Access to thiopyrano[2,3-b]indole via tertiary amine-catalyzed formal (3+3) annulations of β′-acetoxy allenoates with indoline-2-thiones

DABCO-catalyzed formal (3+3) annulations of β'-acetoxy allenoates with indoline-2-thiones are described, which provide facile access to thiopyrano[2,3-b]indole under mild reaction conditions. The reaction might proceed via the SN2'-SN2'-type process between β'-acetoxy allenoate and indole-2-thiolate with the assistance of the DABCO catalyst and K2CO3 additive, followed by intramolecular Friedel-Crafts reaction at the 3-position of indole and central carbon of allene.