Chemical structures of macrocyclic bis ( bibenzyls ) isolated from liverworts ( Hepaticae )

Liverworts (Hepaticae) produce a number of macrocyclic bis(bibenzyls) which show interesting biological activity and are of very valuable for the chemosystematic study of liverworts. The structural elucidation of these characteristic natural products are reviewed.


Introduction
The bryophytes [Musci (mosses), Hepaticae (liverworts) and Anthocerotae (hornworts)], which are the spore-forming terrestrial green plants are morphologically placed between the algae and the pteridophytes (fern) and there are ca.24,000 species in the world.Among the bryophytes almost all liverworts possess cellular oil bodies which are very important markers for the classification in the Hepaticae.Müller [1] reported that oil bodies are composed of sesquiterpenes.In 1967, Huneck and Klein [2] found that some liverworts produced ent-sesquiterpenoids. On the other hand, the mosses and the hornworts do not contain oil bodies.It has been demonstrated that most of the Hepaticae contain mainly mono-, sesqui-and diterpenoids and lipophilic aromatic compounds [bibenzyls, bis(bibenzyls), naphthalenes, phthalides, isocoumarins, cinnamates, benzoates, benzyl and β-phenethyl acrylates and prenyl indoles etc.] which constitute the oil bodies [3][4][5].The characteristic components of the Musci are highly unsaturated fatty acids and triterpenoids.The neolignan is the most important chemical marker of the Anthocerotae.Some liverworts show characteristically fragrant odors and intensely hot and bitter or saccharine-like taste.Generally, liverworts are not damaged by bacteria, fungi, insects, snails, slugs and other small animals.Furthermore, some liverworts cause intense allergenic contact dermatitis and allelopathy.Some bryophytes growing in lake, river and pond accumulate heavy metals.Many bryophyte species have been used as medicinal plants, particularly in China [6].However, only tasting substances and allergens of some species have been fully investigated.We have been interested in these biologically Table 1 The liverworts which contain cyclic bis(bibenzyls) Cyclic bis(bibenzyls) Species References Riccardin A (1a) Riccardia multifida [4,12,13a,13b,13c] Riccardin B (2) Riccardia multifida [4,12,13a,13c] Preissia quadrata [51] Riccardin C (3) Blasia pussila [18][19][20] Dumortiera hirsuta [22] Marchantia paleacea var.
The chemical constituents found in the Hepaticae and those of the bryophytes have been reviewed in Progress in the Chemistry of Organic Natural Products Vol. 42 [3] and Vol.65 [4], respectively.Heterocyclic compounds [5] and biologically active substances [7][8][9][10][11] isolated from liverworts have also been reviewed.Here the distribution of cyclic bis(bibenzyls) in liverworts and their structure elucidation are reviewed.In addition, some biological activity and total synthesis of the isolated cyclic bis(bibenzyls) are also discussed.
The 1 H NMR spectrum (Table 2) of riccardin A (1), C 29 H 26 O 4 (HRMS m/z 438.1831) possessed four benzylic methylenes, a methoxyl group, two phenolic hydroxyl group which was confirmed by the formation of a diacetate (1b) and a trimethyl ether (1c), three meta coupled protons, one of which was heavily shielded, three sets of ortho protons and an additional two sets of ortho protons.The IR spectrum of trimethyl ether (1c) showed neither carbonyl nor hydroxyl absorption bands, indicating the additional oxygen atom of 1a to be an ether.The above spectral evidence coupled with the molecular formula displayed that 1a was a cyclic bis(bibenzyl) derivative with two phenolic hydroxyl groups, a methoxyl group, a biphenyl ether and a biphenyl linkage.The substitution of four benzene rings was deduced by the NOE and double resonance experiments of 1b.The conclusive evidence of the stereostructure of 1a was obtained by X-ray crystallographic analysis of the diacetate (1b) as shown in Fig. 1.In the 1 H NMR spectra of 1a and its trimethoxy derivative (1c), H-3 appears at unusually high field (see Table 2).This is understandable as the result of the paramagnetic effect of two benzene rings A and D between which H-3 is sandwiched.The structure of riccardin B (2) was determined by comparison of 1 H and 13 C NMR spectra with those of riccardin A (1a) and its derivative (Table 2).
Marchantia polymorpha is a common thalloid liverwort, which is widely distributed in the world and shows antihepatic, antimicrobial, diuretic and allergenic contact dermatitis [4,[7][8][9][10][11].The methanol extract of this liverwort contains marchantin A (8a) as a major component, along with marchantin B (9), C (10), D (12), E (13), F (14) and G (15) [12,15,26a,28-30a].The 1 H NMR spectrum of 8a, C 28 H 24 O 5 ([M] + m/z 440.1617), contained signals of four benzylic methylene, three phenolic hydroxyl protons, thirteen protons on benzene rings.The structure of 8a was established by a combination of the chemical degradation of 8a as shown in Scheme 1, NMR spectrometry and X-ray crystallographic analysis.Treatment of 8a with methylene iodide in the presence of cupric oxide gave a methylene dioxide (8c), indicating the presence of two vicinal phenolic hydroxyls.Methylation of 8a gave a trimethyl ether (8b) which was hydrogenated to give an acyclic bis(bibenzyl) derivative (8d), followed by methylation and then by Birch reduction to afford 3-methyhoxybibenzyl (8e) and 3,4,5-trimethoxy-3 -hydroxybibenzyl (8f) which were synthesized by Wittig reaction.IR spectrum of 8b showed neither hydroxyl nor carbonyl absorption bands, indicating the remaining two oxygen atoms were ether oxygen.The direct Birch reduction of 8b yielded 4-hydroxy-3-methoxybibenzyl (8g) and 3-hydroxy-3,4 -dimethoxybibenbzyl (8h) which were also synthesized by Wittig reaction.On the basis of the above chemical and 1 H and 13 C NMR spectral evidence (Tables 2 and 8) of 8a and 8b, the structure of 8a was suggested to be a cyclic bis(bibenzyl) with two ether linkage between C-1 and C-2 and between C-14 and C-11 , possessing three phenolic hydroxyl group at C-13, C-1 and C-6 .In order to confirm the stereochemistry, recrystallization of marchantin itself and its derivatives were carried out.Marchantin series are very viscous Scheme 1.Chemical reactions of marchantin A (8a). gum, however, only the trimethyl ether (8b), which was chromatographed on silica gel-MgSO 4 (1 : 1) furnished crystals suitable for X-ray crystallographic analysis.The ORTEP drawing has been shown in Fig. 2.
The structures of the other marchantins B, D-G (9-15) were determined by comparison of their spectral data (Tables 2-4) with those of marchantin A (8) and chemical correlation.Recently, the stereochemistry of marchantin G (15) was confirmed by its X-ray crystallographic analysis [30b] as shown in Fig. 3.The yield of marchantin A (8a) depends on the Marchantia species.For example 120 g of pure marchantin A has been obtained from 2 kg of the dried M. polymorpha.
The spectral data of the trimethyl ether (18b) of marchantin J (18a) possessed an ethoxyl group resembled those of marchantin E (13), except for replacement of the methoxyl by an ethoxyl group, indicating that marchantin J was marchantin A mono ethyl ether.This was confirmed by 1 H and 13 C NMR spectrometry (Tables 4 and 8) including NOE difference spectra.This is the first record of an ethoxylated compound from liverworts.
The structure of isomarchantin C ( 22) isolated from the Indian Marchantia polymorpha and M. palmata [15] was characterized by analysis of the 1 H and 13 C MR spectra.The same compound was isolated Dumortiera hirsuta and its structure was confirmed by X-ray crystallographic analysis as shown in Fig. 4 [22].R. hemisphaerica and Mannia subpilosa produce marchantia quinone (26) [34,44].Marchantin O (24), the monomethyl ether of marchantin C (10), and marchantin P (25) were obtained from R. hemisphaerica [44,46,47] and South American Marchantia chenopoda, respectively [37].Their structures have been elucidated by analysis of the spectral data including difference NOE of the permethylated and the peracetylated derivatives.
Three novel macrocyclic bis(bibenzyls) named ptychantols A-C (27)(28)(29), which possess a transstilbene moiety were isolated from the stem-leafy liverwort Ptychantus striatus [48].Compound 27 contained two benzylic methylenes, trans ethylenic protons at δ 6.49 and 6.97 (J = 16.5 Hz, H-8 and H-7 , respectively) (Table 5), two phenolic protons, and 14 aromatic protons on four benzene rings.Methylation of 27 gave a dimethyl ether, whose IR spectrum indicated neither hydroxyl nor carbonyl absorption bands, indicating the presence two ether oxygen in 27.Hydrogenation of 27 gave a dihydro derivative, showing that 27 contained one olefinic group.The substitution pattern on benzene rings was suggested by analysis of 1 H and 13 C NMR spectral data (Tables 5 and 9), HMBC and NOESY spectra.The conclusive evidence for the structure of 27 was established by X-ray crystallographic analysis.The ORTEP drawing has been shown in Fig. 5.The 1 H and 13 C NMR data of ptychantol C (29) are indicated in Tables 8 and 9     Mannia fragrans elaborates pakyonol (30) [49].The New Zealand liverwort Schitochila glaucescens elaborates neomarchantin A (31) and B (32) [50].The former compound was also isolated from the German Preissia quadrata [51].Plagiochila sciophila produces not only marchantin C (10) and an acyclic bis(bibenzyl), perrottetin E (59), the latter member of the perrottetin class, which contains single o, pether linkage between two bis(bibenzyl) groups, but also four unique cyclic bis(bibenzyls), plagiochin A-D (33-36) which possess two ortho biphenyl linkage between the two benzyl groups [43].The stereochemistry of compound (33) was established by chemical reactions (methylation and acetylation).The 1 H and 13 C NMR spectrometry (Tables 5 and 9) and NOE studies on its tetramethyl ether as well as by an X-ray crystallographic analysis (see Fig. 6) of the latter.This showed that ring A is perpendicular to ring C and parallel with ring D. The proton at C-3 is strongly shielded by both rings A and D, causing a high field shift to δ 4.84 in the tetramethyl ether of 33.This phenomenon has been also encountered in the series of marchantins and riccardins described earlier.
Further fractionation of the methanol extract of P. fruticosa resulted in the isolation of isoplagiochin A-D (37-40) [18,[52][53][54][55].The number of the phenolic hydroxyl group of 37 was three, determined by acetylation and methylation to give a triacetate and trimethyl ether.The 1 H and 13 C NMR spectra of   6 and 9) showed the presence of two benzylic methylenes and cis-olefinic protons (δ 6.59, 6.63 (each 1H, d, J = 9 Hz, H-7 and H-8).The hydrogenation of 37 gave a dihydro derivative.The stereostructure of 37 was finally established by a combination of COSY, HMQC and HMBC of 37 and NOE difference spectra of a trimethyl and triacetate of 37 and X-ray crystallographic analysis as shown in Fig. 7.The 1 H and 13 C NMR spectra (Tables 6 and 9) of 39 resembled those of isoplagiochin A (37) except for the signal patterns of D-ring, indicating that 39 possessed the same skeleton as that of 37. The molecular formula of 39 was identical to that of 37, suggesting that the former compound contained an additional phenolic hydroxyl group at D-ring in place of an ether oxygen in compound 37.
This assumption was confirmed by the formation of a tetraacetate from 39.The location of the hydroxyl group at C-13 and the whole structure were determined by careful analysis of the 2D NMR spectra (HMBC and HMQC and NOESY).
Pulunasin (41) was isolated from culture cell of Hetroscyphus planus together with isoplagiochin A (37) and its structure elucidated by a combination of comparison of the spectral data with 37 and the analysis of HMBC and NOE spectral data [56].
The similar dimeric bis(bibenzyl), pusilatin E (64) has been isolated from Riccarida mulitifida and its structure was elucidated as the monomethyl ether of pusilatin B (61), because demethylation of 64 with BBr 3 gave 61.The 1 H and 13 C NMR spectral data of 64 are indicated in Tables 7 and 10.Compound 63 was synthesized by coupling reaction of riccardin A (3) with Mn(OAc) 3 [60].

Total synthesis of cyclic bis(bibenzyls)
Riccardin A-C (1a-3) have been synthesized by Gottsegen et al. [62], using a combination of Ullmann, Wittig and Wurtz reactions and Ni(0)-assisted intermolecular coupling reaction.Kodama's group [63-Chart 4. Pusilatin-type cyclic bis(bibenzyls), bibenzyl-phenanthrene and bibenzyls isolated from the liverworts.65] accomplished the total synthesis of riccardin B (2) and marchantin A (8a) in twelve steps using the intramolecular Wadsworth-Emmons olefination and Wittig reaction.Iyoda et al. [66] also reported the total synthesis of riccardin B (2) using nickel-catalysed intramolecular coupling of the acyclic precursors possessing two chlorine atom.The same compound was also synthesized by Norgadi et al. [67] by using a combination of Ullmann, Wittig and Wurtz reactions.Marchantin B (9) and H ( 16) have also been synthesized by Ha et al. [68], applying the same methods as described above.By using Ullmann, Wittig and modified Wurtz reaction, Dienes et al. [69] accomplished the total synthesis of marchantin I (17).Keserü et al. [70] reported the total synthesis of plagiochin C (34) and D (35) by the same methodology as that of the total synthesis of marchantin and riccardin series.Recently, Fukuyama et al. [71,72] accomplished the total synthesis of plagiochins A (33), B (34) and D (35) by a combination of Ullmann, Wadsworth-Emmons, Still-Kelly reactions [71,72].Norgadi reviewed the total synthesis of these types of compounds [73].

General
The 1 H and 13 C NMR spectra were recorded at 600 and 150 MHz, respectively, on a Varian UNITY 600 spectrometer using CDCl 3 with TMS as the internal standard unless otherwise stated.Measurements were performed at 25 • C using 5 mm o.d.sample tubes.For the 1 H-13 C correlation experiment, pulsed field gradient heteronuclear single-quantum correlation (GHSQC)) was used [74,75].The spectra were acquired with 1024 data points and 256 time increments with 8 transients per increment.The relaxation delay was 1.5 s and average 1 J(C-H) was set to 140 Hz.The NMR spectra were also recorded on a JEOL JNM GX 400 (400 MHz for 1 H, 100 MHz for 13 C) spectrometer using CDCl 3 unless otherwise stated.
The mass spectra including high resolution mass spectra were taken with a JEOL JMS AX-500 spectrometer at 70 eV.CD spectra were recorded on a JASCO J-725 spectrometer with MeOH or EtOH.
[α] D was measured in MeOH or CHCl 3 on a JASCO DIP-1000 polarimeter.X-ray crystallographic analysis was carried out on a Mac Science MXC 18 diffractometer with Cu Kα radiation.The structures were solved by direct method using CRYSTAN SIR 92 and refined by full-matrix least squares using CRYSTAN.

Plant materials
The liverworts were collected in Tokushima, in Shikoku Japan.The voucher specimens have been deposited at Faculty of Pharmaceutical Sciences, Tokushima Bunri University.

Extraction and isolation
The liverworts were air-dried and ground mechanically to give powders, which were extracted with methanol.Each methanol extract was filtered and the solvent evaporated to give a viscous material, which was chromatographed on silica gel (n-hexane/ethyl acetate gradient) and/or Sephadex LH 20 [methanol and chloroform (1 : 1)].Further purification of each fraction was carried out by preparative TLC and HLPC to give pure macrocyclic bis(bibenzyls).

Table 10
Measured in 150 MHz (CDCl3).a,b,c,d May be interchangeable in each vertical column.