Selective oxidation of a keramaphidin B model

A crucial step in the Baldwin and Whitehead proposal for explaining the biogenesis of the marine alkaloid manzamine A is the selective oxidation of natural keramaphidin B to an iminium salt 3, which is then hydrolyzed to give the aldehyde 4. Conditions are now presented in which this selective oxidation can be performed on model compound a, leading to the iminium salt 1.8. Although this salt can be considered as a model equivalent of the proposed aldehyde intermediate 4, it was found to be very resistant to hydrolysis as was the corresponding amide 20. From a synthetic point of view, the reported results illustrate the usefulness of the temporary protection of tertiary amines as aminoborane derivatives and constitute a good method for the oxidation of a sterically hindered tertiary nitrogen atom in the presence of a second nitrogen.