Theoretical Study, Synthesis, and Reactivity of Five-Membered-Ring Acyl Sulfonium Cations.

The feasibility of the cyclization of γ-alkylthiobutyric acid derivatives to form previously unknown five-membered-ring acyl sulfonium cations was studied. Experimental results were in good agreement with our DFT calculations that predicted such cyclizations to be easy if starting with acyl iodides and mixed anhydrides of triflic acid. Particularly efficient were the reactions of γ-alkylthiobutyryl fluorides with trimethylsilyl triflate in CDCl₃ solution, which led to cyclic acyl sulfonium triflates. In some cases, the observed acyl sulfonium salts were stable enough to be characterized by NMR spectroscopy. They were found to be both alkyl-transfer reagents and acylating agents. They react with amines to form amides. These findings lend some weight to our hypothesis that the acyl sulfonium derived from methionine may have played a major role in the prebiotic synthesis of the first peptides on the primitive Earth. [ABSTRACT FROM AUTHOR]