Resveratrol oligomer structure in Dipterocarpaceaeous plants

Oligostilbenoids are a group of natural products derived from the oxidative coupling of C6–C2–C6 units found in some plant families. A structurally diverse chemical pool is produced after the successive regioselective and stereoselective oligomerization of resveratrol. This review describes the current status and knowledge of the structure of resveratrol oligomers (ROs) in Dipterocarpaceaeous plants (DPs). Beginning with the recently validated formation of ROs in DPs, each downstream conversion is described from the perspective of the resveratrol coupling mode. Particular emphasis is placed upon the regioselectivity of monomer- and dimer-derived radical–radical coupling processes, which are responsible for producing dimers, trimers, and tetramers with various cyclic frame skeletons, as well as related processes that result in highly condensed scaffolds, such as hexamers and octamers. Trimers in oxidized, dearomatized, and rearranged forms are also summarized, as well as the biogenic relationship between the compounds. Furthermore, emphasis is placed on the O- and C-glucosides of ROs, as well as on the hetero-coupled ROs. In addition, several stereoisomers that originate from asymmetric carbons and the stereochemistry with respect to the conformation due to the chiral axis are described. Besides, NMR spectroscopic properties such as coalescence and anisotropy are briefly described. Approaches to determine absolute configuration are also summarized.


Introduction
In the plant kingdom, resveratrol oligomers (ROs) can be found in a number of plant families, such as the Dipterocarpaceae, Vitaceae, Cyperaceae, Fabaceae, Paeoniaceae, and Gnetaceae families [1][2][3][4]. The Dipterocarpaceaeous plant (DP) is the dominant plant family of Southeast Asia, with a total of 470 species [5,6]. Indeed, plants in this family are a rich source of ROs, which are produced from the successive condensation of resveratrol (1: trans-3,5,4′trihydroxystilbene) (Fig. 1). The first RO was characterized from Hopea odorata in 1966 [7]; in the following 25 years, dozens of structurally related compounds have been identified [1]. In recent years, several hundred ROs have been isolated from DPs with their structures determined accordingly [2]. In essence, this structural diversity stems from patterns of phenoxy radical-radical coupling that yield various fused-ring systems containing asymmetric carbons, which, in turn, give rise to regioisomerism and stereoisomerism. Structural diversity is further expanded by divergent structural modifications, such as oxidation, dearomatization, substituent rearrangement, and glucosylation [8]. Since wide-ranging bioactivity screens have been applied to RO and, in the process, their various activities identified, it remains essential to expand the current chemical library as well as to identify the exact structure of each isolate.

Definition
Resveratrol can be widely found in the plant kingdom and, in particular, in the products of the phenylpropanoid pathway; it is responsible for transforming phenylalanine into 4-coumaroyl-CoA, which finally enters the stilbenoid-biosynthesis pathway [62]. ROs are metabolites found in a small set of phylogenetically distant plant families [4], the BB of which (C 6 -C 2 -C 6 ) is successively oligomerized after generating phenoxy radicals and highly active quinomethides (QM), followed by spontaneous regioselective radical-radical coupling, regiodivergent Friedel-Crafts reactions, nucleophilic trappings, and tautomerizations [8,14].
ROs differ from most other polyphenols (e.g., flavonoids, pyrones, quinones, and their downstream products) by having comparatively less structural diversity due to small variations and the limited patterns of functional groups; by expanding the chemical pool by oligomerization, the production of various frame skeletons is ensured as well as the participation of O-or C-glucoside (monoglucoside of resveratrol: 2-4 ( Fig. 1)) as a BB. Additionally, they differ among themselves with respect to the inversion of the configuration of asymmetric carbons, which originate from the C 2 units of the BBs. This results in the isolation of hundreds of derivatives that have characteristic stereoisomeric structural motifs, such as dihydrobenzofuran-, indane-, and bicyclo-ring systems that are conserved across DPs [2].
The researcher-friendly designating scheme has been applied to standardize the two oxygenated aromatic rings of phenol (A 1 ) and the resorcinol rings (A 2 ) in each resveratrol unit, in combination with a numbering order that denotes 14 carbons (1a-14a) starting from A 1 . The next letter in the alphabet and the next numbering order are B 1 , B 2 , and 1b-14b, respectively, with regard to additional resveratrol units. When we explain the condensation modes of resveratrol units, such as regioselective radical-radical coupling, the numbering orders, 1-14 and 1′-14′, are applied (e.g., coupling modes 8-8′, 8-10′, and 3-8′) according to the molecular species in question.

RO structure in DPs
Structurally diverse ROs can usually be found in planta as dimeric-tetrameric entities; to be sure, higher oligomers also exist. Because dimerization is the initial step in the global biosynthetic scheme of the chemical pool, it is crucial to clarify simple frame skeletons of the smallest oligomer to understand the further oligomerized (and more complex) skeletons of trimer-octamers lying downstream of biogenesis. ROs in DPs are regioselectively biosynthesized due to the coupling of oxidatively generated phenoxyl radicals (1A-1D), where the initial dimerization typically occurs through the 8-10′ coupling mode to produce various dimers that can be represented as (−)-ε-viniferin (5) (Figs. 1, 2) [11,63]. The majority of resveratrol dimers are 8-10′ linked, but many different coupling modes exist in nature (e.g., 8-8′, 3-8′, and 8-12′); these coupling modes (and the potential substrates involved) often vary according to the species being analyzed [2]. The diverse reactivity of further generated reactive QM species, such as 5A-5E, in combination with 1A-1D further contribute to downstream regiodivergent reactions, which, in turn, results in the production of further condensed RO (Figs. 3,4). In addition, the glucoside of 1 not only stores resveratrol in cell tissues and prevents it from being oxidized, but it also contributes to the biosynthesis of RO glucosides, which further expands their chemical diversity. This can be seen in the rich isolation of 2-4 ( Fig. 1).
Generally, asymmetric carbons exist in proportion to the oligomerization degree, e.g., in many cases, the dimers and tetramers of resveratrol have four and eight chiral atoms, respectively. Among the 200 ROs isolated from the DPs, dimers, trimers, and tetramers are common. This trend can be observed in other families, including Vitaceae [1][2][3][4]; however, the further condensed derivatives, such as hexamers and octamers, are not common. Compound 5 is one of the most abundant dimeric resveratrols, which has been isolated from the majority of RO-containing plant families, such as Dipterocarpaceae, Vitaceae, Cyperaceae, Fabaceae, Paeoniaceae, and Gnetaceae, among which DPs are known to produce (−)-form. Indeed, this issue is vital in considering an absolute configuration of biogenetically downstream chemicals. Because the structural diversity of RO can be attributed to skeletal variations and the presence of stereoisomerism, analyzing two-and three-dimensional structures is an interesting challenge, academically speaking.

Two-dimensional structures and the biosynthetic scheme of simply oligomerized resveratrol
In this section, representative ROs are discussed from the viewpoint of oligomerization degree and skeletal diversity with biosynthetic aspects. The listed compounds are prioritized to systematically depict a plausible biogenic relationship between compounds with topological differences. Indeed, our findings, which are based on experiments in the last 2 decades in combination with the existing literature, support the proposed biosynthetic aspects. In this section, each RO is delineated as a planar structure (Figs. 3,4,5,6,7,8).
Alternatively, the biosynthesis of 8-10′ tetramers can be explained by another primary intermediate, 34, which preferentially undergoes intramolecular 5-exo-trig cyclization through C 8b -C 7c isomerizing to 35 instead of 7-exo-trig cyclization as in 39, the latter of which would produce less types of frame skeletons than those from 33. The QM (35) and isomerized 5B-5E QM" (36) follow different cyclization pathways, resulting in the production of vaticanols B (42), C (43) [19] (vat diospyroidol [75]), and 44 [42]. In fact, 42, which is usually present with 43, is another abundant tetramer found in many DPs. It is interesting that the 8-10′ tetramers produced via 35 are specifically isolated from DPs, which further undergo downstream modification resulting in the scaffold characteristics, such as high-order oligomers (e.g., hexamers and heptamers).
The other group of resveratrol tetramers is produced via 7-10′(14′) connectivity between two dimeric resveratrol units. The first isolation is vaticanol K (50) from Vatica chinensis, which possesses an unprecedented fused 2,7-dihydrooxepine-QM skeleton [27]. We presume that the plausible biogenesis, including the concerted intramolecular cyclization of two resveratrol dimers, is followed by desaturation extending conjugation. However, the further isolation of vaticanol L (51) with a 7-10′ bond suggests that the regioselective dimeric dimerization reactions occur due to the nucleophilic trapping of QM [26]. A similar scaffold of resveratrol tetramers with 7-10′(14′) bonds consists of cajyphenol A, which has been previously isolated from Cayratia japonica (Vitaceae) [76]. It has been proposed that cajyphenol A consists of metabolites after the cross coupling of quadrangularin A (8-8′ resveratrol dimer) [77] and its penultimate biosynthetic intermediate, which involves nucleophilic trapping [8]. In the case of 50 and 51, an alternative to quadrangularin A is 8-10′ resveratrol dimer (8 and 9, respectively), which can undergo nucleophilic trapping onto 7 in a fashion similar to cajyphenol A, resulting in regioselective bond formation (Fig. 5). This oligomerization mode, also known as the intramolecular Friedel-Crafts reaction, is insignificant in the biogenesis of resveratrol tetramers.
As described above, major HCRs can be produced by the cross coupling of two ROs through an intermolecular Friedel-Crafts reaction in DPs. Unfortunately, little is known about examples of minor HCRs condensed only by radical couplings. The rare example of this is upunaphenol A (62) [41], which can be produced via radical coupling between 43 and 5 in the 3-8′ mode and/or 5 and 48 in the 8-10′ mode (Fig. 8).

RO glucosides
The chemical diversity of ROs in DPs also stems from modification by glucosylation, in which beta-glucopyranosyl groups are introduced prior to the oligomerization of resveratrol units. Collectively, they are referred to as the O-and C-glucosides of ROs.

O-Glucosides
In the case of O-glucosides of RO (O-G-RO), it is evident that 2 is a vital BB in ROs, where, in the majority of cases, one glucopyranose can be found in said molecules. Currently, O-G-RO bearing aglycon together with 2 have been isolated from many DPs belonging to different families, such as Vatica, Vateria, and Upuna. The majority of O-G-RO co-exist with respective aglycon, such as dimers and hexamers; for instance, in vaticasides A-G [16,17,25] and pauciflorosides A-C [21], O-G-RO are isolated together with their aglycons. Usually, in DPs, O-glucosides are found in less amounts compared with their corresponding aglycons. This suggests that the production of said molecules is the result of 2, which is introduced to the aforementioned biosynthetic pathway instead of 1.
So far, O-G-RO have been determined to be enantiomerically identical with those of co-existing aglycons; however, the cordifolosides A (63) and B (64) with enantiomeric aglycons, which are isolated from Shorea cordifolia [55], are derived from (−)-and (+)-9, respectively (Fig. 9). Moreover, (−)-9 only co-exists as an aglycon in the plant material. Indeed, this suggests that particular DPs are capable of biosynthesizing O-G-RO with enantiomeric aglycons, wherein the aliphatic 8-position of resveratrol is coupled with another aromatic 10′-position of 2.

C-Glucosides
With respect to C-glucosides of RO (C-G-RO), chemical scaffolds have been identified after phytochemical studies on Shorea and Hopea species. In particular, it has also been demonstrated that the BBs, 4-C-β-glucopyranosyl resveratrol (3) and 4, co-exist in the respective species of Shorea (S. hemsleyana and S. uliginosa) and Hopea (H. parviflora and H. utilis). Notably, C-G-RO can be produced via their biosynthetic pathways. These are different from those of nonglucosylated ROs, which, according to previous phytochemical studies, have fewer structural correlations between C-G-RO and RO.
The chemical diversity of C-G-RO with BBs of 3 is the result of the regioselective intramolecular radical coupling and cyclization pathways. Typically, intramolecular cyclization or tautomerization of an intermediate para-quinone methide, which is also described for resveratrol dimers (Fig. 2), outcompetes most intermolecular processes. Some of the resultant dimers undergo further radical coupling through modes of 3-8′, 8-8′, and 8-10′(14′) to produce high-order oligomers with 3. Alternatively, it is assumed that the BBs of 4 are not displaced from 3 in the metabolic pathway; this is supported by the fact that no fused-ring systems involving 4 have been found. Instead, it is acceptable to assume that intermolecular functionalization by oxide of 4 occurs, wherein the electron-rich position of ROs reacts with their 7-position. Indeed, it is assumed that dimer (hopeaside D (70)), trimers (hopeasides C (71) and E (72)), and pentamers (hopeasides A (61), B (C 7e -epimer of 61), and F (73)) are produced after the crossed coupling of 4 with monomer (2), dimers (5 and 10), and tetramers (37 and 42), respectively (Fig. 11) [56,59,60].

ROs as sources of chemical diversification
Increased structural diversity is further produced by minor ROs and subsequent modifications, such as additional oxidation events and/or the attachment of additional atoms or groups. In addition, diversified ROs are further produced by dearomatization, tautomerization, substituent rearrangement, and so forth. Although the absolute quantity of said modified ROs is much less than those of precursor molecules, the isolation of such derivatives indicates a vast array of chemical RO scaffolding. Accordingly, we focus on such molecules to expand on the aspects of ROs.

Dearomatized, rearranged, and/or oxidatively cleaved ROs
The isolation of the dearomatized, rearranged, and/or oxidatively cleaved ROs demonstrates another level of chemical diversity. The first discovery of a dearomatized RO was achieved by the isolation of gnetin A (resveratrol dimer) with dearomatized resorcinol from Gnetum leyboldii (Gnetaceae) [80]. A subsequent report of leachianol C (resveratrol trimer) in Sophora leachiana (Fabaceae) by Ohyama et al. [81] was the first to identify dearomatized phenol in ROs. The finding of gnetin A and leachianols A and B [82], along with the isolation of kobophenol B (resveratrol tetramer) from Carex pumila (Cyperaceae) reported by Kawabata et al. [83], advanced the notion that the resveratrol units in ROs are condensed not only by regioselective radical-radical couplings followed by regiodivergent Friedel-Crafts cyclization, but also by a formal dearomative [3 + 2] annulation, forming the bicyclo[3.2.1]octedione. Alternatively, the dearomatized phenol structure in leachianol C can be regarded as the result of dearomative [3 + 3] annulation in forming the bicyclo[3.3.1]nonedione. The aforementioned resveratrol tetramer with the dearomatized phenol, 38 [33,54], is formed by a different mechanism, where asymmetric dearomatization undergoes intramolecular-stereoselective cyclization (but not intermolecular-annulation reaction) in the biogenetic course.
The further isolation of rearranged 10-8′ trimers bearing a dibenzocycloheptane ring as well as cotylelophenols A (79) and F (80) [43,44] provides insight into the associated biogenic relationships and plausible intermediates (81)(82)(83)(84) (Fig. 13). Indeed, the oxidation of 31 may produce the hypothetical intermediate 81, since 76 is a product of its epoxydation. Following the isomerization of 81, benzofuran-6(3aH)-one 82 can also undergo epoxidation to generate intermediate 83, which is prone to isomerize to 75. A 1,2 aryl migration from 75 yields 79. Upon water trapping, 75 can undergo oxidative cleavage of its C 7a -C 8a bond, followed by the hydrolysis of 4-hydroxybenzoate to obtain 80. Alternatively, 80 can possibly be derived from oxidative cleavage of another olefin of a plausible intermediate, specifically 84, which can be derived from the isomerization of 31.

Hetero-coupled ROs
In 2000, our team discovered and reported the structure of shorealactone (99) (Fig. 15), the ascorbyl-resveratrol dimer derivative isolated from Shorea hemsleyana, which is the first example of hetero-coupled ROs [47,90]. The connectivity of the tricyclic-tetrahydrofuran core was the first instance found in naturally occurring polyphenols. Upon oxidation of 5 and ascorbic acid, the hypothetical QM intermediate, 5B, and monodehydroascorbic acid form a C-C bond, which is followed by concerted regioselective cyclization pathways to generate 99. Later, an identical compound was isolated from the heartwood of Shorea laeviforia by Hirano et al. and given a second name; namely, laevifonol [91].
Upunaphenol L (100) is the first instance of lignostilbenoids in DPs, which is formed by the fusion of the electronrich arene of 42 and a phenylpropan unit [36]. It is assumed that ROs can undergo hetero coupling when the other reactive radical species exist.

RO stereochemistry
It is an interesting challenge to analyze the stereostructure of ROs. The central obstacles to elucidating the relative configuration consist of the following: poor prognosticators of the vicinal coupling constants required for configurational elucidation of the bicyclo five-and seven-membered ring systems; difficulties in affirming NOEs and ROEs in conventional two-dimensional nuclear magnetic resonance (NMR) spectra (NOESY and ROESY, respectively) due to the duplicated proton signals in determining substituent orientation and in elucidating the configuration and conformation of C 2 molecules; the existence of the chiral axis. In addition, diminished NMR-signal intensity due to coalescence is also problematic when variable temperature NMR (VT-NMR) is not available. Another crucial property is anisotropy frequently observed in proton NMR ( 1 H-NMR), the analysis of which in combination with two-dimensional NMR data and three-dimensional molecular modeling would help in elucidating the relative configuration, especially when partial structures are connected through the chiral axis. As described above, DPs produce a number of OS analogues, which typically possess a common skeleton of 1,2-diaryl-dihydrobenzofuran stemming from (−)-5. This RO stereochemical homogeneity suggests that the downstream biosynthetic product of (−)-5 has the same absolute configuration in the 1,2-diaryl-dihydrobenzofuran skeleton. Alternatively, other types of ROs exist in this plant family, such as 63-66) (Figs. 9, 10). The fact that they bear antipodal stereochemistry in each 1,2-diaryl-dihydrobenzofuran and non-heterocyclic bicyclo[3.2.1] system demonstrates the need for various approaches in combination with the solid physicochemical approach to determine the absolute configuration, instead of speculating plausible biosynthetic precursors, such as (−)-5. Although the number of reports on RO stereochemistry is increasing, their absolute configuration is yet to be determined.

Rotational isomerism
Compound 38 is a precedent of the atropisomeric ROs having two configurationally stable two rotational isomers (extended rotamer 38a and compact rotamer 38b) (Fig. 17) at an ambient temperature in the NMR time scale [33,54]. The separation of each conformer is unaccomplished due to exchangeable properties through the chiral axis. Although its peracetate only has one conformer, the deacetylated product has signal duplications due to atropisomerism. Changes in the ratio of the two conformers and the various solvents of the VT-NMR as well as the cross peaks due to conformational exchange observed in the NOESY experiments can be attributed to rotational isomerism. The complete and unequivocal assignment of proton and carbon resonances of the two rotational isomers is demonstrated through structural analysis. The rotational state of the rotamers can be defined using NOESY experiments, which show correlations between H 8c and H 14c . Accordingly, it is possible to differentiate the two rotamers. Each conformation is supported by the anisotropy that is explained by the different chemical shifts of H 2b and H 14c in the two rotamers. Decisive evidence for the absolute configuration can be obtained by the acidcatalyzed rearrangement of 38, resulting in the formation of a monoalkyl ether of the known resveratrol tetramer; namely, (+)-107.
The second instance of an atropisomeric RO is 72 [59]. Because 72 has covalent C-C bonds connecting two partial structures (i.e., 10 and 4)), the configurational relationship between them and the conformational determination due to the rotational isomerism are critical issues in the stereostructure analysis. The NMR spectra in MeOH-d 4 show multiplicity, which possibly stems from the rotational isomerism through the chiral axis, C 14b -C 7c , displaying signals due to a major conformer and a minor one. Moreover, when the material recovered from the MeOH-d 4 solution is redissolved in acetone-d 6 , the 1 H NMR spectrum only shows major conformers. These results indicate that 1 undergoes conformational isomerism in MeOH, an observation that is further confirmed by the NOESY correlations by chemical exchange for the aromatic signals. Another important issue associated with the determining the absolute configuration of 72 is the comparative-configurational analysis, which is conducted using the β-d-glucopyranosyl group and has been previously demonstrated with respect to 71 and 70 [56].

Steric hindrance upon rotation of aromatic rings
The restricted rotation of aromatic substituents is a wellrecognized property, which provides crucial information in elucidating configuration and conformation. Compound 32, for example, the two set of aromatic protons (H 2c /H 6c and H 3c /H 5c ) in a 4-hydroxyphenyl group, is non-equivalent due to the hindered rotation about the C-C bond (C 1c -C 7c ), which can be seen in the NMR with four independent 1 H and 13 C broad signals (H 2c , H 3c , H 5c , and H 6c ; C 2c , C 3c , C 5c , and C 6c ) [16]. The other examples are presented by isoampelopsin F (111: C 7b -epimer of 9) [93], isovaticanol C (112: C 7b -epimer of 43) [21], 79 [44], and arbiraminol D (113: C 7a,7b -diastereomer of 16) [22]. The NMR behavior of 79 is particularly significant because all 4-hydroxyphenyl groups are rotationally restricted, wherein complete structural elucidation is achieved by the aid of the VT-NMR experiment, as was done for 32 (Fig. 18). The energy-minimized structure suggests that the higher field shifts of aromatic protons on rings A 1 -C 1 can be explained by the anisotropic effects caused by the neighboring rings (Fig. 19). For example, at -0 °C, where H 5a and H 6a can be observed at δ 5.90 and 5.70, the higher field shifts are caused by the effect of ring B 1 . The effects of both rings (A 1 and C 1 ) results in the higher field shifts of H 2b,6b and H 3b,5b . At -90 °C, where the aromatic proton on ring C 1 is observed as four separated signals, the higher field shift of H 3c can be observed at δ 6.08, which can be attributed to the anisotropic effect of ring B 1 . As can be seen in the structural elucidation of 79, the exact understanding of the coasealence caused by the hindered rotation of aromatic rings, as well as the accompanying anisotropic effects on aromatic protons, helps in determining the relative configuration and conformation of ROs.
When the partial structures connected through the C-C bond increase in size, the rotational barriers also increase, which results in a stable conformer. Examples of this can be seen in 42 and 37 with the 3-(3,5-dihydroxyphenyl)-6-hydroxy-2-(4-hydroxyphenyl)-2,3-dihydrobenzofuran-4-yl group (1,2-diaryl-dihydrobenzofuran) connected to the dibenzobicyclo[5.3.1]octadiene core, where H 8c and H 14c are situated in syn orientation. The hindered rotation and conformational stability in such molecules can also be enhanced by attractive forces, such as CH-π and OH-π interactions [94,95].
Alternatively, the particular alignment of partial structures could weaken the aforementioned rotational restrictions, which, in turn, can causing coasealence as well as Fig. 17 Energy-minimized conformations of the two atropisomer structures of hetero-coupled ROs difficulties in structural elucidation. Compound 55 exhibited broad signals in the entire region due to unstable conformation at ambient temperatures [25]. Indeed, in the spectrum, reducing the temperature results in a change in the signal features to clear; some substituents did not display signals. The significant features consist of signals for aromatic protons for ring E 1 in various conditions (temperatures and solvents) as well as the completely overlapping methine signals.
Successful isolation of glucosides of 55 (vaticasides E (114) and F [25]) finally enable spectroscopic-data analysis, where clear NMR signals can be attributed to the weakened coasealence due to the enhanced hindered rotation of the C 12 -C 7e bond, which, in turn, results in the successful determination of 55 (Figs. 20, 21).

Approaches to determine absolute configuration
Some ROs with absolute configurations determined by different approaches have been summarized in the existing literature [78]: X-ray crystallographic analysis of their chemical derivatives using anomalous scattering of the bromine atom(s) ((−)-39 [7] and 99 [47]; the comparison of optical rotation and/or circular dichroism ((+)-and (−)-5 [96], (+)-39 [97], 65 and 66 [52,53], 63) and 64) [55], and modified Mosher′s method (85) [84]; the comparison  of the experimental and theoretical ECD spectra (42) [84]; the application of the olefin-cleavage strategy to a known compound to obtain ECDs of the newly separated products (68 and 69 [52,53] as well as laetevirenol D [98]); the regioselective and stereospecific transformations of a hypothetical biogenetic precursor, (+)-5 ((+)-101 [99], (+)-vitisin A [100]); the acid-catalyzed skeletal conversion to obtain monoalkyl ether of the known derivative (38) [33]; assignment based on the comparison of the absolute configuration of the d-β-glucopyranosyl group (70)(71)(72)) [56,59,60]; comparison of experimental and theoretical electronic circular-dichroic spectra of the dehydroxylated derivative (( −)-31) [101]; comparative study using ECD with the help of the ECD of known compounds with previously determined absolute configurations (58 and 59 [78]). Currently, however, the absolute configurations of many ROs are yet to be determined. This is because, in typically cases, ROs are neither crystalline nor secondary alcohols, which is to say they are unsuitable for general methodologies. The application of a comparative study using an ECD database and X-ray analysis using porous complexes [102] is promising with respect to determining the absolute configuration of RO scaffold; however, this depends on a reliable chemical library. To be sure, the object of this review was not to provide a comprehensive example of the absolute configuration determination of ROs. Accordingly, a forthcoming review will be directed toward a better understanding of various methods to solve the issue in question.

Concluding remarks
Even though a considerable amount of knowledge is available with respect to the structural diversity of ROs in DPs (particularly, the 8-8ʹ and 8-10ʹ linked compounds), compounds with other link modes have not yet been comprehensively studied. This includes defining versatile structural motifs stemming from minor couplings and structural modifications (i.e., in terms of introduction of O-atom(s), dearomatization, rearrangement, tautomerization, and a hetero-coupling with other BBs), as well as deducing further stereochemical diversity in the chemical pool. Much work remains in clarifying the differences in physicochemical properties among the diverse stereoisomers that arise from enantiomerism, diastereomerism, and atropisomerism; these will be the subject of future work, as will be defining further RO structural diversity.
Acknowledgements The author gratefully acknowledge Dr. M. Iinuma, Prof. emeritus of Gifu Pharmaceutical University, for supervising study on the chemistry of RO as a core project of his laboratory (Laboratory of Pharmacognosy). I would like to thank all members of his group (past and present) who have participated in the schematic study for their efforts, their ideas, and their enthusiasm, rendering this line of research a true pleasure to pursue; their names are cited in the references.
I am grateful to Dr. Ryuichi Sawa and Dr. Yumiko Kubota, Laboratory of Structural Chemistry and Biology, Institute of Microbial Chemistry (IMC), Japan, for NMR and MS spectral measurements. I also sincerely thank Dr. Yoshikazu Takahashi, IMC, for his support for structural elucidation.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License (https ://creat iveco mmons .org/licen ses/ by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/