Dimeric 9,10-dihydrophenanthrene derivatives from Bletilla striata and their atropisomeric nature

Four pairs of undescribed racemic bi(9,10-dihydro) phenanthrene and phenanthrene/bibenzyl atropisomers, bletistriatins A-D (1-4), along with 22 known compounds were isolated from the rhizomes of Bletilla striata. These dimeric derivatives were constructed through direct C-C connection or an oxygen bridge. The structures of new compounds were fully established by extensive analysis of MS, and 1D and 2D NMR spectroscopic data. Owing to sterically hindered rotation around the biaryl axis, these dimeric 9,10-dihydrophenanthrene derivatives can exist as a pair of enantiomers, but were isolated as racemates. Their racemates were separated to yield enantiomerically pure compounds by HPLC on an optically active stationary phase, and were stereochemically characterized on-line by circular dichroism (CD) spectroscopy (LC-CD coupling). Some isolates were evaluated for cytotoxicity against human cancer cell lines HL-60 and A549. Compounds 13, 17, and 20 showed cytotoxicity against HL-60 and A-549 cell lines with IC₅₀ values ranging from 2.56 to 8.67 μM.