Antibacterial Biphenanthrenes from the Fibrous Roots of Bletilla striata

Four new 9',10'-dihydro-biphenanthrenes, including an unprecedented 1,2'-linked biphenanthrene, 4,7,3',5'-tetramethoxy-9',10'-dihydro(1,2'-biphenanthrene)-2,7'-diol (1), a new 1,3'-linked biphenanthrene, 4,7,7'-trimethoxy-9',10'-dihydro(1,3'-biphenanthrene)-2,2',5'-triol (2), and two new 1,1'-linked biphenanthrenes, 4,7,4'-trimethoxy-9',10'-dihydro(1,1'-biphenanthrene)-2,2',7'-triol (3) and 4,7,3',5'-tetramethoxy-9',10'-dihydro(1,1'-biphenanthrene)-2,2',7'-triol (4), as well as two known biphenanthrenes (5, 6), were isolated from a 95% ethanol extract of the fibrous roots of Bletilla striata. Their structures were determined by spectroscopic and spectrometric methods. Atropisomerism of these compounds was considered based on their chiral optical properties and potential energy surface scans at the ab initio HF/3-21G level, which revealed their racemic mixture form. Compounds 2-6 showed potent antibacterial activities against six Gram-positive bacterial strains.