Pyrazolo[3,4,5-de]phthalazine. Syntheses of a practically unknown heterocyclic system

Simple procedures for the synthesis of 1,3-dihydropyrazolo[3,4,5-de]phthalazines (2), a practically novel type of heterocyclic compounds, have been developed. Compounds equally substituted at positions N1 and N3 can be directly obtained from 4,7-dihalogenated benzalphthalides (1). Those with different R2 and R3 substituents are prepared through intermediate phthalazin-1-ones (3), whose synthesis is also considered.