Here, (1R,4S)-4-aminocyclopent-2-ene-1-carboxylic acid is converted into the corresponding methyl ester by treatment with thionyl chloride in methanol. Methyl (1R,4S)-4-(2,5-dimethyl-1H-pyrrol-1-yl)cyclopent-2-ene-1-carboxylate is then synthesized by reacting the (1R,4S)-4-aminocyclopent-2-ene-1-carboxylate methyl ester with 2,4-pentanedione in the presence of DIEA. The following alkylation of methyl (1R,4S)-4-(2,5-dimethyl-1H-pyrrol-1-yl)cyclopent-2-ene-1-carboxylate with iodopropane is carried out using lithium bis-(trimethylsilyl)amide as a base. The resulting alkylated product is then hydrolyzed to form the (1S,4S)-4-(2,5-dimethyl-1H-pyrrol-1-yl)-1-isopropylcyclopent-2-ene-1-carboxylic acid 4.
