Sandwich and half-sandwich derivatives of platensimycin: Synthesis and biological evaluation

Article abstract of DOI:10.1021/om201146c

The multistep synthesis and biological evaluation of five structurally diverse, chiral and achiral CpMn(CO)₃ (4, 7 and 8), (η⁶-arene)Cr(CO)₃ (5), and [3]ferrocenophane-1-one (6) containing platensimycin (1) derivatives are described in this report. The structures were inspired by the antibiotic platensimycin. All the chiral compounds presented in this report are racemates. The new compounds were unambiguously characterized by ¹H and ¹³C NMR spectroscopy, mass spectrometry, IR spectroscopy, and elemental analysis and in certain cases by X-ray crystallography (4, 16, 18, and 29). The antibacterial and antitumor activity of selected derivatives was tested. Molecular modeling suggests that the derivatives described here may well fit into the active site of the FabF enzyme, which is the biological target of platensimycin. Hence, the antimicrobial activities of our new bioorganometallices 4-8 and the protected amide intermediates 15, 17, 18, 23, 28, 29, and 31 were tested against various Gram-positive and Gram-negative bacterial strains. However, all compounds were inactive up to concentrations of 180 μg/mL. The cytotoxicity of compounds 4 and 6 and the protected amide intermediates 15, 17, 18, 23, 28, 29, and 31 was tested against HepG2 and PT45 mammalian cancer cell lines. Surprisingly, all compounds containing a trimethylsilylethyl ester functionality at the aromatic ring (17, 23, 29, and 31) displayed rather high cytotoxicity between 2 and 9 μM.

Full text of DOI:10.1021/om201146c

Article
pubs.acs.org/Organometallics
Sandwich and Half-Sandwich Derivatives of Platensimycin: Synthesis
and Biological Evaluation
Malay Patra,† Gilles Gasser,†,‡ Michaela Wenzel,§ Klaus Merz,† Julia E. Bandow,§
and Nils Metzler-Nolte*,†
†Lehrstuhl fur Anorganische Chemie I-Bioanorganische Chemie, Fakultat fur Chemie und Biochemie, Ruhr-Universitat Bochum,
̈
̈
̈
̈
Gebaude NC 3 Nord, Universitatsstrasse 150, D-44801 Bochum, Germany,
̈
̈
‡Institute of Inorganic Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland;
§Lehrstuhl fur Biologie der Mikroorganismen, Fakultat fur Biologie und Biotechnologie, Ruhr-Universitat Bochum,
̈
̈
̈
̈
Universitatsstrasse 150, D-44801 Bochum, Germany.
̈
S
* Supporting Information
ABSTRACT: The multistep synthesis and biological evalua-
tion of five structurally diverse, chiral and achiral CpMn(CO)3
(4, 7 and 8), (η6-arene)Cr(CO)3 (5), and [3]ferrocenophane-
1-one (6) containing platensimycin (1) derivatives are
described in this report. The structures were inspired by the
antibiotic platensimycin. All the chiral compounds presented
in this report are racemates. The new compounds were
1
unambiguously characterized by H and 13C NMR spectros-
copy, mass spectrometry, IR spectroscopy, and elemental
analysis and in certain cases by X-ray crystallography (4, 16,
18, and 29). The antibacterial and antitumor activity of
selected derivatives was tested. Molecular modeling suggests
that the derivatives described here may well fit into the active site of the FabF enzyme, which is the biological target of
platensimycin. Hence, the antimicrobial activities of our new bioorganometallices 4−8 and the protected amide intermediates 15,
17, 18, 23, 28, 29, and 31 were tested against various Gram-positive and Gram-negative bacterial strains. However, all
compounds were inactive up to concentrations of 180 μg/mL. The cytotoxicity of compounds 4 and 6 and the protected amide
intermediates 15, 17, 18, 23, 28, 29, and 31 was tested against HepG2 and PT45 mammalian cancer cell lines. Surprisingly, all
compounds containing a trimethylsilylethyl ester functionality at the aromatic ring (17, 23, 29, and 31) displayed rather high
cytotoxicity between 2 and 9 μM.
Despite the success of the “ferrocene derivatization” in
medicinal organometallic chemistry,1,12 other fairly stable
metallocene derivatives such as [3]ferrocenophane, CpMn-
(CO)3 (cymantrene), and (η6-arene)Cr(CO)3 have received,
surprisingly, much less attention. [3]Ferrocenophane was
recently introduced as an attractive pharmacophore in the
development of cytotoxic agents for hormone-refractory breast
cancer cells.13,14 Apart from the carbonyl metal immunoassay
(CMIA),15,16 Schatzschneider et al. have demonstrated that the
conjugation of cymantrene to the cell-penetrating peptide
(CPP) hCT(18-32)-k7 modulates the intracellular distribution
of the CPP and induces cytotoxicity against MCF-7 cells.17
Schmalz et al. reported several (η6-arene)Cr(CO)3 derivatives
of the anti-inflammatory diterpenes as specific inhibitors of the
NOD2 signaling pathway.18 Very recently, Nordlander et al.
disclosed a (η6-benzene)Cr(CO)3 derivative of the antimalarial
INTRODUCTION
■
The derivatization of bioactive organic molecules with organo-
metallics was shown to be an attractive strategy to modify the
inherent properties of biomolecules.1−4 The most prominent
examples are undoubtedly the derivatization of known anticancer
and antimalarial drugs with ferrocene.3,5,6 Ferroquine, a ferrocenyl
derivative of the known antimalarial drug chloroquine, was found
to overcome the resistance problem of chloroquine.7−9 Impor-
tantly, ferroquine has recently successfully passed clinical phase II
trials and is now undergoing field testing. Ferrocifens are another
successful example of such derivatization. These ferrocene-
modified tamoxifens, developed by Jaouen et al., exhibit activity
against hormone-independent breast cancer, where hydroxy-
tamoxifen and tamoxifens are inactive.10 Apart from that,
organometallic compounds have also found application as enzyme
inhibitors.11 With more and more organometallic compounds
receiving attention for applications in medicinal organometallic
chemistry, the need arises for the development of multistep
synthetic routes to complicated structures containing organo-
metallic moieties.
Special Issue: Organometallics in Biology and Medicine
Received: November 17, 2011
© XXXX American Chemical Society
A
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drug chloroquine, which is 2 times more active compared to its
purely organic analogue.19
one of the fatty acid biosynthesis enzymes, namely FabF.20
With this biological target in mind, we performed manual
computer docking experiments to model our organometallic
platensimycin derivatives in a way that they could also fit within
this pocket and hence have potentially a mode of action similar
to that of platensimycin. Of note, we have previously shown, by
manual computer docking experiments, that the S,Sp
enantiomer of 3 fits nicely in the active site of the FabF
enzyme and that the C6−C7 fused ferrocene occupies a pocket
similarly to the C8−C9 fused tetracyclic cage of 1.30 The results
of similar experiments with (S,SP)-4 and (S)-6 are presented in
Figure 2. Both fit well in the active site when superimposed
Since the discovery of the naturally occurring antibiotics
platensimycin (1; Figure 1)20 and platencin (2a; Figure 1),21
Figure 1. Structures of platensimycin (1), platencin (2a), a dialkylamino
derivative (2b), and organometallic derivatives (3−8) of 1.
Figure 2. Manual superimposition of (S,SP)-4 (cyan) (a and c) and
(S)-6 (cyan) (b and d) on 1 (lime green) bound to the FabF enzyme.
Reasonable atomic coordinates for (S,SP)-4 and (S)-6 were obtained
from the X-ray single-crystal structures of rac-4 and rac-29 (the
protecting group was manually substituted by hydrogen). The
obtained models were manually fitted into the reported X-ray crystal
structure of 1 bound to FabF enzyme (PDB code 2gfx).20
numerous synthetic routes have been developed not only to 1
and 2a but also to various derivatives for structure−activity
relationship (SAR) studies.22−24 Strikingly, a recent publication
by Sintim et al. showed that the complex tetracyclic cage of 1
can be replaced by simple dialkyl amine moieties (2b; Figure 1)
without losing the biological activity.25 Concurring with the
emergence of organic derivatives of 1 and 2a, we embarked on
a project to investigate the possibility of replacing its complex
tetracyclic cage by bulky and lipophilic organometallic cores.
The synthesis and biological evaluation of (η6-arene)Cr(CO)3
and ferrocene based derivatives of 1 was reported.26−29
Recently, we published the synthesis of a C6−C7 ferrocene
fused derivative (3; Figure 1) of 1.30 As a continuation of these
initial studies, we report herein the synthesis of five new
derivatives of 1 (4−8; Figure 1) as well as the antimicrobial and
cytotoxic activities of these compounds and the protected
amide derivatives 15, 17, 23, 28, 29, and 31 (Schemes 1−3).
Both of the bioorganometallics 3 and 6 contain ferrocene but
possess significant structural differences. While 3 consists of a
homoannular ferrocene moiety, the [3]ferrocenophane-1-one
of 6 is a heteroannular C3 bridged and slightly strained ferro-
cene moiety. Bioorganometallics 4 and 5 are the homologues
of 3, and the size of the organometallic core increases in
the order ferrocene < cymantrene < (η6-benzene)Cr(CO)3.
Importantly, while compounds 3−6 are chiral molecules, 7
and 8 are relatively simple and achiral in nature. These structurally
diverse metallocene-containing derivatives of 1 extend the
chemical space for further SAR studies on platensimycin
analogues.
manually on 1 bound to its target enzyme FabF. The keto car-
bonyl oxygen in both cases is capable of forming H bonds with
the active site amino acid residue Ala309 as the keto carbonyl of 1
does. As expected, in the case of (S,SP)-4, cymantrene occupied
the same pocket as ferrocene for (S,Sp)-3.30
Synthesis and Spectroscopic Characterization. It
should first be noted that all chiral compounds described in
this report are racemates. The synthesis of the cymantrene-
containing 4, 7, and 8 was initiated from commercially available
cymantrene and is presented in Scheme 1. Carboxylic acid 9
was prepared following a literature procedure.31 Reduction of
the keto group of 9 using Zn(Hg) and concentrated HCl
(Clemmensen reduction) gave 10 in 73% yield.32−34 The
cymantrene fused cyclohexenone 11 was prepared by
polyphosphoric acid mediated intramolecular cyclization of
10.34 Afterward a synthetic route similar to that developed
previously during our synthesis of C6−C7 ferrocene fused
platensimycin derivative 3 was applied to synthesize the C6−
C7 cymantrene fused platensimycin derivative 4 from 11.30
Treatment of 11 with KHMDS and MeI gave exclusively the
1
exo-methylated ketone 12 in 62% yield. As expected, the H
NMR spectrum of 12 showed a doublet for the exo-CH3 group
at 1.19 ppm together with other protons from the cymantrene
moiety. The Michael addition reaction of 12 to methyl acrylate
in the presence of potassium tert-butoxide (KOtBu) led to the
formation of the methyl ester 13 as a single diastereomer in
RESULTS AND DISCUSSION
■
Manual Docking of 4 and 6 into the FabF Enzyme
Pocket. Platensimycin exerts antibacterial activity by inhibiting
B
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a
Scheme 1. Synthesis of Cymantrene-Containing 4, 7, and 8
a
Reagents and conditions: (a) Zn(Hg), concentrated HCl; (b) polyphosphoric acid, 80 °C; (c) KHMDS, HMPA, MeI, THF; (d) KOtBu, methyl
t
acrylate, BuOH, Et2O; (e) 1 M aqueous NaOH, THF, MeOH; (f) methyl 3-amino-2,4-dimethoxybenzoate, HATU, DIPEA, DMF; (g) BBr3,
CHCl3; (h) LiOH, H2O, THF; (i) degassed 1 M aqueous NaOH, THF, MeOH, N2 atmosphere; (j) 2-(trimethylsilyl)ethyl 3-amino-2,4-
dihydroxybenzoate, HATU, DIPEA, DMF; (k) TASF, DMF; (l) methyl 3-amino-2,4-bis(methoxymethoxy)benzoate, HATU, DIPEA, DMF; (m)
LiOH·H2O, THF, H2O, and then 4 M HCl in dioxane.
51% yield. After purification by flash column chromatography,
the H NMR spectrum of 13 showed a clean singlet at 1.17
C3v symmetry were observed in the IR spectrum of 4. Sur-
prisingly, we discovered that the treatment of Me-4 with
aqueous LiOH·H2O/THF in the absence of inert conditions
gave an undesired byproduct, which was structurally charac-
terized as the amide 16 from its ESI-MS (positive detection
mode) and by X-ray crystallography (see Figure 3b).
Importantly, the formation of 16 from Me-4 is very unusual
as the process involves the cleavage of a C(sp2)−N bond. One
possible mechanism could be the formation of an o-quinone
monoimine intermediate from Me-4 or 4 under oxidative basic
reaction conditions which would rapidly hydrolyze to give 16
and an o-quinone derivative (see Figure S4 in the Supporting
Information for details). Alternatively, amide coupling of 14
with 2-(trimethylsilyl)ethyl 3-amino-2,4-dihydroxybenzoate36
gave the trimethylsilyl protected derivative 17 that could then
be deprotected easily by treatment with tris(dimethylamino)-
sulfonium difluorotrimethylsilicate (TASF) to give the desired
analogue 4 in 71% yield.
Synthesis of the achiral bioorganometallics 7 and 8 was
relatively straightforward. Amide coupling of 9 with methyl
3-amino-2,4-bis(methoxymethoxy)benzoate37 gave the protected
amide 18, which was then deprotected using aqueous LiOH
and then 4 N HCl in dioxane to provide 7 in 70% yield. The 1H
NMR spectrum of 7 showed two signals at 5.04 and 5.65 ppm
corresponding to the cymantrene moiety together with two
doublets for the aromatic protons at 6.49 and 7.62 ppm,
respectively. 8 was obtained in 64% yield by Clemmensen
1
ppm corresponding to the endo-CH3 group. As reported earlier
for the similar ferrocene-containing cyclohexanone derivative,30
the Michael addition proceeded with a high degree of
diastereoselectivity due to the preferentially less hindered exo
face (opposite to the Mn(CO)3 unit) approach of the
electrophiles. Treatment of 13 with aqueous NaOH yielded
the desired carboxylic acid 14 in 88% yield. The formation of
14 was confirmed by the disappearance of the COOCH3 signal
1
in its H NMR spectrum. Two peaks centered at m/z 714.80
and 356.85 corresponding to [2M − H]− and [M − H]−
species were observed in the ESI-MS (negative detection
mode) spectrum. HATU-mediated peptide coupling of 14 with
methyl 3-amino-2,4-dimethoxybenzoate30,35 gave amide 15 in
82% yield. Treatment of 15 with BBr3 afforded the desired
bioorganometallic 4, together with its methyl ester Me-4. Me-4
could be converted to 4 by treatment with a deoxygenated 1 M
aqueous NaOH/MeOH/THF mixture under an inert atmos-
1
phere. The H NMR spectrum of 4 showed a singlet at 1.12
ppm corresponding to the endo-CH3 group together with three
broad singlets at 5.04, 5.30, and 5.42 ppm corresponding to the
cymantrene moiety and two doublets at 6.40 and 7.56 ppm for
the benzene ring protons. A clean peak at m/z 507.79 for the
[M − H]− species was observed in the ESI-MS (negative
detection mode) spectrum. Two strong stretching vibration
bands at 2019 and 1921 cm−1 characteristic for the Mn−CO in
C
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reduction of 7 using Zn(Hg) and concentrated HCl. In the 13
C
The synthetic route for the preparation of the [3]ferro-
cenophane-1-one based bioorganometallic 6 is presented in
Scheme 3. [3]Ferrocenophan-1-one (24) was prepared following
a literature procedure.40 Methylation of 24 with KHMDS and
MeI gave 25 in 67% yield. A considerable amount of 24 was
found to be unreacted but could be recovered easily during the
purification by flash column chromatography. Increased amounts
of either KHDMS or MeI did not improve the conversion. The
1H NMR spectrum of 25 showed a doublet at 1.11 ppm for the
CH3 group together with the other expected signals. Treatment
of 25 with tert-butyl acrylate in the presence of KOtBu led to the
formation of tert-butyl ester 26 in 65% yield, together with a
trace amount of the carboxylic acid 27. Attempts to hydrolyze
the tBu ester of 26 with aqueous NaOH or LiOH gave carboxylic
acid 27 together with a considerable amount of unidentified
impurities. However, Et3SiI-mediated ester hydrolysis of 26
proceeded smoothly and the desired carboxylic acid 27 was
obtained in 82% yield. The formation of 27 was confirmed by
the disappearance of the C(CH3)3 signal in its 1H NMR
spectrum. A clean peak at m/z 326.04 corresponding to the [M]+
species was observed in the ESI-MS (positive detection mode)
spectrum. Amide coupling of 27 with methyl 3-amino-2,4-
dimethoxybenzoate yielded the protected amide 28 in 57% yield.
Treatment of 28 with BBr3 afforded a chromatographically
separable mixture of 6 and its methyl ester Me-6. Conversion of
Me-6 to 6 was carried out using deoxygenated 1 M aqueous
NaOH/MeOH/THF under inert conditions. However, slow
decomposition of 6 was observed under these reaction
conditions. The formation of a large amount of side products
was observed, and this is probably due to the oxidative
decomposition of the [3]ferrocenophan-1-one moiety under
the basic reaction conditions. Indeed, as described earlier,
similar problems during the ester hydrolysis of 26 under basic
conditions were noticed. These observations suggest that the
[3]ferrocenophan-1-one moiety is somewhat sensitive to
hydroxyl bases. In order to avoid basic conditions in the depro-
tection step, we employed 2-(trimethylsilyl)ethyl 3-amino-2,4-
dihydroxybenzoate as previously used for the synthesis of 4
and 5. Thus, amide coupling of 27 with 2-(trimethylsilyl)ethyl
3-amino-2,4-dihydroxybenzoate gave compound 29, which
could be then deprotected by treatment with TASF to obtain
the desired bioorganometallic 6 in 78% yield. The presence of 6
NMR spectrum of 8, the signal at 199.4 ppm corresponding to
the keto group of 7 vanished with the appearance of the new
CH2 signal at 26.3 ppm.
As shown in Scheme 2, the (η6-arene)Cr(CO)3 containing
bioorganometallic 5 was prepared in a five-step synthesis
a
Scheme 2. Synthesis of 5
a
Reagents and conditions: (a) Cr(CO)6, Bu2O, THF, 140 °C; (b)
KOtBu, tert-butyl acrylate, BuOH, Et2O; (c) 1 M aqueous NaOH,
t
MeOH, THF; (d) 2-(trimethylsilyl)ethyl 3-amino-2,4-dihydroxyben-
zoate, HATU, DIPEA, DMF; (e) TASF, DMF.
starting from 2-methyl-1-tetralone (19). In the first step, the
metalation of 19 with Cr(CO)6 in a deoxygenated Bu2O/THF
9/1 (v/v) mixture provided compound 20 as a mixture of two
diastereomers (the ratio of exo- and endo-CH3 diastereomers
was ∼60:40) in 61% combined yield.38 The separation of the
diastereomers was not necessary for the next step, since
epimerization occurs. Meyer et al. reported that the base-
catalyzed Michael addition of methyl vinyl ketone with 20
proceeds with a high degree of diastereoselectivity and the
exo- and endo-CH3 diastereomers were indeed obtained in a ratio
of 13:87.39 We used slightly different reagents and reaction
conditions. Treatment of 20 with tert-butyl acrylate in the
presence of KOtBu gave only the endo-CH3 epimer 21 in 79%
yield. The absence of the undesired exo-CH3 epimer within the
detection limit was confirmed from the 1H NMR spectrum. 21
was then treated with aqueous NaOH to obtain the desired
carboxylic acid 22, which was then coupled with 2-(trimethyl-
silyl)ethyl 3-amino-2,4-dihydroxybenzoate36 to yield the inter-
mediated amide 23. Treatment of 23 with TASF in DMF gave
1
was confirmed from its H NMR spectrum, with a singlet at
1.32 ppm corresponding to the CH3 group together with all the
signals expected from the [3]ferrocenophan-1-one and the
benzoic acid unit, respectively. Additionally, in its ESI-MS
(positive detection mode) spectrum, two signals centered at
m/z 477.02 and 499.97 corresponding to [M]+ and [M + Na]+
species unambiguously proved the presence of 6. Compound
31 was synthesized in one step from the carboxylic acid 30
(Scheme 3).30
X-ray Crystallography. The molecular structures of 4, 16,
18, and 29 were confirmed by the determination of their respective
X-ray single-crystal structures. ORTEP plots are presented in
Figures 3 and 4. Table S1 (in the Supporting Information) contains
the relevant crystallographic data and parameters.
The cymantrene-containing compounds 4, 16, and 18
1
the desired bioorganometallic 5 in 85% yield. The H NMR
crystallize in triclinic P1, monoclinic P2 /c, and orthorhombic
̅
1
spectrum of 5 showed, as expected, a singlet at 1.16 ppm
corresponding to the endo-CH3 group together with the signals
for the CH2 protons and all the benzene ring protons. ESI-MS
(negative detection mode) spectrum showed a clean peak at
517.88 corresponding to [M − H]− species of 5.
Pbca space groups, respectively. Neither the metal fragment nor
the organic part in the structures possess any unusual structural
features.31 In the solid state of 4, both phenolic hydroxide
groups form intramolecular hydrogen bonds to the carbonyl
group of the carboxylic acid and the amide moiety (see Figure 3a).
D
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a
Scheme 3. Synthesis of [3]Ferrocenophan-1-one Based Bioorganometallic 6
a
Reagents and conditions: (a) KHMDS, HMPA, MeI, THF; (b) KOtBu, tert-butyl acrylate, tBuOH, Et2O; (c) Et3SiI, CH2Cl2; (d) methyl 3-amino-
2,4-dimethoxybenzoate, HATU, DIPEA, DMF; (e) BBr3, CHCl3; (f) 1 M aqueous NaOH, THF, MeOH, N2 atmosphere; (g) 2-(trimethylsilyl)ethyl
3-amino-2,4-dihydroxybenzoate, HATU, DIPEA, DMF; (h) TASF, DMF.
Me-4 to 4 by treatment with LiOH in the absence of an inert
atmosphere (for hydrogen-bonding interactions in the single
crystals of 4, 16, and 18, see Figures S1−S3 in the Supporting
Information).
Compound 29 crystallized in the monoclinic P21/c space group.
As shown in Figure 4b, the OH groups of the [3]ferrocenophan-1-
one containing amide 29 form intramolecular hydrogen bonds in a
fashion similar to that observed in the solid state of 4. The Cp
rings are slightly tilted because of the comparatively short C3
bridge between them and are in a near-eclipsed conformation. The
bond lengths and angles of the [3]ferrocenophan-1-one moiety fit
well with those reported earlier.41
Cytotoxicity Study. An ideal antimicrobial agent is
supposed to kill bacteria but not host cells. In order to have
a first insight into whether or not our compounds possess any
toxicity toward mammalian cells, we determined the IC50 values
of the organometallics prepared in this study. Compounds 4
and 6 as well as the intermediate protected amide derivatives
15, 17, 18, 23, 28, 29, and 31 were therefore screened against
HepG2 and PT45 mammalian cancer cell lines. The
cytotoxicity of 5 could not be studied due to solubility
Figure 3. ORTEP plots of 4 (a) and 16 (b). Ellipsoids are drawn at the
50% probability level. Except for the OH in 4 and NH in 16, all other
hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and
problems. Both fully deprotected compounds 4 and 6 showed
no cytotoxic activity up to 200 μM concentrations. Surprisingly,
it was found that the protected amide derivatives 15, 17, 18, 23,
28, 29, and 31 exhibited cytotoxicity against both cell lines (see
Table 1 and see Figure S5 in the Supporting Information for
representative examples). When we analyzed the results, it
appeared that the cytotoxicity is related to the lipophilic group
present in the aromatic moiety of the bioorganometallics. The
compounds 17, 23, 29, and 31, which contain the
angles (deg) of 4: Mn(1)−centroid Cp(C1−5) = 1.762, average Mn(1)−
CO = 1.79(4), C(2)−C(9) = 1.516(9), C(3)−C(12) = 1.447(10);
Mn(1)−C(2)−C(9) = 128.7(5), Mn(1)−C(3)−C(12) = 123.6(5),
average CO−Mn(1)−CO = 92.5. Selected bond lengths and angles of
the metal−organic part in 16 are similar to those presented for 4.
As mentioned earlier, the single-crystal X-ray structure determi-
nation of 16 helped us to identify the unexpected compound 16
as the main side product during the attempted transformation of
E
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(Staphylococcus aureus DSM 20231 (type strain), S. aureus
ATCC43300 (MRSA), Bacillus subtilis 168) and Gram-negative
(Escherichia coli DSM 30083 (type strain) and Pseudomonas
aeruginosa DSM 50071 (type strain), Acinetobacter baumannii
DSM 30007 (type strain)) bacterial strains up to 180 μg/mL.
Unfortunately, none of these compounds exhibited antibacterial
activity against any of the bacterial strains. The lack of activity is
perhaps due to insufficient cell permeability of the
bioorganometallics, as recently demonstrated for several other
synthetic and natural congeners of platensimycin having a
modified tetracyclic cage.24,25
CONCLUSION
■
In summary, we have synthesized and characterized structurally
diverse sandwich and half-sandwich derivatives of the naturally
occurring antibiotic platensimycin (1). To the best of our
knowledge, the construction of the quaternary center at the C-2
position of the cymantrene fused cyclohexenone 12 with
complete diastereoselectivity is described for the first time.
Several very interesting synthetic findings were observed in this
study, which should help to pave the way for the preparation of
further highly functionalized bioorganometallics. The newly
synthesized compounds failed to show any antibacterial activity
against either Gram-positive or Gram-negative bacterial strains.
This could be due to inefficient cell permeability of the
bioorganometallics, as observed earlier for some benzene−Cr−
tricarbonyl bioorganometallics.26 However, interestingly, the
trimethylsilylethyl esters containing intermediate amides 17, 23,
29, and 31 showed rather impressive cytotoxicity against
HepG2 and PT45 mammalian cancer cell lines, approaching
IC50 values of the well-known anticancer drug cisplatin.
Importantly, cerulenin, a fatty acid biosynthesis inhibitor, is
currently in preclinical anticancer studies as a part of a
Figure 4. ORTEP plots of 18 (a) and 29 (b). Ellipsoids are drawn at
the 50% probability level. Except for the OH in 29, all other hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles
(deg) of 18: Mn(1)−centroid Cp(C4−8) = 1.766, average Mn(1)−
CO = 1.790 (4), C(8)−C(9) = 1.478(5); Mn(1)−C(8)−C(9) =
123.6(2), average CO−Mn(1)−CO = 92.29. Selected bond lengths
(Å) and angles (deg) of 29: Fe(1)−centroid Cp(C1−5) = 1.654,
Fe(1)−centroid Cp(C6−10) = 1.654, C(3)−C(11) = 1.506(6),
C(11)−C(12) = 1.539(7), C(6)−C(14) = 1.524(7); C(3)−Fe(1)−
C(6) = 98.73(19), C(11)−C(3)−Fe(1) = 117.4(3), C(14)−C(6)−
Fe(1) = 121.8(3).
Table 1. Cytotoxicity Data of Selected Compounds in This
Work against HepG2 and PT45 Mammalian Cancer Cell
a
combination therapy with rosiglitazone.43 IC50 values of 5
1
Lines in the Crystal Violet Assay
and 45 2 μM were determined against prostate cancer cell
lines LNCaP, PC-3, and DU-145 for rosiglitazone in the presence
and absence of cerulenin, respectively. We are currently per-
forming further studies to understand our unexpected findings
and elaborate on the preliminary structure−activity relationship
derived herein.
IC50 value for two different cell lines (μM)
compd
15
HepG2
PT45
39.5 5.3
6.45 0.1
93.4 3.9
3.3 0.2
48.2 5.8
3.6 0.2
8.8 3.4
2.4 0.4
47.0 8.6
7.3 1.9
93.2 22.3
7.3 2.5
55.5 2.8
2.3 0.2
7.8 0.9
0.9 0.2
17
18
23
28
EXPERIMENTAL SECTION
■
29
Materials. All chemicals were of reagent grade quality or better,
obtained from commercial suppliers and used without further
purification. Solvents were used as received or dried over molecular
sieves. All preparations were carried out using standard Schlenk
techniques. 9,31 24,40 methyl 3-amino-2,4-dimethoxybenzoate,30,35
2-(trimethylsilyl)ethyl 3-amino-2,4-dihydroxybenzoate,36 methyl
3-amino-2,4-bis(methoxymethoxy)benzoate,37 was prepared following
the literature procedures.
31
cisplatin42
a
Results represent the averages of two independent experiments with
three replicates each.
trimethylsilylethyl ester functionality and two OH groups at the
aromatic unit, displayed cytotoxicity in the low micromolar
concentration range (2−9 μM). These values are comparable to
those of the well-known inorganic anticancer drug cisplatin.42
However, the cytotoxicity decreased significantly when the OH
and the carboxylic acid groups were protected with methyl or
methoxymethyl groups (for compounds 15, 18, and 28). While
these results give a first hint toward cytotoxic activity, further
biological tests will be required to establish the exact mechanism of
action of these compounds (e.g., whether the compounds are
inducing apoptosis).
Instrumentation and Methods. H and 13C NMR spectra were
1
recorded in deuterated solvents on Bruker DRX 200, 250, 400, or 600
spectrometers at 30 °C. The chemical shifts, δ, are reported in ppm
(parts per million). The residual solvent peaks have been used as an
internal reference. The abbreviations for the peak multiplicities are as
follows: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q
(quartet), m (multiplet), and br (broad). Infrared spectra were
recorded on an ATR unit using a Bruker Tensor 27 FTIR
spectrophotometer at 4 cm−1 resolution. Signal intensity is abbreviated
br (broad), s (strong), m (medium), and w (weak). ESI mass spectra
were recorded on a Bruker Esquire 6000. Crystallographic data were
collected using Bruker-axs SMART 1000 CCD and Rigaku Mercury
375 R/M CCD (XtaLAB mini) diffractometers. The structure was
solved by direct methods (SHELXS-9744) and refined against F2 with
all measured reflections (SHELXL-97,44 Palton-Squeeze45). Crystallo-
Antimicrobial Activity. The antimicrobial activities of our
new metallocene-containing platensimycin derivatives 4−8 and
the protected amide intermediate compounds 15, 17, 18, 23,
28, 29, and 31 were then tested against various Gram-positive
F
dx.doi.org/10.1021/om201146c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
graphic data were deposited as CIF files in the Supporting Information
and also at the CSD (4, 823234; 16, 823236; 18, 823235; 29, 823233)
Elemental microanalyses were performed using a Fisons Carlo Erba
EA1108 instrument (CHNS version). The manual docking of
experiments of the bioorganometallics with FabF enzyme were done
using PyMOL (PyMOL-1_2edu-bin-win32).
Minimum Inhibitory Concentration (MIC) Determination.
The antimicrobial activity tests were performed in a microtiter plate
assay containing 0.2 mL of Luria Broth medium and appropriate
compound concentrations up to 180 μg/mL. Culture media
containing appropriate concentrations of compound were inoc-
ulated with 105 cells/mL and incubated at 37 °C for 18 h. The com-
pounds did not visibly inhibit growth.
82.1, 105.7, 225.1 (Mn−CO). IR bands (ν): 3113 w, 2948 w, 2576 w,
2004 s, and 1906 s (br, Mn−CO), 1712 s, 1686 s, 1480 w, 1424 w,
1408 m, 1316 w, 1222 m, 1186 w, 1022 m, 937 w, 893 w, 838 m,
684 m, 632 s cm−1. ESI-MS (negative detection mode): m/z (%)
288.76 (100) [M − H]−, 578.69 (20) [2M − H]−.
Cyclic Ketone 11.34 A mixture of 10 (300 mg, 1.03 mmol) and
polyphosphoric acid (8 g) was heated at 80 °C for 7 h. Water was
added to the reaction mixture, and this mixture was extracted with
Et2O (2 × 50 mL). The organic phase was washed with 1 M HCl (1 ×
30 mL), water (5 × 50 mL), and brine (1 × 30 mL), dried over
Na2SO4, and filtered. Removal of the solvent gave the desired cyclic
ketone 11 (230 mg, 81%) as a brown sticky solid. The crude product
was used for the next step without further purification. Rf = 0.21 (silica
1
Cell Culture and Cytotoxicity Test. Required cells were cultured
in RPMI 1640 medium supplemented with 10% fetal calf serum
(FCS), 2 mM L-glutamine, penicillin (100 U/mL), and streptomycin
(100 μg/mL) in a 5% CO2 atmosphere. Absolute cell numbers were
determined by the crystal violet assay. Cells were seeded in 96-well
cell-culture treated microtiter plates (MTP). After seeding, the cells
were grown for 24 h under standard conditions. The compounds were
dissolved in culture medium with 0.5% DMSO and applied to the
cells in 1, 5, 20, 50, 100, 500 μM concentrations for 72 h. Every
concentration was tested 6-fold. The cells were fixed with 4%
paraformaldehyde (PFA) in phosphate-buffered saline (PBS) for
15 min at room temperature. PFA was eluted two times with PBS, and
membranes were permeabilized by Triton X-100 (0.1%) in PBS for
10 min. Afterward, aqueous crystal violet solution (0.04%) was added to
the cells and the MTP was mechanically shaken for 1 h. The cells were
washed with H2O (×7), and crystal violet was eluted with 70% EtOH
for 3.5 h. The absorbance was determined at 570 nm; the cell biomass
could be calculated after subtraction of 24 h presubstance incubation
absorbance values. Whenever possible, the inhibitory concentration at
50% growth (IC50) for the assay was determined. The obtained cell
mass (%) data were plotted against the concentration on a half-
logarithmical scale, and a sigmoidal function fit was performed with
Origin 7 (Originlab, Northampton, MA) until the fit converged. IC50
values were directly calculated from the fit function.
Synthesis. General Procedure for the Amide Coupling (GP1).
To a stirred solution of the carboxylic acid in DMF were added
HATU and DIPEA (or NEt3), and the mixture was stirred for
30 min under an argon atmosphere. The protected amine was
then added, and the mixture was stirred at room temperature.
After completion of the reaction, the volume of DMF was
reduced under vacuum, saturated brine was added to it, and the
compound was extracted using ethyl acetate (EtOAc). The
organic layer was washed with distilled water and brine, dried
over anhydrous Na2SO4, filtered, and concentrated. Flash
column chromatography was carried out with the residue to
obtain the desired amide in pure form.
gel, hexane/EtOAc 2.5/1). H NMR (250 MHz, CDCl3): δ (ppm)
1.98−2.16 (m, 2H, CH2), 2.21−2.39 (m, 1H, CH2), 2.45−2.70
(m, 3H, CH2), 4.58−4.70 (m, 1H, C5H3), 4.94−5.01 (m, 1H, C5H3),
5.10−5.21 (m, 1H, C5H3). 13C NMR (250 MHz, CDCl3): δ (ppm)
22.5, 23.1, 38.2, 78.1, 78.4, 86.3, 89.5, 113.4, 197.8, 223.1 (Mn−CO).
IR bands (ν): 3107 w, 2951 w, 2016 s, and 1909 s (br, Mn−CO), 1683 s,
1436 w, 1411 m, 1316 w, 1281 m, 1245 m, 1170 w, 1140 w, 1043 w, 1029
w, 715 m, 665 s, 651 m, 623 s cm−1. ESI-MS (negative detection mode):
m/z (%) 270.96 (100) [M − H]−, 227.02 (50) [M − CO2 − H]−.
Compound 12. To a solution of the ketone 11 (300 mg,
1.10 mmol) in THF (15 mL) was slowly added 0.5 M KHMDS in
toluene (3.3 mL, 1.65 mmol) at −78 °C. The mixture was stirred for
30 min. HMPA (2.3 mL) followed by MeI (1.3 g, 8.82 mmol) was
added to the reaction mixture. After 1.5 h at −78 °C, the reaction was
quenched with aqueous saturated NaHCO3 solution and extracted
with EtOAc (3 × 30 mL). The organic phase was washed with H2O
and brine, dried over anhydrous Na2SO4, filtered, and concentrated.
Flash column chromatography (silica gel, hexane/EtOAc 3/1) gave
the desired product 12 (195 mg, 62%) as a yellow oil. Rf = 0.56 (silica
gel, hexane/EtOAc 3/1). 1H NMR (250 MHz, CDCl3): δ (ppm) 1.19
(d, 3H, CH3), 1.81−1.91 (m, 1H, CH2), 2.11−2.19 (m, 1H, CH2),
2.58−2.78 (m, 3H, CH2 and CH), 4.66−4.71 (m, 1H, C5H3), 4.92−
4.98 (m, 1H, C5H3), 5.22−5.29 (m, 1H, C5H3). 13C NMR (250 MHz,
CDCl3): δ (ppm) 15.1, 20.1, 30.6, 41.2, 78.6, 79.2, 85.3, 88.1, 113.1,
200.3, 223.4 (Mn−CO); IR bands (ν): 2935 w, 2016 s, 1912 s (br,
Mn−CO), 1684 s, 1454 w, 1434 m, 1376 m, 1274 w, 1169 m, 1145 w,
1033 w, 926 w, 858 m, 665 s, 625 s cm−1. ESI-MS (positive detection
mode): m/z (%) 286.85 (90) [M + H]+, 304.10 (100) [M + H2O]+.
Methyl Ester 13. To a stirred solution of 12 (150 mg, 0.52 mmol)
t
t
in a mixture of BuOH/Et2O (5 mL, 1/1 (v/v)) was added BuOK
(117 mg, 1.05 mmol) at 0 °C. After 10 min, methyl acrylate (447 mg,
5.2 mmol) was added to the reaction mixture, and this mixture was
stirred for 1 h at 0 °C. The reaction was quenched with saturated
NH4Cl, and the aqueous layer was extracted with EtOAc (2 × 30 mL).
The organic layer was washed with H2O and brine, dried over
anhydrous Na2SO4, filtered, and concentrated. Flash column
chromatography (silica gel, hexane/EtOAc 3/1 → 2/1) gave 13 as a
light yellow solid (yield 102 mg, 51%). Rf = 0.17 (silica gel, hexane/
4-Cymantrenylbutyric Acid (10).33 A mixture of granulated Zn
(20 g, 0.316 mol) and HgCl2 (3 g, 0.011 mol) in water (20 mL) were
stirred for 30 min. Concentrated HCl (10 mL) was slowly added to
the mixture, which was further stirred for 10 min. The amalgam was
decanted and washed with a 1/1 (v/v) mixture of HCl and water (2 ×
15 mL). To this freshly prepared amalgam were added 9 (4 g, 0.013
mol), benzene (20 mL), H2O (10 mL), acetic acid (5 mL), and
concentrated HCl (10 mL), and the mixture was stirred at 100 °C.
HCl (2 mL) was added in every 2 h. After 10 h, when the TCL (silica
gel, EtOAc/Et2O 1/1) showed that the reaction was complete, the
reaction mixture was cooled to room temperature, diluted with Et2O
(250 mL), and washed repeatedly with water (3 × 50 mL). The
product was then extracted with aqueous 1 M NaOH (50 mL)
solution, washed with Et2O (2 × 50 mL), acidified with 1 M HCl
(55 mL), and extracted with Et2O (2 × 200 mL). The organic phase
was washed with brine, dried over anhydrous Na2SO4, filtered, and
concentrated. 10 was isolated as a light yellow solid (2.8 g, 73%) and
used for the next step without further purification. Rf = 0.49 (silica gel,
EtOAc/Et2O 1/1). 1H NMR (250 MHz, CDCl3): δ (ppm) 2.79−2.97
(m, 2H, CH2), 2.24−2.61 (m, 4H, 2 × CH2), 4.61−4.70 (m, 4H,
C5H4). 13C NMR (250 MHz, CDCl3): δ (ppm) 25.8, 25.9, 27.4, 81.9,
1
EtOAc 3/1). H NMR (250 MHz, CDCl3): δ (ppm) 1.17 (s, 3H,
CH3), 1.62−1.80 (m, 1H, CH2), 1.84−2.08 (m, 3H, CH2), 2.19−2.54
(m, 3H, CH2), 2.68−2.84 (m, 1H, CH2), 3.65 (s, 3H, OCH3), 4.58−
4.62 (s, br, 1H, C5H3), 5.01−5.04 (m, 1H, C5H3), 5.12−5.16 (s, br,
1H, C5H3). 13C NMR (250 MHz, CDCl3): δ (ppm) 19.3, 21.2, 28.6,
29.9, 35.7, 45.1, 51.7, 77.7, 78.4, 87.1, 88.8, 111.7, 173.3, 201.1, 223.5
(Mn−CO). IR bands (ν): 2948 w, 2016 s and 1910 s (Mn−CO), 1731 m,
1681 m, 1474 w, 1452 w, 1434 m, 1355 w, 1339 w, 1304 w, 1206 m,
1182 w, 980 w, 850 w, 665 m, 630 s cm−1. ESI-MS (positive detection
mode): m/z (%) 394.86 (50) [M + Na]+, 310.22 (100) [M − 3CO −
H + Na]+, 282.18 (40) [M − 4CO − H + Na]+. Anal. Calcd for
C17H17MnO6: C, 54.85; H, 4.60. Found: C, 54.92; H, 4.73.
Carboxylic Acid 14. To a stirred solution of 13 (160 mg,
0.43 mmol) in MeOH (10 mL) was added 1 M aqueous NaOH (5.1 mL),
and the mixture was stirred for 1.5 h at room temperature. MeOH was
evaporated, water (30 mL) was added, and the aqueous layer was
washed with Et2O (2 × 20 mL). The aqueous layer was then acidified
with 1 M HCl up to pH ∼2 and extracted with Et2O (2 × 30 mL). The
combined organic phase was washed with H2O and brine, dried over
G
dx.doi.org/10.1021/om201146c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
anhydrous Na2SO4, and filtered, and the solvent was removed under
vacuum. The carboxylic acid 14 was obtained as a light yellow sticky
solid (yield 136 mg, 88%), which was used for the next step without
further purification. For analytical purposes, a small filter column with
silica gel was used with pure EtOAc as eluent.
to the reaction mixture, acidified with 1 M HCl up to pH ∼2, and
extracted with EtOAc. The organic phase was dried over anhydrous
Na2SO4, filtered, and concentrated. Flash column chromatography
(silica gel, EtOAc/MeOH/AcOH 20/1/0.075) afforded 4 (24 mg) as
a light yellow solid. The combined yield of 4 is 101 mg (60%). Single
crystals of 4 were grown by slow evaporation of its solution in a THF/
CHCl3 mixture.
Data for 14 are as follows. Rf = 0.60 (silica gel, EtOAc/MeOH
1
10/1). H NMR (250 MHz, CDCl3): δ (ppm) 1.19 (s, 3H, CH3),
1.62−1.83 (m, 1H, CH2), 1.87−2.84 (m, 3H, CH2), 2.24−2.57 (m,
3H, CH2), 2.71−2.86 (m, 1H, CH2), 4.58−4.62 (s, br, 1H, C5H3),
5.01−5.04 (m, 1H, C5H3), 5.12−5.16 (s, br, 1H, C5H3), 10.7 (1H, br,
COOH). 13C NMR (250 MHz, CDCl3): δ (ppm) 19.2, 21.3, 28.6,
29.7, 35.7, 45.1, 77.8, 78.5, 87.1, 88.6, 111.7, 179.3, 201.1, 223.1 (Mn−
CO). IR bands (ν): 2020 s and 1919 s (br, Mn−CO), 1699 m, 1685 m,
1484 w, 1434 w, 1380 w, 1303 w, 1186 w, 1157 w, 942 w, 846 w,
664 m, 630 s cm−1. ESI-MS (negative detection mode): m/z (%)
714.80 (100) [2M − H]−, 356.85 (50) [M − H]−. Anal. Calcd for
C16H15MnO6: C, 53.65; H, 4.22. Found: C, 53.95; H, 4.16.
Compound 4 from 17. To a stirred solution of amide 17 (100 mg,
0.16 mmol) in DMF (5 mL) was added tris(dimethylamino)sulfonium
difluorotrimethylsilicate (TASF; 116 mg, 0.41 mmol) at room
temperature, and the mixture was heated at 40 °C for 1 h. The
reaction mixture was then cooled to room temperature, brine was
added to the reaction mixture, and this mixture was extracted with
EtOAc (3 × 50 mL). The combined organic phase was dried over
Na2SO4, filtered, and concentrated. Flash column chromatography
(silica gel, EtOAc/MeOH/AcOH 20/1/0.075) gave of compound 4
(yield 59 mg 71%). Rf = 0.35 (silica gel, EtOAc/MeOH/AcOH 20/1/
1
Amide 15. The general procedure GP1 was followed: carboxylic
acid 14 (358 mg, 1 mmol), methyl 3-amino-2,4-dimethoxybenzoate
(422 mg, 2 mmol), HATU (760 mg, 2 mmol), DIPEA (238 mg,
2 mmol), DMF (10 mL), 60 h reaction time, purification by flash
column chromatography (silica gel, hexanes/EtOAc 1/1→0/1). 15
was obtained as a light yellow sticky solid (450 mg, 82%). Rf = 0.63
(silica gel, EtOAc). 1H NMR (250 MHz, CDCl3 and trace of
CD3OD): δ (ppm) 1.08 (s, 3H, CH3), 1.60−1.93 (m, 4H, CH2),
2.03−2.40 (m, 3H, CH2), 2.56−2.86 (m, 1H, CH2), 3.58 (s, 3H,
OCH3), 3.68 (s, 3H, OCH3), 3.70 (s, 3H, OCH3), 4.51 (s, br, 1H,
C5H3), 4.89−4.94 (m, 1H, C5H3), 4.98 (s, br, 1H, C5H3), 6.60 (d, 1H,
benzene ring proton), 7.66 (d, 1H, benzene ring proton). 13C NMR (250
MHz, CDCl3 and trace of CD3OD): δ (ppm) 23.1, 25.0, 34.4, 39.4,
49.2, 55.8, 59.9, 65.5, 81.4, 81.9, 82.4, 91.3, 92.4, 110.7, 116.3, 120.4,
124.1, 135.5, 161.5, 163.5, 170.1, 206.6, 227.2 (Mn−CO). IR bands
(ν): 3252 w, 2943 w, 2019 s (Mn−CO) 1921s (Mn−CO), 1699−
1677 s (br), 1595 m, 1434 m, 1354 w, 1272 s, 1187 m, 1146 s, 1041 w,
942 w, 789 w, 751 m, 665 m, 629 s cm−1. ESI-MS (positive detection
mode): m/z (%) 573.89 (100) [M + Na]+, 551.94 (30) [M + H]+.
Amide 17. The general procedure GP1 was followed: carboxylic
acid 14 (100 mg, 0.27 mmol), methyl 2-(trimethylsilyl)ethyl 3-amino-
2,4-dihydroxybenzoate (188 mg, 0.70 mmol), HATU (260 mg,
0.81 mmol), DIPEA (104 mg, 0.81 mmol), DMF (4 mL), reaction
time 20 h. Flash column chromatography (silica gel, hexane/EtOAc
2.5/1) gave 17 as a light yellow sticky solid (102 mg, 60%). Rf =
0.075). H NMR (250 MHz, DMSO-d6): δ (ppm) 1.12 (s, 3H, CH3),
1.71−1.86 (m, 3H, CH2), 1.98−2.10 (m, 1H, CH2), 2.14−2.28 (m, 1H,
CH2), 2.33−2.47 (m, 2H, CH2), 2.75−2.90 (m, 1H, CH2), 5.02−5.05
(m, 1H, C5H3), 5.28−5.32 (m, 1H, C5H3), 5.40−5.43 (m, 1H, C5H3),
6.39 (d, 1H, benzene ring proton), 7.56 (d, 1H, benzene ring proton), 9.07
(s, 1H, NH), 10.10 (s, br, 1H, OH). 13C NMR (250 MHz, DMSO-d6):
δ (ppm) 18.8, 21.1, 29.9, 30.1, 34.6, 45.1, 78.5, 79.3, 88.2, 88.9, 105.1,
107.4, 112.6, 113.1, 128.8, 158.7, 159.2, 171.5, 171.7, 201.3, 223.9
(Mn-CO). IR bands (ν): 2021 s and 1934 s (br) (Mn−CO), 1654s (br),
1593 m, 1535 m, 1460 w, 1402 w, 1280 m, 1257 s, 1194 w, 1054 m,
904 w, 786 m, 686 m, 613 s cm−1. ESI-MS (negative detection mode):
m/z (%) 508.02 (100) [M − H]−. Anal. Calcd for C23H20MnNO9: C,
54.24; H, 3.96; N, 2.75. Found: C, 53.82; H, 3.97; N, 2.62.
Compound 16 from Me-4. To a stirred solution of Me-4 (50 mg,
0.11 mmol) in a 1/4 (v/v) mixture of H2O and THF (10 mL) was
added solid LiOH·H2O (184 mg, 4.4 mmol). The mixture was stirred
at 45 °C for 14 h. Distilled water (50 mL) was added to the reaction
mixture, and this mixture was acidified with 1 M HCl up to pH ∼2 and
extracted with EtOAc. The organic phase was dried over anhydrous
Na2SO4, filtered, and concentrated. Flash column chromatography
(silica gel, EtOAc/MeOH/AcOH 20/1/0.075) afforded 16 (14 mg,
41%) as a light yellow solid. Single crystals of 16 were grown by slow
1
evaporation of its CDCl3 solution in a NMR tube. H NMR (250
MHz, CDCl3): δ (ppm) 1.14 (s, 3H, CH3), 1.53 (m, 1H, CH2),
1.77 (m, 1H, CH2), 1.99−2.35 (m, 4H, CH2), 2.47 (m, 1H, CH2), 2.85
(m, 1H, CH2), 4.63 (m, 1H, C5H3), 5.09 (m, 1H, C5H3), 5.17 (m, 1H,
C5H3), 5.31 (s, br, 1H, NH), 5.47 (s, br, 1H, NH). ESI-MS (positive
detection mode): m/z (%) 737.00 (80) [2M + Na]+, 396.11 (30)
[M + K]+, 380.21 (100) [M + Na]+.
1
0.34 (silica gel, hexane/EtOAc 3/1). H NMR (250 MHz, CDCl3):
δ (ppm) 0.09 (s, 9H, Si(CH3)3), 1.14 (m, 2H, CH2−CH2−Si(CH3)3),
1.24 (s, 3H, CH3), 1.49−2.21 (m, 4H, CH2), 2.45−2.62 (m, 3H, CH2),
2.86 (m, 1H, CH2), 4.43 (m, 2H, CH2−CH2−Si(CH3)3), 4.62 (m, 1H,
C5H3), 5.04 (m, 1H, C5H3), 5.19 (m, 1H, C5H3), 6.49 (d, 1H, benzene
ring proton), 7.56 (d, 1H, benzene ring proton), 7.89 (s, 1H, NH), 10.85
(s, 1H, OH), 11.84 (s, 1H, OH). 13C NMR (250 MHz, CDCl3):
δ (ppm) −1.2, 17.3, 19.2, 21.5, 30.4, 31.2, 36.2, 45.1, 63.9, 77.9, 78.5,
87.3, 88.7, 104.4, 110.9, 111.7, 114.1, 127.5, 153.9, 154.8, 170.4, 173.3,
200.1, 223.3 (Mn−CO). IR bands (ν): 2952 w, 2021 s (Mn−CO),
1924s (Mn−CO), 1654 m (br), 1595 w, 1534 m, 1452 w, 1387 m,
1250 s, 1145 m, 1040 w, 930 w, 857 s (br), 688 w cm−1. ESI-MS
(positive detection mode): m/z (%) 631.96 (100) [M + Na]+.
Compound 4 from 15. To a stirred solution of the amide 15
(180 mg, 0.33 mmol) in CHCl3 (4 mL) was added BBr3 (0.31 mL,
3.3 mmol) at −78 °C (just before the solution freezes) under an N2
atmosphere. The suspension was stirred for 20 h at room temperature.
The reaction mixture was then poured into distilled water (20 mL),
stirred for a further 30 min, and extracted with EtOAc (3 × 25 mL).
The organic phase was washed with brine (40 mL), dried over
anhydrous Na2SO4, filtered, and concentrated. Flash column
chromatography on silica gel (neat EtOAc → EtOAc/MeOH/AcOH
20/1/0.075) gave Me-4 (50 mg, light yellow sticky solid), followed by
4 (77 mg, light yellow solid).
Amide 18. The general procedure GP1 was followed: carboxylic
acid 9 (1 g, 2.29 mmol), methyl 3-amino-2,4-bis(methoxymethoxy)-
benzoate (1.34 g, 4.94 mmol), HATU (1.9 g, 4.94 mmol), DIPEA
(637 mg, 4.94 mmol), DMF (10 mL), reaction time 60 h. Flash
column chromatography (silica gel, EtOAc/hexanes 1/1 → 1/0) gave
18 as a light brown solid (yield 1.91 g, 71%). Single crystals of 18 were
grown by slow evaporation of its solution in a CH2Cl2/toluene
mixture. Rf = 0.61 (silica gel, EtOAc). 1H NMR (400 MHz, CD3CN):
δ (ppm) 2.79−2.87 (m, 2H, CH2), 3.06−3.14 (m, 2H, CH2), 3.44
(s, 3H, OCH3), 3.51 (s, 3H, OCH3), 3.84 (s, 3H, COOCH3), 4.99 (s, 2H,
OCH2), 5.00−5.06 (m, 2H, C5H4), 5.22 (s, 2H, OCH2), 5.59−5.64
(m, 2H, C5H4), 6.99 (d, 1H, benzene ring proton), 7.71 (d, 1H, benzene
ring proton), 7.82 (s, br, 1H, NH). 13C NMR (400 MHz, CD3CN):
δ (ppm) 29.5, 34.3, 52.1, 56.3, 57.2, 84.8, 87.6, 92.1, 94.8, 100.9, 110.6,
119.1, 122.1, 130.5, 155.1, 157.2, 165.9, 171.1, 196.9, 224.1 (Mn−
CO); IR bands (ν): 2019 s, 1950 s and 1925 s (Mn−CO), 1699 s,
1651 m, 1597 m, 1548 m, 1462 w, 1431 w, 1287 m, 1201 w, 1159 m,
1139 m, 1092 m, 1050 m, 1001 w, 925 m, 855 m, 672 s cm−1. ESI-MS
(positive detection mode): m/z (%) 580.03 (100) [M + Na]+. Anal.
Calcd for C24H24MnNO11: C, 51.72; H, 4.34; N, 2.51. Found: C,
51.79; H, 4.25; N, 2.45.
To a stirred solution of Me-4 (50 mg, 0.11 mmol) in a
deoxygenated 1/3 (v/v) mixture of THF and MeOH (12 mL) was
added 1 M NaOH solution (3.8 mL). The mixture was stirred at 60 °C
for 14 h under an N2 atmosphere. Distilled water (50 mL) was added
Compound 7. To a stirred solution of amide 18 (200 mg, 0.36 mmol)
in a deoxygenated 4/1 (v/v) mixture of THF and H2O (30 mL) was
added LiOH·H2O (809.7 mg, 19.75 mmol). The mixture was heated at
H
dx.doi.org/10.1021/om201146c | Organometallics XXXX, XXX, XXX−XXX

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Article
45 °C for 20 h (TLC shows complete ester hydrolysis). The reaction
mixture was then evaporated to dryness under vacuum. Degassed 4 N
HCl in dioxane was then added slowly to adjust the pH of the reaction
mixture close to zero. The mixture was then stirred at room
temperature. After 15 min the reaction was completed (checked by
TLC), and brine (50 mL) was added to the mixture, which was then
extracted with EtOAc (3 × 30 mL). The combined organic phases
were washed with distilled water (5 × 50 mL) and brine (2 × 25 mL),
dried over anhydrous Na2SO4, and filtered. Removal of the solvent
followed by flash column chromatography (silica gel, EtOAc/MeOH/
AcOH 1/:0/0 → 20/1/0.2) yielded compound 7 as a light brown solid
(115 mg, 70%). Rf = 0.75 (silica gel, EtOAc/MeOH/AcOH 20/1/0.5).
1H NMR (400 MHz, CD3CN): δ (ppm) 2.79−2.87 (m, 2H, CH2),
3.06−3.14 (m, 2H, CH2), 5.00−5.06 (m, 2H, C5H4), 5.59−5.64 (m,
2H, C5H4), 6.46 (d, 1H, benzene ring proton), 7.63 (d, 1H, benzene ring
proton), 7.47 (s, br, 1H, NH). 13C NMR (250 MHz, pyridine-d5): δ
(ppm) 32.2, 37.5, 87.1, 90.2, 94.8, 109.7, 112.2, 117.4, 131.9, 160.5,
160.7, 175.7, 177.2, 198.9, 226.5 (Mn−CO). IR bands (ν): 2948 w,
2024 s, 1924 s (br) (Mn−CO), 1670 m, 1635 m, 1596 m, 1540 m,
1461 w, 1378w, 1291 s, 1155 w, 1084 m, 915 w, 728 w cm−1. ESI-MS
(negative detection mode): m/z (%) 453.95 (100) [M − H]−. Anal.
Calcd for C19H14MnNO9: C, 50.13; H, 3.10; N, 3.08. Found: C, 49.82;
H, 3.45; N, 2.85.
(m, 1H, benzene ring proton), 5.60 (m, 1H, benzene ring proton), 6.15
(d, 1H, benzene ring proton). 13C NMR (250 MHz, CDCl3): δ (ppm)
22.1, 24.3, 28.1, 29.8, 29.9, 33.6, 43.9, 80.6, 89.6, 90.1, 91.8, 92, 94.5,
113.8, 172.5, 199.7, 230.9 (Cr-CO); IR bands (ν): 2976 w, 2933 w,
1964 s (Cr−CO), 1877s (Cr−CO), 1721 s, 1678 m, 1451 w, 1366 w,
1307 m, 1254 w, 1222 m, 1148 s, 894 w, 654 s, 616 s cm−1. ESI-MS
(positive detection mode): m/z (%) 462.86 (30) [M + K]+, 446.91 (50)
[M + Na]+, 424.93 (20) [M + H]+, 368.91 (100) [M − 2CO + H]+.
Compound 22. To a stirred solution of 21 (150 mg, 0.35 mmol) in
a 3/1 (v/v) mixture of MeOH and THF (12 mL) was added 1 M
aqueous NaOH (4.2 mL). The mixture was deoxygenated by bubbling
N2 for 10 min and stirred for 14 h at room temperature. The reaction
mixture was then diluted with distilled water (50 mL), and the
aqueous layer was washed with Et2O (2 × 30 mL). The aqueous layer
was acidified with 1 M HCl up to pH ∼2 and extracted with EtOAc
(2 × 30 mL). The combined organic phase was washed with H2O and
brine, dried over anhydrous Na2SO4, filtered, and concentrated. The
carboxylic acid 22 was obtained as an orange sticky solid (112 mg,
87%) which was used for the next step without further purification.
For analytical purposes, 22 was recrystallized by slow evaporation
1
of its Et2O/EtOAc solution. Rf = 0.4 (silica gel, EtOAc). H NMR
(250 MHz, CDCl3): δ (ppm) 1.26 (s, 3H, CH3), 1.70−2.17 (m, 4H,
CH2), 2.29−2.49 (m, 2H, CH2), 2.58−2.74 (m, 1H, CH2), 3.02−3.20
(m, 1H, CH2), 5.16 (d, 1H, benzene ring proton), 5.32 (m, 1H, benzene
ring proton), 5.60 (m, 1H, benzene ring proton), 6.15 (d, 1H, benzene
ring proton). 13C NMR (250 MHz, CDCl3): δ (ppm) 22.1, 24.3, 28.6,
29.8, 33.6, 43.9, 89.6, 90.1, 91.5, 92.1, 94.5, 113.8, 178.9, 199.7, 230.9
(Cr-CO). IR bands(ν): 3085 w, 2931 w, 1962 s, 1871 s, 1704 s, 1676 s,
1522 w, 1431 m (br), 1285 w, 1222 m, 1044 w, 1008 w, 832 w, 654 s,
614 s cm−1. ESI-MS (negative detection mode): m/z (%) 735.14
(100) [2M − H]−. Anal. Calcd for C17H16CrO6·0.5Et2O: C, 56.29; H,
5.22. Found: C, 56.27; H, 4.74.
Compound 23. The general procedure GP1 was followed:
carboxylic acid 22 (200 mg, 0.54 mmol), 2-(trimethylsilyl)ethyl
3-amino-2,4-dihydroxybenzoate (434 mg, 1.63 mmol), HATU (523 mg,
1.63 mmol), DIPEA (211 mg, 1.63 mmol), DMF (6 mL, deoxy-
genated previously by bubbling N2), reaction time 20 h. On flash
column chromatography (silica gel, hexane/EtOAc 3/1), 23 was
obtained as an orange oil which solidified slowly when coevaporated
with pentane (161 mg, 48%). Rf = 0.2 (silica gel, EtOAc/hexanes 1/3).
1H NMR (250 MHz, CDCl3): δ (ppm) 0.10 (s, 9H, Si(CH3)3), 1.15
(m, 2H, CH2−CH2-Si(CH3)3), 1.29 (s, 3H, CH3), 1.82−2.04 (m, 2H,
CH2), 2.06−2.25 (m, 2H, CH2), 2.44−2.74 (m, 3H, CH2), 3.02−3.22
(m, 1H, CH2), 4.45 (m, 2H, CH2−CH2−Si(CH3)3), 5.16 (d, 1H,
benzene ring proton), 5.32 (m, 1H, benzene ring proton), 5.60 (m, 1H,
benzene ring proton), 6.15 (d, 1H, benzene ring proton), 6.51 (d, 1H,
benzene ring proton), 7.58 (d, 1H, benzene ring proton), 7.97 (s, 1H,
NH), 10.79 (s, br, 1H, OH), 11.87 (s, 1H, OH). 13C NMR (250 MHz,
CDCl3): δ (ppm) −1.46, 17.4, 22.4, 24.3, 30.6, 31.2, 34.1, 44.1, 63.9,
89.6, 90.1, 91.5, 92.1, 94.5, 104.5, 111.1, 113.8, 114.1, 127.6, 153.8,
154.6, 170.4, 173.1, 199.9, 230.8 (Cr−CO). IR bands (ν): 2954 w,
1968 s (Cr−CO), 1885 (Cr−CO), 1654 s (br), 1595 w, 1524 m,
1452 w, 1386 w, 1288 w, 1217 s, 1145 m, 1061 w, 931 w, 857 s, 787 w,
752 s, 654 s cm−1. ESI-MS (positive detection mode): m/z (%) 641.96
(20) [M + Na]+, 310.03 (100) [M + H]2+.
Compound 8. To a solution of 7 (350 mg, 0.875 mmol) in benzene
(10 mL) and AcOH (3 mL) was added 10 g of freshly prepared
Zn(Hg), followed by the addition of distilled H2O (5 mL) and of
concentrated HCl (7 mL). The mixture was deoxygenated by bubbling
N2 for 10 min and heated at 90 °C for 12 h. The reaction mixture was
then filtered, diluted with distilled water (100 mL), and extracted with
EtOAc (3 × 50 mL). The organic phase was washed several times with
water and finally with brine, dried over anhydrous Na2SO4, and
filtered. Removal of the solvent followed by flash column
chromatography (silica gel, EtOAc/MeOH/AcOH 20/1/0.1) yielded
compound 8 as a light brown solid (216 mg, 64%). Rf = 0.44 (silica gel,
1
EtOAc/MeOH/AcOH 20/1/0.1). H NMR (250 MHz, DMSO-d6):
δ (ppm) 1.62−1.90 (m, 2H, CH2), 2.15−2.43 (m, 4H, 2 × CH2),
4.91 (s, br, 2H, C5H4), 4.99 (s, br, 2H, C5H4), 6.46 (s, br, 1H, benzene
ring proton), 7.59 (s, br, 1H, benzene ring proton), 9.05 (s, br, 1H, NH).
13C NMR (250 MHz, DMSO-d6): δ (ppm) 26.3 (min) and 27.3 (maj)
(rotamers, 2 × CH2), 33.6 (min) and 35.1 (maj) (rotamers, CH2),
83.1 (C5H4), 83.6 (C5H4), 83.7 (C5H4), 104.5 (min) and 107.6 (maj)
(rotamers, benzene ring carbon), 105.1 (min) and 108.1 (maj)
(rotamers, benzene ring carbon), 113.2 (benzene ring carbon), 129.5
(benzene ring carbon), 159.6 (2 × benzene ring carbon), 172.2 (COOH),
174.9 (CONH), 226.1 (Mn−CO); IR bands (ν): 2019 s and 1921 s
(Mn−CO), 1634 s (br, COOH, CONH) cm−1. ESI-MS (negative
detection mode): m/z (%) 439.80 (100) [M − H]−.
Compound 20. 2-Methyl-1-tetralone (19; 320 mg, 2 mmol) and
Cr(CO)6 (572 mg, 2.6 mmol) in a degassed 9/1 (v/v) mixture of
Bu2O/THF (30 mL) was heated for 36 h at 140 °C under an N2
atmosphere in the dark. The mixture was then cooled to room
temperature, filtered, and concentrated. Flash column chromatography
(silica gel, hexane/EtOAc 4/1 → 3/1) yielded a diasteriomeric mixture
of 20 (360 mg, 61%) as an orange-yellow solid. The characterization
data matched well with those reported earlier.38
Compound 5. To a stirred solution of amide 23 (300 mg, 0.48 mmol)
in DMF (5 mL) was added tris(dimethylamino)sulfonium difluoro-
trimethylsilicate (TASF; 267 mg, 0.97 mmol) at room temperature, and
the mixture was heated at 40 °C for 40 min. The reaction mixture was
then cooled to room temperature, brine was added, and this mixture was
extracted with EtOAc (3 × 50 mL). The combined organic phase was
dried over Na2SO4, filtered, and concentrated. Flash column chromatog-
raphy (silica gel, EtOAc/MeOH/AcOH 20/1/0.05) of the resulting
residue gave compound 5 (215 mg, 85%) as an orange powder. Rf =
0.43 (silica gel, EtOAc/MeOH/AcOH 20/1/0.05). 1H NMR (250
MHz, DMSO-d6): δ (ppm) 1.16 (s, 3H, CH3), 1.71−1.92 (m, 3H,
CH2), 1.96−2.11 (m, 1H, CH2), 2.19−2.65 (m, 3H, CH2), 3.02−3.24
(m, 1H, CH2), 5.72 (m, 2H, benzene ring proton), 6.08 (m, 1H, benzene
ring proton), 6.23 (m, 1H, benzene ring proton), 6.39 (d, 1H, benzene
ring proton), 7.54 (d, 1H, benzene ring proton), 9.08 (s, 1H, NH), 10.10
tert-Butyl Ester 21. To a stirred solution of 20 (150 mg,
0.52 mmol) in a 1/1 (v/v) mixture of BuOH and Et2O (6 mL) was
t
added tBuOK (116 mg, 1.03 mmol) at 0 °C under an N2 atmosphere.
After 20 min, tert-butyl acrylate (666 mg, 5.2 mmol) was added to the
reaction mixture, and this mixture was stirred for 2.5 h at 0 °C. The
reaction was quenched with saturated NH4Cl, and the aqueous layer
was extracted with EtOAc (2 × 20 mL). The organic layer was washed
with H2O and brine, dried over anhydrous Na2SO4, and filtered. The
solvent and tert-butyl acrylate were removed using a high-vacuum
pump. Flash column chromatography (silica gel, hexane/EtOAc 4/1)
gave 21 as an orange oil (170 mg, 79%). Rf = 0.2 (silica gel, hexane/
1
EtOAc 5/1). H NMR (250 MHz, CDCl3): δ (ppm) 1.22 (s, 3H,
CH3), 1.43 (s, 9H, C(CH3)3), 1.66−1.79 (m, 1H, CH2), 1.85−2.09
(m, 3H, CH2), 2.14−2.31(m, 2H, CH2), 2.53−2.69 (m, 1H, CH2),
3.04−3.20 (m, 1H, CH2), 5.15 (d, 1H, benzene ring proton), 5.32
I
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(s, br, 1H, OH). 13C NMR (250 MHz, DMSO-d6): δ (ppm) 22.4,
24.2, 30.3, 30.6, 33.1, 44.4, 92.6, 93.1, 93.2, 93.6, 97.9, 105.7, 107.8,
113.2, 116.9, 129.3, 158.9, 159.6, 172.2, 172.6, 200.5, 232.5 (Cr−CO).
IR bands (ν): 3376 w, 2932 w, 1962 s (Cr−CO), 1886 s (Cr−CO),
1666 m, 1633 m (br), 1628 m, 1436 w, 1378 w, 1274 m, 1188 w,
1160 w, 1004 w, 1059 w, 822 w, 655 s, 617 s cm−1. ESI-MS negative
detection mode): m/z (%) 517.88 (100) [M − H]−. Anal. Calcd for
C24H21CrNO9: C, 55.50; H, 4.08; N, 2.70. Found: C, 55.23; H, 3.95;
N, 2.57.
Compound 25. To a solution of [3]-ferrocenophanone (24;
225 mg, 0.94 mmol) in 12 mL of THF was added 0.5 M KHMDS in
toluene (3 mL, 1.5 mmol) slowly at −78 °C. The mixture was stirred
for 30 min, and HMPA (2 mL) and MeI (1.3 g, 9.4 mmol) were added
sequentially. After 3 h at −78 °C, the reaction mixture was quenched
with aqueous saturated NaHCO3 solution and extracted with Et2O
(3 × 30 mL). The organic phase was washed with H2O and brine,
dried over anhydrous Na2SO4, filtered and concentrated. Flash column
chromatography (silica gel, hexane/EtOAc 5/1) gave 25 as an orange
solid (first orange band, 113 mg, 67% based on the starting material
consumed) together with 90 mg of the unreacted 24 (2nd orange
band). Rf = 0.44 (silica gel, hexane/EtOAc 5/1). 1H NMR (200 MHz,
CDCl3): δ (ppm) 1.11 (d, 3H, CH3), 2.64−2.95 (m, 2H, CH2), 3.72−
3.90 (m, 1H, CH), 3.94−3.98 (m, 1H, C5H4), 4.10−4.15 (m, 1H,
C5H4), 4.32−4.37 (m, 1H, C5H4), 4.39−4.37 (m, 2H, C5H4), 4.63−
4.68 (m, 1H, C5H4), 4.72−4.76 (m, 1H, C5H4), 5.06−5.10 (m, 1H,
C5H4). 13C NMR (200 MHz, CDCl3): δ (ppm) 15.4, 40.1, 46.1, 66.3,
68.1, 69.8, 70.3, 72.1, 72.3, 72.7, 73.5, 74.9, 85.9, 213.9. IR bands (ν):
3082 w, 2973 w, 2931 w, 2845 w, 1733 w, 1675 s, 1438 m, 1366 w,
1181 m, 1048 w, 1036 s, 973 w, 901 w, 847 m, 827 s, 754 w,
641 w cm−1. ESI-MS (positive detection mode): m/z (%) 253.88
(100) [M]+. Anal. Calcd for C14H14FeO: C, 66.17; H, 5.55. Found: C,
66.05; H, 5.78.
1.76−1.90 (m, 1H, CH2), 2.02−2.18 (m, 1H, CH2), 2.25−2.41
(m, 1H, CH2), 2.66−2.86 (m, 3H, CH2), 4.08 (s, br, 1H, C5H4), 4.13
(s, br, 1H, C5H4), 4.36 (s, br, 2H, C5H4), 4.54 (s, br, 2H, C5H4), 4.64
(s, br, 1H, C5H4), 4.78 (s, br, 1H, C5H4), 12.32 (s, br, 1H, COOH).
13C NMR (250 MHz, CDCl3): δ (ppm) 21.9, 30.3, 33.1, 69.7, 70.4,
70.5, 71.5, 71.8, 72.1, 72.4, 72.7, 76.4, 83.2, 213.3 (signal for COOH is
not visible). IR bands (ν): 2920 w, 1699 s, 1649 s, 1438 w, 1409 w,
1312 m, 1245 w, 1217 s, 1056 w, 1038 w, 955 m, 854 m, 766 w, 676 w,
638 w cm−1. ESI-MS (positive detection mode): m/z (%) 326.04
(100) [M]+. Anal. Calcd for C17H18FeO3·0.5H2O: C, 60.89; H, 5.71.
Found: C, 60.97; H, 5.76.
Compound 28. The general procedure GP1 was followed:
carboxylic acid 27 (600 mg, 1.8 mmol), methyl 3-amino-2,4-
dimethoxybenzoate (777 mg, 3.7 mmol), HATU (1.2 g, 3.7 mmol),
DIPEA (477 mg, 3.7 mmol), DMF (10 mL), reaction time 24 h. Flash
column chromatography (silica gel, hexane/EtOAc 1/1 → 0/1) gave
28 as an orange sticky solid (550 mg, 57%). Rf = 0.11 (silica gel,
1
hexane/EtOAc 1/1). H NMR (250 MHz, CDCl3): δ (ppm) 1.46
(s, 3H, CH3), 1.91−2.05 (m, 1H, CH2), 2.19−2.35 (m, 1H, CH2),
2.40−2.59 (m, 1H, CH2), 2.81 (s, 2H, CH2), 3.11−3.25 (m, 1H, CH2),
3.86 (s, 3H, OCH3), 3.89 (s, 3H, OCH3), 3.90 (s, 3H, OCH3), 4.05
(m, 1H, C5H4), 4.09 (m, 1H, C5H4), 4.32 (m, 2H, C5H4), 4.43
(m, 1H, C5H4), 4.56 (m, 1H, C5H4), 4.67 (m, 1H, C5H4), 4.96
(m, 1H, C5H4), 6.75 (d, 1H, benzene ring proton), 7.02 (s, br, 1H, NH),
6.85 (d, 1H, benzene ring proton). 13C NMR (250 MHz, CDCl3): δ
(ppm) 22.1, 33.1, 33.2, 44.5, 52.1, 55.1, 56.2, 62.1, 69.3, 70.1, 71.7,
71.8, 72.1, 72.3, 72.4, 75.8, 77.2, 82.9, 106.7, 117.1, 120.2, 131.5, 157.5,
159.1, 165.6, 172.4, 214.1. IR bands (ν): 2963 w, 2360 w, 1720 w,
1655 s (br), 1594 m, 1496 w, 1414 w, 1261 s, 1215 w, 1096 s, 1056 m,
1016 s, 795 w, 726 s cm−1. ESI-MS (positive detection mode): m/z
(%) 519.01 (80) [M]+, 541.94 (100) [M + Na]+.
Compound 29. The general procedure GP1 was followed:
carboxylic acid 27 (150 mg, 0.46 mmol), methyl 2-(trimethylsilyl)ethyl
3-amino-2,4-dihydroxybenzoate (309 mg, 1.1 mmol), HATU (443 mg,
1.38 mmol), DIPEA (178 mg, 1.38 mmol), DMF (5 mL), reaction
time 20 h. Flash column chromatography (silica gel, hexane/EtOAc
2.5/1) gave 29 as an orange sticky solid (170 mg, 64%). Single crystals
for X-ray diffraction were grown from a CH2Cl2/pentane mixture at
Compound 26. To a stirred solution of 25 (130 mg, 0.51 mmol) in
t
t
a 1/1 (v/v) mixture of BuOH and Et2O (6 mL) was added BuOK
(85 mg, 0.75 mmol) slowly at room temperature under an N2
atmosphere. After 15 min, tert-butyl acrylate (326 mg, 2.5 mmol)
was added. After 6 h at room temperature TLC (silica gel, hexane/
EtOAc 4/1) showed no more starting ketone present. The reaction
mixture was quenched by adding 20 mL of 0.5 M HCl and extracted
with 40 mL of EtOAc. The organic layer was washed with brine, dried
over anhydrous Na2SO4, filtered, and dried under vacuum (to remove
the excess tert-butyl acrylate). Flash column chromatography was
performed on silica gel. The first orange band was eluted using an
EtOAc/hexanes mixture (1/6, v/v) and was isolated as 26 (127 mg,
65%). The second orange band was eluted using an EtOAc/MeOH/
AcOH mixture (20/1/0.05, v/v) and was isolated as the carboxylic
acid 27 (trace amount, the longer reaction time increases the amount
of carboxylic acid 27 formed). Rf = 0.35 (silica gel, hexane/EtOAc
5/1). 1H NMR (200 MHz, CDCl3): δ (ppm) 1.40 (s, 3H, CH3), 1.50
(s, 9H, C(CH3)3), 1.76−1.93 (m, 1H, CH2), 2.11−2.44 (m, 2H, CH2),
2.79 (s, 2H, CH2), 3.06−3.15 (m, 1H, CH2), 4.05 (s, br, 1H, C5H4),
4.11 (s, br, 1H, C5H4), 4.33 (s, br, 2H, C5H4), 4.42 (s, br, 1H, C5H4),
4.54 (s, br, 1H, C5H4), 4.64 (s, br, 1H, C5H4), 4.91 (s, br, 1H, C5H4).
13C NMR (200 MHz, CDCl3): δ (ppm) 22.2, 28.1, 31.5, 32.4, 44.3,
54.8, 69.2, 69.9, 70.3, 71.7, 72.0, 72.1, 72.2, 72.4, 75.8, 80.6, 82.9,
173.1, 213.7. IR bands (ν): 2971 w, 2927, 1721 s, 1652 s, 1456 w,
1377 m, 1336 w, 1303 m, 1244 w, 1146 s, 1057 m, 1032 m, 935 w,
854 m, 836 m, 759 m cm−1. ESI-MS (positive detection mode): m/z
(%) 326.92 (100) [M − 2CO + H]+, 382.93 (50) [M + H]+. Anal.
Calcd for C21H26FeO3: C, 65.98; H, 6.86. Found: C, 66.27; H, 6.94.
Compound 27. To a stirred solution of 26 (80 mg, 0.2 mmol) in
CH2Cl2 (6 mL) was added Me3SiI (0.054 mL, 0.4 mmol) dropwise at
0 °C under an N2 atmosphere. The mixture was stirred at room
temperature for 2 h. The reaction mixture was then poured into 30 mL
of distilled water and was extracted with EtOAc (2 × 30 mL). The
combined organic phase was washed with H2O and brine, dried over
anhydrous Na2SO4, and filtered, and the solvent was removed. Flash
column chromatography (silica gel, EtOAc/MeOH/AcOH 20/1/
0.05) afforded 27 as a yellow solid (56 mg, 82%). Rf = 0.41 (silica gel,
1
0 °C. Rf = 0.48 (silica gel, hexane/EtOAc 2/1). H NMR (250 MHz,
CDCl3): δ (ppm) 0.09 (s, 9H, Si(CH3)3), 1.15 (m, 2H, CH2−CH2−
Si(CH3)3), 1.59 (s, 3H, CH3), 1.84−2.01 (m, 1H, CH2), 2.42−2.66
(m, 2H, CH2), 2.80−3.02 (m, 3H, CH2 and CH2 of ferrocenopha-
none), 4.09 (m, 2H, C5H4), 4.35 (m, 2H, C5H4), 4.40−4.44 (m, 2H,
C5H4), 4.49 (m, 2H, CH2-CH2− Si(CH3)3), 4.76 (m, 1H, C5H4), 4.84
(m, 1H, C5H4), 6.53 (d, 1H, benzene ring proton), 7.85 (d, 1H, benzene
ring proton), 8.09 (s, br, 1H, NH), 11.10 (s, 1H, OH), 11.88 (s, 1H,
OH). 13C NMR (250 MHz, CDCl3): δ (ppm) −1.4, 17.3, 21.6, 32.9,
34.1, 44.6, 54.6, 63.8, 69.8, 69.9, 70.7, 71.4, 72.1, 72.2, 72.3, 72.4, 75.5,
82.5, 104.6, 111.1, 114.3, 127.4, 153.8, 154.4, 170.5, 173.3, 213.3. IR
bands (ν): 2950 w, 1653 s (br), 1595 w, 1533 m, 1385 s, 1330 s,
1292 w, 1256 s, 1218 m, 1146 s, 1057 s, 1032 w, 857 s, 836 s, 787 s,
636 w cm−1. ESI-MS (positive detection mode): m/z (%) 577.07 (70)
[M]+, 600.03 (100) [M + Na]+. Anal. Calcd for C29H35FeNO6Si: C,
60.31; H, 6.11; N, 2.43. Found: C, 60.45; H, 6.25; N, 2.16.
Compound 6 from Amide 28. To a stirred solution of the amide
28 (240 mg, 0.46 mmol) in CHCl3 (8 mL) was added BBr3 (0.42 mL,
4.6 mmol) at −78 °C (just before the solution freezes) under an
N2 atmosphere. The gray mixture was stirred for 18 h at room tem-
perature. The reaction mixture was poured into distilled water
(20 mL) and extracted with EtOAc (3 × 25 mL) and washed with
brine (30 mL). The organic phase was dried over anhydrous Na2SO4,
filtered, and concentrated. Flash column chromatography on silica
gel (pure EtOAc → EtOAc/MeOH/AcOH 20/1/0.07) gave Me-6
(90 mg, yellow sticky solid), followed by 6 (70 mg, yellow powder).
To a stirred solution of Me-6 (90 mg, 0.18 mmol) in a 1/3 (v/v)
mixture of THF and MeOH (8 mL) was added a 1 M NaOH solution
(5.4 mL), and this mixture was deoxygenated by bubbling N2 for
15 min. The mixture was heated at 60 °C for 16 h under an N2
atmosphere, diluted with distilled water (50 mL), acidified with 1 M
HCl up to pH ∼2, and extracted with EtOAc. The organic phase was
1
EtOAc). H NMR (250 MHz, CDCl3): δ (ppm) 1.35 (s, 3H, CH3),
J
dx.doi.org/10.1021/om201146c | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
dried over anhydrous Na2SO4, filtered, and concentrated. Flash
column chromatography (silica gel, EtOAc/MeOH/AcOH 20/1/
0.07) afforded 6 (25 mg). Combined yield of 6 : 95 mg (43%).
Compound 6 from Amide 29. To a stirred solution of amide 29
(100 mg, 0.17 mmol) in DMF (5 mL) was added tris(dimethyl-
amino)sulfonium difluorotrimethylsilicate (TASF; 117 mg, 0.42 mmol)
at room temperature, and the mixture was heated at 40 °C for 1 h.
The reaction mixture was then cooled to room temperature, brine
was added, and this mixture was extracted with EtOAc (3 ×
50 mL). The combined organic phase was dried over Na2SO4, filtered,
and concentrated. Flash column chromatography (silica gel, EtOAc/
MeOH/AcOH 20/1/0.07) of the resulting residue gave compound 6
(64 mg, 78%) as a yellow powder. Rf = 0.33 (silica gel, EtOAc/
MeOH/AcOH 20/1/0.07). 1H NMR (250 MHz, DMSO-d6):
δ (ppm) 1.32 (s, 3H, CH3), 1.93−2.03 (m, 1H, CH2), 2.11−2.28
(m, 1H, CH2), 2.46−2.62 (m, 1H, CH2), 2.76 (s, 2H, CH2), 2.97−3.15
(m, 1H, CH2), 4.07 (m, 1H, C5H4), 4.16 (m, 1H, C5H4), 4.37 (m, 2H,
C5H4), 4.52 (m, 1H, C5H4), 4.59 (m, 2H, C5H4), 5.06 (m, 1H, C5H4),
6.45 (d, 1H, benzene ring proton), 7.59 (d, 1H, benzene ring proton),
9.10 (s, 1H, NH), 10.22 (s, br, 1H, OH). 13C NMR (250 MHz,
DMSO-d6): δ (ppm) 22.6, 31.5, 32.6, 43.5, 55.2, 69.3, 69.4, 70.7, 71.6,
72.1, 72.4, 72.7, 72.8, 76.6, 83.3, 105.1, 108.2, 113.4, 129.3, 159.5,
159.6, 172.3, 172.7, 213.4. IR bands (ν): 3373 m, 2924 w, 2585 w,
1654 s (br), 1620 m, 1597 s, 1529 s, 1385 m, 1276 m, 1234 m,
1281 m, 1160 m, 1060 s, 1037 w, 950 w, 786 s, 624 w cm−1. ESI-MS
(positive detection mode): m/z (%) 477.02 (100) [M]+, 499.97 (50)
[M + Na]+. Anal. Calcd for C24H23FeNO6·0.5H2O: C, 59.25;, H, 4.97;
N, 2.88. Found: C, 59.09; H, 5.00; N, 2.69.
Compound 31. The general procedure GP1 was followed:
carboxylic acid 30 (95 mg, 0.29 mmol), methyl 2-(trimethylsilyl)ethyl
3-amino-2,4-dihydroxybenzoate (150 mg, 0.56 mmol), HATU
(180 mg, 0.56 mmol), DIPEA (72 mg, 0.56 mmol), DMF (6 mL),
reaction time 20 h. Flash column chromatography (silica gel, hexane/
EtOAc 2.5/1) gave 31 as a red sticky solid (78 mg, 47%). Rf = 0.35 (silica
gel, hexane/EtOAc 2.5/1). 1H NMR (250 MHz, CDCl3): δ (ppm) 0.09
(s, 9H, Si(CH3)3), 1.16 (m, 2H, CH2−CH2−Si(CH3)3), 1.39 (s, 3H,
CH3), 1.71−2.18 (m, 3H, CH2), 2.21−2.78 (m, 5H, CH2), 4.21 (s, 5H,
C5H5), 4.48 (m, 4H, 2H from C5H4 and CH2−CH2−Si(CH3)3), 4.83
(m, 1H, C5H4), 6.43 (d, 1H, benzene ring proton), 7.75 (d, 1H, benzene
ring proton), 7.99 (s, br, 1H, NH), 10.90 (s, 1H, OH), 11.88 (s, 1H,
OH). 13C NMR (250 MHz, CDCl3): δ (ppm) −0.1, 18.3, 21.9, 23.8,
31.8, 33.2, 38.9, 46.8, 65.2, 67.7, 71.5, 71.6, 72.7, 75.6, 92.3, 105.8,
112.5, 115.6, 128.4, 155.3, 156.1, 171.9, 175.1, 209.8. IR bands (ν):
2953 w, 1653 s (br), 1595 w, 1529 m, 1438 w, 1385 s, 1250 s, 1216 m,
1145 s, 1061 m, 1001 m, 931 w, 834 s, 787 s, 693 m cm−1. ESI-MS
(positive detection mode): m/z (%) 591.04 (100) [M]+, 613.95 (20)
[M + Na]+.
ACKNOWLEDGMENTS
■
Help from Mrs. Annegret Knufer in carrying out the IC
̈
50
experiments is gratefully acknowledged. We wish to acknowledge
the International Max Planck Research School for Chemical
Biology (fellowship to M.P.) and the Deutsche Forschungsge-
meinschaft (DFG) (FOR 630) for financial support of this project.
Support from the Research Department Interfacial System
Chemistry (J.E.B. and N.M.-N.) and funding for both groups
from the State of North Rhine-Westphalia (NRW), Germany, and
the EUROPEAN UNION, European Regional Development
Fund, “Investing in your future”, are also gratefully acknowledged.
G.G. thanks the Swiss National Science Foundation (SNSF
professorship, Grant No. PP00P2_133568) and the Alexander von
Humboldt Foundation for funding.
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Corresponding Author
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Notes
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K
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