In the mists of a fungal metabolite: An unexpected reaction of 2,4,5-trimethoxyphenylglyoxylic acid

Article abstract of DOI:10.3390/molecules25081978

The reactions of phenylglyoxylic acids during the synthesis and biological evaluation of fungal metabolites led to the discovery of hitherto unknown compounds with a p-quinone methide (p-QM) structure. The formation of these p-QMs using ¹³C-lab

Full text of DOI:10.3390/molecules25081978

molecules
Article
In the Mists of a Fungal Metabolite: An Unexpected
Reaction of 2,4,5-Trimethoxyphenylglyoxylic Acid
Immo Serbian 1 , Anne Loesche 1 , Sven Sommerwerk 1, Phil Liebing 2 , Dieter Ströhl 1 and
René Csuk 1,
1
*
Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-Str. 2, D-06120 Halle (Saale),
Germany; immo.serbian@chemie.uni-halle.de (I.S.); Anne.Loesche@web.de (A.L.);
Svensommerwerk@gmx.de (S.S.); dieter.stroehl@chemie.uni-halle.de (D.S.)
Chemisches Institut, Otto von Guericke Universität Magdeburg, Universitätsplatz 2, D-39106 Magdeburg,
Germany; Phil.Liebing@ovgu.de
2
*
Correspondence: rene.csuk@chemie.uni-halle.de; Tel.: +49-345-55-25660
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Derek J. McPhee
Received: 13 March 2020; Accepted: 21 April 2020; Published: 23 April 2020
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: The reactions of phenylglyoxylic acids during the synthesis and biological evaluation of
fungal metabolites led to the discovery of hitherto unknown compounds with a p-quinone methide
(p-QM) structure. The formation of these p-QMs using 13C-labelled starting materials revealed a
key-step of this reaction being a retro-Friedel–Crafts alkylation.
Keywords: phenylglyoxylic acid; retro-Friedel–Crafts; fungi; p-quinone methide
1. Introduction
While the secondary metabolites of plants have been studied very intensively, the metabolites
formed in fungi and especially of lichens came only recently in the focus of increased scientific
interest [
(Figure 1, emodine) or 1,8-dihydroxy-3-methyl-anthracene-9,10-dione (chrysophanol) from fungi have
also been isolated from lichens [ ]. Many of these compounds are cytotoxic. For example, the latter
compound blocks the proliferation of colon cancer cells by inhibiting the EGFR/mTor pathway [
1–3]. Furthermore, structures similar to 1,3,8-trihydroxy-6-methyl-anthracene-9,10-dione
4–6
7–
9
6
].
].
Some compounds are similar to “-rubicin” anticancer agents, such as daunorubicin or doxorubicin [4–
Figure 1. Structure emodine, chrysophanol and daunorubicin isolated from fungi or lichens and
p-quinone methide (p-QM) derived natural products rugaurones A–C and cherylline.
p-Quinone methides (p-QMs) are highly reactive compounds possessing a broad range of different
biologic activities [10]. They have also been discussed as the major decomposition products of
Molecules 2020, 25, 1978; doi:10.3390/molecules25081978
www.mdpi.com/journal/molecules

Molecules 2020, 25, 1978
2 of 15
catechol estrogen-o-quinones [11
–
13]. They are well known intermediates in the biosynthesis of natural
products. Their high reactivity and potential has also been exploited in the total synthesis of some
natural products such as the flavonoids rugaurone A–C [14] cherylline [15] and 20-deoxy-elansolid
B1 [16]. Furthermore, the reactions of p-QMs have extensively been studied to generate compounds
of pharmaceutical interest [17–20] and quite recently they were used as starting materials for an
organo-catalytic asymmetric α-alkylation of aldehydes [21].
During our research on metabolites from fungi and lichens we came across the chemical
properties and reactions of phenylglyoxylic acids [22]. Thereby, we encountered several unexpected
and unprecedented reactions yielding 3,3-diaryl substituted benzofuranones which undergo
retro-Friedel–Crafts alkylation in hydrochloric acid forming the p-QM structure. These molecules
are similar to 3,3-diaryloxoindoles and 3,3-substituted oxoindoles that have lately been studied quite
extensively yielding pharmacological interesting molecules [23
–28]; they are also known intermediates
from isatines and a prominent motif in natural product products, as for example in azonazine [29].
In contrast to isatines, benzofuran-2,3-diones are widely overlooked [28
to provide an access to this rare structural motif.
,30]. This was another reason
2. Results and Discussion
Oxidation of 2,4,5-trimethoxy-acetophenone (
trimethoxyphenylglyoxylic acid ( ); this compound has previously been isolated from the fungus
Polyporus tumulosus Cooke [22 32]. As an alternative, a Friedel–Crafts acylation of 2,4,5-trimethoxybenzene
) in the presence of TiCl4 gave a 94% yield of ester 33 35] whose hydrolysis with methanolic KOH
for 2 h resulted in an almost quantitative yield of . Partial deprotection of the methoxy-groups with
AlCl3 of yielded albeit in low yields, while demethylation with either hydrobromic or hydrochloric
acid yielded , a red colored solid in almost quantitative yield. A major issue in these reactions is the
instability of the -keto-acids that are readily decarboxylated [22 36]. These findings parallel previous
reports for trimethoxy-substituted aromatic compounds [36]. Furthermore, -keto acids were used for
the synthesis of oxadiazolopyrazines—selective antibacterial agents against Haemophilus influenzae [37].
Attempts to cyclize under a variety of different conditions invariably led to the formation of in
moderate to excellent yields. The formation of was only observed to a rather minor extend by ESI–MS.
1, Scheme 1) with SeO2 [31] gave 2,4,5-
2
,
(3
4 [ –
2
2
5
6
α
,
α
5
6
7
Scheme 1. Reactions and conditions: (
DCM, ) KOH, MeOH, H2O, r.t. 99%; (
20 ◦C, 2 h, 94%; (
) aq. HCl, 40 ◦C, 3 h, 56%; ( , HCl, 40 ◦C, 4 h, 96%; (
85 ◦C, 1 h, 66%; ( ) SeO2, pyridine, 80 ◦C, 20 h, 44%; (
DCM, oxalyl chloride, DMF, 0 ◦C, 2h, traces.
a b) TiCl4, ClC(=O)CO2Et,
) SeO2, pyridine, 80 ◦C, 4 h, 57%; (
−
c
d
) AlCl3, DCM, microwave, 50 ◦C, 3 h, 13%;
.
(e
f
)
3
g
) aq. HCl, 40 ◦C, 4 h, 49%; (
h
) Ac2O, BF3 Et2O,
i
j) DCM, oxalyl chloride, DMF, r.t. 14 h, 36%; (k)

Molecules 2020, 25, 1978
3 of 15
Friedel–Crafts acylation of sesamol (8) with acetic anhydride/BF3 gave
9
[
38–41] in 66% isolated
yield whose oxidation with SeO2 in pyridine yielded 10. Treatment of 10 with oxalyl chloride in
DCM in the presence of DMF gave a moderate yield of 11 but—interestingly enough - no red-colored
by-products (being analogous to 6) were observed during these reactions.
To gain a deeper insight in the structure and formation of
for. Thus, trimethoxybenzene (
) was allowed to react with 13C-labeled acetylchloride (Scheme 2) in
the presence of TiCl4 and a 97% yield of 13C-labeled
12) was obtained. From its SeO2 oxidation
compound 13C-labeled 2 (13) was obtained in 53% yield.
6,
13C-labeling experiments were called
3
1
(
Scheme 2. Reactions and conditions: (
a
) AcCl, TiCl4, DCM,
20 ◦C, 2 h, 95%; b) SeO2, pyridine, 80 ◦C,
−
4 h, 53%; (c) aq. HCl, 0 ◦C, 2 h, 3%; (d) aq. HCl, ∆,2 h, 48%.
To elucidate the structure of
An ESI/MS of
in MeOH showed a m/z = 345 [M + H]+ corresponding to a molecular composition of
C18H16O7 and indicating a “condensation reaction” of
having taken place. The 1H-NMR spectra
showed the presence of four methoxy groups between = 3.81 and 3.97 ppm and four aromatic
hydrogens between
= 6.06 and 7.26 ppm, respectively. The material obtained from the 13C-labeled
6, a combination of different analytical techniques had to be applied.
6
2
δ
δ
starting material showed only one labeled carbon in the product as indicated by ESI–MS m/z = 346 for
[M + H]+.
Reaction of
2 with AlCl3 or 13 with hydrochloric acid at low temperatures, however, gave access
to an intermediate 15 (from 13); upon warming this reaction mixture, 15 could not be detected any
longer but
6
(from
2
) or 14 (from 13) was formed. For compound 15, different temperatures (
−
50, 30,
−
27 and 40 ◦C) were applied (Figure 2) and the 1H-NMR spectroscopy revealed a change in the spectra.
At room temperature (Figure 2, cyan) an extensive line broadening of one of the methoxy groups
(δ = 3.62 ppm) was observed. In addition, extensive line broadening was seen in the aromatic region.
Integration of the signals suggested the presence of seven methoxy groups. Temperature dependent
NMR spectroscopy revealed that the line broadening observed at room temperature is due to the
presence of a rotational barrier. Furthermore, the NMR spectra of 15 strongly depend on the used
solvent (Figure 3). For example, the aromatic protons in 15 are severely shifted to lower field upon
using deuterated toluene as the solvent probably due to an aromatic solvent induced shift behavior.
An ESI–MS of 15 showed a quasimolecular ion [M + H]+ m/z = 513 corresponding well to the proposed
structure. NMR spectra of the products are depicted in the Supplementary File.

Molecules 2020, 25, 1978
4 of 15
Figure 2. 1H-NMR Spectra of the intermediate 15; temperatures: violet, 40 ◦C; cyan, 27 ◦C; green
−30 ◦C; red, −50 ◦C.
,
Figure 3. NMR Spectra of the intermediate 15 in different solvents; CDCl3 (red), toluene-d8 (green),
DMSO-d6 (blue) at room temperature.

Molecules 2020, 25, 1978
5 of 15
As
6 crystallizes readily from ethyl acetate, crystals suitable for a single crystal X ray analysis
could be obtained. The crystal structure of the black orthorhombic prisms (space group P212121) is
consistent with the NMR data comprising C18H16O7 molecules. The molecular structure is depicted in
Figure 4.
Figure 4. The molecular structure of compound 6 in the crystal. Displacement ellipsoids of C and O
atoms drawn at the 50% probability level, H atoms as spheres of arbitrary size.
The bond lengths within the benzofurane-derived bicyclic system (C1–C8, O2) cover a wide
range of 133.2(3)–150.1(4) pm. C1–C2 (147.4(4) pm), C3–C4 (144.0(4) pm), C3–C8 (142.8(4) pm), C5–C6
(145.8(4) pm), C6-C7 (150.1(4) pm), C1–O2 (140.6(3) pm) and C4–O2 (138.2(3) pm) can be described
as single bonds, while C2–C3 (136.0(3) pm), C4–C5 (133.2(3) pm) and C7–C8 (134.2(3) pm) have
double bond character. The interconnection between the two ring systems, C2–C10, is a single bond at
147.0(4) pm. The C–C separations within the benzene ring (C10–C15) are in a typical narrow range of
137.3(4)–141.1(3) pm. The molecule is not strictly planar as the two ring systems are twisted around the
C2–C10 vector about 41.0(1)◦. In the crystal, the molecules are stacked together to a one-dimensional
supramolecular array by
π interactions (Figure 5), with the closest intermolecular contacts being
C13···O2 (342.2(4) pm) and C8···C15 (344.7(4) pm). The distances between the ring centroids are
356.81(3) pm ((C1–C4, O2)···(C10–C15)) and 369.58(3) pm ((C3–C8)···(C10–C15)), respectively.
Figure 5. Intermolecular
π interactions in compound 6, resulting in a one-dimensional supramolecular
structure extending along the crystallographic a axis.

Molecules 2020, 25, 1978
6 of 15
While the reaction of
2
with trifluoroacetic acid (TFA) in dry DCM did not lead to the formation of
6
2
2
and with conc. sulfuric or phosphoric acid only low yields (<10%) were observed, the reaction of
with HBr (48%) gave 42% of and with conc. HCl (37%) an 56% yield was observed. Reaction of
with 37% HCl in the presence of 5 equivalents 1,2,4-trimethoxybenzene finally led to a 96% yield
. From these observations as well as from the 13C-labeling experiments (vide supra), a tentative
6
of
6
mechanism for this reaction was deduced (as depicted in Scheme 3). In the course of the reaction of
2
with TFA in dry DCM an intermediate 16a was observed. ESI-MS experiments showed the presence
of a quasimolecular ions m/z = 211.1 ([M + 2Na]2+) and m/z = 791.2 ([2M + K]+) corresponding to
M = 376. Subsequently from 16a intermediate 16b is formed; the latter compound was detected in
ESI–MS spectra showing m/z = 549.13 ([M + Na]+) and 1074.73 ([2M + Na]+). Friedel–Crafts reactions
are known to be reversible [42
explaining the very high yield of
used to access a fungal pigment from Peniophora sanguinea Bres [13].
–
48] and compound
6 is very insoluble in the reaction mixture thus
6
in these reactions. Previously, a retro Friedel–Crafts alkylation was
Scheme 3.
Proposed mechanism for the formation of 6 (TMP = 2,4,5-trimethoxyphenyl;
TMB = 1,2,4-trimethoxybenzene).
To verify these assumptions, to a solution of 6 in dry 2M HCl (in ethyl acetate) 3 was added in
10-fold excess and within several hours of stirring at room temperature, the reaction mixture turned
from black to slightly red. ESI–MS investigations of this reaction mixture showed the formation of 15
(m/z = 513, [M + H]+).
Treatment of
reaction (Scheme 4),
6
with Zn/HCl led to the formation of 17. To have a first insight into the scope of this
was allowed to react with anisole or BOC-aniline in the presence of hydrochloric
2
acid and products 18 and 19 were obtained, respectively. Benzoylation of 19 gave 20. Interestingly
enough, reaction of 2 with benzene in the presence of hydrochloric acid furnished a 18% yield of 21.

Molecules 2020, 25, 1978
7 of 15
Scheme 4. Reactions and conditions: (a b
) anisole, HCl, 40 ◦C, 4 h, 74%; ( ) Boc-aniline, HCl, 40 ◦C, 4 h,
43%; (c) BzCl, NEt3, DCM, r.t. 6 h, 69%; (d) Zn/HCl, 0 ◦C, 4 h, 43%; (e) benzene, HCl, 2 h, 18%.
3. Materials and Methods
NMR spectra were recorded using the Varian spectrometers (Varian GmbH, Darmstadt, Germany)
Gemini 2000 or Unity 500 ( given in ppm, J in Hz; typical experiments: APT, H–H-COSY, HMBC, HSQC,
δ
NOESY), MS spectra were taken on a Finnigan MAT LCQ 7000 (electrospray, voltage 4.1 kV, sheath gas
nitrogen) instrument. TLC was performed on silica gel (Merck 5554, detection with cerium molybdate
reagent); melting points are uncorrected (Leica hot stage microscope, Leica GmbH, Wetzlar, Germany)
and elemental analyses were performed on a Foss-Heraeus Vario EL (CHNS, Elementar analysensysteme
GmbH, Langenselbold, Germany) unit. IR spectra were recorded on a Perkin Elmer (Perkin Elmer
Deutschland, Berlin, Germany) FT-IR spectrometer Spectrum 1000 or on a Perkin-Elmer Spectrum Two
(UATR Two Unit). The solvents were dried according to usual procedures. Crystallographic Data were
deposited at the Cambridge Crystallographic Data Center with the depository number CCDC 1569146.
The data are available free of charge at www.ccdc.cam.ac.uk/products/csd upon request.
4. Experimental
4.1. 2,4,5-Trimethoxyphenylglyoxylic Acid (2)
From
7.21 mmol) in pyridine (5 mL) was stirred for 4 h at 80 ◦C. The mixture was poured into NaOH (100 mL,
0.05 m) and extracted with EtOAc (3 50 mL). The aqueous phase was acidified with 2 m hydrochloric
acid and extracted with EtOAc (3 50 mL). The combined organic extracts were washed with brine,
1. A suspension of 2,4,5-trimethoxybenzene (1, 840 mg, 4.00 mmol) and selenium dioxide (800 mg,
×
×
dried (MgSO4), the solvent was evaporated and the residue re-crystallized from EtOAc to yield
(550 mg, 57%) as a yellow solid.
2
From
was added and the mixture was stirred for 2 h. Water (100 mL) was added and the aqueous phase
was extracted with chloroform (3 70 mL). The combined organic phases were dried (MgSO4) and
the solvent was removed to afford (886 mg, 99%) as a yellow solid; an analytical sample showed
m.p. 182–185 ◦C (lit.:[49] 186 ◦C); RF = 0.35 (SiO2, CHCl3/MeOH; 2:1); IR (KBr):
= 3432br, 1736s,
1610s, 1517s, 1480m, 1423w, 1384w, 1288s, 1288s, 1230s, 1190m, 1142s, 1018s cm−1; UV-Vis (MeOH):
(log = 7.22 (s, 1H, 8-H),
) = 237 (4.22), 279 (4.09), 341 (4.01) nm; 1H-NMR (400 MHz, DMSO-d6):
6.82 (s, 1H, 5-H), 3.92 (s, 3H, 9-H), 3.84 (s, 3H, 10-H), 3.76 (s, 3H, 11-H) ppm; 13C-NMR (100 MHz,
DMSO-d6): = 186.0 (C-2), 167.6 (C-1), 157.2 (C-6), 156.4 (C-4), 143.7 (C-7), 113.1 (C-3), 110.2 (C-8),
98.1 (C-5), 57.1 (C-10), 56.2 (C-9), 55.8 (C-11) ppm; MS (ESI, MeOH): m/z (%) = 238.9 ([M
H]−, 100),
4. To a solution of 4 (1.0 g, 3.73 mmol) in methanol (50 mL), powdered KOH (1.5 g, 26.7 mmol)
×
2
ν
λ
max
ε
δ
δ
−

Molecules 2020, 25, 1978
8 of 15
478.8 ([2M
found: C 54.79, H 5.18.
−
H]−, 8), 501.0 ([2M
−
2H + Na]−, 3); analysis calcd for C11H12O6 (240.21): C 55.00, H 5.04;
4.2. 2,4,5-Trimethoxyphenyl-glyoxylic Acid Ethyl-ester (4)
To a solution of 1,2,4-trimethoxybenzene (3, 5.0 g, 29.7 mmol) in DCM (200 mL) at
20 ◦C, TiCl4 (3.6 mL,
−
32.8 mmol) was added followed by slowly adding ethyl chlorooxoacetate (3.5 mL, 31.3 mmol) and
the mixture was stirred for 2 h. Aqueous work-up (2M HCl) followed by extraction with chloroform
(3
yellow crystalline solid; m.p. 89–91 ◦C (lit.:[50] 89.5 ◦C); RF = 0.78 (SiO2, hexanes/CHCl3); IR (ATR):
= 3004w, 2991w, 2943w, 2908w, 2840w, 1732m, 1633m, 1599s, 1581m, 1513s, 1482m, 1461m, 1450m,
1440m, 1418m, 1390w, 1371m, 1298s, 1268s, 1259s, 1228s, 1218s, 1191s, 1179s, 1141vs, 1112m, 1038m,
1021s, 1007s, 954m, 862m, 816m, 805m, 770m, 765m, 743m, 710m, 655m cm−1; UV-Vis (methanol):
(log
) 237 (3.52), 281 (3.40), 346 (3.34) nm;1H-NMR (400 MHz, CDCl3) = 7.38 (s, 1H, 20-H), 6.46 (s, 1H,
50-H), 4.36 (q, J = 7.1 Hz, 2H, EtCH2), 3.95 (s, 3H, OMe(60)), 3.87 (s, 3H, OMe(30)), 3.84 (s, 3H, OMe(40)),
1.38 (t, J = 7.1 Hz, 3H, EtCH3) ppm; 13C-NMR (101 MHz, CDCl3)
= 184.8 (C-2), 166.1 (C-1), 157.3
×
100 mL) and column chromatography (SiO2, hexanes/CHCl3, 1:1) gave 4 (7.48 g, 94%) as pale
ν
λ
max
ε
δ
δ
(C-4‘), 156.5 (C-6‘), 144.2 (C-3‘), 114.2 (C-1‘), 111.0 (C-2‘), 96.5 (C-5‘), 61.6 (C-EtCH2), 56.9 (OMe(C4‘)),
56.3 (OMe(C3‘)), 56.3 (OMe(C2‘)), 14.1 (CEtCH3) ppm; MS (ESI, MeOH): m/z (%) = 269.1 ([M + H]+,
100); analysis calcd for C13H16O6 (268.26): C 58.20, H 6.01; found: C 57.95, H 6.22.
4.3. 2-Hydroxy-4,5-dimethoxyphenyl-glyoxylic Acid (5)
A solution of AlCl3 (166 mg, 1.25 mmol) and 2 (150 mg, 0.62 mmol) in DCM (5 mL) was heated for 3 h
under microwave irradiation at 50 ◦C. Usual aqueous work-up (aq. HCl, 80 mL, 0.125 m) followed
by extraction (ethyl acetate, 2
as yellow solid; m.p. 163–165 ◦C (lit:[22] 142–144 ◦C); RF = 0.25 (SiO2, CHCl3/MeOH 2:1); IR (KBr):
= 3150br, 1713s, 1573s, 1526s, 1482m, 1400m, 1352w, 1308m, 1267s, 1240m, 1205s, 1181m, 1165s,
1035m cm−1; UV-Vis (MeOH): = 238 (4.07), 283 (3.99), 344 (3.93) nm; 1H-NMR (400 MHz,
max (log
DMSO-d6):
= 7.24 (s, 1H, 8-H), 6.61 (s, 1H, 5-H), 3.78 (s, 3H, 10-H), 3.74 (s, 3H, 9-H) ppm; 13C-NMR
(101 MHz, DMSO-d6): = 185.6 (C-2), 167.8 (C-1), 157.1 (C-6), 155.4 (C-4), 142.8 (C-7), 112.3 (C-3), 111.2
H]−, 100), 472.9
×
50 mL) and re-crystallization from chloroform gave 5 (17 mg, 13%)
ν
λ
ε
δ
δ
(C-8), 100.7 (C-5), 56.6 (C-10), 56.0 (C-9) ppm; MS (ESI, MeOH): m/z (%) = 224.9 ([M
−
([2M − 2H + Na]−, 16); analysis calcd for C10H10O6 (226.18): C 53.10, H 4.46; found: C 52.96, H 4.60.
4.4. 5-Methoxy-3-(2,4,5-trimethoxyphenyl)benzofuran-2,6-dione (6)
Method A. Compound
the mixture was heated to 40 ◦C for 3 h, diluted with water (25 mL) and extracted with chloroform
(3 25 mL). The organic extracts were dried (MgSO4), the solvent was removed and the remaining
2 (250 mg, 1.04 mmol) was suspended in conc. hydrochloric acid (25 mL) and
×
residue was subjected to column chromatography (SiO2, CHCl3/MeOH 9:1) to afford 6 (199 mg, 56%)
as a dark red solid.
Method B. Compound
suspended in conc. hydrochloric acid (25 mL) and the mixture was heated to 40 ◦C for 4 h, diluted with
water (25 mL) and extracted with chloroform (3 25 mL). The organic extracts were dried (MgSO4),
2 (200 mg, 0.83 mmol) and 1,2,4-trimethoxybenzene (0.62 mL, 4.16 mmol) were
×
the solvent was removed and the remaining residue was subjected to column chromatography (SiO2,
CHCl3/MeOH 9:1) to afford 6 (274 mg, 96%) as a dark red solid.
Method C. Compound
40 ◦C for 4 h; workup as described above gave
RF = 0.68 (SiO2, CHCl3/MeOH 9:1); IR (ATR):
4
(1.0 g, 3.73 mmol) was suspended in hydrochloric acid (50 mL) and heated to
(636 mg, 49%) as dark red solid; m.p. 238–239 ◦C;
= 2958w, 2941w, 2841w, 1772s, 1735w, 1643s, 1610m,
6
ν
1588vs, 1559s, 1527m, 1517m, 1506m, 1461s, 1454s, 1437m, 1416m, 1388m, 1350s, 1316w, 1274s, 1234vs,
1223s, 1209vs, 1192s, 1180vs, 1167vs, 1134s, 1052w, 1040m, 1023s, 1001s, 985s, 934s, 864s, 854s, 844s,
822m, 814m, 778m, 774m, 766m 701m, 696m, 669m cm−1, UV–Vis (MeOH):
λ
(log
ε
= 311 (3.31),
max
357 (3.08), 498 (3.00) nm; 1H-NMR (400 MHz, CDCl3):
δ
= 7.09 (s, 1H, 60-H), 6.61 (s, 1H, 30-H), 6.36

Molecules 2020, 25, 1978
9 of 15
(s, 1H, 4-H), 6.06 (s, 1H, 7-H), 3.97 (s, 3H, OMe(40)), 3.88 (s, 3H, OMe(50), 3.87 (s, 3H, OMe(20)), 3.82
(s, 3H, OMe(5)) ppm; 13C NMR (101 MHz, CDCl3):
= 181.3 (C-6), 167.4 (C-2), 160.5 (C-7a), 153.82
δ
(C-20), 152.78 (C-40), 152.74 (C-5), 143.74 (C-50), 138.80 (C-3a), 125.59 (C-3), 114.3 (C-60), 109.3 (C-10),
104.2 (C-7), 100.7 (C-4), 97.6 (C-30), 56.8 + 56.6 (OMe(20) + OMe (50)), 56.3 (OMe(40)), 56.3 (OMe(5))
ppm; MS (ESI, MeOH): m/z (%) = 345.2 ([M + H]+, 100); analysis calcd for C18H16O7 (344.32): C 62.79,
H 4.68; found: C 62.55, H 4.83.
4.5. 1-(6-Hydroxybenzo[d][1,3]dioxol-5-yl)ethan-1-one (9)
To a suspension of sesamol (8, 5.52 g, 40.00 mmol) in acetic anhydride (20 mL, 0.21 mol) at 0 ◦C
BF3
aqueous solution of sodium acetate sol. (satd., 40 mL) was added and the mixture was extracted
with diethylether (3 50 mL). The combined organic layers were washed with sat. NaHCO3 sol.
(4 100 mL), water (3 100 mL), brine (1 100 mL) and dried (MgSO4). The solvent was removed
and recrystallization from ethanol gave
(4.74 g, 66%) as an off-white solid; m.p. 111–113 ◦C (lit.:[51
114 ◦C); RF = 0.81 (SiO2, CHCl3/MeOH, 9:1); C; IR (KBr):
= 3446br, 2919m, 1633s, 1485s, 1425s, 1366m,
1322s, 1257s, 1182s, 1118m, 1037s, 958m, 924s, 856m, 844m, 787m, 773m, 540m cm−1; UV–Vis (CHCl3):
max (log = 7.04 (s, 1H, 6-H),
) = 241 (4.16), 278 (3.86), 349 (3.92) nm; 1H-NMR (400 MHz, CDCl3):
6.43 (s, 1H, 3-H), 5.97 (s, 2H, 7-H), 2.51 (s, 3H, 9-H) ppm; 13C NMR (100 MHz, CDCl3):
= 202.0 (C-8),
·
OEt2 (10 mL, 81.03 mmol) was added and the reaction mixture was heated at 85 ◦C for 1 h. An
×
×
×
×
9
]
ν
λ
ε
δ
δ
162.2 (C-2), 154.5 (C-4), 140.6 (C-5), 112.4 (C-1), 107.3 (C-6), 102.0 (C-7), 98.8 (C-3), 26.6 (C-9) ppm; MS
(ESI, MeOH): m/z (%) = 181.1 ([M + H]+, 100); analysis calcd for C9H8O4 (344.32): C 62.79, H 4.68;
found: C 62.57, H 4.90.
4.6. 2-(6-Hydroxybenzo[d][1,3]dioxol-5-yl)-2-oxoacetic Acid (10)
Compound
in dry pyridine (45 mL) and stirred at 80 ◦C for 20 h. Aqueous work-up (400 mL, 1 m), followed by
extraction with EtOAc (8 100 mL) and evaporation of the solvent gave 10 as a brownish solid (1.96 g,
9 (2.5 g, 13.88 mmol) and selenium dioxide (3.08 g, 27.75 mmol) were suspended
×
67%). An analytical sample was obtained by re-dissolving 10 in EtOAc (100 mL), washing with NaOH
(100 mL, 0.2 m), separation of the phases, acidification of the aqueous phase with HCl (2 m, 2.5 mL)
followed by extraction with EtOAc (2
brine, dried (MgSO4) and the solvent was removed at room temperature to yield analytically pure
10 (1.27 g, 44%) as an orange solid; m.p. 138–141 ◦C; RF = 0.22 (SiO2, CHCl3/MeOH, 4:1); IR (KBr):
×
50 mL). The combined organic extracts were washed with
ν
= 3422br, 3284m, 2925w, 1765m, 1734w, 1616m, 1593m, 1481m, 1416m, 1376w, 1289m, 1237s, 1176m,
1102w, 1038m cm−1; UV–vis (MeOH):
(400 MHz, DMSO-d6):
DMSO-d6): = 187.9 (C-2), 167,4 (C-1), 159.8 (C-4), 155.3 (C-6), 141.7 (C-7), 111.7 (C-3), 106.6 (C-8),
102.8 (C-9), 98.3 (C-5) ppm; MS (ESI, MeOH): m/z (%) = 209.0 ([M
H]−, 100), 441.0 ([2M-H + Na]−, 3);
λ
(log
ε
) = 246 (4.04), 286 (3.80), 358 (8.87) nm; H-NMR
1
max
δ
= 7.08 (s,1H, 8-H), 6.53 (s, 1H, 5-H), 6.08 (s, 2H, 9-H) ppm; 13C-NMR (101 MHz,
δ
−
analysis calcd for C9H6O6 (210.14): C 51.44, H 2.88; found: C 51.32, H 3.03.
4.7. [1,3]Dioxolo[4,5-f]benzofuran-6,7-dione (11)
A solution of 10 (4.25 g, 22.12 mmol) was suspended in dry DCM (200 mL) and at 0 ◦C oxalyl
chloride (1.9 mL, 22.00 mmol) and dry DMF (200 µL) were added. The reaction mixture was stirred
at r.t. for 14 h. The volatiles were removed under reduced pressure, the residue is re-dissolved in
dry THF (50 mL) and evaporated to dryness. Column chromatography (SiO2, DCM) gave 11 (1.55 g,
36%) as slightly orange solid; m.p. 203–205 ◦C; RF = 0.69 (SiO2, DCM); IR (KBr): v = 3446br, 1818w,
1765m, 1716m, 1616s, 1480s, 1415m, 1299s, 1233s, 1176m, 1095m, 1037m cm–1; UV–vis (CHCl3):
λ
max
(log
(s, 1H, 7-H), 6.23 (s, 2H, 9-H) ppm; 13C-NMR (126 MHz, DMSO-d6):
(C-1), 156.9 (C-6), 145.2 (C-5), 112.5 (C-3), 103.4 (C-9), 102.7 (C-4), 95.6 (C-7) ppm; MS (ESI, MeOH): m/z
(%) = 225.0 ([M + H + MeOH]+, 68), 247.1 ([M + Na + MeOH]+, 100), 356.0 ([3(M + MeOH) + K + H]2+
ε
= 260 (3.82), 310 (3.56), 414 (3.33) nm; 1H-NMR (500 MHz, DMSO-d6):
= 174.9 (C-2), 161.8 (C-8), 157.3
δ = 7.24 (s, 1H, 4-H), 7.14
δ
,

Molecules 2020, 25, 1978
10 of 15
18), 467.7 ([4(M + MeOH) + K + H]2+, 30); analysis calcd for C9H4O5 (192.13): C 56.26, H 2.10; found:
C 55.97, H 2.27.
4.8. 1-(2,4,5-Trimethoxyphenyl)ethan-1-one-1-13C (12)
To a solution of 1,2,4-trimethoxybenzene (3, 2.0 g, 11.9 mmol) in DCM (50 mL) at
20 ◦C TiCl4
−
(1.44 mL, 13.1 mmol) was added followed by adding dropwise 13C-acetylchloride (0.93 mL, 13 mmol).
The resulting mixture was stirred for 2 h, poured into 2M hydrochloric acid and extracted with
chloroform (3
×
50 mL). The combined organic extracts were washed with brine and dried (MgSO4).
Evaporation followed by column chromatography gave 12 (2.4 g, 95%) as a pale yellow crystalline
◦
1
solid.; m.p. 189–192 C; RF = 0.25 (SiO2, hexanes/ethyl acetate, 1:1); H-NMR (400 MHz, CDCl3):
= 7.42 (d, J = 4.2 Hz, 1H, 6-H), 6.50 (d, J = 1.6 Hz, 1H, 3-H), 3.94 (s, 3H, OMe), 3.91 (s, 3H, OMe), 3.87
(s, 3H, OMe), 2.59 (d, J = 6.2 Hz, 3H, 20-H) ppm; 13C-NMR (101 MHz, CDCl3): = 197.2 (C-10), 155.6 (d,
δ
δ
J = 2.2 Hz, C-4), 153.9 (C-2), 143.0 (d, J = 4.0 Hz, C-5), 119.2 (d, J = 54.5 Hz, C-1), 112.54 (d, J = 1.5 Hz,
C-6), 96.42 (d, J = 3.4 Hz, C-3), 56.3 (OMe), 56.1 (OMe), 56.1 (OMe), 32.03 (d, J = 42.7 Hz, C-20); MS
(ESI): m/z (%) = 212.37 ([M + H]+, 100).
4.9. 2-Oxo-2-(2,4,5-trimethoxyphenyl)acetic-2-13C Acid (13)
A suspension of 12 (2.0 g, 9.47 mmol) and selenium dioxide (2.0 g, 18 mmol) in pyridine (20 mL)
was stirred for 4 h at 80 ◦C. Work-up as described above followed by re-crystallization from EtOAc
gave 13 (1.20 g, 53%) as yellow solid; m.p. 163–165 ◦C; RF = 0.35 (SiO2, CHCl3/MeOH, 2:1); 1H-NMR
(400 MHz, DMSO-d6):
δ
= 7.22 (d, J = 4.2 Hz, 1H, 6-H), 6.82 (d, J = 1.8 Hz, 1H, 3-H), 3.92 (s, 3H, OMe),
3.84 (s, 3H, OMe), 3.76 (s, 3H, OMe) ppm; 13C-NMR (101 MHz, DMSO-d6):
δ
= 186.5 (C-20), 168.1 (d,
J = 74.3 Hz, C-10), 157.7 (d, J = 2.0 Hz, C-2), 156.9 (d, J = 0.8 Hz, C-4), 144.2 (d, J = 4.3 Hz, C-5), 113.6 (d,
J = 59.5 Hz, C-1), 110.6 (d, J = 1.7 Hz, C-6), 98.6 (d, J = 3.0 Hz, C-3), 57.6 (OMe), 56.7 (OMe), 56.3 (OMe)
ppm; MS (ESI, MeOH): m/z (%) = 239.8 ([M − H]−, 100).
4.10. 6-Hydroxy-5-methoxy-3,3-bis(2,4,5-trimethoxyphenyl)benzofuran-2(3H)-one-3-13C (15)
◦
A suspension of 13 (200 mg, 0.83 mmol) in conc. hydrochloric acid (25 mL) was stirred 0 C
for 2 h. The reaction mixture was neutralized with an aqueous sat. soln. of NaHCO3 and extracted
with chloroform (3
×
25 mL). The organic extracts were dried (MgSO4), the solvent was removed
under diminished pressure and the remaining residue was subjected to column chromatography (SiO2,
CHCl3/MeOH, 9:1) to afford 15 as a slightly yellow solid (13 mg, 3%); m.p. 103–105 ◦C; RF = 0.66
(SiO2, CHCl3/MeOH, 9:1); 1H-NMR (500 MHz, toluene-d8):
δ
= 7.17–6.83 (m, 2H, 2
20-H), 3.49 – 3.38 (m, 6H, 2 OMe(20)), 3.40 (s,
OMe(40)), 3.06 (s, 3H, OMe(5)) ppm; 13C-NMR (126 MHz, toluene-d8):
= 175.91 (d, J = 52.5 Hz, C-2), 148.13 (C-20), 146.40 (C-40), 142.8 (C-5), 142.8(C-50), 137.10 (C-7a), 124.4 (d,
×
60-H), 7.01 (s,
1H, 7-H), 6.70 (d, J = 2.6 Hz, 1H, 4-H 6.28 (s, 2H, 2
6H, 2
OMe(50)), 3.30 (s, 6H, 2
×
×
×
×
δ
J = 57.3 Hz, C-10), 122.72 (d, J = 45.6 Hz, C-3b), 108.80 (d, J = 1.7 Hz, C-4), 99.8 (C-30), 97.80 (C-7), 57.41 (C-3),
56.58 (OMe(20)), 55.65 (OMe(50)), 55.55 (OMe(40)), 55.34 (OMe(5)) ppm; MS (ESI, MeOH): m/z (%) = 512.1
([M − H]−, 100), 536.1 ([M + Na]+, 100), 782.0 ([3M + Na + H]2+, 33), 791.1 ([3M + K + H]2+, 26).
4.11. 6-Hydroxy-5-methoxy-3-(2,4,5-trimethoxyphenyl)benzofuran-2(3H)-one (17)
Compound
3.82 mmol) was added at 0 ◦C. The reaction was stirred for 4 h at room temperature, diluted with
water (20 mL) and extracted with EtOAc (3 50mL). The combined organic extracts were washed
6 (250 mg, 0.73 mmol) was suspended in hydrochloric acid (25 mL) and zinc (250 mg,
×
with brine and dried (MgSO4). Purification by column chromatography (SiO2, hexanes/CHCl3, 1:1)
gave 17 (112 mg, 43%) as slightly yellow solid; m.p. 143–147 ◦C; RF = 0.32 (SiO2, hexanes/CHCl3, 1:1);
1H-NMR (500 MHz, CDCl3):
δ
= 6.78 (s, 1H, 7-H), 6.68 (s, 1H, 60-H), 6.56–6.53 (m, 2H, 4-H + 30-H), 5.78
(s, 1H, OH), 4.86 (s, 1H, 3-H3), 3.88 (s, 3H, OMe(40)), 3.82 (s, 3H, OMe(20)), 3.79 (s, 3H, OMe(5)), 3.70 (s,
3H, OMe(50)) ppm; 13C-NMR (126 MHz, CDCl3): = 176.57 (C-2), 151.72 (C-20), 149.96 (C-40), 148.03
δ

Molecules 2020, 25, 1978
11 of 15
(C-7a), 146.24 (C-6), 143.61 (C-5), 143.36 (C-50), 118.20 (C-3a), 116.33(C-10), 114.11 (C-6‘), 106.85 (C-4),
98.63 (C-3‘), 98.44 (C-7), 56.83 (OMe(50)), 56.79 (OMe(20)), 56.62 (OMe(5)), 56.18 (OMe(40)), 46.19 (C-3)
ppm; MS (ESI, MeOH): m/z (%) = 345.6 ([M
H 5.24; found: C 62.26, H 4.98.
−
H]−, 100); analysis calcd for C18H18O7 (346.33): C 62.44,
4.12. 5-Methoxy-3-(2-methoxyphenyl)benzofuran-2,6-dione (18)
Compound (240 mg, 1.0 mmol) and anisole (440 mg, 4 mmol) were suspended in hydrochloric
acid (25 mL) and the mixture was stirred for 4 h at 40 ◦C, diluted with water (10 mL), extracted with
CHCl3 (3 25 mL). The combined organic extracts were washed with brine, dried (MgSO4), the solvent
was evaporated and the residue re-crystallized from EtOAc to yield 18 (211 mg, 74%) as a red crystalline
solid; m.p. 194–196 ◦C; RF = 0.54 (SiO2, CHCl3); 1H-NMR (500 MHz, CDCl3):
= 7.52–7.45 (m, 2H,
60-H + 4‘-H), 7.11 (td, J = 7.6, 1.0 Hz, 1H, 50-H), 7.05 (dd, J = 8.3, 1.0 Hz, 1H, 30-H), 6.27 (s, 1H, 4-H), 6.09
(s, 1H, 7-H), 3.89 (s, 3H, OMe(20)), 3.82 (s, 3H, OMe(5)) ppm; 13C-NMR (126 MHz CDCl3):
= 181.29
2
×
δ
δ
(C6), 160.53 (C2), 157.38 (C20), 154.42 (C5), 140.69 (C7a), 132.40 (C4‘), 131.67 (C6‘), 126.18 (C3a), 121.29
(C5‘), 117.65 (C10), 111.88 (C3‘), 104.49 (C7), 99.83 (C4), 56.41 (OMe(20)), 55.77 (OMe(50)) ppm; MS (ESI,
MeOH): m/z (%) = 285.4 ([M + H]+, 100), 307.3 ([M + Na]+, 38); analysis calcd for C16H12O5 (284.26):
C 67.60, H 4.26; found: C 67.41, H 4.39.
4.13. 3-(4-Aminophenyl)-5-methoxybenzofuran-2,6-dione (19)
To a suspension of
2 (500 mg, 2.08 mmol) in hydrochloric acid (25 mL) Boc-aniline (781 mg,
4 mmol) was added and the mixture was stirred for 4 h at 40 ◦C. Usual aqueous workup followed by
column chromatography (SiO2, CHCl3/MeOH, 95:5) and recrystallization from EtOAc gave 19 (243 mg,
43%) as black crystalline solid; m.p. 243–246 ◦C; RF = 0.14 (SiO2, CHCl3); IR (ATR):
ν = 3456w, 3331m,
3219w, 1780m, 1662m, 1643m, 1628m, 1600s, 1575s, 1551s, 1515s, 1506m, 1470w, 1445m, 1439m, 1415s,
1355m, 1331m, 1227s, 1181s, 1157s, 995m, 945m, 850m, 840s, 832s, 821m, 789m, 773m, 687m, 679m,
1
602m cm−1; UV–Vis (MeOH):
λ
max
(log
ε
) 251 (3.67), 317 (3.80), 507 (3.92) nm; H-NMR (500 MHz,
DMSO-d6):
δ
= 7.71 (d, J = 8.7 Hz, 2H, 20-H), 6.75 (s, 1H, 4-H), 6.72 (d, J = 8.8 Hz, 1H, 30-H), 6.26 (s, 2H,
NH), 6.12 (s, 1H, 7-H), 3.85 (s, 3H, OMe) ppm; 13C NMR (126 MHz, DMSO-d6):
δ
= 179.71 (C-6), 167.28
(C-2), 159.86 (C-7a), 153.98 (C-5), 152.18(C-3), 131.58 (C-20), 130.82 (C-3a), 126.93 (C-10), 116.23 (C-40),
114.06 (C-30), 103.10 (C-7), 99.84 (C-4), 56.03 (OMe) ppm; MS (ESI, MeOH): m/z (%) = 270.2 ([M + H]+,
100), 292.0 ([M + Na]+, 14), 302.1 ([M + H + MeOH]+, 20), 423.5 ([3M + Ca]2+, 18), 560.8 ([2M + Na]+,
90); analysis calcd for C15H11NO4 (269.25): C 66.91, H 4.12, N 5.20; found: C 66.72, H 4.34, N 5.02.
4.14. N-(4-(5-Methoxy-2,6-dioxo-2,6-dihydrobenzofuran-3-yl)phenyl)benzamide (20)
To a solution of 19 (200 mg, 0.74 mmol) in DCM (50 mL) and triethylamine (0.11 mL, 0.8 mmol)
at 0 ◦C benzoylchloride (0.93 mL, 0.8 mmol) was added and the reaction was stirred for 6 h. Usual
aqueous workup followed by column chromatography (SiO2, CHCl3/MeOH, 9:1) gave 20 (191 mg,
69%) as a red solid; m.p. 259–264 ◦C; RF = 0.11 (SiO2, CHCl3); IR (ATR):
ν = 3377w, 3021w, 2973w,
2934w, 1811w, 1782m, 1749w, 1669m, 1629s, 1587s, 1572s, 1512s, 1489s, 1452m, 1421m, 1408s, 1351m,
1321s, 1303m, 1250s, 1232s, 1181s, 1158s, 1124m, 1105m, 1079w, 1066w, 1020w, 991m, 934m, 904m,
854m, 847s, 837s, 827s, 799m, 789m, 720s, 699m, 690m, 669m, 625s, 610s, 546s cm−1; UV–Vis (methanol):
λ
max (log
NH), 8.37–8.32 (m, 2H, 2
(m, 1H, 40’-H), 7.45–7.38 (m, 2H, 2
13C-NMR (126 MHz, pyridine-d5):
(C-5), 143.12 (C-40), 138.18 (C-3a), 136.42 (C-10’), 132.47 (C-40’), 131.38 (2x C-20), 129.19 (2x C-30’), 128.86
ε
) = 229 (3.89), 307 (3.97), 452 (3.92) nm; 1H-NMR (500 MHz, pyridine-d5):
20-H), 8.27–8.21 (m, 2H, 2 20’-H), 8.09–8.05 (m, 2H, 2
30’-H), 6.71 (s, 1H, 4-H), 6.32 (s, 1H, 7-H), 3.77 (s, 3H, OMe) ppm;
= 180.98 (C-6), 167.90 (C=O), 167.75 (C-2), 160.91(C-7a), 156.36
δ
= 11.36 (s, 1H,
×
×
×
30-H), 7.50–7.46
×
δ
(2
×
C-20’), 127.09 (C-3), 125.27 (C-10), 121.63 (2
×
C-30), 105.16 (C-7), 99.25 (C-4), 56.70 (OMe) ppm;
MS (ESI, MeOH): m/z (%) = 374.20 ([M + H]+, 46), 396.07 ([M + Na]+, 18), 406.0 ([M + MeOH + H]+,
100), 428.1 ([M + MeOH + Na]+, 32), 766.7 ([2M + Na]+, 55), 800.7 ([2M + MeOH + Na]+, 70), 832.9

Molecules 2020, 25, 1978
12 of 15
([2M + 2MeOH + Na]+, 64); analysis calcd for C22H15NO5 (373.36): C 70.77, H 4.05, N 3.75; found: C
70.51, H 4.35, N 3.52.
4.15. 6-Hydroxy-5-methoxy-3-phenyl-3-(2,4,5-trimethoxyphenyl)benzofuran-2(3H)-one (21)
To a solution of 2 (240 mg, 1.0 mmol) and benzene (45 µL, 0.5 mmol) hydrochloric acid (25 mL) was
added and the mixture was stirred for 2 h. Usual aqueous work-up followed by column chromatography
(SiO2, hexanes/CHCl3, 1:1) gave 21 (74 mg, 18%) as a pale yellow crystalline solid; m.p. 112–117 ◦C;
RF = 0.63 (SiO2, CHCl3); IR (KBr):
1638s, 1586s, 1527m, 1516m, 1461m, 1427w, 1382w, 1412m, 1388m, 1348s, 1322w, 1233s, 1190s, 1140m,
1043m, 991m, 936m, 852s, 824m, 781m, 689m cm−1; UV-Vis (MeOH):
max (log ) = 239 (3.05), 284 (3.01),
ν = 3432br, 3081w, 3062w, 3008w, 2961w, 2941w, 2842w, 1763w,
λ
ε
331 (3.37) nm1H NMR (500 MHz, CDCl3):
δ
= 7.45 (d, J = 8.0 Hz, 2H, 20’Ha + 20’-Hb), 7.38–7.31 (m, 3H,
3”-Ha + 30’-Hb + 40’-H), 6.81 (s, 1H, 7-H), 6.54 (s, 1H, 4-H), 6.50 (s, 1H, 30-H), 6.41 (s, 1H, 60-H), 5.85 (s,
1H, OH), 3.86 (s, 3H, OMe(20)), 3.81 (s, 3H, OMe(5)), 3.62 (s, 3H, OMe(50)), 3.61 (s, 3H, OMe(40)) ppm;
13C NMR (126 MHz, CDCl3):
δ
= 178.27 (C-2), 151.58 (C-20), 149.83 (C-4), 147.79 (C-7a), 146.46 (C-6),
143.63 (C-5), 143.13 (C-50), 138.09 (C-10’), 129.27 (2
×
C-20’), 128.53 (2
×
C-30’), 128.32 (C-40’), 123.36
(C-10), 120.65 (C-3b), 114.05 (C-60), 108.18 (C-4), 99.07 (C-30), 98.57 (C-7), 57.96 (C-3), 56.9 (OMe(40)),
56.9 (OMe(50)), 56.8 (OMe(5)), 56.2 (OMe(20)) ppm; MS (ESI): m/z (%) = 421.2 ([M
−
H]−, 100); analysis
calcd for C24H22O7 (422.43): C 68.24, H 5.25; found: C 68.10, H 5.37.
5. Conclusions
The reaction
2 in the presence of hydrochloric acid led to the formation of the highly substituted
p-QM
6
. The putative mechanism of this reaction was deduced from 13C-NMR labeling experiments
as well by the ESI-MS identification of several intermediates. The mechanism for the formation of
includes the equilibrium between a Friedel–Crafts alkylation and a retro Friedel–Crafts alkylation.
6
Supplementary Materials: The following are available online: NMR spectra of compounds (1H and 13C (APT)-NMR).
Author Contributions: I.S., S.S. and R.C. conceived and designed the experiments; I.S., A.L. and S.S. performed
the experiments; D.S. performed the NMR measurements; P.L. analyzed the crystal structure; I.S. and R.C. analyzed
the data and wrote the paper. All authors have read and agreed to the published version of the manuscript.
Funding: This research was funded by ScienceCampus Halle (W21010249).
Acknowledgments: We like to thank the late R. Kluge for the ESI-MS spectra and the NMR team for numerous
NMR spectra. The UV-Vis and the IR spectra were recorded by V. Simon. Elemental analyses were performed by
S. Ludwig.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
Huneck, S. The significance of lichens and their metabolites. Naturwissenschaften 1999, 86, 559–570. [CrossRef]
[PubMed]
Molnar, K.; Farkas, E. Current Results on Biological Activities of Lichen Secondary Metabolites: A Review.
Z. Naturforsch. C 2010, 65, 157–173. [CrossRef] [PubMed]
Shukla, V.; Joshi, G.P.; Rawat, M.S.M. Lichens as a potential natural source of bioactive compounds: A review.
Phytochem. Rev. 2010, 9, 303–314. [CrossRef]
Koyama, M.; Kelly, T.R.; Watanabe, K.A. Novel Type of Potential Anticancer Agents - Derived from
Chrysophanol and Emodin - Some Structure Activity Relationship Studies. J. Med. Chem. 1988, 31, 283–284.
[CrossRef] [PubMed]
5.
Koyama, M.; Takahashi, K.; Chou, T.C.; Darzynkiewicz, Z.; Kapuscinski, J.; Kelly, T.R.; Watanabe, K.A.
Intercalating Agents with Covalent Bond Forming Capability—A Novel Type of Potential Anticancer
Agents.2. Derivatives of Chrysophanol and Emodin. J. Med. Chem. 1989, 32, 1594–1599. [CrossRef] [PubMed]

Molecules 2020, 25, 1978
13 of 15
6.
7.
Wang, W.F.; Bai, Z.D.; Zhang, F.S.; Wang, C.H.; Yuan, Y.F.; Shao, J.W. Synthesis and biological activity,
evaluation of emodin quaternary ammonium salt derivatives as potential anticancer agents. Eur. J. Med. Chem.
2012, 56, 320–331. [CrossRef]
Sreelakshmi, V.; Raj, N.; Abraham, A. Evaluation of the Drug-like Properties of Kaempferol, Chrysophanol
and Emodin and their Interactions with EGFR Tyrosine Kinase—An in silico Approach. Nat. Prod. Commun.
2017, 12, 915–920. [CrossRef]
8.
9.
Deitersen, J.; El-Kashef, D.H.; Proksch, P.; Stork, B. Anthraquinones and autophagy—Three rings to rule
them all? Bioorgan. Med. Chem. 2019, 27, 115042. [CrossRef]
Kamil, M.; Haque, E.; Mir, S.S.; Irfan, S.; Hasan, A.; Sheikh, S.; Alam, S.; Ansari, K.M.; Nazir, A.
Hydroxyl Group Difference between Anthraquinone Derivatives Regulate Different Cell Death Pathways via
Nucleo-Cytoplasmic Shuttling of p53. Anti-Cancer Agent Me 2019, 19, 184–193. [CrossRef]
10. Thompson, D.C.; Thompson, J.A.; Sugumaran, M.; Moldeus, P. Biological and Toxicological Consequences of
Quinone Methide Formation. Chem.-Biol. Interact. 1993, 86, 129–162. [CrossRef]
11. Bolton, J.L.; Shen, L. p-Quinone methides are the major decomposition products of catechol estrogen
o-quinones. Carcinogenesis 1996, 17, 925–929. [CrossRef]
12. Pina, I.C.; Sanders, M.L.; Crews, P. Puupehenone congeners from an indo-pacific Hyrtios sponge. J. Nat. Prod.
2003, 66, 2–6. [CrossRef] [PubMed]
13. Gripenberg, J. Fungus Pigments.16. Pigments of Peniophora Sanguinea Bres. Acta Chem. Scand. 1965, 19,
2242–2243. [CrossRef]
14. Sum, T.J.; Sum, T.H.; Galloway, W.R.J.D.; Spring, D.R. Divergent Total Syntheses of Flavonoid Natural
Products Isolated from Rosa rugosa and Citrus unshiu. Synlett 2016, 27, 1725–1727.
15. Hart, D.J.; Cain, P.A.; Evans, D.A. Approaches to Synthesis of Masked Para-Quinone Methides—Applications
to Total Synthesis of (+/-)-Cherylline. J. Am. Chem. Soc. 1978, 100, 1548–1557. [CrossRef]
16. Wang, L.L.; Candito, D.; Drager, G.; Herrmann, J.; Muller, R.; Kirschning, A. Harnessing a p-Quinone
Methide Intermediate in the Biomimetic Total Synthesis of the Highly Active Antibiotic 20-Deoxy-Elansolid
B1. Chem. Eur. J. 2017, 23, 5291–5298. [CrossRef]
17. Mei, G.J.; Xu, S.L.; Zheng, W.Q.; Bian, C.Y.; Shi, F. [4+2] Cyclization of para-Quinone Methide Derivatives
with Alkynes. J. Org. Chem. 2018, 83, 1414–1421. [CrossRef]
18. Parra, A.; Tortosa, M. para-Quinone Methide: A New Player in Asymmetric Catalysis. Chemcatchem 2015, 7,
1524–1526. [CrossRef]
19. Yang, C.; Gao, S.; Yao, H.Q.; Lin, A. Rhodium-Catalyzed Hydroacylation of para-Quinone Methides with
Salicylaldehydes: An Approach to alpha,alpha-Diaryl-2-Hydroxy Acetophenones. J. Org. Chem. 2016, 81,
11956–11964. [CrossRef]
20. Zhang, Z.P.; Dong, N.; Li, X. Bismuth-catalyzed allylation of para-quinone methides. Chem. Commun. 2017
53, 1301–1304. [CrossRef]
,
21. Caruana, L.; Kniep, F.; Johansen, T.K.; Poulsen, P.H.; Jorgensen, K.A. A New Organocatalytic Concept for
Asymmetric alpha-Alkylation of Aldehydes. J. Am. Chem. Soc. 2014, 136, 15929–15932. [CrossRef] [PubMed]
22. Ralph, B.J.; Robertson, A. The Chemistry of Fungi.14. 2-4-5-Trihydroxyphenylglyoxylic Acid from Polyporus
Tumulosus Cooke. J. Chem. Soc. 1950, 3380–3383. [CrossRef]
23. Gasonoo, M.; Klumpp, D.A. Synthesis of functionalized 2-oxindoles by Friedel-Crafts reactions. Tetrahedron
Lett. 2015, 56, 4737–4739. [CrossRef]
24. Kinthada, L.K.; Ghosh, S.; Babu, K.N.; Sharique, M.; Biswas, S.; Bisai, A. Friedel-Crafts alkylations of
electron-rich aromatics with 3-hydroxy-2-oxindoles: Scope and limitations. Org. Biomol. Chem. 2014, 12,
8152–8173. [CrossRef]
25. Kleeblatt, D.; Cordes, C.A.; Lebrenz, P.; Hein, M.; Feist, H.; Matin, A.; Raza, R.; Iqbal, J.; Munshi, O.;
Rahman, Q.; et al. Synthesis and antiproliferative activity of N-glycosyl-3,3-diaryloxindoles. RSC Adv. 2014
4, 22828–22839. [CrossRef]
,
26. Klumpp, D.A.; Yeung, K.Y.; Prakash, G.K.S.; Olah, G.A. Preparation of 3,3-diaryloxindoles by
superacid-induced condensations of isatins and aromatics with a combinatorial approach. J. Org. Chem.
1998, 63, 4481–4484. [CrossRef]

Molecules 2020, 25, 1978
14 of 15
27. Liu, T.; Lai, Y.H.; Yu, Y.Q.; Xu, D.Z. A facile and efficient procedure for one-pot four-component synthesis of
polysubstituted spiro pyrano[2 C-c]pyrazole and spiro 1,4-dihydropyridine catalyzed by a Dabco-based
¨
ionic liquid under mild conditions. New J. Chem. 2018, 42, 1046–1051. [CrossRef]
28. Liu, Q.; Li, S.; Chen, X.Y.; Rissanen, K.; Enders, D. Asymmetric Synthesis of Spiro-oxindole-epsilon-lactones
through N-Heterocyclic Carbene Catalysis. Org. Lett. 2018, 20, 3622–3626. [CrossRef]
29. Wu, Q.X.; Crews, M.S.; Draskovic, M.; Sohn, J.; Johnson, T.A.; Tenney, K.; Valeriote, F.A.; Yao, X.J.;
Bjeldanes, L.F.; Crews, P. Azonazine, a Novel Dipeptide from a Hawaiian Marine Sediment-Derived Fungus,
Aspergillus insulicola. Org. Lett. 2010, 12, 4458–4461. [CrossRef]
30. Jha, O.P. Synthesis of Abutic Acid (5,6-Dimethoxycoumarone-2,3-Dicarboxylic Acid). Indian J. Chem. 1973
11, 989–990.
,
31. Zhuang, J.; Wang, C.Q.; Xie, F.; Zhang, W.B. One-pot efficient synthesis of aryl alpha-keto esters from
aryl-ketones. Tetrahedron 2009, 65, 9797–9800. [CrossRef]
32. Crowden, R.K. Biosynthesis of Polyphenolic Acid Metabolites of Polyporus Tumulosus Cooke.
Can. J. Microbiol. 1967, 13, 181–197. [CrossRef] [PubMed]
33. Davis, A.L.; Ravel, J.M.; Skinner, C.G.; Shive, W. Synthesis and biological activity of some diamino acid
analogs. Arch. Biochem. Biophys. 1958, 76, 139–147. [CrossRef]
34. Hargreaves, K.R.; McGookin, A.; Robertson, A. Polyhydroxyphenylglyoxylic acids. J. Appl. Chem. 1958, 8,
273–285. [CrossRef]
35. Sanchez-Viesca, F.; Flores, J.M. Synthesis of 5-(2,4,5-trimethoxyphenyl)oxazolidine-2,4-dione. Rev. Latinoam.
Quim. 1978, 9, 216–218.
36. Burkett, H.; Cassady, J.; Schultz, F. Acid-Catalyzed Decarbonylation of 2,4,6-Trimethoxybenzaldehyde
Accompanying Condensation Reactions. J. Org. Chem. 1961, 26, 2072–2075. [CrossRef]
37. Beebe, X.; Nilius, A.M.; Merta, P.J.; Soni, N.B.; Bui, M.H.; Wagner, R.; Beutel, B.A. Synthesis and SAR
evaluation of oxadiazolopyrazines as selective Haemophilus influenzae antibacterial agents. Bioorg. Med.
Chem. Lett. 2003, 13, 3133–3136. [CrossRef]
38. Joussot, J.; Schoenfelder, A.; Larquetoux, L.; Nicolas, M.; Suffert, J.; Blond, G. Synthesis of 3-Substituted
Chromones and Quinolones from Enaminones. Synthesis 2016, 48, 3364–3372. [CrossRef]
39. Sui, G.; Sun, J.; Yang, L.; Zhang, W.; Zhou, K.; Zhang, B.; Li, Y.; Liu, G.; Zhou, W. Synthesis and in vitro
antifungal activities of α, β-unsaturated ketones as analogues of Kakuol. Heterocycles 2017, 94, 727–741.
40. Sum, T.H.; Sum, T.J.; Collins, S.; Galloway, W.R.J.D.; Twigg, D.G.; Hollfelder, F.; Spring, D.R. Divergent
synthesis of biflavonoids yields novel inhibitors of the aggregation of amyloid
β (1-42). Org. Biomol. Chem.
2017, 15, 4554–4570. [CrossRef]
41. Sum, T.J.; Sum, T.H.; Galloway, W.R.J.D.; Twigg, D.G.; Ciardiello, J.J.; Spring, D.R. Synthesis of structurally
diverse biflavonoids. Tetrahedron 2018, 74, 5089–5101. [CrossRef]
42. Agranat, I.; Bentor, Y.; Shih, Y.S. Remarkable Reversibility in Aromatic Friedel-Crafts Acylations—Para
Reversible Ortho Acyl Rearrangements of Fluorofluorenones in Polyphosphoric Acid. J. Am. Chem. Soc.
1977, 99, 7068–7070. [CrossRef]
43. Agranat, I.; Oded, Y.N.; Mala’bi, T.; Pogodin, S.; Cohen, S. The linkage between reversible Friedel-Crafts acyl
rearrangements and the Scholl reaction. Struct. Chem. 2019, 30, 1579–1610. [CrossRef]
44. Agranat, I.; Shih, Y.; Bentor, Y. Incursion of Reversibility in Friedel-Crafts Acylations. J. Am. Chem. Soc. 1974
96, 1259–1260. [CrossRef]
,
45. Elliott, M.C.; Wordingham, S.V. Reversibility in Lewis-acid promoted reactions of N-arylcinnamamides.
Synlett 2004, 5, 898–900. [CrossRef]
46. Frangopol, M.; Genunche, A.; Negoita, N.; Frangopol, P.T.; Balaban, A.T. A Study of Reversibility of
Friedel-Crafts C-Acetylations by Means of 14c-Labelling. Tetrahedron 1967, 23, 841–844. [CrossRef]
47. Levy, L.; Pogodin, S.; Cohen, S.; Agranat, I. Reversible Friedel-Crafts acylations of phenanthrene:
Rearrangements of acetylphenanthrenes. Lett. Org. Chem. 2007, 4, 314–318. [CrossRef]
48. Mala’bi, T.; Pogodin, S.; Agranat, I. Reversible Friedel-Crafts Acylations of Anthracene: Rearrangements of
Acetylanthracenes. Lett. Org. Chem. 2009, 6, 237–241. [CrossRef]
49. Smith, L.E.; Haller, H.L. Action of acetic acid on 3,4-dimethoxyphenyldiazonium borofluoride. J. Am. Chem.
Soc. 1934, 56, 237–239. [CrossRef]

Molecules 2020, 25, 1978
15 of 15
50. Guia, M.; Guerzio, M. Synthesis of coumarandiones with ethyl oxalyl chloride. Gazz. Chim. Ital. 1962, 92,
1474–1477.
51. Romeo, A.; Bargellini, G. Flavonic compounds with the methylenedioxy group in the chromone nucleus.
The mechanism of formation of o-hydroxychalcones and flavanones. Ann. Chim. 1952, 42, 361–369.
Sample Availability: Samples of all compounds are available from the authors.
©
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).