## 1 Introduction
Furthermore, the biological importance of condensed \(O\)-heterocycles can be supported by knowing the structure of several synthetic drugs already clinically approved worldwide by relevant regulatory authorities, for example, the Food and Drug Administration (FDA) [39], the European Medicines Agency [40], or the Chinese National Medical Products Administration [41], for the treatment of a wide range of medical conditions [39, 40, 41], including bacterial infections [42]. The 'classic' benzo[\(b\)]furan core is a fundamental building block in the structure of **amiodarone** (**8**) used as a treatment modality for life-threatening ventricular arrhythmias [43], **vilazodone** (**9**; major depressive disorder) [44], as well as **elraplusib** (**10**; orphan  
Figure 3: General chemical structures of biologically active agents containing (**a**) a benzo[\(b\)]furan core (**5**); (**b**) a 2,3-dihydrobenzo[\(b\)]furan (coumaran) scaffold (**6**); (**c**) a 1,3-dihydroobenzo[\(b\)]furan (phthallane) core (**7**). \(R^{1}\), \(R^{2}\), \(R^{3}\) = properly selected substituents attached to suitable positions of particular bicyclic systems; six-membered aromatic ring can also be multi-substituted or condensed with other aromatic or nonaromatic cycle. drug designated for Ewing sarcoma) [45]. The chemical structures of drugs (**8**)-(**10**) are provided in Figure 4. The 2,3-dihydro[\(b\)]benzofuran scaffold is present in the structure of **tasimleleton (11**; non-24 h sleep-wake disorder) [46]**, **ramelteon (12**; insomnia) [47]**, **prucalopride (13**; constipation) [48]** as well as **darifenacin (14**; urge incontinence and/or increased urinary frequency and urgency) [49]**. The chemical structures of these drugs are shown in Figure 5.  
**Escitalopram (15)**, as an (5)-(+)-enantiomer (Figure 6) of **citalopram**, contains a bioisosteric 1,3-dihydrooisobenzo[\(b\)]furan moiety. The compound (**15**) is officially approved for the treatment of major depressive disorder and anxiety [50]. Figure 4: Chemical structure of clinically approved drugs from various PD groups containing a privileged benzo[\(b\)]furan scaffold, i.e., **amiodarone (8)**, **vilazodone (9)**, and **elzplusib (10)**. The privileged benzo[\(b\)]furan scaffold is indicated by a green color in their structures. Figure 5: Chemical structure of clinically approved drugs from various PD groups containing a privileged 2,3-dihydrobenzo[\(b\)]furan scaffold, i.e., **tasimleleton (11)**, **ramelteon (12)**, **prucalopride (13)**, and **darifenacin (14)**. The 2,3-dihydrobenzo[\(b\)]furan scaffold is indicated by a reddish brown color in their structures. Different arrangement of a five-membered ring within the scaffolds (**5**)-(**7**), i.e., a different position of an \(O\)-atom, the presence or absence of a double bond between \(C2\) and \(C3\) and, if present, its contribution to the aromatic nature of a given bicyclic system, as well as the number and position of particular stereogenic centers (Figure 3), results in different structural and physicochemical properties of particular compounds. All given characteristics might notably affect biological activities (PD properties), affinity and selectivity for desired biological targets, PK features, as well as toxicity of the molecules containing those structural motifs. Particular viniferins contain a privileged bicyclic 2,3-dihydrobenzo[\(b\)]furan or benzo[\(b\)]furan structure. They can be found naturally as various oligomeric structures of the plant metabolite (**1**); they are dihydrodimers, dimers, trimers, as well as tetramers [13] structurally properly arranged in a three-dimensional (3D) space. The oligomers, to which attention has been paid within a current review paper, are dihydrodimeric and dimeric molecules. In fact, the most investigated isoforms are the dihydrodimers of **RSV (1)**, i.e., _delta-_**viniferin (\(\delta\)-_viniferin_)** and _epsilon-_**viniferin (\(\varepsilon\)-_viniferin_)**. Their chemical structure is rather specific, allowing them to form several isomeric forms.