Transfer enantioselective

Enantiopure ditihiiranium salt 178 has been reported to transfer enantioselectively the MeS+ group to traws-hex-3-ene to generate the corresponding thiiranium ion which, in turn, reacts with MeCN/HaO allowing the enantioselective synthesis of the vicinally di-substituted alkanes with up to 86% e.e.280. 

Acetone cyanohydrin 97 was used as a cyanide ion source by Herrera and Ricci to accomplish the phase-transfer enantioselective cyanation of in situ-generated... 

Jordan PA, Kayser-Bricker KJ, Miller SJ (2010) Asymmetric phosphorylation through catalytic P(III) phosphoramidite transfer enantioselective synthesis of D-myo-inositol-6-phosphate. Proc Natl Acad Sci USA 107 20620-20624... 

L = P(CH3)3 or CO, oxidatively add arene and alkane carbon—hydrogen bonds (181,182). Catalytic dehydrogenation of alkanes (183) and carbonylation of bensene (184) has also been observed. Iridium compounds have also been shown to catalyse hydrogenation (185) and isomerisation of unsaturated alkanes (186), hydrogen-transfer reactions, and enantioselective hydrogenation of ketones (187) and imines (188). 

Interestingly, phase-transfer catalysts including crown ethers have been used to promote enantioselective variations of Darzens condensation. Toke and coworkers showed that the novel 15-crown-5 catalyst derived from d-glucose 33 could promote the condensation between acetyl chloride 31 and benzaldehyde to give the epoxide in 49% yield and 71% A modified cinchoninium bromide was shown to act as an effective phase transfer catalyst for the transformation as well. ... 

A number of conjugate additions delivering excelent enantioselectivities tlirougli tlie use of organocuprates in tlie presence of stoichiomenic amounts of cliital fnon-transferable) ligands ate known today [7-9],... 

In the elucidation of retention mechanisms, an advantage of using enantiomers as templates is that nonspecific binding, which affects both enantiomers equally, cancels out. Therefore the separation factor (a) uniquely reflects the contribution to binding from the enantioselectively imprinted sites. As an additional comparison the retention on the imprinted phase is compared with the retention on a nonimprinted reference phase. The efficiency of the separations is routinely characterized by estimating a number of theoretical plates (N), a resolution factor (R ) and a peak asymmetry factor (A ) [19]. These quantities are affected by the quality of the packing and mass transfer limitations, as well as of the amount and distribution of the binding sites. 

Unfortunately, the highest enantioselectivity so far obtained for the synthesis of styrene oxide by this route is only 57 % ee with Goodman s sulfide 30 [21]. Thus methylidene transfer is not yet an effective strategy for the synthesis of terminal epoxides. 

Early work on the use of chiral phase-transfer catalysis in asymmetric Darzens reactions was conducted independently by the groups of Wynberg [38] and Co-lonna [39], but the observed asymmetric induction was low. More recently Toke s group has used catalytic chiral aza crown ethers in Darzens reactions [40-42], but again only low to moderate enantioselectivities resulted. 

Arai and co-workers have used chiral ammonium salts 89 and 90 (Scheme 1.25) derived from cinchona alkaloids as phase-transfer catalysts for asymmetric Dar-zens reactions (Table 1.12). They obtained moderate enantioselectivities for the addition of cyclic 92 (Entries 4—6) [43] and acyclic 91 (Entries 1-3) chloroketones [44] to a range of alkyl and aromatic aldehydes [45] and also obtained moderate selectivities on treatment of chlorosulfone 93 with aromatic aldehydes (Entries 7-9) [46, 47]. Treatment of chlorosulfone 93 with ketones resulted in low enantioselectivities. 

Reagents of type 1 are the most important and exhibit the highest reactivity towards carbonyl compounds. The reactivity can be further tuned by altering the substitution on titanium. Reagents of type 2 show lower reactivity, but higher selectivities, but have, so far, only been used occasionally (Section 1.3.3.3.8.2.1.2.). Reagents of type 3, derived from chiral alcohols, accomplish efficient enantioselective allyl transfer (Section 1.3.3.3.8.2,3.3.). 

Enantioselective electron transfer reactions are not possible in principle because the electron cannot possess chirality. Whenever the choice of enantiodifferentiation becomes apparent, it will occur in chemical steps subsequent (or prior) to electron transfer. Thus, enantioselectivities require a chiral environment in the reaction layer of electrochemical intermediates although asymmetric induction was report-... 

In nature, aminotransferases participate in a number of metabolic pathways [4[. They catalyze the transfer of an amino group originating from an amino acid donor to a 2-ketoacid acceptor by a simple mechanism. First, an amino group from the donor is transferred to the cofactor pyridoxal phosphate with formation of a 2-keto add and an enzyme-bound pyridoxamine phosphate intermediate. Second, this intermediate transfers the amino group to the 2-keto add acceptor. The readion is reversible, shows ping-pong kinetics, and has been used industrially in the production ofamino acids [69]. It can be driven in one direction by the appropriate choice of conditions (e.g. substrate concentration). Some of the aminotransferases accept simple amines instead of amino acids as amine donors, and highly enantioselective cases have been reported [70]. 

N-Tosylated P-hydroxy alkylamines (which can be easily hydrolyzed to P-hydroxyamines" ) can be prepared " by treatment of alkenes with the trihydrate of Chloramine-T and a catalytic amount of OSO4. In some cases yields can be improved by the use of phase-transfer catalysis." The reaction has been carried out enantioselectively." In another procedure, certain P-hydroxy secondary alkylamines can be prepared by treatment of alkenes with the osmium compounds... 

Because of the nature of the transition state in the pericyclic mechanism, optically active substrates with a chiral carbon at C-3 or C-4 transfer the chirality to the product, making this an enantioselective synthesis (see p. 1451 for an example in the mechanistically similar Claisen rearrangement). ... 

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