Rearrangement of methyl 11,12-di-O-methyl-6,7-didehydrocarnosate in basic medium. Easy hemisynthesis of miltirone

Abstract

Methyl 11,12-di-O-methyl-6,7-didehydrocarnosate 3, obtained from the abundant natural product carnosol 1, undergoes an interesting rearrangement when treated with potassium tert-butoxide in dimethyl sulfoxide to give 11,12-dimethoxy-20(10→7)abeo-abieta-5(10),8,11,13-tetraen-20-oic acid 4, in which an additional double bond is formed between C-5 and C-10 and the carboxylic acid group has migrated from C-10 to C-7. Deprotection of the two methyl ether moieties in 4 with BBr₃ allows spontaneous air oxidation and decarboxylation of the catechol derivative to give the potent benzodiazepine agonist miltirone 10. The structure of 6 has been unequivocally elucidated by X-ray diffraction analysis and indirect chemical correlation between 4 and 6 has been established.