Enantioselective total synthesis of the macrocyclic spermidine alkaloid (-)-oncinotine

The macrocyclic spermidine alkaloid (-)-oncinotine (1), isolated from Oncinotis nitida (Apocynaceae), was synthesized enantioselectively for the first time based on intramolecular iminium ion cyclization utilizing enantiomerically pure (2S)-N-[(benzyloxy)carbonyl]-2-piperidineacetaldehyde (8) as a chiral starting material. The required 8 was derived from the erythro adduct 16, which was obtained by diastereoselective 1,3-dipolar cycloaddition between 2,3,4,5-tetrahydropyridine 1-oxide (4) and (3S)-3-[(tert-butyldiphenylsilyl)oxy]-4-methyl-1-pentene (15). Wittig condensation of 8 with [8-(methoxycarbonyl)octyl]triphenylphosphonium iodide (21) followed by saponification provided the chiral piperidine moiety 23, which was coupled with the N-propyl-1,4-butanediamine segment 29 by using diethoxyphosphoryl cyanide in the presence of triethylamine to afford the tertiary amide 30. Conversion of 30 to the aldehyde 34 via desilylation and Swern oxidation, followed by hydrogenation over a palladium hydroxide catalyst under high dilution led to in situ formation of the transient iminium ion 35, which was further hydrogenated to form 33 in a single operation. Subsequent removal of the Boc protecting group resulted in (-)-oncinotine (1).