Reversal of enantioselectivity induced by the achiral part of an organocatalyst in a Diels–Alder reaction

Synthesis and Antimicrobial Evaluation of Chiral 3, 5-Diaryl-5, 6-dihydrothiazolo[ 2, 3-c][1, 2, 4]triazoles

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Disclosed herein is a general approach for the synthesis of chiral thiazolo triazoles 5a-e. An efficient 3-step synthetic strategy has been developed to obtain the fused heterocycles in good yields. The key step involves formation of a secondary carbocation under acidic condition and intramolecular attack of the nitrogen of the 1,2,4-triazolo part leads to the formation of fused bicyclic compound in a regioselective manner. A new chiral center was created during the reaction and Chiral HPLC analyses confirmed the presence of the same and the racemic nature of the synthesized compounds. Their antimicrobial activities were evaluated by broth micro-dilution method and expressed as the minimum inhibitory concentration. The preliminary bioassay results demonstrate that most of the target compounds exhibit a significantly wide spectrum activity against S. aureus and E. coli comparable to ampicillin. The efficacies of compounds against C. albicans are either more or similar compared to Griseofulvin.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

A chiral analog of the bicyclic guanidine TBD: synthesis, structure and Brønsted base catalysis

Starting from (S)-β-phenylalanine, easily accessible by lipase-catalyzed kinetic resolution, a chiral triamine was assembled by a reductive amination and finally cyclized to form the title compound 10. In the crystals of the guanidinium benzoate salt the six membered rings of 10 adopt conformations close to an envelope with the phenyl substituents in pseudo-axial positions. The unprotonated guanidine 10 catalyzes Diels–Alder reactions of anthrones and maleimides (25–30% ee). It also promotes as a strong Brønsted base the retro-aldol reaction of some cycloadducts with kinetic resolution of the enantiomers. In three cases, the retro-aldol products (48–83% ee) could be recrystallized to high enantiopurity (≥95% ee). The absolute configuration of several compounds is supported by anomalous X-ray diffraction and by chemical correlation.