Stereoselective Transformation of 2H-1,4-Oxazin-2-ones into 2,(2),5,5-Tri- and Tetrasubstituted Analogues of cis-5-Hydroxy-2-piperidinemethanol and cis-5-Hydroxy-6-oxo-2-piperidinecarboxylic Acid

2,(2),5,5-Tri- and tetrasubstituted analogues of 5-hydroxy-2-piperidinemethanol and 5-hydroxy-6-oxo-2-piperidinecarboxylic acid have been prepared via Diels–Alder reaction of 3-substituted 2H-1,4-oxazin-2-ones with ethene followed by functional group transformation of the resulting imidoyl chloride and lactone groups. Some of the 2-piperidinemethanol products are converted further into potential substance P antagonists.