Elemental sulfur accelerated the reactivity of the 3-position of indole for the construction of chromeno[2,3-b]indoles

An intermolecular cross-cyclization between salicylaldehydes and indoles for the preparation of chromeno[2,3-b]indole derivatives was successfully developed by using elemental sulfur as the promoter and oxidant. Various chromeno[2,3-b]indoles can be prepared using indoles and salicylaldehydes bearing different functional groups. The preliminary mechanistic investigations demonstrated that elemental sulfur not only acted as an additional oxidant but also facilitated the reactivity of the 3-position of indole.