Synthesis and biological evaluation of himanimide C and unnatural analogues

Article abstract of DOI:10.1021/ol047664o

(Chemical Equation Presented) Recently isolated himanimide C (1) can be prepared via a short, flexible, and stereoselective synthesis using a copper-mediated tandem vicinal difunctionalization of dimethyl acetylenedicarboxylate (DMAD, 8) as a key step. Th

Full text of DOI:10.1021/ol047664o

ORGANIC
LETTERS
2005
Vol. 7, No. 4
605-608
Synthesis and Biological Evaluation of
Himanimide C and Unnatural Analogues
Patrice Selle`s*
Research Chemistry, SYNGENTA Crop Protection AG, CH-4002 Basel, Switzerland
patrice.selles@syngenta.com
Received November 12, 2004
ABSTRACT
Recently isolated himanimide C (1) can be prepared via a short, flexible, and stereoselective synthesis using a copper-mediated tandem
vicinal difunctionalization of dimethyl acetylenedicarboxylate (DMAD, 8) as a key step. The flexibility of the synthesis is exemplified by the
preparation of new unnatural himanimide analogues in order to investigate the fungicidal potency of this new family.
Himanimides 1-4 were recently isolated from basidomycete
culture in Chile and were fully characterized by spectroscopic
methods as new maleimide derivatives which inhibit the
growth of bacteria and fungi.1 Himanimides were tested
against filamentous fungi, yeast, and bacteria as well as
different cell lines in order to evaluate potential biological
activity. Himanimide C (1) exhibits good to excellent
antimicrobial activity,2 and the authors suggested that it
could be linked to the N-hydroxylated maleimide moiety.
As we believe this original N-hydroxylated maleimide
moiety could be important for biological activity, it appeared
to us that natural compounds with such simple structures
could be interesting leads in the search for new antifungal
agents for plant pathogens. Herein, a short and flexible
Our disconnection strategy is described in Scheme 1 for
synthesis of himanimide C and related compounds is
himanimide C and analogues. The introduction of the
presented.
hydroxylated amide can be envisaged at the last step from
To evaluate rapidly structure-activity relationships around
the scaffold of himanimides, we required a flexible approach
that would allow us to investigate the benzylic and the aro-
matic region of the molecule as well as the N-hydroxylated
maleimide moiety.
either diesters 5 or the corresponding maleic anhydride.
Preparation of tetrasubstituted olefins 5 can be envisaged
by cross-coupling boronic acid 6 and iodo diesters 7. The
desired iodo diesters 7 could be synthesized via copper-
mediated tandem vicinal difunctionalization of DMAD 8.3,4
Boronic acids 6 can be prepared from readily available
4-bromophenol derivatives.
(1) Aqueveque, P.; Anke, T.; Sterner, O. Z. Naturforsch. 2002, 57c, 257-
262.
(2) At 25 µg/mL, himanimide C exhibits a fungicidal activity against
Alternaria porri, Aspergillus ochraceus, and Pythium irregulare.
(3) Ratemi, E. S.; Dolence, J. M.; Poulter, C. D.; Veradas, J. C. J. Org.
Chem. 1996, 61, 6296-6301
10.1021/ol047664o CCC: $30.25
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magnesium chloride in the presence of CuBr‚Me2S followed
by iodine addition. However, with longer reaction times for
the organocuprate formation, and for the conjugated addition,
we could obtain the tetrasubstituted vinyl iodide 12 as a
single isomer in 50% yield (Table 1).
Scheme 1. Disconnection Strategy
Table 1. Preparation of Tetrasubstituted Olefinsa
compd
n
substituents
yieldsb
12
1
1
0
1
2
0
50
26
20
38
54
33
13a
13b
13c
13d
13e
5′′-fluoro
5′′-phenyl
3′′-methoxy
Our first objective was to prepare stereoselectively the
tetrasubstituted alkenes 7 from DMAD and the organocopper
derived from benzylic Grignard reagents. Such a methodol-
ogy has been reported for the synthesis of natural occurring
maleic anhydrides such as chaetomellic anhydride A3 and
isoglaucanic acid (Scheme 2).4
3′′,7′′-dimethyl
a Reagents and conditions: (i) magnesium chloride added to CuBr‚Me2S,
THF, -40 °C, 2 h then DMAD, THF -78 to -40 °C, 2 h, and quenched
with iodine in THF -40 °C to rt. b Isolated yields (%).
Our observation suggests that the benzylic organometallic
species are less reactive than the corresponding alkyl species
prepared in the former reports3-5 where the anionic character
cannot be stabilized by the adjacent aromatic nucleus.6
To study further the difunctionalization of DMAD and also
to investigate the biological importance of the benzyl moiety
of himanimides, we repeated the reaction for the preparation
of compounds 13a-e (Table 1).7 It is noteworthy that the
best yield was obtained when the homobenzylic magnesium
chloride was used as a nonstabilized organometallic inter-
mediate.
With the tetrasubstituted iodo alkenes (12; 13a-e) in hand,
we next investigated their Suzuki cross-coupling reactions
with boronic acids 6. The commercially available 4-meth-
oxyphenylboronic acid was first used as a model using
standard coupling conditions8 (Table 2).
Scheme 2. Synthesis of Isoglauconic Acid Precursor 104 a
a Reagents and conditions: (i) Grignard reagent added to
CuBr‚Me2S, THF, -40 °C, 30 min then DMAD, THF -78 °C 40
min then iodine in THF added in 30 min and 90 min at -78 °C;
(ii) Pd(OAc)2, PPh3, K2CO3, EtOH, (E)-pent-1-enyl benzodioxa-
borole 11.
These model couplings proceeded in good to excellent
yield in each instance, providing even the hindered tetra-
substituted olefin 16e in 82% yield.
At this stage, attempts to cyclize directly the diester 15
following the Chan procedure9 with hydroxylamine under
basic conditions failed to provide the N-hydroxylated male-
As shown in Scheme 2, Baldwin et al.4 prepared the
intermediate 9 by reacting an alkyl Grignard with DMAD
in the presence of CuBr‚Me2S followed by an in situ quench
with iodine. The tetrasubstituted iodoalkene 9 was then
converted into 10 by cross coupling with (E)-pent-1-enyl
benzodioxaborole 11.
At the outset of our work, the use of benzylmagnesium
halides in such a vicinal difunctionalization of acetylenes
bearing electron-withdrawing substituents was unknown to
our knowledge. In a first attempt, following the Baldwin
procedure,4a we did not succeed in obtaining the correspond-
ing tetrasubstituted olefin by treating DMAD with benzyl-
(5) Alexakis, A.; Cahiez, G.; Normant, J. F. Synthesis 1979, 826-830.
(6) By reacting a 1/1 mixture of butylmagnesium chloride and benzyl-
magnesium chloride under the conditions described in Table 1, we obtained
a 4/1 mixture in favor of the butylalkenes. For a discussion concerning the
relative reactivity of Grignard reagents, see: Sonoda, S.; Houchigai, H.;
Asaoka, M.; Takei, H. Tetrahedron Lett. 1992, 33 (22), 3145-3146.
(7) All of the compounds prepared for this study were characterized by
1
spectroscopic methods: H, 13C NMR, and MS (EI).
(4) (a) Adlington, R. M.; Baldwin, J. E.; Cox, R. J.; Pritchard G. J. Synlett
2002, 5, 820-822. (b) Baldwin, J. E.; Adlington, R. M.; Roussi, F.; Bulger,
P. G.; Marquez, R.; Mayweg, V. W. Tetrahedron 2001, 57, 7409-7416.
(c) Baldwin, J. E.; Beyeler, A.; Cox, R. J.; Keats, C.; Pritchard G. J.;
Adlington, R. M.; Watkin, D. J. Tetrahedron 1999, 55, 7636-7374.
(8) (a) Suzuki, A. Pure Appl. Chem. 1985, 57, 1749-1758. (b) For a
typical example of Suzuki cross-coupling reaction of R-iodo R,â unsaturated
esters, see: Patent WO 9424085 A1, 1994.
(9) Chan, L. C.; Lien, E. J.; Tokes, Z. J. Med. Chem. 1987, 30, 509-
514.
606
Org. Lett., Vol. 7, No. 4, 2005

ring (18e) possessing the ortho disubstituted phenyl ring
adjacent to the second aromatic nucleus. For all the other
compounds the presence of at least one methylene group
introduces more flexibility and minimizes the steric hindrance
between the two aromatic moieties.
Table 2. Suzuki Cross-Coupling Reactiona
We finally investigated the formation of the N-hydroxyl-
ated maleimides 19; 20a-e by treating the previous anhy-
drides in boiling water with hydroxylamine phosphate. It is
noteworthy that under these conditions12 (Table 4) yields
compd
n
substituent
yieldb
15
1
1
0
1
2
0
96
97
98
70
95
82
Table 4. Hydroxylamine Phosphate Treatment12 a
16a
16b
16c
16d
16e
5′′-fluoro
5′′-phenyl
3′′-methoxy
3′′,7′′-dimethyl
a Reagents and conditions: (i) Pd(TPP)4 (2.5% mol) in toluene /ethanol/
Na2CO3 (2 M in water) 3/1/1, reflux (TPP ) triphenylphosphine). b Isolated
yields (%).
compd
n
substituent
yieldb
19
1
1
0
1
2
0
60
60
53
57
53
52
imides. Their synthesis via the corresponding maleic anhy-
drides was therefore investigated.10 Although this route is
less direct, the availability of the maleic anhydrides provides
us a means of evaluating the biological significance of the
N-hydroxylated moiety.
The anhydride preparations were run under basic condi-
tions followed by an acidic workup.11 As seen in Table 3,
the reaction proceeded from low (18e) to good yields (17).
20a
20b
20c
20d
20e
5′′-fluoro
5′′-phenyl
3′′-methoxy
3′′,7′′-dimethyl
a Reagents and conditions: (i) hydroxylamine phosphate; water reflux,
7 h. b Isolated yields (%).
were all in the same range even for the highly hindered 18e
for which we had previously obtained a lower yield during
the cyclization.
Table 3. Saponification-Cyclizationa
While we could have prepared Himanimide 1 from
compound 19 via a sequence of dealkylation-alkylation of
the phenol ring, we decided to use the chemistry described
above to introduce the proper aromatic moiety on the iodo-
tetrasubstituted olefin 12. For this purpose, we prepared the
boronic acid 21 as shown in Scheme 3.
compd
n
substituent
yieldb
Scheme 3. Preparation of Boronic Acid 21a
17
1
1
0
1
2
0
97
51
95
76
70
27
18a
18b
18c
18d
18e
5′′-fluoro
5′′-phenyl
3′′-methoxy
3′′,7′′-dimethyl
a Reagents and conditions: (i) NaOH 2 N; reflux 2-6 h followed by
HCl 1 N, rt. b Isolated yields (%).
The lower yield obtained for the cyclization of 16e is
understandable since considerable steric constraints are
generated by the formation of the unsaturated five-membered
a Reagents and conditions: (i) TPP, diisopropyl azodicarboxylate,
toluene, rt, 3 h; (ii) (1) t-BuLi, THF, -78 °C, (2) B(OMe) 3, -78
°C, (3) HCl 1 N, -20 °C.
(10) Icikawa, Y.; Naganawa, A.; Isobe, M. Synlett 1993, 737-738.
(11) The cyclization reaction can be followed by the color change in
the reaction vessels. The anhydrides obtained exhibit in each case a strong
fluorescence from yellowish green to reddish yellow. The fluorescence
spectra were not registered on these compounds.
Commercially available 4-bromophenol 22 was alkylated
under Mitsunobu-like conditions13 with 3-methyl-2-buten-
1-ol 23. Halogen-metal exchange with t-BuLi of bromide
Org. Lett., Vol. 7, No. 4, 2005
607

24 followed by treatment with trimethyl borate provided after
hydrolysis 21 with an overall yield of 48%. With boronic
acid 21 in hand, synthesis of himanimide C was realized as
described in Scheme 4. The Suzuki cross-coupling reaction
Scheme 5 a
Scheme 4. Synthesis of Himanimide Ca
a Reagents and conditions: (i) methanol, reflux, 2-4 h.
In contrast to the previously reported results,1 we found that
none of them exhibited fungicidal activity in vitro or in
planta.
Furthermore, we demonstrated that compounds from this
family bearing the N-hydroxylated maleimide moiety are
rapidly metabolized in our standard biokinetics metabolism
assay15 with a loss of 98% of the parent compound after 24
h of treatment.
a Reagents and conditions: (i) Pd(TPP)4 (2.5% mol) in toluene/
ethanol/Na2CO3 (2 M in water) 3/1/1, reflux 2 h; (ii) NaOH 2 N;
reflux 4 h followed by HCl 1 N, rt; (iii) hydroxylamine phosphate;
water reflux, 7 h.
In summary, we report a short and flexible synthetic access
to himanimide C (1). Related natural and nonnatural com-
pounds were successfully prepared via this methodology. We
also demonstrated that himanimide C was not active as a
fungicide against plant pathogens in our assays, which may
be linked to the poor metabolic stability.
afforded the diester 25, which was readily transformed by
the complete saponification-cyclization-amide formation
1
sequence into the desired himanimide C 1. The H and 13C
spectroscopic data obtained for the synthetic himanimide C
were identical to those of the natural product, therefore
confirming its structure.
The saponification step was low yielding (40%) due to
the chemical sensitivity of the unsaturated chain. Neverthe-
less, applied on gram scale, this synthesis provided us with
enough material to run the biological tests.
In addition, we exemplified the flexibility of our approach
by preparing the more exotic maleimides 26 and 27 by
treating the anhydrides 18b and 18d with benzyl- or
alkylamines (Scheme 5).
All of the compounds and intermediates were evaluated
in our biological screens against the major plant pathogens.14
Acknowledgment. We are grateful to Dr. John Clough
and Dr. Fiona Murphy for fruitful discussions and proposals.
We thank Y. Laime for performing technical work and
analysis and Dr. Ian Southworth for biokinetic measurements.
Supporting Information Available: Experimental pro-
cedures and analytical data for the preparation of himanimide
C 1 and all intermediates. This material is available free of
charge via the Internet at http://pubs.acs.org.
OL047664O
(14) The compounds were tested in vitro (Agar plates) and in planta
(leaf disks assays) against Oomycetes (Plasmopara Viticola; Phytophthora
infestans), Ascomycetes (Pyrenophora teres, Erysiphe graminis) and
Basidiomycetes (Puccinia recondita, Rhizoctonia solani)
(12) Patent EP 1 085 013 A1, 2000.
(13) Sankara, S. R.; Balasubramanian, K. K. Synth. Commun. 1989, 19
(7-8), 1255-1259.
(15) Biokinetic data were recorded by measuring over a period of time
the disappearance of the parent compound in maize cell cultures.
608
Org. Lett., Vol. 7, No. 4, 2005