Syntheses of 3-(2,4-dimethyl-5-oxocyclopent-1-enyl)propionic acid and 2-methyl-3-oxo-4-isopropylidenecyclopentanecarboxylic acid
2-Methyl-2-prop-2-ynylcyclohexane-1,3-dione (8), prepared by alkylation of the potassium salt of 2-methylcyclohexane-1,3-dione (7) with prop-2-ynyl bromide in dimethylformamide, gave 2-acetonyl-2-methylcyclohexane-1,3-dione (1) on treatment with mercurated Amberlite IR 120 resin in methanol. An attempt to achieve the same result by use of boron trifluoride–ether complex and red mercury(II) oxide in methanol gave methyl 4-(3,5-dimethyl-2-furyl)butyrate (9). Attempts to cyclodehydrate compound (1) with aqueous 5% potassium hydroxide or potassium t-butoxide furnished an isomeric acid which spectral data indicated could be either 3-(2,4-dimethyl-5-oxocyclopent-1-enylpropionic acid (2) or 2-methyl-4-isopropylidene-3-oxocyclopentane-carboxylic acid (4). Both these acids were synthesised independently and the product of alkali cleavage was shown to be compound (2).