Dual organocatalytic activation of isatins and formaldehyde tert-butyl hydrazone: asymmetric synthesis of functionalized 3-hydroxy-2-oxindoles

ASYMMETRIC SYNTHESIS: A GLANCE AT VARIOUS METHODOLOGIES FOR DIFFERENT FRAMEWORKS

Abstract:

Asymmetric reactions have made a significant advancement over the past few decades and involved the production of enantiomerically pure molecules using enantioselective organocatalysis, chiral auxiliaries/substrates, and reagents via controlling the absolute stereochemistry. The laboratory synthesis from an enantiomerically impure starting material gives a combination of enantiomers which are difficult to separate for chemists in the fields of medicine, chromatography, pharmacology, asymmetric synthesis, studies of structure-function relationships of proteins, life sciences and mechanistic studies. This challenging step of separation can be avoided by the use of asymmetric synthesis. Using pharmacologically relevant scaffolds/pharmacophores, the drug designing can also be achieved using asymmetric synthesis to synthesize receptor specific pharmacologically active chiral molecules. This approach can be used to synthesize asymmetric molecules from wide variety of reactants using specific asymmetric conditions which is also beneficial for environment due to less usage and discharge of chemicals into the environment. So, in this review, we have focused on the inclusive collation of diverse mechanisms in this area, to encourage auxiliary studies of asymmetric reactions to develop selective, efficient, environment-friendly and high yielding advanced processes in asymmetric reactions.

A new asymmetric activation strategy for hydrazones as acyl anion equivalents in the bimetallic catalyzed carbonyl-ene reaction

A new asymmetric activation strategy for hydrazones as acyl anion equivalents is developed in the bimetallic catalyzed carbonyl-ene reaction of isatins and hydrazones. Under mild conditions, optically active functionalized 3-hydroxy-2-oxindoles were furnished in up to 98% yield with up to 97% enantioselectivity. In this process, formaldehyde tert-butylhydrazone which is seldom employed in asymmetric carbonyl-ene reactions accelerated by a metallic catalyst can be activated well by a Brønsted base. A possible catalytic cycle is proposed.

Enantio- and Diastereoselective Nucleophilic Addition of N-tert-Butylhydrazones to Isoquinolinium Ions through Anion-Binding Catalysis

A highly enantio- and diastereoselective thiourea-catalyzed dearomatization of isoquinolines employing N-tert-butylhydrazones as neutral α-azo carbanions and masked acyl anion equivalents has been developed. Experimental and computational data supports the generation of highly ordered complexes wherein the chloride behaves as a template for the catalyst, the hydrazone reagent, and the isoquinolinium cation, providing excellent stereocontrol in the formation of two contiguous stereogenic centers. The ensuing selective and high-yielding transformations provide appealing dihydroisoquinoline derivatives.

Room Temperature Ionic Liquids in Asymmetric Hetero-Ene Type Reactions: Improving Organocatalyst Performance at Lower Temperatures

Room temperature ionic liquids (RTILs) have been widely used as (co)solvents in several catalytic processes modifying, in most of the cases, the catalyst activity and/or the selectivity for the studied reactions. However, there are just a few examples of their use in hydrogen bonding organocatalysis. In this paper, we show the positive effect of a set of imidazole-based ionic liquids ([bmim]BF₄ and [hmim]PF₆) in the enantioselective addition of formaldehyde tert-butylhydrazone to prochiral α-keto esters catalyzed by a sugar-based chiral thiourea. Reactions performed in the presence of low percentages of RTILs led to an increase of the catalyst activity, thereby making possible to work at lower temperatures. Thus, the chiral tert-butyl azomethyl tertiary alcohols could be obtained with moderate to good conversions and higher enantioselectivities for most of the studied substrates when using up to 30 vol% of [hmim]PF₆ as a cosolvent in processes performed in toluene.

α-Keto hydrazones in asymmetric aminocatalysis: reactivity through β-amino aza-dienamine intermediates

α-keto hydrazones activated by a bifunctional amino-thiourea catalyst generate β-amino aza-dienamine intermediates which react with nitroalkenes in highly enantioselective fashion.

Formaldehyde tert-butyl hydrazone as a formyl anion equivalent: asymmetric addition to carbonyl compounds

The asymmetric 1,2-addition of formyl anion equivalents to carbonyl compounds is a powerful synthetic tool that ideally provide access to highly functionalizable α-hydroxy aldehydes in an enantioselective fashion. In this context, the nucleophilic character of formaldehyde hydrazones, together with their remarkable stability as monomeric species, has been exploited for the functionalization of diverse carbonyl compounds, using initially auxiliary-based methodologies and, more recently, catalytic enantioselective versions. This feature article highlights our research progress employing formaldehyde tert-butyl hydrazone as a versatile formyl anion equivalent, in combination with bifunctional H-bonding organocatalysis. The design and optimization of different catalytic systems, focusing on a dual activation of both reagents, is reviewed, as well as the racemization free unmasking of the formyl group and representative product transformations for the construction of valuable, densely functionalyzed chiral building blocks.

Bifunctional Squaramide Organocatalysts for the Asymmetric Addition of Formaldehyde tert-Butylhydrazone to Simple Aldehydes

The nucleophilic addition of formaldehyde tert-butylhydrazone to simple aldehydes (a formal hetero-carbonyl-ene reaction) can be performed with good reactivity and excellent enantioselectivity by virtue of the dual hydrogen-bonding activation exerted by amide-squaramide organocatalysts. The resulting hydroxydiazenes (azo alcohols) were isolated in high yields as enantiomerically enriched azoxy compounds after a regioselective azo-to-azoxy transformation. Subsequent derivatization provides an entry to relevant amino alcohols, oxazolidinones, and derivatives thereof.

Asymmetric organocatalytic synthesis of tertiary azomethyl alcohols: key intermediates towards azoxy compounds and α-hydroxy-β-amino esters

A series of peracylated glycosamine-derived thioureas have been synthesized and their behavior as bifunctional organocatalysts has been tested in the enantioselective nucleophilic addition of formaldehyde tert-butyl hydrazone to aliphatic α-keto esters for the synthesis of tertiary azomethyl alcohols. Using the 1,3,4,6-tetra-O-acetyl-2-amino-2-deoxy-β-d-glucosamine derived 3,5-bis-(trifluoromethyl)phenyl thiourea the reaction could be accomplished with high yields (75-98%) and moderate enantioselectivities (50-64% ee). Subsequent high-yielding and racemization-free tranformations of both aromatic- and aliphatic-substituted diazene products in a one pot fashion provide a direct entry to valuable azoxy compounds and α-hydroxy-β-amino esters.

Sustainable Synthesis of Oximes, Hydrazones, and Thiosemicarbazones under Mild Organocatalyzed Reaction Conditions

Pyrrolidine catalyzes very efficiently, presumably via iminium activation, the formation of acyloximes, acylhydrazones, and thiosemicarbazones derived from aromatic and aliphatic aldehydes using equimolar amounts of reagents and green solvents. Experimental simplicity and excellent yields after a simple filtration are the main advantages of the method, being an alternative to those currently available especially for the acyl derivatives, which do not work under uncatalyzed conditions. Its application to the synthesis of acyloximes by direct condensation between aldehydes and acylhydroxylamines is unprecedented.

Catalytic asymmetric synthesis of biologically important 3-hydroxyoxindoles: an update

Oxindole scaffolds are prevalent in natural products and have been recognized as privileged substructures in new drug discovery. Several oxindole-containing compounds have advanced into clinical trials for the treatment of different diseases. Among these compounds, enantioenriched 3-hydroxyoxindole scaffolds also exist in natural products and have proven to possess promising biological activities. A large number of catalytic asymmetric strategies toward the construction of 3-hydroxyoxindoles based on transition metal catalysis and organocatalysis have been reported in the last decades. Additionally, 3-hydroxyoxindoles as versatile precursors have also been used in the total synthesis of natural products and for constructing structurally novel scaffolds. In this review, we aim to provide an overview about the catalytic asymmetric synthesis of biologically important 3-substituted 3-hydroxyoxindoles and 3-hydroxyoxindole-based further transformations.

Asymmetric organocatalytic synthesis of quaternary α-hydroxy phosphonates: en route to α-aryl phosphaisoserines

The dual activation of acyl phosphonates and formaldehyde tert-butyl hydrazone by a BINAM-derived bis-urea catalyst is the key to achieve high reactivities and enantioselectivities in the synthesis of densely functionalized quaternary α-hydroxy phosphonates. Subsequent high-yielding transformations in a 'one-pot' fashion provide direct access to valuable azoxy compounds and quaternary α-aryl-phosphaisoserines.

A catalytic asymmetric carbonyl–ene reaction of β,γ-unsaturated α-ketoesters with 5-methyleneoxazolines

A catalytic asymmetric carbonyl–ene reaction of β,γ-unsaturated α-ketoesters with 5-methyleneoxazolines was accomplished. The process was based on the utilization of a chiral N,N′-dioxide/Mg^II catalyst, providing the desired products with excellent outcomes.

Stereoselective reaction of 2-carboxythioesters-1,3-dithiane with nitroalkenes: an organocatalytic strategy for the asymmetric addition of a glyoxylate anion equivalent

Under mild reaction conditions γ-nitro-β-aryl-α-keto esters with up to 92% ee were obtained, realizing a formal catalytic stereoselective conjugate addition of the glyoxylate anion synthon.

An intermolecular C–H functionalization method for the synthesis of 3-hydroxy-2-oxindoles

An intermolecular C–H functionalization with a denitrosation-triggered cyclization method is developed for the synthesis of 3-hydroxy-2-oxindoles.