γ-Hydroxyalkynyl Ketones as Useful Scaffolds for the Preparation of Combinatorial Libraries of Furans, Isoxazoles and Pyrazoles

Using 5-hydroxy-hex-3-yne-2-one (9) as a model substrate, the utility of the gamma-hydroxyl alkynyl ketone scaffold for the preparation of combinatorial libraries of furans, isoxazoles, and pyrazoles is described. The addition of hydrazoic acid to the acetylenic ketone 9 forms in a single step both 3-azido-2,5-dimethyl furan (10) and alpha,5-dimethyl-3-isoxazolemethanol (11). Reaction of a mixture of (E)- and (Z)-bromoenediones (5 and 6), accessible from 3-bromo-2,5-dimethyl furan (17), with 1,1-dimethylhydrazine afforded a nearly quantitative yield of 1,3-dimethyl-5-acetylpyrazole (18). When the (E)- and (Z)-vinyl bromides 5 and 6 were reacted with sodium azide, 3-acetyl-5-methylisoxazole (7) formed via the intermediate (Z)-3-azido-3-hexene-2,5-dione (4), was the only product.