Search

Cellulose-derived chiral pyrrolidines were synthesized in excellent yields, regioselectivities, and stereoselectivities via a 1,3-dipolar cycloaddition reaction between levoglucosenone and azomethine ylides. An unprecedented isomerization event led to a new family of pyrrolidines with an unusual relative stereochemistry. Preliminary results showed that these compounds are promising organocatalysts for iminium ion-based asymmetric Diels–Alder reactions. [ABSTRACT FROM AUTHOR]

Abstract: An experimental and computational study was conducted to assess the effect of chlorine and bromine substitution in Diels–Alder reactions involving chiral α-halo enones as dienophiles. An important rate enhancement was observed in the case of acyclic dienes, while the use of cyclic dienes resulted in prolonged reaction times and lower yields. DFT calculations suggest that these reactions are governed by finely balanced geometric and electronic features at the TSs. [Copyright &y& Elsevier]

Microwave-assisted asymmetric Diels-Alder reactions using levoglucosenone-derived chiral auxiliaries have been investigated. The cycloaddition reaction between the corresponding chiral acrylate and cyclopentadiene showed important rate enhancements under microwave irradiation compared to conventional heating conditions, whereas the yields and diastereoselectivities were not significantly affected. [ABSTRACT FROM AUTHOR]

Abstract: Chiral alcohols were designed and easily prepared from levoglucosenone, a biomass-derived valuable synthon. These alcohols were tested as chiral auxiliaries in asymmetric Diels–Alder reactions between the corresponding acrylates with acyclic and cyclic dienes. The regio, stereo, and facial selectivity varied from very good to excellent, depending upon the benzylic substitution of the auxiliary and the diene employed. As a consequence, after removal of the auxiliary, the resulting carboxylic acid derivatives were obtained in 72–99% ee. [Copyright &y& Elsevier]

Abstract: New chiral auxiliaries derived from levoglucosenone were developed in a simple and efficient way and evaluated as chiral inductors in asymmetric Diels–Alder reactions between the corresponding acrylate derivatives and cyclopentadiene. The results showed an important influence of the absolute configuration of the C(2) center of the auxiliary on the level of stereoinduction obtained. [Copyright &y& Elsevier]

A synthetic strategy has been developed for the preparation of immobilized 1,2-aminoalcohols starting from easily available and renewable chiral building blocks. They were tested as chiral ligands for the asymmetric diethylzinc addition to carbonyl compounds. Enantioselectivities were comparable to those observed for non-immobilized analogs. These results provide strong evidence for the flexibility of our approach to generate highly valuable supported chiral ligands derived from cellulose-rich materials. [ABSTRACT FROM AUTHOR]

Abstract: An appealing inversion in the regiochemical outcome of Diels–Alder reactions between levoglucosenone (1) and its α-bromo derivative (5) with isoprene (2) was studied computationally. Based on different DFT calculations we concluded that both reactions should display the same regioselectivity. This result was further validated experimentally. [Copyright &y& Elsevier]

Diels–Alder cycloaddition between an enantiomerically pure protected cis -cyclohexadiene diol metabolite and optically pure levoglucosenone derived from cellulose gave rise to complex pentacyclic natural product like structures in a chemically succinct process. The adducts were fully characterized and the observed results were in accord with theoretical calculations performed on the possible course of the reaction. [ABSTRACT FROM AUTHOR]

Abstract: The first application of chiral auxiliaries synthesized from levoglucosenone (a biomass-derived anhydrosugar) in asymmetric 1,3-dipolar cycloaddition reactions of azomethine ylides is herein reported. The corresponding pyrrolidinic cores were obtained in excellent levels of regio and stereocontrol, good to excellent π-facial selectivities, and could be isolated enantiomerically pure by column chromatography. Unexpected NMR observations coupled with DFT calculations allowed the stereochemical assignment of the synthesized adducts. The stereochemical assignment performed in silico was further unambiguously validated by structural X-ray diffraction analysis. [Copyright &y& Elsevier]

The first application of levoglucosenone-derived chiral auxiliaries in asymmetric 1,3-dipolar cycloaddition reactions of azomethine ylides is given. [ABSTRACT FROM AUTHOR]