L. Biancalana et al. / Journal of Organometallic Chemistry 848 (2017) 214e221
217
have been investigated for their possible biological applications
31]. In particular, Cu(II) [32], Ru(II) [33], Zn(II) and Sn(II) [34]
4. Experimental
[
compounds have been demonstrated to exhibit antifungal, anti-
microbial or antibacterial activity, the activity being sometimes
enhanced compared to that of dhaH itself. It should be noticed also
that structurally-related pyrones have been conjugated to Pt(II) or
Ru(II) arene scaffolds with the aim of obtaining a synergic effect in
terms of anticancer activity [35].
4.1. General experimental details
3 2
$xH
O (99.9%) was purchased from Alfa Aesar, then [(h6-p-
RuCl
2 2
cymene)RuCl ] [40] and [(h -p-cymene)RuCl(k N,O-L-serinate)], 1
6
2
[6], were prepared according to the literature. The organic reactants
and solvents were obtained from Alfa Aesar, Sigma Aldrich or TCI
Europe, and were of the highest purity available. Acetyl chloride
In order to assess the suitability of 2 to cytotoxicity studies, we
investigated the stability of this compound in aqueous medium at
3
and Et N (over 4 Å MS) were stored under nitrogen as received.
ꢁ
3
7 C along 72 h. Due to insolubility in water, the stability of 2 was
1.0 M NaOH solution in water was prepared from Normex solution
(Carlo Erba) and standardized by potassium hydrogen phthalate
titration before use. The synthesis of dehydroacetic acid (dhaH) and
1
2
evaluated by H NMR in a DMSO:D O 9:1 v/v solution, being DMSO
a solvent of choice in drug research [36]. NaCl 0.11 M was added to
the solution, thus matching the chloride concentration normally
employed for in vitro tests. Under these conditions, rapid release of
3
the reaction of 1 with MeCOCl/Et N were performed under a ni-
trogen atmosphere using standard Schlenk techniques and solvents
distilled from appropriate drying agents. All the other operations
were carried out in air with common laboratory glassware. NMR
spectra were recorded at 298 K on a Bruker Avance II DRX400 in-
strument equipped with a BBFO broadband probe. Chemical shifts
(expressed in parts per million) are referenced to the residual sol-
ꢀ
dha from 2 took place, with only minor amounts (<10%) of the
starting material still in solution from 7 h onwards (see Scheme 4
and Table 4). On the other hand, the analogous acetylacetonate
compound 3 resulted significantly more inert toward ligand
dissociation, as ca. 70% of Ru-acac persisted in solution after 72 h
1
13
(
see Scheme 5 and Table 5).
vent peaks ( H, C) [41]. Spectra were assigned with the assistance
1
1
1
13
Earlier studies on the behavior of ruthenium arene complexes in
of DEPT-135, H- H (COSY) and H- C (gs-HSQC and gs-HMBC)
correlation experiments [42]. Infrared spectra were recorded on a
Perkin Elmer Spectrum One FT-IR spectrometer, equipped with a
UATR sampling accessory. Carbon, hydrogen and nitrogen analyses
were performed on a Carlo Erba mod. 1106 instrument.
aqueous solution evidenced that N,N- and N,O-bidentate ligands
usually remained bound to the metal centre [1f,37]. In contrast with
this general trend, a significant release over time has been reported
for a variety of O,O-ligands, including fluoro-substituted diketo-
nates [38], quinolones [19a], maltolate [39] and 3-hydroxy-4-
pyr(id)ones [35]. For instance, ca. 40% release of quinolone li-
gands was detected after 24 h in aqueous solution. It is noteworthy
that a high degree of dissociation may correlate with a low cyto-
toxicity against a panel of cancer cell lines [35]. On account of these
considerations, we decided not to proceed with biological assays on
compound 2.
4.2. Synthesis and characterization of compounds
4.2.1. Reaction of [(
CH C(O)Cl/NEt
In a 25-mL Schlenk tube, Et
mixture of 1 (58 mg, 0.16 mmol) and acetyl chloride (55
.77 mmol) in CHCl (8 mL). The resulting yellow solution was
stirred at reflux temperature for 4 h then at room temperature
overnight. The mixture was then extracted with H
h k L-serinate)], 1, with
6-p-cymene)RuCl( 2N,O-
3
3
3
N (1.1 mL, 7.9 mmol) was added to a
L,
m
0
3
2
O (3 ꢂ 15 mL)
and volatiles were removed under vacuum from the organic phase,
3
. Conclusions
affording a yellow-orange solid. X-ray quality crystals of 2 were
collected from a CHCl
3
solution of this solid layered with hexane
In the course of our studies on the structural modification of
ꢁ
and settled aside at ꢀ20 C. Anal. Calcd. for C18
4
H21ClO Ru: C, 49.37;
Ru(II) arene complexes for medicinal purposes, we have seren-
dipitously found that acetyl chloride can be converted into dehy-
droacetic acid in one pot, in the presence of triethylamine. This
reaction, albeit occurring in low yield, represents an unusual case of
acyl chloride self condensation. A Ru(II) arene - dehydroacetate
conjugate has been prepared and structurally characterized. This
compound manifests fast and extensive release of the bidentate
O,O-ligand in aqueous medium, the degree of dissociation being
superior to that exhibited by acetylacetonate in an analogous sys-
tem. This observation resembles previous reports on the relative
lability, in aqueous environment, of various bidentate O,O-donors
coordinated to Ru(II) arene complexes.
H, 4.83. Found: C, 4; 49.84, 4.70.
4
.2.2. Dehydroacetic acid (dhaH)
In a 25-mL Schlenk tube, acetyl chloride (0.10 mL,1.4 mmol) was
added dropwise to a solution of Et N (0.40 mL, 2.9 mmol) in CHCl
3
3
)
ꢀ
1
(
9 mL) and the resulting colorless solution (cMeCOCl ¼ 0.15 mol L
was stirred at room temperature for 15 h. A pale yellow solution
was obtained, thus volatiles were removed under vacuum and the
residue was re-dissolved in CH
2 2
Cl . The organic phase was extrac-
ted with H O (x3) and then loaded on top of a silica column. The
2
title compound was obtained as a colorless solid following elution
with EtOAc:hexane 2:1 v/v and solvent removal under vacuum
ꢁ
(
40 C). Yield: 11 mg, 19%. Anal. Calcd. for C
8
H
8
O
4
: C, 57.14; H, 4.79.
ꢀ
1
Found: C, 57.22; H, 4.65. IR (solid state): ῦ/cm ¼ 3087w, 2962w,
927w, 1730s-sh ( C7¼O), 1708s ( C5¼O), 1638s ( C1¼C6), 1611m-sh,
547s ( C2¼C3), 1449 m, 1429m-sh, 1416m-sh, 1371 m, 1349 m,
254s, 1170w, 1031 m, 1008m-sh, 995s, 963 m, 923 m, 856s, 806w,
2
n
n
n
1
n
1
1
7
78 m, 711w, 704w. H NMR (CDCl
.93 (s, 1H, C2-H), 2.66 (s, 3H, C8-H), 2.27 (s, 3H, C4-H). C{ H}
):
/ppm ¼ 205.4 (C7), 181.2 (C1), 169.2 (C3), 161.4 (C5),
01.6 (C2), 100.0 (C6), 30.2 (C8), 20.8 (C4). See Chart 1 for structure
3
):
d
/ppm ¼ 16.69 (s, 1H, OH),
13 1
5
NMR (CDCl
3
d
1
and atom numbering.
A comparable yield of dhaH was obtained when the reaction
was performed under protection from the light or with 10 eq. of
Et N. The development of a red color with massive precipitation of
3
Scheme 4. NMR detected species as a function of time in the dmso/H
of 2 at 37 C.
2
O/NaCl solution
ꢁ