Absolute Configuration of Lactams and Oxazolidinones Using Kinetic Resolution Catalysts

Synthesis and application of methyl itaconate-anthracene adducts in configuration assignment of chiral secondary alcohols by 1H NMR

Novel chiral derivatising agents (CDAs) such as methyl itaconate-anthracene adducts (MIAs) were reported for the absolute configuration determination of chiral secondary alcohols by the ¹H NMR technique. These adducts were facilely prepared through well-known reactions, and furthermore, commercially available starting materials. According to these synthetic routes, the desired MIAs were afforded in 6 steps with 49% overall yield from dimethyl itaconate. Moreover, the represented MIAs provided significantly large differences of chemical shift values (Δδ^SR). No racemisation from the tertiary characteristics of the adjacent alpha carbon was observed.

Using the Competing Enantioselective Conversion Method to Assign the Absolute Configuration of Cyclic Amines with Bode's Acylation Reagents

The competing enantioselective conversion (CEC) method is a quick and reliable means to determine absolute configuration. Previously, Bode's chiral acylated hydroxamic acids were used to determine the stereochemistry of primary amines, as well as cyclic and acyclic secondary amines. The enantioselective acylation has been evaluated for 4-, 5-, and 6-membered cyclic secondary amines, including medicinally relevant compounds. The limitations of the method were studied through computational analysis and experimental results. Piperidines with substituents at the 2-position did not behave well unless the axial conformer was energetically accessible, which is consistent with the transition state geometries proposed by Bode and Kozlowski. Control experiments were performed to investigate the cause of degrading selectivity under the CEC reaction conditions. The present study expands the scope of the CEC method for secondary amines and provides a better understanding of the reaction profile.

Heteroatom-Directed Acylation of Secondary Alcohols To Assign Absolute Configuration

Birman's HBTM catalyst is effective for the enantioselective acylation and kinetic resolution of benzylic secondary alcohols. The enantioselective acylation has now been extended to secondary alcohols bearing electron-withdrawing groups such as halides and other heteroatoms. The level of selectivity is modest to good and is sufficient for determining configuration using the competing enantioselective conversion method. A mathematical analysis identifies conditions for achieving maximum differences in conversion and, consequently, assigning configuration with greater confidence. The new method is effective for halohydrins and secondary-tertiary 1,2-diols and was used to confirm the configuration of two inoterpene natural products.

Evans Enolates: Structures and Mechanisms Underlying the Aldol Addition of Oxazolidinone-Derived Boron Enolates

The soft enolization of an acylated oxazolidinone using di-n-butylboron triflate (n-Bu₂BOTf) and trialkylamines and subsequent aldol addition was probed structurally and mechanistically using a combination of IR and NMR spectroscopies. None of the species along the reaction coordinate show a penchant for aggregating. Complexation of the acylated oxazolidinone by n-Bu₂BOTf was too rapid to monitor, as was the subsequent enolization with Et₃N (triethylamine). The preformed n-Bu₂BOTf·Et₃N complex, displaying muted Lewis acidity and affiliated tractable rates, reveals a rate-limiting complexation via a transition structure with a complicated counterion. n-Bu₂BOTf·i-Bu₃N bearing a hindered amine shifts the rate-limiting step to proton transfer. Rate studies show that the aldol addition with isobutyraldehyde occurs as proffered by others.

Determination of the Absolute Configuration of β-Chiral Primary Alcohols Using the Competing Enantioselective Conversion Method

A method for determining the absolute configuration of β-chiral primary alcohols has been developed. Enantioenriched alcohols were acylated in the presence of either enantiomer of the enantioselective acylation catalyst HBTM, and the faster reaction was determined by measuring product conversion using 1H NMR spectroscopic analysis. An empirical mnemonic was developed that correlates the absolute configuration of the alcohol to the faster reacting catalyst. Successful substrates for this method include primary alcohols that bear a "directing group" on the stereogenic center; directing groups include arenes, heteroarenes, enones, and halides.

The assignment of the configuration for α-hydroxy acid esters using a CEC strategy

A simple and efficient (1)H NMR method for determining the absolute configuration of chiral α-hydroxy acid esters using a competing enantioselective conversion (CEC) strategy was developed. The α-hydroxy acid esters were acylated in the presence of Feng's chiral N,N'-dioxide-scandium(iii) complex, and the faster reaction was identified when one enantiomer of the chiral α-hydroxy acid ester was treated with both enantiomers of the ligand by NMR analysis of the reaction mixture without further purification. A mnemonic is presented to aid the assignment of the absolute configuration of the substrates.

Evans Enolates: Solution Structures of Lithiated Oxazolidinone-Derived Enolates

The results of a combination of (6)Li and (13)C NMR spectroscopic and computational studies of oxazolidinone-based lithium enolates-Evans enolates-in tetrahydrofuran (THF) solution revealed a mixture of dimers, tetramers, and oligomers (possibly ladders). The distribution depended on the structure of the oxazolidinone auxiliary, substituent on the enolate, and THF concentration (in THF/toluene mixtures). The unsolvated tetrameric form contained a D(2d)-symmetric core structure, whereas the dimers were determined experimentally and computationally to be trisolvates with several isomeric forms.

Elucidating absolute configuration of unsaturated alcohols via enantioselective acylation reactions

Enantioselective nucleophilic acylation catalysis provides a simple method of determining absolute configuration for unsaturated alcohols. Extension of this technique to natural products and synthetic compounds, as well as current limitations of this approach, are also described.

Determination of absolute configuration of secondary alcohols using thin-layer chromatography

A new implementation of the competing enantioselective conversion (CEC) method was developed to qualitatively determine the absolute configuration of enantioenriched secondary alcohols using thin-layer chromatography. The entire process for the method requires approximately 60 min and utilizes micromole quantities of the secondary alcohol being tested. A number of synthetically relevant secondary alcohols are presented. Additionally, (1)H NMR spectroscopy was conducted on all samples to provide evidence of reaction conversion that supports the qualitative method presented herein.