First total synthesis of chromanone A, preparation of related compounds and evaluation of their antifungal activity againstCandida albicans, a biofilm forming agent

Article abstract of DOI:10.1039/d1ra02553h

A straightforward and convenient approach for the first total syntheses of chromanone A and a related 7-OMe substituted natural product is reported. These unique C-3 substituted 2-hydroxymethyl chromones were recently isolated as fungal metabolites. Chromanone A was synthesized in 25.3% overall yield from the readily available pyrocatechol, whereas the second natural product was prepared in 39.7% global yield. A small library of chromones, including both natural products and some of their synthetic heterocyclic precursors, was evaluated againstCandida albicansATCC 10231, a biofilm forming agent. It was found that 8-methoxy-3-methyl-4-oxo-4H-chromene-2-carbaldehyde, a partially oxidized form of chromanone A, exhibited a minimum inhibitory concentration of 7.8 μg mL^?1and significantly inhibited the yeast's virulence factors, including the adherence to buccal epithelial cells and the secretion of phospholipases, as well as the formation of germ tubes and the generation of the hyphal pseudomycelium. In addition, despite the heterocycle exhibiting non-significant inhibition of the formation of theCandidabiofilm, it completely inhibited the growth ofC. albicansin preformed biofilms at 62.5 μg mL^?1

Full text of DOI:10.1039/d1ra02553h

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First total synthesis of chromanone A, preparation
of related compounds and evaluation of their
antifungal activity against Candida albicans,
a biofilm forming agent†
Cite this: RSC Adv., 2021, 11, 19587
a
b
a
*
Ivan Cortes,
Estefanıa Cordisco,
Teodoro S. Kaufman,
´
´
´
b
b
a
*
*
Maximiliano A. Sortino,
Laura A. Svetaz
and Andrea B. J. Bracca
A straightforward and convenient approach for the first total syntheses of chromanone A and a related 7-
OMe substituted natural product is reported. These unique C-3 substituted 2-hydroxymethyl chromones
were recently isolated as fungal metabolites. Chromanone A was synthesized in 25.3% overall yield from
the readily available pyrocatechol, whereas the second natural product was prepared in 39.7% global
yield. A small library of chromones, including both natural products and some of their synthetic
heterocyclic precursors, was evaluated against Candida albicans ATCC 10231, a biofilm forming agent. It
was found that 8-methoxy-3-methyl-4-oxo-4H-chromene-2-carbaldehyde, a partially oxidized form of
chromanone A, exhibited a minimum inhibitory concentration of 7.8 mg mLꢀ1 and significantly inhibited
the yeast's virulence factors, including the adherence to buccal epithelial cells and the secretion of
phospholipases, as well as the formation of germ tubes and the generation of the hyphal
pseudomycelium. In addition, despite the heterocycle exhibiting non-significant inhibition of the
formation of the Candida biofilm, it completely inhibited the growth of C. albicans in preformed biofilms
Received 31st March 2021
Accepted 24th May 2021
DOI: 10.1039/d1ra02553h
rsc.li/rsc-advances
at 62.5 mg mLꢀ1
.
In this context, there is a need for antimicrobial agents capable
of attacking microorganisms within the biolms. Hence, the
search for new compounds endowed with this property is
currently relevant.
1. Introduction
Biolms are structured microbial communities, embedded in
self-produced polymeric matrix of protein and poly-
a
The chromone skeleton is
meaning promising motif for drug development. Not
a
“privileged structure”,2a
saccharides. They are frequently found on interfaces or attached
to surfaces, and are the scenario of complex interactions. These
enduring structures are elusive to the defense system of the
host, reluctant to undergo phagocytosis, and offer the
producing microorganism a markedly increased resistance to
antibiotics and chemical disinfectants.1a Therefore, biolms are
an important form of microbial resistance and transmission of
chronic infections.1b
a
surprisingly, substituted chromones are known to display
a variety of useful biological properties, including antifungal
activity.2b,c
In 2009, Gamal-Eldeen and coworkers reported the isolation
of chromanone A (A, Fig. 1),3 from an algicolous marine Peni-
cillium species, cultivated on a solid biomalt medium. In turn,
this fungus was isolated as an endophyte of the Egyptian green
alga Ulva sp. The natural product inhibits the activity of CYP1A
at a level of 4 mg mLꢀ1, being a potential cancer chemo-
preventive agent.3
The biolms are ubiquitous, and their presence on surfaces
such as biomedical implants, may put at risk complex surgical
interventions and ultimately compromise the life of the patient.
The structure of the natural product was assigned based on
an analysis of 1D and 2D (HSQC, HMBC) NMR spectra. Being
a member of the very small family of the naturally occurring 2-
hydroxymethylchromones which bear a C-3 functionalization,
chromanone A is a structurally unique chromone. Its special
structural characteristics are shared only by a handful of natural
products, such as its isomer B, recently isolated from Rhino-
cladiella sp., a fungus obtained from the marine sponge Ircinia
oros.4a These features are also found in heterocycles C4b and D,4c
aInstituto de Quımica Rosario (IQUIR, CONICET-UNR), Facultad de Ciencias
´
´
´
Bioquımicas y Farmaceuticas, Universidad Nacional de Rosario, Suipacha 531,
S2002LRK Rosario, Argentina. E-mail: kaufman@iquir-conicet.gov.ar; bracca@
iquir-conicet.gov.ar
b
´
´
Area Farmacognosia, Facultad de Ciencias Bioquımicas y Farmaceuticas, Universidad
Nacional de Rosario, Suipacha 531, S2002LRK Rosario, Argentina. E-mail: lsvetaz@
ioyf.unr.edu.ar
† Electronic supplementary information (ESI) available: NMR spectra of the
intermediates and nal products, bioactivity testing methodology and results of
inhibition assays. See DOI: 10.1039/d1ra02553h
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Fig.
1 Chromanone A (A) and related naturally occurring 3-
Scheme 1 Retrosynthetic analyses of chromanone A (1).
substituted-2-hydroxymethyl chromone derivatives.
as well as in boeravinone Q (E)5a and its congener mirabijalone
C (F).5b
aromatic side of the carbonyl motif of propiophenone 4 (bond
b) set aside the three-carbon side chain and determined that the
logical starting material for this approach was the commercially
available pyrocatechol (6).
For simplicity, the Route a was explored rst. Thus, ortho-
vanillin (5) was subjected to a 1,2-addition to the carbonyl with
excess ethyl Grignard reagent (Scheme 2), freshly prepared from
iodoethane and activated magnesium. The reaction was
executed at room temperature (RT), giving alcohol 7 in 97%
yield.8
Subsequently, 7 was submitted to a selective oxidation of its
secondary alcohol moiety. Different conditions [PDC, CH2Cl2;
H2N–NH2$xH2O, DMSO/I2, MeCN/H2O (5 : 1, v/v); DMSO/Ac2O
(2.3 equiv.); IBX (1.5 equiv.), EtOAc]9 were tested; however, the
reaction proved most successful with IBX in EtOAc, furnishing
ketone 3 in up to 81% yield in just one hour at room tempera-
ture.10 Unfortunately, this transformation proved to lack
robustness, being highly dependent on the experimental
conditions, including the source of the oxidizing reagent. This
characteristic frequently resulted in an exacerbated concomi-
tant oxidation of the ortho-phenol, ultimately causing tarry
materials and low yields.
Our work is focused on the synthesis of structurally unique
heterocyclic natural products,6 as well as in their evaluation.7a,b
Further, among chromone derivatives, we have developed the
total synthesis of the structure assigned to aspergillitine,7c a 2,3-
dimethyl chromone derivative isolated from a marine Asper-
gillus species.
In pursuit of these interests, here we report an efficient
approach to the rst total syntheses of chromanone A (A) and of
its isomer, the related natural product B. The results of the
evaluation of both natural products and their heterocyclic
synthetic intermediates, as antifungal agents against the bio-
lm forming yeast Candida albicans ATCC 12031, are also
discussed.
2. Results and discussion
2.1 Chemistry
The initial step toward chromanone A as the rst target (1), was
a retrosynthetic analysis of the product (Scheme 1). Initially,
focus was made on the oxygen bearing functionality of the 2-
hydroxymethyl feature; a functional group interconversion was
proposed, conjecturing that the proper oxidation stage of the C-
2 substituent could be set either through reduction (aldehyde,
ester, 2a) or by means of a selective oxidation.7a The latter
possibility revealed the 2,3-dimethylchromone 2b as a suitable
precursor.
Then, two different alternatives were considered. In one of
them (Route a), the heterocyclic ring of 2a,b was disassembled
as shown, to unveil a propiophenone derivative (3), which was
further submitted to a C–O disconnection on the aliphatic side
of the carbonyl moiety (bond a), to uncover ortho-vanillin (5) as
a suitable starting material.
In the second case (Route b), an additional methyl ether C–O
disconnection was considered on the chromone 2, which sug-
gested the catechol derivative 4 as the most appropriate
Scheme 2 Reagents and conditions (a) CH3CH2I, Mg0, Et2O, RT (97%);
(b) IBX, EtOAc, RT, 1 h (81%); (c) EtO2C–COCl, Et3N, CH2Cl2, MW (100
synthetic intermediate. In turn, a C–O disconnection on the ꢁC), 1 h (42%); (d) NaBH4, CaCl2, EtOH, 0 ꢁC, 24 h (41%).
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Next, a one-pot Kostanecki–Robinson cyclization protocol
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Despite this rst approach to 1 afforded the natural product
was practiced on the propiophenone 3 in order build the in just four steps and 13.5% overall yield, the lack of robustness
chromone system. The reaction of aromatic ketones with a gly- of the selective oxidation of 7, coupled to the moderate yields of
colate ether was considered as an alternative, to access a 2- the cyclization toward 8 and its selective ester moiety reduction
alkoxymethylene chromone; however, it generally proved to stages, suggested the need to devise an improved alternative.
proceed in low yield11a and was soon discarded due to inability
Therefore, we resorted to the second synthetic plan toward 1,
of the product to withstand the harsh conditions required to commencing with the selective Friedel–Cras ortho-acylation of
break the ether bond and the lack of selectivity of the reaction. pyrocatechol (6) under BF3$OEt2 promotion.15 However, since in
In addition, model experiments with the scarcely precedented our hands the reported procedure proved hard to be repro-
use of glyoxal11b as a two-carbon synthon, which would have duced, a series of optimization experiments were run in order to
delivered a 2-hydroxymethyl chromone directly, met with nd the proper reaction conditions.
failure; no reaction was observed, despite different basic
In the process, it was soon found that ZnCl2 and AlCl3 are not
suitable promoters, and that the use of 1,2-dichlorobenzene as
conditions (KOH, KtBuO and NH4OH in EtOH) were explored.
Therefore, the ketone 3 was exposed to ethyl chloroox- solvent negatively affects the reaction performance. It was also
oacetate,12 which can be regarded as an oxidized form of glyoxal discovered that under conventional thermal conditions (Table
ꢁ
or glycolate ethers. Luckily, the reaction took place in CH2Cl2, in 1), the transformation hardly proceeded aer 3 days at 110 C
the presence of triethylamine at 100 ꢁC under microwaves (entry 1), whereas only small amounts of product were recov-
irradiation (MW), giving chromone 8 in a reproducible but ered when the reaction was performed at 180 ꢁC for 5 h (entry 2).
rather moderate (42%) yield.
On the other hand, employing microwaves radiation, no
ꢁ
Finally, the required adjustment of the oxidation state of the product was isolated aer 10 min at 180 C (entry 3), whereas
C-2 substituent was carried out by means of a selective reduc- slightly milder conditions (170 ꢁC, 5 min) resulted in 33% yield
tion of 8; however, since the use of NaBH4 as reducing agent13 of product (entry 4), when 10 equiv. EtCO2H were used.
did not perform as expected, this challenging transformation Increasing the amount of acid caused a further increase in the
was carried out with Ca(BH4)2.14 The reagent was prepared in yield to 61% (entry 5), while unexpectedly, additional amounts
situ by adding a stoichiometric amount of CaCl2 to NaBH4 in of acid produced a drastic yield reduction (entry 6). In addition,
EtOH.14c Under these conditions, chromanone A (1, compound performing the reaction at 160 ꢁC for 10 min (entry 7) or in the
A of Fig. 1) was obtained in 41% yield. Its spectroscopic data in presence of propionic anhydride (entry 8) did not improve the
CDCl3 conrmed its structure, whereas the NMR spectra taken results.
in MeOH-d4 were in full agreement with those reported for the
Therefore, the reaction was best performed as in entry 5, in
natural product.3 Signal enhancement (NOE) of the CH2OH a solventless condition and under microwaves irradiation,
moiety (0.8%) was observed upon irradiation of the hydrogen affording consistently over 60% yield of the expected product
atoms of the 3-Me group, as well as between the hydrogen atoms 4.16 The use of 6 instead of guaiacol for this approach was based
of the 8-OMe group and H-7 (1.7%). Notably, the spectra taken on literature precedents, which suggested that the latter should
in both solvents were alike, and differences exceeding 10 ppm not be a suitable starting material because it would afford the
were found between them, particularly for the carbon atoms of unwanted propiophenone isomer.17a,b
the isocyclic ring attached to oxygen.
Next, 4 was cyclocondensed and further cyclized with Ac2O
under Kostanecki-Robinson conditions, uneventfully affording
Table 1 Optimization of the synthesis of propiophenone 4
Entry no.
EtCO2H (equiv.)
BF3$Et2O (equiv.)
Temp. (ꢁC)
Power MW (W)
Time (min)
Yield of 4 (%)
1
2
3
4
5
6
7
8
9
1
1
1
1.5
1
1
1
1
110
180
180
170
170
170
160
170
—
—
3 days
7
23
0
33
61
27
50
29
13
9.5
10
12
20
12
6.4a
300
10
5
5
5
200
100
100
100
100
100
10
5
a
Propionic anhydride was employed, in the presence of BHT (0.05 equiv.).
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the expected 2,3-dimethylchromone 9 (ref. 7c) in 76% yield
(Scheme 3). A subsequent Williamson O-methylation of 9 with
MeI/K2CO3 in reuxing acetone gave 94% yield of 2.17c This was
followed by a selective oxidation with the versatile I2/DMSO
reagent system, which conveniently furnished aldehyde 10 in
83% yield. The transformation, which was carried out aerobi-
cally, required the addition of TsOH.18a
Finally, the reduction of the formyl moiety with NaBH4 in
EtOH uneventfully provided the expected product 1 in 70%
yield. Interestingly, unlike the reduction of compound 8, in this
case the use of Ca(BH4)2 proved to be unnecessary, since the
formyl group attached to C-2 is much more reactive than the C-4
ketone moiety.
This optimized approach gave 1 in 25.3% overall yield, aer
ve synthetic steps. The NMR spectroscopic data of the
synthetic compound (in MeOH-d4) were in excellent agreement
with those of the literature,3 and the heterocycle obtained
Scheme 4 The Kornblum oxidation-based mechanistic proposal for
the conversion of 2,3-dimethylchromone 2 into 10 with the I2/DMSO
through Route a, conrming the structure of the natural reagent system.
product.
The intimate details of the mechanism of the key I2/DMSO-
mediated oxidation toward 10 remain unclear; however, based
on some previous literature precedents, a polar rather than
a free-radical reaction mechanism can be proposed. Further,
considering that the 2-methylchromone moiety may be regar-
ded as a vinylogous a-methylketone,18 it can be conjectured that
a Kornblum-like oxidation is at the heart of the mechanistic
sequence (Scheme 4). In this scenario, at rst the carbonyl
group of the substrate (2) would be activated by the added
TsOH, giving rise to intermediate I. In turn, this intermediate
would be subjected to deprotonation to provide the dienol II,
being followed by an iodination with I2, to generate the reactive
iodide III.
intermediate IV. The latter would undergo a proton abstraction,
resulting in the aldehyde product 10. Interestingly, it has been
reported that reaction of a primary iodide like III with KO2 in
DMSO afforded a hydroxymethyl chromone, albeit in rather low
yield (27%).19
During the reaction two molecules of HI and one of SMe2 are
produced. Although DMSO can oxidize iodide ions to iodine,
generating SMe2 (DMSO + 2Iꢀ / SMe2 + I2 + H2O), it is assumed
that the presence of oxygen (air) under the strenuous reaction
conditions (130 ꢁC) would serve to reoxidize all the SMe2 formed
in the transformation and/or to regenerate the iodine in the
presence of DMSO.
Next, in the presence of DMSO which plays the dual role of
solvent and oxidant, the iodide III would undergo a SN2-type
reaction with the nucleophilic oxygen atom of the reagent,
losing iodide and forming the alkoxysulfonium salt
The general guidelines provided by the retrosynthetic anal-
ysis of Scheme 1 were used to synthesize compound 16 (Fig. 1,
compound B) and to access additional heterocyclic intermedi-
ates for bioactivity testing.
The synthetic sequence commenced with the commercially
available propiophenone 12 (Scheme 5) which was sequentially
Scheme 3 Reagents and conditions: (a) MeCH2CO2H, BF3$OEt2, MW
(160 ꢁC), 5 min (61%); (b) 1. Ac2O, NaOAc, reflux, 3 h; 2. Et3N, 115 ꢁC, Scheme 5 Reagents and conditions: (a) 1. Ac2O, NaOAc, reflux, 3 h; 2.
14 h; 3. 1 M HCl, 40 ꢁC, 3 h (76% overall); (c) K2CO3, MeI, Me2CO, reflux, Et3N, 115 ꢁC, 14 h; 3. 1 M HCl, 40 ꢁC, 6 h (77% overall); (b) MeI, K2CO3,
24 h (94%); (d) I2, TsOH, DMSO, O2, 130 ꢁC, 2 h (83%); (e) NaBH4, EtOH, Me2CO, reflux, 24 h (93%); (c) I2, TsOH, DMSO, O2, 130 ꢁC, 2 h (78%); (d)
0 ꢁC / RT, 1 h (70%).
NaBH4, EtOH, 0 ꢁC / RT, 1 h (71%).
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exposed to Ac2O/NaOAc,20 Et3N and HCl. Under these condi- C. albicans (1 ꢂ 103 CFU per mL) plotted against the logarithm
tions, it experienced an exhaustive acetylation, followed by of the corresponding concentration of each compound.
a Baker–Venkataraman rearrangement and cyclization, and
The assay revealed that the natural product (1) is a very weak
a nal dehydration and deacetylation to afford 13 in 77% overall inhibitor, being considered inactive. It barely caused 50%
yield.
inhibition at a concentration of 250 mg mLꢀ1 (MIC > 250 mg
The phenol was methylated under conventional conditions, mLꢀ1); quite a similar prole was exhibited by its phenolic
with MeI in reuxing acetone, using K2CO3 as base to afford 14 precursor 9 (63% inhibition at 250 mg mLꢀ1). On the contrary,
(93%). This is a natural product, which has been recently iso- the aldehyde 10 proved to be a very good inhibitor with MIC ¼
lated from Rhinocladiella sp.4b and from the co-culture of 7.8 mg mLꢀ1
.
a marine-derived Actinomycete (Streptomyces rochei MB037) and
Regarding the 7-substituted chromones, compounds 13, 14
the fungus Rhinocladiella similis 35. Compound 14 proved to and 16 demonstrated to be inactive. Compound 14 exhibited
display weak antibacterial activity against Staphylococcus aureus 72.8% inhibition at 250 mg mLꢀ1 (MIC > 250 mg mLꢀ1), whereas
and Pseudomonas aeruginosa.21
aldehyde 15 was moderately active, with a MIC value of 62.5 mg
Next, 14 was selectively oxidized with the DMSO–I2 reagent mLꢀ1. The MFC values for the isomeric active aldehydes 10 and
system, providing aldehyde 15 in 78% yield.22 Finally, the 15 were also determined according to the established protocol,23
aldehyde was selectively reduced with the aid of the NaBH4, being 125 mg mLꢀ1 in both cases.
affording the alcohol 16 (Fig. 1, compound B) in 71% yield. This
The results suggested that the presence of a formyl moiety is
synthetic route provided the natural product 16 with an overall relevant for the observed activity, and that the latter can be
yield of 39.7% in just four steps.
modulated by the position of the oxygenated substituent in the
homocyclic ring.
Next, the effect of the heterocycles on the virulence factors of
C. albicans were examined. The tests included inhibition of the
adherence to buccal epithelial cells, inhibition of the formation
of the germ-tube, morphogenesis of C. albicans on solid media
and the inhibition of lytic enzymes.
The rst step by which a microorganism can initiate an
infection is through adherence to an epithelial surface; this
ability enables it to exist in biolms and is in clear association
with its virulence.24 In order to evaluate whether the active
compounds are able to affect this process, at sub-lethal
concentrations, compounds 10 and 15 were submitted to the
assay of inhibition of the adherence to buccal epithelial cells
(BEC).
The results (Fig. 3A) showed that the number of yeasts
adhered to 100 BEC decreased from 2972 ꢃ 233 in the untreated
control cells to 177 ꢃ 60, in the presence of compound 10 at
MFC/2; interestingly, the compound was still active at MFC/32
(number of yeasts adhered to 100 BEC ¼ 1724 ꢃ 399).
Further, as seen in Fig. 3B, treatment with sub-lethal concen-
trations of compound 15 also caused a remarkable decrease in
yeast adherence to BEC at levels ranging from MFC/2 (482 ꢃ
187) to MFC/8 (1334 ꢃ 233).
2.2 Evaluation of bioactivity
The methods employed are fully described in the ESI.† Initially,
antifungal susceptibility tests were carried out, with the deter-
mination of the minimum inhibitory concentration (MIC) and
the minimum fungicidal concentration (MFC). The antifungal
activity of the compounds was assessed in the concentration
range 0.49–250 mg mLꢀ1 with the standardized CLSI microbroth
dilution method for yeasts23 against C. albicans ATCC 10231.
Initially, the aldehyde 10 and the natural product (1) were
screened; however, in order to obtain a better insight into the
structural factors implied in the bioactivity, their precursors 2
and 9 as well as the related 7-substituted 2,3-dimethylchro-
mones 13 and 14 were also tested, along with the aldehyde 15
and the natural product 16.
The results are collected in Fig. 2, which depicts the
percentage of growth inhibition of a standardized inoculum of
Fig. 3 Adherence of Candida albicans ATCC 10231 to human buccal
Fig. 2 Semi-logarithmic plot of the growth inhibition of Candida epithelial cells (BEC) after incubation in media containing sub-lethal
albicans ATCC 10231 in the presence of different concentrations of the concentrations of 10 (A) and 15 (B). The adherence to BEC is expressed
chromone test compounds. The dashed line indicates the 50% inhi- as the number of adhered yeasts/100 BEC. Wilcoxon test ¼ ****(p <
bition level.
0.0001), ns: not significant.
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These results clearly indicated that the amount of adhered secretion of C. albicans esterases, since their values of the Pz
fungal cells to BEC was signicantly lower (Wilcoxon test, p < index (Pz ¼ 0.82 ꢃ 0.02) were the same as the control.
0.0001) in yeasts treated with both aldehydes than in the
In view of these promising observations, the interaction of
untreated cells, suggesting that the presence of these hetero- the chromone derivatives on formation of the C. albicans
cycles causes some degree of resistance to the colonization of biolm and on preformed biolms was examined. In the rst
BEC by C. albicans.
case, an inoculum prepared according to Pierce et al. was
In the germ-tube inhibition assay, the effect of different used, employing yeast extract–peptone–dextrose (YPD)
geometrically distributed sub-lethal concentrations (MFC/64– medium.25
MFC/2) of compounds 10 and 15 on the formation of germ
tubes (GT) in C. albicans was assessed.
Fig. 5A displays the data related to the inhibition of the
formation of the C. albicans biolm in the presence of different
The results (Fig. 4A) showed that the presence of compound concentrations of compounds 10 and 15. A compound was
10 at concentrations between MFC/2 and MFC/32, inhibited the considered active if it signicantly inhibited biolm formation
formation of hyphae (GT% ¼ 48.5 ꢃ 2.1% at MFC/32) in a dose- at concentrations below the MIC (sub-inhibitory). However, it
dependent way with respect to the control (GT% ¼ 92 ꢃ 1.4%). was detected that none of the heterocycles managed to
The maximum degree of inhibition was recorded at MFC/2 completely avoid formation of the biolm.
(GT% ¼ 12.0 ꢃ 1.4%).
In the presence of 10 at a level of 250 mg mLꢀ1, the inhibition
Compound 15 proved to be a less powerful inhibitor was 67.6% (54.5% at 125 mg mLꢀ1), while 15 exhibited meager
(Fig. 4B), which inhibited hyphae formation up to a concentra- values of 45.6% and 23.7% inhibition at the same concentra-
tion of MFC/8 (GT% ¼ 59 ꢃ 1.4%). At MFC/2, the observed GT% tions, respectively. Not unexpectedly, at the corresponding MIC
was 16.0 ꢃ 1.4%. The results were all statistically signicant values, their degree of inhibition lacked statistical signicance;
according to the Holm–Sidak test (p < 0.05).
therefore, based on these results, none of the heterocycles
The morphogenesis of C. albicans on solid media was inhibited biolm formation.
studied by examining the effects of compounds 10 and 15 on
On the other hand, when the antifungal activity against
the formation of pseudomycelium in C. albicans in the nutrient- preformed C. albicans biolms was evaluated, it was observed
poor Spider medium, that induces pseudohyphal morphogen- that compounds 10 and 15 displayed almost complete inhibi-
esis. It was observed that the aldehyde 10 effectively reduced tion at concentrations of 62.5 and 250 mg mLꢀ1, respectively.
formation of the hyphal pseudomycelium up to a concentration Further, compound 10 caused over 60% inhibition when the
of MFC/4. This effect was apparent by the smoother aspect of concentration was halved. The effect was concentration
the colonies and the notable reduction of hyphae at the edges. dependent, as reected in the progressive increase in cell
On the other hand, in the presence of compound 15, the C. viability with reducing concentrations of both compounds.
albicans colonies showed their typical lamentation at the
edges, suggesting that this heterocycle is not an effective against planktonic cells of C. albicans is 7.8 mg mLꢀ1, the
inhibitor of pseudomycelium formation. aldehyde can completely inhibit the growth of the biolm yeast
The results indicate that, while the MIC of compound 10
Finally, in the study of lytic enzyme inhibition, it was colonies at a concentration of 62.5 mg mLꢀ1, suggesting that it
detected a statistically signicant inhibition of phospholipases may be a useful and promising advantage in the development of
secretion in the presence of compound 10 at MFC/2 (Pz ¼ 0.92 ꢃ more effective anti-biolm agents.
0.01) and MFC/4 (Pz ¼ 0.82 ꢃ 0.02) with respect to the control
(Pz ¼ 0.74 ꢃ 0.02). Contrarily, however, no signicant changes
in the Pz index were observed between the untreated (control)
cells and those exposed to compound 15. This signaled that the
secretion of phospholipases was not inhibited.
On the other hand, it was also observed that at sub-lethal
concentrations, compounds 10 and 15 did not inhibit
Fig. 4 Candida albicans ATCC 10231 germ tube formation (% cells)
either without (control) or with exposure to sub-lethal concentrations Fig. 5 Activity of compounds 10 (-) and 15 (B) against the C. albi-
of compounds 10 (A) and 15 (B). Holm–Sidak test ¼ *(p < 0.05), ns: not cans ATCC 10231 biofilm. (A) Inhibition of the biofilm formation. (B)
significant.
Antifungal activity against the pre-formed biofilm.
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The reactions were monitored by TLC, employing
aluminium supported silica gel 60 GF254 plates run in different
hexanes:EtOAc mixtures. The chromatographic spots were
revealed by exposure to UV light (254 and 365 nm), followed by
spraying with the ethanolic p-anisaldehyde/sulfuric acid
reagent and gentle heating to improve selectivity.
The ash column chromatographies were developed under
positive pressure with slurry-packed silica gel 60 H for thin layer
chromatography (particle size < 55 mm), employing gradient of
solvent polarity techniques with hexanes:EtOAc.
3. Conclusions
Two facile and expedient approaches to the rst total synthesis
of chromanone A, a unique chromone isolated from an algico-
lous fungus Penicillium sp., endophyte of the Egyptian green
alga Ulva sp., were developed. In the most efficient sequence,
the heterocycle was accessed in only ve steps and 25.3% overall
yield from pyrocatechol, a commercial and easily available
starting material.
The key stages of the synthesis included a selective Friedel–
Cras acylation of the starting catechol under BF3$Et2O
promotion and the aerobic oxidation of the C-2 methyl group of
a chromone intermediate by the I2/DMSO reagent system. To
the best of our knowledge, this is the rst report of the use of
this approach and this reagent system for the synthesis of 2-
formyl- and 2-hydroxymethyl chromones. A 7-methoxy isomer of
the natural product, which is also a fungal natural product, was
prepared following the same strategy. Neither of the synthetic
sequences required the use of protecting groups, favouring the
efficiency of the syntheses.
4.2 Equipment
The melting points were determined on an Ernst Leitz Wetzlar
model 350 hot-stage microscope and are informed uncor-
rected. The FT-IR spectra were acquired on a Shimadzu Pres-
tige 21 spectrophotometer, as thin lms held between two
NaCl disks or as solid dispersions in KBr (solid samples). The
NMR spectra were recorded in CDCl3 with a Bruker Avance 300
NMR spectrometer (300.13 MHz for 1H and 75.48 MHz for 13C).
The chemical shis are reported in the d scale, in ppm
downeld from TMS (d ¼ 0.0 ppm). The residual solvent peaks
of CDCl3 (dH ¼ 7.26 ppm, dC ¼ 77.16 ppm), were used as
internal references. The scalar coupling constant (J) and width
and half-height (w1/2) values are given in Hertz. NOE and 2D
NMR experiments (HSQC and HMBC) were also acquired in
order to ensure unambiguous signal assignment. The HRMS
were obtained with a Bruker MicroTOF-Q II instrument from
UMyMFOR (Buenos Aires, Argentina). The microwave-assisted
reactions were carried out in a CEM Discover microwave
reactor. The light microscopy observations and determina-
tions were performed with an Eclipse E100 (Nikon Corp.,
Tokyo, Japan) instrument. The microplates were read in
a VERSA Max microplate reader (Molecular Devices, Sunny-
vale, CA, USA).
The tests of antifungal activity against C. albicans revealed
that 8-methoxy-3-methyl-4-oxo-4H-chromene-2-carbaldehyde,
the 2-formyl analogue of chromanone A, exhibited a MIC ¼
7.8 mg mLꢀ1 (MFC ¼ 125 mg mLꢀ1). In addition, at sub-lethal
concentration levels, this compound signicantly inhibited
various C. albicans virulence factors, including the secretion of
phospholipases and the yeast adherence to buccal epithelial
cells, as well as the formation of the hyphal pseudomycelium
and germ tubes. On the other hand, the formyl derivative
completely inhibited the growth of C. albicans in preformed
biolms at 62.5 mg mLꢀ1
.
Taken together, the results indicate that the 2-formyl
analogue of chromanone A not only kills C. albicans, but also
inhibits its virulence factors, suggesting that it is able to target
both, the cell growth and the pathogenic process. These
features turn the aldehyde into a potentially useful lead to
develop more effective agents to ght this yeast in biolm
scenarios. These ndings also support the hypothesis that
marine fungi may be important sources of inspiration for the
development of new agents to combat biolm-forming micro-
organisms, and perhaps help to overcome difficulties or failure
of current therapy for fungal infections.
4.3 2-(1-Hydroxypropyl)-6-methoxyphenol (7)
A stirred solution of ortho-vanillin (5, 1500 mg, 9.86 mmol) in
anhydrous Et2O (10 mL) was purged with nitrogen, cooled to
0
ꢁC in an ice bath and treated dropwise with a solution of
EtMgI (2.5 M, 24.6 mmol) in Et2O, prepared in situ from ethyl
iodide and magnesium turnings. The mixture was allowed to
warm to room temperature and then additionally stirred for 1
hour. At the end of the reaction (assessed by TLC), a saturated
solution of NH4Cl was added (10 mL) and the organic products
were extracted with CH2Cl2 (3 ꢂ 20 mL). The combined organic
4. Experimental section
4.1 General information
The chemical transformations were carried out under dry argon extracts were washed with brine (10 mL), dried over anhydrous
atmospheres, employing freshly distilled anhydrous solvents MgSO4, and concentrated under reduced pressure. Chroma-
and oven-dried glassware. Anhydrous CH2Cl2 was obtained tography of the residue provided 7 (1742 mg, 97%) as
from an MBraun solvent purier and dispenser system. Abso- a yellowish oil.8b IR (lm, ~n): 3443, 2965, 1620, 1593, 1487 and
lute EtOH was prepared by reuxing the commercial solvent 1454 cmꢀ1. 1H NMR (300 MHz, CDCl3): d ¼ 6.85–6.75 (3H, m, H-
over clean Mg/I2 and distilling from the resulting magnesium 3, H-4, H-5), 6.31 (1H, s, OH), 4.79 (1H, q, J ¼ 6.3, H-10), 3.90
alkoxide. Anhydrous Et3N was prepared by distillation of the (3H, s, OMe), 2.55 (1H, d, J ¼ 5.6, OH) 1.97–1.78 (2H, m, H-20)
commercial product from CaH2. The anhydrous solvents were and 0.96 (3H, t, J ¼ 7.4, Me). 13C NMR (75 MHz, CDCl3): d ¼
stored in dry Young ampoules. The rest of the solvents and 146.8 (C-6), 143.1 (C-1), 129.5 (C-2), 119.6 (C-3)*, 119.3 (C-4)*,
reagents were used as received.
109.8 (C-5), 73.4 (C-10), 56.1 (OMe), 30.1 (C-20) and 10.3 (C-Me).
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(3H, s, OMe), 2.65 (1H, bs, w1/2 ¼ 13, OH), and 2.12 (3H, s, Me-3).
13C NMR (75 MHz, CDCl3): d ¼ 178.3 (C-4), 160.5 (C-2), 148.5 (C-
8), 146.2 (C-8a), 124.5 (C-6), 123.6 (C-4a), 117.4 (C-3), 116.8 (C-5),
113.8 (C-7), 60.4 (CH2), 56.2 (OMe) and 9.2 (Me-3).
4.4 1-(2-Hydroxy-3-methoxyphenyl)propan-1-one (3)
A stirred solution of 7 (34 mg, 0.19 mmol) in EtOAc (1 mL) was
treated with 2-iodoxybenzoic acid (78 mg, 0.28 mmol) and the
mixture was warmed to 80 ꢁC for 1 h. Once the reaction was
completed (aer TLC monitoring), the solvent was removed
under reduced pressure and the resulting crude was chroma-
tographed, affording 3 (27.4 mg, 81%) as a yellowish solid,26 mp:
1H NMR (300 MHz, MeOH-d4): d ¼ 8.00 (1H, dd, J ¼ 1.5, 7.8,
H-5), 7.03 (1H, bs, H-6), 7.02 (1H, dd, J ¼ 1.5, 7.8, H-7), 4.62
(2H, s, H-10), 3.98 (3H, s, OMe) and 2.07 (3H, s, Me). 13C NMR (75
MHz, MeOH-d4): d ¼ 180.1 (C-4), 166.0 (C-8a), 164.2 (C-8), 159.4
(C-2), 127.7 (C-6), 118.1 (C-4), 117.3 (C-4a), 116.0 (C-7), 101.0 (C-
3), 60.7 (CH2), 56.5 (OMe) and 9.2 (Me-3). The NMR signals of
the synthetic compound in MeOH-d4 were in agreement with
those of the natural product (compound A).3
69–71 C (Lit.: 72–73 C).10 IR (KBr, ~n): 3420, 2918, 1636, 1458,
1435, 1368, 1317, 1273, 1252, 1227, 1088, 1022, 986, 822, 806,
770, 739, 719 and 625 cmꢀ1. 1H NMR (300 MHz, CDCl3): d ¼ 7.37
(1H, dd, J ¼ 1.5, 8.0, H-60), 7.05 (1H, d, J ¼ 8.0, H-40), 6.84 (1H, t, J
¼ 8.0, H-50), 3.91 (3H, s, OMe), 3.05 (2H, q, J ¼ 7.4, H-2) and 1.24
(3H, t, J ¼ 7.4, Me). 13C NMR (75 MHz, CDCl3): d ¼ 207.5 (C-1),
152.8 (C-20), 149.0 (C-30), 121.0 (60), 119.3 (C-40), 118.2 (C-50),
116.7 (C-10), 56.2 (OMe), 31.9 (C-2) and 8.2 (Me). HRMS (ESI+)
calcd for C10H13O3 [M+]: 181.0859, found 181.0859.
ꢁ
ꢁ
4.7 1-(2,3-Dihydroxyphenyl)propan-1-one (4)
A mixture of pyrocatechol (6, 100 mg, 0.91 mmol), BF3OEt
(128.9 mg, 0.91 mmol) and propionic acid (807 mg, 10.9 mmol)
reacted together under microwave irradiation at 160 ꢁC, without
any solvent for a short time. Aer cooling to room temperature,
the reaction mixture was dissolved in dichloromethane (10 mL)
and brine (about 20 mL). Aer extraction of the organic phase, it
was washed with aqueous NaHCO3 (20 mL), dried with MgSO4,
ltered and evaporated to give a crude product. The crude
product was chromatographed to afford 11 (92 mg, 61%), as
a yellow solid, mp: 54–56 ꢁC (Lit.: 56–57 ꢁC).16a IR (KBr, ~n): 3566,
3491, 2978, 2940, 1643, 1636, 1597, 1456, 1452, 1369, 1271, 1109,
1076, 1045, 880, 820 and 731 cmꢀ1. 1H NMR (300 MHz, CDCl3):
d ¼ 7.31 (1H, dd, J ¼ 1.4, 8.2, H-60), 7.12 (1H, dd, J ¼ 1.4, 7.9, H-40),
6.81 (1H, t, J ¼ 8.0, H-50), 5.75 (2H, bs, w1/2 ¼ 76, 2 ꢂ OH). 3.04
(2H, q, J ¼ 7.3, CH2) and 1.24 (3H, t, J ¼ 7.3, Me). 13C NMR (75
MHz, CDCl3): d ¼ 207.7 (C-1), 149.5 (C-20), 145.5 (C-30), 120.5 (C-
60), 120.0 (C-40), 119.1 (C-10), 118.9 (C-50), 31.7 (CH2) and 8.2 (Me).
4.5 Ethyl 8-methoxy-3-methyl-4-oxo-4H-chromene-2-
carboxylate (8)
Under an argon atmosphere, anhydrous triethylamine (198 mg,
1.95 mmol) and ethyl chlorooxoacetate (67 mg, 0.49 mmol) were
successively added to a stirred solution of 3 (44 mg, 0.24 mmol)
in anhydrous CH2Cl2 (1 mL), placed in a microwave vial. The
mixture was heated at 100 ꢁC in a microwave oven for 1 h; aer
its completion was conrmed by TLC, it was cooled to room
temperature and diluted with brine (10 mL). The reaction
products were extracted with EtOAc (3 ꢂ 20 mL). The combined
organic extracts were washed with brine (5 mL), dried over
MgSO4 and concentrated under reduced pressure. The resulting
residue was subjected to chromatography, giving 8 (26 mg, 42%)
as a yellowish oil. 1H NMR (300 MHz, CDCl3): d ¼ 7.75 (1H, dd, J
¼ 1.5, 8.0, H-5), 7.32 (1H, t, J ¼ 8.0, H-6), 7.18 (1H, dd, J ¼ 1.5,
8.0, H-7), 4.49 (2H, q, J ¼ 7.2, CH2Me), 4.01 (3H, s, OMe), 2.37
(3H, s, Me) and 1.46 (3H, t, J ¼ 7.2, CH2Me). 13C NMR (75 MHz,
CDCl3): d ¼ 178.8 (C-4), 161.8 (CO2Et), 148.9 (C-2), 148.7 (C-8),
146.0 (C-8a), 125.1 (C-6), 123.9(C-3), 123.5 (C-4a), 116.7 (C-5),
114.7 (C-7), 62.6 (CH2Me), 56.5 (OMe), 14.1 (CH2Me) and 10.3
(Me-3). HRMS (ESI+) calcd for C14H15O5 [M+]: 263.0914, found
263.0911.
4.8 8-Hydroxy-2,3-dimethyl-4H-chromen-4-one (9)
A solution of 4 (110 mg, 0.66 mmol) in Ac2O (2973 mg, 29.13
mmol) was treated with anhydrous NaOAc (271 mg, 3.31 mmol)
and the mixture was heated under reux for 3 h. The excess Ac2O
was distilled under reduced pressure (6 mm Hg), the residue was
suspended with AcOEt and the remainder of NaOAc was ltered
off under reduced pressure, supplying a mixture of acetates. The
acetates were treated with anhydrous Et3N (0.5 mL) and the
mixture was heated for 14 h at 115 ꢁC. Aer that period, the
reaction was cooled to room temperature, the solvent was
removed under reduced prꢁessure and the residue was treated
4.6 2-(Hydroxymethyl)-8-methoxy-3-methyl-4H-chromen-4-
one (1)
A cold (5 ꢁC) mixture of CaCl2 (13.0 mg, 0.11 mmol) and NaBH4 with 1 M HCl solution at 5 C (5 mL). The resulting suspension
(3.5 mg, 0.09 mmol) in an EtOH : THF (1 : 1 v/v, 500 mL) solvent was stirred for 6 h at 40 ꢁC and the white precipitate formed was
ꢁ
mixture was added to a stirred solution of 8 (20 mg, 0.08 mmol) ltered under reduced pressure and washed with water at 5 C.
in EtOH : THF (1 : 1 v/v, 500 mL), placed in an ice-water bath. Chromatography of the product provided 9 (96.1 mg, 76% over-
Aer 24 h, no starting material was observed by TLC and the all), as a yellowish solid, mp: 195–197 ꢁC (Lit.: 203–204 C).27 IR
ꢁ
reaction was quenched with acetone (1 mL), the solvents were (KBr, ~n): 3462, 3447, 3416, 1636, 1616, 1578, 1558, 1362, 1269,
removed under reduced pressure and the crude product was 1159, 1134, 762 and 619 cmꢀ1. 1H NMR (300 MHz, (CD3)2CO): d ¼
chromatographed, yielding 1 (6.8 mg, 41%) as a yellow solid, 9.05 (1H, s, OH), 7.59–7.51 (1H, m H-5), 7.26–7.19 (2H, m, H-6, H-
mp: 152–154 ꢁC. IR (KBr, ~n): 3397, 2955, 2916, 2849, 1734, 1717, 7), 2.45 (3H, s, Me-3) and 2.00 (3H, s, Me-2). 13C NMR (75 MHz,
1684, 1636, 1609, 1570, 1497, 1458, 1398, 1375, 1362, 1285, (CD3)2CO3): d ¼ 177.5 (C-4), 162.3 (C-2), 147.0 (C-8), 146.2 (C-8a),
1265, 1225, 1179, 1163, 1115, 1072 and 1020 cmꢀ1. 1H NMR (300 125.2 (C-6), 124.5 (C-4a), 118.9 (C-7), 117.1 (C-3), 116.1 (C-5), 18.3
MHz, CDCl3): d ¼ 7.76 (1H, dd, J ¼ 1.4, 8.0, H-5), 7.29 (1H, t, J ¼ (Me-2) and 10.0 (Me-3). HRMS (ESI+) calcd for C11H11O3 [M+]:
8.0, H-6), 7.12 (1H, dd, J ¼ 1.4, 8.0, H-7), 4.75 (2H, s, H-10), 3.98 191.0703, found 191.0705.
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reux for 3 h. The excess Ac2O was distilled under reduced
pressure (6 mm Hg), the residue was suspended with AcOEt and
the remainder of NaOAc was ltered off under reduced pres-
sure, supplying a mixture of acetates. The acetates were treated
with anhydrous Et3N (1 mL) and the mixture was heated for 14 h
at 115 ꢁC. Aer that period, the reaction was cooled to room
temperature, the solvent was removed under reduced pressure
4.9 8-Methoxy-2,3-dimethyl-4H-chromen-4-one (2)
A solution of 9 (69 mg, 0.36 mmol) and K2CO3 (70,2 mg, 0.51
mmol) in anhydrous acetone (1 mL) was heated under reux
and treated with MeI (66.9 mg, 0.47 mmol) for 24 h. Then the
solvent was evaporated, brine (10 mL) added, and the product
was extracted with EtOAc (3 ꢂ 20 mL). The combined extracts
were concentrated under reduced pressure and the residue was
chromatographed to afford 2 (70 mg, 94%), as a white solid, mp:
149–151 ꢁC (Lit.: 154 ꢁC).17c 1H NMR (300 MHz, CDCl3): d ¼ 7.72
(1H, dd, J ¼ 1.4, 8.1, H-5), 7.22 (1H, t, J ¼ 8.0, H-6), 7.07 (1H, dd, J
¼ 1.4, 7.9, H-7), 3.95 (3H, s, OMe), 2.43 (3H, s, Me-3) and 2.04
(3H, s, Me-2). 13C NMR (75 MHz, CDCl3): d ¼ 177.7 (C-4), 161.6
(C-2), 148.3 (C-8a), 146.3 (C-8), 124.1 (C-6), 123.6 (C-4a), 117.0 (C-
3), 116.8 (C-5), 113.4 (C-7), 56.2 (OMe), 18.6 (Me-2) and 10.1 (Me-
3). HRMS (ESI+) calcd for C12H13O3 [M+]: 205.0859, found
205.0859.
ꢁ
and the residue was treated with 1 M HCl solution at 5ꢁ C (40
mL). The resulting suspension was stirred for 6 h at 40 C and
the precipitate formed was ltered under reduced pressure and
washed with water at 5 ꢁC. Chromatography of the product
provided 13 (880 mg, 77% overall) as a yellowish solid, mp: 280–
282 ꢁC (Lit.: 281 ꢁC).28 IR (KBr, ~n): 3993, 2924, 2853, 1653, 1558,
1
1541, 1458, 1246, 1101, 860 and 689 cmꢀ1. H NMR (300 MHz,
acetone-d6): d ¼ 9.7 (1H, bs, w1/2 ¼ 42, OH), 7.94 (1H, d, J ¼ 8.7,
H-5), 6.81 (1H, dd, J ¼ 2.2, 8.7, H-6), 6.71 (1H, d, J ¼ 2.2, H-8),
2.31 (3H, d, J ¼ 0.6, Me-2) and 1.96 (3H, d, J ¼ 0.6, Me-3). 13C
NMR (75 MHz, acetone-d6): d ¼ 177.4 (C-4), 163.2 (C-7), 162.3 (C-
2), 158.5 (C-8a), 127.9 (C-5), 116.6 (C-3), 116.4 (C-4a), 115.3 (C-6),
102.7 (C-8), 18.4 (Me-2) and 9.9 (Me-3).7c
4.10 8-Methoxy-3-methyl-4-oxo-4H-chromene-2-
carbaldehyde (10)
A mixture of 2 (52.9 mg, 0.26 mmol), I2 (13.15 mg, 0.10 mmol),
TsOH (44.6 mg, 0.26 mmol) in DMSO (1 mL) was heated at
130 ꢁC under oxygen atmosphere for 2 h. Aer this time, the
reaction solution was washed with saturated NH4Cl (5 mL) and
extracted with CH2Cl2 (3 ꢂ 5 mL), the organic phase was dried
with MgSO4, ltered and the remaining DMSO was distilled
under reduced pressure evaporated to give a crude product. The
crude product was chromatographed to afford 10 (47 mg, 83%),
as a yellow solid, mp: 150–152 ꢁC. IR (KBr, ~n): 3539, 3420, 3233,
1724, 1636, 1622, 1605, 1582, 1493, 1458, 1441, 1406, 1377,
1356, 1271, 1227, 1194, 1159, 1121, 1076, 1001, 918, 824 and
4.13 7-Methoxy-2,3-dimethyl-4H-chromen-4-one (14)
A solution of 13 (800 mg, 4.21 mmol) and K2CO3 (813.8 mg, 5.89
mmol) in anhydrous acetone (20 mL) was heated under reux
and treated with MeI (776 mg, 5.47 mmol) for 24 h. Then the
solvent was evaporated, brine added, and the product was
extracted with EtOAc. The combined extracts were concentrated
under reduced pressure and the residue was chromatographed
to afford 14 (795 mg, 93%) as a as a yellowish solid,21 mp: 125–
127 (Lit.: 127 ꢁC).29 IR (KBr, ~n): 3420, 2924, 1636, 1605, 1443,
1404, 1352, 1244, 1204, 1111, 1030, 856, 824 and 689 cmꢀ1. 1H
NMR (300 MHz, CDCl3): d ¼ 8.09 (1H, d, J ¼ 8.8, H-5), 6.92 (1H,
dd, J ¼ 2.5, 8.8, H-6), 6.77 (1H, d, J ¼ 2.5, H-8), 3.88 (3H, s, OMe),
2.38 (3H, d, J ¼ 0.5, Me-3) and 2.04 (3H, d, J ¼ 0.5, Me-2). 13C
NMR (75 MHz, CDCl3): d ¼ 177.4 (C-4), 163.5 (C-7), 161.2 (C-2),
157.6 (C-8a), 127.2 (C-5), 116.6 (C-3), 116.6 (C-4a), 113.9 (C-6),
99.7 (C-8), 55.7 (OMe), 18.4 (Me-2) and 10.0 (Me-3).
1
752 cmꢀ1. H NMR (300 MHz, CDCl3): d ¼ 10.20 (1H, s, CHO),
7.73 (1H, dd, J ¼ 1.2, 8.4, H-5), 7.33 (1H, t, J ¼ 8.1, H-6), 7.20 (1H,
dd, J ¼ 1.4, 8.0, H-7), 4.01 (3H, s, OMe) and 2.43 (3H, s, Me-2).
13C NMR (75 MHz, CDCl3): d ¼ 186.5 (C-10), 179.2 (C-4), 150.5
(C-2), 149.1 (C-8), 145.9 (C-8a), 125.3 (C-6, C-3), 123.6 (C-4a),
116.6 (C-5), 114.9 (C-7), 56.4 (OMe) and 8.2 (Me-2). HRMS
(ESI+) calcd for C12H11O4 [M+]: 219.0652, found 219.0652.
4.11 2-(Hydroxymethyl)-8-methoxy-3-methyl-4H-chromen-4-
one (1)
4.14 7-Methoxy-3-methyl-4-oxo-4H-chromene-2-
carbaldehyde (15)
To a solution of 13 (24 mg, 0.11 mmol) in EtOH (1 mL) was
added NaBH4 (5.0 mg, 0.13 mmol) under an argon atmosphere
at 0 ꢁC and stirred the reaction mixture at RT for 1 h. Aer
completion of the reaction, the reaction mixture was quenched
with acetone (1 mL). Then, acetone was distilled under reduced
pressure evaporated and the crude product was chromato-
graphed to afford chromanone A (1, 17 mg, 70%), as a yellow
solid. The NMR spectroscopic data of this compound exhibited
concordance with those recorded for compound A3 (Fig. 1) and
for 1 when obtained from 5.
A mixture of 14 (20.4 mg, 0.10 mmol), I2 (5.07 mg, 0.04 mmol)
and TsOH (17.2 mg, 0.10 mmol) was heated at 130 ꢁC under an
oxygen atmosphere for 3 h in DMSO (1 mL). Aer this time, the
reaction was diluted with saturated NH4Cl solution (10 mL) and
the product was extracted with CH2Cl2 (3 ꢂ 10 mL). The
combined organic phases were dried with MgSO4, and
concentrated under reduced pressure. The remaining DMSO
was distilled under vacuum and the crude product was chro-
matographed to afford 15 (17 mg, 78%) as a yellowish solid,22
mp: 118–121 ꢁC. IR (KBr, ~n): 2920, 2849, 1699, 1624, 1443, 1281,
1
1261, 1206, 1159, 1111, 1020, 851, 768 and 623 cmꢀ1. H NMR
4.12 7-Hydroxy-2,3-dimethyl-4H-chromen-4-one (13)
(300 MHz, CDCl3): d ¼ 10.20 (1H, s, CHO), 8.11 (1H, d, J ¼ 8.9, H-
A solution of 2,4-dihydroxypropiophenone (12, 1000 mg, 6.0 5), 6.99 (1H, dd, J ¼ 2.4, 8.9, H-6), 6.93 (1H, d, J ¼ 2.4, H-8), 3.92
mmol) in Ac2O (25 mL, 265 mmol) was treated with anhydrous (3H, s, OMe) and 2.44 (3H, s, Me-3). 13C NMR (75 MHz, CDCl3):
NaOAc (2468 mg, 30.1 mmol) and the mixture was heated under d ¼ 185.7 (C-10), 178.3 (C-4), 165.1 (C-7), 157.2 (C-8a), 150.6 (C-2),
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´
Ruano, R. U. Gutierrez, F. E. Jimenez-Montejo,
127.4 (C-5), 126.6 (C-3), 116.8 (C-4a), 115.7 (C-6), 99.9 (C-8), 55.9
(OMe) and 8.0 (Me-3).
´
´
M. J. Fragoso-Vazquez, J. Correa-Basurto, M. C. Cruz-Lopez,
F. Delgado and J. Tamariz, Med. Chem. Res., 2019, 28, 831–
848.
4.15 2-(Hydroxymethyl)-7-methoxy-3-methyl-4H-chromen-4-
one (16)
3 A. M. Gamal-Eldeen, A. Abdel-Lateff and T. Okino, Environ.
Toxicol. Pharmacol., 2009, 28, 317–322.
A stirred solution of 13 (10.5 mg, 0.05 mmol) in EtOH (1 mL)
under an argon atmosphere was cooled to 0 ꢁC and treated with
NaBH4 (2.73 mg, 0.07 mmol). The reaction mixture was further
stirred at RT for 1 h, when it was quenched with acetone (1 mL).
The volatiles were removed under reduced pressure and the
crude product was chromatographed to afford 13 (7.5 mg, 71%),
as a white solid, mp: 122–125 ꢁC. IR (KBr, ~n): 3323, 2924, 2853,
1638, 1597, 1449, 1244, 1177, 1042, 1024 and 827 cmꢀ1. 1H NMR
(300 MHz, CDCl3): d ¼ 8.06 (1H, d, J ¼ 8.9, H-5), 6.92 (1H, dd, J ¼
2.4, 8.9, H-6), 6.74 (1H, d, J ¼ 2.4, H-8), 4.67 (2H, s, H-10), 3.87
(3H, s, OMe), 2.75 (1H, bs, w1/2 ¼ 16, OH) and 2.07 (3H, s, Me-3).
13C NMR (75 MHz, CDCl3): d ¼ 178.0 (C-4), 163.9 (C-7), 160.5 (C-
2), 157.5 (C-8a), 127.2 (C-5), 117.3 (C-3), 116.4 (C-4a), 114.5 (C-6),
99.6 (C-8), 60.5 (C-10), 55.7 (OMe) and 9.2 (Me-3). The spectro-
scopic data of this compound were in full agreement with those
recorded for the natural product (compound C).4b
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´
7 (a) I. Cortes, C. Borini Etichetti, J. Girardini, T. S. Kaufman
All the authors were involved in the manuscript, performing
conceptualization (IC, TSK, ABJB, LAS), data curation (IC, MAS,
LAS), formal analysis (IC, EC, TSK, ABJB), funding acquisition
(TSK, ABJB, MAS, LAS), investigation (IC, EC, TSK, ABJB),
methodology (IC, EC, TSK, LAS, MAS, ABJB), project adminis-
tration (TSK, ABJB, LAS, MAS), resources (TSK, ABJB, LAS, MAS),
use of soware (IC, TSK, ABJB), supervision (ABJB, TSK, MAS,
LAS), results validation (TSK), results visualization (IC, EC,
TSK), writing – original dra (IC, EC, TSK, ABJB) and writing –
review & editing (IC, EC, TSK, LAS, MAS, ABJB).
and A. B. J. Bracca, Synthesis, 2019, 51, 433–440; (b)
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors gratefully acknowledge Consejo Nacional de
´
´
Investigaciones Cientıcas y Tecnicas (CONICET, PUE IQUIR
´
´
2016) and Agencia Nacional de Promocion Cientıca y Tec-
´
nologica (ANPCyT, PICT 2017-0149 and PICT 2016-1833) for
nancial support. IC and EC also thank CONICET for their
Doctoral fellowships.
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RSC Adv., 2021, 11, 19587–19597 | 19597