A series of nineteen nitrogen-containing lupane triterpenoids was obtained by modification of C2, C3, C20 and C28 positions of betulonic acid and their α-glucosidase inhibiting activity was investigated. Being a leader compound from our previous study, 2,3-indolo-betulinic acid was used as the main template for different modifications at C-(28)-carboxyl group to obtain cyano-, methylcyanoethoxy-, propargyloxy- and carboxamide derivatives. 20-Oxo- and 29-hydroxy-20-oxo-30-nor-analogues of 2,3-indolo-betulinic acid were synthesized by ozonolysis of betulonic acid followed by Fischer indolization reaction. To compare the influence of the fused indole or the seven-membered A-ring on the inhibitory activity, lupane A-azepanones with different substituents at C28 were synthesized. The structure-activity relationships revealed that the enzyme inhibition activity dramatically increased (up to 4730 times) when the carboxylic group of 2,3-indolo-betulinic acid was converted to the corresponding amide. Thus, the IC50 values for glycine amide and L-phenylalanine amides were 0.04 and 0.05 μM, respectively. This study also revealed that 2,3-indolo-platanic acid is 4.5 times more active than the parent triterpenoid with IC50 of 0.4 μM. Molecular modeling suggested that improved potency is due to additional polar interactions formed between C28 side chain and a sub-pocket of the α-glucosidase allosteric site.
Keywords: Acarbose; Betulinic acid; Indole; Lupanes; Natural products; Synthesis; Triterpenoids; Type II diabetes; α-Glucosidase inhibitor.
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