Electroreductive Intramolecular Coupling of Chiral α-Imino Esters:  Stereoselective Synthesis of Mixed Ketals of cis-2,4-Disubstituted Azetidine-3-ones

The electroreduction of chiral aromatic α-imino esters prepared from (S)-α-amino acids, such as (S)-valine, (S)-leucine, and (S)-phenylalanine, in the presence of chlorotrimethylsilane and triethylamine afforded four-membered cyclized products, mixed ketals of cis-2,4-disubstituted azetidine-3-ones, stereospecifically (>99% de, 85−99% ee). The best result of the electroreductive cyclization was obtained using Bu₄NClO₄ as a supporting electrolyte and a Pt cathode. The absolute stereochemistry of the obtained single stereoisomers was confirmed to be 2R,3R,4S by X-ray crystallography. Calculations for the transition states of the cyclization support the stereospecific formation of the (2R,3R,4S)-isomers.