Highly Oxygenated Triterpenoids and Rare Tetraterpenoids from Abies chensiensis and Their Antibacterial Activity

Nine new cycloartane triterpenoids (1, 2, 4, 7, 8, 12, 15, 17, and 18) and two new rare tetraterpenoids (24 and 25) formed via a [4 + 2] Diels–Alder cycloaddition between a lanostane triterpenoid and a monoterpenoid, along with 14 previously known triterpenoids, were isolated from the bark of the branches of Abies chensiensis. The structures and absolute configurations of new compounds were elucidated based on spectroscopic data, X-ray diffraction analysis, and electronic circular dichroism. Some of the isolates were evaluated for their antibacterial activity by determining their minimum inhibitory concentrations and growth inhibition curves and examining for morphological alterations. Among the compounds tested, the new cycloartane triterpenoid 8 was the most active against Bacillus subtilis. Thus, morphological alterations of B. subtilis on treatment with 8 were observed by scanning electron microscopy, showing that the cells were irregular, wrinkled, and disrupted.