Bifunctional Lewis Base-Catalyzed (3 + 2) Cycloadditions of Pyrazolone-Derived MBH Carbonates with Arynes

The (3 + 2) cycloaddition of arynes with allylic ylides
remains
a formidable challenge because both intermediates are highly reactive
and prone to spontaneous quenching. Here, we report a (3 + 2) cycloaddition
of pyrazolone MBH carbonates with arynes, enabling the efficient synthesis
of diverse indene-fused spiropyrazolones. The key is employing a new
bifunctional Lewis base catalyst to facilitate the cycloaddition of in situ generated allylic pyridinium ylides with arynes.