Showing papers on "Chiral Lewis acid published in 1995"

Chiral lewis acid catalysed asymmetric nucleophilic ring opening of cyclohexene oxide

Abstract: Titanium and zirconium complexes of bispicolinic amides catalyze the ring opening of cyclohexene oxide with trimethylsilyl azide as nucleophile. The product, 1-azido-2-trimethylsilyloxycyclohexane, was obtained in up to 71% enantioselectivity when a catalyst prepared from (S,S)-N,N′-bis(2-pyridinecarboxamide)-1,2-diphenylethane and zirconium tetra-t-butoxide was employed under optimum conditions, which included the addition of a small amount of diethylamine in the catalyst preparation step. The nature of the catalyst is still unknown, but it seems probable that oligomeric metal species are involved.

Preparation of a new asymmetric lanthanoid complex catalyst for the Mukaiyama aldol reaction

Abstract: Several chiral Lewis acid catalysts were prepared from lanthanoid trifluoromethanesulfonate (triflate) and a chiral sulfonamide ligand, and were applied to the Mukaiyama aldol reaction. The reaction proceeded in a catalytic manner with moderate enantiomeric excess.

A Novel Approach to the Synthesis of Taxol. A Synthesis of Optically Active 3,7-dibenzyloxy-4,8-di-t-butyl-dimethylsiloxy-5,5-dimethyl-6-p-methoxybenzyloxy-2-octanone by Way of Stereoselective Aldol Reactions.

Abstract: Optically active 3,7-dibenzyloxy-4,8-di-t-butyldimethylsiloxy-5,5-dimethyl-6-p-methoxybenzyloxy-2-octanone (2) has been synthesized by way of diastereoselective aldol reaction between 4-benzyloxy-5-t-butyldimethylsiloxy-2,2-dimethyl-3-p-methoxy-benzyloxypentanal (3) and ketene silyl acetal 4 using MgBr2·OEt2 as a catalyst. The chiral pentanal 3 was synthesized either by asymmetric aldol reaction of both prochiral aldehyde 5 and ketene silyl acetal 4 using a chiral Lewis acid or by diastereoselective aldol reaction between the chiral aldehyde 6 derived from L-serine and the lithium enolate derived from methyl isobutyrate.

Asymmetric chemical transformations

Abstract: In situ generation of asymmetric catalysts for highly enantioselective processes, Henri Brunner catalytic hydroboration, David A. Evans and Gregory C. Fu. catalytic enantiselective 1,2- and 1,4- addition reactions, Ben L. Fierenga assymetric catalysis of carbonyl reductions, David J. Mathre catalytic assymetric ene reactions, Takeshi Nakai chiral semicorrin and bis-oxazoline ligands for asymmetric catalysts, Andreas Pfaltz assymetric catalysis of metal carbene transformations, William R. Winchester, and Michael P. Doyle chiral lewis acid catalysts in organic synthesis, Hisashi Yamamoto.

Preparation of a New Asymmetric Lanthanoid Complex Catalyst for the Mukaiyama Aldol Reaction.

Abstract: Several chiral Lewis acid catalysts were prepared from lanthanoid trifluoromethanesulfonate (triflate) and a chiral sulfonamide ligand, and were applied to the Mukaiyama aldol reaction. The reaction proceeded in a catalytic manner with moderate enantiomeric excess.

Enantioselective Diels-Alder Reaction Using Chiral Lewis Acid Prepared from Grignard Reagent and a Chiral 2-(2-p-Toluenesulfonylamino)phenyl- 4-phenyloxazoline.

Abstract: Enantioselective Diels-Alder reaction was conducted successfully using a chiral magnesium complex prepared from Grignard reagent and a chiral 2-(2-p-toluenesulfonylamino)phenyl-4-phenyloxazoline derived from easily available D-phenylglycine as a chiral auxiliary. The endo-isomer was exclusively obtained with 91%ee from the reaction of 3-crotonoyl-1,3-oxazolidin-2-one and cyclopentadiene.

A Novel Approach to the Synthesis of Taxol. A Synthesis of Optically Active 3,7-Dibenzyloxy-4,8-di-t-butyl-dimethylsiloxy-5,5-dimethyl-6-p- methoxybenzyloxy-2-octanone by Way of Stereoselective Aldol Reactions.

Abstract: Optically active 3,7-dibenzyloxy-4,8-di-t-butyldimethylsiloxy-5,5-dimethyl-6-p-methoxybenzyloxy-2-octanone (2) has been synthesized by way of diastereoselective aldol reaction between 4-benzyloxy-5-t-butyldimethylsiloxy-2,2-dimethyl-3-p-methoxy-benzyloxypentanal (3) and ketene silyl acetal 4 using MgBr2·OEt2 as a catalyst. The chiral pentanal 3 was synthesized either by asymmetric aldol reaction of both prochiral aldehyde 5 and ketene silyl acetal 4 using a chiral Lewis acid or by diastereoselective aldol reaction between the chiral aldehyde 6 derived from L-serine and the lithium enolate derived from methyl isobutyrate.

Chiral Lewis Acid-Catalyzed Asymmetric Hetero Diels-Alder Reaction of ( E)-2-Oxo-1-phenylsulfonyl-3-alkenes with Vinyl Ethers.

Abstract: (E)-2-Oxo-1-phenylsulfonyl-3-alkenes are effectively activated with the aid of a catalytic amount of chiral titanium reagents in hetero Diels-Alder reactions with vinyl ethers to produce (2R,4R) or...

Asymmetric Synthesis of Homopropargylic Alcohols from Aldehydes and Allenyltri‐n‐butylstannane.

Abstract: The reaction of allenyltri- n -butylstannane with aldehydes and a chiral Lewis acid prepared from (R)-BINOL and Ti(O- i -Pr) 4 gives homopropargylic alcohols with 82 ->99% ee.