A solution of 4-(4-acetyl-8-hydroxytridecyl)-benzoic acid (3.62 g., 10 millimole), Example 1, Step F, triethylamine (1.74 ml., 12.5 millimole), and distilled water (18 ml., 1.0 mole) in acetonitrile (100 ml.) is treated with N-t-butyl-5-methylisoxazolium perchlorate (3.0 g., 12.5 millimole). The resulting solution is evaporated in vacuo (water aspirator) at 20°-23° C. for 4 hours providing a tacky residue which is triturated with water (150 ml.) at 0°-5° C. for 15 minutes. After decanting the aqueous phase, the oily residue is dissolved in benzene-ether (1:1) (200 ml.). The organic extract is dried over sodium sulfate, filtered, and evaporated in vacuo at 35°-40° C. providing the desired active ester, N-t-butyl-3-[4-(4-acetyl-8-hydroxytridecyl)benzoyloxy]crotonamide, as a pale yellow oil.
