Various aspects of the reaction of a chiral catalyst or reagent with a racemic or enantiopure substrate

Enantiomer-selective carbamoylation of racemic alpha-hydroxy gamma-lactones with chiral Cu(II) Catalysts: an example of a highly active Lewis acid catalyzed reaction

Enantiomer-selective carbamoylation of racemic alpha-hydroxy gamma-lactones with half equivalents of isocyanates in the presence of chiral Cu(II) catalysts was studied. Among a series of catalyst bearing chiral bis(oxazoline) (box) and pyridine(bisoxazoline) ligands, [Cu(tBu-box)]X(2) [X=OSO(2)CF(3) (3 a), SbF(6) (3 b)] showed the highest enantioselectivity in the reaction of pantolactone (1 a). Use of n-C(3)H(7)NCO, a small alkyl isocyanate, in CH(2)Cl(2) solution was important to achieve a high level of enantiomer selection. The tBu-box-Cu(II) catalyst efficiently differentiated two enantiomers of beta-substituted alpha-hydroxy gamma-lactones under the optimized reaction conditions, resulting in a stereoselectivity factor (s=k(fast)/k(slow)) of up to 209. Furthermore, this catalyst is highly active, so that the carbamoylation can be conducted with a substrate-to-catalyst molar ratio of 2000-3000. A catalytic cycle of this reaction is also proposed.

The total synthesis of (-)-cyanthiwigin F by means of double catalytic enantioselective alkylation

Double catalytic enantioselective transformations are powerful synthetic methods that can facilitate the construction of stereochemically complex molecules in a single operation. In addition to generating two or more stereocentres in a single reaction, multiple asymmetric reactions also impart increased enantiomeric excess to the final product in comparison with the analogous single transformation. Furthermore, multiple asymmetric operations have the potential to independently construct several stereocentres at remote points within the same molecular scaffold, rather than relying on pre-existing chiral centres that are proximal to the reactive site. Despite the inherent benefits of multiple catalytic enantioselective reactions, their application to natural product total synthesis remains largely underutilized. Here we report the use of a double stereoablative enantioselective alkylation reaction in a concise synthesis of the marine diterpenoid (-)-cyanthiwigin F (ref. 8). By employing a technique for independent, selective formation of two stereocentres in a single stereoconvergent operation, we demonstrate that a complicated mixture of racemic and meso diastereomers may be smoothly converted to a synthetically useful intermediate with exceptional enantiomeric excess. The stereochemical information generated by means of this catalytic transformation facilitates the easy and rapid completion of the total synthesis of this marine natural product.

Regiodivergent epoxide opening: a concept in stereoselective catalysis beyond classical kinetic resolutions and desymmetrizations

An approach to highly regiodivergent epoxide openings (REOs) is presented. The very popular kinetic resolutions of epoxides and openings of meso-epoxides constitute subclasses of such REOs. REOs are attractive for parallel resolutions, double asymmetric reactions of enantiomerically enriched epoxides, and for semisynthetic applications in the functionalization of natural products. They have been notoriously difficult to realize by means of SN2 mechanisms. Our titanocene-catalyzed radical REO addresses this issue by decoupling epoxide opening and radical trapping and is firmly based on a mechanistic study of the reductive epoxide opening.

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.

Asymmetric Amplification by Kinetic Resolution Using A Racemic Reagent: Example in Amine Acetylation

The reaction of a racemic reagent on a mixture of enantiomers with small ee (ee=enantiomeric excess) has been studied for amine acylation. A substantial asymmetric amplification could be realized, for example, from 67 to >95.5 ee. The combination of asymmetric amplifications is subsequently discussed. Two sequential asymmetric amplifications, one using a racemic reagent and another using a positive nonlinear effect allowed us to start from 1.5 % ee and end with a large amount of a product of 97 % ee.

Kinetic Resolution Displaying Zeroth Order Dependence on Substrate Consumption: Copper-Catalyzed Asymmetric Alcoholysis of Azlactones

Kinetic resolution of 4-alkyl-2-aryl-5-oxo-4,5-dihydrooxazole-4-carboxylic acid esters (azlactones 1) were achieved by copper-DTBM-SEGPHOS catalyzed alcoholysis reaction with good selectivity (12 examples). Variation of ee of unreacted substrates 1 and products 2 with conversion was found to follow the theoretical line of zeroth-order kinetic resolution, for which the selectivity profiles and graphical analysis were presented for the first time. The efficiency of resolution in zeroth-order reaction is higher than first-order reaction. For example, the reaction with 1a afforded (S)-1a (99% ee) and (R)-2a (74% ee) at 57% conversion, where the k(rel) values were calculated to be 6.7 as zeroth-order kinetic resolution and 37 as first-order kinetic resolution.

3,3‘-Bis(trisarylsilyl)-Substituted Binaphtholate Rare Earth Metal Catalysts for Asymmetric Hydroamination

Chiral 3,3'-bis(trisarylsilyl)-substituted binaphtholate rare earth metal complexes (R)-[Ln{Binol-SiAr3}(o-C6H4CH2NMe2)(Me2NCH2Ph)] (Ln = Sc, Lu, Y; Binol-SiAr3 = 3,3'-bis(trisarylsilyl)-2,2'-dihydroxy-1,1'-binaphthyl; Ar = Ph (2-Ln), 3,5-xylyl (3-Ln)) and (R)-[La{Binol-Si(3,5-xylyl)3}{E(SiMe3)2}(THF)2] (E = CH (4a), N (4b)) are accessible via facile arene, alkane, and amine elimination. They are efficient catalysts for the asymmetric hydroamination/cyclization of aminoalkenes, giving TOF of up to 840 h(-1) at 25 degrees C for 2,2-diphenyl-pent-4-enylamine (5c) using (R)-2-Y. Enantioselectivities of up to 95% ee were achieved in the cyclization of 5c with (R)-2-Sc. The reactions show apparently zero-order rate dependence on substrate concentration and first-order rate dependence on catalyst concentration, but rates depend on total amine concentrations. Activation parameters for the cyclization of pent-4-enylamine using (R)-2-Y (deltaH(S)(double dagger) = 57.4(0.8) kJ mol(-1) and deltaS(S)(double dagger) = -102(3) J K(-1) mol(-1); deltaH(R)(double dagger) = 61.5(0.7) kJ mol(-1) and deltaS(R)(double dagger) = -103(3) J K(-1) mol(-1)) indicate a highly organized transition state. The binaphtholate catalysts were also applied to the kinetic resolution of chiral alpha-substituted aminoalkenes with resolution factors f of up to 19. The 2,5-disubstituted aminopentenes were formed in 7:1 to > or = 50:1 trans diastereoselectivity, depending on the size of the alpha-substituent of the aminoalkene. Rate studies with (S)-1-phenyl-pent-4-enylamine ((S)-15e) gave the activation parameters for the matching (deltaH(double dagger) = 52.2(2.8) kJ mol(-1), deltaS(double dagger) = -127(8) J K(-1) mol(-1) using (S)-2-Y) and mismatching (deltaH(double dagger) = 57.7(1.3) kJ mol(-1), deltaS(double dagger) = -126(4) J K(-1) mol(-1) using (R)-2-Y) substrate/catalyst combination. The absolute configuration of the Mosher amide of (2S)-2-methyl-4,4-diphenyl-pyrrolidine and (2R)-methyl-(5S)-phenyl-pyrrolidinium chloride, prepared from (S)-15e, were determined by crystallographic analysis. Catalyst (R)-4a showed activity in the anti-Markovnikov addition of n-propylamine to styrene.

Quasienantiomers and Quasiracemates: New Tools for Identification, Analysis, Separation, and Synthesis of Enantiomers

The old adage "never mix pure organic compounds" holds in spades for enantiomers. After going to all the trouble to make enantiopure molecules, who in their right mind would ever mix them to make a racemate? Quasienantiomers are almost enantiomers, but not quite. Yet unlike enantiomers, the interest is not so much in separating them but in mixing them to make quasiracemates. This backwards thinking opens new possibilities for identification, analysis, separation and synthesis of enantiomers. A short history is provided, the terms are defined and illustrated, and recent applications of quasienantiomers, quasiracemates and related species are reviewed.

Preparation of methyl (1R,2S,5S)- and (1S,2R,5R)-2-amino-5-tert-butyl-cyclopentane-1-carboxylates by parallel kinetic resolution of methyl (RS)-5-tert-butyl-cyclopentene-1-carboxylate

Comparison of the kinetic and parallel kinetic resolutions of methyl (RS)-5-tert-butyl-cyclopentene-1-carboxylate allows for the efficient synthesis of both (1R,2S,5S)- and (1S,2R,5R)-enantiomers of methyl 2-amino-5-tert-butyl-cyclopentane-1-carboxylate.

Novel catalytic kinetic resolution of racemic epoxides to allylic alcohols

[formula: see text] The kinetic resolution of racemic epoxides via catalytic enantioselective rearrangement to allylic alcohols was investigated. Using the Li-salt of (1S,3R,4R)-3-(pyrrolidinyl)methyl-2-azabicyclo [2.2.1] heptane 1 as catalyst allowed both epoxides and allylic alcohols to be obtained in an enantioenriched form.

Stereocontrol at the steady state in radical cyclizations of acyclic dihalides

The first examples of manipulating stereocontrol solely by reaction topography in radical cyclizations starting from acyclic precursors are reported. The kinetic model for acyclic compound stereoselection is verified experimentally by conducting a series of radical cyclizations of 1,3-dihalo-2-(1-phenyl-3-butynyl)propanes with triphenyltin hydride and measuring the ratios of the products. Monohalide intermediates are observed for the first time, and evidence that bromide- and iodide-substituted radicals have different cyclization rate constants is provided.

Non-sequential processes for the transformation of a racemate into a single stereoisomeric product: proposal for stereochemical classification

Non-sequential processes which allow the transformation of a racemate into a single stereoisomeric product without the occurrence of an "undesired" isomer are classified according to their underlying stereochemistry. A re-definition of the term "de-racemization" is proposed.