Synthesis of 6-methoxy-1,1,10-trimethyl-1,2,3,4-tetrahydroanthracene, a synthetic intermediate of a linear abietane diterpene, umbrosone

Abstract

Methyl 13-acetyl-12-methoxy-8,11,13-podocarpatrien-18-oate, prepared from (+)-dehydroabietic acid, was converted into methyl 12-methoxy-7-oxo- 5,8,11,13-podocarpatetraen-18-oate (9) by a series of reactions: haloform reaction, decarboxylation, Jones oxidation, and dehydrogenation. Reduction of 9 with sodium borohydride, followed by treatment of the resulting 7-hydroxy compound with boron trifluoride etherate, afforded a rearranged ester, methyl 6-methoxy-1,10-dimethyl-1,2,3,4-tetrahydroanthracene-1-carboxylate. This was further converted into 6-methoxy-1,1,10-trimethyl-1,2,3,4- tetrahydroanthracene (14) by means of the following reactions: lithium aluminum hydride reduction, pyridinium chlorochromate oxidation, and Huang- Minlon reduction. Compound 14 was finally converted into the desired orthoquinone (1) according to the procedure of Ghosh and Ghatak. The synthetic 1 was shown to he identical with natural umbrosone by spectral comparisons.