A Practical Synthesis of (S)-2-Cyclohexyl-2-phenylglycolic Acid via Organocatalytic Asymmetric Construction of a Tetrasubstituted Carbon Center

Light Induced Diastereoselective Ketoesterification To Access 6,5-Fused Tetrahydrobenzofuranones in Batch and Continuous Flow Conditions

Ketoesterification stands as a pivotal technique in organic synthesis, particularly due to its essential role in the construction of numerous natural products and bioactive compounds. In this study, we have successfully accomplished a visible-light-induced cyclization and diastereoselective direct ketoesterification of cyclohexadienones, facilitating access to cis 6,5-fused tetrahydrobenzofuranone derivatives. The utilization of TEMPO radical quenching experiments has provided insights, suggesting an ionic mechanism underlying this methodology. Additionally, the regioselective addition of 2-oxo-2-phenylacetate to the least hindered side in a cis-selective fashion makes this protocol more appealing toward natural product development. Incorporation of a continuous flow reaction into the batch protocol has notably bolstered the efficiency and reaction rate. Furthermore, the demonstration of gram-scale reactions in the flow setup and synthetic utility with NaOH underscore the scalability and practical applicability of this approach.

Organocatalysts based on natural and modified amino acids for asymmetric reactions

Small organic molecules predominantly containing C, H, O, N, S and P element are found promising molecule to accelerate chemical reactions and are named organocatalysis. In addition, these organocatalysts are easy availability, stable in water and air, inexpensive, and low toxicity, which confer a huge direct application in organic synthesis when compared to transition metal catalyzed reactions and becoming powerful tools in the construction of a selective chiral product. Interest on organocatalysis is spectacularly increased since last two decades, due to the novelty of the concept and selectivity. Based on the nature of the organocatalysts used, they are classified in to four major classes, among them one of the types is amino acids derived organocatalysts. Natural amino acids are playing important role in building blocks of protein construction, and also intermediate products of the metabolism. α-Amino acid is a molecule, that contains both amine and carboxyl functional group. Their particular structural characteristic determines their role in protein synthesis, and bifunctional asymmetric catalysts for stereoselective synthesis. Two functional groups present on a single carbon acting as an acid and base, which promote chemical transformations in concert similar to the enzymatic catalysis. The post translational derivatives of natural α-amino acids include 4-hydroxy-L-proline and 4-amino-L-proline scaffolds, and its synthetic variants based organocatalysts, whose catalytic activity is well documented. This chapter discussed past and present development of the organocatalysts derived from natural and modified amino acids for various important organic transformations reviewed.

Tripeptide-Catalyzed Asymmetric Aldol Reaction Between α-ketoesters and Acetone Under Acidic Cocatalyst-Free Conditions

Here, we report the tripeptide-catalyzed asymmetric aldol reaction between α-ketoesters and acetone under acidic cocatalysts-free conditions. H-Pro-Tle-Gly-OH 3g-catalyzed reactions between α-ketoesters and acetone resulted in up to 95% yield and 88% ee. Analysis of the transition state using density functional theory (DFT) calculations revealed that the tert-butyl group in 3g played an important role in enantioselectivity.

CuH-Catalyzed Enantioselective Ketone Allylation with 1,3-Dienes: Scope, Mechanism, and Applications

Chiral tertiary alcohols are important building blocks for the synthesis of pharmaceutical agents and biologically active natural products. The addition of carbon nucleophiles to ketones is the most common approach to tertiary alcohol synthesis but traditionally relies on stoichiometric organometallic reagents that are difficult to prepare, sensitive, and uneconomical. We describe a mild and efficient method for the copper-catalyzed allylation of ketones using widely available 1,3-dienes as allylmetal surrogates. Homoallylic alcohols bearing a wide range of functional groups are obtained in high yield and with good regio-, diastereo-, and enantioselectivity. Mechanistic investigations using density functional theory (DFT) implicate the in situ formation of a rapidly equilibrating mixture of isomeric copper(I) allyl complexes, from which Curtin-Hammett kinetics determine the major isomer of the product. A stereochemical model is provided to explain the high diastereo- and enantioselectivity of this process. Finally, this method was applied to the preparation of an important drug, ( R)-procyclidine, and a key intermediate in the synthesis of several pharmaceuticals.

Asymmetric synthesis of 1H-pyrrol-3(2H)-ones from 2,3-diketoesters by combination of aldol condensation with benzilic acid rearrangement

An efficient two-step protocol for the asymmetric synthesis of 1H-pyrrol-3(2H)-one derivatives in 99% ee from conveniently accessed 2,3-diketoesters has been developed.

Cascade reaction for the construction of CF3 containing tetrasubstituted furan ring

Activated olefins are well known nucleophiles, but proton abstraction from the substituted α-methyl group in nitroolefin is rare.

Cross-Aldol Reaction of Isatin with Acetone Catalyzed by Leucinol: A Mechanistic Investigation

Comprehensive mechanistic studies on the enantioselective aldol reaction between isatin (1 a) and acetone, catalyzed by L-leucinol (3 a), unraveled that isatin, apart from being a substrate, also plays an active catalytic role. Conversion of the intermediate oxazolidine 4 into the reactive syn-enamine 6, catalyzed by isatin, was identified as the rate-determining step by both the calculations (ΔG(≠) =26.1 kcal mol(-1) for the analogous L-alaninol, 3 b) and the kinetic isotope effect (kH /kD =2.7 observed for the reaction using [D6 ]acetone). The subsequent reaction of the syn-enamine 6 with isatin produces (S)-2 a (calculated ΔG(≠) =11.6 kcal mol(-1) ). The calculations suggest that the overall stereochemistry is controlled by two key events: 1) the isatin-catalyzed formation of the syn-enamine 6, which is thermodynamically favored over its anti-rotamer 7 by 2.3 kcal mol(-1) ; and 2) the high preference of the syn-enamine 6 to produce (S)-2 a on reaction with isatin (1 a) rather than its enantiomer (ΔΔG(≠) =2.6 kcal mol(-1) ).

Organocatalytic methods for C-C bond formation

Beyond doubt organocatalysis belongs to the most exciting and innovative chapters of organic chemistry today. Organocatalysis has emerged not only as a complement to metal-catalyzed reactions or to biocatalysis over the past decade, but also new asymmetric organocatalyzed reactions have been discovered that could not be accomplished by metal- or biocatalyzed reactions so far. This review gives a brief overview of organocatalyzed asymmetric C-C bond formation processes currently available.

Organocatalyzed enantioselective aldol reaction of 1H-pyrrole-2,3-diones

The enantioselective cross aldol reaction of 1-benzyl 4,5-dioxo-2-aryl-4,5-dihydro-1H-pyrrole-3-carboxylates and ketones was studied with proline derivatives or cinchona alkaloid-derived primary amines as the catalysts for the first time. trans-4-Benzoyloxy-L-proline (15) was found to be the best catalyst for acyclic ketones. For cyclohexanone, the best results were achieved with 9-deoxy-9-epi-aminoquinine (18) as the catalyst in the presence of racemic 1,1'-binaphthyl-2,2'-diyl hydrogen phosphate (19) as the cocatalyst. Using these protocols, 3-alkyl-3-hydroxy-1H-pyrrol-2(3H)-one derivatives were obtained in excellent yields and good to high ee values (up to 94% ee).

Developing novel organocatalyzed aldol reactions for the enantioselective synthesis of biologically active molecules

Aldol reaction is one of the most important methods for the formation of carbon-carbon bonds. Because of its significance and usefulness, asymmetric versions of this reaction have been realized with different approaches in the past. Over the last decade, the area of organocatalysis has made significant progresses. As one of most studied reactions in organocatalyses, organocatalyzed aldol reaction has emerged as a powerful tool for the synthesis of a large number of useful products in optically enriched forms. In this review, we summarize our efforts on the development of novel organocatalyzed aldol reactions for the enantioselective synthesis of biological active molecules. Literatures closely related to our studies are also covered.

Enamine catalysis

The reversible reaction of primary or secondary amines with enolizable aldehydes or ketones affords nucleophilic intermediates, enamines. With chiral amines, catalytic enantioselective reactions via enamine intermediates become possible. In this review, structure-activity relationships and the scope as well as current limitations of enamine catalysis are discussed.

"One-pot" access to 4H-chromenes with formation of a chiral quaternary stereogenic center by a highly enantioselective iminium-allenamine involved oxa-Michael-aldol cascade

An unprecedented organocatalytic highly enantioselective cascade Michael-aldol reaction has been developed in high yields under mild reaction conditions. The "one-pot" process affords an efficient approach to the synthetically and biologically important chiral 4H-chromenes bearing a quaternary stereogenic center. The study significantly expands the scope of less explored organocatalytic iminium-allenamine chemistry.

Highly diastereo- and enantioselective organocatalytic michael addition of α-ketoamides to nitroalkenes

The first organocatalytic enantio- and diastereoselective conjugate addition of α-ketoamides to nitroalkenes has been achieved using a bifunctional amino thiourea catalyst. In this new approach, the substrate amide proton plays a critical role in the formation of the Michael anti-adducts in high yields and high stereoselectivities. To illustrate the high synthetic potential of this methodology, the diastereo- and enantioselective synthesis of a hexasubstituted cyclohexane via a Michael-Michael-Henry cascade reaction is described.

Organocatalytic enantioselective synthesis of both diastereomers of alpha-hydroxyphosphinates

Racemic alpha-acylphosphinates and formylphosphinate hydrate were used directly as the substrates in a proline derivative-catalyzed cross aldol reaction with ketones. Because of the preexisting phosphorus stereogenic center, a mixture of two diastereomers of the corresponding alpha-hydroxyphosphinates was obtained in this reaction. Good to high enantioselectivities (up to 99% ee) were obtained simultaneously for the two diastereomers in good yields. Good diastereoselectivities were also obtained when the reaction generates an additional carbon stereogenic center.

The direct catalytic asymmetric aldol reaction

Asymmetric aldol reactions are a powerful method for the construction of carbon-carbon bonds in an enantioselective fashion. Historically this reaction has been performed in a stoichiometric fashion to control the various aspects of chemo-, diastereo-, regio- and enantioselectivity, however, a more atom economical approach would unite high selectivity with the use of only a catalytic amount of a chiral promoter. This critical review documents the development of direct catalytic asymmetric aldol methodologies, including organocatalytic and metal-based strategies. New methods have improved the reactivity, selectivity and substrate scope of the direct aldol reaction and enabled the synthesis of complex molecular targets (357 references).