Highly enantioselective synthesis of terutroban key intermediate via asymmetric hydrogenation

A chiral (R) key intermediate of the biologically active form of terutroban has been prepared by asymmetric hydrogenation. The catalysts are based on very easily accessible ruthenium complexes modified by chiral phosphorous ligands. The use of the chiral catASium^®T ligands family allowed us to realize this transformation efficiently in terms of yield and enantioselectivity (ee up to 92%) with high substrate/catalyst ratios.     

Highly enantioselective hydrogenation of new 2-functionalized quinoline derivatives

The asymmetric hydrogenation of a new series of 2-functionalized quinolines has been developed in the presence of in situ generated catalysts obtained from [Ir(cod)Cl]₂/(R)-bisphosphine/I₂ combinations. The enantioselectivity levels were as high as 84-94% ee for the synthesis of 1,2,3,4-tetrahydroquinolines.     

Highly enantioselective Rh-catalyzed hydrogenations with heterocombinations of pentafluorobenzyl- and methoxybenzyl-derived binaphthyl phosphites

The Rh-catalyzed hydrogenations of dimethyl itaconate and methyl acetamido acrylate using selected heterocombinations of pentafluorobenzyl- and methoxybenzyl-derived binaphthyl phosphites proved to be highly enantioselective (ee 93-99%). In these selected cases the Rh-heterocomplexes, which were formed in a statistical amount (ca. 50% by ³¹P NMR), turned out to be more active and selective than the two homocomplexes.     

Stereoselective synthesis of syn β-hydroxy cycloalkane carboxylates: transfer hydrogenation of cyclic β-keto esters via dynamic kinetic resolution

The transfer hydrogenation of bicyclic and monocyclic β-keto esters using HCO₂H/Et₃N as the hydrogen source and TsDPEN-based Ru(II) catalysts proceeds with dynamic kinetic resolution to afford the corresponding cyclic β-hydroxy esters with moderate to excellent levels of diastereo- and enantioselectivities. The mild reaction conditions used make possible to preserve in most cases the syn relative configuration of the products, providing a complementary tool to known approaches to the synthesis of anti isomers.     

Copper(II)-catalyzed enantioselective conjugate addition of nitro esters to 2-enoyl-pyridine N-oxides

A highly enantioselective Michael addition of nitro esters to 2-enoyl-pyridine N-oxides was developed by using chiral copper catalysts. The Michael addition products can be obtained in high yields and with up to 96% ee.     

An enantioselective total synthesis of (S)-(−)-licochalcone E: determination of the absolute configuration

The absolute configuration of (−)-licochalcone E (1) was determined to be (S) via the first enantioselective total synthesis of the compound. The chirality in (S)-(−)-licochalcone E (1) was installed by asymmetric methylation of the Evans' oxazolidinone derivatives.     

Highly enantioselective catalytic hydrogenation of a 5-amino-3,5-dioxopentanoic ester

The highly enantioselective hydrogenation of methyl 4-tert-butylcarbamoyl-3-oxo-butyrate to the corresponding secondary alcohol, representing an interesting chiral building block, for example, for the synthesis of statins, has been investigated in the presence of homogeneous chiral Rh(I) and Ru(II) complexes bearing phosphine ligands. The highest enantioselectivity (up to 96%) was achieved with a [Ru((R)-BINAP)(p-cymene)Cl]Cl complex (sub./cat. ratio 100:1, 5 bar H₂, rt, MeOH).     

Asymmetric synthesis of (−)-tetrahydrolipstatin

Attempts toward the asymmetric synthesis of (−)-tetrahydrolipstatin are described. A palladium catalyzed Wacker-type reaction to convert an alkene to a ketone, highly diastereoselective reduction of a β-hydroxy ketone, selective oxidation of a diol, and modular synthesis are the key features of the successful approach.     

Transition-metal catalyzed enantioselective hydrofunctionalization of alkynes

A significant number of reactions regarding hydrofunctionalization of alkynes have appeared in the literature over the years. This digest focus on the transition-metal catalyzed enantioselective alkyne hydrofunctionalization reactions. Based on the reaction mechanism, these reactions described herein are classified into three types. Strategies, substrate scope, along with mechanisms are highlighting.     

A short enantioselective synthesis of (+)-eleutherin, (+)-allo-eleutherin and a formal synthesis of (+)-nocardione B

A short enantioselective synthesis of (+)-eleutherin, (+)-allo-eleutherin and the formal synthesis of (+)-nocardione B is described. The synthesis is completed in six steps in overall yields of 8% for eleutherin and 14% for allo-eleutherin. The synthetic strategy features an efficient combination of the Dötz annulation reaction with a chiral alkyne and an oxa-Pictet Spengler reaction as the keys steps in the stereodivergent synthesis of (+)-eleutherin and (+)-allo-eleutherin. The synthesis of (S)-(+)-2-(2′-hydroxypropyl)-5-methoxy-1,4-naphthoquinone entails the formal synthesis of (+)-nocardione B.     

Highly enantioselective Michael addition of isobutyraldehyde to nitroalkenes

The asymmetric catalytic Michael reaction between isobutyraldehyde and nitroalkanes with chiral primary amine thiourea organocatalysts was described. In the presence of 10 mol % of 1-((1R,2R)-2-amino-1,2-diphenylethyl)-3-benzylthiourea, the desired products were achieved in excellent enantioselectivity (up to>99% ee) with up to 98% yield.     

MM3 force field prediction of the enantioselective preference in the asymmetric synthesis of a chiral 2-cyclohexen-1-ol using a chiral lithium amide reagent

Enantioselective preference in the asymmetric synthesis where cyclohexene oxide is transformed enantioselectively to chiral (S)- or (R)-2-cyclohexen-1-ol by the reaction with the appropriate chiral lithium amide reagent has been evaluated theoretically using the MM3 force field. The plausible possible structures for each precursor (reaction intermediate complex) leading to a (S)- or (R)-2-cyclohexen-1-ol have been optimized with the extended MM3 force field applicable to the lithium amide functional group, and the populations of their (S)- or (R)-reaction intermediate complexes at an ambient temperature (298 K) were calculated. The initial structure for evaluating the reaction intermediates of this asymmetric synthesis was constructed on the basis of the optimized ab initio transition state structure (MP2/6-31+G^∗) comprising lithium amide LiNH₂ and propene oxide. To the thus obtained transition state structure composed of LiNH₂ and propene oxide, the other remaining Cartesian coordinates for the actual reaction intermediates composed of the chiral lithium amides and cyclohexene oxide were added to make the reaction intermediate structure. The conformational search for the reaction intermediate has been carried out by using the Stochastic search Algorithm, and the optimized geometries and their conformational energies (steric energies) have been calculated by the MM3 force field. The populations calculated from the conformational energies of the reaction intermediate leading to the (S)- or (R)-2-cyclohexen-1-ol were shown to be linearly well correlated with the experimentally reported enantiomer excess (% ee) values. The critical factors to control the enantioselectivity were investigated on the basis of the optimized structures of the reaction intermediate complexes. The MM3 force field approach was shown to be applicable to the theoretical evaluation of the enantioselectivity and be useful for designing a new functional chiral lithium amide reagent for the asymmetric synthesis.     

Straightforward enantioselective synthesis of (+)-ancistrofuran

This paper reports the straightforward enantioselective synthesis of (+)-ancistrofuran starting from a readily available enantiopure building block. A stereofacial directed diastereoselective addition of an organocerate for the installation of the required stereogenic center served as the key step. In addition, the efficient conversion of an intermediate hydroxy aldehyde to the one-carbon homologated cyanide was developed. The sequence was achieved by the mild formation of a cyanohydrin and the subsequent one-pot two-step Barton-McCombie double deoxygenation of the hydroxyl groups.     

A convergent Pd-catalyzed asymmetric allylic alkylation of dl- and meso-divinylethylene carbonate: enantioselective synthesis of (+)-australine hydrochloride and formal synthesis of isoaltholactone

The use of a mixture of dl- and meso-divinylethylene carbonate as an electrophile in palladium-catalyzed asymmetric allylic alkylation reactions is reported. From the diastereomeric mixture of meso and chiral racemic starting materials, a single product is obtained in high optical purity employing either oxygen or nitrogen nucleophiles. The resulting dienes have proven to be versatile synthetic intermediates as each carbon is functionalized for further transformation and differentiated by virtue of the reaction. A mechanism for this intriguing transformation is proposed and a concise enantioselective total synthesis of (+)-australine hydrochloride is reported as well as a formal synthesis of isoaltholactone.     

Application of asymmetric phase-transfer catalysis in the enantioselective synthesis of cis-5-substituted proline esters

A simple, highly stereoselective three-step sequence for the enantioselective synthesis of cis-5-substituted proline esters is described. This sequence features an asymmetric PTC Michael addition, followed by acid catalysed imine exchange and catalytic hydrogenation. Application of this chemistry in the synthesis of the nonpeptide cholecystokinin antagonist (+)-RP-66803 11 is also described.     

Catalytic enantioselective allylation at the activated benzylic position of prochiral aryl cyano esters: access to quaternary stereogenic centers

Palladium catalyzed asymmetric allylic alkylation of prochiral aryl cyano esters has been carried out in the presence of various chiral ligands. The base and additives have been varied and allowed to produce the allyl derivative presenting a highly functionalized quaternary stereogenic center. The chiral pocket ligands of Trost appears the most appropriate to produce the desired chiral derivative.     

Enzymatic synthesis of gallic acid esters

Esters of gallic acid were synthesized by enzymatic means using tannase fromAspergillus niger. Alcohols ranging from C₁ through C₁₂ and diols from C₃ through C₆ were tested and found to form esters in the presence of the enzyme.