N-N-Coupled Indolo-sesquiterpene Atropo-Diastereomers from a Marine-Derived Actinomycete

Four new indolo-sesquiterpenes dixiamycins A (1) and B (2), oxiamycin (3), and chloroxiamycin (4) were isolated from a marine-derived Actinomycete and characterized, together with the known compound xiamycin A (5). Dixiamycins A (1) and B (2) are the first examples of atropisomerism of naturally occurring N-N-coupled atropo-diastereomers, with a dimeric indolo-sesquiterpene skeleton and a stereogenic N-N axis between sp3-hybridized nitrogen atoms. Solution TDDFT ECD calculations were utilized to ascertain the axial chirality of 1 and 2, and rotational barriers and transitions states of the inversion were calculated. Oxiamycin (3) contains a seven-membered 2,3,4,5-tetrahydrooxepine ring. The two dimeric compounds 1 and 2 showed better antibacterial activities than the monomers 35.