Sulfur-Mediated Decarboxylative Thioamidation of α,β-Unsaturated Carboxylic Acids to Lactam-Derived Thioamides

An efficient and practical protocol has been developed for the synthesis of γ-lactam- and ε-caprolactam-derived thioamides through sulfur-mediated decarboxylative coupling of α,β-unsaturated carboxylic acids with DBN or DBU, accompanied by amidine ring opening. The transformation employs readily available and inexpensive starting materials and proceeds under transition-metal- and external-oxidant-free conditions. Control experiments and mechanistic studies support the involvement of sulfur-derived polysulfide intermediates and subsequent amidine ring opening. The operational simplicity and the use of elemental sulfur make this protocol a useful approach to lactam-derived thioamides.