Absolute and atropisomeric structure of ES-242s, N-methyl-D-aspartate receptor antagonists

ES-242 derivatives and cycloheptapeptides from Cordyceps sp. strains BCC 16173 and BCC 16176

Five new ES-242 analogues ( 1- 5) were isolated together with nine known compounds ( 6- 14) from the insect pathogenic fungus Cordyceps sp. BCC 16173. A closely related strain, BCC 16176, provided cordyheptapeptide A ( 15) and small amount of its new analogue, cordyheptapeptide B ( 16), along with known ES-242s. Structures of the new bioxanthracenes, 1- 5, were determined to be 6'- O-desmethyl analogues of 6 (ES-242-4), 8, 9 (ES-242-2), 12, and 13, respectively, primarily by spectroscopic analyses. Cordyheptapeptide B ( 16) has an N-methyl- l-phenylalanine residue instead of the N-methyl- l-tyrosine in 15.