A Rh(III)-catalyzed strategy involving the [4+1] annulation of 2-arylphthalazine-1,4-diones with α-diazo carbonyl compounds was developed, accessing a series of unprecedented hydroxy-dihydroindazolo-fused phthalazines in good to excellent yields. By varying the additive, phthalazino-fused cinnolines were synthesized under Rh-catalyzed conditions via [4+2] annulation between the same starting materials. Notably, such two strategies showed a good functional group tolerance and high atom efficiency.

A mild Rh-catalyzed method was developed for the synthesis of hydroxyimino functionalized indazolo[1,2-a]cinnolines and phthalazino[2,3-a]cinnolines by reductive [4 + 2] annulation between 1-arylindazolones and 2-aryl-2,3-dihydrophthalazine-1,4diones with varied nitroolefins. The targeted oxime decorated tetracyclic fused cinnolines were synthesized via sequential C−H activation/olefin insertion/reduction under reducing-agent-free conditions.

An Ir-catalyzed tandem strategy for the synthesis of indazolone-fused cinnolines by [4 + 2] annulation of 1-arylindazolones with α-diazo carbonyl compounds.

A tender-hearted Pd­(II)-catalyzed C–H activation of 1-arylindazolones followed by an oxidative [4 + 2] annulation reaction has been accomplished, engaging allenoates as annulating partners. Using this strategy, two different regioisomeric forms of cinnoline-fused indazolones possessing internal and exocyclic double bonds were synthesized in acetic acid and dioxane, respectively. Mild and aerobic conditions, avoiding the use of any metal–oxidant, highlights the rewards of this oxidative annulation protocol.

A phenyliodine(III) diacetate mediated umpolung reactivity of the tertiary amines with suitably substituted o-hydroxybenzyl and phenyl groups is exploited to facilitate o-C(sp)-H functionalization to afford diaryl ethers. The presence of an o-CHO and secondary amine functionalities in the resulting diaryl ether, generated in situ, were utilized for synthesis of dibenzoxazepines and dibenzoxazepinones. Mild conditions and relative broad substrate scope, and potential for further diversification of the diaryl ethers are highlights of this methodology.