4-Thiopyrones, halogenations

Quinolin-4-one forms a 3-bromo derivative, but coumarin gives the addition compound (116) which is easily re-aromatized (116 — 117). 4-Thiopyrones are halogenated in position 3. Pyridazinones and cinnolinones are also readily halogenated in the expected position. 

Electrophiles such as halogen attack 4-thiopyrones in position 3 while alkylation occurs on the exocyclic oxygen or sulfur atom to form a thiopyrylium cation. Nitration with nitric acid is unsuccessful because the acid protonates the ketone and the thiopyrylium produced is no longer nucleophilic (Scheme 18). 

Halogenated thiins are relatively sparse, and are usually prepared by chlorination of dihydropyrones (equation 112) (79JOC3144) or thiopyrones (78LA1280). 

---