Investigation On Visible-Light-Induced Dearomatization Of Indole/Furan Derivatives To Synthesize Sulfonyl Polycycles

Among the numerous types of drug molecules and biologically active natural products,most have a fused or spiro cyclic heterocyclic skeleton.For a long time,synthetic organic chemists have been committed to the dearomatization of aromatic compounds to construct these privileged frameworks.Among them,indole derivatives,as the most widely distributed alkaloid precursors in nature,their dearomative transformation has been concerned by many researchers.Since indole C3 is electron-rich and C2 is electron-deficient,there are two general reaction modes:（1）Ipso cyclization of the side chain with an electrophilic group at the C3 position to obtain a spiroindolenine;（2）Friedel-Crafts alkylation occurs when an external electrophilic reagent attacks the C3 position,and then the nucleophilic side chain at the C3 position is cyclized at the C2 position to obtain a polycyclic indoline.In recent years,the rapid development of visible-light-induced organic synthesis reactions has provided effective tools for constructing various molecular structures,however,these compounds are often difficult to prepare using traditional thermal reaction.Herein,we developed a visible-light-induced[2+2+1]dearomative cyclization reaction of indole/furan-alkynes using inorganic salt NaHSO₃ as a sulfonyl source,and synthesized a series of high strained sulfonyl polycyclic compounds.The specific content is as follows:This article mainly studies the single-electron oxidation of indole/furan-alkyne compounds by excited photocatalyst under the irradiation of blue LED,followed by intramolecular addition to obtain alkenyl radical intermediate,and then SO₂ in the system was captured by alkenyl radical.After cyclization to obtain a high strained sulfonyl polycyclic compound.We have synthesized four types of corresponding compounds with medium to excellent yields（up to 97%）.In addition,the mechanistic studies show that the intramolecular electron donor-acceptor pathway is also available.Unlike other types of radical dearomatization reactions,this method does not need introduce additional radical precursors—indole/furan can generate radicals by themself.Compared with the previous reactions,the single electron oxidation pathway reveals the potential reactivity of indole and furan rings,this method thus provides a useful,complementary method for achieving photoredox dearomatization reactions.