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Optimal conditions for enzymatic esterification of betulinic acid (5) with acid anhydride [125]. Reagents and conditions: (i) phthalic anhydride (1.11 eq.), K2CO3, celite 545, Novoenzym 435 (145.6 mg), and chloroform:hexane/(1:1), 53.9 °C, 20.3 h.
