A set of unique nucleoside analogs, containing 'spirocyclic orthoester-type' scaffolds, were synthesized from a common isothiazolo[4,3-d]pyrimidine-riboside precursor. The key reaction, using 1,2-di-heteroatomic nucleophiles (e.g., 1,2-ethandithiol) and BF3•OEt2, converts an exocyclic imine into the spirocyclic analogs. The novel structural scaffold is confirmed through the use of one- and two-dimensional 1H and 13C NMR experiments.