**1. Introduction**

The genus *Caesalpinia* comprises more than 500 species around the world, existing essentially in tropical and subtropical zones. These species are correlated to ethnopharmacological uses due to their biological properties, which include analgesic, adaptogenic, antiangiogenic, antiulcer, anthelmintic, antibacterial, insecticidal, antifungal, anti‐inflammatory, antipyretic, antioxidant, antiproliferative, antiviral, antimalarial, immunomodulatory, and immunosup‐ pressive activities, as well as glutathione S‐transferase (GST) inhibition, xanthine oxidase (XO) inhibition suppression of melanin synthesis, inhibition of viral neuraminidases, and other properties which will be further discussed ahead [1–5].

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Associated to these biological properties, these plants are chemically composed by different classes of metabolites including steroids, organic acids, chromenes, diterpenes, triterpenes, polyphenols, tannins, anthraquinones, alkaloids, and flavonoids, which comprise the natural product diversity of this genus.

Besides these compounds, species from the genus *Caesalpinia* interestingly produce unusual compounds such as uncommon biflavonoids and a rare subclass of flavonoids, named homoi‐ soflavonoids. The first group is more distributed within plants, while homoisoflavonoids are restrict only to some vegetal species including those from Fabaceae and Asparagaceae [6]. These compounds are also encountered, although less common, in other families as Gentianaceae, Polygonaceae, Portulacaceae, and Orchidaceae. There are two different work concerning about homoisoflavonoids, which relate the existence of approximately 240 natu‐ rally occurring compounds [6, 7].

In this sense, it is important to define the general characteristics of flavonoids, once they are the core subunits of biflavonoids and cover the rare class of homoisoflavonoids. In general, flavonoids are low molecular weight polyphenols, brightly colored due to their absorptions of UV light, and the most common structures are associated to antioxidant properties [8–10]. Flavonoids, classified as phytoalexins, are produced as a response to microbial infection in plants. They have a notorious participation into the scientific scenario due to the beneficial association to the humans' daily basis intake of nutrients as functional foods improving human health [8, 10].

The consumption of functional foods, or nutraceuticals, is strongly associated to these com‐ pounds. In addition, the ingestion of flavonoids from functional foods implicates in lowering blood triglycerides and homocysteine, decreasing blood pressure, acting against inflammatory, platelet antiaggregation processes, and the improvement of endothelial function [11]. These compounds are also associated to another range of biological properties lowering the incidence of cancer, including prostate, stomach, breast, and lung cancers [12]. In addition, various pro‐ tective effects of flavonoids have demonstrated them as important multi‐target agents [13, 14].

In that regard, the genus *Caesalpinia* is considered a rich source of common flavonoids. However, this genus is also associated to unique biflavonoids constituted by homoisofla‐ vonoids subunits and a considerable amount of representatives from the class of naturally occurring homoisoflavonoids. Up to date, there are reports pointing to the existence of about 240 naturally occurring homoisoflavonoids [6, 7].

An interesting point is that homoisoflavonoids can also be found as dimers. Biflavonoids com‐ pounds are dimers of flavonoids assembled in diverse manners by different species. The number of possibilities for these structures (involving all classes of flavonoids) points to more than 20,000 different molecules. However, not all these have been encountered in nature so far, summing to 500 representatives [15]. From these, less than 10 are constituted by homoisoflavonoids subunits.

As homoisoflavonoids and their dimers from the genus *Caesalpinia* are unique compounds, this chapter proposes to gather the available data from the literature in a systematic overview associating them to biological properties aiming to demonstrate these compounds as notable representatives composing the chemical space associated to natural products.
