The constitution and stereochemistry of erythroxydiols X (III) and Y (IV) is indicated by their spectroscopic properties and chemical reactions, in particular their conversion by acid into the same major products, erythroxydiol Z (IV: Δ³⁽⁴⁾-isomer) and (X). The structures and absolute configurations of the three diols are supported by conversion of deoxyrosenonolactone (XIX) and the diol (X), respectively, into the diene (XIV) and its antipode