Total Syntheses and Biological Evaluation of Both Enantiomers of Several Hydroxylated Dimeric Nuphar Alkaloids

Herein, we describe the first total syntheses of five members of the dimeric nuphar alkaloids: (+)-6,6′-dihydroxythiobinupharidine (+)-1a, (+)-6-hydroxythiobinupharidine (+)-1-b, (-)-6,6′-dihydroxythionuphlutine (-)-2-a, (-)-6,6′-dihydroxyneothiobinupharidine (-)-3a, and (+)-6,6′-dihydroxyneothionuphlutine (+)-4-a. The latter two have not been found in nature. We have also made each of their enantiomers (-)-1a-b, (+)-2a, (+)-3a, and (-)-4a. The key step in these syntheses was the dimerization of an α-aminonitrile (a hydrolytically stable surrogate for its corresponding hemiaminal) with chiral Lewis acid complexes. We have also reassigned the literature structures of (+)-1-a-1b - for those instances in which the NMR spectra were obtained in CD₃OD - to their corresponding CD₃O-adducts. Our efforts provide for the first time apoptosis data for (-)-3a, (+)-4a, and all five non-natural enantiomers prepared. The data indicate high apoptotic activity regardless of the enantiomer or relative stereochemical configuration at C7 and C7′. Nuphars are speedy killers: For the first time several of the hydroxylated dimeric nuphar alkaloids and their unnatural enantiomers have been synthesized. First-ever apoptosis data for many of these compounds is provided.