Asymmetric Mannich reactions of beta-keto esters with acyl imines catalyzed by cinchona alkaloids

Cinchona alkaloids catalyze the enantioselective Mannich reaction of beta-keto esters with acyl aryl imines. The reaction requires 10 mol % of cinchonine or cinchonidine. The reaction products are obtained in good yields (81-99%), high enantioselectivities (80-96% ee), and in diastereoselectivities that range from 1:1 to >95:5. The cinchonine-catalyzed reaction provides access to highly functionalized building blocks used in the asymmetric synthesis of a dihydropyrimidone and beta-amino alcohol.     

Direct catalytic asymmetric mannich reactions of malonates and beta-keto esters

The first catalytic asymmetric direct Mannich reaction of malonates and beta-keto esters has been developed. Malonates react with an activated N-tosyl-alpha-imino ester catalyzed by chiral tert-butyl-bisoxazoline/Cu(OTf)(2) to give the Mannich adducts in high yields and with up to 96% ee. These reactions create a chiral quaternary carbon center and it is demonstrated that this new direct Mannich reactions provides for example a new synthetic procedure for the formation of optically active beta-carboxylic ester alpha-amino acid derivatives. A series of different beta-keto esters with various ester substituents has been screened as substrates for the catalytic asymmetric direct Mannich reaction and it was found that the best results in terms of yield, diastereo- and enantioselectivity were obtained when tert-butyl esters of beta-keto esters were used as the substrate. The reaction of different beta-keto tert-butyl esters with the N-tosyl-alpha-imino ester gave the Mannich adducts in high yields, diastereo- and enantioselectivities (up to 95% ee) in the presence of chiral tert-butyl-bisoxazoline/Cu(OTf)(2) as the catalyst. To expand the synthetic utility of this direct Mannich reaction a diastereoselective decarboxylation reaction was developed for the Mannich adducts leading to a new synthetic approach to attractive optically active beta-keto alpha-amino acid derivatives. Based on the stereochemical outcome of the reactions, various approaches of the N-tosyl-alpha-imino ester to the chiral bisoxazoline/Cu(II)-substrate intermediate are discussed.     

A facile direct anti-selective catalytic asymmetric Mannich reaction of aldehydes with preformed N-Boc and N-Cbz imines

Anti-selective Mannich reactions of N-Boc and N-Cbz protected imines with unmodified aldehydes proceeded smoothly under the catalysis of a secondary amine-thiourea catalyst, which led to high yields (70%-95%) and excellent enantioselectivity (up to 96 : 4 dr and >99% ee) under conventional organic synthetic operations.     

Organocatalytic asymmetric Mannich reactions with N-Boc and N-Cbz protected alpha-amido sulfones (Boc: tert-butoxycarbonyl, Cbz: benzyloxycarbonyl)

Different malonates and beta-ketoesters can react with N-tert-butoxycarbonyl- (N-Boc) and N-benzyloxycarbonyl- (N-Cbz) protected alpha-amido sulfones in an organocatalytic asymmetric Mannich-type reaction. The reaction makes use of a simple and easily obtained phase-transfer catalyst and proceeds under very mild and user-friendly conditions. The optimised protocol avoids the preparation and the isolation of the relatively unstable N-Boc and N-Cbz imines that are generated in situ from the bench-stable alpha-amido sulfones. The corresponding Mannich bases are generally obtained in good yields and enantioselectivities, and can be readily transformed into key compounds, such as optically active beta3-amino acids in one easy step. Enantioenriched N-Boc and N-Cbz protected beta-amino acids that are suitable for peptide synthesis are also available from the Mannich adducts through simple manipulations. Control experiments showed the dual role of the enolate-catalyst ion pair in this reaction, as well as the crucial role of the presence of water to achieve high enantioselectivities.     

The enantio- and diastereoselective synthesis of the first phospho-statine derivative

The title compound was synthesized by the diastereoselective addition of the lithium or sodium anion of dimethyl methylphosphonate to $\text{N}$-trityl-L-phenylalaninal.     

Cobalt-catalyzed reductive Mannich reactions of 4-acryloylmorpholine with N-tosyl aldimines

Using cobalt catalysis, diethylzinc promotes the conjugate reduction of 4-acryloylmorpholine to produce the corresponding ethylzinc enolate, which reacts with N-tosyl aldimines to afford beta-aminoamides.     

Zinc-mediated ring-expansion and chain-extension reactions of beta-keto esters

[Chemical reaction: see text] The reaction of cyclic beta-keto esters with CF3CO2ZnCH2I provided the corresponding ring-expanded products in moderate to good yields. Although alpha-substituted acyclic beta-keto esters reacted with much less efficient, chain-extension reaction of simple beta-keto esters also proceeded effectively to generate gamma-keto esters in high yields.     

Direct catalytic diastereoselective Mannich reactions: the synthesis of protected α-hydroxy-β-aminoketones

The combination of Mg(ClO₄)₂, 2,2′-bipyridine and N-methylmorpholine generates an effective catalyst system for the direct addition of α-carbonate-substituted ketones to aryl N-Ts imines. Methyl-carbonate-substituted ketones deliver acyclic α-hydroxy-β-aminoketone derivates, while ketones substituted with α-iso-propenyl-carbonates furnish cyclic carbamate adducts. In both cases the anti-configured Mannich products dominate.     

Organic solvent-free, enantio- and diastereoselective, direct Mannich reaction in the presence of water

An organocatalyst-mediated, asymmetric Mannich reaction in the presence of water without using organic solvents has been developed. A highly reactive siloxytetrazole hybrid catalyst has been developed for the reaction of dimethoxyacetaldehyde, while the sodium salt of siloxyproline is an effective catalyst of alpha-imino glyoxylate. Excellent enantioselectivity can be realized, and the usage of organic solvents can be reduced compared to the conventional reactions in organic solvents.     

Manganese(III) acetate-mediated alkylation of beta-keto esters and beta-keto amides: an enantio- and diastereo-selective approach to substituted pyrrolidinones

beta-Keto esters and beta-keto amides can be efficiently alkylated on reaction with enol ethers and manganese(III) acetate in the presence of copper(II) acetate. These intermolecular radical addition reactions can be used to construct quaternary carbon centres in excellent yield and this method has been utilised in a diastereoselective approach to substituted pyrrolidinones.     

Direct catalytic enantio- and diastereoselective aldol reaction using a Zn-Zn-linked-BINOL complex: a practical synthesis of syn-1,2-diols

[reaction: see text] The direct catalytic enantio- and diastereoselective aldol reaction with 2-hydroxy-2'-methoxyacetophenone proceeded smoothly using as little as 1 mol % of a dinuclear zinc catalyst, Zn-Zn-linked-BINOL complex 2, to afford alpha,beta-dihydroxy ketones in a highly syn-selective manner (up to syn/anti 97/3) and in excellent yields (up to 95%) and ees (up to 99%). Efficient transformations of the alpha,beta-dihydroxy ketone into an alpha,beta-dihydroxy ester and an alpha,beta-dihydroxy amide via regioselective rearrangements are also described.     

Highly enantio- and diastereoselective organocatalytic cascade aza-Michael-Michael reactions: a direct method for the synthesis of trisubstituted chiral pyrrolidines

An unprecedented highly enantio- and diastereoselective cascade aza-Michael-Michael reaction of alpha,beta-unsaturated aldehydes with trans-gamma-Ts protected amino alpha,beta-unsaturated ester has been developed; the simple and practical process, efficiently catalyzed by chiral diphenylprolinol TMS ether, serves as a powerful access to highly functionalized trisubstituted chiral pyrrolidines.     

Preparation of beta-keto esters and beta-diketones by C-acylation/deacetylation of acetoacetic esters and acetonyl ketones with 1-acylbenzotriazoles

Acyl-, aroyl-, and heteroaroyl-acetic esters 6a-f and 8a-l are prepared by reactions of 1-acylbenzotriazoles 1a-k with acetoacetic esters 5 or 7a,b in the presence of sodium hydride followed by regioselective deacetylation. Similar C-acylation/deacetylation of acetylacetone and benzoylacetone affords beta-diketones 10a-d and 13a-c, respectively.     

Highly enantio- and diastereoselective one-pot synthesis of acyclic epoxy alcohols with three contiguous stereocenters

Two highly enantio- and diastereoselective one-pot procedures for the synthesis of epoxy alcohols with up to three contiguous stereocenters are reported. Route one involves asymmetric addition of an alkylzinc reagent to an enal followed by diastereoselective epoxidation. Route two entails asymmetric vinylation of an aldehyde with divinylzinc reagents and subsequent diastereoselective epoxidation. The oxidant for the epoxidation is generated by exposure of the allylic alkoxide intermediate and the remaining organozinc reagent to dioxygen. Upon addition of catalytic titanium tetraisopropoxide, the directed epoxidation yields the epoxy alcohols with good to excellent yields.     

Catalytic enantio- and diastereoselective aldol reactions of glycine-derived silicon enolate with aldehydes: an efficient approach to the asymmetric synthesis of anti-beta-hydroxy-alpha-amino acid derivatives

We have developed an efficient method for the asymmetric synthesis of anti-beta-hydroxy-alpha-amino acid derivatives based on highly enantio- and diastereoselective aldol reactions of the silicon enolate derived from N-trifluoroacetylglycinate with aldehydes using a chiral zirconium catalyst. The resulting N-trifluoroacetyl group is easily cleaved under either acidic or basic conditions and can be used directly as a protecting group for further transformations.