The structure and stereochemistry of phorbol, a novel tetracyclic diterpene containing a reactive hydroxycyclopropyl carbinol system, is discussed. It undergoes protonation at the homoallylic 12-hydroxy-group and gives crotophorbolone (an isopropenyl cyclohexanone) and crotophorbolone K (a dimethyl cyclobutanone). With boron trifluoride–acetic acid, phorbol triacetate gives an acetoxy-isopropyl cyclohexanone, and treatment with lithium aluminium hydride–aluminium chloride, followed by reacetylation, produces an enol acetate of crotophorbolone acetate. These reactions appear to be a result of cation formation at the 12-position.Treatment with lead tetra-acetate (1 mol.) in aqueous methanol yields the bisdehydro-compounds tiglophorbol, tiglophorbol A, and tiglophorbol B. In the first, ring C is a boat spanned by a 9,12-five-membered heterocyclic ring with the 11-keto- and the 14-hydroxy-group linked as a hemi-acetal group. Tiglophorbol A is a 12-hydroxy-crotophorbolone and tiglophorbol B its 11,12-acyloin isomer. Tiglophorbol A is converted into tiglophorbol T by a second molar proportion of lead tetra-acetate. In tiglophorbol T, ring C is ruptured; one part is linked with the 14-hydroxy-group to give a five-membered ring lactone, and the other (aldehydic) part has formed a hemiacetal with the 4-hydroxy-group. The course of these reactions is considered.Crotophorbolone undergoes rupture at the 9,11-bond, with acetoxylation at C-9, when treated with lead tetra-acetate in acetic acid. The remainder of ring C forms a spiro-lactone with the 14-hydroxy-group. Various structures which arise by alteration of the functionality of phorbol are mentioned.