Asymmetric construction of benzylic quaternary carbons from chiral malonates: Formal synthesis of both (−)- and (+)-aminoglutethimides AG and analogues

Desymmetric Partial Reduction of Malonic Esters

Desymmetrization of easily available disubstituted malonic esters is a rewarding strategy to access structurally diverse quaternary stereocenters. Particularly, asymmetric reduction of malonic esters would generate a functional group with a lower oxidation state than the remaining ester, thus allowing for more chemoselective derivatization. Here, we report a new set of conditions for the zinc-catalyzed desymmetric hydrosilylation of malonic esters that afford aldehydes as the major product. Compared with alcohol-selective desymmetrization, the partial reduction uses a higher concentration of silanes and new pipecolinol-derived tetradentate ligands, proposedly to switch the pathway of zinc hemiacetal intermediates from elimination to silylation. As a result, high aldehyde-to-alcohol ratios and enantioselectivity of aldehydes are obtained from malonic esters with a large collection of substituents. Together with the abundant reactivity of aldehydes, the partial reduction has enabled an expeditious synthesis of bioactive compounds and natural metabolites containing a quaternary stereocenter.

Enantioselective Synthesis of Nitriles Containing a Quaternary Carbon Center by Michael Reactions of Silyl Ketene Imines with 1-Acrylpyrazoles

The enantioselective construction of quaternary carbon centers is a marked challenge in asymmetric catalysis research. It is extremely difficult when a chiral catalyst can not distinguish the facial selectivity of the substrate through bond interactions. Here we realized an enantioselective Michael reaction of silyl ketene imines to 1-acrylpyrazoles using a chiral N,N'-dioxide-Co(II) complex. The protocol is highly efficient for the construction of nitrile-, aryl-, and dialkyl-bearing carbon centers and has been successful applied in the divergent synthesis of pharmaceuticals and natural products. The through-space dispersion interactions between unbound silyl ketene imines and the 1-acrylpyrazole-bonded catalyst play a key role in facilitating the reactivity and the enantioselectivity of this process.

Easy Access to Esters with a Benzylic Quaternary Carbon Center from Diallyl Malonates by Palladium-Catalyzed Decarboxylative Allylation

Diallyl 2-alkyl-2-arylmalonates underwent palladium-catalyzed decarboxylative allylation quickly under mild conditions. In contrast, no reaction took place with diallyl 2,2-dialkylmalonates under the same conditions. Electron-donating phosphine ligands were found to be vital for this reaction. Most of the solvents used did not affect the catalytic cycle. Catalysis in [bmim][BF4], a well-known ionic liquid, was inhibited as a result of formation of a hydrogen bond between a carboxylate anion and a [bmim]+ cation; however, the reaction in [bdmim][BF4], in which the acidic proton of [bmim][BF4] was replaced with a methyl group, proceeded smoothly. The catalytic mechanism was investigated using a tetradeuterated substrate and an enzymatically synthesized enantio-enriched allyl methyl 2-methyl-2-phenylmalonate. Even the electron-deficient phosphite ligand was found to be active for catalysis of diallyl 2-methyl-2-(2- or 4-nitrophenyl)malonates.

Catalytic Enantioselective Synthesis of Quaternary All-Carbon Stereogenic Centers. Preparation of α,α‘-Disubstituted β,γ-Unsaturated Esters through Cu-Catalyzed Asymmetric Allylic Alkylations

[reaction: see text] Peptide-based chiral ligands, readily prepared from commercially available materials, are used to promote Cu-catalyzed asymmetric allylic alkylations of alpha,beta-unsaturated esters bearing a gamma-phosphate with various alkylzinc reagents. These transformations lead to the formation of alpha,alpha'-dialkyl-beta,gamma-unsaturated esters in high yields as well as high regio- (re) and enantioselectivities (ee).

Efficient lipase-catalyzed enantioselective desymmetrization of prochiral 2,2-disubstituted 1,3-propanediols and meso 1,2-diols using 1-ethoxyvinyl 2-furoate

An efficient lipase-catalyzed desymmetrization of prochiral 2,2-disubstituted 1,3-propanediols was developed using 1-ethoxyvinyl 2-furoate 1b, for which the well-known method using vinyl or isopropenyl acetate has had limited success due to low reactivity and easy racemization of the products through acyl group migration. The reagent 1b is highly reactive and converts various prochiral 1,3-diols to the monoesters having a chiral quaternary carbon center with 82-99% ee. These products were stable against racemization under acidic conditions, and their furoyl groups were compatible with oxidative conditions. Prolonging the reaction time led to the kinetic resolution of the monoesters resulting in an increase of their optical purity. The similar desymmetrization of meso cis-1,2-cycloalkanediols gave the monoesters with 82-97% ee without racemization.

Direct Synthesis of Imides from Dicarboxylic Acids using Microwaves

1,4- and 1,5-dicarboxylic acids, when treated with amines in a domestic microwave oven, afford good yields of the corresponding imides.