Stereoselective Olefin Cyclisations of tert-a-Acyliminium Ions

Tert a-acyliminium ions are shown to exhibit excellent properties for inducinq stereoselective olefin Cy~lisations.Startinq from the appropriate alkene precursor bicyclic and tetracyclic ring systems are formed in qood to excellent yields.In the last few years a-acyliminium ion intermediates were found to operate as remarkably qood initiating centres for stereoselective olefin cyclisations.'Recently the utility of = a-acyliminium ions was demonstrated in the stereocontrolled synthesis of perhydrohistrionic~toxin.~In this communication more qeneral results are communicated on the exnloration of tert n-ncyliminium ion initiated olefin cyclisations.Nucleophilic substitution reactions at the imidecarbonyl have been shown to result in mixtures of tertiary hydroxylactams and the correspondinq tautomeric open keto amides.As Chiron and ~r a f f 3b'c have demonstrated, the tautomeric equilibrium +=g is among other factors dependinq on the nature of the N-substituent.Under the conditions investiqated the cycllc form $ is a necessary startinq point for the cyclisation sequence A + C + D ; therefore useful synthetic results are only expected if the formation of C favourably competes with the rate of interconversion between