Abstractmagnified imageThe racemic N‐benzylisothiazol‐3‐amine 1‐oxide (2) was demonstrated to be an efficient partner in Diels–Alder reactions (Schemes 2–4) and a good dipolarophile in 1,3‐dipolar cycloaddition reactions with nitrile oxides (Scheme 5). Polycyclic isothiazole S‐oxides with different substitution patterns were obtained from 2 in good yield and in a fully regioselective way. Improved diastereoselection was observed performing the Diels–Alder or 1,3‐dipolar cycloaddition reactions in H2O.