Studies of samarium diiodide mediated tandem radical-ionic reactions.

摘要

This thesis describes the studies of samarium diiodide (SmI{dollar}sb2{dollar}) mediated tandem radical-ionic reactions. The first chapter reports a study toward synthesis of a penitrem D model 19. The key steps are two sets of tandem radical-ionic reactions for the formation of C- and E-ring of 19. The model study for the formation of E-ring showed that 5-exo-radical cyclization and the following reduction of the secondary radical by SmI{dollar}sb2{dollar} proceeds efficiently to give D-E-F-rings moiety of 19 in good yields. The model study for the formation of C-ring showed that the yield of the desired 6-exo-cyclization product was low, and the highest yield obtained so far is 50% by using the substrate 96 in benzene-HMPA. The dibromide 133 was prepared as a key precursor for the double tandem radical-ionic reactions; however, due to the reduction of benzenesulfonylamide group in 133, the desired product was not isolated. Stepwise formation of C- and E-rings gave 154 which corresponds B-E-rings moiety of 19. The overall yield from 133 to 154 is 7%. The second chapter reports mechanistic studies of some of SmI{dollar}sb2{dollar} mediated reactions: (1) TEMPO was introduced as an efficient quenching reagent for primary and secondary alkylsamarium intermediates; (2) A chiral HMPA analog 198 was prepared, and the reducing ability of SmI{dollar}sb2{dollar} in the presence of 198 was examined; (3) The rate of reduction of primary alkyl radical by SmI{dollar}sb2{dollar} was re-examined by using 5-hexenyl radical clock; (4) The effect of HMPA on the stability of primary alkylsamarium intermediates was studied; (5) The reaction of tertiary radical and SmI{dollar}sb2{dollar}-HMPA was studied.