Biological Evaluation of Phellinsin A Analogues as Chitin Synthase II Inhibitors

Abstract

Chitin is the β-1,4-linked homopolymer of N-acetyl-Dglucosamine (GlcNAc) that constitutes an important structural component of the cell walls of nearly all zoopathogenic and phytopathogenic fungi, and plays a crucial role in the determination of cell morphology. Chitin have been known to be synthesized by chitin synthase I, II, and III in Saccharomyces cerevisiae. Chitin synthase II is an essential enzyme for the formation of primary septum between mother and daughter cells, and chitin synthase III is responsible for the formation of chitin ring at bud emergence, whereas chitin synthase I is a repair enzyme of the damaged cells on cell division. Accordingly, specific inhibitors of chitin synthase II and III are expected to be an attractive target for the development of antifungal agents. In the course of exploring a chitin synthase II inhibitor from microbial sources, we isolated a new phenolic lignan bearing a γ-lactone ring from the cultured broth of Phellinus sp. PL3. The lignan compound named phellinsin A (3a) exhibited inhibition of chitin synthase II with an IC50 value of 27 μg/mL and showed 2.5 times stronger inhibitory activity than polyoxin D, and its structure was established by NMR analysis and synthesis. Our efforts toward the development of a potent antifungal agent have focused on examining structure-activity relationships for phellinsin A. We have modified the aryl group in phellinsin A by changing the number of phenolic OH groups in order to investigate the effect of the number of free phenolic OH groups in γ-lactone analogues of phellinsin A (3a) on chitin synthase II inhibitory activity. Herein, we describe the biological evaluation of phellinsin A analogues 3. The γ-lactone analogues 3c-f of phellinsin A were prepared by oxidative dimerization of cinnamic acid derivatives 1 into the corresponding dilactones 2, followed by monohydrolysis of the dilactones as shown in Scheme 1. The γlactones 3a and 3b were prepared by demethylation of compounds 3d and 3e using BBr3, respectively. The γ-lactone analogues 3 of phellinsin A were examined for structure-activity relationships of phellinsin A (3a). Inhibitory activities of the compounds against chitin synthase II were evaluated by measurement of the formation of chitin with UDP-[C]-N-acetyl-D-glucosamine. Table 1 showed the inhibitory activities of phellinsin A analogues at 140 and 280 μg/mL concentrations. The activities were largely dependent on the presence of free phenolic OH substituents in γ-lactone analogues 3. Generally, the compound 3f with non-phenolic OH groups did not show