Synthesis and Antifungal Activity of Chromones and Benzoxepines from the Leaves of Ptaeroxylon obliquum

Article abstract of DOI:10.1021/acs.jnatprod.0c00587

This study reports the first total synthesis of the bioactive oxepinochromones 12-O-acetyleranthin (8) (angular isomer) and 12-O-acetylptaeroxylinol (9) (linear isomer). The antifungal activity of these compounds and their derivatives was determined against Candida albicans and Cryptococcus neoformans. Most compounds had good selectivity between the two fungi and showed moderate to good activity. 12-O-Acetyleranthin (8) had the highest activity against C. albicans, with an MIC value of 9.9 μM, while 12-O-acetylptaeroxylinol (9), the compound present in Ptaeroxylon obliquum, had the highest activity against C. neoformans, with an MIC value of 4.9 μM.

Full text of DOI:10.1021/acs.jnatprod.0c00587

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Article
Synthesis and Antifungal Activity of Chromones and Benzoxepines
from the Leaves of Ptaeroxylon obliquum
Modibo S. Malefo, Thanyani E. Ramadwa, Ibukun M. Famuyide, Lyndy J. McGaw, Jacobus N. Eloff,
Molahlehi S. Sonopo, and Mamoalosi A. Selepe*
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ABSTRACT: This study reports the first total synthesis of the bioactive
oxepinochromones 12-O-acetyleranthin (8) (angular isomer) and 12-O-
acetylptaeroxylinol (9) (linear isomer). The antifungal activity of these
compounds and their derivatives was determined against Candida albicans
and Cryptococcus neoformans. Most compounds had good selectivity
between the two fungi and showed moderate to good activity. 12-O-
Acetyleranthin (8) had the highest activity against C. albicans, with an MIC
value of 9.9 μM, while 12-O-acetylptaeroxylinol (9), the compound present
in Ptaeroxylon obliquum, had the highest activity against C. neoformans, with
an MIC value of 4.9 μM.
nfections caused by pathogenic or opportunistic fungi are
Idifficult to cure and are considered a great health threat to
humankind worldwide,1 mainly due to the continuous spread
of invasive fungal infections and the associated high mortality
rate, which surpasses one and a half million annually.1−4 These
infections and deaths mostly affect people with underlying
conditions such as cancer or HIV/AIDS.5 Species of
Cryptococcus, Candida, Aspergillus, Mucormycosis, and Pneumo-
cystis are at the forefront of invasive fungal infections.6,7
Although there are a number of agents available that can be
used to treat fungal infections, only a few are approved for
clinical use. The antifungals that are currently used are divided
into four classes on the basis of their targets and mechanism of
action: the polyenes, e.g., amphotericin B (1), the azoles, e.g.,
fluconazole (2), the echinocandins, e.g., caspofungin (3), and
the pyrimidine analogues, e.g., olorofim (4) (Figure 1).1−4,6−8
The azoles have been used as the first-line treatment for
candidiasis infection for many years.9,10 However, very recently
there have been reports that show that candidiasis is showing
reduced susceptibility toward the azoles.11 This led to the use
of the echinocandins as the mainstay treatment for invasive
Candida infections, particularly to patients with compromised
immune systems since they display favorable tolerability and a
reduced toxicity profile.12 However, some candidiasis-causing
species such as Candida albicans, C. glabrata, and C. parapsilosis
are resistant to these drugs when used for a prolonged
duration, and they can only be administered intrave-
nously.6,13,14 Owing to the high mortality rate, high costs,
and the rapid development of drug resistance of fungal
pathogens against the current antifungal treatment, there is an
urgent need for the development of new drugs with new
mechanisms of action, reduced toxicity profiles, and reduced
susceptibility to drug resistance.3,15,16 Current investigational
antifungal agents with different skeletons from the existing
ones include a triterpenoid analogue, ibrexafungerp (5),
derived from enfumafungin (Figure 1).17,18 The two
triterpenoids act as specific inhibitors of glucan synthesis.17,19
One of the most frequently explored classes of natural
products is the chromones. Chromone scaffolds are found in
many compounds with a variety of biological activities
including antifungal, anti-inflammatory, antiallergenic, antiviral,
antitubulin, antihypertensive, antimalarial, neuroprotective,
and anti-HIV activity.16,20−22 The benzoxepinochromones
represent a rare class of chromone compounds that were first
isolated by Dean et al. from the sneezewood tree in 1966.23
Few compounds carrying the benzoxepinochromone architec-
ture have been isolated since, mostly from the species
Ptaeroxylon obliquum (Rutaceae) (Thunb.) Radlk. and
Eranthus hiemalis (Ranunculaceae) Salisb. The isolated
benzoxepinochromones from P. obliquum include desokarenin,
Received: May 25, 2020
© XXXX American Chemical Society and
American Society of Pharmacognosy
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Figure 1. Structures of the current commonly used antifungal drugs and the investigational triterpenoid ibrexafungerp (5).
karenin, ptaeroxylinol (7), 12-O-acetylptaeroxylinol (9), and
ptaeroglycol.23−27 Those isolated from E. hiemalis include
eranthin (6) (Figure 2), its glucoside, and eranthinglycol.28,29
Bruder et al.31 The key intermediate noreugenin (11) was
prepared from phloroacetophenone (10) via the Kostanecki−
Robinson reaction on a gram scale (2−4 g). Compound 11
was allylated with allyl bromide and subsequently subjected to
Claisen rearrangement conditions at 220 °C in N,N-diethylani-
line to give a mixture of regioisomers 12 and 13 in a ratio of
11:2. Purification by column chromatography gave 8-allyl
isomer 12 in a 33% yield and 6-allyl isomer 13 in a 6% yield.
The structures of the regioisomers 12 and 13 were confirmed
by 1D and 2D NMR data analysis. Other conditions that were
tested for the Claisen rearrangement such as using DMF as a
solvent or conducting the reaction in a microwave oven or neat
still led to lower yields of 8- and 6-allyl isomers 12 and 13.
Having successfully prepared regioisomers 12 and 13, access to
the total synthesis of 12-O-acetylptaeroxylinol (9) and 12-O-
acetyleranthin (8) could be achieved rapidly. Thus, the 7-O-
alkylation of 12 and 13 with 3-bromo-2-(tert-
butyldiphenylsiloxy)methylpropene followed by ring-closing
metathesis (RCM) with the Grubbs second-generation
(Grubbs II) catalyst in dry CH2Cl2 and subsequent desilylation
with TBAF afforded the requisite eranthin (6) and
ptaeroxylinol (7), respectively.31 Finally, acetylation of 6 and
7 afforded 12-O-acetyleranthin (8) and 12-O-acetylptaerox-
ylinol (9) in 22% and 35% yields, respectively.
Figure 2. Structures of oxepinochromones 6, 7, 8, and 9.
Important to note is that the benzoxepinochromones isolated
from P. obliquum are mostly linear isomers, while those isolated
from E. hiemalis are angular isomers.
Continuation of the work on P. obliquum led to the isolation
of the O-acetyl derivative of eranthin 8, named obliquumol,
and other known terpenoids.26,27 The isolated compounds
were tested against various fungal species. Obliquumol
emerged as the most potent antifungal compound as reported
by Van Wyk and colleagues.26,27,30 Therefore, the oxepino-
chromone moiety could represent a new scaffold of antifungal
leads.27 Inspired by the high potency of obliquumol against
several fungal strains, including the Candida species, it was
decided to prepare the natural oxepinochromones and their
derivatives in order to provide a sufficient amount of material
for further evaluation of their antifungal properties and
establish the scaffolds responsible for the antifungal activity.
Since both the angular and linear oxepinochromones 8 and 9
were previously reported from P. obliquum,24,26,27,32 distinction
between the two natural compounds can be a challenge and
may lead to misassignment due to the similarities of the two
structures. Therefore, it was important to characterize the two
synthesized compounds using different analytical methods that
include HRMS and 1D and 2D NMR spectroscopy and use the
data as reference for confirming the structures of the isolated
compounds unequivocally. The molecular formula of com-
pound 8 was determined to be C17H16O6 on the basis of the
HRESIMS (m/z 317.1052 [M + H]+; calcd for C17H17O6,
RESULTS AND DISCUSSION
■
The synthesis of the two oxepinochromones 12-O-acetyler-
anthin (8) and 12-O-acetylptaeroxylinol (9) was undertaken as
shown in Scheme 1 following the reported procedure by
1
317.1020) and the 13C NMR data. The H NMR data (Table
B
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a
Scheme 1. Total Synthesis of 12-O-Acetyleranthin (8) and 12-O-Acetylptaeroxylinol (9)
a
Reagents and conditions: (a) Py, Ac2O, 90% (10a); (b) K2CO3, CH3CN, heat; (c) H2O, K2CO3, CH3OH, reflux, 61% (11) over 2 steps; (d) allyl
bromide, K2CO3, DMF, 63% (11a); (e) N,N-diethylaniline, reflux, 33% (12) and 6% (13); (f) 3-bromo-2-(tert-butyldiphenylsiloxy)-
methylpropene, K2CO3, DMF, 42% for 8-allyl (12a) and 70% for 6-allyl (13a); (g) Grubb’s II, CH2Cl2, 40 °C; (h) TBAF, THF, 0 °C, 65% (6) and
40% (7) over 2 steps; AcCl, K2CO3, CH2Cl2, 35% (8) and 22% (9).
1
Table 1. H NMR (400 MHz) and 13C NMR (100 MHz) Data of the Synthesized Compounds 6 and 8 and Comparison with
Obliquumol in CDCl3.
6
8
obliquumol26,27
δH (J in Hz)
position
δH (J in Hz)
δC
δH (J in Hz)
δC
δC
2
3
4
4a
5
6
6a
8
166.8
108.7
183.1
107.0
160.1
104.6
164.4
70.8
166.8
108.8
183.1
107.2
160.3
104.7
164.3
71.0
167.14
108.66
182.68
106.72
155.79
99.29
6.05, s
6.06, s
5.99, q (0.7)
6.46, s
4.72, s
6.49, s
6.47, s
164.39
71.04
4.65, d (1.4)
4.61, tt (1.6, 1.6)
9
138.5
124.2
21.4
111.0
153.8
65.4
133.8
127.4
21.6
111.4
153.7
66.5
170.7
20.9
20.5
133.23
128.30
21.14
115.82
158.06
66.45
170.61
20.42
20.78
10
11
11a
11b
12
13
14
2-Me
HO-5
5.99, t (5.3)
3.61, d (5.3)
6.05−6.03, m
3.63, d (5.5)
6.02, tt (5.5, 1.2)
3.52, d (5.6, 1.2, 1.6)
4.03, s
4.46, s
4.41, brs
2.05, s
2.38, s
12.66, s
2.01, s
2.34, d (0.7)
12.94, brs
2.38, s
12.61, s
20.5
1) displayed three one-proton singlets at δH 12.66 (HO-5)
(representing the hydrogen-bonded hydroxy group), δH 6.49
(H-6) in the aromatic region, and δH 6.06 (H-3) characteristic
of a 2-substituted benzopyran-4-one moiety, while the signal at
δH 6.05−6.03 (H-10, m) indicated the presence of a vinylic
and an aromatic CH signal at δC 104.7. The oxygenated
olefinic carbon at δC 166.8 and the olefinic carbon at δC 108.8
were indicative of the presence of the chromone moiety. The
signals at δC 71.0 and 66.5 were indicative of the presence of
the oxymethylene carbons, and the signal at δC 21.6 indicated
the allylic methylene group. The signals at δC 133.8 and 127.4
were indicative of a vinylic group, and those at δC 20.9 and
20.5 indicated the presence of two methyl groups. Analysis of
the COSY spectrum showed correlations of H-10/H2-11. The
HMBC spectrum indicated the H-6 (δH 6.49)/C-4a, C-5, C-
6a, and C-11a correlations as illustrated in Figure 3. The
additional HMBC correlations between H2-11 (δH 3.63)/C-6a,
C-9, C-10, C-11a, and C-11b confirmed the presence of the
oxepine ring. This was further corroborated by the correlation
of H-10 (δH 6.05−6.03)/C-8, C-9, and C-11a. These
correlations imply that the oxepine ring is attached at C-11a,
consistent with the angular structure of 8. On the other hand,
1
proton. The H NMR data also indicated the presence of two
sets of oxymethylene protons at δH 4.65 (d, J = 1.4, H-8) and
δH 4.46 (s, H-12); an allylic methylene group at δH 3.63 (d, J =
5.5, H-11); a methyl group at δH 2.38 (3H, s, 2-Me); and a
signal for the methyl of the acetoxy group at δH 2.05 (3H, s,
CO2CH3). Seventeen signals were observed in the 13C NMR
spectrum and allocated by means of HSQC, HMBC, and
DEPT-135 NMR data (Table 1), with the signal of the
ketocarbonyl carbon appearing at δC 183.1 and that of the
acetoxy carbonyl appearing at δC 170.7. The six aromatic
carbon signals consisted of three C−O signals at δC 164.3,
160.3, and 153.7; two tertiary carbons at δC 111.4 and 107.2,
C
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obliquumol compared rather favorably with those of the
synthesized compound 9 (Table 2) at δC 99.4, 116.0, and
158.2, representing C-11, C-5a, and C-5, respectively. These
findings suggest the structure of obliquumol correlated with
that of the linear isomer 12-O-acetylptaeroxylinol (9) and not
the angular isomer 12-O-acetyleranthin (8). Although eranthin
(6) has been isolated and synthesized previously,28,31 it is
noteworthy that its O-acetyl derivative 8 has not previously
been synthesized nor isolated.
Figure 3. Key correlations, HMBC (blue arrow) and COSY (thick
black line), observed for compounds 8 and 9.
Having successfully synthesized the angular and linear
oxepinochromones 6, 7, 8, and 9, the next task was to prepare
derivatives of these compounds in order to evaluate their
biological activities and determine the scaffolds responsible for
the antifungal activities. First, the benzoxepine 22, devoid of
the chromone moiety, was prepared. As shown in Scheme 2,
the synthesis of 22 began with the selective protection of 10
with MOMCl to give 2-hydroxy-4,6-dimethoxymethoxy-
acetophenone (14) in 50% isolated yield.33 The allylation of
14 with allyl bromide via the Williamson ether synthesis
proceeded smoothly to give the allyl ether 15 in a 79% yield,
which set the stage for the Claisen rearrangement reaction.
Upon thermally heating the allyl ether 15 at 220 °C in N,N-
diethylaniline for 24 h, compound 16 was obtained in 50%
yield. Treatment of 16 with CH3I in the presence of K2CO3
generated 17 in 98% yield, which was followed by the
deprotection with 3 M HCl/CH3OH (1:10) to give phenol 18
in 79% yield. Methylation of phenol 18 afforded dimethoxy-
acetophenone 19 in 89% yield, followed by the removal of the
MOM protecting group with 3 M HCl/CH3OH (1:5) to
afford 20 in 81% yield. Treating compound 20 with 3-bromo-
2-(tert-butyldiphenylsiloxy)methylpropene afforded the fully
protected 21 in 59% yield.31 Subjecting compound 21 to RCM
with Grubbs II catalyst, followed by removal of the TBDPS
group with TBAF, gave the benzoxepine 22 in 32% yield.31 It
is worth mentioning that attempts to purify the product
formed after RCM were hampered by instability when the
mixture was exposed to silica gel and air; hence it was
H-3 (δH 6.06, s) correlated to Me-2, indicating that the methyl
group was linked to C-2 of the benzopyran system. The
presence of the acetyl group was identified by the strong
HMBC correlation between H2-12 and the ester carbonyl
carbon at δC 170.7.
Similarly, the molecular formula for compound 9 was
identical to that of compound 8 from the HRESIMS
(317.1052 [M + H]+; calcd for C17H17O6, 317.1020) and
13C NMR data (Table 2). The NMR data of compound 9 were
similar to those of compound 8, except for the deshielded
signal at δH 6.54 in the aromatic region. This signal (6.54 ppm)
was assigned to H-11 based on its long-range HMBC
correlations to C-10a, C-11a, C-5a, and C-4a as illustrated in
Figure 3. The correlation of H2-6 (δH 3.59) to C-5, C-8, C-7,
and C-5a confirmed the presence of the oxepine ring and its
attachment at C-5a. The data of the synthesized compound 9
were consistent with those of the natural compound 12-O-
acetylptaeroxylinol (9) reported by Agostinho et al., except for
a few signals that were interchanged.24
1
Comparison of the H and 13C NMR data of the isolated
natural compound obliquumol with those of the synthesized
12-O-acetyleranthin (8) showed some inconsistencies (Table
1). There was a clear deviation of the C-6, C-11a, and C-11b
carbon signals. These resonances for the synthesized 8
occurred at δC 104.7, 111.4, and 153.7 and for the natural
product obliquumol at δC 99.29, 115.82, and 158.06,
respectively.26,27 The carbon signals for the natural product
1
Table 2. H NMR (400 MHz) and 13C NMR (100 MHz) Data of the Synthesized Compounds 7 and 9 and Comparison with
a
the Isolated 12-O-Acetylptaeroxylinol in CDCl3
7
9
12-O-acetylptaeroxylinol24
position
δH (J in Hz)
6.04, s
δC
δH (J in Hz)
δC
δH (J in Hz)
δC
2
3
4
4a
5
5a
6
7
8
9
10a
11
11a
12
13
14
2-Me
HO-5
167.2
108.7
182.8
106.7
158.1
115.8
21.1
125.4
138.0
71.0
167.1
108.8
182.8
106.9
158.2
116.0
21.2
128.4
133.2
71.2
164.5
99.4
155.9
66.6
170.7
20.9
20.5
167.2
108.8
182.5
114.1
158.2
133.2
21.3
6.05, brs
6.07, s
b
b
3.57, d (5.6)
6.00, td (5.6, 1.1)
3.59, d (5.5)
6.05−6.08, m
3.60, d (5.5)
6.09, t (5.5)
128.4
116.1
b
c
4.74, d (1.1)
6.52, s
4.65, d (1.5)
6.54, s
4.48,
s
66.1
164.6
99.3
155.9
65.7
167.2
99.4
6.56, s
154.3
b
c
4.03, s
4.46, s
4.67,
s
71.2
170.8
20.9
20.6
2.05 s
2.36, s
13.08, s
2.08 s
2.38, s
13.10, s
2.35, s
13.08, s
20.5
a−c
Signals with the same supercripted b and c letters in the same column are interchangeable.
D
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a
Scheme 2. Synthesis of Benzoxepine 22
a
Reagents and conditions: (a) ZnBr2, dimethoxymethane (DMM), AcCl, DIPEA, CH2Cl2, 50%; (b) allyl bromide, K2CO3, DMF, 79%; (c) N,N-
diethylaniline, 220 °C, 50%; (d) MeI, K2CO3, DMF, 150 °C, 98%; (e) 3 M HCl/CH3OH (1:10), reflux, 79%; (f) MeI, K2CO3, DMF, 150 °C,
89%; (g) 3 M HCl/CH3OH (1:10), reflux, 81%; (h) 3-bromo-2-(tert-butyldiphenylsiloxy)methylpropene, K2CO3, DMF, 59%; (i) Grubb’s II,
CH2Cl2, 40 °C; (j) TBAF, THF, 0 °C, 32%.
desilylated without any further purification. Derivatives 23 and
25 were prepared as shown in Scheme 3, by reacting
compounds 12 and 24 with 3-methylbut-2-enal (prenal)
under Ca(OH)2, respectively.33
Table 3. MIC (μM) Values of Ptaeroxylon obliquum
Derivatives against Candida albicans and Cryptococcus
neoformans Incubated for Different Times
MIC (μM)
a
Candida albicans
Cryptococcus neoformans
Scheme 3. Synthesis of Benzopyrans 23 and 25
compound
24 h
48 h
24 h
48 h
6
7
8
11.4
11.4
9.9
11.4
11.4
9.9
5.7
5.7
9.9
5.7
5.7
9.9
9
19.8
16.3
13.5
11.2
41.9
86.7
0.42
19.8
16.3
13.5
22.5
83.8
86.7
0.84
4.9
4.9
11
12
22
23
25
Amp-B
16.3
13.5
11.2
10.5
21.7
0.42
16.3
13.5
11.2
10.5
43.3
0.42
regioisomer 9 had the best activity against C. neoformans with
an MIC value of 4.9 μM. Interestingly, oxepinochromone 9
showed selectivity between the C. albicans and C. neoformans
fungi (Table 3). This selectivity may be attributed to the
position of the oxepine ring. However, oxepinochromone 8
showed equipotency across the strains, thus failing to show
selectivity between the two strains. Given the results, it was
decided to explore other benzoxepinochromones (6 and 7)
devoid of the acetoxy group. Removing the acetyl group in 8
and 9 was found to slightly decrease the activity as seen in
compounds 6 and 7 (Table 3). Both compounds 6 and 7
showed equipotent activity (11.4 μM) for C. albicans and C.
neoformans (5.7 μM). The benzoxepine 22 lacking the
chromone moiety also had activity comparable to those of 6
and 7 against C. albicans, which indicates the importance of the
a
Reagents and conditions: (a) 3-methylbut-2-enal, Ca(OH)2, 10% for
23 and 44% for 25.
The synthesized compounds were tested in vitro against the
fungi Candida albicans and Cryptococcus neoformans. The
minimum inhibitory concentrations (MICs, μM) of all tested
compounds are collated in Table 3. In this study amphotericin
B was used as a positive control. Of the nine screened
compounds, compound 25 was least potent against both fungi,
and compound 23 was ineffective against C. albicans.
Benzoxepinochromone 8 was the most active compound
against C. albicans with an MIC value of 9.9 μM, while its
E
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197.6 (C, ArCOCH3), 168.3 (2 × C, OCOCH3), 168.2 (C,
OCOCH3) 151.7 (C, C-4), 148.2 (2 × C, C-2, C-6), 125.0 (C, C-
1), 114.1 (2 × CH, C-3, C-5), 31.2 (CH3, ArCOCH3), 21.1 (CH3,
OCOCH3), 21.0 (2 × CH3, OCOCH3); HRMSESI m/z [M + Na]+
317.0637 (calculated for C14H14O7 Na 317.0632).
oxepine moiety for the antifungal activity of the compounds.
However, the compounds with a chromone scaffold, 11 and
12, lacking the oxepine moiety, showed low potency. This
trend was also observed with the compounds carrying the
dimethylpyran moiety, 23 and 25, which had lower activity
against both fungal genera. In most cases the activity appears to
be fungicidal and not fungistatic.
5,7-Dihydroxy-2-methyl-4H-chromen-4-one, Noreugenin
(11). To a solution of 2,4,6-triacetoxyacetophenone (10a) (8.58 g,
29.2 mmol, 1.0 equiv) in CH3CN (35 mL) was added K2CO3 (20.0 g,
144.7 mmol, 4.9 equiv), and the reaction mixture was refluxed for 2 h.
Upon complete consumption of the starting material, a 1:1 mixture of
CH3OH and a saturated aqueous K2CO3 solution (20 mL) was
added, and the mixture was heated at reflux for 3 h. After the mixture
had cooled, it was acidified with 3 M HCl, and the precipitate that
formed was filtered and washed with cold 3 M HCl to give the
product 11 as a beige solid (3.53 g, 18.4 mmol, 61%): 1H NMR (400
MHz, DMSO-d6) δ 12.83 (1H, s, HO-5), 10.81 (1H, s, HO-7), 6.32
(1H, d, J = 2 Hz, H-8), 6.17 (2H, s, H-6, H-3), 2.34 (3H, s, CH3-2);
13C NMR (100 MHz, DMSO-d6) δ 181.7 (C, C-4), 167.7 (C, C-2),
In conclusion, the present work resulted in the total
synthesis of 12-O-acetylptaeroxylinol (9) and the new
compound 12-O-acetyleranthin (8). These compounds were
synthesized in a seven-step reaction sequence, involving
Kostanecki−Robinson reaction, Williamson etherification,
Claisen rearrangement, ring-closing metathesis, desilylation,
and acetylation. The structure of the antifungal compound
obliquumol occurring naturally in Ptaeroxylon obliquum that
was previously assigned as 12-O-acetyleranthin (8) is corrected
to 12-O-acetylptaeroxylinol (9). The oxepinochromones 8 and
9 and their derivatives were tested for antifungal activity
against C. albicans and C. neoformans. Most tested compounds
had good to moderate activity against these pathogens.
Compounds carrying the oxepine moiety had good activity
and selectivity between the two fungal genera, while those with
the dimethylpyran ring were less potent. Therefore, the
benzoxepine compounds are earmarked for further develop-
ment.
164.1 (C, C-7), 161.5 (C, C-5), 157.8 (C, C-8a), 107.9 (CH, C-3),
103.4 (C, C-4a), 98.7 (CH, C-6), 93.7 (CH, C-8), 19.9 (CH3, CH3-
2); HRMSESI m/z [M + H]+ 193.0517 (calculated for C10H9O4
193.0495). All spectroscopic data were in agreement with the
reported data.31
7-Allyloxy-5-hydroxy-2-methyl-4H-chromen-4-one (11a).
To a solution of noreugenin (11) (1.60 g, 8.52 mmol, 1.0 equiv) in
DMF (26 mL) was added K2CO3 (1.80 g, 12.8 mmol, 1.5 equiv). The
mixture was stirred for 10 min under argon, allyl bromide (0.81 mL,
9.37 mmol, 1.1 equiv) was added, and the mixture was stirred at rt for
3 h. Upon complete consumption of material, the mixture was filtered
and washed with EtOAc (25 mL). The filtrate was washed with
saturated NH4Cl solution and extracted with EtOAc (3 × 30 mL).
The combined organic phase was washed with brine, dried over
anhydrous Na2SO4, and concentrated in vacuo. The residue was
purified by silica gel column chromatography using hexanes/EtOAc
(8:2) to afford compound 11a as a light yellow solid (1.20 g, 5.34
mmol, 63%, Rf = 0.4); IR νmax 1739, 1664, 1624, 1368, 1342, 1203,
1167 cm−1; 1 H NMR (400 MHz, CDCl3) δ 6.36 (1 H, d, J = 2.2 Hz,
H-8), 6.34 (1 H, d, J = 2.2 Hz, H-6), 6.08−5.98 (1 H, m, H-2′), 6.02
(1 H, s, H- 3), 5.42 (1 H, dd, J = 17.3 and 1.2 Hz, H-3′a), 5.33 (1H,
dd, J = 10.5 and 1.2 Hz, H-3′b), 4.58 (2H, d, J = 5.3 Hz, H-1′), 2.28
(3H, s, CH3-2); 13C NMR (100 MHz, CDCl3) δ 182.4 (C, C-4),
166.8 (C, C-2), 164.2 (C, C-7), 162.1 (C, C-5), 158.0 (C, C-8a),
132.1 (CH, C-2′), 118.4 (CH2, C-3′), 108.8 (CH, C-3), 105.3 (C, C-
4a), 98.5 (CH, C-6), 93.1 (CH, C-8), 69.2 (CH2, C-1′), 20.5 (CH3,
CH3-2); HRMSESI m/z [M + H]+ 233.0848 (calculated for
C13H13O4 233.0808). All spectroscopic data were in agreement with
the reported data.31
8-Allyl-5,7-dihydroxy-2-methyl-4H-chromen-4-one (12).
The allyl ether 11a (1.0 g, 4.3 mmol, 1 equiv) was refluxed in N,N-
diethylaniline (5 mL) at 220 °C for 12 h. After cooling the reaction
mixture to rt, EtOAc (30 mL) was added and the mixture was washed
with 1 M HCl (10 mL). A saturated NH4Cl solution (30 mL) was
added, and the product was extracted with EtOAc (3 × 30 mL),
washed with brine, dried over anhydrous Na2SO4, and concentrated in
vacuo. The residue was filtered through a short pad of silica gel using
hexanes/EtOAc (9:1) to remove the excess N,N-diethylaniline, and
the solvent was evaporated in vacuo. The product was purified by
silica gel column chromatography using hexanes/EtOAc (8:2) as the
eluent to afford 12 (0.32 g, 1.4 mmol, 32%, Rf = 0.44) as a yellow
solid and 13 (60 mg, 0.27 mmol, 6.19%, Rf = 0.29) as a yellow oil: 1H
NMR (400 MHz, DMSO-d6) δ 12.78 (1H, s, HO-5), 10.78 (1H, s,
HO-7), 6.23 (1H, s, H-6), 6.15 (1H, s, H-3), 5.91−5.82 (1H, m, H-
2′), 4.98−4.92 (2H, m, H-3′), 3.34 (2H, d, J = 6.3 Hz, H-1′), 2.34
(3H, s, CH3-2); 13C NMR (100 MHz, DMSO-d6) δ 182.1 (C, C-4),
167.5 (C, C-2), 161.6 (C, C-7), 159.4 (C, C-5), 155.2 (C, C-8a),
135.8 (CH, C-2′), 114.8 (CH2, C-3′), 107.7 (CH, C-3), 103.9 (C, C-
8), 103.3 (C, C-4a), 98.2 (CH, C-6), 26.2 (CH2, C-1′), 19.9 (CH3,
CH3-2); HRMS-ESI m/z [M + H]+ 233.0819 (calculated for
C13H13O4 233.0808).
EXPERIMENTAL SECTION
■
General Experimental Procedures. All reactions that require
anhydrous conditions were carried out under argon in oven-dried
glassware. Reagent grade solvents and chemicals used for syntheses
were purchased from Sigma-Aldrich (Pty) Ltd. (Johannesburg, South
Africa) or Merck Sigma-Aldrich (Pty) Ltd. (Johannesburg, South
Africa) and were used without further purification. Thin layer
chromatography (TLC) was performed on Merck silica gel 60 F254
aluminum plates and were observed under UV light (254 nm). The
plates were developed in vanillin/KMnO4 stain followed by heating.
Preparative TLC was carried out on Merck silica gel glass plates and
visualized under UV light (254 nm). Column chromatographic
purifications were carried out on silica gel (230−400 mesh) using the
eluent system detailed for each procedure. The synthesized
compounds were analyzed by IR spectroscopy, mass spectrometry
(MS), and NMR spectroscopy. The IR spectra were recorded on an
Alpha Bruker Optics FT-IR spectrometer, and all data were reported
in wavenumbers. HRMS data were acquired on a Waters UPLC
coupled to QTOF Synapt G2 spectrometry using electrospray
ionization in the positive or negative mode. The NMR spectra were
recorded at rt on a Bruker Avance III 300 or 400 MHz spectrometer.
1H and 13C NMR chemical shifts were referenced to residual
protonated solvents peaks: δH 7.26, δC 77.0 for CDCl3; δH 2.50, δC
39.5 for DMSO-d6. Chemical shifts are reported in ppm (δ), and
spin−spin coupling constants (J) in hertz (Hz). The multiplicities of
1H and 13C resonances are expressed by the following abbreviations: s
(singlet), d (doublet), dd (doublet of doublet), t (triplet), dt (doublet
of triplet), quartet (q), m (multiplet). 1D NMR (1H, 13C, and DEPT-
135) and 2D NMR (HSQC, HMBC, NOESY, and COSY) spectra
were used for the complete assignment of NMR signals.
1-[2,4,6-Tris(acetyloxy)phenyl]ethanone (10a). To a stirred
solution of phloroacetophenone (10) (3.00 g, 16.1 mmol, 1.0 equiv)
in pyridine (6.0 mL, 74 mmol, 4.6 equiv) at rt was added Ac2O (6.40
mL, 68.8 mmol, 4.3 equiv), and the reaction mixture was stirred for 4
h. The product was precipitated by adding crushed ice into the
reaction mixture. The precipitate was filtered and washed with ice
cold H2O. The white solid (4.30 g, 90%, 14.6 mmol) was used in the
next step without further purification. 1H NMR (400 MHz, CDCl3) δ
6.92 (2H, s, H-3, H-5), 2.46 (3H, s, ArCOCH3), 2.28 (3H, s,
OCOCH3), 2.27 (6H, s, OCOCH3); 13C NMR (100 MHz, CDCl3) δ
F
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1
6-Allyl-5,7-dihydroxy-2-methyl-4H-chromen-4-one (13). H
NMR (400 MHz, DMSO-d6) δ 13.08 (1H, s, HO-5), 10.81 (1H, s,
HO-7), 6.40 (1H, s, H-8), 6.15 (1H, s, H-3), 5.91−5.81 (1H, m, H-
2′), 4.95−4.90 (2H, m, H-3′), 3.25 (2H, d, J = 6.1 Hz, H-1′), 2.34
(3H, s, CH3-2); 13C NMR (100 MHz, DMSO-d6) δ 181.8 (C, C-4),
167.4 (C, C-2), 161.9 (C, C-7), 158.6 (C, C-5), 155.8 (C, C-8a),
135.7 (CH, C-2′), 114.6 (CH2, C-3′), 108.9 (C, C-6), 107.8 (CH, C-
3), 103.1 (C, C-4a), 92.9 (CH, C-8), 26.0 (CH2, C-1′), 19.9 (CH3,
CH3-2); HRMSESI m/z [M + H]+ 233.0819 (calculated for
C13H13O4 233.0808). All spectroscopic data were in agreement with
the reported data.31
(5-Hydroxy-2-methyl-4-oxo-8,11-dihydro-4H-oxepino[2,3-
h]chromen-9-yl)methyl acetate, 12-O-Acetyleranthin (8).
K2CO3 (0.088 g, 0.28 mmol, 3.0 equiv) was added to a stirred
mixture of eranthin 6 (0.025 g, 0.092 mmol, 1.0 equiv) and AcCl (6.6
μL, 0.092 mmol, 1.0 equiv) in CH2Cl2 (10 mL) under an argon
atmosphere. The reaction mixture was stirred at rt for 4 h. A saturated
NH4Cl solution (10 mL) was added, and the mixture was extracted
with CH2Cl2 (3 × 10 mL). The combined organic phase was washed
with brine, dried over anhydrous Na2SO4, filtered, and concentrated
in vacuo. The product was purified by PTLC using hexanes/EtOAc
(3:7) to give 8 (10.0 mg, 35%, 0.0316 mmol, Rf = 0.58) as white solid:
IR νmax 3463, 3018, 2968, 1740, 1662, 1434, 1369, 1222, 1103, 900,
782, 518 cm−1; 1H NMR (400 MHz, CDCl3) δ 12.66 (1H, s, HO-5),
6.49 (1H, s, H-6), 6.06 (1H, s H-3), 6.06−6.03 (1 H, m, H-10), 4.65
(2H, d, J = 1.4 Hz, H-8), 4.46 (2H, s, H-12), 3.63 (2H, d, J = 5.5 Hz,
H-11), 2.38 (3H, s, CH3-2), 2.05 (3H, s, COCH3); 13C NMR (100
MHz, CDCl3) δ 183.1 (C, C-4), 170.7 (C, COCH3), 166.8 (C, C-2),
164.3 (C, C-6a), 160.3 (C, C-5), 153.7 (C, C-11b), 133.8 (C, C-9),
127.4 (CH, C-10), 111.4 (C, C-11a), 108.8 (CH, C-3), 107.2 (C, C-
4a), 104.7 (CH, C-6), 71.0 (CH2, C-8), 66.5 (CH2, C-12), 21.6
(CH2, C-11), 20.9 (CH3, COCH3), 20.5 (CH3, CH3-2); HRMSESI
m/z [M + H]+ 317.1052 (calculated for C17H17O6 317.1020)
7-{2″-[(tert-Butyldiphenylsiloxy)methyl]allyloxy}-8-allyl-5-
hydroxy-2-methyl-4H-chromen-4-one (12a). To a solution of
allyl ether 12 (0.30 g, 1.3 mmol, 1.0 equiv) in DMF (10 mL) was
added K2CO3 (0.27 g, 1.9 mmol, 1.5 equiv), and the mixture was
stirred for 15 min at rt under argon. 3-Bromo-2-(tert-
butyldiphenylsiloxy)methylpropene (0.65 g, 1.7 mmol, 1.3 equiv)
was added to the mixture, which was then stirred at rt for 5 h. The
reaction mixture was filtered, diluted with EtOAc (20 mL), and
washed with a saturated NH4Cl solution (20 mL). The mixture was
extracted with EtOAc (3 × 20 mL), washed with brine, dried over
anhydrous Na2SO4, and concentrated in vacuo. The mixture was
purified by PTLC using hexanes/EtOAc (9:1) to afford the target
compound 12a (0.29 g, 0.54 mmol, 42%, Rf = 0.48) as a colorless oil:
IR νmax 2933, 2859, 1741, 1659, 1467, 1422, 1382, 1325, 1268, 1237,
7-{2″-[(tert-Butyldiphenylsiloxy)methyl]allyloxy}-6-allyl-5-
hydroxy-2-methyl-4H-chromen-4-one (13a). Compound 13a
was obtained as a colorless oil (0.029 g, 0.54 mmol, 70%, Rf =
0.56) after purification with PTLC using hexanes/EtOAc (9:1) from
the treatment of allyl ether 13 (0.18 g, 0.76 mmol, 1.0 equiv) with 3-
bromo-2-(tert-butyldiphenylsiloxy)methylpropene (0.39 g, 0.92
mmol, 1.2 equiv) in the presence of K2CO3 (0.160 g, 1.14 mmol,
1.5 equiv) in DMF (5 mL) following the procedure for synthesizing
12a above. IR νmax 2928, 2857, 1740, 1660, 1451, 1374, 1343, 1267,
1
1193, 1109, 820, 743, 704, 613, 502 cm−1; H NMR (400 MHz,
CDCl3) δ 12.79 (1H, brs, HO-5), 7.68−7.66 (4H, m, SiAr2), 7.44−
7.36 (6H, m, SiAr2), 6.34 (1H, s, H-6), 6.02 (1H, s, H-3), 5.82−5.72
(1H, m, H-2′), 5.38 (1H, brs, H-3″a), 5.26 (1H, d, J = 1.2 Hz, H-
3″b), 4.90−4.83 (2H, m, H-3′), 4.59 (2H, s, H-1″), 4.28 (2H, s, H-
4″), 3.37 (2H, d, J = 6.2 Hz, H-1′), 2.35 (3H, s, CH3-2), 1.07 (9H, s,
(CH3)3CSi); 13C NMR (100 MHz, CDCl3) δ 182.9 (C, C-4), 166.8
(C, C-2), 161.6 (C, C-7), 160.7 (C, C-5), 154.9 (C, C-8a), 142.7 (C,
C-2″), 135.6 (CH, C-2′), 135.5 (4 × CH, SiAr2), 133.2 (2 × C,
SiAr2), 129.7 (2 × CH, SiAr2), 127.7 (4 × CH, SiAr2), 114.6 (CH2,
C-3′), 112.8 (CH2, C-3″), 108.3 (CH, C-3), 105.9 (C, C-8), 104.7
(C, C-4a), 95.8 (CH, C-6), 69.2 (CH2, C-1″), 64.4 (CH2, C-4″), 29.7
(3 × CH3, (CH3)3CSi), 26.6 (CH2, C-1′), 20.5 (CH3, CH3-2), 19.2
(C, (CH3)3CSi); HRMSESI m/z [M + H]+ 541.2401 (calculated for
C33H37O5Si 541.2404). All spectroscopic data were in agreement with
the reported data.31
1
1229, 1204, 1109, 819, 706 cm−1; H NMR (400 MHz, CDCl3) δ
7.68−7.66 (4H, m, SiAr2), 7.43−7.36 (6H, m, SiAr2), 6.37 (1H, s, H-
8), 6.03 (1H, s, H-3), 5.87−5.81 (1H, m, H-2′), 5.36 (1H, s, H-3″a),
5.26 (1H, s, H-3″b), 4.92 (1H, dd, J = 17.1 and 1.6 Hz, H-3′a), 4.85
(1H, dd, J = 10.0 and 1.6 Hz, H-3′b), 4.61 (2H, s, H-1″), 4.28 (2H, s,
H-4″), 3.34 (2H, d, J = 6.1 Hz, H-1′), 2.33 (3H, s, CH3-2), 1.08 (9H,
s, (CH3)3CSi); 13C NMR (100 MHz, CDCl3) δ 182.4 (C, C-4), 166.4
(C, C-2), 161.9 (C, C-7), 158.6 (C, C-5), 156.6 (C, C-8a), 142.7 (C,
C-2″), 135.7 (CH, C-2′), 135.5 (4 × CH, SiAr2), 133.2 (2 × C,
SiAr2), 129.8 (2 × CH, SiAr2), 127.7 (4 × CH, SiAr2), 114.5 (CH2,
5-Hydroxy-9-hydroxymethyl-2-methyl-8,11-dihydro-4H-
oxepino[2,3-h]chromen-4-one, Eranthin (6). To a solution of
diene 12a (0.29 g, 0.54 mmol, 1.0 equiv) in dry CH2Cl2 (30 mL) was
added [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]-
(tricyclohexylphosphine) benzylideneruthenium(IV) dichloride
(Grubbs’ II catalyst) (0.05 g, 0.06 mmol, 0.1 equiv) in one portion
under argon. The reaction mixture was heated at 45 °C for 2 h, and
the solvent was evaporated after cooling. The residue was dissolved in
THF (5 mL), and a TBAF solution (1 M in THF; 0.59 mL, 0.59
mmol, 1.1 equiv) was added dropwise at 0 °C under argon. After
stirring the reaction mixture for 1 h at 5 °C, EtOAc (20 mL) was
added, followed by a saturated aqueous NH4Cl solution (20 mL). The
mixture was extracted with EtOAc (3 × 20 mL). The combined
organic layer was washed with brine, dried over anhydrous Na2SO4,
and evaporated in vacuo. The resulting oil was purified by silica gel
column chromatography using hexanes/EtOAc (3:7) as the eluent to
afford 6 (95.0 mg, 0.346 mmol, 65%, Rf = 0.4) as a green solid: IR
νmax 3446, 2923, 2853, 1655, 1615, 1586, 1486, 1414, 1265, 1180,
C-3′), 112.9 (CH2, C-3″), 111.1 (C, C-6), 108.8 (CH, C-3), 105.2
(C, C-4a), 90.4 (CH, C-8), 68.9 (CH2, C-1″), 64.6 (CH2, C-4″), 26.8
(3 × CH3, (CH3)3CSi), 26.4 (CH2, C-1′), 20.4 (CH3, CH3-2), 19.3
(C, (CH3)3CSi); HRMSESI m/z [M + H]+ 541.2401 (calculated for
C33H37O5Si 541.2404). All spectroscopic data were in agreement with
the reported data.31
5-Hydroxy-8-hydroxymethyl-2-methyl-6,9-dihydro-4H-
oxepino[3,2-g]chromen-4-one, Ptaeroxylinol (7). The treatment
of diene 13a (0.28 g, 0.51 mmol, 1.0 equiv) with Grubbs II catalyst
(0.045 g, 0.053 mmol, 0.1 equiv) in dry CH2Cl2 (30 mL), followed by
TBAF solution (1 M in THF; 0.56 mL, 0.56 mmol, 1.1 equiv) in THF
(5 mL), afforded the target compound 7 (0.056 g, 0.20 mmol, 40%, Rf
= 0.18) as light green crystals, after purification via silica gel column
chromatography using hexanes/EtOAc (3:7) as the eluent while
1
following the procedure for 6 above. H NMR (400 MHz, CDCl3) δ
13.08 (1H, s, HO-5), 6.52 (1H, s, H-11), 6.04 (1H, s, H-3), 6.00 (1H,
tt, J = 5.6, 1.1 Hz, H-7), 4.74 (2H, d, J = 1.1 Hz, H-9), 4.03 (2H, s, H-
12), 3.57 (2H, d, J = 5.6 Hz, H-6), 2.35 (3H, s, CH3-2); 13C NMR
(100 MHz, CDCl3) δ 182.8 (C, C-4), 167.2 (C, C-2), 164.6 (C, C-
10a), 158.1 (C, C-5), 155.9 (C, C-11a), 138.0 (C, C-8), 125.4 (CH,
C-7), 115.8 (C, C-5a), 108.7 (CH, C-3), 106.7 (C, C-4a), 99.3 (CH,
C-11), 71.0 (CH2, C-9), 65.7 (CH2, C-12), 21.1 (CH2, C-6), 20.5
(CH3, CH3-2); HRMSESI m/z [M + H]+ 275.0898 (calculated for
C15H15O5 275.0921). All spectroscopic data were in agreement with
the reported data.31
1
1158, 1101, 1073, 1024, 846, 640, 554 cm−1; H NMR (400 MHz,
CDCl3) δ 12.61 (1H, s, OH-5), 6.45 (1H, s, H-6), 6.04 (1H, s, H-3),
5.99 (1H, t, J = 5.3 Hz, H-10), 4.72 (2H, s, H-8), 4.03 (2H, s, H-12),
3.61 (2H, d, J = 5.3 Hz, H-11), 2.38 (3H, s, CH3-2); 13C NMR (100
MHz, CDCl3) δ 183.1 (C, C-4), 166.8 (C, C-2), 164.4 (C, C-6a),
160.1 (C, C-5), 153.8 (C, C-11b), 138.5 (C, C-9), 124.2 (CH, C-10),
111.0 (C, C-11a), 108.7 (CH, C-3), 107.0 (C, C-4a), 104.6 (CH, C-
6), 70.8 (CH2, C-8), 65.4 (CH2, C-12), 21.4 (CH2, C-11), 20.5 (CH3,
CH3-2); HRMSESI m/z [M + H]+ 275.0898 (calculated for
C15H15O5 275.0921). All spectroscopic data were in agreement with
the reported data.31
(5-Hydroxy-2-methyl-4-oxo-6,9-dihydro-4H-oxepino[3,2-g]-
chromen-8-yl)methyl acetate, 12-O-Acetylptaeroxylinol (9).
The treatment of 7 (40 mg, 0.15 mmol, 1.0 equiv) with K2CO3 (40
mg, 0.29 mmol, 1.9 equiv) and AcCl (10 μL, 0.15 mmol, 1.0 equiv) in
G
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CH2Cl2 (10 mL) under an argon atmosphere afforded oxepinochro-
mone 9 (10.0 mg, 23%, 0.0345 mmol, Rf = 0.68) as colorless crystals
after purification by PTLC using hexanes/EtOAc (3:7) as the eluent
while following the procedure for 8 above. IR νmax 3467, 2924, 2853,
1739, 1650, 1619, 1484, 1447, 1398, 1327, 1235, 1169, 1110, 1083,
1037, 966, 845, 796, 578, 544, 505 cm−1; 1H NMR (400 MHz,
CDCl3) δ 13.08 (1H, s, HO-5), 6.54 (1H, s, H-11), 6.08−6.05 (1H,
m, H-7), 6.05 (1H, brs, H-3), 4.65 (2H, d, J = 1.5 Hz, H-9), 4.46 (2H,
s, H-12), 3.59 (2H, d, J = 5.5 Hz, H-6), 2.36 (3H, s, CH3-2), 2.05
(3H, s, COCH3); 13C NMR (100 MHz, CDCl3) δ 182.8 (C, C-4),
170.7 (C, COCH3), 167.2 (C, C-2), 164.5 (C, C-10a), 158.2 (C, C-
5), 155.9 (C, C-11a), 133.2 (C, C-8), 128.4 (CH, C-7), 116.0 (C, C-
5a), 108.8 (CH, C-3), 106.9 (C, C-4a), 99.4 (CH, C-11), 71.2 (CH2,
C-9), 66.6 (CH2, C-12), 21.2 (CH2, C-6), 20.9 (CH3, COCH3), 20.5
(CH3, CH3-2); HRMSESI m/z [M + H]+ 317.1052 (calculated for
C17H17O6 317.1020).
3′), 3.52 (3H, s, OCH3), 3.47 (3H, s, OCH3), 3.36 (2H, dt, J = 6.1
and 1.5 Hz, H-1′), 2.66 (3H, s, ArCOCH3); 13C NMR (100 MHz,
CDCl3) δ 203.5 (C, ArCOCH3), 163.6 (C-4), 163.6 (C, C-2), 160.9
(C, C-4), 159.1 (C, C-6), 136.3 (CH, C-2′), 114.1 (CH2, C-3′),
109.6 (C, C-3), 106.8 (C-1), 94.5 (CH2, OCH2O), 93.8 (CH2,
OCH2O), 91.2 (CH, C-5), 56.7 (CH3, OCH3), 56.4 (CH3, OCH3),
33.2 (CH3, ArCOCH3), 26.5 (CH2, C-1′). All spectroscopic data
were in agreement with the reported data.34
1-[3-Allyl-2-methoxy-4,6-bis(methoxymethoxy)phenyl]-
ethanone (17). Iodomethane (0.59 mL, 9.50 mmol, 1.3 equiv) was
added to a solution of 3-allyl-2-hydroxy-4,6-dimethoxymethoxy-
acetophenone (16) (2.16 g, 7.30 mmol, 1.0 equiv) and K2CO3
(5.05 g, 36.5 mmol, 5 equiv) in DMF (10 mL). The reaction mixture
was heated at reflux for 1 h, cooled to rt, and diluted with EtOAc (50
mL) and NH4Cl solution (50 mL). The mixture was extracted with
EtOAc (3 × 50 mL). The organic layer was washed with brine, dried
over anhydrous Na2SO4, and concentrated in vacuo. The product was
purified by silica gel column chromatography using hexanes/EtOAc
(10:1) as the eluent to give target compound 17 (1.83 g, 5.90 mmol,
97.7%, Rf = 0.29) as a yellow oil: IR νmax 2939, 1741, 1691, 1594,
1474, 1395, 1314, 1256, 1201, 1147, 1109, 1064, 1040, 997, 920, 832,
1-[2-Hydroxy-4,6-bis(methoxymethoxy)phenyl]ethanone
(14). ZnBr2 (3.00 mg, 0.0131 mmol, 0.0049 equiv) was stirred in
dimethoxymethane (0.59 mL, 6.7 mmol, 2.5 equiv) under an argon
atmosphere, and then AcCl (0.47 mL, 6.7 mmol, 2.5 equiv) was
added dropwise to the stirred solution. The solution was stirred at rt
for an additional 2 h and transferred into an ice-cold solution of the
predried phloroacetophenone (10) (0.50 g, 2.7 mmol, 1.0 equiv) and
DIPEA (0.93 mL, 5.4 mmol, 2.0 equiv) in CH2Cl2 (30 mL) under an
argon atmosphere. The mixture was stirred for 3 h, diluted with
saturated NH4Cl solution (30 mL), and stirred for an additional 15
min. The two phases were partitioned, and the aqueous phase was
extracted with CH2Cl2 (5 × 30 mL). The combined organic extracts
were washed with brine and dried over anhydrous Na2SO4. The
solvent was evaporated to give a yellow oil, which was purified by
silica gel column chromatography using hexanes/EtOAc (5:1) to
afford 14 (0.35 g, 1.4 mmol, 50%, Rf = 0.84) as a colorless oil that
1
557 cm−1; H NMR (400 MHz, CDCl3) δ 6.71 (1H, s, H-5), 5.99−
5.91 (1H, m, H-2′), 5.18 (2H, s, OCH2O), 5.14 (2H, s, OCH2O),
5.01−4.96 (2H, m, H-3′), 3.71 (3H, s, OCH3), 3.47 (3H, s, OCH3),
3.46 (3H, s, OCH3), 3.36 (2H, dt, J = 6.0 and 1.6 Hz, H-1′), 2.51
(3H, s, COCH3); 13C NMR (100 MHz, CDCl3) δ 202.1 (C,
COCH3), 157.3 (C, C-4), 156.3 (C, C-2), 153.3 (C, C-6), 136.8
(CH, C-2′), 120.7 (C, C-1), 116.4 (C, C-3), 114.6 (CH2, C-3′), 97.8
(CH, C-5), 95.0 (CH2, OCH2O), 94.3 (CH2, OCH2O), 63.4 (CH3,
OCH3), 56.4 (CH3, OCH3), 56.2 (CH3, OCH3), 32.6 (CH3,
COCH3), 27.7 (CH2, C-1′).
1-[3-Allyl-6-hydroxy-2-methoxy-4-(methoxymethoxy)-
phenyl]ethanone (18). A solution of compound 17 (1.47 g, 4.73
mmol, 1.0 equiv) in CH3OH (30 mL), and 3 M HCl (1.5 mL) was
heated at 60 °C for 8 h. The reaction was quenched with a saturated
NH4Cl solution (30 mL), and the product was extracted with EtOAc
(3 × 50 mL). The organic layer was washed with brine, dried over
anhydrous Na2SO4, and concentrated in vacuo. The crude product
was purified by silica gel column chromatography using hexanes/
EtOAc (8.5:1.5) as the eluent to give target compound 18 (1.00 g,
3.75 mmol, 79%, Rf = 0.58) as a light yellow oil: 1H NMR (400 MHz,
CDCl3) δ 13.19 (1H, s, HO-6), 6.47 (1H, s, H-5), 6.01−5.94 (1H, m,
H-2′), 5.20 (2H, s, OCH2O), 5.01−4.96 (2H, m, H-3′), 3.74 (3H, s,
OCH3), 3.45 (3H, s, OCH3), 3.36 (2H, d, J = 5.7 Hz, H-1′), 2.69
(3H, s, COCH3); 13C NMR (100 MHz, CDCl3) δ 203.5 (C,
COCH3), 164.4 (C, C-6), 161.8 (C, C-4), 161.1 (C, C-2), 136.9
(CH, C-2′), 114.6 (CH2, C-3′), 114.0 (C, C-3), 109.9 (C, C-1), 98.7
(CH, C-5), 93.9 (CH2, OCH2O), 63.1 (CH3, OCH3), 56.6 (CH3,
OCH3), 30.9 (CH3, COCH3), 27.7 (CH2, C-1′); HRMSESI m/z [M
+ H]+ 267.1259 (calculated for C14H19O5 267.1224).
1
solidified at rt. H NMR (400 MHz, CDCl3) δ 13.71 (1H, s, HO-2),
6.26 (1H, d, J = 2.4 Hz, H-3), 6.24 (1H, d, J = 2.4 Hz, H-5), 5.25
(2H, s, OCH2O), 5.17 (2H, s, OCH2O), 3.52 (3H, s, OCH3), 3.47
(3H, s, OCH3), 2.65 (3H, s, ArCOCH3); 13C NMR (CDCl3, 100
MHz) δ 203.2 (C, ArCOCH3), 166.8 (C, C-2), 163.4 (C, C-4), 160.3
(C, C-6), 106.9 (C, C-1), 97.5 (CH, C-3), 94.4 (CH2, OCH2O), 94.0
(CH2, OCH2O), 94.0 (CH, C-5), 56.7 (CH3, OCH3), 56.4 (CH3,
OCH3), 33.0 (CH3, ArCOCH3); HRMSESI m/z [M + H]+ 257.1034
(calculated for C12H17O6 257.1019). All spectroscopic data were in
agreement with the reported data.33
1-[2-Allyloxy-4,6-bis(methoxymethoxy)phenyl]ethanone
(15). Compound 15 was obtained as a light yellow oil (0.092 g, 3.1
mmol, 79%, Rf = 0.41) after purification by silica gel column
chromatography using hexanes/EtOAc (8:2) as eluents from the
treatment of 2-hydroxy-4,6-dimethoxymethoxyacetophenone (14)
(0.10 g, 0.39 mmol, 1.0 equiv) with allyl bromide (0.37 mL, 0.429
mmol, 1.1 equiv) and K2CO3 (0.081 g, 0.59 mmol, 1.5 equiv) in DMF
1
(10 mL) following the procedure for preparing 11a. H NMR (400
MHz, CDCl3) δ 6.45 (1H, d, J = 2.0 Hz, H-5), 6.30 (1H, d, J = 2.0
Hz, H-3), 6.03−5.93 (1H, m, H-2′), 5.37−5.33 (1H, m, H-3′a),
5.27−5.26 (1H, m, H-3′b), 5.14 (2H, s, OCH2O), 5.13 (2H, s,
OCH2O), 4.51 (2H, dt, J = 5.2 and 1.5 Hz, H-1′), 3.47 (3H, s,
OCH3), 3.45 (3H, s, OCH3), 2.48 (3H, s, ArCOCH3); 13C NMR
(100 MHz, CDCl3) δ 201.5 (C, ArCOCH3), 159.5 (C, C-4), 156.8
(C, C-2), 155.3 (C, C-6), 132.6 (CH, C-2′), 117.7 (CH2, C-3′),
116.2 (C, C-1), 96.2 (CH, C-5), 95.0 (CH, C-3), 94.8 (CH2,
OCH2O), 94.4 (CH2, OCH2O), 69.3 (CH2, C-1′), 56.3 (CH3,
OCH3), 56.1 (CH3, OCH3), 32.5 (CH3, ArCOCH3); HRMSESI m/z
[M + H]+ 297.1338 (calculated for C15H21O6 297.1332)
1-[3-Allyl-2,6-dimethoxy-4-(methoxymethoxy)phenyl]-
ethanone (19). A mixture of 18 (1.00 g, 3.77 mmol, 1.0 equiv),
K2CO3 (2.61 g, 18.8 mmol, 5.0 equiv), and CH3I (0.31 mL, 4.9 mmol,
1.3 equiv) in DMF (5 mL) was heated at reflux for 1 h. The reaction
mixture was cooled to rt and diluted with EtOAc (20 mL) and
saturated NH4Cl solution (30 mL). The organic and aqueous phases
were partitioned, and the aqueous phase was extracted with EtOAc (3
× 50 mL). The organic layer was washed with brine, dried over
anhydrous Na2SO4, and concentrated in vacuo. The product was
filtered through a short pad of silica gel using hexanes/EtOAc
(8.5:1.5) as the eluent to give target compound 19 (0.94 g, 3.4 mmol,
89%, Rf = 0.55) as a yellow oil: IR νmax 2967, 1740, 1691, 1597, 1446,
1373, 1206, 1148, 1104, 1053, 1012 cm−1; 1H NMR (400 MHz,
CDCl3) δ 6.53 (1H, s, H-5), 6.01−5.91 (1H, m, H-2′), 5.19 (2H, s,
OCH2O), 4.99−4.94 (2H, m, H-3′), 3.79 (3H, s, OCH3), 3.71 (3H,
s, OCH3), 3.47 (3H, s, OCH3), 3.34 (2H, dt, J = 5.9, and 1.6 Hz, H-
1′), 2.49 (3H, s, COCH3); 13C NMR (100 MHz, CDCl3) δ 202.4 (C,
COCH3), 157.6 (C, C-4), 156.5 (C, C-2), 156.0 (C, C-6), 136.9
(CH, C-2′), 119.5 (C, C-1), 114.9 (CH2, C-3′), 114.5 (C, C-3), 94.3
(CH, C-5), 94.3 (CH2, OCH2O), 63.4 (CH3, OCH3), 56.1 (CH3,
1-[3-Allyl-2-hydroxy-4,6-bis(methoxymethoxy)phenyl]-
ethanone (16). Compound 16 was obtained as a yellow oil (0.10 g,
0.33 mmol, 50%, Rf = 0.42) after purification by silica gel column
chromatography using hexanes/EtOAc (5:1) as eluent from the
Claisen rearrangement reaction of 15 (0.20 g, 0.67 mmol, 1.0 equiv)
in N,N-diethylaniline (5 mL) at 220 °C for 12 h while following the
1
procedure for the preparation of 12. H NMR (400 MHz, CDCl3) δ
13.86 (1H, s, HO-2), 6.39 (1H, s, H-5), 5.99−5.89 (1H, m, H-2′),
5.26 (2H, s, OCH2O), 5.23 (2H, s, OCH2O), 5.03−4.93 (2H, m, H-
H
https://dx.doi.org/10.1021/acs.jnatprod.0c00587
J. Nat. Prod. XXXX, XXX, XXX−XXX

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Article
OCH3), 55.8 (CH3, OCH3), 32.5 (CH3, COCH3), 27.6 (CH2, C-1′);
HRMSESI m/z [M + H]+ 281.1367 (calculated for C15H21O5
281.1385).
brine and dried over anhydrous MgSO4. The solvent was evaporated,
and the crude product was purified by silica gel column
chromatography using hexanes/EtOAc (7:3) to give 23 as a white
1
1-(3-Allyl-4-hydroxy-2,6-dimethoxyphenyl)ethanone (20).
Compound 20 was obtained as a light yellow oil (0.63 g, 2.7 mmol,
81%, Rf = 0.19) after purification by silica gel column chromatography
using hexanes/EtOAc (8.5:1.5) as eluent from MOM deprotection of
19 (0.92 g, 3.3 mmol, 1.0 equiv) in CH3OH (20 mL), and 3 M HCl
(4 mL) at 70 °C following the procedure for the preparation of 18. IR
νmax 3316, 2940, 1740, 1668, 1593, 1493, 1408, 1323, 1257, 1200,
1146, 1001, 912, 819, 562, 497 cm−1; 1H NMR (400 MHz, CDCl3) δ
6.26 (1H, s, H-5), 6.05−5.95 (1H, m, H-2′), 5.16−5.11 (2H, m, H-
3′), 3.73 (3H, s, OCH3), 3.69 (3H, s, OCH3), 3.38 (2H, dt, J = 5.8
and 1.7 Hz, H-1′), 2.50 (3H, s, COCH3); 13C NMR (100 MHz,
CDCl3) δ 202.7 (C, COCH3), 157.5 (C, C-4), 156.8 (C, C-2), 156.4
(C, C-6), 136.2 (CH, C-2′), 118.3 (C, C-1), 116.2 (CH2, C-3′),
110.9 (C, C-3), 95.9 (CH, C-5), 63.4 (CH3, OCH3), 55.7 (CH3,
OCH3), 32.5 (CH3, COCH3), 27.6 (CH2, C-1′); HRMSESI m/z [M
+ H]+ 237.1097 (calculated for C13H17O4 237.1121).
1-(4-{2″-[(tert-Butyldiphenylsiloloxy)methyl]allyloxy}-3-
allyl-2,6-dimethoxyphenyl)ethanone (21). Compound 21 was
obtained as a light yellow oil (0.82 g, 1.5 mmol, 58.9%, Rf = 0.5) after
purification by PTLC using hexanes/EtOAc (9:1−8:2) from the
treatment of 20 (0.60 g, 2.6 mmol, 1.0 equiv) with 3-bromo-2-(tert-
butyldiphenylsiloxy)methylpropene (0.79 g, 2.1 mmol, 0.86 equiv)
and K2CO3 (0.35 g, 2.6 mmol, 1.0 equiv) in DMF (10 mL) following
the procedure used to synthesize 12a. IR νmax 3016, 2970, 2944, 1738,
1436, 1366, 1228, 1216, 1206, 1111, 538 cm−1; 1H NMR (400 MHz,
CDCl3) δ 7.68−7.66 (4H, m, SiAr2), 7.45−7.35 (6H, m, SiAr2), 6.24
(1H, s, H-5), 5.87−5.77 (1H, m, H-2′), 5.39 (1H, d, J = 0.8 Hz, H-
3″a), 5.26 (1H, d, J = 1.1 Hz, H-3″b), 4.87−4.82 (2H, m, H-3′), 4.56
(2H, s, H-1″) 4.29 (2H, s, H-4″), 3.75 (3H, s, OCH3), 3.69 (3H, s,
OCH3), 3.25 (2H, d, J = 6.0 Hz, H-1′), 2.49 (3H, s, COCH3), 1.00
(9H, s, (CH3)3CSi); 13C NMR (100 MHz, CDCl3) δ 202.3 (C,
COCH3), 158.8 (C, C-4), 156.7 (C, C-2), 156.0 (C, C-6), 143.4 (C,
C-2″), 136.8 (CH, C-2′), 135.4 (4 × CH, SiAr2), 133.3 (2 × C,
SiAr2), 129.7 (2 × CH, SiAr2), 127.7 (4 × CH, SiAr2), 118.5 (C, C-
1), 114.4 (CH2, C-3′), 114.4 (C, C-3), 112.3 (CH2, C-3″), 92.3 (CH,
C-5), 69.1 (CH2, C-1″), 64.5 (CH2, C-4″), 63.3 (CH3, OCH3), 55.8
(CH3, OCH3), 32.5 (CH3, COCH3), 27.5 (CH2, C-1′), 26.8 (3 ×
CH3, (CH3)3CSi), 19.3 (C, (CH3)3CSi); HRMSESI m/z [M + H]+
545.2715 (calculated for C33H41O5Si 545.2717).
solid (26.0 mg, 10% 0.872 mmol, Rf = 0.7): H NMR (400 MHz,
CDCl3) δ 12.97 (1H, s, HO-5), 6.71 (1H, d, J = 10.0 Hz, H-1′), 6.00
(1H, s, H-3), 5.93−5.83 (1H, m, H-2″), 5.60 (1H, d, J = 10.0 Hz, H-
2′), 5.04−4.94 (2H, m, H-3″), 3.42 (2H, dt, J = 6.3 and 1.4 Hz, H-
1″), 2.34 (3H, s, CH3-2), 1.44 (6H, s, CH3-3′); 13C NMR (100 MHz,
CDCl3) δ 182.9 (C, C-4), 166.4 (C, C-2), 156.8 (C, C-7), 154.9 (C,
C-8a), 154.7 (C, C-5), 135.9 (CH, C-2″), 127.8 (CH, C-2′), 115.8
(CH, C-1′), 114.7 (CH2, C-3″), 108.5 (CH, C-3), 105.3 (C, C-8),
105.2 (C, C-6), 104.8 (C, C-4a), 77.8 (C, C-3′), 28.2 (2 × CH3,
CH3-3′), 26.5 (CH2, C-1″), 20.4 (CH3, CH3-2); HRMSESI m/z [M
+ H]+ 299.1285 (calculated for C18H19O4 299.1285).
1-(3-Allyl-4,6-dihydroxy-2-methoxyphenyl)ethanone (24).
Compound 24 was obtained as a light yellow solid (0.16 g, 0.70
mmol, 67%, Rf = 0.44) after purification by silica gel column
chromatography using hexanes/EtOAc (8.5:1.5) as the eluent from
MOM deprotection of 17 (0.320 g, 1.04 mmol, 1.0 equiv) in CH3OH
(3.5 mL) and 3 M HCl (1 mL) at 70 °C following the procedure for
1
the preparation of 18. H NMR (400 MHz, CDCl3) δ 13.20 (1H, s,
HO-6), 6.24 (1H, s, H-5), 6.09−5.99 (1H, m, H-2′), 5.83 (1H, s,
HO-4), 5.18−5.12 (2H, m, H-3′), 3.74 (3H, s, OCH3), 3.41 (2H, dt,
J = 5.7 and 1.8 Hz, H-1′), 2.69 (3H, s, COCH3); 13C NMR (100
MHz, CDCl3) δ 203.5 (C, COCH3), 164.4 (C, C-6), 161.9 (C, C-4),
161.8 (C, C-2), 136.2 (CH, C-2′), 116.3 (CH2, C-3′), 111.0 (C, C-
3), 109.8 (C, C-1), 100.5 (CH, C-5), 63.0 (CH3, OCH3), 31.0 (CH3,
COCH3), 27.8 (CH2, C-1′); HRMSESI m/z [M + H]+ 223.0965
(calculated for C12H15O4 223.0964).
1-(8-Allyl-5-hydroxy-7-methoxy-2,2-dimethyl-2H-chromen-
6-yl)ethanone (25). Compound 25 was obtained as a light yellow
oil (17 mg, 44%, 0.059 mmol, Rf = 0.76) after purification by PTLC
using hexanes/EtOAc (7:3) as the eluent from the reaction of phenol
24 (30.0 mg, 0.135 mmol, 1.0 equiv) with prenal (64.0 μL, 0.675
mmol, 5.0 equiv) and added Ca(OH)2 (20.0 mg, 0.270 mmol, 2.0
equiv) in CH3OH according to the procedure used to prepare 23. 1H
NMR (400 MHz, CDCl3) δ 13.55 (1H, s, HO-5), 6.68 (1H, d, J =
10.0 Hz, H-4), 6.01−5.91 (1H, m, H-2′), 5.51 (1H, d, J = 10.0 Hz, H-
3), 5.04−4.98 (2H, m, H-3′), 3.74 (3H, s, OCH3), 3.31 (2H, dt, J =
6.0 and 1.6 Hz, H-1′), 2.67 (3H, s, COCH3), 1.43 (6H, s, CH3-2);
13C NMR (100 MHz, CDCl3) δ 203.5 (C, COCH3), 161.2 (C, C-7),
159.2 (C, C-5), 158.5 (C, C-8a), 136.9 (CH, C-2′), 126.7 (CH, C-3),
116.0 (CH, C-4), 114.8 (CH2, C-3′), 112.9 (C, C-8), 109.0 (C, C-6),
105.8 (C, C-4a), 78.0 (C, C-2), 63.1 (CH3, OCH3), 31.0 (CH3,
COCH3), 28.4 (2 × CH3, CH3-2), 27.5 (CH2, C-1′); HRMSESI m/z
[M + H]+ 289.1489 (calculated for C17H21O4 289.1442).
Antifungal Assay. Fungal Test Organisms. The test fungi,
Candida albicans (ATCC 10231) and Cryptococcus neoformans
(isolated from a cheetah), were obtained from the culture collection
of the Phytomedicine programme, Department of Paraclinical
Sciences, Faculty of Veterinary Science, University of Pretoria. The
fungi were maintained on Sabouraud dextrose agar (Oxoid,
Basingstoke, UK).
Microdilution Assay. A reported microdilution assay with
tetrazolium violet added as growth indicator was used to determine
the MIC values for the compounds.35,36 Compounds were dissolved
in DMSO and added to a sterile 96-well microtiter plate in triplicate.
A serial dilution was done along the ordinate of the wells with sterile
distilled water. Fungal cultures diluted in Sabouraud dextrose broth
were then added to each well. The final concentrations of each
compound ranged between 25 and 0.2 μg/mL. Amphotericin B and
DMSO served as positive and negative controls, respectively. Forty
microliters of 0.2 mg/mL p-iodonitrotetrazolium violet (Sigma)
dissolved in sterile water was added to each well of the microplate.
The plates were incubated for 1−2 days at 35 °C and 100% relative
humidity. The MIC was recorded as the lowest concentration of the
compound that inhibited fungal growth after 24 and 48 h.
1-(2,5-Dihydro-3-hydroxymethyl-6,8-dimethoxybenzo[b]-
oxepin-7-yl)ethanone (22). The treatment of diene 21 (1.00 g,
1.84 mmol, 1.0 equiv) with (Grubbs II catalyst) (0.162 g, 0.191
mmol, 0.10 equiv) in dry CH2Cl2 (30 mL), followed by TBAF
solution (0.520 mL, 2.02 mmol, 1.1 equiv) in THF, afforded the
target benzoxepine 22 (0.16 g, 32%, 0.59 mmol, Rf = 0.45) as a light
yellow oil, after purification through a short pad of silica gel using
hexanes/EtOAc (9:1−5:5) as the eluent while following the
procedure for preparing 6. IR νmax 3381, 2941, 2600, 2492, 1690,
1596, 1471, 1394, 1322, 1253, 1197, 1145, 1098, 1031, 843, 806, 747,
1
567, 466 cm−1; H NMR (400 MHz, CDCl3) δ 6.47 (1H, s, H-9),
5.91 (1H, t, J = 5.3 Hz, H-4), 4.67 (2H, brs, H-2), 3.98 (2H, brs, H-
1′), 3.78 (3H, s, OCH3), 3.68 (3H, s, OCH3), 3.44 (2H, d, J = 5.5 Hz,
H-5), 2.48 (3H, s, COCH3); 13C NMR (100 MHz, CDCl3) δ 202.4
(C, COCH3), 160.9 (C, C-9a), 155.6 (C, C-8), 154.6 (C, C-6), 138.2
(C, C-3), 123.9 (CH, C-4), 121.9 (C, C-7), 120.6 (C, C-5a), 101.1
(CH, C-9), 71.6 (CH2, C-2), 65.7 (CH2, C-1′), 63.3 (CH3, OCH3),
55.8 (CH3, OCH3), 32.4 (CH3, COCH3), 22.0 (CH2, C-5);
HRMSESI m/z [M + H]+ 279.1214 (calculated for C15H19O5
279.1234).
8-Allyl-5-hydroxy-2-methyl-3′,3′-dimethylpyrano[7,6]-
chromone (23). To a solution of 12 (0.20 g, 0.86 mmol, 1.0 equiv)
in CH3OH (10 mL) was added Ca(OH)2 (0.120 g, 1.72 mmol, 2.0
equiv) followed by prenal (0.40 mL, 4.3 mmol, 5 equiv). The mixture
was stirred under an argon atmosphere for 3 days at rt and diluted
with EtOAc (15 mL) and 1 M HCl (15 mL). The two phases were
partitioned, and the aqueous phase was extracted with EtOAc (3 × 15
mL). The combined organic extracts were washed with H2O and
I
https://dx.doi.org/10.1021/acs.jnatprod.0c00587
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Article
(9) Bartroli, J.; Turmo, E.; Alguero, M.; Boncompte, E.; Vericat, M.
L.; Conte, L.; Ramis, J.; Merlos, M.; Garcia-Rafanell, J.; Forn, J. J.
Med. Chem. 1998, 41, 1869−1882.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.jnatprod.0c00587.
(10) Bartroli, J.; Turmo, E.; Alguero, M.; Boncompte, E.; Vericat, M.
L.; Conte, L.; Ramis, J.; Merlos, M.; Garcia-Rafanell, J.; Forn, J. J.
Med. Chem. 1998, 41, 1855−1868.
1H NMR and 13C NMR spectra for the synthesized
compounds (PDF)
(11) Ji, L.; Zhou, A.; Yu, X.; Dong, Z.; Zhao, H.; Xue, H.; Wu, W. J.
Antibiot. 2020, 73, 309−313.
(12) Mohammad, H.; Eldesouky, H. E.; Hazbun, T.; Mayhoub, A.
S.; Seleem, M. N. Sci. Rep. 2019, 9, 18941.
AUTHOR INFORMATION
(13) Chen, Y.-H.; Cheng, I. L.; Lai, C.-C.; Tang, H.-J. Int. J.
Antimicrob. Agents 2019, 53, 789−794.
■
Corresponding Author
́
(14) Du Pre, S.; Beckmann, N.; Almeida, M. C.; Sibley, G. E. M.;
Mamoalosi A. Selepe − Department of Chemistry, University of
Pretoria, Pretoria 0002, South Africa; orcid.org/0000-
0002-5081-178X; Phone: +27 12 420 2345;
Email: mamoalosi.selepe@up.ac.za; Fax: +27 12 420 4687
Law, D.; Brand, A. C.; Birch, M.; Read, N. D.; Oliver, J. D. Antimicrob.
Agents Chemother. 2018, 62, e00231−18.
(15) Huang, M.; Kao, K. C. FEMS Microbiol. Lett. 2012, 333, 85−93.
(16) Aggarwal, N.; Sharma, V.; Kaur, H.; Ishar, M. P. S. Int. J. Med.
Chem. 2013, 2013, 815453.
Authors
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Modibo S. Malefo − Department of Chemistry, University of
Pretoria, Pretoria 0002, South Africa
Thanyani E. Ramadwa − Phytomedicine Programme,
Department of Paraclinical Sciences, Faculty of Veterinary
Science, University of Pretoria, Pretoria 0110, South Africa
Ibukun M. Famuyide − Phytomedicine Programme,
Department of Paraclinical Sciences, Faculty of Veterinary
Science, University of Pretoria, Pretoria 0110, South Africa
Lyndy J. McGaw − Phytomedicine Programme, Department of
Paraclinical Sciences, Faculty of Veterinary Science, University of
Pretoria, Pretoria 0110, South Africa
Jacobus N. Eloff − Phytomedicine Programme, Department of
Paraclinical Sciences, Faculty of Veterinary Science, University of
Pretoria, Pretoria 0110, South Africa
Molahlehi S. Sonopo − Radiochemistry, South African Nuclear
Energy Corporation (Necsa), Brits R104, South Africa
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Complete contact information is available at:
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is based on the research supported by the National
Research Foundation of South Africa (Grant Numbers: UID
113964 and UID 117853), the University of Pretoria, and the
South African Nuclear Energy Corporation (Necsa SOC Ltd).
The University of Pretoria provided postdoctoral funding to
I.M.F.
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