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967 KiB

Open AccessArticle

Chiral Zn(II)-Bisamidine Complex as a Lewis-Brønsted Combined Acid Catalyst: Application to Asymmetric Mukaiyama Aldol Reactions of α-Ketoesters

by Ryo Gotoh and Masahiro Yamanaka

Molecules 2012, 17(8), 9010-9022; https://doi.org/10.3390/molecules17089010 - 30 Jul 2012

Cited by 16 | Viewed by 7233

Abstract

Focusing on the steric and electronic properties of the resonance-stabilized amidine framework, a cationic metal-bisamidine complex was designed as a conjugated combined Lewis-Brønsted acid catalyst. The chiral Zn(II)-bisamidine catalyst prepared from the 2,2'-bipyridyl derived bisamidine ligand, ZnCl₂, and AgSbF₆ promoted [...] Read more.

Focusing on the steric and electronic properties of the resonance-stabilized amidine framework, a cationic metal-bisamidine complex was designed as a conjugated combined Lewis-Brønsted acid catalyst. The chiral Zn(II)-bisamidine catalyst prepared from the 2,2'-bipyridyl derived bisamidine ligand, ZnCl₂, and AgSbF₆ promoted asymmetric Mukaiyama aldol reaction of α-ketoester and α,α-disubstituted silyl enol ether to afford the α-hydroxyester having sequential quarternary carbons in good yield, albeit with low enantioselectivity. Addition of 1.0 equivalent of the fluoroalcohol having suitable acidity and bulkiness dramatically increased the enantioselectivity (up to 68% ee). DFT calculations suggested that this additive effect would be caused by self-assembly of the fluoroalcohol on the Zn(II)-bisamidine catalyst. Full article

(This article belongs to the Special Issue Stereoselective Synthesis)

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477 KiB

Open AccessArticle

A Further Contribution to the Study of Sagittamide A: Synthesis of a Pivotal Intermediate Belonging to a Rare L-Series

by Anne Humbert, Karen Plé, Dominique Harakat, Agathe Martinez and Arnaud Haudrechy

Molecules 2012, 17(7), 7709-7721; https://doi.org/10.3390/molecules17077709 - 25 Jun 2012

Cited by 1 | Viewed by 4090

Abstract

A key saggitamide intermediate corresponding to a rare sugar framework has been obtained. This approach should help to establish the overall configuration of more complex structures of the sagittamide family. Full article

(This article belongs to the Special Issue Stereoselective Synthesis)

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379 KiB

Open AccessCommunication

Ring Cleavage Reactions of Methyl α-D-Allopyranoside Derivatives with Phenylboron Dichloride and Triethylsilane

by Masaru Kojima, Yutaka Nakamura, Yuusuke Ito and Seiji Takeuchi

Molecules 2011, 16(12), 10303-10313; https://doi.org/10.3390/molecules161210303 - 13 Dec 2011

Cited by 4 | Viewed by 6761

Abstract

In the course of our studies on the regioselective carbon-oxygen bond cleavage of the benzylidene acetal group of hexopyranosides with a reducing agent, we found that a combination of a Lewis acid and a reducing agent triggered a ring-opening reaction of the pyranose [...] Read more.

In the course of our studies on the regioselective carbon-oxygen bond cleavage of the benzylidene acetal group of hexopyranosides with a reducing agent, we found that a combination of a Lewis acid and a reducing agent triggered a ring-opening reaction of the pyranose ring of methyl α-D-allopyranosides. The formation of an acyclic boronate ester by the attachment of a hydride ion at C-1 indicated that the unexpected endocyclic cleavage of the bond between the anomeric carbon atom and the pyranose ring oxygen atom proceeded via an oxacarbenium ion intermediate produced by the chelation between O5/O6 of the pyranoside and the Lewis acid, followed by nucleophile substitution with a hydride ion at C1. Full article

(This article belongs to the Special Issue Stereoselective Synthesis)

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245 KiB

Open AccessArticle

Synthesis of New Substituted Chromen[4,3-c]pyrazol-4-ones and Their Antioxidant Activities

by Abdullah Sulaiman Al-Ayed

Molecules 2011, 16(12), 10292-10302; https://doi.org/10.3390/molecules161210292 - 12 Dec 2011

Cited by 38 | Viewed by 7997

Abstract

A series of new coumarin derivatives 4 containing a 4-arylbut-3-en-2-one moiety were synthesized by condensation of 3-acetylcoumarin 1 with aryl aldehydes 2 in chloroform in the presence of piperidine. The interactions of 3-formyl-4-chlorocoumarin (3) with nitrogen-containg nucleophiles leading to the corresponding [...] Read more.

A series of new coumarin derivatives 4 containing a 4-arylbut-3-en-2-one moiety were synthesized by condensation of 3-acetylcoumarin 1 with aryl aldehydes 2 in chloroform in the presence of piperidine. The interactions of 3-formyl-4-chlorocoumarin (3) with nitrogen-containg nucleophiles leading to the corresponding substituted chromen-[4,3-c]pyrazol-4-ones 5 are described. The structures of the obtained compounds were established on the basis of 1D NMR, 2D NMR and IR and further the compounds were evaluated for possible antioxidant activities. The coumarinic chalcone 4a has been found to be the most active (IC₅₀ = 2.07 μM) in this study. Full article

(This article belongs to the Special Issue Stereoselective Synthesis)

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