N‐Methyl Pseudoephedrine Mediated Asymmetric Syntheses of Dihydroquinoxalinones for the Preparation of Flavane Analogues.

Kim, Yongtae; Kang, Kyoung Hee; Choi, Eui Ta; Lee, Min Hee; Park, Yong Sun

doi:10.1002/chin.200728151

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“…However, when the starting ester was equilibrated with triethyl amine before treatment with a nucleophile, equilibration provided an 89:11 ratio of the two diastereomers. Subsequent treatment of the equilibriated mixture with sodium p -methoxyphenoxide (PMPO − Na + ) gave ( R )- 59 with a 90:10 ratio in 77% yield, which was then converted to ( R )- 60 with the same er (Scheme ) . The dependence of the product ratio on the diastereomeric ratio of the α-bromo compounds shows equilibration of the α-bromo esters to be slower than the substitution reaction.…”

Section: Auxiliary-based Controlmentioning

confidence: 99%

“…Subsequent treatment of the equilibriated mixture with sodium p-methoxyphenoxide (PMPO -Na + ) gave (R)-59 with a 90:10 ratio in 77% yield, which was then converted to (R)-60 with the same er (Scheme 16). 20 The dependence of the product ratio on the diastereomeric ratio of the R-bromo compounds shows equilibration of the R-bromo esters to be slower than the substitution reaction. With 1,2phenylenediamine as the nucleophile, a subsequent intramolecular cyclization provided the R-substituted dihydroquinoxalinone 61 with an er of 98:2.…”

Section: Auxiliary-based Controlmentioning

confidence: 99%

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Dynamic Thermodynamic Resolution: Advantage by Separation of Equilibration and Resolution

2009

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In the investigation of a chemical reaction, researchers typically survey variables such as time, temperature, and stoichiometry to optimize yields. This Account demonstrates how control of these variables, often in nontraditional ways, can provide significant improvements in enantiomeric ratios for asymmetric reactions. Dynamic thermodynamic resolution (DTR) offers a convenient method for the resolution of enantiomeric products in the course of a reaction. This process depends on an essential requirement: the equilibration of the penultimate diastereomers must be subject to external control. As a general case, the reaction of A(R), A(S) with B under the influence of the chiral species, L*, gives resolved products C(R) and C(S). In the first step of dynamic resolution under thermodynamic control, the enantiomeric reactants A(R) and A(S) and L* form the diastereomers A(R)/L* and A(S)/L*. The equilibrium between A(R) and A(S) can be rapid, slow, or not operative, and L* can represent a ligand, an auxiliary, or a crystallization process that provides a chiral environment. Second, the populations of the diastereomers are controlled, usually by thermal equilibration. Finally, the reaction of the diastereomers with a reagent B provides the enantiomeric products C(R) and C(S). The control of the diastereomeric equilibrium distinguishes DTR from other resolution techniques. By contrast, physical resolutions separate thermodynamically stable, nonequilibrating diastereomers, and dynamic kinetic resolutions utilize kinetic control for reactions of rapidly equilibrating diastereomers. The dynamic thermodynamic resolutions discussed in this Account illustrate cases of significantly improved enantioselectivities using this technique. Although many of the well-recognized cases come from organolithium chemistry, the principles are general, and we also present cases facilitated by other chemistries. This approach has been used to control enantioselectivities in a number of different reactions, with improvements in enantiomeric ratios up to 99% from essentially racemic reactants.

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Section: Auxiliary-based Controlmentioning

confidence: 99%

Section: Auxiliary-based Controlmentioning

confidence: 99%

Dynamic Thermodynamic Resolution: Advantage by Separation of Equilibration and Resolution

2009

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N‐Methyl Pseudoephedrine Mediated Asymmetric Syntheses of Dihydroquinoxalinones for the Preparation of Flavane Analogues.

Abstract: Dynamic resolution of esters (I) allows the preparation of various title compounds without the need of optically pure amino acid derivatives. -(KIM, Y.; KANG, K. H.; CHOI, E. T.; LEE, M. H.; PARK*, Y. S.; Bull. Korean Chem.

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Dynamic Thermodynamic Resolution: Advantage by Separation of Equilibration and Resolution

Dynamic Thermodynamic Resolution: Advantage by Separation of Equilibration and Resolution

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