4141-24-6Relevant academic research and scientific papers
Conformational Analysis of 5-Substituted 1,3-Dioxanes. 7. Effect of Lithium Bromide Addition
Juaristi, Eusebio,Díaz, Francisco,Cuéllar, Geiser,Jiménez-Vázquez, Hugo A.
, p. 4029 - 4035 (2007/10/03)
The position of equilibria, established by means of BF3, between diastereomeric cis- and trans-5- substituted-2-phenyl-1,3-dioxanes, in solvents THF and CHCl3, and in the presence of 0, 1, and 10 equiv of LiBr has been determined. The observed ΔG° values show that the addition of salt to the reaction medium influences the position of equilibrium. Lithium bromide effects on the conformational behavior are discussed in terms of lithium ion complexation events that lead to increased stability of the axial isomers when the substituent at C(5) is CO2H, CO2CH3, CONHCH3, and CH2- OH. By contrast, disruption of the intramolecular hydrogen bond present in the axial 5-acetamido derivative (cis-9 substituent equal to NHCOCH3) modifies the preference for the axial conformation in salt-free 9 to a net dominance of the equatorial isomer in the presence of LiBr. Interpretation of the experimental observations was based on models that are apparently supported by semiempirical AM1 calculations. The results derived from the present study contribute to our understanding of the processes involved in molecular recognition and may model salt effects in physiological events.
1,3-Dioxan-5-ones: synthesis, deprotonation, and reactions of their lithium enolates
Majewski, Marek,Gleave, D. Mark,Nowak, Pawel
, p. 1616 - 1626 (2007/10/02)
A general synthetic route to 2-alkyl- and 2,2-dialkyl-1,3-dioxan-5-ones, using tris(hydroxymethyl)nitromethane as the starting material, is described.Deprotonation of these compounds was studied.It was established that these dioxanones could be deprotonated with LDA; however, the reduction of the carbonyl group via a hydride transfer from LDA, giving the corresponding dioxanols, often competed with deprotonation.The reduction could be minimized by using Corey's internal quench procedure to form silyl enol ethers and was less pronounced in 2,2-dialkyldioxanones (ketals) than in 2-alkyldioxanones (acetals).Self-aldol products were observed when dioxanone lithium enolates were quenched with H2O.Addition reactions of lithium enolates of dioxanones to aldehydes were threo-selective as predicted by the Zimmerman-Traxler model.Dioxanones having two different alkyl groups at the 2-position were deprotonated enantioselectively by chiral lithium amide bases with enantiomeric excess (ee) of up to 70percent. - Key words: 1,3-dioxan-5-ones, enantioselective deprotonation, chiral lithium amides.
