We were able to show the predominant incorporation of a single enantiomer and intact incorporation of multiply labelled synthetic diketide precursors (14 and 16), which established the intermediacy of cyclopropanated diketide and led to our proposal for the unprecedented biological cyclopropanation, viaPKS (polyketide synthase) having a novel cyclopropanase domain, in the biosynthesis of FR-900848 (1).