Mulberry Diels−Alder-type adducts (MDAAs) are a group of rare natural polyphenols biosynthetically derived from [4 + 2]-cycloaddition of chalcones and dehydroprenylphenols. In this study, kuwanons G (1) and H (2), two bioactive MDAAs with unique dehydroprenylflavonoid dienes, were totally synthesized for the first time in a biomimetic manner. The key features of the convergent route include the use of the Baker−Venkataraman rearrangement, alkylation of β-diketone, intramolecular cyclization, and Suzuki−Miyaura coupling to achieve the subunit diene.
Chromatographic fractionation of the EtOH extracts of the marine-derived fungus Aspergillus versicolor A18 has led to the isolation of 11 homo/hetero-dimers of aromatic bisabolane sesquiterpenoids including eight diphenyl ether-coupled aromatic bisabolanes (1a/1b and 5–10) and three homodimers (2–4), together with their monomers including three aromatic bisabolanes (11–13) and two diphenyl ethers (14 and 15). Their structures and absolute configurations were elucidated by extensive spectroscopic analysis including HRESIMS, 1D/2D NMR, calculated ECD, and the optical rotatory data. Among the four new compounds, (+/−)-asperbisabol A (1a/1b), asperbisabol B (2), and asperbisabol C (3), the enantiomers 1a and 1b represent an unprecedented skeleton of diphenyl ether-coupled aromatic bisabolane sesquiterpenoids with a spiroketal core moiety. The neuroprotective effects of selected compounds against sodium nitroprusside (SNP)-induced injury were evaluated in PC12 cells by the MTT assay. Five compounds (1a, 6, and 8–10) showed remarkable neuroprotective activities at 10 μM, being more active than the positive control edaravone.
Phytochemical investigation on the plant of Wikstroemia alternifolia led to the isolation of 26 compounds including two new ones, wikstralternifols A and B (1 and 7). Their structures including the absolute configuration were elucidated by spectroscopic data together with analysis of experimental and calculated ECD data. All compounds were isolated from this plant for the first time, and their main structural types were lignans, sesquiterpenoids, and flavonoids. In the sodium nitroprusside-induced rat pheochromocytoma PC-12 cell model, the neuroprotective activities of the selected sesquiterpenoids (1 and 4) and lignans (7−14) were screened at the concentration of 10 M, and 7−14 displayed better activities than the positive control edaravone.
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