Stereoselective Synthesis of C-Fused Pyranoindoles, Pyranobenzofurans and Pyranobenzothiophene Scaffolds Using Oxa-Pictet—Spengler Type Reaction of Vinylogous Carbonates.

C-fused pyranoheterocycles can be readily assembled using an intramolecular oxa-Pictet–Spengler type reaction of vinylogous carbonates in a highly stereoselective manner. Required indole and benzofuran rings tethered to vinylogous carbonates are prepared by a tandem Sonogashira coupling–nucleopalladation reaction. The entire process can also be carried out in a ‘one-pot’ manner starting from homopropargyl alcohol. The C-fused pyranoindoles could be converted to spirooxindoles as well as to ring expanded products under oxidative conditions.