An efficient synthesis of (2S,3R)-3-hydroxylysine via ruthenium catalyzed asymmetric hydrogenation

Highly efficient synthesis of enantioenriched β-hydroxy α-amino acid derivatives via Ir-catalyzed dynamic kinetic asymmetric hydrogenation

Asymmetric hydrogenation of aryl α-dibenzylamino β-ketoesters proceeded smoothly to provide the corresponding chiral aryl β-hydroxy α-amino derivatives with excellent diastereo- and enantioselectivities (>99/1 dr, up to >99% ee).

Comprehensive Account on the Synthesis of (-)-Balanol and its Analogues

BACKGROUND

A variety of diseases have been associated with hyperactivation of protein kinase C (PKC) enzymes such as cancer, diabetes, asthma, cardiovascular and central nervous system disorders. There is a dire need to selectively inhibit these enzymes by synthesizing new potent inhibitors. Balanol, a fungal metabolite belonging to the PKC inhibitor family, is especially included in this aspect. Tremendous effort has been put towards the synthesis of balanol by different research groups.

OBJECTIVE

The aim of this review is to provide a detailed description of synthetic approaches adopted for the synthesis of key fragments of balanol (azepane and benzophenone). All the factors that resulted in excellent yield and high enantioselectivity have also been mentioned.

CONCLUSION

It has been shown throughout this review that the synthesis of hexahydroazepine and benzophenone cores of balanol was achieved by employing a variety of important key steps with commercially available starting precursors, which make this total synthesis more valuable. Moreover, this article provides ideas to the synthetic as well as pharmaceutical chemists for the synthesis of (-)-balanol and its analogues.

Highly Diastereo- and Enantioselective Access to syn-α-Amido β-Hydroxy Esters via Ruthenium-Catalyzed Dynamic Kinetic Resolution-Asymmetric Hydrogenation

Dynamic kinetic resolution (DKR) of racemic aryl α-amino β-ketoesters via Ru-diphosphine-catalyzed asymmetric hydrogenation was realized at 70 °C under 50 atm of hydrogen, affording syn α-amido β-hydroxy esters in high yields (up to 96%) with high reactivity (TON up to 940) and diastereo- and enantioselectivities (up to 99:1 dr, 98% ee). These hydrogenation products provide valuable chiral synthons in many natural products and pharmaceuticals. Gram-scale DKR asymmetric hydrogenation (DKR-AH) was also performed with retained reactivity and stereoselectivity, revealing the synthetic utility of this method.

A new cost-effective Ru-chloramphenicol base derivative catalyst for the asymmetric transfer hydrogenation/dynamic kinetic resolution of N-Boc α-amino-β-ketoesters and its application to the synthesis of the chiral core of vancomycin

Herein we describe the application of Ru-chloramphenicol base complexes catalyzed highly diastereo- and enantioselective transfer hydrogenation of N-Boc α-amino-β-ketoesters for the asymmetric synthesis of anti-N-Boc-β-hydroxy-α-amino esters.

Stereocontrolled synthesis of syn-β-Hydroxy-α-amino acids by direct aldolization of pseudoephenamine glycinamide

β-Hydroxy-α-amino acids figure prominently as chiral building blocks in chemical synthesis and serve as precursors to numerous important medicines. Reported herein is a method for the synthesis of β-hydroxy-α-amino acid derivatives by aldolization of pseudoephenamine glycinamide, which can be prepared from pseudoephenamine in a one-flask protocol. Enolization of (R,R)- or (S,S)-pseudoephenamine glycinamide with lithium hexamethyldisilazide in the presence of LiCl followed by addition of an aldehyde or ketone substrate affords aldol addition products that are stereochemically homologous with L- or D-threonine, respectively. These products, which are typically solids, can be obtained in stereoisomerically pure form in yields of 55-98 %, and are readily transformed into β-hydroxy-α-amino acids by mild hydrolysis or into 2-amino-1,3-diols by reduction with sodium borohydride. This new chemistry greatly facilitates the construction of novel antibiotics of several different classes.

A mild and efficient direct α-amination of β-dicarbonyl compounds using iodosobenzene and p-toluenesulfonamide catalyzed by perchlorate zinc hexahydrate

A direct α-amination of β-dicarbonyl compounds has been achieved by using iodosobenzene (PhIO) as an oxidant and p-toluenesulfonamide (TsNH(2)) as an aminating reagent in the presence of a catalytic amount of perchlorate zinc hexahydrate. The present amination reaction proceeds quickly at rt (<30 min needed for most tested substrates) to provide the corresponding α-N-tosylamido β-dicarbonyl compounds in high to excellent yields.

Highly diastereo- and enantioselective synthesis of monodifferentiated syn-1,2-diol derivatives through asymmetric transfer hydrogenation via dynamic kinetic resolution

The first enantio- and diastereoselective approach to alpha-alkoxy-substituted syn-beta-hydroxyesters through highly efficient catalytic asymmetric transfer hydrogenation via dynamic kinetic resolution reactions from the corresponding racemic beta-ketoesters is described. In this atom-economical process, two contiguous stereogenic centers are generated simultaneously with an excellent diastereoselectivity (up to 99/1) and enantioselectivity (up to 99%), allowing a rapid access to a wide variety of aromatic and heteroaromatic monodifferentiated syn-1,2-diols.

Chiral version of the Burgess reagent and its reactions with oxiranes: application to the formal enantiodivergent synthesis of balanol

An efficient formal synthesis of a (-)-balanol intermediate ( 25a) from cyclohexadiene oxide was accomplished in eight steps. An asymmetric version of the Burgess reagent allows for an enantiodivergent approach to both enantiomers of balanol from a racemic starting material.

Efficiency in nonenzymatic kinetic resolution

The Walden memorial at the Technical University in Riga is pictured in the frontispiece to mark the recent centennial of the Walden inversion. This is a rare public monument to key events from the first era of exploration in stereocontrolled synthesis, and may be the only such monument to use the language of organic chemistry expressed at the molecular level. The reaction of racemic substrates with chiral nucleophiles is one of many methods currently known to achieve kinetic resolution, a phenomenon that ranks as the oldest and most general approach for the synthesis of highly enantioenriched substances. The first nonenzymatic kinetic resolutions as well as the original forms of the Walden inversion were studied in the 1890s. All of these investigations were conducted within the first generation following the demonstration that carbon is tetrahedral, and provided abundant evidence that the principles and importance of enantiocontrolled syntheses were understood. However, a reliable, rapid technique to quantify results and guide the optimization process was still lacking. Many decades passed before this problem was solved by the advent of HPLC and GLPC assays on chiral supports, which stimulated explosive growth in the synthesis of nonracemic substances by kinetic resolution. The Walden monument is accessible to passers-by for hands-on inspection as well as for contemplation and learning. In a similar way, kinetic resolution is experimentally accessible and can be thought-provoking at several levels. We follow the story of kinetic resolution from the early discoveries through fascinating historical milestones and conceptual developments, and close with a focus on modern techniques that maximize efficiency.

Dynamic Kinetic Resolution Catalyzed by Ir Axially Chiral Phosphine Catalyst: Asymmetric Synthesis of anti Aromatic β-Hydroxy-α-amino Acid Esters

The anti selective hydrogenation of alpha-amino-beta-keto esters via dynamic kinetic resolution was achieved for the first time by using the iridium-MeOBIPHEP catalyst in asymmetric synthesis of anti aromatic beta-hydroxy-alpha-amino acid esters with excellent diastereo- and enantioslectivities. Acetic acid as a solvent and sodium acetate as an additive affected dramatically the yield and the enantioselectivity, respectively. The product anti aromatic beta-hydroxy-alpha-amino acid esters are useful for synthesis of pharmaceuticals and natural products.

Asymmetric catalytic hydrogenation. Design of new Ru catalysts and chiral ligands: from laboratory to industrial applications

This Account covers the design of Ru catalysts and ligands. Two classes of chiral phosphine ligands are prepared: the electron-rich trans-2,4-substituted phosphetanes, readily available from optically pure 1,3-diol cyclic sulfates, and atropoisomeric ligands (SYNPHOS, MeO-NAPhePHOS, bearing heterotopic biaryl moieties, and a chiral water-soluble diguanidinium binaphthyl diphosphine, Digm-BINAP). Applications of these ligands to rhodium- and ruthenium-mediated hydrogenation of ketones and olefins have been reported with high enantioselectivities. The recognition abilities of Ru-SYNPHOS for a wide range of ketones is superior to those observed with BINAP, MeO-NAPhePHOS, and MeO-BIPHEP. Several biologically active compounds have been prepared through dynamic kinetic resolution. This work gives access to a number of highly active catalysts of the type [Ru(biphosphane)(H)(eta(6)-cot)]BF(4). These catalysts have demonstrated their utility in the enantioselective hydrogenation of the tetrasubstituted cyclopentenone "dehydrodione", which leads to the commercially important perfume component Paradisone (Firmenich).

Stereoselective synthesis of iso-dolaproine via dynamic kinetic resolution

[see reaction]. An efficient multigram-scale synthesis of optically pure Boc-(2S,3R,4S)-iso-dolaproine is reported using dynamic kinetic resolution (DKR). The catalytic asymmetric hydrogenation of ethyl (4S)-3-(2'-pyrrolidinyl)-3-oxo-2-methyl propanoate hydrochloride using in situ generated Ru[(S)-MeO-BIPHEP]Br2 catalyst affords the anti beta-hydroxy alpha-methyl ester quantitatively. The two new stereogenic centers are simultaneously controlled with high diastereoselectivity.

Synthetic applications of the ruthenium-catalyzed hydrogenation via dynamic kinetic resolution

This review summarizes some recent work in the field of homogeneous ruthenium-catalyzed hydrogenation via dynamic kinetic resolution. The hydrogenation reaction of various α-substituted β-oxo esters using chiral ruthenium(II) catalysts is presented as well as the synthesis of some key intermediates of pharmaceutical interest.Key words: dynamic kinetic resolution, ruthenium, hydrogenation, α-substituted β-keto ester.

Asymmetric synthesis of the balanol heterocycle via a palladium-mediated epimerization and olefin metathesis

[formula: see text] The enantioselective formal synthesis of balanol, a potent protein kinase C inhibitor, was accomplished from D-serine utilizing a Pd-catalyzed equilibration of diastereomeric 5-vinyloxazolines to set the stereochemistry of the vicinal amino and hydroxyl groups. A ruthenium-catalyzed ring-closing metathesis was employed to form the seven-membered nitrogen heterocyle.