Detailed analysis of HSQC, HMBC, and NOESY spectra identified two aryl units, A-ring and B-ring.. The methoxy signal showed HMBC cross-peak with the carboxyl carbon atδ 167.0 and NOESY c
Trang 1New phenolic compounds from the lichen
Parmotrema praesorediosum (Nyl.)
Hale (Parmeliaceae)
Introduction
Lichens, symbiotic combination of fungi and algae, comprise more
than 20 000 species that are found in most of the environmental
habitats from the tropics to polar regions Lichens produce a great
variety of metabolites, most of them occur only in lichens and the
others are also present in fungi and higher plants Characteristic
secondary metabolites of lichens are depsides, depsidones,
diphenyl ethers, benzofuran, usnic acid, and anthraquinone
derivatives.[1–3]
Parmotrema is one of the largest genera of the family
Parmeliaceae This genus of foliose lichen is widely distributed in
tropical regions and composed of c 350 species worldwide.[4]
Pre-vious phytochemical examination on Parmotrema praesorediosum
(Nyl.) Hale, collected on a betel nut tree, in southern Thailand,
re-vealed that this species contained some lactone fatty acids.[5]
Sam-ple collected in south Korea contained atranorin, chloroatranorin,
and fatty acids, i.e protopraesorediosic acid and praesorediosic
acid.[4]
In the course of our systematic research on lichen substances
from the Vietnamese flora, we have examined P praesorediosum
(Nyl.) Hale that is distributed in the south of Vietnam In our previous
study on this species, nine compounds, including methyl
haematommate, butyl haematommate, methyl chlorohaematommate,
methylβ-orsellinate, methyl divaricatinate, atranol, atranorin,
(+)-(12R)-isousnic acid, and (+)-(12R)-usnic acid were isolated.[6] Further
chemical investigation on this species by using modern separation
techniques has led to the isolation of seven novel phenolic
com-pounds, four diphenyl ethers (1–4) and three phthalide derivatives
(5–7) In this paper, we report the isolation and structure elucidation
of these compounds
Results and discussion
The thalli of P praesorediosum were extracted with MeOH at room
temperature A combination of chromatographic fractionation
of the extract led to the isolation of seven phenolic compounds
(1–7) (Fig 1) Their structures were elucidated as the following
Compound 1 was obtained as a yellow solid Its molecular
for-mula was established as C17H16O7through the protonated
mole-cule at m/z 333.0970 [M + H]+ in the HR-ESI-MS spectrum IR
absorptions implied the presence of hydroxyl (3383 cm 1), ester
carbonyl (1730 cm 1), and aromatic (1645, 1455 cm 1) functionali-ties Its1H NMR spectrum showed one hydrogen bonded hydroxyl group atδ 12.06 (1H, s), a formyl group at δ 10.39 (1H, d, J = 1.0 Hz),
an aromatic proton atδ 6.55 (1H, brs), two meta-coupled aromatic protons atδ 6.31 (1H, d, J = 2.5 Hz) and 6.17 (1H, brd, J = 2.5 Hz), a methoxy group atδ 3.50 (3H, s), and two methyl groups at δ 2.22 (3H, d, J = 0.5 Hz) and 2.00 (3H, brs) The combination of13C and DEPT NMR spectra of 1 (Table 1) revealed 17 carbons including a formyl group (δ 193.6), a methoxycarbonyl [δ 167.0 (―COO), 52.4 (―OCH3)], 12 aromatic carbons (δ 102–164), five of which were ox-ygenated, and two methyl groups (δ 20.8 and 17.0)
Detailed analysis of HSQC, HMBC, and NOESY spectra identified two aryl units, A-ring and B-ring The HMBC spectrum (Fig 2) showed the correlations from the chelated hydroxyl group (δ 12.06, 4-OH) to carbon signals atδ 110.6 (C-3), 164.0 (C-4), and 113.9 (C-5) and from the aldehydic proton (δ 10.38) to 3 and
C-4 The aromatic singlet proton (δ 6.55, H-5) showed cross-peaks with C-1 (δ 114.9), C-3, C-4, and C-9 (δ 20.8), and three protons of the methyl group (δ 2.22, H3-9), in turn, were correlated with C-1, C-5, and C-6 (δ 147.7) The methoxy signal showed HMBC cross-peak with the carboxyl carbon atδ 167.0 and NOESY correlation with H3-9, implying that the methoxycarbonyl group was situated
at C-1 Furthermore, C-2 was supposed to be oxygenated from its chemical shift (δ 158.1) in the13
C NMR spectrum Therefore, the A-ring
of 1 was established as 3-formyl-4-hydroxy-1-methoxycarbonyl-6-methyl-2-oxygenated benzene
The second aryl unit, B-ring, was a tetrasubstituted benzene core The HMBC spectrum of 1 showed correlations from a meta-coupled aromatic proton atδ 6.17 (H-1′) to carbon signals at δ 153.3 (C-2′), 102.1 (C-3′), 136.3 (C-5′), and 17.0 (C-7′), from another proton at δ 6.31 (H-3′) to C-1′ (δ 109.5), C-2′, C-4′ (δ 148.8), and C-5′, and from methyl protons (H3-7′) to C-1′, C-5′, and C-6′ (δ 131.1) Therefore,
* Correspondence to: Nguyen Kim Phi Phung, Department of Organic Chemistry, University of Science, National University —Ho Chi Minh City, 227 Nguyen Van
Cu Str., Dist 5, Ho Chi Minh City, Vietnam E-mail: kimphiphung@yahoo.fr
a Department of Science, Dong Nai University, Vietnam
b Department of Organic Chemistry, Kobe Pharmaceutical University, Japan
c Department of Organic Chemistry, University of Science, National University —Ho Chi Minh City, Vietnam
Received: 15 March 2015 Revised: 14 July 2015 Accepted: 21 July 2015 Published online in Wiley Online Library: 25 August 2015
(wileyonlinelibrary.com) DOI 10.1002/mrc.4316
Trang 2the B-ring of 1 was established as 2′,4′,5′-trioxygenated benzene
bearing a methyl group at 6′
Comparison of the chemical shifts of C-1′–C-6′ of
2,4-dihydroxy-6-methylphenoxy part with those of related compounds suggested
that the A-ring was linked to the B-ring through an ether linkage
between C-2 and C-5′.[7,8]
This connection was well supported by the key NOESY correlation between H-8 of the A-ring and H3-7′ of
the B-ring Thus, compound 1 was characterized as methyl
2-(2,4-
dihydroxy-6-methylphenoxy)-3-formyl-4-hydroxy-6-methylbenzoate and named praesorether A
Compound 2 was also obtained as a yellow solid Its molecular
formula was determined as C27H26O11from the HR-ESI-MS
spec-trum The1H NMR spectral data of 2 were similar to those of 1
(Table 1) except for the absence of two meta-coupled aromatic
protons of B-ring and the presence of two newly appeared singlets
of aromatic protons and some additional signals, i.e one methoxy
group (δ 3.88, 3H, s), one methylene group (δ 3.98 and 3.97, each
1H, br s), and one methyl group (δ 2.58, 3H, s) The13
C-NMR spec-trum of 2 exhibited, besides the signals because of the same A
and B rings as 1, signals for one methoxycarbonyl group (δ 172.4,
52.2), six aromatic carbons including two oxygenated carbons
[δC161.5 (C-2″), 160.0 (C-4″)], three quaternary carbons [δC142.5
(C-6″), 119.5 (C-5″), 109.4 (C-1″)], and one CH [δC101.5 (C-3″)], one
methylene carbon (δ 20.7) and one methyl carbon (δ 19.1) These
data indicated the presence of a third aromatic C-ring linked to
the B-ring in 2 This C-ring was established as the 2
″,4″-dihydroxy-1″-methoxycarbonyl-6″-methylphenyl moiety with a methylene
group at C-5″ by the analysis of its HMBC spectrum, which showed
the correlations from the methyl protons (δ 2.58, H3-8″) to C-1″, C-5″,
and C-6″, from the methylene (H2-8″) to C-4″, C-5″, C-6″, from an
ar-omatic singlet proton (δ 6.33, H-3″) to C-1″, C-2″, C-4″, and C-5″, and
from the hydroxyl (δ 10.56, 2″-OH) to C-1″, C-2″, and C-3″ The sub-stitution pattern of the ring C was further supported by the ROESY correlation between H3-8″ and H2-8′
The HMBC experiments showed that these methylene protons (H2-8′) correlated to aromatic carbons of B-ring at δ 153.3 (C-2′), 113.1 (C-3′), and 149.2 (C-4′) From these findings, the methylene carbon of the C-ring was linked to the B-ring at C-3′ (Fig 2) Conse-quently, compound 2 was established as a new diphenyl ether de-rivative and named praesorether B
Compound 3 was isolated as a yellow solid Its molecular formula was determined as C29H26O14from the HR-ESI-MS spectrum that meant compound 3 contained two carbon and three oxygen atoms more than that of 2 The1H NMR spectral data (Table 1) of 3 closely resembled those of 2, suggesting that they had the same basic framework except for the lack of one aromatic methine proton and one methyl group and the appearance of an acetalic proton
atδ 5.20 and a methoxy group at δ 3.13 The comparison of13
C NMR data of these two compounds showed that 3 also possessed three aromatic rings connected thorough an ether linkage and a methylene bridge as in 2, but it lacked one methyl group and contained three more carbon atoms including one carboxyl car-bon (δ 169.6), one acetalic carbon (δ 101.6), and one methoxy carbon (δ 56.8), implying the presence of a lactone ring in 3 Al-though the HMBC spectrum could not afford further information relating to the position of this lactone ring, the ROESY correla-tions of the methoxy protons (7′-OCH3) with the acetalic proton (H-7′) and also with the formyl proton at δ 10.12 (H-8) of the A-ring indicated theγ-lactone ring was fused to the B-ring at C-1′ and C-6′ (Fig 3) Complete analysis of the 2D NMR data for 3 re-sulted in its formulation as shown, and it was named praesorether C
Figure 1 Structures of isolated compounds 1 –7.
Trang 3Compound 4 was isolated as a yellow solid Its molecular formula
was determined as C43H40O16from the HR-ESI-MS The 1H NMR
spectrum of 4 (Table 1) was similar to that of 1; however, all signals
ascribable to A and B-rings appeared in duplicate, and furthermore
some additional signals of one aromatic methine proton (δ 6.24, 1H,
s), two methylene protons (δ 3.89 and 3.88, each 1H, s; 3.81, 2H, s),
and protons of a methyl group (δ 2.38, 3H, s) were observed The
same observation was also recorded for the13C and DEPT NMR
spectra of 4 with the appearance of six more aromatic carbons (δ
156.5, 152.9, 137.8, 119.4, 112.5, and 109.0), two methylenes (δ
21.6, 19.3), and a methyl (δ 20.8)
These spectral features suggested that 4 composed of two sets of
praesorether A (1) basic skeleton (parts a and b) and a fifth aromatic
ring C The position of functional groups in Aa, Ba, Ab, Bb, and
C-rings was confirmed by analysis of HSQC, HMBC, and ROESY
correlations The connection of the two units, Ba and Bb-rings of 1
with the fifth aromatic ring C through two methylene groups, was
elucidated by the HMBC spectrum with the correlations of the first
methylene protons (δ 3.81, H2-8′a) to carbon signals at δ 114.9
(C-3′a), 149.1 (C-4′a), and 152.7 (C-2′a) of the Ba-ring and to carbon signals atδ 112.5 (C-2″), and 152.9 (C-1″) of the C-ring, as well as cor-relations of the second methylene protons (δ 3.88 and 3.89, H2-8′b)
to carbon signals atδ 114.1 (C-3′b), 150.0 (C-4′b), and 153.6 (C-2′b) of the Bb-ring and to signals atδ 119.4 (C-4″), 137.8 (C-5″) and 156.5 (C-3″) of the C-ring Thus, the first methylene carbon linked the C-ring to the Ba-ring at C-2″ and C-3′a and the second methylene carbon linked the C-ring to the Bb-ring at C-4″ and C-3′b This was further supported by the ROESY cross peak between H2-8′b (δ 3.88 and 3.89) and H3-7″ (δ 2.38) (Fig 3) These information fully established the chemical structure of compound 4 as shown and it was named praesorether D
Compound 5 was obtained as a white amorphous solid Its mo-lecular formula C12H14O6was deduced from the protonated mole-cule [M + H]+at m/z 255.0862 in the HR-ESI-MS spectrum The IR spectrum showed characteristic absorptions for a hydroxyl group (3243 cm 1), a lactone group (1785 cm 1) and substituted aromatic system (1630 and 1363 cm 1) The1H NMR spectrum of 5 displayed signals of three methoxy groups atδ 3.53, 3.62, and 3.86 (each 3H,
Table 1 NMR data for compounds 1 4
Moiety a Moiety b
δ H J (Hz) δ C δ H J (Hz) δ C δ H J (Hz) δ C δ H J (Hz) δ C δ H J (Hz) δ C
5 6.55 brs 113.9 6.52 d (0.5) 112.9 6.71 s 116.2 6.48 s 112.4 6.47 s 112.7
8 10.39 d (1.0) 193.6 10.44 brs 195.6 10.12 s 193.7 10.45 s 195.7 10.43 s 195.7
9 2.22 d (0.5) 20.8 2.15 s 20.5 2.31 s 21.2 2.11 s 20.5 2.13 s 20.5
4-OH 12.06 s 12.11 s 11.92 brs 12.09 brs 12.09 brs
7-OCH 3 3.50 s 52.4 3.18 s 52.0 3.23 s 52.2 3.00 s 51.9 3.00 s 51.8
1 ′ 6.17 brd (2.5) 109.5 6.24 s 108.9 103.4 6.22 s 108.9 6.16 s 108.2
3 ′ 6.31 d (2.5) 102.1 113.1 117.6 114.9 114.1
7 ′ 2.00 brs 17.0 1.99 s 16.7 5.20 s 101.6 2.08 s 16.6 1.97 s 16.7
8 ′ 3.97 brs 20.7 4.05 brs 20.0 3.81 s 19.3 3.88 brs 21.6
7 ″-OCH 3 3.88 s 52.2 3.91 s 52.0
a CDCl 3
b
acetone-d 6
Trang 4s), two methylene protons atδ 4.84 and 4.88 (each 1H, d, J = 14.0 Hz,
H2-8), an aromatic proton atδ 6.88 (1H, s, H-7), an acetalic methine
proton atδ 6.33 (1H, s, H-3), and a phenolic hydroxyl proton at δ
9.08 (1H, s, 4-OH) The combination of13C and DEPT NMR spectra
of 5 revealed 12 carbons including one carboxyl carbon (δ 168.8),
one acetalic methine carbon (δ 102.2), five aromatic quaternary
car-bons (δ 159.7, 153.3, 128.6, 124.5, and 116.1), one aromatic CH
car-bon (δ 97.6), one oxymethylene carbon (δ 70.0), and three methoxy
groups (δ 59.3, 56.3, and 56.1) (Table 2)
These spectral data suggested that 5 could be a 3-oxyphthalide
with three substituents on the benzene ring (Fig 4) The HMBC
ex-periments showed the correlations from the methoxy protons atδ
3.53 to a methylene group (δ 70.0, C-8) and from the methylene
protons (H2-8) to two oxygenated aromatic carbons (δ 159.7, C-6;
153.3, C-4), one aromatic quaternary carbon (δ 116.1, C-5), and also
to this methoxy carbon (δ 59.3), indicating the methoxymethyl
group at C-5 The sole aromatic proton atδ 6.88 was located at
C-7 by its HMBC correlations with the carboxyl carbon (δ 168.8, C-1) and other aromatic carbons (δ 159.7, C-6; 124.5, C-3a; and 116.1, C-5) The other substituents, methoxy and phenolic hydroxyl groups were located at C-6 and C-4, respectively, by the analysis
of 2D NMR spectra (HMBC and NOESY) Finally, the HMBC correla-tions of the acetalic methine proton atδ 6.33 with the carboxyl car-bon (C-1) and a methoxy carcar-bon (δ 56.3) confirmed the methoxy group at C-3 on the five-member ring lactone The absolute config-uration of C-3 was not determined Thus, compound 5 was assigned as 4-hydroxy-3,6-dimethoxy-5-methoxymethylphthalide and named praesalide A
Compounds 6 and 7, designated praesalides B and C, were phthalide derivatives closely related to 5 The HR-MS measure-ments of 6 and 7 established the molecular formulas of C13H16O6 and C14H18O6 The1H and13C NMR data of 6 and 7 (Table 2) were similar to those of 5 except for the presence of an ethoxy group
at C-8 in 6 and two ethoxy groups at C-3 and C-8 in 7 instead of methoxy groups in 5 This was supported by the analysis of their 2D NMR (COSY, HSQC, HMBC, and NOESY) spectral features These results suggested the structures of 6 and 7 as 3-ethoxy-4-hydroxy-6-methoxy-5-methoxymethylphthalide and 3-ethoxy-5-ethoxymethyl-4-hydroxy-6-methoxyphthalide, respectively
Experimental
General experimental procedures The NMR spectra were measured on a Varian NMR System-500 or INOVA-500 spectrometer, at 500 MHz for1H NMR and 125 MHz for
13
C NMR The HR-ESI-MS were recorded on an Exactive mass spec-trometer (Thermo Fisher Scientific) The optical rotations were mea-sured on a Jasco DIP-370 digital polarimeter The IR spectra were measured on Shimadzu FTIR-8200 infrared spectrophotometer TLC was carried out on silica gel 60F254or silica gel 60 RP-18 F254S (Merck) and spots were visualized by spraying with a solution of 5% vanillin in ethanol, followed by heating at 100 °C Gravity col-umn chromatography was performed with silica gel 60 (0.040– 0.063 mm, Merck)
Figure 3 HMBC and ROESY correlations of 3 and 4.
Figure 2 HMBC and NOESY/ROESY correlations of 1 and 2.
Trang 5All NMR experiments were acquired at ambient temperature.
Chemical shifts are expressed in ppm with reference to the internal
TMS (0.000)
1
H and 13C NMR spectra were obtained using a Varian NMR
System-500 or INOVA-500 spectrometer.1H spectra: On a Varian
NMR System-500 spectrometer, spectral width (SW) 8012.8 Hz,
ac-quisition time (AT) 2.045 s, number of data points (NP) 32 768 K,
fil-ter band width (FB) 4000 Hz, block size (BS) 32, steady-state
transients (SS) 0, relaxation delay (D1) 1 s, spectrometer frequency
(SF) 499.73 MHz, pulse 90 width (PW) 7.9μs, temperature (TE)
25 °C, line broadening (LB) not use; On a INOVA-500, SW
7996.8 Hz, AT 4.097 s, NP 65 530 K, FB 4000 Hz, BS 32, SS 1, D1 1 s,
SF 499.83 MHz, PW 11.5μs, TE 25 °C, LB not use.13
C spectra: On a Varian NMR System-500 spectrometer, SW 31 250.0 Hz, AT 1.049 s,
NP 65 536 K, FB 17 000 Hz, BS 32, D1 1 s, SF 125.671 MHz, PW
9.0μs, TE 25 °C, LB 0.5 Hz; On a INOVA-500, SW 30 165.9 Hz, AT
1.3 s, NP 78 460 K, FB 17 000 Hz, BS 32, D1 1.7 s, SF 125.694 MHz,
PW 17.5μs, TE 25 °C, LB 0.5 Hz
HSQC spectra were done using the INOVA-500: D1 1.301 s, AQ
0.199 s, width 7303.9 Hz, 2D width 30 165.9 Hz, TE 25 °C, FT size
4096 × 2048 HMBC was done using the INOVA-500: D1 1.000 s,
AQ 0.128 s, width 7383.5 Hz, 2D width 30 165.9 Hz, TE 25 °C, FT size
2048 × 2048 ROESY: D1 1.000 s, AQ 0.140 s, width 7292.6 Hz, 2D
width 7292.6 Hz, TE 25 °C, FT size 2048 × 2048
Plant material The lichen thalli of P praesorediosum were collected on the bark of Dipterocarpus sp at Tan Phu forest, Dong Nai province, Vietnam in June 2009 The geographical location where the lichen was col-lected is at an altitude of 110 m, 11°20′–11°50′ N and 107°09′–107°
35′ E The botanical species of P praesorediosum (Nyl.) Hale (syno-nym of Parmelia praesorediosa Nyl.) was identified by MSc Vo Thi Phi Giao, Faculty of Biology, University of Science, National Univer-sity—Ho Chi Minh City A voucher specimen (No US-B020) was de-posited in the Herbarium of The Department of Organic Chemistry, Faculty of Chemistry, University of Science, National University—Ho Chi Minh City, Vietnam
Extraction and isolation The thallus material (5.0 kg) was washed under flow of tap water and then was air-dried at ambient temp to obviate thermally in-duced decomposition prior to be ground into a fine powder The ground powder sample (3.0 kg) was macerated by methanol at room temperature to afford a crude methanol extract (450 g) This crude one (450 g) was applied to silica gel solid phase extraction, successively eluted with the following solvents: petroleum ether (60–90 °C) (PE), chloroform (C), ethyl acetate (EA), acetone (A), and methanol (M) to afford corresponding extracts: extract PE (40 g), ex-tract C (105 g), exex-tract EA (50 g), exex-tract A (45 g), and exex-tract M (37 g)
The chloroform extract (105 g) was subjected to silica gel column chromatography, eluted by the solvent system of petroleum ether– ethyl acetate with increasing ethyl acetate to give 23 fractions from C1 to C23 Fraction C19 (6.1 g) was applied on silica gel column and eluted with a gradient solvent system of chloroform–acetone (95:5)
to give three fractions (C19a, C19b, and C19c) Fraction C19a (1.0 g) was silica gel rechromatographed, eluted with chloroform–acetone (98:2) and subjected to pre TLC using chloroform–methanol (9:1 and 95:5) as eluent to afford 1 (5.0 mg) Fraction C19b (3.2 g) was sil-ica gel rechromatographed, eluted with chloroform–acetone (98:2)
to give six fractions (C19ba to C19bf) Fraction C19bc (454.3 mg)
Table 2 NMR data for compounds 5 –7 (CDCl 3 )
δ H J (Hz) δ C δ H J (Hz) δ C δ H J (Hz) δ C
8 4.84 d (14.0) 70.0 4.83 d (14.0) 70.0 4.86 d (14.0) 68.1
4.88 d (14.0) 4.88 d (14.0) 4.92 d (14.0)
1 ′ 3.62 s 56.3 3.86 dq (9.5, 7.0) 65.3 3.86 dq (9.5, 7.0) 65.3
3.94 dq (9.5, 7.0) 3.94 dq (9.5, 7.0)
1 ″ 3.53 s 59.3 3.53 s 59.3 3.70 q (7.0) 67.6
Figure 4 HMBC and NOESY correlations of 5.
Trang 6was subjected to pre TLC (chloroform–methanol, 95:5) to afford 2
(28.1 mg) Fraction C19ba (169.6 mg) was subjected to pre TLC
(chloroform–methanol, 95:5, 9:1 and n-hexane–diethyl ether, 5:5)
to afford 3 (18.7 mg) and 4 (7.0 mg) Fraction C20 (23.9 g) was
re-peatedly subjected to silica gel column chromatography, eluted
with chloroform–methanol (10:0–9:1) to obtain eight fractions
(from C20a to C20h) The fraction C20c (5.8 g) was subjected to silica
gel chromatography, eluting with solvent of chloroform–methanol to
get six fractions (from C20ca to C20cf) Fractions C20cb (979.3 mg)
was silica gel rechromatographed, eluted with n-hexane–diethyl
ether and continuously subjected to pre TLC (n-hexane–diethyl ether
(2:8) and chloroform–methanol (98:2)) to afford three compounds 5
(8.0 mg), 6 (71.7 mg), and 7 (6.2 mg)
Praesorether A (1) [methyl
2-(2,4-dihydroxy-6-methylphenoxy)-3-formyl-4-hy-droxy-6-methylbenzoate]
Yellow solid IR (KBr)νmaxcm 1: 3383, 1730, 1645, 1455, 1265
HR-ESI-MS m/z 333.0970 [M + H]+, (Calcd for C17H16O7+ H, 333.0975)
and m/z 355.0789 [M + Na]+, (Calcd for C17H16O7+ Na, 355.0794)
1
H and13C NMR (CDCl3) data see Table 1 HMBC and NOESY see
Fig 2
Praesorether B (2)
Yellow solid IR (KBr)νmaxcm 1: 3371, 1730, 1706, 1646, 1465, 1263
HR-ESI-MS m/z 527.1544 [M + H]+, (Calcd for C27H26O11+ H,
527.1554) and m/z 549.1363 [M + Na]+, (Calcd for C27H26O11+ Na,
549.1373).1H and13C NMR (acetone-d6) data see Table 1 HMBC
and ROESY see Fig 2
Praesorether C (3)
Yellow solid [α]D23+ 3.5 (c 0.68, CHCl3) IR (KBr)νmaxcm 1: 3394,
1732, 1651, 1455, 1276 HR-ESI-MS m/z 599.1396 [M + H]+, (Calcd
for C29H26O14+ H, 599.1402) and m/z 621.1213 [M + Na]+(Calcd
for C29H26O14+ Na, 621.1220).1H and 13CNMR (CDCl3) data see
Table 1 HMBC and ROESY see Fig 3
Praesorether D (4)
Yellow solid IR (KBr)νmaxcm 1: 3366, 1706, 1645, 1458, 1268
HR-ESI-MS m/z 813.2394 [M + H]+, (Calcd for C43H40O16+ H, 813.2396)
and m/z 835.2169 [M + Na]+, (Calcd for C43H40O16+ Na, 835.2214)
1
H and13C NMR (acetone-d6) data see Table 1 HMBC and NOESY
see Fig 3
Praesalide A (5) (4-hydroxy-3,6-dimethoxy-5-methoxymethylphthalide)
White amorphous solid [α]D26+ 24.7 (c 0.23, CHCl3) IR (KBr)νmax
cm 1: 3243, 1785, 1630, 1363 HR-ESI-MS m/z 255.0862 [M + H]+,
(Calcd for C12H14O6+ H, 255.0869) and m/z 277.0680 [M + Na]+,
(Calcd for C12H14O6+ Na, 277.0688).1H and13C NMR (CDCl3) data
see Table 2 HMBC and NOESY see Fig 4
Praesalide B (6) (3-ethoxy-4-hydroxy-6-methoxy-5-methoxymethylphthalide)
White amorphous solid [α]D25 2.0 (c 1.33, CHCl3) IR (KBr)νmax
cm 1: 3240, 1769, 1625, 1341 HR-ESI-MS m/z 269.1018 [M + H]+,
(Calcd for C13H16O6+ H, 269.1025) and m/z 291.0837 [M + Na]+
(Calcd for C13H16O6+ Na, 291.0845).1H and13C NMR (CDCl3) data
see Table 2
Praesalide C (7) (3-ethoxy-5-ethoxymethyl-4-hydroxy-6-methoxyphthalide)
White amorphous solid [α]D
26
+ 9.5 (c 0.61, CHCl3) IR (KBr)νmaxcm 1:
3235, 1766, 1624, 1339 HR-ESI-MS m/z 283.1174 [M + H]+(Calcd for
C14H18O6+ H, 283.1182) and m/z 305.0992 [M + Na]+ (Calcd for
C14H18O6+ Na, 305.1001).1H and13C NMR (CDCl3) data see Table 2
Conclusions
From the chloroform soluble fraction of the methanol extract of the lichen P praesorediosum (Nyl.) Hale seven novel compounds are iso-lated, including four diphenyl ethers praesorether A (1), praesorether B (2), praesorether C (3), and praesorether D (4), as well as three stable phthalides praesalide A (5), praesalide B (6), and praesalide C (7) Their chemical structures were established pri-marily by NMR and MS spectroscopy
Diphenyl ethers connected with benzyl moiety such as 2–4 are quite unique and have not been isolated from the lichens with ex-ception of a depsidone furfuric acid of Pseudevernia furfuracea.[9] The one-pot synthesis of furfuric acid by the acid-catalysed alkyl-ation led to the proposal that this compound is an artifact formed during isolation procedure.[10]Nevertheless, it is not excluded com-pounds 2–4 could be genuine lichen compounds Therefore, it is of great interest to study the mechanism of their formation
A literature search revealed that just four natural phthalide deriv-atives have been reported from lichens: Buellolide and canesolide from Buellia canescens; 5,7-dihydroxy-6-methylphthalide from Anamylopsora pulcherrima; and 7-hydroxy-5-methoxy-6-methylphthalide from Usnea aciculifera.[11–13]However, this type
of compound was often found from fungi, such as rubralide C closely related to 5, which has been isolated from the marine sediment-derived fungus Penicillium pinophilum SD-272.[14] The majority of secondary metabolites found in lichens are produced
by the fungal partner However, most of secondary metabolites known from lichens, so-called lichen substances such as depsides, depsidones, xanthones, diphenyl ethers, and pulvinic acid are unique to these organisms and related to the symbiosis as a small minority occur in other fungi or higher plants.[2,15]The isolation of the phthalides of unusual compound classes could offer some in-sights to the secondary metabolites of the lichens, and lichens could be a potent source for searching unusual compounds Acknowledgements
We are grateful to the Government of Vietnam (Project 322, MOET) for the fellowship to B.L.C.H We are grateful to MSc Vo Thi Phi Giao for identification of the Parmotrema specimens Thanks are also due
to Dr C Tode (Kobe Pharmaceutical University) for1H and13C NMR spectra, and to Dr A Takeuchi (Kobe Pharmaceutical University) for mass spectral measurements This research was financially sup-ported by Vietnam’s National Foundation for Science and Technol-ogy Development (NAFOSTED) grant #104.01-2013.17
Conflict of interest
The authors have declared that there is no conflict of interest
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Supporting information
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