Search results for "Selectivity"

Stereoselective Conjugate Addition of Mixed Organoaluminum Reagents to ?,?-Unsaturated N-Acyloxazolidinones Derived from Carbohydrates.

2005

The stereoselective synthesis of β-branched carboxylic acid derivatives was accomplished by conjugate addition of mixed organoaluminum reagents to chiral α,β-unsaturated N-acyloxazolidinones. Mixed organoaluminum reagentswere generated in situ by transmetalation of Grignard or organolithium compounds with methylaluminum dichloride. Efficient stereocontrol was achieved using different bicyclic glycosamine-derived oxazolidinones, yielding alternatively (R)- or (S)-configured β-branched carboxylic acid derivatives.

Mixed Dialkylaluminum Chlorides and Mixed Trimethylorganoaluminates in Chemoselective 1,4 Addition Reactions to Alkylidene Malonic Acid Diethyl Ester

2000

Mixed alkyl-methyl- and aryl-methylorganoaluminum chlorides 6 were formed by reaction of methylaluminum dichloride with organolithium or Grignard compounds and used for chemoselective 1,4 addition of higher alkyl, aryl, alkenyl and alkinyl groups to alkylidine malonic esters 1 and 2. As an alternative, mixed trimethylorganoaluminates 7 can also be applied for these Michael addition reactions. For conjugate addition of alkenyl groups to alkylidene malonates 1 and 2, alkenyl diisopropylalanes 10 obtained from alkynes and diisopropylaluminum hydride proved the most efficient reagents. Using these novel mixed organoaluminum compounds, β-branched malonic (carboxylic) acid derivatives 3c, 8, 9 an…

ChemInform Abstract: Stereoselective Synthesis of syn-α-Methyl-β-hydroxy Esters.

2000

Abstract Boron enolates of an ethyl ketone structurally related to erythrulose react with achiral aldehydes in a highly stereoselective fashion to yield 1,2- syn /1,3- syn stereoisomers. Oxidative cleavage of the aldol adducts yields enantiopure O -formylated syn -α-methyl-β-hydroxy esters, easily cleaved to the corresponding hydroxyl-free compounds. The aforementioned ketone behaves therefore as a chiral propionate enolate equivalent.

Modular one-pot synthesis of tetrasubstituted pyrroles from alpha-(alkylideneamino)nitriles.

2007

2,3,4,5-Tetrasubstituted pyrroles have been prepared with high regioselectivity by a formal cycloaddition of alpha-(alkylideneamino)nitriles and nitroolefins followed by elimination of HCN and HNO2. The reaction allows the convergent construction of the pyrrole ring in four steps from a nitroalkane and three aldehydes.

Stereoselective Synthesis of 2-Substituted Pyrrolidines

2000

Using O-pivaloyl protected D-galactopyranosylamine and D-arabinopyranosylamine, (S) or (R) configured α-substituted homoallylamines are synthesized with high diastereoselectivity by reaction of the corresponding aldimines with allyltributylstannane. Electrophile-induced endo-trig-cyclization of these N-glycosylhomoallylamines gave the 2-substituted pyrrolidines of high diastereomeric purity.

ChemInform Abstract: Stereoselective Synthesis of 2-Substituted Pyrrolidines.

2000

Using O -pivaloyl protected D-galactopyranosylamine and D-arabinopyranosylamine, ( S ) or ( R ) configured α-substituted homoallylamines are synthesized with high diastereoselectivity by reaction of the corresponding aldimines with allyltributylstannane. Electrophile-induced endo -trig-cyclization of these N -glycosylhomoallylamines gave the 2-substituted pyrrolidines of high diastereomeric purity.

Aziridination of Aromatic Aldimines Through Stabilized Ammonium Ylides: A Molecular Electron Density Theory Study

2019

ChemInform Abstract: Calixcrowns : Synthesis and Properties

2009

The synthesis and properties of calix[n]crowns (n = 4–8), calix[n]biscrowns and their related compounds, resorcinarene crowns, have been discussed and reviewed. These macrocycles exhibit remarkable ionophoric properties toward alkali and alkaline earth metal cations, as well as, to tertiary amines. The selectivity and efficiency of calixcrowns in binding cations have been attributed to their structural features, which include substituent effects and size of the crown ether moiety and, conformation of the parent calixarene.

An analysis of the regioselectivity of 1,3-dipolar cycloaddition reactions of benzonitrile n-oxides based on global and local electrophilicity and nu…

2009

The regioselectivity of the 1,3-dipolar cycloaddition (13DC) reactions of benzonitrile N-oxides (BNOs) with electrophilic and nucleophilic alkenes has been analyzed by using global and local nucleophilicity and electrophilicity reactivity indices defined within the conceptual DFT. The BNOs react with electron-deficient and electron-rich ethylenes, but the regioselectivities of these polar reactions are different. Whereas the reactions with electron-rich ethylenes are completely regioselective, yielding 5-isoxazolines, a change in the regioselectivity is observed in the reactions with electron-deficient ethylenes, which yield a mixture of 4- and 5-isoxazolines. Analysis of the energies, geom…

Stereo and regioselectivity in the phenylation of cationic allylpalladium(II) α-diimine complexes by tetraphenylborate anion

1993

The reaction of the cationic complex [Pd(4-methoxy-1,3-η3-cyclohexenyl)(py-2-CHNC6H4OMe-4)]+ (1) with BPh4− in the presence of fumaronitrile yields trans-3-methoxy-6-phenylcyclohexene (2a) and trans-4-methoxy-3-phenylcyclohexene (2b), in ca. 1 : 1 molar ratio. The trans stereochemistry of these products implies that the phenylation of the allyl ligand involves prior transfer of a phenyl group from BPh4− to the metal, followed by reductive coupling of the organic moieties. In the reactions of [Pd(η3-1,1-R1,R2-C3H3)(NN′)]+ (3) [NN′ 4-MeOH4C6NCHCHNC6H4OMe-4; py-2-CHNR (R  C6H4OMe-4, Me, or CMe3), 2,2′-bipyridine (bipy); R1  H, R2  Ph, Me; R1  R2  Me; with BPh4− in the presence of …