[3+2] Cycloaddition to Pyrido[1,2-b]indazoles

Significance

The present paper is inspired by the studies of Masumura and co-workers (Chem. Lett. 1980, 1133), who synthesized pyrido[1,2-b]indazoles via aryne cycloaddition with N-acyl­pyridinium imides. The yield in this original report was low due to a facile side reaction leading to N-acyl-2-(2-pyridyl)anilines. Larock and co-workers reasoned that changing the protecting group from N-acyl to N-tosyl would lead to higher yields due to facile elimination of tosyl sulfinate. This was shown to be true, and the products were obtained in moderate to good yields. The cycloaddition of 3-substituted pyridinium imides gave rise to two regioisomeric cycloadducts with poor C2 versus C6 (of the pyridine) regioselectivity. Lowering the amount of CsF or testing other N-protecting groups led to lower product yields.