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Using a bioactivity-guided fractionation procedure, five cembranolides, 11- epi -sinulariolide acetate ( 1 ), 11-dehydrosinulariolide ( 2 ), sinulariolide ( 3 ), dihydrosinularin ( 4 ), and 3,4:8,11-bisepoxy-7-acetoxycembra-15(17)-en-1,12-olide ( 5 ), along with two nucleosides, 2′-deoxyadenosine and thymidine, were isolated from the Formosan soft coral Sinularia flexibilis . Moreover, 7,8-epoxy-11- epi -sinulariolide acetate ( 1a ), 11-sinulariolide acetate ( 3a ), dihydrosinulariolide ( 3b ), 3,4:8,11-bisepoxy-7-hydroxycembra-15(17)-en-1,12-olide ( 3c ), 11-acetoxyl-15(17)-dihydrosinulariolide ( 3d ), 7,8-epoxy-11-sinulariolide acetate ( 3e ), and 3,4:8,11-bisepoxy-7-hydroxycembra-15(17)-dihydro-1,12-olide ( 3f ) were derived from compounds 1 and 3 , respectively. These structures were deduced on the basis of physical and chemical evidence. Among them, 1a , 3d , 3e , and 3f are new cembranolide analogues. The structure of compound 1 was further confirmed by X-ray analysis. In addition, the isolated cembranolides and the analogues under went a cytotoxicity assay, and the structure-activity relationship (SAR) of these compounds was studied. [ABSTRACT FROM AUTHOR]