Functionalization of Betulinic Acid with Polyphenolic Fragments for the Development of New Amphiphilic Antioxidants

authors	Sousa, JLC; Goncalves, C; Ferreira, RM; Cardoso, SM; Freire, CSR; Silvestre, AJD; Silva, AMS
nationality	International
journal	ANTIOXIDANTS
author keywords	betulinic acid; oleanane-type compounds; polyhydroxylated compounds; amphiphilic antioxidants; radical scavenging activity; antioxidant activity
abstract	The present work aimed at the valorization of biomass derived compounds by their transformation into new added-value compounds with enhanced antioxidant properties. In this context, betulinic acid (BA) was decorated with polyphenolic fragments, and polyhydroxylated (E)-2-benzylidene-19,28-epoxyoleanane-3,28-diones 4a-d were obtained. For that, the synthetic strategy relied on base-promoted aldol condensation reactions of methyl betulonate, which was previously prepared from natural BA, with appropriate benzaldehydes, followed by cleavage of the methyl protecting groups with BBr3. It is noteworthy that the HBr release during the work-up of the cleavage reactions led to the rearrangement of the lupane-type skeleton of the expected betulonic acid derivatives into oleanane-type compounds 4a-d. The synthesized compounds 4a-d were designed to have specific substitution patterns at C-2 of the triterpene scaffold, allowing the establishment of a structure-activity relationship. The radical scavenging ability of 4a-d was evaluated using the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH center dot) and 2,2 '-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid radical cation (ABTS(center dot+)) scavenging assays. In particular, derivative 4c, bearing a catechol unit, revealed to be the most efficient scavenger against both free radicals DPPH center dot and ABTS(center dot+). Subsequently, we designed two analogues of the hit derivative 4c in order to achieve more potent antioxidant agents: (i) the first analogue carries an additional unsaturation in its lateral chain at C-2 (analogue 5) and (ii) in the second analogue, E-ring was kept in its open form (analogue 6). It was observed that the presence of an extended pi-conjugated system at C-2 contributed to an increased scavenging effect, since analogue 5 was more active than 6, alpha-tocopherol, and 4c in the ABTS(center dot+) assay.
publisher	MDPI
isbn	2076-3921
year published	2021
volume	10
issue	2
digital object identifier (doi)	10.3390/antiox10020148
web of science category	12
subject category	Biochemistry & Molecular Biology; Chemistry, Medicinal; Food Science & Technology
unique article identifier	WOS:000622046100001