The synthesis and antibacterial activity of totarol derivatives. part 1: modifications of ring-C and pro-drugs

Abstract

A series of analogues of, and potential pro-drugs derived from, the potent antibacterial diterpene totarol (1) were synthesized in order to elucidate the minimum structural requirements for antibacterial activity and to seek compounds with good bioavailability in vivo. These analogues varied in the structural features of their aromatic rings and the prodrugs were O-glycosylated derivatives. They were tested in vitro against three Gram-positive bacteria: β-lactamase-positive and high level gentamycin-resistant Enterococcus faecalis, penicillin-resistant Streptococcus pneumoniae, and methicillin-resistant Staphylococcus aureus (MRSA); and against the Gram-negative multi-drug-resistant Klebsiella pneumoniae. None of the analogues was more potent than totarol itself, which is effective against these Gram-positive bacteria at MIC values of 7μM. The results were evaluated in terms of a structure–activity relationship and this showed that a phenolic moiety was essential for potent antibacterial activity. Amongst the pro-drugs, totaryl α-d-mannopyranoside (22) proved the most active in vitro (MIC 18μM). The in vivo antibacterial activities of compounds 1, 22 and totarol β-lactoside (23) were assessed in a mouse model of infection, but they were found to be ineffective. Compounds 1 and 22 were shown to be cytotoxic towards proliferating human cell cultures, CH 2983, HeLa, and MG 63, but only at concentrations of >30μM.