Highly Diastereoselective, Nucleophilic Alkenoylation of Enantiopure α-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