Construction of carbocyclic arrays containing nitrogen via intramolecular imino Diels-Alder reactions in polar media. A comparative study: 5.0 M lithium perchlorate-diethyl ether versus water

The intramolecular Diels-Alder reaction of iminium ions has been examined in polar media such as 5.0 M lithium perchlorate-diethyl ether and water. Cycloaddition of 3 in 5.0 M, lithium perchlorate-diethyl ether containing 10 mol % camphorsulfonic acid proceeds, not by in situ generation of iminium ion 1, but rather via N-(acyloxy)iminium ion 7 which subsequently cyclizes to tricyclic compounds 4 and 5. Direct formation of imines 15 and 30 was realized by reduction of the corresponding lactams (13 and 29, respectively) followed by exposure to 2.0 equiv of tetra-n-butylammonium fluoride. Exposure of the trifluoroacetic acid salt 18 of imine 15 to 5.0 M lithium perchlorate-diethyl ether at ambient temperature gave rise to tricyclic amine 16 in which the diene underwent isomerization prior to [4+2] cycloaddition. In contrast, use of water provided tricyclic amine 19. Similarly, exposure of iminium salt 31 to water afforded tricyclic amine 32. The polar sovent of choice for intramolecular imino Diels-Alder reactions employing substrates such as 18 and 31 is water.