4,6-Dioxo-10-propyl-4H,6H-benzo[1,2-b:5,4-b']dipyran-2,8-carboxylic acid:
2 ml of benzyltrimethylammonium hydroxide was added to a solution of 18.3
g 2-propylresorcine and dimethyl acetylenedicboxylate in 500 ml of ethanol
and the mixture obtained was heated for 20 hours at reflux. The solution was
cooled, 30 g sodium hydroxide in 150 ml of water was added and all mass
was heated for 2 hours at reflux. On cooling the reaction mixture was poured
into 2 L of water and washed with ethyl acetate. The water layer was acidified
with conc. hydrochloric acid and a precipitate fallen was collected, washed with water, and dried in vacuum at 80°C to give 21 g cream-colored solid
product.
21 g above product was in portion added to 200 ml of chlorosulfonic acid by
ice cooling. The temperature of reaction was not higher 10°C. Then the
reaction mixture was heated for 2.5 hours at 50°C. On cooling it was poured
into 3 L of ice/water, the fallen precipitate was filtered off, washed with water
and dried in vacuum. The solid product was heated with 100 ml of ethanol, an
insoluble part was collected and boiled with 100 ml of ethyl acetate. Insoluble
product was centrifuged and dried in vacuum at 60°C. Yield of title product
hydrate 10.5 g. 4,6-Dioxo-10-propyl-4H,6H-benzo[1,2-b:5,4-b']dipyran-2,8-
carboxylic acid melted at 310°-311°C (deg).
