The trunkwood of Croton diasii Pires (Euphorbiaceae) contains diasin, a novel rearranged Labdanic diterpene which includes furan, carbomethoxy, β- and δ-lactone moieties. All ¹³C NMR signals were assigned and used in conjunction with PMR data, for the elucidation of the relative configuration of all chiral centers. A possible biogenetic pathway to diasin is discussed.