ChemInform Abstract: Highly Diastereoselective, Nucleophilic Alkenoylation of Enantiopure . alpha.-Oxy- and α-Aminoalkanals by Lithiated (1-(p- Toluenesulfonyl)alk-2-enyl) Carbamates. Revision of the Stereochemistry and Application to the Synthesis of a Dihydroxyethylene Dipeptide Isostere.