Isolated from Hypericum species H. chinese L. var. salicifolium, biyouyanagin A was assigned structure 1a or 1b on the basis of NMR spectroscopic analysis. This novel natural product exhibited significant anti-HIV properties and inhibition of lipopolysaccharide-induced cytokine production. Described herein are the total syntheses of biyouyanagin A and several analogues (3-11), structural revision of biyouyanagin A to 2b, and the biological properties of all synthesized compounds. The total synthesis proceeded through cascade sequences that efficiently produced enantiomerically pure key building blocks 15b (ent-zingiberene) and 18 (hyperolactone C) and featured a novel [2 + 2] photoinduced cycloaddition reaction which occurred with complete regio- and stereoselectivity. Biological investigations with the synthesized biyouyangagins A (2-11) and hyperolactones C (12-16) revealed that the activity of biyouyanagin A most likely resides in its hyperolactone C structural domain.