“…Despite the fact that it is often not valorized and hence discarded, resin is a very interesting fraction from both an academic and an industrial point of view. Indeed, it is not only rich in sesquiterpenoids and triterpenes of potential commercial value [6], it also contains a variety of interesting secondary metabolites of the shrub, like guayulins [6][7][8] ((1S,2E,6E,9S,10R)-3,7,11,11-tetramethylbicyclo[8.1.0]undeca-2,6-dien-9-ol) [7], whereas the guayulin C ((1AR,1BR,2R, 4AR,7S,7AR)-2-hydroxy-1,1,2-trimethyl-5-methylidene-decahydro-1H-cyclopropa [E]azulen-7-yl (2E)-3-phenylprop-2-enoate) and guayulin D ((1AR,1BR,2R,4AR,7S,7AR)-2-hydroxy-1,1,2-trimethyl-5-methylidene-decahydro-1H-cyclopropa[E]azulen-7-yl 4-methoxybenzoate) are likely formed by the oxidation of the guayulines A and B, respectively [6,8]. Scheme 1 reports the structures of these molecules.…”