It has been convincingly argued(1-3) that molecular machines that manipulate individual atoms, or highly reactive clusters of atoms, with Angstrom precision are unlikely to be realized. However, biological molecular machines routinely position rather less reactive substrates in order to direct chemical reaction sequences, from sequence-specific synthesis by the ribosome(4) to polyketide synthases(5-7), where tethered molecules are passed from active site to active site in multi-enzyme complexes. Artificial molecular machines(8-12) have been developed for tasks that include sequence-specific oligomer synthesis(13-15) and the switching of product chirality(16-19), a photo-responsive host molecule has been described that is able to mechanically twist a bound molecular guest(20), and molecular fragments have been selectively transported in either direction between sites on a molecular platform through a ratchet mechanism(21). Here we detail an artificial molecular machine that moves a substrate between different activating sites to achieve different product outcomes from chemical synthesis. This molecular robot can be programmed to stereoselectively produce, in a sequential one-pot operation, an excess of any one of four possible diastereoisomers from the addition of a thiol and an alkene to an alpha,beta-unsaturated aldehyde in a tandem reaction process. The stereodivergent synthesis includes diastereoisomers that cannot be selectively synthesized(22) through conventional iminium-enamine organocatalysis. We anticipate that future generations of programmable molecular machines may have significant roles in chemical synthesis and molecular manufacturing.