Himgaline

Condensation of (S)-3-aminobutan-1-ol with 1,3-cyclohexane-dione followed by an intramolecular alkylation afforded bicyclic enamine 32, which was converted into enone 35 through a diastereoselective hydrogenation. Mukaiyama-Michael addition of a bicyclic silyl enol ether to 35 and subsequent stereochemistry inversion by means of an oxidation/reduction strategy provided lactone 41. After reduction of lactone 41 with LAH, Swern oxidation was carried out to give enone 46 upon a spontaneous intramolecular aldol reaction and cleavage of the ketal protecting group. SmI(2) -mediated carbonyl-alkene reductive coupling of 46 proceeded smoothly in refluxing tetrahydrofuran to deliver pentacyclic intermediate 49, which was oxidized with 2-iodoxybenzoic acid and then treated with trifluoroacetic acid to furnish (-)-GB 13. The overall yield was 6.1% over 19 linear steps. By following the known procedure, our synthetic (-)-GB 13 was converted into himgaline. In addition, by starting from lactone 41, the first total synthesis of (+)-GB 16, a newly isolated member of the gabulimima alkaloid family, was achieved. This synthesis features an intramolecular condensation between an amine and a 1,3-diketone moiety.

Neuroactive metabolites from the bark of Galbulimima belgraveana occur in variable distributions among trees and are not easily accessible through chemical synthesis because of elaborate bond networks and dense stereochemistry. Previous syntheses of complex congeners such as himgaline have relied on iterative, stepwise installation of multiple methine stereocenters. We decreased the synthetic burden of himgaline chemical space to nearly one-third of the prior best (7 to 9 versus 19 to 31 steps) by cross-coupling high fraction aromatic building blocks (high Fsp2) followed by complete, stereoselective reduction to high fraction sp3 products (high Fsp3). This short entry into Galbulimima alkaloid space should facilitate extensive chemical exploration and biological interrogation.

A full account of our total synthesis of the galbulimima alkaloids GB 13 and himgaline is provided. Using a strategy adapted from the proposed biosynthesis of the GB alkaloid family, a linear precursor underwent successive intramolecular Diels-Alder, Michael, and imine aldol cyclizations to form the polycyclic alkaloid core. We now show that modification of this strategy can also deliver an advanced intermediate en route to the related alkaloid himandridine. The success of the key imine aldol cyclization is acutely sensitive to substrate structure and solvent, including a case in which cyclization was spontaneous in protic solvents. A detailed computational investigation of the course of the reaction closely correlates with, and suggests a rationale for, the observed patterns of imine aldol reactivity.