Asymmetric 1,4-Reduction of Pyrano[2,3‑<i>b</i>]indoles with Hantzsch Ester by Chiral Phosphoric Acid

A highly\nregioselective and enantioselective transfer hydrogenation\nof pyrano[2,3-<i>b</i>]indoles with Hantzsch ester has been\nsuccessfully realized using spiro-chiral phosphoric acid, affording\na series of optically active 1,4-reductive adducts, 4,9-dihydropyrano[2,3-<i>b</i>]indoles, in 34–99% yields with 61–99% <i>ee</i>.