Oxygen → Methyl rac-4,4-ethylenedioxy-1,2,3,4-tetrahydro-2-hydroxy-5,8-dimethoxynaphthalene-2-carboxylate

Participants

Reaction Steps

(ii)(b) A solution of the lithium enolate of methyl rac-4,4-ethylenedioxy-1,2,3,4-tetrahydro-5,8-dimethoxy-naphthalene-2-carboxylate was prepared in tetrahydrofuran as described in part (ii)(a) hereinbefore from 9.84 g of methyl rac-4,4-ethylenedioxy-1,2,3,4-tetrahydro-5,8-dimethoxynaphthalene-2-carboxylate. The enolate was added over a period of 5 minutes at -78° C. to a stirred solution of 11.2 ml of dry triethylphosphite in 60 ml of tetrahydrofuran through which a rapid stream of oxygen was passing. The passage of oxygen was maintained for 50 minutes and the temperature was held at -78° C. The reaction was then quenched by the addition of 8.8 ml of acetic acid. The cooling bath was removed and, after 5 minutes, 200 ml of water were added. After a further 20 minutes, most of the tetrahydrofuran was evaporated and the product was extracted into four 100 ml portions of ethyl acetate. The combined ethyl acetate extracts were washed with 200 ml of 10% aqueous potassium hydrogen carbonate, dried over magnesium sulphate, filtered and evaporated to give a yellow oil which was dissolved in 130 ml of ether and left to crystallise. There were obtained 6.86 g (66%) of methyl rac-4,4-ethylenedioxy-1,2,3,4-tetrahydro-2-hydroxy-5,8-dimethoxynaphthalene-2-carboxylate in the form of colourless crystals of melting point 74°-75° C.

Provenance & Evidence