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

Abstract

Herein, we describe the first total syntheses of five members of the dimeric nuphar alkaloids:(+ +)-6,6'-dihydroxythiobinupharidine (+ +)-1a,( + +)-6-hydroxythiobinupharidine (+ +)-1b,( À)-6,6'-dihydroxythionuphlutine (À)-2a,( À)-6,6'dihydroxyneothiobinupharidine (À)-3a,a nd (+ +)-6,6'-dihydroxyneothionuphlutine (+ +)-4a.T he latter two have not been found in nature.W eh ave also made each of their enantiomers (À)-1a-b,(+ +)-2a,(+ +)-3a,and (À)-4a.The key step in these syntheses was the dimerization of an a-aminonitrile (a hydrolytically stable surrogate for its corresponding hemiaminal) with chiral Lewis acid complexes.W eh ave also reassigned the literature structures of (+ +)-1a-1b-for those instances in whicht he NMR spectra were obtained in CD 3 OD-to their corresponding CD 3 O-adducts.O ur efforts providef or the first time apoptosis data for (À)-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'.

Synthesis and Sulfur Electrophilicity of the Nuphar Thiaspirane Pharmacophore

“…The ability to form the pharmacophore in isolation from other functional groups and not embedded in a dimeric structure allowed us to study its reactivity with nucleophiles. Obviously, the tetrahydrothiophene is stable to alcoholic nucleophiles 1 , 8 —it can be formed in methanol and can be characterized by NMR in methanol- d 4 . No hemiaminal is observed in equilibrium even at −50 °C (see Supporting Information ), reflecting the stability of the Nuphar thiaspirane noted previously (see below).…”

“…Therefore, the promiscuity of an unstabilized iminium and the preservation of a stabilized iminium cannot be the basis for the different activity of C6-hydroxy versus C6′-hydroxy dimers. A recent report by MacMillan, Eastman, and Wu demonstrates that all diastereomers and enantiomers of dihydroxydimers 1a , 2a , 3a , 4a have similar apoptotic activity, 8 suggesting a common mechanism of action between the stereoisomers, at least for the dihydroxy-dimers.…”

“…Cytotoxicity assays indicate that a hemiaminal adjacent to the tert -alkyl thioether (C6) is necessary for induction of apoptosis, whereas a thioether adjacent to a quaternary center (C6′) displays low potency. 7 , 8 For example, 6-OH-TNL ( 2b , Figure 1 a) is strongly apoptotic below 1 μM, whereas 6′-OH-TNL ( 2c ) is completely inactive at 10 μM. 8 Simplified analogues that replace the thioether with a hydroxyl are also reported to lose all activity in fungicidal experiments.…”

“… 7 , 8 For example, 6-OH-TNL ( 2b , Figure 1 a) is strongly apoptotic below 1 μM, whereas 6′-OH-TNL ( 2c ) is completely inactive at 10 μM. 8 Simplified analogues that replace the thioether with a hydroxyl are also reported to lose all activity in fungicidal experiments. 14 One explanation put forth for these structural requirements is the enhanced stability of the iminium adjacent to a thioether in 6-hydroxydimers ( Figure 1 b), 15 − 17 which may allow this otherwise strong carbon electrophile to penetrate the cell and reach a specific target.…”

“… 2 Dihydroxy-thiaspirane dimers would then be expected to not show activity or exhibit the low activity of 6′-hydroxydimers since they also give rise to an unstabilized iminium. Yet 6,6′-dihydroxy- 1 ( 1a ) and 6-hydroxy- 1 ( 1b ) inhibit invasion of collagen-matrix by B-16 melanoma cells 11 and promote apoptosis 7 , 8 at nearly identical levels. Therefore, the promiscuity of an unstabilized iminium and the preservation of a stabilized iminium cannot be the basis for the different activity of C6-hydroxy versus C6′-hydroxy dimers.…”