Synthesis of organochalcogen propargyl aryl ethers and their application in the electrophilic cyclization reaction: an efficient preparation of 3-halo-4-chalcogen-2H-benzopyrans.J Org Chem. 2009 May 01; 74(9):3469-77.JO
We herein described the synthesis of various organochalcogen propargyl aryl ethers via reaction of lithium acetylide intermediate with electrophilic chalcogen (sulfur, selenium, tellurium) species. Various aryl and alkyl groups directly bonded to the chalcogen atom were used as electrophile. The results revealed that the reaction does not significantly depend on the electronic effects of substituents in the aromatic ring bonded to the chalcogen atom of the electrophilic chalcogen species. Additional versatility in this process was demonstrated with respect to a diverse array of functionality in the aromatic ring at propargyl aryl ethers. These propargyl aryl ethers, bearing the chalcogen group, underwent highly selective intramolecular cyclizations when treated with I(2) or ICl affording 3-iodo-4-chalcogen-2H-benzopyrans. The results demonstrated that the cyclization efficiency was significantly influenced by the steric effects of aromatic ring, since the cyclization reaction gave low yields with aromatic rings having a substituent at orto position than those having no substituent. The reactivity of 3-iodo-4-chalcogen-2H-benzopyrans was also studied. 4-Selenobutyl benzopyrans were treated under Neghishi cross-coupling conditions providing the corresponding 3-aryl benzopyran derivatives in good yields. In addition, using the copper catalyzed cross-coupling reactions with thiols, in the absence of any cocatalyst, we were able to introduce a thiol function in 3-iodo-benzopyran derivatives.