Metal-Free One-Pot Annulative Coupling of 2-Hydroxybenzaldehydes with ß-Ketothioamides: Access to Diverse 2-Arylimino-2H-Chromenes.

A concise and practical one-pot sustainable approach for expedient synthesis of 2-arylimino-2H-chromenes by two-component cascade [4 + 2] annulative coupling of easily available 2-hydroxybenzaldehydes with ß-ketothioamides has been developed in good yields for the first time. Remarkably, metal- and additive-free conditions, use of simple K2CO3 as a mild base, open atmosphere, exclusive regioselectivity, step/atom economy, nonhazardous reagents, and easy purification are added characteristics to the strategy. This annulative protocol will not only provide an efficient method to access diverse chromene scaffolds, but also enrich the research domain of ß-ketothioamides.

Electrochemical Synthesis of 1,2,3-Thiadiazoles from α-Phenylhydrazones.

We have developed an electrochemical approach for the synthesis of fully substituted 1,2,3-thiadiazoles from α-phenylhydrazones at room temperature, which is very challenging and complementary to the conventional thermal reactions. The key step involves anodic oxidation of phenylhydrazone derivatives at a constant current followed by N,S-heterocyclization. The protocol is remarkable in that it is free of a base and free of an external oxidant and can be converted to a gram scale for postsynthetic drug development with functional thiadiazoles. Most importantly, the electrochemical transformation reflected efficient electro-oxidation with an operationally friendly easy procedure with ample functional molecules. Cyclic voltammograms support the mechanism of this electro-oxidative cyclization process.

Visible-Light Photocatalysis of Eosin Y: HAT and Complementing MS-CPET Strategy to Trifluoromethylation of ß-Ketodithioesters with Langlois' Reagent.

A metal- and oxidant-free photoinduced strategy for thioxo sulfur-selective trifluoromethylation of ß-ketodithioesters at room temperature is reported. Excellent Z/E-stereoselectivity has been achieved with cheap and viable Langlois' reagent (CF3SO2Na, sodium triflinate) in the presence of eosin Y, which acts as a hydrogen atom transfer (HAT) catalyst. The reaction proceeds via disulfide intermediate disulfanediylbis(3-(alkylthio)-1-phenylprop-2-en-1-one) (a dimer of ß-ketodithioester) followed by complementing proton-coupled electron transfer-mediated reverse HAT cycle of eosin Y. This operationally simple and efficient protocol allows direct access to triflinated α-oxoketene dithioacetals in good to excellent yields bearing diverse synthetically useful functional groups of different electronic and steric nature.

Radical-Cascade Avenue to Access 1,2-Dithioles Employing Dithioesters and Edman's Reagent.

An operationally simple and efficient domino etiquette has been developed for the facile construction of 1,2-dithioles employing easily accessible dithioesters as a three-atom CCS synthon and aryl isothiocyanates as a two-atom CS unit in the absence of any catalyst and additive at room temperature under open air. The reaction proceeded efficiently affording the desired 1,2-dithioles in good yields having various functional groups of a diverse electronic and steric nature. This approach avoids possible toxicity and tiresome workup conditions and features easy to handle, cheap, and readily accessible reagents, O2 as a green oxidant, and gram-scale ability. Notably, the final S-S bond formation and cascade ring construction follow a radical pathway, which has been recognized via a radical trapping experiment with BHT during the course of the reaction. Notably, the exocyclic C═N bond at position 3 of 1,2-dithiole possesses Z stereochemistry.

Photo-oxidative Ruthenium(II)-Catalyzed Formal [3 + 2] Heterocyclization of Thioamides to Thiadiazoles.

An operationally simple and sustainable one-pot photo-oxidative formal [3 + 2] heterocyclization of ß-ketothioamides with aryldiazonium salts catalyzed by Ru(bpy)3Cl2 has been realized to provide 2,4-disubstituted 5-imino-1,2,3-thiadiazoles in good to high yields under mild reaction conditions for the first time. The reaction proceeded via an α-phenylhydrazone adduct of thioamides leading to 1,2,3-thiadiazoles via N-S bond formation at room temperature. Notably, the products possess Z-stereochemistry with regard to the exocyclic C═N double bond at the 5-position of the ring.