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18-Methoxy-18-oxooctadecanoic acid is a long alkane linker with a terminal carboxylic acid group. The terminal carboxylic acid group can react with primary amine groups in the presence of activators (e.g. HATU) to form a stable amide bond.

The general procedure for the synthesis of monomethyl octadecanedioate from dimethyl octadecanedioate was as follows: compound 8 (5.8 mmol) was added to 0.07 M Ba(OH)? solution in anhydrous methanol (85 mL) and the reaction mixture was stirred at 40 °C for 24 hours. After completion of the reaction, the mixture was filtered under reduced pressure. The resulting precipitate was washed with ether and dissolved in deionized water. Subsequently, the solution was acidified to pH 3 with hydrochloric acid and extracted twice with dichloromethane. The organic phases were combined, the solvents were removed by vacuum evaporation and the resulting residue was purified by fast column chromatography using petroleum ether:ethyl acetate (8:2, v/v) as eluent to give the final compound 9 in 52% yield.

[1] Journal of Lipid Research, 2015, vol. 56, # 1, p. 129 - 141
[2] Indian Journal of Chemistry, Section B: Organic Chemistry Including Medicinal Chemistry, 1979, vol. 17B, p. 396