N-Ethoxycarbonyl-2-ethoxymethylene-3-oxobutanamide (2.3 g) was dissolved in toluene (15 ml) and treated dropwise with 1,1-di-ethoxyprop-1-ene (2 g). The mixture was stirred at 40° C. for 2 hours and evaporated to dryness. The residual oil was purified by medium pressure liquid chromatography on K60 silica using ethyl acetate/petroleum ether b.p. 60°-80° C. 15/85 v/v and 20/80 v/v as eluting solvents, to give after trituration with n-hexane trans-2,2-diethoxy-3,6-dimethyl-4-ethoxy-N-ethoxycarbonyl-3,4-dihydro-2H-pyran5-carboxamide (0.6 g) m.p. 84°-7° C. The n.m.r. spectrum in deuterochloroform displayed the --CH(OEt)-- signal at 3.9 ppm (doublet of quartets, 1H J=3 Hz and 1 Hz), which is indicative of the trans arrangement of the 3 and 4 substituents.
