Resin acids. XXII. Unusual decarboxylation induced by a degenerate acyloin rearrangement

Abstract

Pyrolysis of a dihydroxylactone 2b, prepared by oxidation of the levopimarie acid-formaldehyde adduct 1, unexpectedly resulted in decarboxylation to methyl 13a-hydroxy-14-oxoabietan-18-oate (5) or to methyl 8 , 14adihydroxy-12-abieten-18-oate (8) depending on the conditions. Decarboxylation of methyl 12a-carboxy-13ahydroxy-14-oxoabietan-18-oate (4a) also gave 5. These remarkable and unusually facile decarboxylations were traced to a degenerate acyloin rearrangement in 13-hydroxy-14-oxoabietanes which involves reversible migration of the C-12-C-13 and C-8-C-13 bonds. In the methyl ester 4b, prepared by treatment of 2b with methanolic HC1, the stable orientation of the carbomethoxy group was shown to be axial. Transformations of 5 and 8 which shed light on the stereochemistry of various 14-oxygenated abietanes are described.