Organocatalytic asymmetric synthesis of β,β-diaryl ketones via one-pot tandem dehydration/1,6-addition/decarboxylation transformation of β-keto acids and 4-hydroxybenzyl alcohols

Computational insights into the binding modes, keto-enol tautomerization and stereo-electronically controlled decarboxylation of oxaloacetate in the active site of macrophomate synthase

Oxaloacetic acid (OAA) is a β-ketocarboxylic acid, which plays an important role as an intermediate in some metabolic pathways, including the tricarboxylic acid cycle, gluconeogenesis and fatty acid biosynthesis. Animal studies have indicated that supplementing oxaloacetic acid shows an increase of lifespan and other substantial health benefits including mitochondrial DNA protection, and protection of retinal, neural and pancreatic tissues. Most of the chemical transformations of OAA in the metabolic pathways have been extensively studied; however, the understanding of decarboxylation of OAA at the atomic level is relatively lacking. Here, we carried out MD simulations and combined quantum mechanical/molecular mechanical (QM/MM) calculations as an example to systematically elucidate the binding modes, keto-enol tautomerization and decarboxylation of OAA in the active site of macrophomate synthase (MPS), which is a Mg(II)-dependent bifunctional enzyme that catalyzes both the decarboxylation of OAA and [4+2] cycloaddition of 2-pyrone with the decarboxylated intermediate of OAA (pyruvate enolate). On the basis of our calculations, it was found that the Mg2+-coordinated oxaloacetate may exist in enol forms and keto forms. The four keto forms can be transformed into each other by simply rotating the C2-C3 single bond, nevertheless, the keto-enol tautomerization strictly requires the assistance of pocket water molecules. In addition, the decarboxylation is stereo-electronically controlled, i.e., it is the relative orientation of the terminal carboxyl anion that determines the rate of decarboxylation. As such, the chemistry of oxaloacetate in the active site of MPS is complex. On one hand, the most stable binding mode (K-I) may undergo enol-keto tautomerization to isomerize to the enol form, which may further react with the second substrate; on the other hand, K-I may isomerize to another binding mode K-II to proceed decarboxylation to generate pyruvate enolate and CO2. Starting from K-I, the enol-keto tautomerization corresponds to a barrier of 16.2 kcal mol-1, whereas the decarboxylation is associated with an overall barrier of 19.7 kcal mol-1. These findings may provide useful information for understanding the chemistry of OAA and the catalysis of related enzymes, and they are basically in agreement with the available experimental kinetic data.

Molecular Iodine-Catalyzed N-Benzylic Sulfonamides C-N Bond Cleavage for the Decarboxylative Substitution of β-Keto Acids

Abstract:

A molecular iodine-catalyzed system for the decarboxylative substitution reactions of β-keto acids with N-benzylic sulfonamides via sp3 C-N bond cleavage has been disclosed. This procedure provides a series of α-functionalized ketones in good to excellent yields. Furthermore, the practicability of this method could be manifested efficiently in a gram-scale synthesis.

Water-mediated decarboxylative radical nitrosation of β-keto acids with tert-butyl nitrite: access to α-oximino ketones

A practical catalyst-free decarboxylative radical nitrosation system of β-keto acids with tert-butyl nitrite in water has been described.

Copper-catalyzed asymmetric 1,6-conjugate addition of in situ generated para-quinone methides with β-ketoesters

A Cu-catalyzed asymmetric 1,6-conjugate addition of in situ generated para-quinone methides (p-QMs) with β-ketoester has been developed to construct the ketoester skeleton bearing adjacent tertiary-quaternary carbon stereocenter in good yields...

Iron catalyzed tandem ring opening/1,6-conjugate addition of cyclopropanols with p-quinone methides: new access to γ,γ-diaryl ketones

An iron(III) catalyzed tandem ring opening/1,6-conjugate addition of cyclopropanols to p-quinone methides leading to γ,γ-diaryl ketones has been described. This catalytic protocol provides a novel and efficient method to access γ,γ-diaryl ketone derivatives in good to excellent yields with high functional group tolerance. Importantly, γ,γ-diaryl ketone can be further functionalized to give a versatile set of useful products.

β-Keto acids in asymmetric metal catalysis and organocatalysis

β-Keto acids, ideal surrogates of inactive ketones, play an important role in organic synthesis. The asymmetric decarboxylative reaction using β-keto acids is the one which is being studied the most. Herein we present a comprehensive review on this research topic, which is generally classified according to different catalytic systems and chiral induction modes. Additionally, some extended utilities of these methodologies for synthesizing bioactive compounds were also summarized. This review will facilitate the synthetic community to understand the role of β-keto acids in asymmetric reactions, providing many new opportunities for further exploration in this field.

Isothiourea-Catalyzed Enantioselective α-Alkylation of Esters via 1,6-Conjugate Addition to para-Quinone Methides

The isothiourea-catalyzed enantioselective 1,6-conjugate addition of para-nitrophenyl esters to 2,6-disubstituted para-quinone methides is reported. para-Nitrophenoxide, generated in situ from initial N-acylation of the isothiourea by the para-nitrophenyl ester, is proposed to facilitate catalyst turnover in this transformation. A range of para-nitrophenyl ester products can be isolated, or derivatized in situ by addition of benzylamine to give amides at up to 99% yield. Although low diastereocontrol is observed, the diastereoisomeric ester products are separable and formed with high enantiocontrol (up to 94:6 er).

Switching of Enantioselectivity in the Cu-Catalyzed Asymmetric Decarboxylative Aldol Reaction of Tryptanthrin with β-Keto Acids: An Unexpected Counteranion Effect

Cu-bisoxazoline-catalyzed enantioselective decarboxylative aldol reaction of tryptanthrin with aryl-substituted β-keto acids is developed, providing a straightforward approach to deliver a series of phaitanthrin A analogues. Both enantiomers of the products can be obtained with good to high enantioselectivity in the presence of a single chiral ligand by simply changing the copper salts. Based on the X-ray crystallographic analysis of chiral Cu(II)-bisoxazoline complexes, the tentative stereochemical models are presented to account for the observed counteranion-induced switching in enantioselectivity.

Organocatalytic 1,6-hydrophosphination of para-quinone methides: enantioselective access to chiral 3-phosphoxindoles bearing phosphorus-substituted quaternary carbon stereocenters

An efficient organocatalytic enantioselective 1,6-hydrophosphonylation of para-quinone methides has been achieved for the construction of phosphorus-substituted quaternary carbon stereocenters.

The catalyst-free decarboxylative dearomatization of isoquinolines with β-keto acids and sulfonyl chlorides in water: access to dihydroisoquinoline derivatives

An efficient and concise catalyst-free one-pot synthetic protocol for obtaining dihydroisoquinoline derivatives has been developed via the three-component condensation of isoquinolines with β-keto acids and sulfonyl chlorides. This transformation involving decarboxylative dearomatization worked well under mild and water-mediated conditions. The protocol tolerates diverse functional groups, furnishing the dihydroisoquinoline products in good to excellent yields.