Lewis Acid-Promoted Generation of an Iminium ion from Ethyl 2-(N-Benzyloxycarbonylamino)-methylene-3-oxobutanoate and Its Reactions with Ketene Silyl Acetals - Saito, Seiki; Uedo, Eiji; Kato, Yoshitaka; Murakami, Yoshinori; Ishikawa, Teruhiko (Synlett, 1996)