Phosphine-Mediated\nReductive Insertion of α‑Keto\nEsters and Isatins into Phthalic Anhydride Derivatives

Herein, we report an unprecedented P(NMe<sub>2</sub>)<sub>3</sub>-mediated reductive insertion of 1,2-dicarbonyl compounds\nincluding\nα-keto esters and isatins into phthalic anhydride-derived alkenes\nand phthalic anhydrides, which furnishes the corresponding isochroman-1-ones\nand isochroman-1,4-diones, respectively, in moderate to excellent\nyields with high chemo- and regioselectivity. Furthermore, the asymmetric\nversion of the ring expansion reaction could be realized by using\na chiral auxiliary strategy. Mechanistically, the nucleophilic attack\nof the Kukhtin-Ramirez adduct, generated from P(NMe<sub>2</sub>)<sub>3</sub> and 1,2-dicarbonyl compound, to the anhydride derivative,\nfollowed by a cascade ring-opening and ring-closure process, affords\nthe ring expansion product. The reaction represents a novel metal-free\ncarbon insertion ring expansion of aliphatic rings and also the first\n[1 + 5] annulation involving the Kukhtin-Ramirez adducts.