The influence of A steric interactions on the conformational features of benzo[g]pyridazino[1,2-b]phthalazine-6,13-dione derivatives

Abstract

Diazatetracyclic compounds related to the anthracyclinone system have been synthesized via[4 + 2] cycloadditions of benzo[g]phthalazine-1,4-dione with the appropriate dienes. The geometry of the ring A moiety in methyl-substituted adducts 2a–g has been studied by X-ray diffraction and NMR spectroscopic methods. The conformational properties of ring A are controlled by the carbonyl groups on ring B through A steric interactions. Substitution at C-1 and/or C-4 freezes the conformational equilibrium to give terminal tetrahydropyridazine rings with the substituents axially oriented or exhibiting a boat-like geometry. Similar results have been found in the C-2-C-3 double-bond isomerization products 5a–d, which are formed from the respective adducts only in the absence of substituents vicinal to the nitrogen atoms.