, 2002