Allylic alcohols and allylic amines were directly utilized in a Pd-catalyzed hydrogen-bond-activated allylic amination under mild reaction conditions in the absence of any additives. The cooperative action of a Pd-catalyst and a hydrogen-bonding solvent is most likely responsible for its high reactivity. The catalytic system is compatible with a variety of functional groups and can be used to prepare a wide range of linear allylic amines in good to excellent yields. Furthermore, this methodology can be easily applied to the one-step synthesis of two drugs, cinnarizine and naftifine, on a gram scale.
A Pd-catalyzed asymmetric allylic amination of 4-substituted 2-acetoxybut-3-enoates with amines has been developed for the regiospecific synthesis of chiral α,β-unsaturated γ-amino esters. The desired chiral aminated products can be obtained in up to 98% yield, and 99% ee and can be conveniently transformed to chiral γ-amino acid/alcohol derivatives and chiral γ-lactams, which can then be subjected to the synthesis of several types of chiral drugs and drug candidates. The preferential formation of chiral γ-amino esters may be attributed to the bulky substituents on the right side of the allyl substrates. This work provides an efficient strategy for the synthesis of chiral α,β-unsaturated γ-amino esters and their derivatives.
Asymmetric hydrogenation of various α-substituted acrylic acids was carried out using RuPHOX-Ru as a chiral catalyst under 5 bar H2, affording the corresponding chiral α-substituted propanic acids in up to 99% yield and 99.9% ee. The reaction could be performed on a gram-scale with a relatively low catalyst loading (up to 5000 S/C), and the resulting product (97%, 99.3% ee) can be used as a key intermediate to construct bioactive chiral molecules. The asymmetric protocol was successfully applied to an asymmetric synthesis of dihydroartemisinic acid, a key intermediate required for the industrial synthesis of the antimalarial drug artemisinin.
An asymmetric domino reaction was developed utilizing readily available cyclic N-sulfonylimines and simple aldehydes to construct biologically important and synthetically challenging piperidine derivatives consisting of three contiguous stereocenters. trans-Perhydroindolic acid proved to be an efficient organocatalyst in this reaction (up to 89% yield, 80:20 dr, and 99% ee). The absolute configuration of the catalytic product was determined by X-ray crystallography studies. The product could be conveniently converted to synthetically useful intermediates, such as (3R,4S)-4-ethyl-3-methyl-6-phenylpiperidinyridin-2-one (8), via a simple transformation.
Chiral β-aryl-α,β-unsaturated amino alcohols were synthesized via a Pd-catalyzed asymmetric allylic amination of 4-aryl-1,3-dioxolan-2-one using planar chiral 1,2-disubstituted ferrocene-based phosphinooxazolines as ligands. Under the optimized reaction conditions, a series of substrates were examined and the products were obtained in good to excellent yields (up to 92%) and enantioselectivities (up to 98% ee) under mild reaction conditions. The desired products were determined to be of (R)-configuration and could subsequently be transformed into compounds with interesting biological activity using simple transformations.
A highly enantioselective Pd-catalyzed asymmetric allylic substitution cascade of cyclic N-sulfonylimines with an accompanying asymmetric desymmetrization has been developed for the construction of fused tetrahydroindole derivatives bearing two chiral centers. Mechanistic studies confirmed that the cascade reaction proceeds by initial allylic alkylation and subsequent allylic amination. The first alkylation is a chirality-control step and represents an asymmetric desymmetrization of cis-cyclic allyl diacetates. The reaction has been performed on a gram scale, and the desired products can take part in several transformations.
An organocatalyzed asymmetric tandem reaction of cyclic N-sulfonylimines and α,β-unsaturated aldehydes was developed. These substrates follow an alternative reaction pathway to that of reactions involving saturated aldehydes, affording similar piperidine derivatives.
(2S,3aR,7aS)-Perhydroindolic acid, the key intermediate in the synthesis of trandolapril, and its trans-isomers, were readily prepared. These prolinelike molecules are unique in that they contain a rigid bicyclic structure, with two hydrogen atoms trans to each other at the bridgehead carbon atoms. These molecules were used successfully as chiral organocatalysts in asymmetric domino Michael addition/cyclization reactions of aldehyde esters with b,g-unsaturated a-keto esters. They proved to have excellent catalytic behavior, allowing for the synthesis of multi-substituted, enantiomerically enriched hemiacetal esters. Under optimal conditions (using 10 mol% catalyst loading), a series of b,g-unsaturated a-keto esters was examined with up to 99% de, ee and yield, respectively. Additionally, the enantiomerically enriched hemiacetal esters could be readily transformed into their corresponding bioactive pyranoA C H T U N G T R E N N U N G [2,3-b]pyrans (possessing a multi-substituted bicyclic backbone).
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