Yicathins B and C and Analogues: Total Synthesis, Lipophilicity and Biological Activities

Article abstract of DOI:10.1002/cmdc.201900735

Natural products have always been an important source of new hits and leads in drug discovery, with the marine environment being regarded as a significant source of novel and exquisite bioactive compounds. Yicathins B and C are two marine-derived xanthones that have shown antibacterial and antifungal activity. Herein, the total synthesis of these yicathins and six novel analogues is reported for the first time. As marine natural products tend to have very lipophilic scaffolds, the lipophilicity of yicathins and their analogues was evaluated in the classical octanol/water system and a biomimetic model-based system. As the xanthonic nucleus is a “privileged structure”, other biological activities were evaluated, namely antitumor and anti-inflammatory activities. An interesting anti-inflammatory activity was identified for yicathin analogues that paves the way for the design of dual activity (anti-infective and anti-inflammatory) marine-inspired xanthone derivatives.

Full text of DOI:10.1002/cmdc.201900735

Accepted Article
Title: Yicathins B and C and analogues: total synthesis, lipophilicity and
biological activities
Authors: Daniela R. P. Loureiro, Álvaro F. Magalhães, José X.
Soares, Joana Pinto, Carlos M. G. Azevedo, Sara Vieira, Ana
Henriques, Helena Ferreira, Nuno Neves, Hassan Bousbaa,
Salette Reis, Carlos M. M. Afonso, and Madalena M. M. Pinto
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of the final Version of Record (VoR). This work is currently citable by
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemMedChem 10.1002/cmdc.201900735
Link to VoR: http://dx.doi.org/10.1002/cmdc.201900735
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ChemMedChem
FULL PAPER
Yicathins B and C and analogues: total synthesis, lipophilicity
and biological activities
Daniela R. P. Loureiro,+[a,b] Álvaro F. Magalhães,+[a] José X. Soares,+[c] Joana Pinto,[a] Carlos M. G.
Azevedo,[a] Sara Vieira,[d,e] Ana Henriques, [f] Helena Ferreira, [d,e] Nuno Neves, [d,e,g] Hassan Bousbaa, [f]
Salette Reis,[c] Carlos M. M. Afonso,*[a,b] and Madalena M. M. Pinto,[a,b]
[a]
D. R. P. Loureiro+, Á. F. Magalhães+, Dr. J. Pinto, Dr. C. M. G. Azevedo, Prof. Dr. C. M. M. Afonso, Prof. Dr. M. M. M. Pinto
Department of Chemical Sciences, Laboratory of Organic and Pharmaceutical Chemistry
Faculty of Pharmacy, University of Porto
Rua de Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal
E-mail: cafonso@ff.up.pt
[b]
D. R. P. Loureiro+, Prof. Dr. C. M. M. Afonso, Prof. Dr. M. M. M. Pinto
Interdisciplinary Center of Marine and Environmental Investigation (CIIMAR/CIMAR)
Edifício do Terminal de Cruzeiros do Porto de Leixões, Av. General Norton de Matos s/n, 4050-208 Matosinhos, Porto, Portugal
J. X. Soares+, Prof. Dr. S. Reis
LAQV-REQUIMTE, Department of Chemical Sciences, Laboratory of Applied Chemistry
Faculty of Pharmacy, University of Porto
[c]
[d]
Rua de Jorge Viterbo Ferreira, 228, 4050-313 Porto, Portugal
S. Vieira, Dr. H. Ferreira, Dr. N. Neves
3B's Research Group, I3Bs – Research Institute on Biomaterials, Biodegradables and Biomimetics
University of Minho
Headquarters of the European Institute of Excellence on Tissue Engineering and Regenerative Medicine, AvePark, Parque de Ciência e Tecnologia, Zona
Industrial da Gandra, 4805-017 Barco, Guimarães, Portugal
S. Vieira, Dr. H. Ferreira, Dr. N. Neves
ICVS/3B’s–PT Government Associate Laboratory
Braga/Guimarães, Portugal
[e]
[f]
A. Henriques, Dr. H. Bousbaa
CESPU, Instituto de Investigação e Formação Avançada em Ciências e Tecnologias da Saúde
Rua Central de Gandra, 1317, 4585-116 Gandra, PRD, Portugal
Dr. N. Neves
The Discoveries Centre for Regenerative and Precision Medicine
Headquarters at University of Minho, Avepark, 4805-017 Barco, Guimarães, Portugal
These authors contributed equally to this work.
[g]
[+]
Supporting information for this article is given via a link at the end of the document.
Abstract: Natural products had always be an important source of new
activities, making them a valuable source of new hits, leads and
drugs.[2]
hits and leads in drug discovery. The marine environment has been
regarded as a significant souce of novel and exquisite bioactive
compounds. Yicathins B and C are two marine derived xanthones that
have shown antibacterial and antifungal activities. Herein, the total
synthesis of these yicathins is reported for the first time as well as six
novel analogues. As marine natural products tend to bear very
lipophilic scaffolds, the lipophilicity of yicathins and its analogues was
evaluated using the classical octanol:water system and a biomimetic
model based system. As the xanthonic nucleus is a “privileged
structure”, other biological activities were evaluated, namely antitumor
and anti-inflammatory activities. An interesting anti-inflammatory
activity was identified for yicathins analogues that paves the way for
the design of dual activity (anti-infective and anti-inflammatory) marine
inspired xanthones derivatives.
Xanthones are O-heterocyclic compounds with the dibenzo-γ-
pyrone scaffold (Figure 1, nucleus highlighted in blue). Xanthones
are widely distributed in Nature, including the marine
environment.[3] Marine xanthones derivatives are mostly isolated
from fungi and bacteria, which live in symbiotic relationships with
micro- and macroorganisms.[3-4]
Introduction
Marine microorganisms are able to develop different and original
pathways to the biosynthesis of secondary metabolites, providing
novel scaffolds which are quite different from those found in
terrestrial sources.[1] In addition to its chemical uniqueness, these
kind of metabolites frequently show important pharmacological
Figure 1. Structure of yicathin B (1) and yicathin C (2) and its six analogues 3,
4, 5, 6, 7, and 8. Xanthonic scaffold highlighted in blue.
1
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As “privileged structures” in Medicinal Chemistry, xanthone
scaffold allows a variety of substitution patterns that lead to
diverse pharmacological activities, such as anti-infective,
anticancer, and anti-inflammatory activities.[5] Yicathin B (1) and
yicathin C (2) (Figure 1) are two paradigmatic examples of
bioactive marine xanthones which have shown a quite interesting
antibacterial and antifungal activity.[6] In addition to the reported
wide range of biological activities, xanthone derivatives can also
have dual activity. For instance, marine xanthones sharing
structural similarity to yicathins have been reported showing
antimicrobial and antitumor and/or anti-inflammatory activities.[7]
Marine natural products (MNPs) are usually difficult to access and
the extraction provides relatively small amounts.[8] Laboratory
synthesis can surpass the supply issues, and additionally allows
the obtention of different substitution patterns besides those
furnished by Nature.[9]
Bioactive natural products are often potent, but present
inadequate ADMET properties. Lipophilicity is one of the most
important biophysicochemical properties affecting the ADMET.[10]
In the case of MNPs, this property requires even more attention,
as these scaffolds tend to be very lipophilic.[1] Traditionally,
lipophilicity is evaluated by the partition coefficients between
octanol and aqueous systems (Log P and Log D).[11] However,
biomimetic models, such as micelles, have proved to be an
advantageous alternative to the classical octanol-based systems,
as they take into account all the involved interactions,
hydrophobic and electrostatic, that occur in the partition of a drug
with biological membranes.[12]
Building blocks 9, 10, 11, 12 and 13 were used for the synthesis
of yicathins and its analogues (Scheme 2). Building block 9 was
obtained in two steps from 4-bromo-3,5-dimethoxybenzoic acid
14. This acid was reduced with BH3:THF to the alcohol 15,[14]
which was subsequently protected with TBDMS-Cl to achieve
compound 9 (Scheme 2 a, b).[15] The building blocks 10 and 11,
two benzaldehydes, were obtained in two steps from orcinol 16
and resorcinol 17, respectively. The phenolic groups were
protected with MOM-Cl,[16] followed by an arylformylation
(Scheme 2 c, d).[16] Other protecting groups were also explored
(MEM and Bn), but lower yields were obtained (data not shown).
The building block 12 was obtained by protecting the phenol
groups of methyl 2,6-dihydroxy-4-methylbenzoate 20 (Scheme 2
f).[16] The building block 13 was synthetized in two steps from 2,6-
dihydroxybenzoic acid 21, which was esterified to the methyl ester
22,[17] followed by protection with MOM-Cl to provide 13 (Scheme
2 e, f).[16]
Herein, we report the first total synthesis of yicathin B (1) and
yicathin C (2), along with six analogues (3, 4, 5, 6, 7, 8) (Figure
1). Their lipophilicity was assessed in silico using the classical
octanol model (Log P and Log D7.4 values) and experimentally
using the micelles biomimetic model (Log KP 7.4 values). As the
xanthonic nucleus can interact with more than one target,[5b] in
vitro antitumor and anti-inflammatory activities were evaluated.
Scheme 2. Synthesis of building blocks 9, 10, 11, 12 and 13. Reagents and
conditions: a) BH3:THF, THF, 99%; b) TBDMS-Cl, Imidazole, DMF, 88 %; c)
MOM-Cl, NaH, DMF, 5: 91%, 10: 95%; d) n-BuLi, TMEDA, THF, DMF, 10: 79%,
11: 85%; e) (CH3O)2SO2, K2CO3, acetone, 76%; f) MOM-Cl, NaH, DMF, 12:
72 %, 13: 71 %.
Results and Discussion
Retrosynthetic analysis used to plan the synthesis of yicathins B
and C is based on a benzophenone intermediate, which are
frequent important precursors of xanthones.[13] The synthetic
pathway that requires benzophenone 7, which was converted to
the target yicathins (1 and 2) (Scheme 1). This benzophenone
was obtained by a convergent synthesis using suitable building
blocks (Scheme 1). In addition, six yicathins analogues (3, 4, 5, 6,
7, 8) were also prepared using this synthetic route.
Using these building blocks, yicathins and its analogues were
synthesized accordingly to Scheme 3. The acyl substitution of the
benzaldehydes 10 and 11 by an aryllithium intermediate,
prepared in situ from 9, yielded the diarylmethanol derivatives 23
and 24 (Scheme 3 a).[18] These diarylmethanol derivatives were
then oxidized to the benzophenones 25 and 26 using DMP as
19]
oxidant reagent (Scheme 3 b).[18a,
This oxidation was also
performed with another oxidant, MnO2, but the main formed
products were the result of the cleavage of the precursor
diarylmethanol (data not shown). The acyl substitution of the
methyl esters 12 and 13 by the aryllithium intermediate, prepared
in situ from 9, yielded the benzophenones 25 and 26 (Scheme 3
c).[18] This one-step pathway provided the desired compounds in
lower yields, which can be explained by the low carbon
electrophilicity of the esters 12 and 13. Benzophenones 25 and
26 were then deprotected, under acidic conditions, resulting the
Scheme 1. Retrosynthesis of yicathins B (1) and C (2). SNAr: aromatic
nucleophilic substitution; SNAc: acyl nucleophilic substitution.
benzophenones
7 and 8 (Scheme 3
d).[20] Subsequently,
compounds 5 and 6 were synthesized by a microwave assisted
intramolecular nucleophilic aromatic substitution of 7 and 8
2
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(Scheme 3 e).[21] Yicathin C (2) and analogue 4 were provided by
oxidation of 5 and 6, respectively, using a modified Jones reagent
(Scheme 3 f).[22] Finally, yicathin B (1) and analogue 3 were
obtained by Fisher esterification of the carboxylic acids 2 and 4
(Scheme 3 g).[23]
reflected on the high standard deviation values (up to 1 unit). Log
P and Log D7.4 values were similar for all compounds, with
exception of the compounds 2 and 4, whose Log D7.4 were low.
This is due to the deprotonation of the carboxylic acid group of 2
and 4 with consequent reduction of the free acid concentration.
Log KP 7.4 values were higher than Log P and Log D7.4 values.
Compound 2 (Log KP 7.4 3.48 vs. Log D7.4 0.2) illustrates well the
difference between biomimetic and classical model. This is
ascribed to the fact that micelles system is able to take into
account not only the hydrophobic interactions, but also the
electrostatic interactions, which are not evaluated by classical
octanol system. In accordance, the electrostatic interactions play
a minor role on the partition of the more hydrophobic compounds,
like 1 and 3. Log KP 7.4 values for all evaluated compounds were
within the limits preconized for lipophilicity by the most common
“drug-like” guidelines.[25]
Figure 2. Predicted Log P and Log D7.4 and experimentally determined Log KP
7.4 for yicathins and its analogues (1-8). Error bars in Log P and Log D7.4 indicate
mean ± SD of predicted values by fifteen in silico methods. Error bars in Log KP
7.4 indicate mean ± SD of three replicates.
The cell growth inhibitory activities of yicathins and derivatives (1
to 8) were evaluated for their in vitro growth inhibitory effect on
three human tumor cell lines: A375 (melanoma), MCF-7 (breast
adenocarcinoma), and NCI-H460 (non-small-cell lung cancer)
(Table 1). All the compounds required higher than 150 µM to
reduce growth rates to 50% (GI50) in human cell lines tested,
namely, compounds 7 and 6. Overall, melanoma A376 cell line
was more sensitive to the test compounds than MCF-7 and NCI-
H460 cell lines. Compound 2 displayed the greatest growth
inhibitory activity in the three human cells lines studied (range
between 48.70 to 86.21 μM). Comparison of the GI50 values of the
compounds 7 and 8 (benzophenone analogues), seems to
indicate that the absence of a methyl group (R = H) of compound
8 is associated with an increase in the growth inhibitory activity.
Scheme 3. Synthesis of yicathins and analogues. Reagents and conditions: a)
1. n-BuLi, THF, -78 ºC, 2. 10 or 11, -78 ºC to rt, 23: 53%, 24: 87%; b) 1. n-BuLi,
THF, -78 ºC, 2. 12 or 13, -78 ºC to rt, 25: 21%, 26: 19%; c) DMP, DCM, 25: 90%,
26: 87%; d) p-TsOH, MeOH, 7: 45%, 8: 39%; e) MW, NaOH, H2O/MeOH, 130
ºC, 5: 93%, 6: 93%; f) H5IO6/CrO3, wet ACN, 2: 48%, 4: 57%; g) H2SO4, MeOH,
1: 45%, 3: 46%
Log P and Log D7.4 values of yicathins and its analogues (1-8)
were predicted using different in silico approaches (Supporting
Information, Table S1 and S2). The biomimetic partition
coefficients of yicathins and its analogues (1-8), Log KP 7.4, were
experimentally measured using micelles as lipidic phase and
phosphate-buffered saline buffer as aqueous phase (PBS: 10
mmol L-1, I = 0.15 mol L-1, pH 7.4).[12a] The determination of Log
KP 7.4 was performed by derivative spectrophotometry (Table S3
and Figure S22, Supporting Information).[24] Figure 2 summarizes
the in silico Log P and Log D7.4, and the experimental Log KP 7.4 in
micelles/buffer. The results obtained for each compound using
different in silico methods provided very different values, which is
Table 1. GI50 concentrations (µM) of the synthesized compounds in A375, MCF-
7 and NCI-H460 cell lines.
GI50 ± SD (µM)[a]
Compd
A375
MCF7
NCI-H460
1
2
47.70 ± 2.62
48.70 ± 4.24
79.83 ± 18.45
86.21 ± 2.30
98.93 ± 9.83
73.92 ± 2.28
3
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cells (without LPS), which can be related with their anti-
inflammatory activity. These results were corroborated by DNA
quantification analyses, as a similar amount of this nucleic acid
was obtained for non-stimulated or LPS-stimulated macrophages
in the presence of the different compounds.The stimulation of
macrophages with LPS (100 ng/mL) led to a significant IL-6
production (non-stimulated macrophages did not produce this
pro-inflammatory cytokine; Figure 3C). However, the amount of
this cytokine in the medium was significantly reduced in the
presence of some of the tested compounds. Compound 7
presented the strongest anti-inflammatory activity (higher IL-6
reduction), followed by 5, booth showing a significant reduction
with a minimum concentration of 5 μM. In spite of lower
concentrations of 6 and 3 did not present anti-inflammatory
activity in this in vitro scenario, its higher concentration (20 μM)
presented the highest IL-6 reduction, followed by 2, 5 and 7
(Figure 3C). The compounds 1, 4 and 8 were not effective in the
reduction of IL-6, even for the highest tested concentration (20
μM). Dexamethasone (10 μM) was the most efficient control in
reducing the IL-6 production. Celecoxib (10 μM) and diclofenac
(10 μM) also led to a significant reduction of the IL-6 amount, while
salicylic acid (10 μM) did not show ability to significantly reduce
the IL-6 concentration in this in vitro inflammatory model. None of
the tested compounds showed a similar anti-inflammatory activity
of dexamethasone (corticosteroid). However, the compounds in
some concentrations led to a higher decrease of the pro-
inflammatory cytokine amount than well-known nonsteroidal anti-
inflammatory drugs (NSAIDs), namely diclofenac, salicylic acid
and celecoxib.
3
61.59 ± 7.25
110.95 ± 34.47
115.94 ± 16.95
87.57 ± 9.23
>150
100.78 ± 1.64
134.36 ± 13.65
101.54 ± 7.53
>150
4
74.40 ± 1.23
55.71 ± 4.16
102.76 ± 3.90
137.12 ± 1.07
63.61 ± 1.70
0.010 ± 0.002
5
6
7
>150
138.65 ± 0.92
74.74 ± 2.74
0.018 ± 0.005
8
105.32 ± 0.45
0.012 ± 0.001
Doxorubicin
[a] Results are expressed as mean ± SD. Doxorubicin was used as positive
control
The anti-inflammatory activity of yicathins and its analogues was
evaluated based on their capacity to decrease the concentration
of the pro-inflammatory cytokine IL-6 on LPS-stimulated
macrophages. Figure 3 presents the metabolic activity, DNA
concentration and IL-6 amount obtained for LPS-stimulated
macrophages in the absence or in the presence of the different
compounds.
Conclusions
Total synthesis of yicathin B (1), yicathin C (2), and six new
yicathins analogues was described for the first time. The desired
compounds were obtained with
a suitable overall yield.
Lipophilicity of yicathins and its analogues was evaluated using in
silico and experimental biomimetic methodologies. The obtained
partition coefficients were quite different, namely for ionized
compounds at physiological pH. Nevertheless, the lipophilicity of
the synthetized compounds were within the limits preconized by
the most common “drug-like” guidelines. Additional biological
activities were explored for the yicathins and its analogues.
Antitumor activity was screened in three human tumor cell lines,
but the compounds did not revealed a significant ability to inhibit
cell growth, being 48.70 μM the lowest found value of GI50.
Nevertheless, compounds 2, 5, and 7 shown a significant in vitro
anti-inflammatory activity, which was comparable with well-known
NSAIDs, like diclofenac and celecoxib. Unfortunately, a detailed
and ascertain SAR cannot be established due to the limited
number of compounds. However, one hit compound was
identified which are being currently explored by us in the design
of dual activity (anti-infective and anti-inflammatory) marine
inspired xanthones derivatives.
Figure 3. Metabolic activity (A), relative DNA concentration (B) and IL-6
cytokine percentages (C) of LPS-activated macrophages cultured in the
presence of different concentrations of the tested compounds and clinically
used anti-inflammatory drugs (dexamethasone, diclofenac, salicylic acid and
celecoxib) for 24 h of culture at 37 °C. Results are expressed as mean ± SD
and statistically significant differences are * (p < 0.0471), ** (p < 0.0077), *** (p
< 0.0007), **** (p < 0.0001) in comparison to the LPS-stimulated control (0 μM)
for each different tested compound.
The compounds were not cytotoxic for all tested concentrations
as the cell metabolic activity was not affected (Figure 3A) and the Experimental Section
DNA was preserved in the presence of the different compounds
General. All reagents and solvents were purchased from TCI (Tokyo
Chemical Industry Co. Ltd., Chuo-ku, Tokyo, Japan), Acros Organics (Geel,
Belgium), Sigma-Aldrich (Sigma-Aldrich Co. Ltd., UK), or Alfa Aesar
in an inflammatory scenario (Figure 3B). Additionally, all the
compounds, with exception of some tested conditions, allowed
cells to reach the metabolic activity state of the non-stimulated
4
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(Thermo Fisher GmbH, Kandel, Germany) and were used directly without
any further purification. Anhydrous solvents were dried according to the
published procedures.[26] All reactions were monitored by TLC, carried out
on Merck silica gel 60 (GF254) precoated plates using with appropriate
mobile phases, and/or by GC-MS. Purification of the synthesized
compounds was usually performed by flash column chromatography using
Merck silica gel 60 (0.040–0.063 mm). MW reactions were performed in a
CEM Discovery SP® from CEM Corporation (Matthews, NC, US). All MW
reactions were performed in a closed vessel at fixed temperature, which
was monitored by an integrated infrared sensor. Melting points (mp) were
measured in a Köfler microscope (Wagner and Munz, Munich, Germany)
equipped with a Crison TM 65 (Crison Instruments, Barcelona, Spain) and
were uncorrected. IR spectra were measured on an ATI Mattson Genesis
series FTIR (software: WinFirst v. 2.10) spectrophotometer in KBr
microplates (cm-1). EIMS spectra were recorded on a ThermoQuest
Finnigan GC 2000 series/GCQ plus. Injections were performed using
compounds directly dissolved in ethyl acetate or previously derivatized
with MSTFA at 80 °C for 30 minutes. HRMS spectra were recorded as ESI
(electrospray ionization) mode either on VG Autoespec MicroTOF FOCUS
spectrometer (Bruker Daltonics, Bremen, Germany) or on LTQ OrbitrapTM
XL hybrid mass spectrometer (Thermo Fischer Scientific, Bremen,
Germany). 1H and 13C NMR spectra were taken in CDCl3 or DMSO-d6
(Deutero GmbH, Kastellaun, Germany) at room temperature on Bruker
Avance 300 instrument (300.13 or 500.16 MHz for 1H and 75.47 or 125.77
MHz for 13C, Bruker Biosciences Corporation, Billerica, MA, USA) or
Bruker AVANCE III (400.14 MHz for 1H and 100.62 MHz for 13C). Chemical
shifts are expressed in δ (ppm) values relative to tetramethylsilane (TMS)
round-bottom flask, compound 5 (50 mg, 0.18 mmol) was dissolved in 35
mL of wet ACN (75 %) and 10 mL of the H5IO6/CrO3 solution was added.
The reaction mixture stirred for 26 hours at room temperature. NaHSO3
was added and the solution was extracted with chloroform (3 x 15 mL).
The organic layer was washed with brine, dried over Na2SO4, filtered and
the solvent evaporated. The crude product was purified by silica gel flash
chromatography (hexane/EtOAc/formic acid 6:4:0.1). Compound 2 was
obtained as a yellow solid (50 mg, 48 %).
°
Yicathin C (2): yellow solid; mp. 204–205 C; IR (KBr): ꢀꢁ = 3446, 2924,
2853, 1718, 1652, 1615, 1559, 1478, 1419, 1386, 1329, 1301, 1250, 1203,
1141, 1115, 1087, 1006, 895, 824, 770 cm-1; 1H NMR (500.16 MHz,
DMSO-d6): d = 13.85 (s, OH), 12.74 (s, OH), 7.54 (d, J = 1.4 Hz, H), 7.36
(d, J = 1.4 Hz, H), 6.83 (q, J = 1.3 Hz, H), 6.63 (q, J = 1.3 Hz, H), 3.93 (s,
3H), 2.39 (s, 3H); 13C NMR (125.77 MHz, DMSO-d6): d = 180.8, 164.8,
160.9, 160.4, 157.0, 154.7, 148.9, 135.9, 112.9, 111.4, 110.2, 107.1, 107.0,
105.8, 53.0, 21.9; EIMS: m/z 445 (0.5, [M+TMS]+.), 429 (10), 369 (10),313
(22), 312 (100), 297 (21), 283 (33), 241 (15), 217 (15); HRMS (ESI): m/z
calcd for C16H11O6 [M-H]-: 299.05611; found: 299.05606.
Compound 4: yellow solid (30 mg, 57 %); mp: 238–240 °C; IR (KBr):
ꢀꢁ = 3447, 2922, 1708, 1654, 1594, 1473, 1384, 1352, 1261, 1100 cm-1; 1H
NMR (300.16 MHz, CDCl3): d = 12.99 (s, OH), 12.96 (s, COOH), 7.38 (t, J
= 8.3 Hz, H), 7.34 (s, H), 7.12 (s, H), 6.64 (d, J = 8.4 Hz, H), 6.28 (d, J =
8.2 Hz, H), 3.80 (s, 3H) ;13C NMR (125.77 MHz, DMSO-d6): d = 181.8,
165.8, 161.3, 159.6, 157.0, 155.2, 136.5, 129.4, 110.0, 109.9, 108.8, 107.2,
106.6, 106.5, 56.0; EIMS: m/z 430 (0.5, [M+TMS]+•), 418 (2), 417 (14),
(416 (32), 415 (100), 400 (11), 372 (2), 357 (8), 355 (12), 299 (16), 298
(58), 283 (19), 269 (18), 253 (14), 227 (13), 187 (12), 165 (7), 74 (6), 73
(8); HRMS (ESI): m/z calcd for C15H9O6 [M-H]-: 285.04046; found:
285.04129.
as an internal reference. Coupling constants are reported in hertz (Hz).13
C
NMR assignments were made by 2D HSQC and HMBC experiments.
Synthesis of Yicathin B (1) and methyl 8-hydroxy-1-methoxy-6-
methyl-9-oxo-9H-xanthene-3-carboxylate (3): In a round-bottom flask,
compound 2 (140 mg, 466 µmol) was dissolved in 10 mL of methanol and
H2SO4 (78 µL, 1.40 mmol) was added. The reaction mixture was heated
under reflux overnight. Water and NaHCO3 were added to the flask and
the solution extracted with ethyl acetate (3 x 5 mL). The organic layer was
washed with brine, dried over Na2SO4, filtered and the solvent evaporated.
The crude product was purified by crystallization with DCM and MeOH.
Compound 1 was obtained as a yellow solid (66 mg, 45 %).
Synthesis of 1-hydroxy-6-(hydroxymethyl)-8-methoxy-3-methyl-9H-
xanthen-9-one (5) and 8-hydroxy-3-(hydroxymethyl)-1-methoxy-9H-
xanthen-9-one (6): In a typical experiment, 4 mL of a solution of NaOH
2M in H2O/MeOH (9:1) was added to compound 7 (210 mg, 660 µmol) in
a 10 mL MW reaction vessel. The reaction mixture was microwave heated
at 130 °C for 5 minutes. The mixture was cooled to room temperature, and
HCl 5% were added until a pH of 4-5. The precipitate was collected by
filtration under reduced pressure. Compound 5 was obtained as a pale
yellow solid (175 mg, 93 %).
Yicathin B (1): yellow solid; mp: 155–156 °C; IR (KBr): ꢀꢁ = 3004, 2955,
2920, 2850, 1735, 1659, 1564, 1508, 1474, 1428, 1360, 1333, 1305, 1272,
1242, 1209, 1189, 1155, 1139, 1114, 1090, 989, 880, 835, 823, 807 cm-1;
1H NMR (400.14 MHz, DMSO-d6): d = 12.73 (s, OH), 7.56 (s, 1H), 7.37 (s,
1H), 6.79 (s, 1H), 6.60 (s, 1H), 4.00 (s, 3H, OCH3), 3.94 (s, 3H, OCH3),
2.40 (s, 3H); 13C-NMR (100.63 MHz, DMSO-d6): d = 180.8, 164.8, 160.8,
160.4, 157.0, 154.7, 148.9, 135.8, 112.9, 111.4, 110.2, 107.0, 106.9, 105.7,
54.9, 53.0, 21.9; EIMS: m/z 387 (1, [M+TMS]+.), 371 (18), 356 (14), 312
(10), 299 (16), 298 (83), 297 (10), 271 (22), 270 (100), 269 (18), 242 (17),
241 (51), 227 (10), 143 (18), 134 (11); HRMS (ESI): m/z calcd for C17H15O6
[M+H]+: 315.08631; found: 315.08600.
Compound 5: pale yellow solid; mp: 179–180 °C; IR (KBr): ꢀꢁ = 3472, 2983,
2920, 2853, 1655, 1607, 1559, 1504, 1477, 1464, 1428, 1361, 1327, 1298,
1265, 1224, 1210, 1133, 1112, 1091, 1069, 1005, 983, 963, 904, 826 cm-
1; 1H NMR (300.13 MHz, DMSO-d6): d = 13.06 (s, OH), 7.06 (s, H), 6.95 (s,
H), 6.82 (s, H), 6.60 (s, H), 5.59 (t, J = 3.4 Hz, OH), 4.64 (d, J = 3.4 Hz,
2H), 3.91 (s, 3H), 2.38 (s, 3H); 13C NMR (75.47 MHz, DMSO-d6): d = 181.0,
161.0, 160.1, 157.4, 154.8, 152.9, 148.2, 111.0, 108.7, 106.8, 106.7, 106.1,
103.9, 62.3, 56.3, 21.9; EIMS: m/z 431 (1, [M+TMS]+•), 416 (15), 415 (26),
311 (22), 310 (17), 283 (60), 253 (20), 237 (15), 89 (32), 73 (100), 59 (42);
HRMS (ESI): m/z calcd for C16H15O5 [M+H]+: 287.09140; found: 287.09283.
Compound 3: yellow solid (8.7 mg, 46 %); mp. 178–181 °C; IR (KBr): ꢀꢁ =
3446, 2921, 2851, 1728, 1647, 1636, 1617, 1592, 1559, 1472, 1458, 1435,
1419, 1325, 1307, 1231, 1209, 1114, 1098, 762 cm-1; 1H NMR (500.16
MHz, DMSO-d6): d = 12.82 (s, H), 7.71 (t, J = 8.3 Hz, H), 7.62 (s, H), 7.42
(s, H), 7.03 (d, J = 8.3 Hz, H), 6.81 (d, J = 8.3 Hz, H), 4.02 (s, H), 3.95 (s,
3H).13C NMR (125.77 MHz, DMSO-d6): d = 181.5, 164.8, 161.1, 160.5,
157.1, 154.9, 137.2, 136.1, 113.0, 110.7, 110.2, 109.4, 106.0, 102.8, 56.7,
53.1; EIMS: m/z 372 (2, [M+TMS]+•), 358 (12), 357 (48), 322 (21), 285 (24),
284 (100), 283 (12), 257 (22), 256 (99), 255 (18), 202 (12), 198 (11), 185
(13), 165 (11), 73 (10). HRMS (ESI): m/z calcd for C16H11O6 [M-H]-:
299.05501; found: 299.05582.
Compound 6: pale yellow solid (311 mg, 90 %); mp: 236–238 °C; IR (KBr):
ꢀꢁ = 3471, 2920, 2851, 1655, 1607, 1558, 1504, 1476, 1464, 1428, 1361,
1327, 1264, 1224, 1209, 1133, 1111, 1091, 1068, 826, 790 cm-1; 1H NMR
(400. 14 MHz, DMSO-d6): d = 13.12 (s, OH), 7.64 (t, J = 8.3, 1H), 7.06 (d,
J =1.2 Hz, 1H), 6.99 – 6.92 (m, 2H), 6.74 (dd, J = 8.3, 0.9 Hz, 1H), 5.56 (t,
J = 5.8 Hz, OH), 4.64 (d, J = 5.6, 2H), 3.92 (s, 3H, OCH3); 13C NMR (100.63
MHz, DMSO-d6): d = 181.6, 161.3, 160.2, 157.5, 155.0, 153.2, 136.8,
110.4, 108.8, 108.8, 106.6, 106.2, 104.0, 62.4, 56.4; EIMS: m/z 416 (0.5,
[M+TMS]+•), 401 (52), 341 (21), 315 (21), 314 (18), 312 (20), 311 (18), 299
(30), 298 (16), 297 (36), 296 (16), 287 (16), 286 (18), 285 (27), 272 (18),
270 (28), 269 (72), 253 (15), 252 (25), 239 (27), 237 (16), 223 (19), 89 (31),
73 (100), 59 (31); HRMS (ESI): m/z calcd for C15H13O5 [M+H]+: 273.07685;
found: 273.07611.
Synthesis of Yicathin C (2) and 8-hydroxy-1-methoxy-6-methyl-9-oxo-
9H-xanthene-3-carboxylic acid (4): In a typical experiment, H5IO6 (1.0 g,
4.6 mmol) and chromium (VI) oxide (4.6 mg, 46 µmol) was placed in a 50
mL Erlenmeyer, and then dissolved with 10 mL of wet ACN (75 %). In a
5
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Synthesis of (2,6-dihydroxy-4-methylphenyl)(4-(hydroxymethyl)-2,6-
dimethoxyphenyl)methanone (7) and (2,6-dihydroxyphenyl)(4-
Keywords: xanthone • marine • total synthesis • lipophilicity •
anti-inflammatory
(hydroxymethyl)-2,6-dimethoxyphenyl)methanone (8): In
a round-
bottom flask of 50 mL, compound 25 (1.1 g, 2.11 mmol) was dissolved in
24 mL of MeOH. p-TsOH (603 mg, 3.17 mmol) was added and the mixture
turned red. The reaction mixture was stirred at 50 °C for 5 hours. H2O was
added to the mixture and the solution was extracted with EtOAc (3 x 10
mL). The organic layers were washed with brine, dried over Na2SO4,
filtered and the solvent evaporated. The crude product was purified by
silica gel flash chromatography (hexane/EtOAc/formic acid 7:3:0.1).
Compound 7 was obtained as a yellow solid (300 mg, 45 %).
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Compound 7: yellow solid; mp: 170–171°C; IR (KBr): ꢀꢁ = 3269, 3020,
2937, 1637, 1612, 1589, 1496, 1460, 1415, 1357, 1326, 1299, 1258, 1229,
1216, 1197, 1065, 1035, 1006, 961, 923, 914, 822, 719, 622 cm-1; 1H NMR
(300.13 MHz, DMSO-d6): d = 11.59 (s, 2OH), 6.64 (s, 2H), 6.10 (s, 2H),
5.28 (t, J = 4.8 Hz, OH), 4.51 (d, J = 4.8 Hz, 2H), 3.65 (s, 6H s), 2.16 (s,
3H); 13C NMR (75.47 MHz, DMSO-d6): d = 198.8, 162.3, 155.7, 148.4,
144.8, 120.4, 109.1, 107.6, 101.9, 63.0, 55.7, 21.7; EIMS: m/z 535 (2,
[M+TMS]+•), 520 (1), 519 (13), 416 (10), 415 (28), 283 (15), 282 (14), 281
(54), 267 (10), 209 (13), 191 (10), 163 (11), 149 (15), 147 (11), 89 (15), 75
(15), 73 (100); HRMS (ESI): m/z calcd for C17H19O6 [M+H]+: 319.11761;
found: 319.11894.
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°
Compound 8: yellow solid (292 mg, 39 %); mp: 111–113 C; IR (KBr):
ꢀꢁ = 3545, 3385, 2940, 2360, 2342, 1589, 1456, 1418, 1351 1275, 1255,
1239, 1121, 827, 766, 752, 710 cm-1; 1H NMR (300.13 MHz, DMSO-d6): d
= 11.59 (s, 2OH), 7.26 (t, J = 8.2, H), 6.65 (s, 2H), 6.27 (d, J = 8.2, 2H),
5.32 (s, OH), 4.52 (s, 2H), 3.67 (s, 6H); 13C NMR (75.47 MHz, DMSO-d6):
d = 199.6, 162.4, 155.8, 145.0, 137.2, 120.4, 111.2, 107.0, 101.9, 63.1,
55.7; EIMS: m/z 520 (1, [M+TMS]+•), 505 (2), 402 (14), 401 (30), 312 (13),
311 (10), 270 (12), 269 (28), 268 (26), 267 (100), 265 (14), 244 (12), 195
(10), 177 (17), 149 (15), 147 (11), 90 (12), 75 (11), 73 (83); HRMS (ESI):
m/z calcd for C16H15O6 [M-H]+: 303.08741; found: 303.0881.
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This work was supported through national funds provided by
FCT/MCTES - Foundation for Science and Technology from the
Minister of Science, Technology and Higher Education (PIDDAC)
and European Regional Development Fund (ERDF) through the
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COMPETE
–
Programa
Operacional
Factores
de
Competitividade (POFC) programme, under the projects
PTDC/MAR-BIO/4694/2014 (reference POCI-01-0145-FEDER-
016790; Project 3599 – Promover a Produção Científica e
Desenvolvimento Tecnológico
e a Constituição de Redes
Temáticas (3599-PPCDT)) and PTDC/SAU-PUB/28736/2017
(reference POCI-01-0145-FEDER- 028736) in the framework of
the programme PT2020. Daniela R. P. Loureiro thanks for a
research grant PTDC/MAR-BIO/4694/2014-BI-2017-003. José X.
Soares thanks for the FCT PhD Programmes and by Programa
Operacional Capital Humano (POCH), specifically by the
BiotechHealth Programme (Doctoral Programme on Cellular and
Molecular Biotechnology Applied to Health Sciences), reference
PD/00016/2012; through the FCT and POCH for the PhD grants
(SFRH/BD/98105/2013 and SFRH/BD/116167/2016). The
authors would like to thank Sara Cravo and Gisela Adriano for the
technical support. To Centro de Apoio Cientifico e Tecnolóxico á
Investigation (C.A.C.T.I., University of Vigo, Pontevedra, Spain)
for HRMS analysis. To Centro de Materiais da Universidade do
Porto (CEMUP, Porto, Portugal) for HRMS, and to Departamento
de Química da Universidade de Aveiro (Portuguese NMR
network) for the NMR analysis.
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7
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Entry for the Table of Contents
Total synthesis of two new marine xanthones - Yicathin B and C - and six additional analogues were reported for the first time.
Lipophilicity of the synthetized compounds were evaluated using biomimetic models. In vitro antitumor and anti-inflammatory
activities of Yicathins and its analogues were also evaluated.
8
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