The five-component reaction of ethyl 2-[(2-oxopropyl) sulfanyl] acetate, aromatic aldehydes, and ammonium acetate affords two diastereomers of 5, 7-diaryl-5, 6, 7, 8-tetrahydro-1H- pyrido [3, 4-b][1, 4] thiazin-2 (3H)-ones via a novel tandem Mannich-enamine-substitution sequence. Presumably, they are generated from ethyl 2-[(4-oxo-2, 6-diaryl-3-piperidinyl) sulfanyl] acetates. During the formation of the trans-5, 7-diaryl-5, 6, 7, 8-tetrahydro-1H- ...