4-O-tert-butyl 1-O-(2-hydroxyethyl) 2,3-dimethylbutanedioate;4-O-tert-butyl 1-O-[3-(methanesulfonamido)-3-oxopropyl] 2,3-dimethylbutanedioate;3-[[2,3-dimethyl-4-[(2-methylpropan-2-yl)oxy]-4-oxobutanoyl]amino]propanoic acid;3-[2-[2,3-dimethyl-4-[(2-methylpropan-2-yl)oxy]-4-oxobutanoyl]oxyethoxy]propanoic acid;3-[2,3-dimethyl-4-[(2-methylpropan-2-yl)oxy]-4-oxobutanoyl]oxypropanoic acid;1-ethoxyethyl 4-(methanesulfonamido)-2,3-dimethyl-4-oxobutanoate