N-(4'-Chlorophenyl)-2-ethoxymethylene-3-oxobutanamide (1.3 g) and tris (6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedianato)ytterbium (0.26 g) were stirred in toluene (10 ml) whilst 1,1-diethoxyprop-1-ene (2 g) was added dropwise. The mixture was stirred at room temperature for two hours and evaporated to dryness. The residual gum was purified by medium pressure liquid chromatography on K60 silica using ethyl acetate/petroleum ether b.p. 60°-80° C., 15/85 v/v as eluting solvent to give two stereoisomers. The first isomer after trituration with n-hexane gave trans-N-(4'-chlorophenyl)-2,2-diethoxy-3,6-dimethyl-4-ethoxy-3,4-dihydro-2H-pyran-5-carboxamide (0.2 g), m.p. 104°-106° C. The second isomer, again after trituration with n-hexane, gave cis-N-(4'-chlorophenyl)-2,2-diethoxy-3,6-dimethyl-4-ethoxy-3,4-dihydro-2H-pyran-5-carboxamide (1.3 g), m.p. 125°-127° C.
