Levels Of Diastereoselectivity Research Articles

Regulation of diastereoselectivity through the epimerisation of a cyclic intermediate in the reaction of cyclic ketene ortho ester with aldehydes

A highly stereocontrolled synthesis of 3 was accomplished. This involves the conversion of the cyclic intermediate 5 (from the reaction of dihydropyran 1 with aldehyde promoted by Lewis acid catalyst) to 6 in the presence of bases and introduction of an ester functionality from hydrolysis of the ortho ester intermediate. The method described herein is successful with a variety of aldehydes and affords products in high yields (55–89%) with useful levels of diastereoselectivity (10–30∶1). Experiments are described which unambiguously established the stereochemistry for the coupling products. Synthetic manipulations of products by functional group transformations to useful compounds are also reported.

On the Stereochemistry of the Dihydropyrone Diels−Alder Reaction

The Diels−Alder reaction of activated dihydropyrones with electron-rich dienes proceeds with synthetically useful levels of diastereoselectivity. Product ratios vary with the nature and location of substituents on diene and dienophile, as well as with reaction conditions. Lewis acid catalyzed reactions of related pyrone and benzopyrone dienophiles are also diastereoselective.

A highly diastereoselective synthesis of 3-substituted isoindolin-1-one dericatives

A highly diastereoselective method for the synthesis of 3-substituted isoindolin-1-ones has been developed through application of a tricyclic lactam substrate as an N-acyliminium ion precursor. Ring-opening of the tricyclic lactam with triethylsilane as hydride source generates the targets with up to exclusive levels of diastereoselectivity. This approach compliments that reported in the preceding paper.

Diastereoselective manipulations of bicyclo[m.1.0]alkane derivatives. 3. Nucleophilic additions to the carbonyl carbon of (1R,8R)-bicyclo[6.1.0]nonan-2,6-dione2-(2S,3S)-1,4-di-O-methyl-1,2,3,4-butanetetrol ketal

Abstract Conformations of the title compound were studied using a Monte Carlo search technique. Two principal conformational motifs were observed for the bicyclic carbocycle in which both faces of the carbonyl appear susceptible to nucleophilic attack. The title compound was synthesized in eight steps from cis-1,5-cyclooctanediol. Additions of nucleophiles (e.g., CH3Li) to the title compound gave adducts in good yields, but with low levels of diastereoselectivity, in agreement with computational prediction.

Highly Diastereoselective Reduction of Bicyclic and Steroidal Conjugated Enones with Dibal in THF-DME

Reduction of four bicyclic and two steroidal conjugated enones employing diisobutylaluminum hydride (DIBAL) in tetrahydrofuran (THF)/1,2-dimethoxyethane (DME) led to allylic alcohols with high levels of diastereo-selectivity.

1,5-Asymmetric induction of chirality: diastereoselective addition of organoaluminium reagents and allylstannanes into aldehyde groups in the side-chain of π-allyltricarbonyliron lactone complexes

π-Allyltricarbonyliron lactone complex 5, bearing an aldehyde group in the side-chain, can be easily prepared from commercially available (2E,4E)-ethyl hexadienoate and reacts with organoaluminium and allylstannane nucleophiles to afford secondary alcohols. In analogy with the corresponding ketone-substituted complexes, the lactone tether acts via the Fe(CO)3 moiety as a source of asymmetric induction. The levels of diastereoselectivity are generally reduced, however, compared with those obtained using ketone complexes. This can be attributed, at least in part, to the carbonyl appendage adopting both s-cis and s-trans conformations. The level of 1,5-asymmetric induction is strongly dependent upon the nature of the nucleophile in the case of the organoaluminium reactions and upon the reaction temperature in the case of BF3-mediated addition of allylstannanes into the aldehyde group.

Alkylation ofl- andd-Menthyl Pentanoate with (20S)-20-Iodomethyl-4α-methyl-5α-pregnane

Abstract The alkylation of l- and d-menthyl valerate with n-hexyl iodide and with (20S)-20-iodomethyl-4α-methyl-5α-pregnane gave modest levels of diastereoselection. The chirality of the ester group favoured slight degree of selectivity. The chiral centers had limited impact on diastereoselectivity

The reduction of functionalized pyrazolium salts as a stereoselective route to functionalized pyrazolidines

Functionalized pyrazolium salts 1 react with metal complex hydrides (LiAlH 4 and NaBH 4) to give pyrazolidines 2 with good-to-excellent levels of diastereoselectivity. The stereochemical course of the reduction is essentially anti, leading to trans,trans-pyrazolidines. The relative configuration of the crystalline pyrazolidine 2c has been determined by X-ray crystallographic analysis.

Stereoselective reaction of arylsulfanyl-stabilized homoenolates with aldehydes

The reaction of arylsulfanyl-stabilized lithium homoenolates with aromatic aldehydes gave anti-adducts with a high level of diastereoselectivity (up to 96% de) and fair enantioselectivity (up to 47% ee) in the presence of (–)-sparteine.

Studies on diastereoselectivity of the cobalt(I) catalyzed cycloisomerization of substituted ε-acetylenic β-ketoester

Cobalt (I) catalyzed cycloisomerization of a ε-acetylenic β′-alkyl β-ketoester led to highly functionalized methylenecyclopentanes with stereocontrol of two contiguous centers. The level of diastereoselectivity is moderate to high (de = 12–92 %).

Stereoselective bromination of dehydroamino acids with controllable retention or inversion of olefin configuration

Dehydroamino acid esters react with brominating reagents to produce syn-α-bromo imines as the major products, which undergo tautomerization to mixtures of the diastereomeric (E)- and (Z)-β-bromo-α,β-dehydroamino acids upon treatment with base. Herein, we examine the characteristics of dehydroamino acid substrates 1 that affect both the diastereofacial selectivity of the bromination and the configuration of the resulting olefin product(s) 3. In these studies, we demonstrate E- and Z-diastereoselectivity for this reaction using Kinetic and thermodynamic reaction conditions. In these studies, we demonstrate E- and Z-diastereoselectivity for these reaction using kinetic and thermodynamic reaction condition. Z-vinyl bromides were shown to be the thermodynamically more stable products and were formed from the kinetic E-isomers by DABCO-induced isomerisation. The reaction conditions detailed herein allow for the stereodivergent preparation of of both E- and Z-vinyl bromide products from either diastereomeric E- and Z-olefin starting material with useful levels of diastereoselectivity

A New Strategy for Obtaining High Level of Diastereoselectivity in the Asymmetric Diels-Alder Reaction of Chiral Dienophiles

A New Strategy for Obtaining High Level of Diastereoselectivity in the Asymmetric Diels-Alder Reaction of Chiral Dienophiles

Diastereoselective synthesis of 2,5-dialkyl tetrahydrofuran-3-ones by a copper-catalysed tandem carbenoid insertion and ylide rearrangement reaction

Cu(acac) 2-catalysed cyclisations of the α-diazo ketones 3, result in the diastereoselective formation ( > 97:3) of the trans-2,5-dialkyl tetrahydrofuran-3-ones 4. The yields and levels of diastereoselection are catalyst, solvent, and temperature dependent.

Asymmetric synthesis of functionalised pyrrolidines. Highly diastereoselective cyclisations mediated by sulfide and sulfoxide ligands

Silver(I)-catalysed cyclisation of sulfide 1b and sulfoxides 4 and 5 to give the corresponding 2-vinylpyrrolidines 2b, 6a and 7a, respectively proceeds with a high (up to 99% diastereoisomeric excess) level of diastereoselectivity; in all of these cases the sense of asymmetric induction observed has been shown to be the same.

A Route to 3,5-Dialkylated Carbohydrates: The Claisen Rearrangement of a 3-C-Methylated Aldose

ABSTRACT The thermal Claisen rearrangement of the Z-isomer (5-Z) of 3,5,6-trideoxy-1,2-O-isopropylidene-3-C-methyl-α-D-ribo-hept-5-eno-1,4-furanose with triethyl orthoacetate in the presence of propionic acid gave two rearrangement products with a high level of diastereoselectivity (approximately 8.5 to 1). On the other hand, the re-arrangement executed by using the E-isomer resulted in the formation of the products without significant stereoselectivity (approximately 1.6 to 1 ratio). The S-configuration for the newly introduced stereogenic center in the major product was secured by chemical correlation with our previously reported compound, which was the major rearrangement product of 5,6-dideoxy-1,2-O-isopropylidene-α-D-ribo-hept-5-eno-1,4-furanose. The transition state argument applied to the similar models accounts well for the stereoselectivity observed in the case of 5-Z.

Diastereoselectivity in nucleophilic addition reactions to (.alpha.,.beta.-dialkoxyacyl) silanes: an operationally useful route to optically active 1,2,3-syn-triols

[syn-α-Alkoxy-β-(silyloxy)acyl] silanes undergo chelation-controlled nucleophilic addition reactions to produce Cram-type addition products (1,2,3-syn-triol derivatives) with useful levels of diastereoselectivity

Stereochemistry of the 1,2-Wittig rearrangement: A synthesis of syn-1,3-diol monoethers

The [1,2] Wittig rearrangement of β-alkoxyalkyl allyl ethers has been studied and found to provide syn-1,3-diol derivatives in 14–32% yield and with useful levels of diastereoselection.