| Bryophytes, considered being the first plant type to successfully make the transition from water to land, are taxonomically placed between the algae and the pteridophytes. Morphologically, they are divided into three classes, liverworts (Hepaticae), mosses (Musci) and hornworts (Anthocerotae). Among the bryophytes, liverworts are a rich source of terpenoids and aromatic compounds with intriguing biological activities such as cytotoxic, antifungal, allelopathic, insect antifeedant, andantioxidant properties, which might because they possess appealing, characteristic cell organelles:cellular oil bodies.In this research, the liverworts Scapania irrigua (Nees.) Dumort, collected from Anhui Provence, Marsupella alpine (Gott. et Limpr.) Bernet collected from Guangxi Province, and Chiloscyphus polyanthus var. rivularis (Schrad.) Nees collected from Jilin Province, were phytochemically investigated. A total of 58 compounds, including 23 diterpenoids,33 sesquiterpenoids,1 triterpenoid, and 1 monoterpenoid, were isolated and identified on the basis of NMR, MS, single crystal X-ray diffraction analysis, and time-dependent density functional theory (TDDFT) ECD calculations. 40 of them were new compounds, containing 17 new diterpenoids and 23 new sesquiterpenoids. Ent-longipinane-type sesquiterpenoids isolated from M. alpine exhibited moderate to weak AChE inhibitory activity. Some labdane-type diterpenoids isolated from S. irrigua exhibited inhibitory activities against a small panel of human cancer cell lines. This work represents the first phytochemical investigations on the plants of S. irrigua.Separation of the liverwort S. irrigua afforded seventeen new labdane-type diterpenoids, scapairrins A-Q (1-17), including six pairs of diastereoisomers, as well as four known analogues 5α,8α,9α-trihydroxy-13E-labden-12-one (18),1α,5α,8α-trihydroxy-13E-labden-12-one (19),5α,8α-dihydroxy-13E-labden-12-one (20) and scapaundulin C (21). Cytotoxicity testing showed that compounds 7-10 exhibited inhibitory activities against a small panel of human cancer cell lines.Acetylcholinesterase (AChE) inhibitory activity-guided fractionation of the Chinese liverwort M. alpine afforded six new [marsupellins A-F (22-27)] and three known ent-longipinane-type sesquiterpenoids:(-)-ent-12β-hydroxylongipinan-3-one (28),9-acetoxymarsupellol (29), acetoxymarsupellone (30) as well as one known phytane:phytol (31). Compounds 22-30 exhibited moderate to weak AChE inhibitory activity.Separation of the liverwort C. polyanthus var. rivularis afforded twenty-seven compounds, including twenty-one ent-eudesmane type sesquiterpenoids, (32-46; 51-56), two ent-cyperane type sesquiterpenoids (47-48), one guaiane type sesquiterpenoid (50), one labdane-type diterpenoid (57), one triterpenoid (58) and one monoterpenoid (49), among them, compounds 32-48 are new compounds.The metabolic pathways are dependent on plant genomes or enzymes, eventually leading to the formation of structurally diverse secondary metabolites which are mainly derived from a small number of primary molecules. In recent years, however, there is a growing recognition of the importance of both biotic and abiotic stresses on the diversity of natural products. Among these stimuli, UV/vis irradiation is very significant, especially for compounds containing strong chromophores which easily undergo photoinduced reactions.Recently, we disclosed two unprecedented 19-nor-7,8-secolabdane diterpenoids with tetracyclo[4.4.03;5.02’8] decane skeletons, pallambins A and B, and a pair of structurally related isomers, pallambins C and D, from the Chinese liverwort Pallavicinia ambigua. We carried out the photochemical reaction and found that pallambins C and D can be converted to pallambins A and B. The photochemical mechanism, intramolecular diradical rearrangement reaction, was investigated by time-lapse monitoring of the end product formations with HPLC and UV, as well as detailed theoretical calculations. |
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