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.     

Stereoselective synthesis of C-phenyl d- and l-glycero heptopyranosides

d- and l-glycero-C-Phenyl heptopyranosides 1, 2 and 3 are synthesized for the first time from (R)-glyceraldehyde and l-ascorbic acid derivatives. The main strategy involves intramolecular nucleophilic substitution and double bond functionalisation.     

Stereoselective synthesis of norephedrine and norpseudoephedrine by using asymmetric transfer hydrogenation accompanied by dynamic kinetic resolution

Each of the enantiomers of both norephedrine and norpseudoephedrine were stereoselectively prepared from the common, prochiral cyclic sulfamidate imine of racemic 1-hydroxy-1-phenyl-propan-2-one by employing asymmetric transfer hydrogenation (ATH) catalyzed by the well-defined chiral Rh-complexes, (S,S)- or (R,R)-Cp*RhCl(TsDPEN), and HCO(2)H/Et(3)N as the hydrogen source. The ATH processes are carried out under mild conditions (rt, 15 min) and are accompanied by dynamic kinetic resolution.     

Sequential kinetic resolution catalyzed by halohydrin dehalogenase

A sequential kinetic resolution catalyzed by halohydrin dehalogenase was employed for the synthesis of two valuable enantiopure building blocks. Resolution of methyl 4-chloro-3-hydroxybutanoate methylester ((R,S)-2) with use of a Trp249Phe mutant of halohydrin dehalogenase yielded methyl 4-cyano-3-hydroxybutanoate methylester ((S)-4) with 96.8% ee (40% yield) and (S)-2 with 95.2% ee (41% yield). This reaction is carried out in aqueous solution under mild conditions and provides access to a useful statin side-chain building block.     

Stereoselective total synthesis of (+)-cryptocarya diacetate by an iterative Jacobsen’s hydrolytic kinetic resolution protocol

A combination of iterative Jacobsen’s hydrolytic kinetic resolution and reduction of a ketone for the construction of a 1,3-polyol moiety are key steps en route to a total synthesis of (+)-cryptocarya diacetate.     

Stereoselective synthesis of sugar allenes and their hydrosilylation catalyzed by biscobaltoctacarbonyl

Stereoselective introduction of allenyl group to glycals has been developed with propargyltrimethylsilane under the catalysis of a Lewis acid to produce largely the -sugar allenes in good yields. The subsequent hydrosilylations of these sugar allenes were catalyzed by biscobaltoctacarbonyl to obtain the corresponding vinylsilanes. Stereochemical selectivity is also discussed.     

Stereoselective synthesis of 1,3-dienylstannanes by palladium catalyzed cross-coupling reactions

(Z)-α-Bromovinylstannanes are new difunctional group reagents, which undergo palladium catalyzed cross-coupling reactions with (E)-alkenylzirconium complexes to give stereoselectively 1,3-dienylstannanes in good yields.     

Spontaneous enzymatically mediated dynamic kinetic resolution of 8-amino-5,6,7,8-tetrahydroquinoline

A spontaneous dynamic kinetic resolution of 8-amino-5,6,7,8-tetrahydroquinoline 1 was observed in the presence of Candida antarctica Lipase B, in which a >60% yield of (R)-acetamide [(R)-2] was isolated from the racemic amine. The spontaneous formation of ketone 3, followed by a condensation/hydrolysis sequence with the remaining (S)-amine 1, via enamine 4, provides the necessary racemization pathway.     

Stereoselective synthesis of pseudoglycosides catalyzed by TeCl4 under mild conditions

Catalytic amounts of tellurium(IV) tetrachloride were used to promote the O-glycosylation of 3,4,6-tri-O-acetyl-d-glucal to give the corresponding 2,3-unsaturated-O-glycosides. With simple alcohols, the desired compounds were obtained in good yields and excellent anomeric selectivity in a short reaction time using only 2 mol % of the catalyst. The application of the method in the synthesis of a small set of glycopyranosides with rigid or flexible linkers gave the corresponding α anomers as products in good yields. Further applications of some of the synthesized compounds in allylation reaction of aldehydes gave the corresponding homoallylic alcohols in good yields.     

Oxidative kinetic resolution of racemic alcohols catalyzed by chiral ferrocenyloxazolinylphosphine-ruthenium complexes

Oxidative kinetic resolution of racemic secondary alcohols by using acetone as a hydrogen acceptor in the presence of a catalytic amount of [RuCl(2)(PPh(3))(ferrocenyloxazolinylphosphine)] (2) proceeds effectively to recover the corresponding alcohols in high yields with an excellent enantioselectivity. When 1-indanol is employed as a racemic alcohol, the oxidation proceeds quite smoothly even in the presence of 0.0025 mol % of the catalyst 2 to give an optically active 1-indanol in good yield with high enantioselectivity (up to 94% ee), where turnover frequency (TOF) exceeds 80,000 h(-1). From a practical viewpoint, the kinetic resolution is investigated in a large scale, optically pure (S)-1-indanol (75 g, 56% yield, >99% ee) being obtained from racemic 1-indanol (134 g) by employing this kinetic resolution method twice.     

Stereoselective synthesis of dihydrofurans under phase-transfer catalyzed conditions

Treatment of cyclic or acyclic -haloenones with various carbon nucleophiles involving active methylene functions afforded the corresponding dihydrofurans in a stereoselective manner with good to high yields. This reaction system can provide a general and practical methodology for the construction of dihydrofuran rings.     

Dynamic kinetic resolution of amino acid amide catalyzed by D-aminopeptidase and alpha-amino-epsilon-caprolactam racemase

Amino acid amide racemizing activity was discovered in alpha-amino-epsilon-caprolactam (ACL) racemase (EC 5. 1. 1. 15) from Achromobacter obae. The enzymatic synthesis of d-alanine from l-alanine amide has been demonstrated by use of d-aminopeptidase (DAP; EC 3. 4. 11. 19) from Ochrobactrum anthropi C1-38 and ACL racemase. The conversion of 45 mM l-alanine amide was carried out at 30 degrees C for 7 h; l-alanine amide was completely converted to d-alanine, and no l-alanine was detected. The result of successive enzymatic reaction shows that the combination of ACL racemase and DAP can be applied for dynamic kinetic resolution of dl-amino acid amides to yield d-amino acids.     

Enzymatic kinetic resolution of racemic ketones catalyzed by Baeyer–Villiger monooxygenases

A set of racemic cyclic and linear ketones, as well as 2-phenylpropionaldehyde, were tested as substrates in the enzymatic Baeyer–Villiger oxidation catalyzed by two Baeyer–Villiger monooxygenases: phenylacetone monooxygenase (PAMO) and 4-hydroxyacetophenone monooxygenase (HAPMO). Excellent enantioselectivites (E > 200) can be obtained in the kinetic resolution processes depending on the substrate structure and the reaction conditions. The parameters affecting the biocatalytic properties of these enzymes were also studied, in order to establish a deeper understanding of these novel biocatalysts.     

Parallel kinetic resolution of 4-alkynals catalyzed by Rh(I)/Tol-BINAP: synthesis of enantioenriched cyclobutanones and cyclopentenones

A Rh(I)/(R)-Tol-BINAP catalyst reacts with a racemic mixture of 4-alkynals to selectively furnish a cyclobutanone from the (R)-substrate and a cyclopentenone from the (S)-substrate. This method is noteworthy from several standpoints, including the scarcity of parallel kinetic resolutions (especially those that are catalyzed or that involve carbon-carbon bond formation) and the dearth of catalytic processes that generate enantioenriched cyclobutanones.     

Dynamic kinetic resolution catalyzed by Ir axially chiral phosphine catalyst: asymmetric synthesis of anti aromatic beta-hydroxy-alpha-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.     

Synthesis of D- and L-carbocyclic nucleosides via rhodium-catalyzed asymmetric hydroacylation as the key step

D- and L-carbocyclic nucleosides were obtained by a new procedure involving an enantioselective rhodium/duphos-catalyzed hydroacylation reaction as the key step. The 3-hydroxymethyl-cyclopentanol intermediate was obtained by stereoselective reduction of ketone and by dynamic kinetic resolution (DKR).     

Biocatalyst CAL-B catalyzed synthesis of modified nucleosides: An overview

Nucleosides are valuable biologically active compounds, which display antitumour and antiviral activities. Various types of bioactive nucleosides are designed to improve their therapeutic efficacy, However this strategy faces the difficult selectivity issues of nucleoside chemistry. Therefore, the goal of this review is to give an idea of the opportunities provided by biocatalyst CAL-B procedures in the preparation of different types of bioactive nucleoside compounds. The function of Candida Antarctica lipase-B (CAL-B) in organic synthesis is reviewed. This enzyme has been found to be a principally efficient and robust lipase catalyzing an unexpected variety of reactions including many different regio- chemo, and diastereo-selective synthesis. Moreover, the structure of Candida Antarctica lipase-B is an example of an enzyme for which its specificity has been predicted based on acylation and deacylation mechanism on substrates.     

Efficient asymmetric oxidation of sulfides and kinetic resolution of sulfoxides catalyzed by a vanadium-salan system

The asymmetric oxidation of sulfides to chiral sulfoxides with hydrogen peroxide in good yield and high enantioselectivity has been catalyzed very effectively by chiral vanadium-salan [N,N'-alkyl bis(salicylamine)] complex. The salan ligand shows results superior in terms of reactivity and enantioselectivity to those of salen [N,N'-alkylene bis(salicylideneimine)] analogue, and provides the sulfoxide with opposite configuration. The high enantioselectivity of this reaction is the direct result of the asymmetric oxidation. The efficient kinetic resolution of racemic sulfoxides catalyzed by the vanadium-salan system is also described.