Synthesis of 3,4-Dihydrophthalazin-1(2H)-one by Zinc Reduction of Phthalazin-1(2H)-one

3,4-Dihydrophthalazin-1(2H)-one (II) was first prepared by H.Sund ¹⁾, by electrochemical reduction of phthalazin-1(2H)-one (I) (phthalazone). In the recent years we have described new syntheses of II, through easily hydrolyzable mono- or diacetyl- derivatives, namely condensation of 2-bromomethylbenzoylchloride with N,N'-diacetylhydrazine ²⁾ catalytic reduction of I in acetic acid ²⁾ or of 2-acetylphthalazone in acetic anhydride ³⁾ The reduction of I with zinc is known to proceed with ring contraction and formation of N-aminophthalimidine (III)⁴⁾ (zinc and sodium hydroxide at about 100ddot]C) or of phthalimidine (IV) ⁵⁾ (zinc and hydrochloric acid under unspecified conditions).

Having observed that II is transformed into N-aminophthalimidine when heated with hydrazine hydrate ⁶⁾ or sodium hydroxide ²⁾, the zinc reduction of I was reinvestigated with the aim to find out in which cases the intermediate formation of II can be dem-onstratedo