Synthesis of cedranoid sesquiterpenes. III. Functionalization at carbon 4

Abstract

Dimethyl 6,6-dimethyl-5,7-dioxobicyclo[2.2.2]oct-2-ene-2,3-dicarboxylate (8) on irradiation in acetophenone gives dimethyl 6,6-dimethyl-4,7-dioxotricyclo[3.2.1.02,8]octane-1,8-dicarboxylate (13), which on treatment with lithium dimethylcuprate followed by monodecarbomethoxylation gives methyl 4,4-endo-8-trimethyl-3,6-dioxo-cis-bicyclo[3.3.0]octane-1-carboxylate (17). Similar irradiation of dimethyl 4,6,6-trimethyl-5,7-dioxobicyclo[2.2.2]oct-2-ene-2,3-dicarboxylate (24) and its 7,7-ethylenedioxy derivative (25) followed by treatment with DBU and concentrated H2SO4, respectively, gives dimethyl 3-hydroxy-4,4,8-trimethyl-6-oxo-cis-bicyclo[3.3.0]-octa-2,7-diene-1,2-dicarboxylate (30). This, on acetylation, reduction with NaBH4/CeCl3, methanolysis, monodecarbomethoxylation, and hydrogenation, gives methyl endo-6-hydxoxy 4,4-endo-8-trimethyl-3-oxo-cis-bicyclo[3.3.0]octane-1-carboxylate (38), while on reduction with Li/NH3 followed by monodecarbomethoxylation it gives a methyl 6-hydroxy-4,4-exo-8-trimethyl-3-oxo-cis-bicyclo[3.3.0]octane-1-carboxylate (33).