Chemical constituents and acetylcholinesterase inhibitory activity from the stems of Bauhinia pentandra

Abstract Two steroids (1 and 2), two oxepin derivatives (3 and 4) and seven flavonoids (5–11) were isolated from the stems of Bauhinia pentandra. Their structures were elucidated on the basis of NMR spectroscopic data, and by comparison with data previously reported in the literature. The ethanol extract from the stems of B. pentandra and the compounds, 4, 5, 6, 7, 8 and 11 have been evaluated as acetylcholinesterase inhibitors, and among these, the compound 5 exhibited the strongest activity. In addition, all the isolated compounds are reported for the first time as constituents of B. pentandra. Graphical Abstract


Introduction
The Bauhinia genus belongs to the Fabaceae family and comprises about 600 species distributed in tropical and subtropical regions of the world, of which approximately 60 species are found in Brazil (Hua et al. 2018a;Aquino et al. 2019). Alkaloids, bibenzyls, cyanoglycosides, dibenzofurans, lactones, oxepin derivatives, steroids, terpenes, and flavonoids are the main compounds isolated from this genus (Pettit et al. 2006;Boonphong et al. 2007;de Almeida et al. 2017;G ois et al. 2017;Hua et al. 2018aHua et al. , 2018b, and studies have suggested that the therapeutic properties of these species are mostly due to the presence of flavonoids (Cechinel Filho 2009).
Bauhinia pentandra (Bong.) D.Dietr., popularly known in the Northeastern Brazil as 'moror o de Espinho', is used popularly as a tonic and depurative and in the treatment of diabetes (Silva et al. 2013), and previous chemical studies report essential and fatty oils, cyanoglycoside, flavonoids, phenylacetic derivatives and lactones in this species (Salatino et al. 1999;Duarte-Almeida et al. 2004;Silva et al. 2013;de Almeida et al. 2015;de Almeida et al. 2017).
Phenolic compounds are capable of inhibiting acetylcholinesterase, an enzyme that plays a key role in the evolution of Alzheimer's disease, and in the search for bioactive natural compounds from Bauhinia species, which are rich in these compounds, study aimed to undertake the chemical investigation of the ethanol extract from the stems of B. pentandra (EESBP), and to evaluate the acetylcholinesterase inhibitory activity of this extract (EESBP) and isolated compounds4, 5, 6, 7, 8 and 11.
Since phenolic compounds are reported to have acetylcholinesterase inhibitory activity (Cakmak and Gulc¸in 2019), the ethanol extract from the stems of B. pentandra (EESBP) and the isolated compounds bauhiniastatin 4 (4), 7,4 0 -dihydroxyflavone (5), liquiritigenin (6), naringenin (7), isoliquiritigenin (8) and fisetin (11) have been evaluated as acetylcholinesterase inhibitors using the modified Ellman's method (Ellman et al. 1961;Rhee et al. 2001). In this experiment, the formation of a white halo around the spots in thin-layer chromatography plates is indicative of acetylcholinesterase inhibition, and among the evaluated compounds, 7,4 0 -dihydroxyflavone (5) exhibited the strongest activity, with an inhibition halo diameter (0.9 cm) equal to that of eserine, which was used as a reference standard (Table 1). In addition, our obtained data are in agreement with the finding of Yang's data, who observed that 7,4 0 -d ihydroxyflavone (5) presents acetylcholinesterase inhibitory activity . Concerning the activity of the extract (EESBP), it was less active than isolated compounds 4, 5, 6, 7, 8 and 11, however, this is a reasonable result for a crude extract containing a wide array of compounds. In addition, with regard to literature searches, no acetylcholinesterase inhibitory activity investigations were found on oxepin derivatives.

Conclusion
This work demonstrated that B. pentandra stems are rich in phenolic compounds, such as oxepine derivatives (3 and 4) and mainly flavonoids (5-11), as the other species of the genus Bauhinia. In addition, compound 5 displayed good acetylcholinesterase inhibitory activity and can be used in Alzheimer's disease therapy.