Multistep tandem reactions (or cascade reactions) are a sequence of chemical transformations (usually more than two steps) that happens consecutively to convert a starting material to a complex product. This kind of organic reactions are designed to construct difficult structures encountered in natural product total synthesis.
In the total synthesis of spiroketal ionophore antibiotic routiennocin 1 (Fig. 1), the central spiroketal skeleton was constructed by a multistep tandem reaction (Fig. 2).  Fragment A and fragment B were coupled in a single step to form the key intermediate G that could be further elaborated to afford the final product routiennocin.
Four chemical transformations happened in this tandem reaction. First, treating fragment A with n-butyllithium formed carbon anion that attacked the alkyliodide part of fragment B to generate intermediate C (step 1). Then a 3, 4-dihydropyran derivative D was formed through base-mediated elimination reaction on intermediate C (step 2). The protecting group on 1, 3-diol moiety in intermediate D was removed by acid treatment to give the diol product E (step 3). The spiroketal product G was generated via intramolecular ketal formation reaction. This multistep tandem reaction greatly simplified the construction of this complex spiroketal structure and eased the path towards the total synthesis of routiennocin.