Divergent Cyclizations of 1-R-Ethynyl-9,10-anthraquinones: Use of Thiourea as a “S²–” Equivalent in an “Anchor-Relay” Addition Mediated by Formal C–H Activation

The EtONa-mediated reaction of peri-R-ethynyl-9,10-anthraquinones with thiourea yields 2-R-7H-dibenzo[de,h]quinolin-7-ones and 2-R-anthra[2,1-b]thiophene-6,11-diones. Although 2-R-7H-dibenzo[de,h]quinolin-7-ones were observed previously in reactions with other N-centered nucleophiles (hydrazine, guanidine, and urea), the formation of 2-R-anthra[2,1-b]thiophene-6,11-diones is a new reactivity path. DFT computations analyzed factors responsible for the switch in reactivity and the relative importance of two possible pathways: (1) the "anchor-relay" mechanism mediated by nucleophilic attack at the carbonyl and (2) direct attack at the alkyne. The two pathways converge on a vinyl sulfur anion, set up for a 5-endo-trig cyclization at the ortho-position. Subsequent rearomatization/oxidation provides the fused thiophene product via formal C-H activation. The calculations suggest that the latter pathway, the direct attack at the alkyne, is more likely, due to the relatively high barrier for the 8-endo-dig cyclization (pathway 1). Computational insights led to a more selective synthesis of fused thiophenes, based on the reaction of acetylenic anthraquinones with sodium sulfide. This reaction does not require prefunctionalization at the ortho-position since direct C-H activation is efficient. The absence of fused five-membered heterocycles in earlier work was investigated computationally. The other N-centered nucleophiles form stronger anion-π complexes with the electron-deficient quinone core, promoting carbonyl attack over direct alkyne attack.