Mechanism of С-Н cyclization of alkynylanthraquinones into thienoanthraquinones with the participation of sodium sulfide

Article abstract of DOI:10.1016/j.tet.2017.09.023

The CH-cyclization of alkynyl-9,10-anthraquinones 1 into thienoanthraquinones 2 is shown to be initiated by the neutral S₂ molecule and not by the sulfide anion as it is assumed in the existing model of CH-cyclization with the nucleophiolic substitution of H. The formation of S₂ takes place in the reaction medium due to the partial reduction of the substrate 1 by Na₂S into the corresponding anthrahydroquinone, which is inactive in the main reaction. We present a new cyclization mechanism with the key stage involving the electrophilic displacement of H, which results in the formation of the reaction product 2 and elimination of S. In the reaction medium atomic sulfur is transformed into S₂, which initiates the next cyclization act. This determines the chain nature of cyclization with the participation of S₂. Within this model, the route to increase the yield of target thienoanthraquinones is proposed through the introduction of a neutral quinone able to generate S₂ by means of easy reduction.