Antibacterial constituents from the roots, stem bark and leaves of Manilkara obovata (Sabine & G. Don) J. H. Hemsl. (Sapotaceae)

Abstract

• Obovacoumaric acid and obovatol were isolated from the methanolic extracts of Manilkara obovata . • A new ethylcanophyllate was obtained by esterification of canophyllic acid. • The structures were elucidated by NMR and single crystal X-ray crystallography. • Antibacterial effects of the methanolic extracts of Manilkara obovata . The investigation of the methanolic extracts of the roots, stem bark and leaves of Manilkara obovata (Sabine & G. Don) J. H. Hemsl. (Sapotaceae), synonym Manilkara argentea Pierre ex Dubard, has led to the isolation and characterization of a new 4-phenylcoumarin derivative, in form of a racemic mixture of obovacoumaric acid ( 1a ) and the previously reported callophyllic acid ( 1b ) as well as a new taraxerane type triterpene named obovatol ( 2 ), alongside twenty-five known compounds. In addition, ethylcanophyllate ( 3a ), obtained by esterification of canophyllic acid ( 3 ) is reported here for the first time. The structures of compounds were determined by comprehensive spectroscopic analyses of their 1D and 2D NMR, EI/ESI-MS and single crystal X-ray diffraction (SC-XRD) analysis for 1a and 1b , accompanied by comparison with reported records. In addition, the antibacterial activity of crude extracts, fractions and some of the isolated compounds including the hemi-synthesized ethylcanophyllate ( 3a ) was determined against Salmonella typhi , Staphylococcus aureus ATCC4330, Enterobacter cloacae , Pseudomonas aeruginosa HM801, Streptococcus pneumoniae ATCC491619 and Escherichia coli ATCC25322 applying the broth microdilution method. While the methanolic stem bark extract showed good to significant activity against all bacterial test strains with MICs between 7.8 and 62.5 μg/mL, its fractions A and B were less active, indicating synergistic effects. This effect was also observed for the methanolic root extract, which gave a significant MIC against Staphylococcus aureus ATCC4330 with MIC < 3.9 μg/mL, but only MICs of 31.2 and 7.8 μg/mL for its fractions D and E. Likewise, though fractions F 3 and F 4 of the methanolic leaf extract displayed good MICs of 31.2 and 15.6 μg/mL, respectively, against Staphylococcus aureus ATCC4330, tested compounds were not stronger active. Interestingly, the hemi-synthesized ethylcanophyllate ( 3a ) based on the completely antibacterial inactive canophyllic acid ( 3 ) displayed a good MIC of 15.6 μg/mL against Staphylococcus aureus ATCC4330, pointing here to a dominant role of the ester function in compound 3a in the structure-activity relationship.