Phenolic Compounds From the Stems and Leaves of Berchemia lineata (L.) DC

Eight new phenolic compounds, named bercheminols A-H (1–8), and eleven known analogues were isolated from the stems and leaves of Berchemia lineata (L.) DC. Their structures including the absolute configurations were elucidated by extensive spectroscopic analysis, chemical method, and quantum chemical calculations. Compound 1 possesses an unprecedented 3,4-dihydro-11H-benzo[b]pyrano[4,3-e] oxepin-11-one skeleton. The other new compounds belong to three structural types of natural products, including naphthopyrones (2–5), flavonoids (6–7), and bibenzyl (8). The α-glucosidase inhibitory activities of the isolated compounds were assayed. As a result, vittarin-B (9), rubrofusarin-6-O-β-D-glucopyranoside (11), quercetin (14), kaempferol (15), and dihydrokaempferol (17) showed moderate inhibitory activities against α-glucosidase with IC50 values of 22.5, 28.0, 36.5, 32.7, and 31.9 μM, respectively.


INTRODUCTION
Berchemia lineata (L.) DC. belongs to the genus of Berchemia, defined as the plant origin of "Tiebaojin" in Guangxi Traditional Chinese Medicine Standard. "Tiebaojin" is an important ethnic medicine commonly used in Guangxi Zhuang nationality and southwest minority areas of China. It can be used for the treatment of pulmonary tuberculosis hemoptysis, icteric hepatitis, abdominal pain, traumatic injury, snake bite, etc. Through investigation, it is found that the actual source of the medicinal materials of "Tiebaojin" commonly used clinically in the Zhuang region of Guangxi mainly include B. lineata, B. floribunda, B. polyphylla Wall. ex Laws., B. polyphylla var. leioclada (Zhang et al., 2011;Jing et al., 2017). Previous phytochemical studies on B. floribunda and B. polyphylla showed that they mainly include flavonoids, glycosides, lignans, quinones, and terpenoids (Chen et al., 2006). However, there are few studies on the chemical constituents of B. lineata, only some report chromones, flavonoids, and lignans (Shen et al., 2010a;Shen, et al., 2010b;Li et al., 2016;Jiang et al., 2019). To further search for new active compounds from B. lineata, phytochemical investigations of an extract of the stems and leaves of this plant afforded eight new phenolic compounds and eleven known analogues (Figure 1). This study reported the isolation, structure identification, and biological activity of these compounds.

Plant Material
The stem and leaves of Berchemia lineata (L.) DC. were purchased from Nanning, Guangxi Zhuang autonomous Region, P. R. China, and were identified by Prof. Hongli Teng, Guangxi Zhuang Medicine International Hospital. The voucher specimen was deposited in the herbarium of the School of Pharmaceutical Sciences, South Central University for Nationalities.

α-Glucosidase Inhibitory Assay
According to the literature method (Özgünseven et al., 2021) with some modifications, 20 μL compounds at different concentrations reacted with 20 μL of 4-nitrophenyl-α-Dglucopyranoside (20 mM) and 20 μL of α-glucosidase (0.5 U/ mL) in a 96-well plate at 37°C for 30 min. Na 2 CO 3 (0.2%, 80 μL) was then added to terminate the reaction and the absorbance value was measured at 405 nm using a microplate reader. IC 50 values were calculated from the graph of inhibition percentage against the logarithm of the concentrations of compounds.

Determination of Sugar
Compounds 4, 5, 6, 7, and 8 (0.5 mg)were refluxed with 2 ml of 4N HCl-dioxane (1 : 1) for 2 h, and then cooled to room temperature. The mixtures were extracted three times with 2 ml EtOAc. The aqueous layers were dried and refluxed with 2 | 1 H NMR, 13 C NMR, and HMBC data of compounds 6-7 in CD 3 OD (δ in ppm, J in Hz  0.5 ml pyridine and 1 mg L-cysteine methyl ester hydrochloride at 60 C for 1 h. 5 μL 2-methylphenyl isothiocyanate was added to the reaction mixtures and continued to reflux at 60 C for 1 h (Min et al., 2003;Tanaka, T., 2007). These dithiocarbamate derivatives of D-glucose and L-rhamnose were prepared in the same way.   Figure 2) from H 2 -12 to δ C 105.1 (s, C-2), 132.0 (s, C-3) and 122.2 (s, C-4), from H-4 to δ C 105.1 (s, C-2), from CH 3 -11 [δ H 2.31 (s)] to δ C 105.1 (s, C-2), and δ C 173.1 (s, C-1) suggested that the trisubstituted double bond was connected to C-12, and the trisubstituted and enolized double bonds were connected through C-3 to C-2. The remaining ester carbonyl carbon should be connected to C-2. Therefore, substructure A was established as depicted in Figure 1. Furthermore, HMBC correlations from H-4 to δ C 106.5 (d, C-6) and 132.8 (s, C-10) suggested that substructure A was connected to C-5. ROESY correlation of MeO to H-8 and HMBC correlation of MeO to δ C 153.6 (s, C-9) indicated that the methoxy group was connected to C-9. Except for the presence of two double bonds, one carbonyl and one phenyl, two DOUs were needed to satisfy the molecular formula of 1. Therefore, compound 1 still had two more rings. Owing to insufficient HMBC correlations, the connection positions of the two rings could not be determined, which ring was formed through ether bond, and which ring was formed by an ester bond. Therefore, there were two possible structures for 1. The ether bond was formed between C-1 and C-10, and C-2 was connected to C-13 through the carbonyl C-15 to form the ester bond in the candidate structure 1a.

RESULTS AND DISCUSSION
In contrast, C-2 was connected to C-10 through the carbonyl C-15 to form the ester bond, and the ether bond was formed between C-1 and C-13 in the candidate structure 1b. To further assign the connection positions of the two rings, NMR calculations with DP4+ analysis for two possible isomers were carried out. As a result, 1b was the most likely structure based on DP4+ probability with 100% ( Figure 3). Ultimately, its absolute configuration was Frontiers in Chemistry | www.frontiersin.org April 2022 | Volume 10 | Article 889441 5 confirmed as (13S) by ECD calculations (Figure 4). Therefore, the structure of 1 was defined and named bercheminol A, which possessed an unprecedented 3, 4-Dihydro-11H-benzo [b]pyrano [4,3-e] oxepin-11-one skeleton. From the biogenic analysis, compound 1 comes from the acetate-malonate (AA-MA) pathway. The reasonable biosynthetic pathway is shown in Scheme 1. First, one acetyl coenzyme A (CoA) and five malonyl-CoA undergo Claisen condensation and decarboxylation to obtain polyketone i. Then, i react with acetoacetyl-CoA through a series of reactions, including Claisen condensation, hydrolysis and decarboxylation to obtain the key intermediate ii. Starting from ii, phenolic compound iii is obtained through aldol condensation, dehydration, enolization, and reduction. Finally, iii is etherified, oxidized, and methylated to give compound 1.
Compound 2 was obtained as a brown amorphous powder. . The 13 C-NMR and DEPT spectra exhibited 17 carbon signals, including 5 sp 2 methines, 1 methyl, 1 methylene, 1 methoxy, and 4 sp 2 quaternary carbons, 3 oxygenated sp 2 tertiary carbons, one oxygenated methine, and one ester carbonyl carbon. The above NMR data suggested that 2 might belong to naphthopyranone. By comparison of the NMR data of 2 with those of pannorin B (Kaur et al., 2015) suggested that their structures closely resembled except for two major differences. First, the signal of 1, 2, 3-trisubstituted benzene ring in 2 replaced Frontiers in Chemistry | www.frontiersin.org April 2022 | Volume 10 | Article 889441 6 a pair of meta-coupled aromatic protons in pannorin B, suggesting that the methoxy and hydroxyl groups at C-10 and C-12 in the latter were replaced by the hydrogen and methoxy group, respectively, which was further supported by HMBC spectrum. Second, the chemical shifts of C-6 have shifted upfield 24.7 ppm, respectively, compared to pannorin B, suggesting that the hemiacetal carbon at C-6 in pannorin B was replaced by an oxygenated methine which was further confirmed by 1 H-1 H COSY correlation of CH 3 CH(O)CH 2and HMBC spectrum. Ultimately, its absolute configuration FIGURE 4 | Calculated and experimental ECD spectra of 1-3 and 6-7. SCHEME 1 | Plausible biosynthetic pathway for 1.
Compound 3 was obtained as a brown amorphous powder. The molecular formula was determined as C 16 H 12 O 4 by the HR-ESI-MS of the protonated molecular ion peak at m/z269.08072 [M + H] + (calcd 269.08084), suggesting 14 mass units less than 2. Compared with NMR data of 2, 3 had one less methoxy signal, suggesting that 3 was demethylated of 2 which was further confirmed by HMBC spectrum and ECD calculations. Thus, the structure of 3 was defined and named bercheminol C.
Compound 4 was obtained as a white amorphous powder. The molecular formula was determined as C 21 H 22 O 8 by the HR-ESI-MS of the protonated molecular ion peak at m/z 403.13870 [M + H] + (calcd 403.13874). Acid hydrolysis of 4 and HPLC analysis of dithiocarbamate derivative of sugar provided D-glucose. The configuration of the D-glucose was determined β by the coupling constant of 7.5 Hz of an anomeric proton (Cai et al., 2021). In addition to the NMR signal of glucose, the 1 H-NMR spectrum of the aglycone of 4 contained an isolated aromatic proton signal [δ H 7.50(1H, s, H-6)], a set of signals of 1, 2, 3trisubstituted benzene ring [δ H 7.62 (1H,t,J = 7.8 Hz,7.34 (1H,d,J = 7.8 Hz,7.50 (1H,d,J = 7.8 Hz,], an olefinic proton [δ H 6.31 (1H, s, H-3)], and two tertiary methyls [δ H 2.55, 2.86 (each 3H, s)]. The 13 C-NMR and DEPT spectra of the aglycone of 4 revealed the presence of five sp 2 methines, two methyls, and eight non-protonated carbons, including four sp 2 quaternary carbons, a conjugated carbonyl carbon [δ C 182.6 (C-4)], three oxygenated tertiary carbons [δ C 167.6 (C-2), 158.1(C-1a), 156.4 (C-10)]. The above NMR data suggested that 4 might belong to angular naphthopyrone (Shen et al., 2007;Chovolou et al., 2011). By careful comparison of NMR data of 4 with those of pleuropyrone A, their structures resembled each other. The major difference between them was the presence of signals of a 1, 2, 3-trisubstituted benzene ring in 4, instead of a pair of metacoupled aromatic proton signals in pleuropyrone A (Min et al., 2003). This deduction was further confirmed by HMBC correlations and H-7/H-8/H-9 of 1 H-1 H COSY. Therefore, the structure of 4 was defined and named bercheminol D.
Compound 5 was obtained as a white amorphous powder. The molecular formula was determined as C 22 H 24 O 9 by the HR-ESI-MS of the protonated molecular ion peak at m/z 433.14941 [M + H] + (calcd 433.14931) suggesting 14 mass units more than pleuropyrone A. By careful comparison of NMR data of 5 with those of pleuropyrone A, it was found that the only difference was the presence of an additional methoxy signal. It was suggested that 5 was an 8-O-methylated derivative of pleuropyrone A, which was further confirmed by the HMBC correlation from the methoxy group to C-8. Therefore, the structure of 5 was defined and named bercheminol E.
Compounds 6 and 7 were obtained as brown amorphous powders. The negative mode of HR-ESI-MS of 6 and 7 showed their quasimolecular ions at m/z 493.13760 and 493.13589 [M-H]respectively, suggesting the molecular formula C 23 H 26 O 12 . The detailed analyses of their NMR data indicated that they also possessed identical planar structures and belonged to dihydroflavonol glycoside containing L-rhamnopyranosyl moiety, which was further supported by acid hydrolysis of 6 and 7. By comparison of the NMR data of 6 and 7 with those of neoastilbin (De Britto et al., 1995), it was found that they are different from neoastilbin in the substituent pattern of the C ring. The ring C of 3,4dihydroxyphenyl in neoastilbin was replaced by 3, 5-dimethoxy-4hydroxyphenyl in 6 and 7. HMBC correlations from H-2′ and 6′ to C-2, C-1′ and C-4′, and MeO to C-3′ and C-5′ confirmed these findings. Careful analyses of the NMR data of 6 and 7 indicated that the difference between 6 and 7 were the absolute configurations of C-2 and C-3, which were the same as those of neoastilbin and astilbin. Compared to a (2R, 3R) configuration, the chemical shifts of H-1″ and H-2″ of the O-rhamnopyranosyl at C-3 in the (2S, 3S) configuration were located downfield, the chemical shifts of H-5″ and CH 3 -6″ were located upfield (De Britto et al., 1995). According to this rule, the absolute configurations of C-2 and C-3 of 6 and 7 were determined as (2S, 3S) and (2R, 3R), respectively. The presence of negative and positive cotton effect at about 330 nm in the CD spectrum of 6 and 7, respectively, and ECD calculations confirmed these findings (Figure 4). Therefore, the structures of 6 and 7 were defined and named as bercheminols F and G.
Compound 8 was obtained as a yellow amorphous powder. The molecular formula was determined as C 21 H 26 O 8 by the HR-ESI-MS of the protonated molecular ion peak at m/z 407.17007 (calcd 407.17004). Acid hydrolysis of 8 and HPLC analysis of dithiocarbamate derivative of sugar provided D-glucose. The configuration of D-glucose was determined as β based on the coupling constant of 7.5 Hz of an anomeric proton. Detailed analyses of its NMR data indicated that the aglycone of 8 was determined as vittarin A (Wu et al., 2005). Therefore, 8 was 3-Oglucosylated derivative of vittarin A, which was further confirmed by the HMBC correlation from the anomeric proton of glycopyranosyl to C-3. Therefore, the structure of 8 was defined and named bercheminol H.