Total synthesis of (±)-12-methoxyabieta-8,11,13-trien-6-one, a versatile intermediate for diterpene syntheses

Abstract

The title compound (1) has been synthesized from perhydro-4α-methoxy-5,5,8a-trimethylnaphthalen-1-one (12). The formyl derivative (14) was subjected to Robinson annelation with 4- diethylaminobutan-2-one methiodide to obtain the adduct (15) and this, on heating with sodium methoxide in methanol, yielded the tricyclic ketone (16). The enolate of (16), generated by treatment with lithium di-isopropylamide, reacted with acetone in the presence of anhydrous zinc chloride to afford an aldol (21) which, on heating with toluene-p-sulphonic acid in benzene, provided the dienone (22); subsequent treatment with sulphuric acid in methanol produced the dimethoxyabietatriene (6) whose conversion into the desired ketone (1) was accomplished by treatment with trichloromethylsilane and sodium iodide followed by oxidation of the resulting compound with Jones' reagent and then methylation with dimethylsulphate and alkali. Elimination of C-6 carbonyl group of the ketone (1) and subsequent oxidation with chromium trioxide and acetic acid yielded the ketone (3) which, on demethoxylation with silicon tetrachloride and sodium iodide, yielded sugiol (2).