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Chiral alcohols are important synthetic intermediates and building blocks for the synthesis of drugs, agrochemicals, and natural products. Asymmetric reduction of prochiral ketones has been the most investigated method for accessing chiral alcohols. In this regard, organocatalyzed asymmetric reduction, as a complementary method to transition-metal- and enzyme-catalyzed reactions, has attracted tremendous interest in the past decades due to the reactions with such catalysts being metal-free and easy to operate, and principally, the ease of recovery and the ability to reuse the catalysts. Following up on a comprehensive overview on organocatalyzed asymmetric reductions of prochiral ketones in early 2018, this short review is intended to summarize the recent progress in this area from the beginning of 2018 until the end of August 2021. 1 Introduction 2 Boron-Based Chiral Organocatalysts 2.1 Boron-Containing Chiral Schiff Base Catalysts 2.2 Chiral Alpine-Borane Catalysts 2.3 Boron-Containing Chiral Frustrated Lewis Pair Catalysts 2.4 Chiral Borate Ester–Amine Complex Catalysts 3 Phosphorus-Based Chiral Organocatalysts 3.1 Chiral Phosphoric Acid Organocatalysts 3.2 Chiral Phosphinamide and Phosphoramide Organocatalysts 4 Chiral Ionic Liquid Organocatalysts 5 Chiral-Oxazoline-Based Organocatalysts 6 Conclusion and Outlook [ABSTRACT FROM AUTHOR]

The oxidative transformation of α,β-unsaturated ketones was investigated under visible-light-induced photocatalytic conditions using rhodamine 6G as an organophotocatalyst. In this organocatalysis, the mild co-oxidant bromotrichloromethane (BrCCl3) acts not only as a quencher toward the activated photocatalyst species, having reductant properties, but also as a brominating reagent for the intermediate radicals. This study shows that bromine atom transfer from BrCCl3 to intermediate radicals is a key step in the reaction mechanism of our oxidation method. [ABSTRACT FROM AUTHOR]

The syntheses of fluorinated cyclopentanone derivatives, which have pharmaceutical and agrochemical importance are reported. The catalytic reaction of copper(I) and nickel(II) difluorocarbenes with silyl dienol ethers afforded 4,4-difluoro- and 5,5-difluorocyclopent-1- en-1-yl silyl ethers, respectively. The fluorine-directed and -activated Nazarov cyclization of 1-fluorovinyl vinyl ketones, which were prepared from silyl dienol ethers and free difluorocarbene, proceeded efficiently to afford 2-fluorocyclopent-2-en-1-ones. Moreover, fluorine-directed Nazarov cyclizations of 2,2-difluorovinyl vinyl ketones and 1-(trifluoromethyl) vinyl vinyl ketones afforded 3-fluorocyclopent-2-en-1-ones and 5-(trifluoromethyl)cyclopent-2-en-1-ones, respectively. In addition, derivatization of ring-difluorinated products also provided 3-fluorocyclopent- 2-en-1-ones. [ABSTRACT FROM AUTHOR]

The reaction of aromatic nitroso derivatives with enolizable carbonyl compounds (nitroso aldol reaction) to give either α-hydroxyamino or α-aminoxy carbonyl compounds is an important synthetic method. This review illustrates the recent advances in rendering the process regio- and enantioselective as well as catalytic. By employing metal and organic catalysts a range of α-amino (α-oxyamination) and α-hydroxy (α-aminoxylation) carbonyl derivatives can be generated with total regioselectivity and high levels of enantiomeric excess. [ABSTRACT FROM AUTHOR]

Plant products (+)-sesamin, (+)-sesaminol, (+)-methylpiperitol, (+)-aschantin, and (+)-5'-hydroxymethylpiperitol were synthesized in a highly stereocontrolled manner through L-proline-catalyzed bifunctional-urea-accelerated cross-aldol reaction, followed by biomimetic construction of the furofuran lignan skeleton through a quinomethide intermediate. [ABSTRACT FROM AUTHOR]

O-3,5-Di(adamantan-1-yl)benzoyltartaric acid catalyzes the conjugate addition of bis(neopentyl glycolato)diboron to α,β-unsaturated ketones with good enantioselectivity. The addition of magnesium sulfate as a dehydrating agent and benzoic acid as a co-catalyst was found to be effective. [ABSTRACT FROM AUTHOR]

Stereodifferentiating agents such as chiral auxiliaries, reagents and additives as well as ligands for metal catalysis and organocatalysts are essential in asymmetric synthesis. While it is commonly agreed that amino acids, terpenes and alkaloids from the chiral pool make highly useful starting materials for many important stereodifferentiating agents, carbohydrates, which are the most abundant chiral natural products, are still regularly regarded as ill-suited for this purpose. Many chemists are surprisingly ignorant of the wealth of carbohydrate-derived agents available for stereoselective synthesis. This review highlights the most successful carbohydrate- derived auxiliaries, reagents, complex ligands and organocatalysts, and provides an overview of their usefulness in asymmetric transformations. [ABSTRACT FROM AUTHOR]

O-3,5-Di(adamantan-1-yl)benzoyltartaric acid catalyzes the conjugate addition of bis(neopentyl glycolato)diboron to α,β-unsaturated ketones with good enantioselectivity. The addition of magnesium sulfate as a dehydrating agent and benzoic acid as a co-catalyst was found to be effective. [ABSTRACT FROM AUTHOR]