Asymmetric condensation catalytic activity

Complexation of (124) and (125) with [ Rh(COD)Cl 2] in the presence of Si(OEt)4, followed by sol-gel hydrolysis condensation, afforded new catalytic chiral hybrid material. The catalytic activities and selectivities of these solid materials have been studied in the asymmetric hydro-gen-transfer reduction of prochiral ketones and compared to that of the homogeneous rhodium complexes containing the same ligands (124) and (125) 307... 

The promising results of triazolium salt catalysis inspired our research group to synthesize a variety of chiral triazolium salts for the asymmetric benzoin condensation (Enders et al. 1996b Enders and Breuer 1999 Teles et al. 1999). Extensive investigations have shown that the enantiomeric excesses and catalytic activities are highly dependent on the substitution pattern of the triazolium system. The most active catalyst (S, S)-97, which is readily available from an intermediate of the industrial chloramphenicol synthesis, provided benzoin (857) in its (R)-configuration with 75% ee and a good yield of 66%. Remarkably, only... 

Unfortunately the bicyclic triazolium salt that had successfully been used in our research group for the enantioselective intermolecular benzoin condensation (Enders and Kallfass 2002) did not show any catalytic activity in the intramolecular reaction. We thus searched for alternative, easily accessible enantiopure polycyclic y-lactams as precursors for the synthesis of novel triazolium salts (Enders et al. 2006c for a related study see Takikawa et al. 2006). The rigid polycyclic structure of the catalysts should allow high asymmetric inductions. A first tar-... 

For example, onium ion-tagged prolines have been synthesized and their catalytic activity in direct asymmetric aldol condensation was initially studied in molecular solvents.The observed superior performance of IL-proline relative to Peg-proline or free proline, in terms of yields and enantiomeric excess, evidenced that the ionic moiety in IL-proline plays more than a silent or simply supporting role in the reaction (Scheme 4.4). ... 

Since then, optically active a-aminophosphonates have been obtained by a variety of methods including resolution, asymmetric phosphite additions to imine double bonds and sugar-based nitrones, condensation of optically active ureas with phosphites and aldehydes, catalytic asymmetric hydrogenation, and 1,3-dipolar cycloadditions. These approaches have been discussed in a comprehensive review by Dhawan and Redmore.9 More recent protocols involve electrophilic amination of homochiral dioxane acetals,10 alkylation of homochiral imines derived from pinanone11 and ketopinic acid,12 and alkylation of homochiral, bicyclic phosphonamides.13... 

The Pictet-Spengler reaction is the method of choice for the preparation of tetrahydro-P-carbolines, which represent structural elements of several natural products such as biologically active alkaloids. It proceeds via a condensation of a carbonyl compound with a tryptamine followed by a Friedel-Crafts-type cyclization. In 2004, Jacobsen et al. reported the first catalytic asymmetric variant [25]. This acyl-Pictet-Spengler reaction involves an N-acyliminium ion as intermediate and is promoted by a chiral thiourea (general Brpnsted acid catalysis). 

The methodology of the catalytic asymmetric aldol reaction has been further extended to the aldol-type condensation of (isocyanomethyl)phosphonates (12) with aldehydes, providing a useful method for the synthesis of optically active (l-aminoalkyl)phosphonic acids, which are a class of biologically interesting phosphorous analogs of a-amino acids (Scheme 8B1.6) [21,22], Higher enantioselectivity and reactivity are obtained with diphenyl ester 12b than with diethyl ester 12a (Table 8B1.6). 

Cyclopropanation, Horner-Wadsworth Emmons Reaction, and Darzens Condensation Although induction in the cyclopropanation of alkenes was reported early, this work was disputed [49]. Other reports of cyclopropanations have yielded, at best, low asymmetric inductions [llh,50]. The first example of a catalytic asymmetric Horner-Wadsworth Emmons reaction, which is promoted by a chiral quaternary ammonium salt, was reported recently by the Shioiri group (Scheme 10.10) [51]. The reaction of the prochiral ketone 74 gives optically active a,P-unsaturated ester 76 with 57% ee. 

The Strecker reaction is defined as the addition of HCN to the condensation product of a carbonyl and amine component to give a-amino nitriles. Lipton and coworkers reported the first highly effective catalytic asymmetric Strecker reaction, using synthetic peptide 43, a modification of Inoue s catalyst (38), which was determined to be inactive for the Strecker reactions of aldimines (see Scheme 6.5) [62], Catalyst 43 provided chiral a-amino nitrile products for a number of N-benzhydryl imines (42) derived from substituted aromatic (71-97% yield 64->99% ee) and aliphatic (80-81% yield <10-17% ee) aldehydes, presumably through a similar mode of activation to that for hydrocyanations of aldehydes (Table 6.14). Electron-deficient aromatic imines were not suitable substrates for this catalyst, giving products in low optical purities (<10-32% ee). The a-amino nitrile product of benzaldehyde was converted to the corresponding a-amino acid in high yield (92%) and ee (>99%) via a one-step acid hydrolysis. 

The first example of the asymmetric synthesis of P-chiral trialkyl phosphates (12) via trialkyl phosphite, in which the keystone is dynamic kinetic resolution in the condensation of a dialkyl phosphorochloridite (13) and an alcohol by the catalytic assistance of a chiral amine has been reported (Figure 2)." 2,4-Dinitrophenol (DNP) was employed as an activating reagent with ben-zyloxy-bis-(diisopropylamino) phosphite to synthesize the cyclic phosphate derivatives (14) from a series of alkane diols HO-(CH2)n-OH (n=2-6). Included was a cyclic phosphate derivative of carbohydrate (15). The mechanism of activation by 2,4-DNP and cyclization was also described (Figure 3). ... 

In biosynthesis, pyruvic acid, a representative 1,2-dicarbonyl compound, is used as a key C2 and C3 donor unit. The use of related 1,2-dicarbonyl compounds, such as a-keto esters and ct-keto anilides, as nucleophiles in catalytic asymmetric synthesis, however, is rather limited due to their high reactivity as electrophiles. Chemoselective activation of 1,2-dicarbonyl compounds as nucleophiles is required to avoid undesired self-condensation reactions of 1,2-dicarbonyl compounds. Applications of 1,2-dicarbonyl compounds as donors in asymmetric Michael reactions remained unsolved until a recent report by Sodeoka et oL Indeed, these authors have described the first example of a diastereo- and... 

Isolated reports mentioned the use of NHC-Ru species for the asymmetric Diels-Alder reaction of methacrolein and cyclopentadiene/ cyclopropana-tion of styrene with ethyl diazoacetate/ or dehydrative condensation of Al-(2-pyridyl)benzimidazole and allyl alcohol. Whittlesey and co-workers described the stoichiometric activation of C-F bonds in perfluoroaromatic substrates such as hexafluorobenzene, perfluorotoluene, and pentafluoro-pyridine using the dihydrido complex 49 (Scheme 7.9). Modification of the NHC nitrogen substituents from N-alkyl to N-aryl groups and replacement of the chelating diphosphine with two triphenylphosphine ligands afforded complexes 50 that performed the catalytic hydrodefluorination of polyfluoroarenes with alkylsilanes with reasonable turnover number (up to 200) and frequency (up to 0.86 h ). ... 

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