**Contents**



## **About the Editor**

**Giovanni Ribaudo**, PhD. My research activity is based on the combination of tools from synthetic, analytical (HPLC, NMR, mass spectrometry), and computational medicinal chemistry. My main research topics consist of the design and screening of small molecules interacting with peculiar DNA arrangements and in the study of nature-inspired phosphodiesterase (PDE) inhibitors targeting the central nervous system. I received my Master's degree in Medicinal Chemistry and Technology at the University of Padova (Italy) in 2011. In 2015, I graduated with my PhD in Pharmaceutical Sciences at the same Institution, after carrying out part of the research activity at the State University of New York in Albany (NY, USA). Between 2015 and 2019, I worked as a post-doc at the University of Padova in collaboration with a company operating in the field of the chemistry of natural compounds. In 2019, I joined the Department of Molecular and Translational Medicine of the University of Brescia.

### *Editorial* **Synthesis of Flavonoids or Other Nature-Inspired Small Molecules**

**Giovanni Ribaudo**

Department of Molecular and Translational Medicine, University of Brescia, 25123 Brescia, Italy; giovanni.ribaudo@unibs.it

Natural compounds are endowed with an intriguing variety of scaffolds, functional groups and stereochemical properties. Natural evolution continuously gives rise to extremely complicated arrangements of polysubstituted rings and chiral centers. In addition to the interest that such molecules trigger from the point of view of synthetic organic chemistry per se, the world of natural compounds has been explored by medicinal chemists considering the valuable biological activities that they bear, inspiring the design and development of novel drugs.

The Special Issue of *Molbank*, entitled "Synthesis of Flavonoids or Other Nature-Inspired Small Molecules", was launched in Spring 2020, and collected 10 contributions by the end of 2021 with a wide geographical reach. The Guest Editor is grateful to all the authors that actively contributed to the Special Issue by submitting the results of their research activity in the field of the synthesis of natural and nature-inspired compounds.

Among small molecules of natural origin, flavonoids represent an outstanding chemical class. Together with their semi-synthetic derivatives, flavonoids have been extensively studied in light of their antioxidant, antiproliferative, antibacterial and anti-inflammatory activities, to name a few. Therefore, this Special Issue aimed to collect contributions related to the following topics:


In fact, in addition to flavonoids, several other classes of natural or nature-inspired molecules, such as alkaloids and terpenoids, are particularly attractive from the synthetic and medicinal chemistry perspectives.

In the first paper, the preparation and characterization of a semi-synthetic hydrazone derivative of quercetin, studied as a phosphodiesterase inhibitor and synthesized through the single-step modification of the natural precursor, was reported by our research group from the University of Brescia (Italy) [1]. In the second paper, Prof. Unang Supratman and colleagues from Universitas Padjadjaran (Indonesia) described a polyoxygenated dimertype xanthone isolated from the stem bark of *Garcinia porrecta* [2]. In the third paper, a new onoceranoid triterpene was isolated from the fruit peels of *Lansium domesticum* by Dr. Tri Mayanti and colleagues from Universitas Padjadjaran (Indonesia), and the compound was tested for its antiproliferative activity [3]. In the fourth paper, Dr. Mariia Nesterkina and colleagues from Odessa National Polytechnic University (Ukraine), described the synthesis of 2-propyl-*N*- -[1,7,7-trimethylbicyclo[2.2.1]hept-2-ylidene]pentanehydrazide, a camphor derivative with anticonvulsant properties [4]. In the fifth paper, the synthesis and characterization of a quinoline derivative were described by the group of Dr. Paolo Coghi and Prof. Vincent Kam Wai Wong from the Macau University of Science and Technology (China). Furthermore, the compound was tested for its antiproliferative activity against several cell lines [5]. In the sixth paper, the novel tetranortriterpenoid kokosanolide D was isolated from *Lansium domesticum* by Dr. Tri Mayanti and colleagues from Universitas

**Citation:** Ribaudo, G. Synthesis of Flavonoids or Other Nature-Inspired Small Molecules. *Molbank* **2022**, *2022*, M1313. https://doi.org/10.3390/ M1313

Received: 4 January 2022 Accepted: 7 January 2022 Published: 10 January 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Padjadjaran (Indonesia), and the compound was characterized through a combination of spectroscopic techniques [6]. In the seventh paper, Dr. Mithun Rudrapal and colleagues from the Dhariwal Institute of Pharmaceutical Education & Research (India) described flavonoids from *Cordia dichotoma* and their antidiabetic activity, based on computational and experimental data [7]. In the eight paper, a novel curcumin analogue developed as a potential fluorescent dye for biological systems was synthesized and characterized by Prof. Marco Edilson Freire de Lima and colleagues from Universidade Federal Rural do Rio de Janeiro (Portugal) [8]. In the ninth paper, Dr. Diana Becerra, Dr. Juan-Carlos Castillo and colleagues from Universidad Pedagógica y Tecnológica de Colombia and Universidad de los Andes (Colombia) reported the synthesis of 2-oxo-2*H*-chromen-7-yl 4-chlorobenzoate obtained by *O*-acylation of 7-hydroxy-2*H*-chromen-2-one [9]. Eventually, in the tenth paper, lupeol extracted from *Bombax ceiba* was used by Dr. Thuc-Huy Duong from the Ho Chi Minh City University of Education (Vietnam), Dr. Jirapast Sichaem from Thammasat University (Thailand), and colleagues, as a starting material to produce semi-synthetic derivatives exhibiting α-glucosidase inhibitory activity [10].

All these interesting contributions demonstrate the flourishing interest of the international scientific community in the identification and optimization of novel synthetic routes for producing nature-inspired bioactive compounds. As a conclusive note, the Guest Editor would like to sincerely thank the Reviewers and the Assistant Editors for their valuable support and for having made the realization of this Special Issue possible.

**Acknowledgments:** The Guest Editor would like to sincerely thank Jessica Tecchio for her support.

**Conflicts of Interest:** The author declares no conflict of interest.

#### **References**


## *Short Note* **2-(3,4-Dihydroxyphenyl)-4-(2-(4-nitrophenyl) hydrazono)-4***H***-chromene-3,5,7-triol**

#### **Alessandra Gianoncelli 1, Alberto Ongaro 1, Giuseppe Zagotto 2, Maurizio Memo <sup>1</sup> and Giovanni Ribaudo 1,\***


Academic Editor: Fang-Rong Chang Received: 10 June 2020; Accepted: 28 June 2020; Published: 29 June 2020

**Abstract:** On the basis of the knowledge from traditional herbal and folk medicine, flavonoids are among the most studied chemical classes of natural compounds for their potential activity as phosphodiesterase 5 (PDE5) inhibitors. We here describe the preparation of a semi-synthetic hydrazone derivative of quercetin, 2-(3,4-dihydroxyphenyl)-4-(2-(4-nitrophenyl)hydrazono)-4*H*chromene-3,5,7-triol, that was obtained via a single-step modification of the natural compound. The product was characterized by NMR, mass spectrometry and HPLC. Preliminary molecular modeling studies suggest that this compound could efficiently interact with PDE5.

**Keywords:** quercetin; flavonoids; semi-synthetic; PDE; sildenafil; molecular modeling

#### **1. Introduction**

Phosphodiesterase (PDE) inhibitors contrast the degradation of 3- ,5- -cyclic adenosine monophosphate (cAMP) and/or 3- ,5- -cyclic guanosine monophosphate (cGMP), thus promoting several downstream effects. The inhibitors of PDE5 isoform, in particular, interfere with cGMP hydrolysis and induce smooth-muscle relaxation in specific tissues [1,2]. These compounds find clinical applications in the treatment of erectile dysfunction and pulmonary hypertension, and they are being studied as potential treatments against other diseases [3–5]. Repurposing of approved drugs is becoming an attractive strategy for identifying new applications for compounds with proved safety [6], and in this context PDE5 inhibitors are currently under investigation to contrast neurodegeneration [7,8], depression [9], diabetes [10] and rare pathologies such as Duchenne muscular dystrophy [11].

The development of synthetic PDE5 inhibitors, such as sildenafil (Figure 1A) and its analogues, has been paralleled by the exploration of the potential activity of natural compounds from traditional and folk medicine [12–14]. Flavonoids, in particular, have been known as PDE inhibitors for decades [15]. Natural glycosylated flavonoids and aglycones [16–19], as well as semi-synthetic flavones [20] and isoflavones [21–23], have been studied in silico, in vitro and in vivo for their inhibitory activity towards PDE5 and other isoforms.

The single-step derivatization procedure to obtain 2-(3,4-dihydroxyphenyl)-4-(2-(4-nitrophenyl) hydrazono)-4*H*-chromene-3,5,7-triol (**1**) from quercetin is here reported. This semi-synthetic compound was characterized by NMR, mass spectrometry and HPLC. Its interaction pattern with PDE5 was investigated in silico and compared to that of quercetin and sildenafil.

#### **2. Results and Discussion**

#### *2.1. Chemistry*

Quercetin was previously reported to possess inhibitory activity on several PDE isoforms [24], and the vasorelaxant effect of this natural flavonoid and of the corresponding metabolites was demonstrated to be due to the interference with the cGMP pathway [25]. Chan et al. investigated the effects of quercetin derivatives on PDE isoforms [26], and we previously explored the semi-synthetic derivatization of flavonoids to enhance their interaction with PDE5 in silico and in vitro [21,22].

We here report the preparation of a hydrazone derivative, 2-(3,4-dihydroxyphenyl)-4-(2-(4 nitrophenyl)hydrazono)-4*H*-chromene-3,5,7-triol (**1**), that was obtained via a single-step modification procedure from quercetin (Figure 1B). Rollas et al. recently discussed the biochemical relevance, in terms of reactivity and bioactivity aspects, of hydrazones [27]. Hydrazones are generally prepared from carbonyl compounds by reaction with an opportune hydrazine in acidic conditions [28,29]. Hydrochloric, acetic or 3-chloroperbenzoic acid are usually adopted for the synthesis of hydrazones, and the use of catalysts has been reported [30,31]. Compound **1** was synthesized by reacting quercetin with an excess of 4-nitrophenylhydrazine in a 1:1 mixture of acetic acid and ethanol. Following this procedure, the compound was isolated by filtration in a good yield (56%).

**Figure 1.** Chemical structure of sildenafil (**A**) and synthetic scheme for the preparation of compound **1** (**B**).

Compound **1** was characterized by NMR, mass spectrometry and HPLC (see Figures S1–S4 in the Supplementary material).

#### *2.2. Molecular Modeling*

The interaction of compound **1** with PDE5 was investigated in silico following a protocol reported previously [22]. For comparison, sildenafil and quercetin were also docked to the same 3D model and the predicted interaction patterns demonstrated a good co-localization of the ligands within the protein. The calculated binding energy value was particularly encouraging for compound **1** (−10.3 kcal/mol), exceeding that predicted for sildenafil (−9.7 kcal/mol) and quercetin (−9.5 kcal/mol) (Figure 2A). More in detail, docking experiments showed that the three compounds bind to the same region of the protein, consisting in the catalytic site (Figure 2B–D and Figures S5–S10 in the Supplementary materials). Most importantly, according to the predicted models, compound **1**, sildenafil and quercetin

interact with the same group of residues in such a PDE5 domain. In particular, Ile778, Val782, Ala783, Leu804, Ile813, Met816, Gln817 and Phe820, which were previously reported to be relevant interacting residues for known PDE5 inhibitors [32], were highlighted within the < 5 Å region from the docked ligands (Figure 2).

**Figure 2.** Results of the docking study for compound **1** to PDE5, in comparison with sildenafil and quercetin. (**A**): calculated binding energy values obtained from docking experiments. (**B**): docking pose of sildenafil. (**C**): docking pose of quercetin. (**D**): docking pose of compound **1**.

Furthermore, the stability of the complex predicted for compound **1** and PDE5 was assessed using molecular dynamics simulations [33]. The results show that the complex reached stability after 8 ns, and it was retained during the remaining simulation time (see Figures S11–S12 in the Supplementary materials)

#### **3. Materials and Methods**

#### *3.1. Chemistry*

#### 3.1.1. General

Commercially available chemicals were purchased from Sigma–Aldrich (Saint Louis, MO, USA) and used as received, unless otherwise stated. 1H and 13C{1H} NMR spectra were recorded on an Avance III 400 MHz spectrometer (Bruker, Billerica, MA, USA). All spectra were recorded at room temperature; the solvent for each spectrum is given in parentheses. Chemical shifts are reported in ppm and are relative to tetramethylsilane (TMS) internally referenced to the residual solvent peak. Datasets were edited with iNMR (Nucleomatica, Molfetta, Italy). The multiplicity of signals is reported as a singlet (s), doublet (d), triplet (t), quartet (q), multiplet (m), broad (b) or a combination of any of these. Mass spectra were recorded by direct infusion ESI on a Xevo G2-XS (Waters, Milford, MA, USA). The purity profile (96%) was assayed by HPLC using a Pro-Star system (Varian, Palo Alto, CA, USA) equipped with a 1706 UV–VIS detector (254 nm, Bio-rad, Hercules, CA, USA) and an C-18

column (5 μm, 4.6 × 250 mm, Agilent Technologies, Santa Clara, CA, USA). An appropriate ratio of water (A) and acetonitrile (B) was used as mobile phase with an overall flow rate of 1 mL/min; the general methods for the analyses are reported here: 0 min (95% A–5% B), 5 min (95% A–5% B), 25 min (5% A–95% B), 35 min (5% A–95% B) and 40 min (95% A–5% B).

#### 3.1.2. Synthesis of 2-(3,4-dihydroxyphenyl)-4-(2-(4-nitrophenyl)hydrazono)-4*H*-chromene-3,5,7-triol (**1**)

A round-bottom flask was charged with quercetin (50.0 mg, 0.17 mmol) and ethanol (5 mL). A solution of 4-nitrophenylhydrazine (76.0 mg, 0.50 mmol) in acetic acid (5 mL) was added dropwise to this mixture and the reaction was refluxed under stirring for 6 h. After cooling to room temperature, the concentration of the solvent induced the formation of a precipitate. The solid, collected by filtration, was triturated using diethyl ether and the resulting product was isolated as a brown solid (42.0 mg). Yield: 56%, mp. 264–267 ◦C, HPLC r.t. 21.8 min. 1H-NMR (DMSO-*d*6, 400 MHz): δH, 12.48 (1H, bs, OH), 9.96 (1H, bs, OH), 8.99 (1H, bs, OH), 8.05 (2H, d, *J* 8.1 Hz, Hr and Ht), 7.67 (1H, d, *J* 1.8 Hz, Ho), 7.52 (1H, dd, *J* 8.0 Hz, *J* 1.8 Hz, Hk), 6.89 (1H, d, *J* 8.0 Hz, Hl), 6.73 (2H, d, *J* 8.1 Hz, Hq and Hu), 6.37 (1H, s, Hf of Hh), 6.15 (1H, s, Hh or Hf). 13C {1H}-NMR (DMSO-*d*6, 100 MHz): δ<sup>C</sup> 177.2, 176.3, 169.5, 164.3, 161.1, 159.2, 156.5, 155.4, 148.1, 147.2, 145.5, 138.4, 136.2, 126.3, 122.4, 120.4, 116.0, 115.5, 110.9. ESI-MS found 438.454 (C21H16N3O8 <sup>+</sup>. [M + H]+), calc. 438.366.

#### *3.2. Molecular Modeling*

The structure of PDE5 was obtained from the RCSB Protein Data Bank (www.rcsb.org, PDB ID: 2H42). The target and ligands were prepared for the blind docking experiment which was performed using Autodock Vina (Molecular Graphics Laboratory, Department of Integrative Structural and Computational Biology, The Scripps Research Institute, La Jolla, CA, USA) [34]. Output data (energies, interaction patterns) were analyzed and scored using a UCSF Chimera molecular viewer [35], which was also used to produce the artworks. Molecular dynamics simulations were carried out using PlayMolecule (Accelera, Middlesex, UK) starting from the output model of docking experiments. A ligand was prepared by running a Parametrize function based on GAFF2 force field [36]. The complex was prepared for the simulation using ProteinPrepare and SystemBuilder functions, setting pH = 7.4, AMBER force field and default experiment parameters [37]. A simulation of 25 ns was carried out using SimpleRun, with default settings [38].

#### **4. Conclusions**

In this short note, we reported the preparation of 2-(3,4-dihydroxyphenyl)-4-(2-(4-nitrophenyl) hydrazono)-4*H*-chromene-3,5,7-triol (**1**), a semi-synthetic hydrazone derivative of quercetin that was obtained via a single-step approach. Preliminary in silico studies suggest that this compound could efficiently interact with the catalytic domain of PDE5 and that the effect on enzymatic inhibition of quercetin derivatives bearing this or other hydrazone substituents should be evaluated in vitro.

**Supplementary Materials:** The following are available online, Figures S1 and S2: NMR spectra, Figure S3: ESI-MS spectrum, Figure S4: HPLC profile, Figures S5–S10: docking studies, Figures S11 and S12: molecular dynamics simulations.

**Author Contributions:** Conceptualization, A.G. and G.R.; methodology, G.R.; software, A.O. and G.R.; investigation, A.O.; data curation, A.G.; writing—original draft preparation, A.G., A.O. and G.R.; writing—review and editing, M.M. and G.Z.; supervision, A.G. and G.R.; funding acquisition, A.G. and G.R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was granted by University of Brescia.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


**Sample Availability:** Samples of the compounds of compound **1** are available from the authors.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

*Short Note*

#### **5,5**- **-Oxybis(1,3,7-trihydroxy-9***H***-xanthen-9-one): A New Xanthone from the Stem Bark of** *Garcinia porrecta* **(Clusiaceae)**

#### **Ayu N. Safitri 1, Nurlelasari 1, Tri Mayanti 1, Darwati <sup>1</sup> and Unang Supratman 1,2,\***


Received: 15 July 2020; Accepted: 7 August 2020; Published: 12 August 2020

**Abstract:** A new polyoxygenated dimer-type xanthone, namely 5,5- -oxybis(1,3,7-trihydroxy-9*H*xanthen-9-one (**1**), has been isolated from the stem bark of *Garcinia porrecta*. The structure of **1** was determined based on spectroscopic data, including 1D and 2D-NMR as well as high resolution mass spectroscopy analysis.

**Keywords:** *Garcinia porrecta*; Clusiaceae; xanthone

#### **1. Introduction**

The famous *Garcinia* genus, representing a major source of triterpenes, flavonoids, xanthones, and phloroglucinols which have pharmacological activities as antioxidants, antibacterial, antiviral, anti-HIV, and significant anticancer activity [1].

The genus *Garcinia* belongs to the Clusiaceae family, which consists of more than 400 species widely distributed in the Polinesia mainland, India, Indochina, Indonesia, West and Central Africa, and Brazil [2]. Indonesia is known as one of the countries rich in diversity of *Garcinia*, there are 64 species of *Garcinia* scattered across several islands in Indonesia [3]. Various parts of *Garcinia* plants have been used in traditional medicine for the treatment of sprue (mouth ulcer), diarrhea, dysentery and skin disease [4]. Investigations into biologically active compounds from Indonesia *Garcinia* plants have resulted in some bioactive compounds being isolated from *G. mangostana* [5–7], *G*. *celebica* [8,9] and *G. cowa* [10]. Previous investigation on the stem bark of *G. porrecta* had led to the isolation of dulxanthone E–G, which showed strong cytotoxic activity against murine leukemia L1210 cells [6]. In this paper, we reported the isolation and structure elucidation of new polyoxygenated dimer-type xanthone, 5,5- -Oxybis(1,3,7-trihydroxy-9*H*-xanthen-9-one) (**1**) (Figure 1).

**Figure 1.** Chemical structure of compound **1**.

#### **2. Results**

#### *Extraction and Isolation*

The chopped dried stem bark of *G. porrecta* (2 Kg) was macerated at room temperature with *n*-hexane (5 × 2 L), ethyl acetate (5 × 2 L), and methanol (5 × 2 L). The solvents were removed by a rotary evaporator to give a crude *n*-hexane extract (21 g), ethyl acetate (12.5 g), and methanol (25 g). The ethyl acetate extract (12.5 g) was fractionated by vacuum liquid chromatography on silica gel using a gradient of *n*-hexane-ethyl acetate-methanol solvent to give eight fractions (A–H). Fraction E (1.93 g) was separated with silica gel column chromatography using *n*-hexane:methylene chloride:acetone (5:3:2) as the solvent system to give nine subfractions (E1–E9). Subfraction E8 (140.7 mg) was purified by column chromatography on RP-18 silica using 10% gradient MeOH:H2O to give **1** (30.6 mg).

5,5- -Oxybis(1,3,7-trihydroxy-9*H*-xanthen-9-one) (**1**), yellow amorphous powder, [α] 20D +12.4 (*c* 0.1, MeOH); UV (MeOH) λmax: 322 and 262 nm; HR-TOFMS *m*/*z* 503.0667 [M + H]<sup>+</sup> (calcld. for C26H15O11, 503.0614); IR (KBr) νmax: 3412, 2962, 1755, 1484, 1174 cm<sup>−</sup>1; 1H-NMR (acetone-*d*6, 600 MHz) δH: 6.2 (1H, s, H-2, H-2- ), 6.3 (1H, s, H-4, H-4- ), 6.9 (1H, s, H-8, H-8- ), 7.5 (1H, s, H-6, H-6- ), 13.2 (1H, s, OH-1); 13C-NMR and DEPT-135 (acetone-*d*6, 150 MHz), δc: 179.6 (C-9), 179.5 (C-9- ), 164.8 (C-3- ), 164.7 (C-1- ), 163.5 (C-1), 163.2 (C-3), 157.9 (C-4a, C-4a'), 153.5 (C-5a, C-5a'), 151.6 (C-7, C-7- ), 143.3 (C-5, C-5- ), 122.7 (C-8a'), 112.8 (C-8a), 108.2 (C-6, C-6- ), 102.5 (C-8, C-8- ), 102.2 (C-9a), 102.1 (C-9a'), 97.7 (C-2), 97.6 (C-2- ), 93.5 (C-4), 93.4 (C-4- ).

#### **3. Discussion**

Compound **1** was isolated as a yellow amorphous powder. The UV spectrum showed absorption bands at λmax 322 and 262 nm attributable to a conjugated system [11,12]. Its molecular composition was established to be C26H14O11 with twenty degrees of unsaturation from HR-TOFMS *m*/*z* 503.0667 [M + H]+, calculated for C26H15O11 (*m*/*z* 503.0614) and NMR spectral data (Table 1). The IR spectrum exhibited bands at νmax 3412 cm−<sup>1</sup> (hydroxyl), 2962 cm−<sup>1</sup> (C-H stretching of aliphatic) and 1755 cm−<sup>1</sup> (carbonyl).

The 13C-NMR spectrum demonstrated the presence of a total of 26 carbon signals, which were classified by their chemical shifts, DEPT, and HSQC spectra (Figures S3 and S4) as eight sp2 methine carbons, two carbonyl carbon at δ<sup>C</sup> 179.53 and 179.53, 16 sp<sup>2</sup> quaternary carbons (including two sp<sup>2</sup> carbons and 14 sp<sup>2</sup> oxygenated carbons). These functionalities accounted for 14 out of the total 20 degrees of unsaturations. The remaining of six degrees of unsaturation were consistent with six cyclics of bixanthones [13,14].

The 1H-NMR and HSQC spectra of **1** (Figures S1 and S4), showed proton signals indicative of a tetrasubstituted aromatic group δ<sup>H</sup> 6.2 (2H, s, H-2, H-2- ), 6.3 (2H, s, H-4, H-4- ), 6.9 (2H, s, H-8, H-8- ) and 7.5 (2H, s, H-6, H-6- ) and showed a hydroxyl proton at δ<sup>H</sup> 13.2 (1H, s, OH-1). Low shimming quality could explain the missing splitting of H-2/H-2- , H-4/H-4- /H-6/H-6- , and H-8/H-8 signals in the 1H-NMR spectrum.

A comparison of the NMR data of **1** with 1,3,7-trihydroxyxanthone, gentisein, isolated from *Gentiana lutea* [14] indicated that the structure of compound **1** is very similar to gentisein. The main difference was the presence of dimer skeleton at C-5. The substitution of the xanthone skeleton was determined by HSQC and HMBC spectra (Figures S4 and S5). The HMBC correlations (Figure 2) from H-6 (δ<sup>H</sup> 108.2) with C-5a (δ<sup>C</sup> 153.5), C-5- (δ<sup>C</sup> 143.3), C-5 (δ<sup>C</sup> 143.3) and C-7 (δ<sup>C</sup> 151.6), and of H-6- (δ<sup>H</sup> 108.2) with C-5a' (δ<sup>C</sup> 153.5), C-5 (δ<sup>C</sup> 143.3), (δ<sup>C</sup> 143.3) and C-7- (δ<sup>C</sup> 151.6) suggested that the substituent of dimer xanthone with the xanthone at C-5 or C-5- . The hydroxyl group was located at C-1 based on HMBC correlations from OH-1 (δ<sup>H</sup> 13.2) to C-1 (δ<sup>C</sup> 163.5), C-2 (δ<sup>C</sup> 97.7) and C-9a (δ<sup>C</sup> 102.2) (Figure 2 and Figure S5). Therefore, the structure of **1** was assigned as 5,5- -Oxybis(1,3,7-trihydroxy-9*H*-xanthen-9-one).

¦ ¦

**Figure 2.** Selected HMBC correlations for **1**.


**Table 1.** NMR data of compound **1** and gentisein acetone-*d*6.

#### **4. Materials and Methods**

#### *4.1. General Experimental Procedures*

UV spectra were recorded on Vilber Lourmat UV/VIS spectrophotometer. Mass spectra were measured with a Waters Xevo QTOFMS instrument (Waters, Milford, MA, USA). IR spectra were measured on a One Perkin Elmer infrared-100. NMR data were recorded on a Bruker Avance-600 spectrometer at 600 MHz for 1H and 150 MHz for 13C using Tetramethylsilane (TMS) as an internal standard (Billerica, MA, USA). Chromatographic separations were carried out on silica gel G60 (0.063–0.200 mm) (Merck, Darmstadt, Germany), RP18 (0.04–0.063 mm) (Merck, Darmstadt, Germany). Precoated silica gel GF254 plates (0.25 mm, Merck, Darmstadt, Germany) were used for Thin Layer Chromatography (TLC), and detection was achieved by spraying with 5% AlCl3 in ethanol, followed by heating.

#### *4.2. Plant Material*

The stem bark of *G. porrecta* was collected from Bogor Botanical Garden, Bogor, Indonesia in April 2018. The plant was identified and deposited in the Herbarium Bogoriense (No. IV.K.78a), Center of Biological Research and Development, National Institute of Science, Bogor, Indonesia.

#### **5. Conclusions**

A new polyoxygenated dimer-type xanthone, namely 5,5- -Oxybis(1,3,7-trihydroxy-9*H*-xanthen-9-one) (**1**), was isolated from the stem bark of *G*. *porrecta*, belonging to Clusiaceae family. This polyoxygenated dimer-type xanthone was found in the *Garcinia* genus for the first time.

**Supplementary Materials:** The following are available online, Figure S1. 1H-NMR spectrum of **1** (600 MHz in acetone-*d*6), Figure S2. 13C-NMR spectrum of **1** (150 MHz in acetone-*d*6), Figure S3. DEPT-135◦ spectrum of **1** (150 MHz in acetone-*d*6), Figure S4. HSQC Spectrum of **1**, Figure S5. HMBC spectrum of **1**, Figure S6. Infrared Spectrum of **1** (in KBr), Figure S7. HR-TOF-MS Spectrum of **1**, Figure S8. TLC Profile of **1**.

**Author Contributions:** Conceptualization, D., N., U.S.; Data curation, T.M.; N.; A.N.S.; Formal Analysis, N.; Investigation, A.N.S.; Methodology, A.N.S., N., T.M.; Supervision, D., U.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Directorate General of Scientific Resources, Technology, and Higher Education, Ministry of Research, Technology, and Higher Education, Indonesia (PDUPT, number, 2788/UN6.D/LT/2019, by D.S.).

**Acknowledgments:** Authors also thank Mohamad Nurul Azmi from Universiti Sains Malaysia for the assistance of NMR measurements, Kansi Haikal at the Central Laboratory, Universitas Padjadjaran for QTOFMS Measurements, Suharto from Bogor Botanical Garden, Bogor for the plant sample.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
