Experiments on the total synthesis of Lysolipin I. Part II. Michael addition of 1,3‐cyclohexanedione to quinone acetals

Abstract

AbstractBase‐catalyzed reaction of 1,3‐cyclohexanedione (3) with the quinone monoacetals 4 and 7 leads to the polycyclic products 5 and 8, respectively, and in the case of 4 to variable amounts of dibenzofuranone 6. The 2‐arylcyclohexanedione 9, on the other hand, is isolated from the reaction of 3 and bisacetal 11 catalyzed by ZnCl2 (Scheme 2). Treatment of the adduct 8 with (CH3O)2SO2/K2CO3 results in cleavage of teh heterocyclic ring by a retro‐Michael reaction affording teh liable enone 23 which was further transformed to 24 by selective hydrogenation. The 8‐acetoxydibenzofuranone 22 is obtainable from 8 by acid treatment and acetylation (Scheme 4). The reactions of the silylenol ethers 27 and 35 with quinone monoacetals were very complex (Scheme 6). The desired arylcyclohexanone derivatives 28 and 36 were formed in very low yields. Under certain conditions (elevated temperature or strong Lewis acids as catalysts), single‐electron transfer or addition to the ene‐acetal rather than to the enone function of the quinone monoacetals became predominant. In connection with this study, the sensitive 2‐methoxy‐p‐benzoquinone monoacetals 15 (Scheme 3) and 29 (Scheme 6) have been prepared and characterized.