Racemization of α-Alkyl-β-Keto Esters and Enantioselective Total Synthesis of Two C-2′′′Epimers of Plant Glycerolipid Santinol C

Enantio- and Diastereoselective Mannich Reactions of ß-Dicarbonyls by Second Stage Diastereoconvergent Crystallization

The synthesis of chiral α-monosubstituted-ß-dicarbonyls is a challenging task in asymmetric catalysis due to the rapid, typically uncontrolled, product racemization or epimerization under most reaction conditions. For this reason, diastereoselective additions of unsubstituted ß-dicarbonyls to π-electrophiles are unusual. Herein, we disclose a simple catalytic crystallization-driven enantio- and diastereoselective Mannich reaction for the synthesis of stereodefined α-monosubstituted-ß-keto esters, dissymmetric ß-diesters, dissymmetric ß-diketones, and ß-keto amides that productively leverages product epimerization in solution. Mechanistic studies suggest a scenario where the initial enantioselective, diastereodivergent skeletal assembly is catalyzed by a chiral tertiary amine organocatalyst, which then facilitates second stage crystallization-induced diastereoconvergence to provide the challenging α-stereocenter in excellent stereoselectivity.

Recent advances of N-heterocyclic carbenes in the applications of constructing carbo- and heterocyclic frameworks with potential biological activity

In recent years, N-heterocyclic carbenes (NHCs) have established themselves as a masterful and promising type of organocatalyst for the speedy construction of medicinally and biologically significant molecules from common and accessible small molecules. In particular, various cyclic scaffolds, including carbocycles and heterocycles, have been synthesized using NHCs via cycloaddition reaction. An exhaustive review focused on the chemistry of NHCs in these cyclic molecules has yet to be reported. In this contribution, a general picture of the utilization of NHCs in constructing twelve kinds of bioactive cyclic skeletons is firstly presented. We provide a systematic and comprehensive overview from the perspective of cycloaddition reactions; moreover, the limitations, challenges, and future prospects were discussed.

Stereodivergent Nucleophilic Additions to Racemic β-Oxo Acid Derivatives: Fast Addition Outcompetes Stereoconvergence in the Archetypal Configurationally Unstable Electrophile

Additions of carbon nucleophiles to racemic α-stereogenic β-oxo acid derivatives that deliver enantiomerically enriched tertiary alcohols are valuable, but uncommon. This article describes stereodivergent Cu-catalyzed borylative cyclizations of racemic β-oxo acid derivatives bearing tethered pro-nucleophilic olefins to deliver highly functionalized cyclopentanols containing four contiguous stereogenic centers. The reported protocol is applicable to a range of β-oxo acid derivatives, and the diastereomeric products are readily isolable by typical chromatographic techniques. α-Stereogenic-β-keto esters are typically thought to have extreme or spontaneous configurational fragility, but mechanistic studies for this system reveal an unusual scenario wherein productive catalysis occurs on the same time scale as background substrate racemization and completely outcompetes on-cycle epimerization, even under the basic conditions of the reaction.