Synthesis and antimalarial activity of prodigiosenes

Article abstract of DOI:10.1039/c3ob42548g

Several analogues of the natural compound prodigiosin with modified A- and C-rings were synthesised as were some of their tin, cobalt, boron and zinc complexes. The antimalarial activity of these prodigiosenes was evaluated in vitro using the 3D7 Plasmodium falciparum strain. The presence of a nitrogen atom in the A-ring is needed for antimalarial activity but the presence of an alkyl group at the β′-position of the C-ring seems detrimental. Dibutyl tin complexes exhibit IC₅₀ values mostly in the nanomolar range with equal or improved activity compared to the free-base prodigiosene ligand, despite the fact that the general toxicity of such tin complexes is demonstrably lower than that of the free-bases. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3ob42548g

Organic &
Biomolecular Chemistry
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PAPER
Synthesis and antimalarial activity of
prodigiosenes†
Estelle Marchal,a Deborah A. Smithen,a Md. Imam Uddin,a Andrew W. Robertson,a
David L. Jakeman,a,b Vanessa Mollard,c Christopher D. Goodman,c
Kristopher S. MacDougall,d Sherri A. McFarland,*d Geoffrey I. McFadden*c and
Alison Thompson*a
Cite this: Org. Biomol. Chem., 2014,
12, 4132
Several analogues of the natural compound prodigiosin with modified A- and C-rings were synthesised as
were some of their tin, cobalt, boron and zinc complexes. The antimalarial activity of these prodigiosenes
was evaluated in vitro using the 3D7 Plasmodium falciparum strain. The presence of a nitrogen atom in
the A-ring is needed for antimalarial activity but the presence of an alkyl group at the β’-position of the
C-ring seems detrimental. Dibutyl tin complexes exhibit IC50 values mostly in the nanomolar range with
equal or improved activity compared to the free-base prodigiosene ligand, despite the fact that the
general toxicity of such tin complexes is demonstrably lower than that of the free-bases.
Received 20th December 2013,
Accepted 6th May 2014
DOI: 10.1039/c3ob42548g
www.rsc.org/obc
Prodigiosins (Fig. 1, 1–4) constitute a class of natural pro-
ducts isolated from bacterial strains such as Serratia marces-
Introduction
Malaria is an infectious disease caused by a parasite (Plasmo- cens.8 These tripyrrolic red pigments have been widely studied
dium) that occurs in tropical and subtropical regions. It is due to their notable biological activity, including antibacter-
responsible for more than 750 000 annual deaths and ial,9 immunosuppressive10 and anticancer11 properties.
225 million infections.1 Although there is significant promise Although the antimalarial activity of natural prodigiosins was
for future vaccines,2 the only current course of action to reported several years ago,12 the parasiticidal activity of ana-
prevent infection from the bite of a parasite-carrying mosquito logues of the prodigiosin family (termed prodigiosenes)13 was
is the use of prophylaxis and/or personal protection.3 Moreover reported only recently, with encouraging results.14 We herein
the emergence of drug-resistant forms of the parasite toward report the antimalarial activity of prodigiosenes bearing modi-
antimalarial drugs is an ever-burdening public health threat,
and it curbs the likelihood of eradicating the disease. Chloro-
quine, widely used during the 1950s, is now almost ineffective
and is administered in only a few countries in Central America
and central Asia.4 More worrisome is that resistance to artemi-
sinin, the current first-line treatment against severe malaria,
has emerged.5 Although much effort has been directed
towards the discovery of new antimalarial drugs,6 the efficacy
and safety of the artemisinins remains difficult to reach.7
aDepartment of Chemistry, Dalhousie University, PO BOX 15000, Halifax, NS B3H
4R2, Canada. E-mail: Alison.Thompson@dal.ca; Tel: +1-902-494-3305
bCollege of Pharmacy, Dalhousie University, PO Box 15000, Halifax, NS B3H 4R2,
Canada
cSchool of Botany, University of Melbourne, VIC 3010, Australia.
E-mail: gim@unimelb.edu.au; Tel: +61 414 189 905
dDepartment of Chemistry, Acadia University, Wolfville, Nova Scotia, B4P 2R6,
Canada. E-mail: sherri.mcfarland@acadiau.ca; Tel: +1-902-585-1320
†Electronic supplementary information (ESI) available: General procedures for
antimalarial assays, growth and isolation of prodigiosin, plus spectral characteri-
sation data for all new compounds. See DOI: 10.1039/c3ob42548g
Fig. 1 Selected examples of natural prodigiosins (1–4) and prodigio-
sene analogues (A and B).
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fications at the A- and C-rings (Fig. 1, A), so as to begin to
probe a structural activity relationship (SAR) as well as the
minimal active pharmacophore. The modified C-ring features:
(i) an extra methyl group at the β′-position allowing facile syn-
thetic access via Knorr-type pyrroles; and (ii) an alkyl or carbo-
nyl group at the β-position. The importance of the A-ring
pyrrole in the antimalarial activity is assessed via substitution
with other aromatic and non-aromatic groups.
Some organometallic complexes exhibit noteworthy anti-
malarial properties.6c For example, ferroquine is a ferrocenyl
complex of chloroquine15 that was evaluated in a phase II clini-
cal trial.16 Given this, plus the fact that tin-complexes of prodi-
giosenes previously demonstrated low toxicity profiles,9g we
also report the synthesis and the first antimalarial evaluation
of some metal complexes of prodigiosenes (Fig. 1, B) and
discuss their complementary behaviour compared to the
corresponding free-base dipyrrins.
Scheme 2 Synthesis of A-ring modified prodigiosenes using a Suzuki–
Miyaura coupling.
expected product (27) was not obtained even when employing
reaction conditions that had been successful in coupling alkyl
boronic acids and halogeno-arenes.18
We thus moved to a Negishi-type reaction19 in order to
obtain a prodigiosene substituted with an alkyl group in lieu
of an A-ring heterocycle: the ethyl-substituted dipyrrin salt 28
was rendered in 53% yield, after quenching with HCl and a
simple precipitation from methanol (Scheme 3).
To extend the conjugation of the dipyrrin core we attempted
a Stille coupling of tetravinyltin and dipyrrin 7 (Scheme 4).20
The reaction was performed under microwave-promoted con-
ditions at 85 °C for 10 min, after which time the starting
material (7) was completely consumed. However, the extra con-
jugation seemed to bring instability to the system as the free-
base dipyrrin 29 decomposed within a few days. Attempts to
form the HCl salt of 29, usually a stabilisation strategy for
dipyrrins, also failed.
An analogue of dipyrrin 28 (Scheme 3), without the B-ring
methoxy substituent and bearing a methyl group in place of
the ethyl substituent, was also prepared (Scheme 5, 33). This
was obtained via deprotection and decarboxylation of pyrrole
3017a to quantitatively give the α-free pyrrole 31. Then, conden-
sation with aldehyde 3221 gave the dipyrrin 33 in excellent
yield.
Results and discussion
Synthesis of prodigiosenes
Synthesis of prodigiosenes with an A-ring pyrrole. Prodigio-
senes were obtained starting from the corresponding formyl
pyrroles (Scheme 1),17 to give compounds with alkyl (14, 15),
carboxy (16–19) and acyl groups (20–21) at the β-position of the
C-ring.
Synthesis of A-ring modified prodigiosenes and dipyrrins.
A-ring modified prodigiosenes were prepared by taking advan-
tage of the Pd-catalysed coupling, as the last step, to enable
the incorporation of a range of A-ring motifs. Thus, dipyrrins
23 and 5 were subjected to Suzuki–Miyaura reaction conditions
using various boronic acids (Scheme 2).
The coupling was successful when an aromatic boronic acid
was used and it provided prodigiosenes 24–26 substituted with
a phenyl or an indolyl A-ring. However, the Boc protecting
group on the indolyl A-ring proved to be more stable to the
reaction conditions than was a similarly protected pyrrolyl sub-
stituent, as the latter is usually deprotected in situ. Thus, an
extra deprotection step using potassium carbonate in metha-
nol was necessary in order to obtain compounds 25 and 26.
Unfortunately, when n-propyl boronic acid was used, the
Scheme 3 Synthesis of a dipyrrin substituted with an alkyl group.
Scheme 1 Synthesis of prodigiosenes 14–22. aKOH, THF–H2O, 70 °C,
Scheme 4 Attempted synthesis of a dipyrrin substituted with an allyl
then HCl.
group.
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Scheme 5 Synthesis of dipyrrin 27.
Synthesis of metal complexes of prodigiosenes and dipyr-
rins. Incorporation of metals into the backbone of known
antimalarial drugs has provided promising results in terms of
activity and toxicity.6c Consequently we explored the synthesis
of several metal complexes of prodigiosenes, so as to be able
to evaluate the antimalarial activity of this new structural
class. Prodigiosenes can be considered as a dipyrrin extended
with a pyrrolyl substituent, and are thus good candidates for
metal complexation.22 Indeed both nitrogen atoms of the
dipyrrin moiety could be involved in chelation to the metal
centre. Alternatively the three nitrogen atoms of the entire
dipyrrinato-pyrrolyl skeleton could all be coordinated. Homo-
leptic dimeric zinc complexes of prodigiosenes11d,23 and the
natural compound 124 are known, as is an oxidised Cu(II):pro-
digiosin complex24 proposed to be partially responsible for the
anticancer properties of prodigiosin.25 Several homoleptic zinc
complexes, MP2, of our prodigiosenes (P) were thus prepared
(Scheme 6).26
C-ring benzyl ester prodigiosenes with non-modified (16)
and modified A-rings (24, 25, 28, 33) were obtained as their
zinc-complexes (34–38) in moderate-to-good yields. Complex
39 with an alkyl chain in the C-ring β-position, was obtained
quantitatively using the same conditions. As expected, and in
keeping with previous work involving dipyrrins and prodigio-
senes,24,27 all zinc complexes were obtained as single discrete
ZnP2 complexes, with no mass spectral evidence to the
contrary.
Scheme 7 Metal complexes of prodigiosenes; n.r. = no reaction.
Dipyrrins form complexes with numerous M(II)27,28 and
M(III) metals.28c,29 We consequently attempted the synthesis of
transition metal complexes of prodigiosenes (Scheme 7). Using
the same protocol as for the formation of prodigiosene Zn
complexes but using Co(OAc)2,26 a complex of cobalt(II) (MP2,
40) was obtained in 30% yield using benzyl ester prodigiosene
16 as starting material. Surprisingly, although dipyrrins are
known to form metal(III) complexes, the formation of prodigio-
sene complexes of Co(III),29b Fe(III)29b and In(III)29f failed in our
hands, perhaps due to steric hindrance at the C-ring α-position
(Scheme 7).30
To investigate the influence of the nitrogen atom of the pro-
digiosene A-ring upon consequent antimalarial activity, we
protected the A-ring pyrrolic moiety with a methyl group. As
the fully delocalised system of the prodigiosene prevents selec-
tive protection of the A-ring pyrrole, we took advantage of the
fact that prodigiosenes easily form metal complexes to seques-
ter the B- and C-rings: the protected prodigiosene could then
be methylated at the A-ring. As such, the dipyrrinato unit of
prodigiosene 16 was protected (Scheme 8).31,32
F-BODIPY 41 was then subjected to classic methylation con-
ditions using NaH and methyl iodide (Conditions A). Unfortu-
nately only 21% of the protected prodigiosene 42 was isolated
after 2 days of reaction along with 24% of starting material
(41). Changing the conditions to involve NaOH, TBAB and
methyl iodide in DCM33 was equally unsuccessful and gave
only 14% of the expected compound 42 (Conditions B). Fortu-
nately, the use of zinc(II) proved to be a successful strategy for
protecting the prodigiosene for this purpose: methylation of
the zinc complex 34 (Scheme 8) occurred smoothly in the pres-
ence of NaH and methyl iodide to give prodigiosene 43 in 69%
yield after a quench using aqueous HCl, followed by purifi-
cation. Without quenching with HCl, the prodigiosene could
be isolated as the zinc metal complex 44 (Scheme 8).
We have previously reported that prodigiosene tin com-
plexes 47 and 48 (Scheme 9) exhibit a low acute systemic
Scheme 6 Synthesis of zinc complexes of prodigiosenes and dipyrrins.
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discrete MP entities as demonstrated using LRMS spec-
trometry. These tin complexes exhibited considerable fluo-
rescence, in keeping with previously reported compounds.34
Biological activity
All synthesised prodigiosenes, as well as the natural product
1,35 were evaluated against the human 3D7 strain of Plasmo-
dium falciparum. Antimalarial activity was determined as the
half maximal inhibitory concentration (IC50). So as to facilitate
discussion of valuable activity levels, efficient antimalarial
activity can be appreciated as excellent (IC50 < 1 μM) and good
(1 μM < IC50 < 20 μM),36 following previously published
criteria.
The antimalarial activity of C-ring ester prodigiosenes was
first evaluated (Table 1). Benzyl, ethyl and isopropyl esters
16–18 exhibited promising IC50 values around 1 μM, but fell
significantly short of the activity of the natural product prodi-
giosin 1 (11 nM) or chloroquine (15 nM). In agreement with
previously reported data that showed that substitution with
either a very short or an excessively long alkyl chain dramati-
cally decreases antimalarial activity of prodigiosenes,14 the
long chain ester 19 was not effective against 3D7. Interestingly
dibutyl tin complexes of A-ring ester-bearing prodigiosenes 47
and 49 exhibited improved IC50 values (116 nM and 521 nM,
respectively) compared to their corresponding free-base prodi-
giosenes 16 and 17 (IC50 = 0.9 μM and 1.5 μM, respectively) but
the cobalt-prodigiosene dimer (CoP2, where P = prodigiosene,
40), the diphenyl tin complex (48), the zinc dimer (ZnP2, 34)
and BF2 (41) complexes were not as effective as their free-base
ligand (16).
Scheme 8 Methyl protection of the A-ring pyrrole of prodigiosene.
We then turned our attention to the influence of the nature
of the A-ring upon the antimalarial activity of prodigiosenes.
Indeed, a previous study affirmed the importance of the A-ring
Table 1 In vitro antimalarial activity of C-ring ester prodigiosenes
R
IC50 (free-base)
M
IC50 (complex)
Scheme 9 Preparation of tin complexes of prodigiosenes.
Chloroquine
Prodigiosin
-Benzyl
-Benzyl
-Benzyl
-Benzyl
-Benzyl
-Ethyl
-ipropyl
-C14H28
15 nM
11 nM (1)
0.9 μM (16)
toxicity compared to the natural product free-base prodigiosin
1.9g The suggestion that tin complexes of prodigiosenes could
be better tolerated than prodigiosin itself lead us to evaluate
the antimalarial activity of several prodigiosene tin complexes.
Prodigiosenes substituted at the C-ring with alkyl (Scheme 9,
14, 15) and carboxyl groups (16, 17), as well as A-ring modified
prodigiosenes (25, 26) and the natural product itself (1), were
thus subjected to tin complexation using Bu2SnO and
Ph2SnO.34 Stable tin complexes 45–52 were obtained in excel-
lent yield after purification over alumina. Akin to the zinc com-
plexes, tin complexes of prodigiosenes were obtained as
SnBu2
Coa
116 nM (47)
5 < IC50 < 50 μM (40)
5 < IC50 < 50 μM (48)
≈50 μM (34)
No effect (41)
521 nM (49)
SnPh2
Zna
BF2
b
1.5 μM (17)
1.0 μM (18)
No effect (19)
SnBu2
a The Co and Zn complexes are dimeric, i.e. MP2 where
P =
prodigiosene with chelating dipyrrinato units and uncomplexed N–H
moieties on the A-ring pyrroles. b For the BF2 complex the dipyrrinato
unit complexes with boron leaving an uncomplexed N–H on the A-ring
pyrrole.
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pyrrole for parasiticidal activity while substituents at α- and β′- substitution with a methyl group does not seem to be detri-
positions of the C-ring can be varied.14 For this purpose we mental to antimalarial activity (compare 33 and 28).
compared the IC50 values of several benzyl ester prodigiosenes
The antimalarial activity of C-ring alkyl prodigiosenes was
substituted with various R2 groups (Table 2). Protection of the also explored (Table 3). Prodigiosene 14 is a close analogue of
pyrrole A-ring with a methyl group (43) considerably decreased the natural prodigiosin (1), substituted with only one extra
the antimalarial activity of the benzyl ester prodigiosene 16 methyl group at the β′-position of the C-ring (Fig. 1). We pre-
(compare IC50 of 938 nM for 16 vs. 5 > IC50 > 50 μM for 43) viously showed that the presence of the extra methyl group
showing that the presence of the N–H moiety of the A-ring is allows a shorter and easier synthesis compared to prodigiosin
essential. This is supported by the moderate IC50 values of pro- itself,37 and that analogue 14 possesses similar properties to
digiosenes substituted with a phenyl (24) or ethyl group (28). the natural compound (i.e., anticancer, transmembrane anion
Replacement of the A-ring pyrrole with an indole slightly transport and DNA cleavage activity).17b Here, compound 14
decreases the antimalarial activity (compare IC50 = 0.9 μM for exhibits an excellent IC50 value in the same range as prodigio-
16 and IC50 = 5.6 μM for 25) but as shown in Table 1, yet again sin 1 (Table 3), demonstrating that the extra methyl group is
the activity can be increased by complexation of the prodigio- not detrimental for antimalarial activity, further complement-
sene with dibutyl tin (51, IC50 4.7 μM, compared to 5.6 μM for ing its role as a model for the natural product prodigiosin.
25). As previously, zinc complexes have almost no effect on
As shown in Table 3, substitution of the A-ring pyrrole of
plasmodium 3D7 (44, 35, 37, 36, 38). Interestingly, although prodigiosene 14 with an indolic group decreases the activity
omission of the methoxy group at the B-ring of prodigiosenes slightly, yet both are in the excellent range (Table 3, compare
induces a decreased cytotoxic activity against cancer cells,37 IC50 = 19 nM for 14 and IC50 98 nM for 26). For prodigiosin 1
Table 2 In vitro antimalarial activity of dipyrrins and prodigiosenes with a modified A-ring
R1
R2
IC50 (free-base)
M
IC50 (complex)
Prodigiosin
-OMe
-OMe
-OMe
-OMe
-OMe
-OMe
-Me
11 nM (1)
0.9 μM (16)
5 < IC50 < 50 μM (43)
5 < IC50 < 50 μM (24)
5 < IC50 < 50 μM (28)
5.6 μM (25)
-2-Pyrrolyl
-1-Me-2-pyrrolyl
-Phenyl
Zna
≈50 μM (44)
No effect (35)
No effect (37)
4.7 μM (51)
≈50 μM (36)
No effect (38)
Zna
-Ethyl
Zna
-2-Indolyl
-2-Indolyl
-Me
SnBu2
Zna
4.7 μM (33)
Zna
a The Zn complexes are dimeric, i.e. MP2 where P = prodigiosene with chelating dipyrrinato units and uncomplexed N–H moieties on the A-ring
pyrroles.
Table 3 In vitro antimalarial activity of C-ring alkyl and carbonyl prodigiosenes
R1
R2
IC50 (free-base)
M
IC50 (complex)
Prodigiosin
-C5H11
-C5H11
11 nM (1)
19 nM (14)
98 nM (26)
SnBu2
SnBu2
SnBu2
Zna
7.1 nM (50)
18 nM (45)
1.4 μM (52)
5 < IC50 < 50 μM (39)
56 nM (46)
2-Pyrrolyl
2-Indolyl
2-Indolyl
2-Pyrrolyl
2-Pyrrolyl
2-Pyrrolyl
2-Pyrrolyl
-C5H11
-CH2CO2Me
-(CO)C4H8CO2Me
-(CO)C2H4CO2Et
-(CO)C4H8CO2H
1.5 μM (15)
SnBu2
5 < IC50 < 50 (20)
5 < IC50 < 50 (21)
≈50 μM (22)
a The Zn complex is dimeric, i.e. MP2 where P = prodigiosene with a chelating dipyrrinato unit and an uncomplexed N–H moiety on the A-ring
pyrrole.
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and compounds 14 and 26, complexation with dibutyl tin did alongside a variety of other substituents, against the 3D7 Plas-
not improve their antimalarial activity. However tin complexa- modium falciparum strain. We demonstrate that the extra
tion was beneficial for prodigiosene 15, bearing an ethanoate methyl group allows a short and facile synthesis of prodigio-
side-chain. Indeed the free-base prodigiosene 15 exhibited an sene analogues, and that this addition is not detrimental to
IC50 of 1.5 μM and its dibutyl tin complex (46) an IC50 of antimalarial activity. Indeed, the close analogue 14 exhibiting
56 nM. Once again zinc complexation decreased the antimalar- only the extra methyl group compared to the natural product 1
ial activity (zinc complex 39 exhibited an IC50 > 5 μM). Com- seems as effective as prodigiosin 1 itself against 3D7 and is
pounds 20–22 show good anticancer properties,11d but are deemed to exhibit excellent36 activity against 3D7. This com-
poorly cytotoxic against the plasmodium 3D7, indicating a pound previously showed similar anticancer properties to pro-
clear decoupling of activity.
These results can be assembled to probe a brief structure material will be used as a model for the natural compound (1).
activity relationship. Although it would be premature to debate This study also demonstrated the importance of the pres-
digiosin 117b and we anticipate that this easily accessible
modes of action for prodigiosenes and their complexes against ence of the nitrogen atom in the A-ring for antimalarial
malaria parasites, from these experiments it appears that pro- activity. Prodigiosenes with alkyl substituents at the C-ring
digiosenes substituted at the β′-position of the C-ring with an β-position were more cytotoxic against 3D7 than prodigiosenes
alkyl chain better inhibit the malaria parasite 3D7 than prodi- with carbonyl substituents. Zn, Co, BF2 and diphenyl tin com-
giosenes substituted with an ester group. This suggests that plexes of prodigiosenes were, at best, poorly effective against
decreasing the acidity of the prodigiosene, induced by the the human malaria parasite in vitro. However, dibutyl tin com-
presence of the ester group, is detrimental to the inhibition plexes of prodigiosenes were as effective or more effective than
activity (prodigiosin pKa = 8.2, pKa C-ring ester prodigiosene = their free-base counterparts, perhaps due to the lipophilicity of
6.5).17b The natural product prodigiosin is known to interact the dibutyl tin moiety. Many of the prodigiosenes reported
with DNA by intercalation with a preference for the AT herein exhibit activity against 3D7 that has been classified as
sequences.38 From our studies of prodigiosene–DNA inter- “good”, with several in the “excellent” class:36 analogue 14
action (CT-DNA melting studies and UV/vis titrations of shows particular promise, with activity at nanomolar concen-
CT-DNA), we found that ethyl ester prodigiosene 17 (which trations, as do the tin complexes 45 and 47. Cognisant of the
demonstrates good antimalarial activity) and prodigiosenes 20 low toxicity profile of prodigiosene tin complexes,9g these
and 22 (which demonstrate poor antimalarial activity) exhibit results could open the door to new antimalarial agents.
similar DNA-binding profiles, presumably via an intercalative
mode (see ESI†). As such, the antimalarial activity of 17 could
be related to its ability to intercalate DNA strongly. However,
20 and 22 also interact with DNA strongly, yet their lack of anti-
malarial activity points toward either a target other than
Experimental
General information
DNA; differential uptake/efflux/metabolism; or an alternative All chemicals were purchased and used as received unless
parasite–drug interaction.
otherwise indicated. Moisture-sensitive reactions were per-
The SAR study also suggests that the presence of the nitro- formed in flame-dried glassware under a positive pressure of
gen atom at the A-ring is necessary for a good anti-plasmodial nitrogen or argon. Solutions of air- and moisture-sensitive
activity in vitro. Contrary to what was observed for the anti- compounds were introduced via syringe or cannula through a
cancer properties of prodigiosenes,37 it appears that the pres- septum. Flash chromatography was performed using Silicycle
ence of the B-ring methoxy group is not essential for ultra pure silica (230–400 mm) or 150 mesh Brockmann III
antimalarial activity against 3D7. We also observed that activated neutral or basic alumina oxide as indicated. The
dibutyl tin complexation of prodigiosenes could in some cases NMR spectra were recorded using a 500 MHz or 300 MHz
increase the inhibition property of the free-base prodigiosene. spectrometer instrument using CDCl3 as solvent and are
The increased activity in that case could, in part, be explained reported in part per million (δ) using the solvent signals at
1
by increased lipophilicity affecting uptake.
7.26 ppm for H and at 77.16 ppm for 13C as an internal refer-
ence with J values given in hertz. Mass spectra were obtained
using TOF and LCQ Duo ion trap instruments operating in
ESI+ mode. Melting points were determined using a Fisher-
Johns melting point apparatus, and are uncorrected. Com-
Conclusions
Although the antimalarial properties of natural prodigiosins pounds 1,35 5,17b 6,17c 7 and 8,17a 9 and 10,17b 11 and 12,11d
have long been reported,12a the antimalarial activity of prodi- 13,39 14,17b 15,17c 16 and 17,17a 18 and 19,17b 20,11d 21 and
giosene analogues has been poorly studied. Recently, the cyto- 22,39 30 and 3117a 32,21 34,17b 47–4934 were prepared according
toxic activity of prodigiosene analogues substituted at the to literature procedures.
α-and β′-position of the C-ring against D6 and Dd2 falciparum
(Z)-Benzyl 2-((3-methoxy-5-(((trifluoromethyl)sulfonyl)oxy)-
strains was reported and showed promising results.14 Here we 1H-pyrrol-2-yl)methylene)-3,5-dimethyl-2H-pyrrole-4-carboxylate
report the antimalarial activity of analogues of the natural pro- 23. To a suspension of (Z)-benzyl 5-((3-methoxy-5-oxo-1H-
digiosin 1 bearing an extra methyl group at the β′-position, pyrrol-2(5H)-ylidene)methyl)-2,4-dimethyl-1H-pyrrole-3-carboxy-
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late17a (2.0 g 5.67 mmol), in dry DCM (300 mL) at 0 °C was 138.0, 139.4, 145.0, 161.0, 165.0, 169.4 (1 carbon non
slowly added Tf2O (2.7 mL, 15.9 mmol). After 4 h stirring at accounted for). HRMS-ESI (m/z): [M
+
H]+ calcd for
this temperature, the reaction was quenched with sat. aqueous C28H26N3O3, 452.1969; found, 451.1967.
NaHCO3 (400 mL), then extracted with DCM (3 × 200 mL). The
(Z)-2-(5-((3,5-Dimethyl-4-pentyl-2H-pyrrol-2-ylidene)methyl)-4-
combined organic layers were washed with brine, and then methoxy-1H-pyrrol-2-yl)-1H-indole 26. Obtained following the
dried (Na2SO4). After evaporation of the solvents under general procedure 1. Then the crude material was dissolved in
reduced pressure, the crude material was purified using flash a mixture of methanol–chloroform–water 3/3/1.5 mL and
column chromatography (SiO2, EtOAc–hexane 2/8) to give a K2CO3 (3 eq.) was added. The reaction mixture was stirred over-
yellow solid (1.5 g, 55%). 1H NMR (CDCl3, 300 MHz) 2.42 (s, night at reflux temperature then water (50 mL) was added and
3H), 2.57 (s, 3H), 3.90 (s, 3H), 5.30 (s, 2H), 5.43 (s, 1H), 7.12 (s, the product was extracted with ethyl acetate (3 × 40 mL). The
1H), 7.30–7.44 (m, 5H), 11.01 (br s, 1H). 13C NMR (CDCl3, combined organic layers were washed with brine then dried
125 MHz) 11.7, 15.2, 59.0, 65.7, 87.6, 113.9, 118.7 (q, J = 319 (Na2SO4). After evaporation of the solvent under reduced
Hz), 119.0, 126.1, 128.2, 128.2, 128.7, 133.5, 135.4, 136.6, pressure the crude was purified using column chromatography
145.0, 161.9, 164.9, 168.2. HRMS-ESI (m/z): [M + H]+ calcd for (Al2O3 type III basic, EtOAc–hexanes 1/9) to give a red glass
1
C21H20F3N2O6S1, 485.0989; found, 485.0996.
(57%, 94 mg). H NMR (CDCl3, 500 MHz) 0.83 (t, J = 7.2 Hz,
3H), 1.20–1.30 (m, 6H), 1.72 (s, 3H), 2.10 (s, 3H), 2.16 (t, J =
7.2 Hz, 2H), 4.07 (s, 3H), 6.34 (s, 1H), 6.77 (d, J = 7.8 Hz, 1H),
General procedure 1 for the synthesis of prodigiosenes 24–27
Compound 5 or 23 (0.48 mmol, 1 eq.) was dissolved in DME 6.87 (s, 1H), 6.92 (t, J = 7.8 Hz, 1H), 7.00 (t, J = 7.8 Hz, 1H),
(9 mL) then LiCl (60 mg, 1.44 mmol, 3 eq.) and boronic acid 7.10 (s, 1H), 7.48 (d, J = 7.8 Hz, 1H). 13C NMR (CDCl3,
(121 mg, 0.57 mmol, 1.2 eq.) were added. The solution was 125 MHz) 9.8, 10.5, 14.2, 22.6, 24.2, 30.4, 32.0, 58.7, 95.9,
degassed by bubbling with N2, and then palladium tetrakis- 104.3, 111.7, 115.8, 119.6, 120.8, 123.2, 124.2, 126.1, 128.6,
(PPh3) (56 mg, 10 mol%) was added. Then a degassed 2 M 132.2, 134.3, 136.0, 137.8, 139.5, 157.7, 168.7. UV (DCM) λmax
solution of Na2CO3 was added (1.0 mL, 1.92 mmol, 4 eq.) and (ε): 578 nm (118 000 L mol−1 cm−1). HRMS-ESI (m/z): [M + H]+
the suspension was stirred at 85 °C for 18 h. After cooling the calcd for C25H30N3O1, 388.2383; found, 388.2377.
solution was poured into water (100 mL) and extracted with
DCM (3 × 50 mL). The combined organic layers were washed 3,5-dimethyl-2H-pyrrole-4-carboxylate hydrochloride 28. Under
with brine (100 mL), and then dried (Na2SO4). nitrogen to a Schlenk tube containing (Z)-benzyl 2-((5-bromo-
(Z)-Benzyl 2-((5-ethyl-3-methoxy-1H-pyrrol-2-yl)methylene)-
(Z)-Benzyl 2-((3-methoxy-5-phenyl-1H-pyrrol-2-yl)methylene)- 3-methoxy-1H-pyrrol-2-yl)methylene)-3,5-dimethyl-2H-pyrrole-4-
3,5-dimethyl-2H-pyrrole-4-carboxylate 24. Obtained following carboxylate 7 (200 mg, 0.48 mmol) and Pd(dppf)Cl2 (10.4 mg,
general procedure 1 and then purification using chromato- 0.014 mmol) was added dioxane (4 mL). The resulting solution
graphy (Al2O3 basic type III, EtOAc–hexane 1/9) as an orange was degassed with nitrogen (3 times vacuum/nitrogen refill
solid (23%, 62 mg). Mp 169 °C. 1H NMR (CDCl3, 500 MHz) cycle) then Et2Zn (960 μL, 0.96 mmol) was added. The reaction
2.44 (s, 3H), 2.64 (s, 3H), 3.93 (s, 3H), 5.32 (s, 2H), 6.09 (s, 1H), vessel was heated at 100 °C for 1 hour, then cooled to room
6.98 (s, 1H), 7.33–7.35 (m, 1H), 7.39 (t, J = 7.2 Hz, 2H), temperature. The reaction was quenched by the addition of a
7.43–7.48 (m, 5H), 7.98 (d, J = 7.2 Hz, 2H), 11.00 (br s, 1H). 13
C
few drops of methanol, then HCl 1 M (20 mL) was added and
NMR (CDCl3, 125 MHz) 11.8, 15.5, 58.5, 65.5, 95.1, 113.3, the reaction mixture was extracted with DCM (3 × 30 mL). The
115.1, 127.0, 127.6, 128.0, 128.2, 128.6, 128.7, 130.1, 132.6, combined organic layers were washed with brine (30 mL), then
134.8, 136.8, 141.9, 143.5, 165.4, 166.6, 168.5. HRMS-ESI (m/z): dried (Na2SO4). After evaporation of the solvents under vacuum
[M + H]+ calcd for C26H25N2O3, 413.1860; found, 413.1864.
the resulting solid was triturated with MeOH, filtered (Milli-
(Z)-Benzyl 2-(2-((5-(1H-indol-2-yl)-3-methoxy-1H-pyrrol-2-yl)- pore) and washed 3 times with MeOH to give an orange solid
1
methylene)-3,5-dimethyl-2H-pyrrol-4-yl)-2-oxoacetate 25. Obtained (103 mg, 53%). Mp 192 °C. H NMR (CDCl3, 500 MHz) 1.36 (t,
following the general procedure 1. Then the crude material J = 7.5 Hz, 3H), 2.53 (s, 3H), 2.83 (s, 3H), 3.05 (q, J = 7.5 Hz,
was dissolved in a mixture of methanol–chloroform–water 3/3/ 2H), 4.00 (s, 3H), 5.30 (s, 2H), 5.77 (s, 1H), 7.30 (s, 1H),
1.5 mL and K2CO3 (3 eq.) was added. The reaction mixture was 7.33–7.42 (m, 5H), 13.60 (br s, 1H), 13.95 (br s, 1H). 13C NMR
stirred for 2 days at reflux temperature then water (50 mL) was (CDCl3, 125 MHz) 12.2, 12.5, 15.2, 23.1, 59.2, 66.1, 95.1, 116.7,
added and the mixture was extracted with DCM (3 × 40 mL). 118.2, 120.7, 124.1, 128.3, 128.7, 136.2, 144.9, 154.0, 164.2,
The combined organic layers were washed with brine then 167.4, 168.1 (1 carbon non accounted for). HRMS-ESI (m/z):
dried (Na2SO4). After evaporation of the solvent under reduced [M − Cl]+ calcd for C22H25N2O3, 365.1860; found, 365.1868.
pressure the crude product was purified using column chrom-
(Z)-Benzyl
2-((3,5-dimethyl-1H-pyrrol-2-yl)methylene)-3,5-
atography (Al2O3 type III basic, EtOAc–hexanes 2/8) to give a dimethyl-2H-pyrrole-4-carboxylate hydrobromide 33. To
a
red solid (67%, 203 mg). Mp 113 °C. 1H NMR (CDCl3, mixture of THF–MeOH (1.6/1.6 mL) was added benzyl 2,4-
500 MHz) 2.25 (br s, 3H), 2.40 (s, 3H), 4.05 (s, 3H), 5.23 (s, 2H), dimethyl-1H-pyrrole-3-carboxylate 31 (372 mg, 1.62 mmol) and
6.26 (s, 1H), 6.98–7.03 (m, 3H), 7.11–7.15 (m, 2H), 7.31–7.36 3,5-dimethyl-1H-pyrrole-2-carbaldehyde
(32)
(100
mg,
(m, 5H), 7.54 (d, J = 8.0 Hz, 1H). 13C NMR (CDCl3, 125 MHz) 0.81 mmol). To the resulting solution was added HBr (48%,
11.9, 13.3, 58.9, 65.5, 96.7, 106.7, 111.6, 113.9, 114.9, 120.2, 156 μL, 1.78 mmol) and a precipitate formed instantly. The
121.4, 124.3, 126.2, 128.0, 128.2, 128.6, 133.3, 134.2, 136.7, reaction mixture was cooled to 0 °C for an hour, and then
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filtered (Millipore). The crude solid was washed with Et2O (3 ×
Zinc(II) complex 38. Obtained following general procedure 2
15 mL) to give the product as a yellow solid (300 mg, quant.) then, after concentration, the residue was triturated with
Mp 226 °C. 1H NMR (CDCl3, 500 MHz) 2.40 (s, 3H), 2.59 (s. MeOH, filtered (Millipore) and washed 3 times with MeOH to
3H), 2.71 (s, 3H), 2.91 (s, 3H), 5.32 (s, 2H), 6.23 (s, 1H), 7.24 (s, give a yellow solid (30 mg, 68%). 1H NMR (CDCl3, 500 MHz)
1H), 7.34–7.42 (m, 5H), 13.24 (br s, 1H), 13.58 (br s, 1H). 13C 1.93 (s, 6H), 2.19 (s, 6H), 2.33 (s, 6H), 2.55 (s, 6H), 5.25 (s, 4H),
NMR (CDCl3, 125 MHz) 12.4, 12.5, 15.0, 15.4, 66.4, 117.8, 6.08 (s, 2H), 7.16 (s, 2H), 7.27–7.300 (m, 2H), 7.34 (t, J = 7.2 Hz,
119.4, 121.3, 125.0, 128.4, 128.5, 128.7, 128.8, 135.9, 147.3, 4H), 7.40 (d, J = 7.2 Hz, 4H). 13C NMR (CDCl3, 125 MHz) 12.0,
149.2, 156.4, 160.5, 163.5. HRMS-ESI (m/z): [M − Br]+ calcd for 12.4, 16.8, 17.7, 65.4, 116.6, 119.7, 123.1, 128.0, 128.3, 128.6,
C21H23N2O2, 335.1754; found, 335.1746.
134.4, 137.0, 139.2, 143.8, 146.0, 158.0, 163.1, 165.6.
HRMS-APCI (m/z): [M + H]+ calcd for C42H43N4O4Zn, 731.2570;
found, 731.2577.
General procedure 2 for the synthesis of Zn complexes
Zinc(II) complex 39. Obtained following general procedure 2
To a solution of prodigiosene (1 eq.) in CHCl3 (0.04 M) was then, after concentration, the residue was purified using
added a solution of Zn(OAc)2·2H2O (2.4 eq.) and NaOAc·3H2O column chromatography (Al2O3 type III basic, DCM–hexanes
(2.4 eq.) in MeOH (0.2 M). After 5 h stirring at room tempera- 3/7) to give a red film (55 mg, quant.). 1H NMR (CDCl3,
ture, water (20 mL) was added and the solution was extracted 500 MHz) 0.79 (t, J = 7.5 Hz, 6H), 1.15–1.25 (m, 8H), 1.30
with DCM (3 × 20 mL). the combined organic layers were (quint., J = 7.5 Hz, 4H), 1.94 (s, 6H), 2.25–2.30 (m, 10H), 3.98
washed with water (40 mL) and brine (40 mL), and then dried (s, 6H), 6.26 (s, 2H), 6.71 (d, J = 1.5 Hz, 2H), 6.85 (d, J = 8.0 Hz,
(Na2SO4).
2H), 6.97 (t, J = 7.0 Hz, 2H), 7.07 (t, J = 7.0 Hz, 2H), 7.35 (s,
Prodigiosene zinc(II) complex 35. Obtained following 2H), 7.43 (d, J = 8.0 Hz, 2H), 9.09 (br s, 2 H). 13C NMR (CDCl3,
general procedure 2 then, after concentration, the residue was 125 MHz) 10.2, 14.1, 14.9, 22.6, 24.8, 30.1, 31.7, 58.2, 96.2,
purified using flash chromatography (Al2O3 III basic, EtOAc– 103.6, 111.6, 118.4, 119.9, 120.3, 122.7, 128.0, 128.4, 130.2,
hexane 1/9 then 2/8) to give a red solid (48 mg, 90%). Mp 133.3, 135.7, 137.3, 138.5, 148.5, 158.2, 163.9. LRMS 837.3
1
181 °C. H NMR (CDCl3, 500 MHz) 2.12 (s, 6H), 2.53 (s, 6H), [M + H]+. HRMS-APCI (m/z): [M + H]+ calcd for C50H57N6O2Zn,
3.91 (s, 6H), 5.21–5.24 (ABq, J = 12.5 Hz, 4H), 5.84 (s, 2H), 7.01 837.3829; found, 837.3818.
(t, J = 7.7 Hz, 4H), 7.11 (t, J = 7.5 Hz, 2H), 7.27–7.30 (m, 6H),
Prodigiosene cobalt(II) complex 40. To a solution of prodi-
7.33–7.35 (m, 6H), 7.38–7.39 (m, 4H). 13C NMR (CDCl3, giosene 16 (50 mg, 0.12 mmol) in CHCl3 (12 mL) was added a
125 MHz) 12.4, 17.3, 58.3, 65.4, 95.7, 116.8, 121.6, 126.9, 127.9, solution of Co(OAc)2·4H2O (77 mg, 0.31 mmol) and
128.1, 128.2, 128.6, 128.8, 131.4, 134.3, 134.4, 137.0, 143.6, NaOAc·3H2O (42 mg, 0.31 mmol) in MeOH (4 mL). After
157.9, 161.9, 165.6, 166.7. HRMS-ESI (m/z): [M + H]+ calcd for 30 min stirring at room temperature, water (20 mL) was added
C52H46N4O6Zn1, 886.2703; found, 886.2674.
and the solution was extracted with DCM (3 × 20 mL). The
Prodigiosene zinc(II) complex 36. Obtained following combined organic layers were washed with brine, and then
general procedure 2 then, after concentration, the residue was dried (Na2SO4). After concentration under vacuum the residue
purified using flash chromatography (Al2O3 III neutral, EtOAc– was purified using flash chromatography (Al2O3 III neutral,
hexane 3/7) to give a red solid (36 mg, 68%). Mp 266 °C. 1H EtOAc–hexane 5/5) to give a red solid (31 mg, 30%). Mp =
NMR (CDCl3, 500 MHz) 2.22 (s, 6H), 2.62 (s, 6H), 4.00 (s, 6H), 262 °C. As this complex is paramagnetic no NMR spectra could
5.21 (s, 4H), 6.21 (s, 2H), 6.82–6.85 (m, 4H), 7.01 (t, J = 7.5 Hz, be obtained. HRMS-APCI (m/z): [M
2H), 7.14 (t, J = 7.5 Hz, 2H), 7.27–7.38 (m, 10H), 7.47 (d, J = 8.0 C48H45CoN6O6, 860.2727; found, 860.2697.
+
H]+ calcd for
Hz, 2H), 7.53 (s, 2H), 8.91 (s, 2H). 13C NMR (CDCl3, 125 MHz)
2-((Benzyloxy)carbonyl)-5,5-difluoro-9-methoxy-1,3-dimethyl-
12.4, 17.5, 58.6, 65.6, 97.1, 107.2, 111.6, 117.7, 119.7, 120.6, 7-(1H-pyrrol-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-
121.1, 124.3, 128.0, 128.2, 128.3, 128.6, 131.7, 133.9, 136.7, ium-5-uide 41. The HCl salt of prodigiosene 16 (300 mg,
137.8, 143.6, 154.5, 158.1, 165.2, 166.8 (1 carbon non 0.68 mmol) was dissolved in anhydrous DCM (40 mL) under
accounted for). HRMS-APCI (m/z): [M
C56H49N6O6Zn, 965.3000; found, 965.2970.
+
H]+ calcd for nitrogen. Triethylamine (570 μL, 4.08 mmol) was added and
the reaction was stirred for 5 min before BF3·OEt2 (970 μL,
Zinc(II) complex 37. Obtained following general procedure 2 6.8 mmol) was added. The reaction mixture was stirred for
then, after concentration, the residue was triturated with 16 h at room temperature, and then quenched via the addition
MeOH, filtered (Millipore) and washed with MeOH (3 × 5 mL) of 1 M HCl (50 mL). The crude mixture was extracted with
1
to give a yellow solid (63 mg, 61%). Mp/dec > 220 °C. H NMR DCM (3 × 50 mL). The combined organic layers were washed
(CDCl3, 500 MHz) 0.88 (t, J = 7.7 Hz, 6H), 2.19 (s, 6H), 2.22 (q, with brine (50 mL), and dried (Na2SO4). After evaporation of
J = 7.7 Hz, 4H), 2.52 (s, 6H), 3.89 (s, 6H), 5.24 (s, 4H), 5.59 (s, the solvent the crude material was purified using flash column
2H), 7.25 (s, 2H), 7.27–7.30 (m, 2H), 7.34 (t, J = 7.5 Hz, 4H), chromatography (Al2O3 neutral type III, DCM–hexanes, 5/5, 6/4
7.40 (d, J = 7.5 Hz, 4H). 13C NMR (CDCl3, 125 MHz) 12.3, 12.8, then 7/3) to give a dark purple film (95%, 292 mg). Mp 280 °C.
17.6, 25.6, 58.3, 65.3, 95.2, 115.9, 120.7, 127.9, 128.2, 128.6, 1H NMR (CDCl3, 500 MHz) 2.42 (s, 3H), 2.80 (s, 3H), 3.99 (s,
130.1, 133.5, 137.1, 142.2, 156.8, 165.8, 167.3, 169.2. 3H), 5.32 (s, 2H), 6.14 (s, 1H), 6.37–6.39 (m, 1H), 6.97–6.99 (m,
HRMS-APCI (m/z): [M + H]+ calcd for C44H47N4O6Zn, 791.2782; 1H), 7.16 (s, 1H), 7.18–7.19 (m, 1H), 7.31–7.34 (m, 1H),
found, 791.2745.
7.37–7.40 (m, 2H), 7.43–7.44 (m, 2H) 10.49 (br s, 1H). 13C NMR
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(CDCl3, 125 MHz) 12.0, 14.5, 58.7, 65.7, 97.3, 111.7, 114.9, 13C NMR (CDCl3, 125 MHz) 11.6, 15.1, 37.8, 58.5, 65.4, 97.0,
117.3, 118.8, 123.5, 126.5, 128.1, 128.2, 128.7, 129.4, 129.5, 108.8, 111.7, 112.8, 115.5, 127.5, 128.0, 128.2, 128.6, 128.8,
136.4, 136.7, 151.5, 153.0, 164.4, 165.1. HRMS-ESI (m/z): 129.8, 136.9, 141.3, 142.9, 160.0, 165.6, 167.3 (1 carbon non
[M + H]+ calcd for C24H22BF2N3O3, 449.1717; found, 449.1705.
accounted for). HRMS-ESI (m/z): [M
2-((Benzyloxy)carbonyl)-5,5-difluoro-9-methoxy-1,3-dimethyl- C25H26N3O3, 416.1969; found, 416.1984.
7-(1-methyl-1H-pyrrol-2-yl)-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diaza- Prodigiosene zinc(II) complex 44. To a suspension of NaH
+
H]+ calcd for
borinin-4-ium-5-uide 42. Method A: to a suspension of NaH (60% in grease, 9 mg, 0.23 mmol) in anhydrous THF (4 mL)
(60% in grease, 5.5 mg, 0.13 mmol) in anhydrous THF (3 mL) under nitrogen was added prodigiosene 34 (50 mg,
under nitrogen was added prodigiosene 41 (50 mg, 0.06 mmol) at 0 °C. After 30 min stirring at room temperature,
0.11 mmol) at 0 °C. After 30 min stirring at room temperature MeI (14 μL, 0.23 mmol) was added and the reaction was stirred
MeI (20 μL, 0.33 mmol) was added and the reaction was for 24 hours. Then, water (20 mL) was added and the reaction
heated at 40 °C for two days. After cooling to room temperature mixture was extracted with ethyl acetate (3 × 20 mL). The com-
a saturated solution of ammonium chloride (20 mL) was bined organic layers were washed with brine (50 mL), and then
added and the reaction mixture was extracted with ethyl dried (Na2SO4). After evaporation of the solvent under reduced
acetate (3 × 20 mL). The combined organic layers were washed pressure the crude material was purified using column chrom-
with brine (50 mL) then dried (Na2SO4). After evaporation of atography (Al2O3 type III basic, EtOAc–hexanes 4/6) to give a
the solvent under reduced pressure the crude product was dark red film (39%, 29 mg). 1H NMR (CDCl3, 500 MHz) 2.10 (s,
purified using column chromatography (Al2O3 type III basic, 6H), 2.51 (s, 6H), 3.45 (s, 6H), 3.92 (s, 6H), 5.21, 5.25 (ABq, J =
EtOAc–hexanes 3/7) to give a dark pink solid (21%, 11 mg).
12.5 Hz, 4H), 5.76 (s, 2H), 5.77–5.79 (m, 2H), 6.09–6.10 (m,
Method B: to a solution of prodigiosene 41 (75 mg, 2H), 6.45 (s, 2H), 7.29 (d, J = 7.2 Hz, 2H), 7.34 (t, J = 7.2 Hz,
0.17 mmol) in DCM (1.5 mL) was added MeI (11.4 μL, 4H), 7.39 (d, J = 7.2 Hz, 4H). 13C NMR (CDCl3, 125 MHz) 12.4,
0.18 mmol), followed by TBAB (5.5 mg, 0.017 mmol) and an 17.2, 35.4, 58.3, 65.3, 96.3, 108.4, 111.7, 116.4, 120.8, 125.6,
aqueous solution of NaOH 12 M (80 μL, 12 eq.). The reaction 127.9, 128.2, 128.5, 128.7, 130.4, 134.6, 137.1, 142.9, 152.8,
mixture was heated at 40 °C for 24 hours then cooled to room 157.4, 165.7, 166.1. HRMS-APCI (m/z): [M + H]+ calcd for
temperature and quenched with HCl (1 M, 20 mL). The reac- C50H49N6O6Zn, 893.3000; found, 893.2997.
tion mixture was extracted with DCM (3 × 20 mL). The com-
General procedure 3 for the formation of tin complexes
bined organic layers were washed with brine then dried
(Na2SO4). After evaporation of the solvent under reduced Prodigiosene (1 eq. 0.15 mmol) was dissolved in MeOH
pressure the crude material was purified using column chrom- (12 mL) (if the compound is not soluble in methanol a
atography (Al2O3 type III basic, EtOAc–hexanes 3/7) to give a 1/1 mixture of methanol and DCM can be used), then Bu2SnO
dark pink solid (14%, 11 mg). Mp 203 °C. 1H NMR (CDCl3, or Ph2SnO (2 eq., 0.30 mmol) was added. The reaction mixture
500 MHz) 2.44 (s, 3H), 2.77 (s. 3H), 3.77 (s, 3H), 3.97 (s, 3H), was stirred at reflux temperature overnight. After cooling to
5.30 (s, 2H), 5.88 (s, 1H), 6.29–6.31 (m, 1H), 6.86 (s, 1H), room temperature, a saturated solution of NaHCO3 (20 mL)
7.28–7.34 (m, 3H), 7.36–7.39 (m, 2H), 7.42–7.43 (m, 2H). 13C was added and the reaction mixture was extracted with DCM
NMR (CDCl3, 125 MHz) 12.1, 14.8, 36.5, 58.7, 65.8, 98.8, 110.0, (3 × 30 mL). The combined organic layers were washed with
117.8, 118.0, 118.1, 118.1, 125.4, 128.1, 128.2, 128.7, 129.1, brine, and then dried (Na2SO4). After evaporation of the
130.4, 136.6, 139.6, 151.4, 155.8, 164.2, 164.9. HRMS-ESI (m/z): solvent under reduced pressure the crude material was quickly
[M + Na]+ calcd for C25H24B1F2N3Na1O1, 486.1771; found, purified using flash chromatography (Al2O3 basic type III,
486.1758.
DCM 100%).
(Z)-Benzyl 2-((4-methoxy-1′-methyl-1H,1′H-[2,2′-bipyrrol]-5-yl)-
Dibutyl tin(IV) complex 45. Obtained following the general
methylene)-3,5-dimethyl-2H-pyrrole-4-carboxylate 43. To a sus- procedure 3 as a thick purple oil (89%, 76 mg). 1H NMR
pension of NaH (60% in grease, 6 mg, 0.15 mmol) in anhy- (CDCl3, 500 MHz) 0.76 (t, J = 7.2 Hz, 6H), 0.90 (t, J = 7.5 Hz,
drous THF (3 mL) under nitrogen was added prodigiosene 34 3H), 1.20–1.26 (m, 4H), 1.29–1.35 (m, 5H), 1.36–1.51 (m, 9H),
(50 mg, 0.06 mmol) at 0 °C. After 30 min stirring at room 2.17 (s, 3H), 2.34 (s, 3H), 2.37 (t, J = 7.5 Hz, 2H), 3.96 (s, 3H),
temperature, MeI (11 μL, 0.17 mmol) was added and the reac- 6.03 (s, 1H), 6.37–6.39 (m, 1H), 6.69 (d, J = 3.5 Hz, 1H),
tion was stirred during 24 hours. Then, an HCl solution (1 M, 6.88–6.89 (m, 2H). 13C NMR (CDCl3, 125 MHz) 10.1, 13.6, 14.3,
20 mL) was added and the reaction mixture was extracted with 14.7, 22.8, 23.4, 24.7, 26.5, 27.1, 30.5, 31.9, 58.3, 91.5, 109.3,
ethyl acetate (3 × 20 mL). The combined organic layers were 112.7, 114.1, 126.3, 127.4, 130.0, 132.5, 134.1, 134.4, 149.6,
washed with brine (50 mL) then dried (Na2SO4). After evapor- 153.7, 166.5. HRMS-ESI (m/z): [M
+
H]+ calcd for
ation of the solvent under reduced pressure the crude material C29H43N3O1Sn1, 569.2423; found, 569.2420.
was purified using column chromatography (Al2O3 type III
Dibutyl tin(IV) complex 46. Obtained following the general
basic, EtOAc–hexanes 2/8) to give an orange solid (69%, procedure 3 as a thick purple oil (82%, 28 mg). 1H NMR
1
33 mg). H NMR (CDCl3, 500 MHz) 2.41 (s, 3H), 2.53 (s. 3H), (CDCl3, 500 MHz) 0.76 (t, J = 7.2 Hz, 6H), 1.20–1.26, (m, 4H),
3.89 (s, 3H), 4.13 (s, 3H), 5.31 (s, 2H), 5.92 (s, 1H), 6.21–6.22 1.42–1.51 (m, 8H), 2.21 (s, 3H), 2.38 (s, 3H), 3.42 (s, 2H), 3.67
(m, 1H), 6.71–6.72 (m, 1H), 6.80–6.81 (m, 2H), 7.33 (t, J = (s, 3H), 3.97 (s, 3H), 6.03 (s, 1H), 6.39–6.40 (m, 1H), 6.74 (s,
7.2 Hz, 1H), 7.38 (t, J = 7.2 Hz, 2H), 7.44 (d, J = 7.2 Hz, 2H). 1H), 6.90–6.91 (m, 2H). 13C NMR (CDCl3, 125 MHz) 10.2, 13.6,
4140 | Org. Biomol. Chem., 2014, 12, 4132–4142
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14.7, 23.5, 26.5, 27.0, 30.8, 52.0, 58.4, 91.8, 110.2, 113.1, 114.2,
118.5, 127.2, 130.7, 132.3, 133.7, 134.7, 148.5, 154.8, 167.2,
Acknowledgements
172.3. HRMS-ESI (m/z): [M + H]+ calcd for C27H37N3O3Sn1, E. Marchal is supported by a trainee award from The Beatrice
571.1851; found, 571.1869.
Hunter Cancer Research Institute with funds provided by
Prodigiosin tin(IV) complex 50. Obtained following the Cancer Care Nova Scotia as part of The Terry Fox Foundation
1
general procedure 3 as a purple film (quant., 6.1 mg). H NMR Strategic Health Research Training Program in Cancer
(CDCl3, 500 MHz) 0.76 (t, J = 7.5 Hz, 3H), 0.91 (t, J = 7.0 Hz, Research at CIHR. G. McFadden is supported by a Program
3H), 1.18–1.26 (m, 4H), 1.33–1.36 (m, 5H), 1.41–1.58 (m, 9H), Grant from the National Health and Medical Research Council
2.35 (s, 3H), 2.39 (t, J = 7.5 Hz, 2H), 3.95 (s, 3H), 6.02 (s, 1H), of Australia and a Discovery Project from the Australian
6.39 (dd, J = 3.5, 2.0 Hz, 1H), 6.59 (s, 1H), 6.73 (dd, J = 3.5, 2.0 Research Council. We thank the Australian Red Cross for sup-
Hz. 1H), 6.79 (s, 1H), 6.91 (s, 1H). 13C NMR (CDCl3, 125 MHz) plying human blood.
13.6, 14.2, 14.6, 22.8, 23.4, 26.2, 26.5, 27.1, 30.2, 31.9, 58.4,
91.9, 110.2, 113.1, 116.9, 125.1, 127.5, 129.0, 130.6, 132.3,
135.0, 148.6, 154.9, 167.4. HRMS-ESI (m/z): [M + H]+ calcd for
C28H41N3O1Sn1, 555.2266; found, 555.2266.
Notes and references
Dibutyl tin(IV) complex 51. Obtained following the general
procedure 3 as a thick purple oil (80%, 48 mg). 1H NMR
(CDCl3, 500 MHz) 0.63 (t, J = 7.5 Hz, 6H), 1.10 (sext., J = 7.5 Hz,
4H), 1.33 (quint., J = 7.5 Hz, 4H), 1.54–1.65 (m, 4 H), 2.50 (s,
3H), 2.69 (s, 3H), 4.04 (s, 3H), 5.32 (s, 2H), 6.28 (s, 1H),
6.99–7.02 (m, 2H), 7.14–7.17 (m, 2H), 7.31 (d, J = 8.0 Hz, 1H),
7.35 (d, J = 7.5 Hz, 1H), 7.40 (t, J = 8.0 Hz, 2H), 7.47 (d, J = 7.5
Hz, 2H), 7.67 (d, J = 8.0 Hz, 1H). 13C NMR (CDCl3, 125 MHz)
12.4, 13.5, 17.6, 24.3, 26.2, 27.0, 58.7, 65.6, 93.8, 103.1, 114.2,
116.7, 117.4, 118.8, 121.7, 123.0, 128.1, 128.3, 128.7, 128.9,
132.7, 134.7, 136.9, 137.5, 141.4, 144.4, 155.1, 156.6, 165.5,
168.0. HRMS-APCI (m/z): [M + H]+ calcd for C36H42N3O5,
684.2243; found, 684.2209.
Dibutyl tin(IV) complex 52. Obtained following the general
procedure 3 as a purple thick oil (87%, 53 mg). 1H NMR
(CDCl3, 500 MHz) 0.63 (t, J = 7.2 Hz, 6H), 0.91 (t, J = 6.7 Hz,
3H), 1.08–1.11 (m, 4H), 1.30–1.38 (m, 7H), 1.44–1.48 (m, 3H),
1.53–1.61 (m, 4H), 2.20 (s, 3H), 2.37–2.40 (m, 5H), 4.00 (s, 3H),
6.26 (s, 1H), 6.87 (s, 1H), 6.98 (t, J = 7.5 Hz, 1H), 7.02 (s, 1H),
7.10 (t, J = 7.5 Hz, 1H), 7.30 (d, J = 8.2 Hz, 1H), 7.65 (d, J =
8.2 Hz, 1H). 13C NMR (CDCl3, 125 MHz) 10.1, 13.5, 14.2,
15.1, 22.8, 23.6, 24.7, 26.2, 27.1, 30.2, 31.9, 58.3, 92.8, 99.9,
113.8, 116.4, 118.2, 121.1, 121.6, 125.5, 129.3, 132.8, 136.1,
137.3, 138.6, 143.8, 151.4, 154.5, 165.2. UV (DCM) λmax (ε):
498 nm (55 600 L mol−1 cm−1). LRMS 620.3 [M + H]+.
HRMS-ESI (m/z): [M + H]+ calcd for C33H46N3O1Sn1, 620.2657;
found, 620.2683.
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a
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