Substrate Control in the Gold(I)-Catalyzed Cyclization of beta-Propargylamino Acrylic Esters and Further Transformations of the Resultant Dihydropyridines
Abstrakt
N-Protected beta-propargylamino acrylic esters with a push-pull olefinic bond afforded good to high yields of dihydropyridines upon treatment with 5% tris(2-furyl)phosphine-gold(I) chloride/silver(I) tetrafluoroborate [(TFP)AuCl/AgBF4] in anhydrous benzene. Carbamate and sulfonyl groups were employed for nitrogen protection.
On a model enyne, the p-methoxybenzenesulfonyl (MBS) group was found to be a better protective group than tosyl in terms of cyclization yield, and also the yield of elimination to the corresponding 2,3,4-trisubstituted pyridines. Boc-protected dihydropyridines underwent partial deprotection/oxidation under the cyclization conditions, which enabled a more straightforward, one-pot preparation of the corresponding pyridines.
In another application, an appropriately substituted derivative protected as a stable methoxycarbamate was subjected to catalytic hydrogenation affording the known precursor of paroxetine. The chemoselectivity of enyne cyclization (dihydropyridine vs. pyrrole) is governed, among other factors, by C-3 substitution.
Dihydropyridines were obtained as sole products regardless of the catalyst/conditions when C-3 was unsubstituted.