Bifunctional\nLewis Base-Catalyzed (3 + 2) Cycloadditions\nof Pyrazolone-Derived MBH Carbonates with Arynes

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