Synthesis of cedranoid sesquiterpenes. II. Functionalization at carbon 12

Abstract

Dimethyl 6,6-dimethyl-7-oxotricyclo[3.2.1.02,8]octane-1,8-dicarboxylate (6) undergoes homoconjugate addition with lithium diphenylcuprate to give dimethyl 3-hydroxy-4,4-dimethyl-exo-8-phenyl-cis-bicyclo[3.3.0]oct-2-ene-1,2-dicarboxylate (12a), which on monodecarboxymethylation, ozonolysis, and esterification with diazomethane gives dimethyl 6,6-dimethyl-7-oxo-cis-bicyclo-[3.3.0]octane-1,exo-2-dicarboxylate (15). Reaction of 6 with diethylaluminum cyanide gives dimethyl exo-8-cyano-3-hydroxy-4,4-dimethyl-cis-bicyclo[3.3.0]oct-2-ene-1,2-dicarboxylate (16), which on hydrolysis and esterification with diazomethane also gives 15. Sensitized photolysis of dimethyl 5-oxo-6,6-dimethylbicyclo[2.2.2]octa-2,7-diene-2,3-dicarboxylate (19) gives dimethyl 4,4-dimethyl-3-oxotricyclo[3.2.1.02.8]oct-6-ene-1,8-dicarboxylate (20), which on reduction with lithium in ammonia gives dimethyl 6,6-dimethyl-7-oxobicyclo[3.3.0]oct-3-ene-1,exo-2-dicarboxylate (31); conversion of 31 to its 2-ene isomer 32 followed by catalytic hydrogénation gives 15 and its endo-2 epimer 33.