A solution of compound C (89.5 g), methyl propionylacetate (32.8 g) and p-toluenesulphonic acid (1.0 g) in cyclohexane (900 ml) was heated under reflux with azeotropic removal of water for 20 hours. Volatile material was then removed from the reaction mixture by evaporation and the residue was dissolved in ethyl acetate (1 l). The solution was washed with saturated sodium hydrogen carbonate solution (500 ml), water (500 ml), saturated sodium chloride solution (500 ml), and then dried (MgSO4). The solvent was removed by evaporation and the residue dissolved in a eutectic mixture of 26.5% v/v diphenyl and 73.5% v/v diphenyl oxide (750 ml). The solution was heated to 260° C. and the temperature maintained for 30 minutes. The solution was cooled and diluted with hexane (1 l) to give 6-[2-(t-butyldiphenylsilyl)oxyethoxy]-2-ethyl-1,5-naphthyridin-4(1H)-one (D) (55.4 g), as a white solid, m.p. 209°-210° C.; NMR (d6 -DMSO): 1.1(s,9H), 1.35(t,3H), 2.8(q,2H), 4.15(t,2H), 4.65(t,2H), 6.3(br s,1H), 7.25(d,1H), 7.5-7.6(m,7H), 7.65-7.8(m,4H), 8.1(d,1H).
