6.12 g. of the sodium salt of antibiotic X-537A was dissolved in pyridine (10 ml.) by warming slightly. The solution was cooled in an ice bath and treated with propionyl chloride, then allowed to stand at room temperature for 2 hours. The mixture was poured into water, extracted with ethyl acetate, and the organic phase washed in succession with 1N hydrochloric acid (several times) and sodium carbonate solution (twice). Evaporation of the ethyl acetate solution left a residue which was crystallized from methylene chloride/hexane to afford 3-methyl-2-propionyloxy-6-{7-ethyl-4-hydroxy-3,5-dimethyl-6-oxo- 7[5-ethyl-3-methyl-5-(5-ethyl-5-hydroxy-6-methyl-2-tetrahydropyranyl)-2-tetrahydrofuryl]heptyl}benzoic acid, sodium salt, m.p. 185°-186.5° [α]D25 -20.00° (C=1.00 in methanol).
