trans-4-ethyltetrahydro-5-oxo-3-furoic acid

(3R,4R)-4-ethyl-5-oxooxolane-3-carboxylic acid

(3R,4R)-4-ethyl-5-oxo-oxolane-3-carboxylic acid

(3R,4R)-4-ethyl-5-keto-tetrahydrofuran-3-carboxylic acid