STRUCTURE SYNTHESIS OF (+)-OBLIQUIN

With hydrobromic acid, (+)-obliquin (II) gives mainly the hydrobromide (IV; R = Br) together with a little of a hemiketal (VIII) that is tautomeric with 6-hydroxy-7-(3-methyl-2-oxobutoxy)coumarin (IX: R = H), the orientation of which is established spectroscopically and by synthesis. Etherification of 6,7-dihydroxycoumarin with 3,3-dimethylallyl bromide gives 6-hydroxy-7-(3,3- dimethylallyloxy)coumarin, identical with the natural coumarin, prenyletin, and the isomeric 6-(3,3-dimethylallyl) ether. Epoxidation of prenyletin acetate affords an oxide (XVI) which is transformed by sodium iodide and 1-iodopropane in dimethyl sulphoxide into the acetate of the oxobutoxycoumarin and thence by hydrolysis into the hemiketal (VIII). Alkaline hydrolysis of the oxide (XVI) leads to a racemic alcohol (XVIII), and a parallel series of reactions leads to the alternative alcohol (XX): hydration of (+)-obliquin with sulphuric acid gives an optically active alcohol corresponding to the former racemate only. The racemic alcohol (XVIII) is resolvable through the (-)-menthyloxyacetate, and the resulting (+)-alcohol is identical with that prepared from (+)-obliquin. Dehydration with camphorsulphonyl chloride yields (+)-obliquin identical with the natural compound.