Eight new polycyclic phloroglucinol meroterpenoids guajamers A-H (1-8), a methylated benzoylphloroglucinol meroterpenoid guajamer I ( 9) representing a new skeleton, and two known analogues (10 and 11) were isolated from the leaves of Psidium guajava. The structures of new molecules were elucidated by detailed analysis of spectroscopic data, and those of 1, 2, 8, and 9 were unambiguously confirmed by single-crystal X-ray diffraction study. Structurally, compounds 1-8 were sesquiterpene and monoterpenebased meroterpenoids with rearranged skeletons, while compound 9 was the first case of 3-alkyl-5-formyl-benzoylphloroglucinolcoupled sesquiterpene containing an unusual C-1-spiro-fused 6/6/9/4 polycyclic skeleton. In addition, all the isolated compounds were evaluated for their antibacterial activity against three bacterial strains, and most of them (compounds 2-7, 10, and 11) showed antibacterial activity against Staphylococcus aureus and Staphylococcus epidermidis with MIC values of 8-32 μmol/L. These findings suggested that meroterpenoids isolated from Psidium guajava can be considered as potential antibacterial leading compounds for pharmaceutical industry.

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