10.1002/hlca.19870700426
The study, titled "Stereoselective Alkylation at C(α) of Serine, Glyceric Acid, Threonine, and Tartaric Acid Involving Heterocyclic Enolates with Exocyclic Double Bonds," investigates the stereoselective alkylation of various chiral, non-racemic α-amino acids and their derivatives using heterocyclic enolates with exocyclic double bonds. The researchers converted these acids into methyl dioxolane, oxazoline, and oxazolidine carboxylates. These compounds were then deprotonated to form lithium enolates, which were stable enough to undergo alkylation with or without cosolvents like HMPA or DMPU. The products were obtained in good to excellent yields and with high diastereoselectivities, except for the tartrate-derived acetonide. The study demonstrated that the configuration of the products could be determined through NOE-NMR measurements and chemical correlation, revealing that the dioxolane-derived enolates were alkylated preferentially from the face already substituted, while the dihydrooxazol- and oxazolidine-derived enolates were alkylated from the opposite face. This work provides a method for constructing quaternary stereogenic centers without racemization, using readily available enantiomerically pure precursors like hydroxy- and amino-acids.
