Thioarylation of 6‐Amino‐2,3,6‐trideoxy‐d‐manno‐oct‐2‐ulosonic Acid (IminoKdo): Access to 3,6‐Disubstituted Picolinates and Mechanistic Insights

Abstract

In this work, we present a metal‐free coupling protocol for the regio‐ and stereoselective C3‐thioarylation of 6‐amino‐2,3,6‐trideoxy‐d‐manno‐oct‐2‐ulosonic acid (iminoKdo). The developed procedure enables the coupling of electron‐rich, electron‐deficient, and hindered arylthiols, providing a series of C3‐modified iminoKdo derivatives in moderate to good yields. Elucidation of active species through controlled experimental studies and time‐lapse 31P NMR analysis provides insights into the reaction mechanism. We demonstrate that, following a tandem Staudinger/aza‐Witting reaction of an azido‐containing keto ester, an inseparable equimolar mixture of imine/enamine is formed. The enamine then undergoes a Stork‐like nucleophilic attack with the in situ‐formed disulfide reagent, resulting in the formation of the coupling products. Additionally, we describe a rarely reported acid‐promoted aromatization of the C3‐thioarylated iminoKdo skeleton into 3,6‐disubstituted picolinates, which are reminiscent of dichotomines.