A solution of compound C (10 g), methyl propionylacetate (11.5 g) and p-toluenesulphonic acid (0.1 g) in benzene (40 ml) was heated under reflux with azeotropic removal of water for 20 hours. Volatile material was removed by evaporation and the residue added to a refluxing eutectic mixture of 26.5% v/v diphenyl and 73.5% v/v diphenyl oxide (30 ml). The solution was heated under reflux for 1 hour, cooled and diluted with hexane (100 ml). The precipitated solid was filtered off and triturated with hot methanol (100 ml) to give 2-ethyl-6-[2-(trimethylsilyl)ethoxy]-1,5-naphthyridin-4-(1H)-one (D) (in 62% yield) as a white solid, m.p. 193° C.; NMR: -0.1(s,9H), 0.85(t,2H), 1.3(t,3H), 2.8(q,2H), 4.1(t,2H), 6.55(s,1H), 6.9(d,1H), 8.2(d,1H).
