Enzyme-catalysed differentiation of hydroxy groups in a C2v-shaped tetraol-sulfide, combined with a E-stereoconvergent Ramberg-Bäcklund process, allowed to prepare pure (2S)-2,3-dimethyl-1-iodobutane, which could be coupled with a 3,5-cyclopregnane-20-thiomethanol derivative so as to give an efficient precursor of the title vegetal hormone.