Synthetic studies with 7-functionalised norbornenes, and their synthesis by a silicon-controlled carbocation rearrangement

As part of a projected synthesis of gelsemine (1), the synthesis of dimethyl 7-oxobicyclo[2.2.1]hept-5-ene-2-endo,3-exo-dicarboxylate (5) is described. The route involves the silicon-controlled rearrangement of 5-exo-bromo-3-exo-methoxycarbonyl-7-anti-trimethylsilyicybicyclo[2.2.1]heptane-2,6-carbolactone (9), catalysed by silver ions in methanol, to give dimethyl 7-syn-hydroxybicyclo[2.2.1]hept-5-ene-2-endo,3-exo-d icarboxylate (11). Some related reactions show that a 7-trimethylsilyl group in a norbornene can be used to synthesise other 7-functionalised norbornenes. Thus the epoxide (13) of methyl 7-anti-trimethylsily bicyclo[2.2.1]hept-5-ene-2-endo-carboxylate (12) rearranges on treatment with boron trifluoride–ether to give methyl 7-anti-hydroxybicylo[2.2.1]hept-5-ene-2-exo-carboxylate (14), and treatment of (12) with bromine gives mainly the rearranged bromide, methyl 7-anti-bromobicyclo[2.2.1]hept-5-ene-2-exo-carboxylate (16). Some limitations in this approach to 7-functionalised norbornenes are also described.