This project proposal targets the synthesis of unconventional derivatives of known (carba)nucleoside drugs. In particular, we put forward the synthesis of a so far unknown class of spirocyclic analogues, in which small carbocyclic rings are spiro-fused to a dearomatized nucleobase heterocycle. For this, several synthetic methods and strategies will be explored in parallel, with differing levels of ambition, ranging from multistep transformations that build up the scaffolds from common nucleobases, to direct spiroannulations. The developed synthetic methodology builds on recent research results obtained within our group in the general area of (dearomative) allyl cation (3+2) and (2+1) cycloadditions. We have developed several sulfur-heterocycle based reagents that generate high energy cationic intermediates. These can act either as potent 1,3-dipoles or as vinyl carbenoids in innovative cycloadditions, depending on the mode of activation. If successful, the newly developed methodology will be a valuable addition to the drug discovery chemistry tool box. It will allow an innovative derivatization and decoration of common aromatic heterocyclic scaffolds, not necessarily limited to the show case nucleosides targeted herein.