Eschenmoser coupling reactions starting from primary thioamides. When do they work and when not?

Synthesis of hydroxy-thiazoline substituted pyridine derivatives via [3 + 2] annulation of 1,4-dithiane-2,5-diol with cyanopyridine

A series of hydroxy-thiazoline substituted pyridine compounds were synthesized via the annulation of 1,4-dithiane-2,5-diol with cyanopyridine catalyzed by organic bases. The yields could reach up to 95%. The reaction required no solvent, and the products were obtained from raw materials and catalysts simply by grinding the mixture at room temperature for 10 min. The reaction could be well tolerated by variously substituted cyanide compounds. The universal applicability of this method was proven by gram-scale reaction and product derivatization.

Autocatalysis in Eschenmoser Coupling Reactions

The Eschenmoser coupling reaction (ECR) of thioamides with electrophiles is believed to proceed via thiirane intermediates. However, little is known about converting the intermediates into ECR products. Previous mechanistic studies involved external thiophiles to remove the sulfur atom from the intermediates. In this work, an ECR proceeding without any thiophilic agent or base is studied by electrospray ionization-mass spectrometry. ESI-MS enables the detection of the so-far elusive polysulfide species Sn , with n ranging from 2 to 16 sulfur atoms, proposed to be the key species leading to product formation. Integrating observations from ion mobility spectrometry, ion spectroscopy, and reaction monitoring via flow chemistry coupled with mass spectrometry provides a comprehensive understanding of the reaction mechanism and uncovers the autocatalytic nature of the ECR reaction.