The stereochemistry of the reduction of cyclic enaminones

The stereo- and regiochemistry of di-, tri-, and tetracyclic enaminones upo n catalytic hydrogenation on Pd and Pt catalysts seems to be mainly a funct ion of the catalyst and the medium. The highest stereoselectivity was obser ved for multiflorine on Pd/C in which 99% of equatorial alcohol were formed in this case, the formation of alcohols proceeds via a ketonic intermediat e. On platinum, irrespective of the solvent used (EtOH, H2O, AcOH, HCl), th e hydrogenation reaction proceeds through ketonic (piperidone system) and d ehydro (pyridone system) intermediates. In EtOH or H2O solution, the dehydr o product remains unchanged, whereas the ketonic intermediate is reduced to a mixture of epimeric alcohols. In HCl and acetic acid, both intermediates are hydrogenolyzed to a product with a methylene group, but the ketonic on e is additionally reduced to a mixture of epimeric alcohols. Reductions wit h complex metal hydrides provide mixtures of epimeric alcohols with a predo minance of equatorial orientation. The structures of products were determin ed by NMR spectroscopy and/or by GC-MS analysis.