On the Allylic Oxidation of Ent -kaurenic Acid Methyl Ester with Lead Tetra-acetate

In this paper, results obtained upon treatment of the methyl ester of ent-kaur-16-en-19-oic acid (1b) with lead tetra-acetate under reflux in glacial acetic acid solution and an argon atmosphere are described. After 30 minutes reflux, GC-MS analysis of the product indicated the presence of three substances, the initial compound (1b, 7 %), the methyl ester of ent-kaur-16-en-15-O-acetyl-19-oic acid (2a, C₂₃H₃₀0₄, 45 %), and its isomer, the methyl ester of ent-kaur-15-en-17- 0-acetyl-19-oic acid (3a, 47 %). Flash chromatography over silica gel containing 20% AgNO₃ yielded 2b (456 mg), which was identified by direct comparison with an authentic sample, and 3a (472 mg), whose structure was established by uni- and bi-dimensional NMR analysis. Hydrolysis of 3a with dry ammonia in MeOH yielded the methyl ester of ent-kaur-15-en-17-hydroxy-19-oic acid (3b, C₂₁H₃₂ 0₃); treatment of this compound with chromic acid/pyridine complex rendered the aldehyde (4).