Asymmetric Transfer Hydrogenation of Arylidene-Substituted Chromanones and Tetralones Catalyzed by Noyori–Ikariya Ru(II) Complexes: One-Pot Reduction of C═C and C═O bonds

Advances in the synthesis of rearranged homoisoflavonoids

Rearranged homoisoflavonoids constitute a unique group of natural products, renowned for their structural diversity and complexity. These compounds, derived from modifications in the 3-benzylchroman skeleton, are categorized into four subclasses: brazilin, caesalpin, protosappanin, and scillascillin homoisoflavonoids. This review examines the advancements in the total synthesis of these complex structures, aiming to highlight the challenges and opportunities encountered. A comparative analysis of the strategies employed thus far to synthesize these compounds provides a comprehensive understanding of the progress in this field.

Catalytic Asymmetric Transfer Hydrogenation of Acylboronates: BMIDA as the Privileged Directing Group

Developing a general, highly efficient, and enantioselective catalytic method for the synthesis of chiral alcohols is still a formidable challenge. We report in this article the asymmetric transfer hydrogenation (ATH) of N-methyliminodiacetyl (MIDA) acylboronates as a general substrate-independent entry to enantioenriched secondary alcohols. ATH of acyl-MIDA-boronates with (het)aryl, alkyl, alkynyl, alkenyl, and carbonyl substituents delivers a variety of enantioenriched α-boryl alcohols. The latter are used in a range of stereospecific transformations based on the boron moiety, enabling the synthesis of carbinols with two closely related α-substituents, which cannot be obtained with high enantioselectivities using direct asymmetric hydrogenation methods, such as the (R)-cloperastine intermediate. Computational studies illustrate that the BMIDA group is a privileged enantioselectivity-directing group in Noyori-Ikariya ATH compared to the conventionally used aryl and alkynyl groups due to the favorable CH-O attractive electrostatic interaction between the η6-arene-CH of the catalyst and the σ-bonded oxygen atoms in BMIDA. The work expands the domain of conventional ATH and shows its huge potential in addressing challenges in symmetric synthesis.

α-Alkylation and Asymmetric Transfer Hydrogenation of Tetralone via Hydrogen Borrowing and Dynamic Kinetic Resolution Strategy Using a Single Iridium(III) Complex

Here we present a novel strategy for the synthesis of enantiomerically enriched tetrahydronaphthalen-1-ols. The reaction proceeds via an alkylation (via hydrogen borrowing) and ammonium formate-mediated asymmetric transfer hydrogenation (via dynamic kinetic resolution), giving alkylated tetralols in high yields and good enantio- and diastereoselectivity across a diverse range of both alcohol and tetralone substrates. Additionally, these products were successfully derivatized to several complex molecules, demonstrating the utility of the tetrahydronaphthalen-1-ol.

Ru-catalyzed asymmetric hydrogenation of α,β-unsaturated ketones via a hydrogenation/isomerization cascade

Ru-catalyzed asymmetric hydrogenation of α-substituted α,β-unsaturated ketones has been developed for the enantioselective synthesis of chiral α-substituted secondary alcohols with high diastereo- and enantioselectivities (up to >99 : 1 dr, 98% ee). Mechanistic experiments suggest that the reaction proceeds via a Ru-catalyzed asymmetric hydrogenation of the CO bond in concert with a base-promoted allylic alcohol isomerization, and the final stereoselectivities were controlled by a DKR process during the asymmetric hydrogenation of the ketone intermediate.

Synthesis and Structure Revision of Naturally Occurring Homoisoflavane (+)-Dracaeconolide B

Dracaeconolide B (1), a naturally occurring homoisoflavane, was isolated from the red resin of Dracaena cochinchinensis. Efforts have been made to elucidate the exact structure of compound 1 since it was confirmed that dracaeconolide B did not contain a 7-hydroxy-5,8-dimethoxy moiety. The structure of dracaeconolide B was revised by synthesis of three homoisoflavanes containing a 5,6,7-trioxygenated moiety each and analysis by NMR spectroscopy. The revised structure of dracaeconolide B was proposed as 3-(4-hydroxybenzyl)-7-hydroxy-5,6-dimethoxychromane. Noyori's Ru-catalyzed asymmetric transfer hydrogenation was used to synthesize (+)-dracaeconolide B. The absolute configuration of the compound was revised to S based on the results obtained by the electronic circular dichroism calculation. We examined the antiangiogenic activity of (S)- and (R)-dracaeconolide B and of synthetic 5,6,7- and 5,7,8-trioxygenated homoisoflavanes. The results can potentially help in the synthesis of related natural products and support drug discovery to treat neovascular eye diseases.

Stereoselective Synthesis of Flavonoids: A Brief Overview

Stereoselective synthesis has been emerging as a resourceful tool because it enables the obtaining of compounds with biological interest and high enantiomeric purity. Flavonoids are natural products with several biological activities. Owing to their biological potential and aiming to achieve enantiomerically pure forms, several methodologies of stereoselective synthesis have been implemented. Those approaches encompass stereoselective chalcone epoxidation, Sharpless asymmetric dihydroxylation, Mitsunobu reaction, and the cycloaddition of 1,4-benzoquinone. Chiral auxiliaries, organo-, organometallic, and biocatalysis, as well as the chiral pool approach were also employed with the goal of obtaining chiral bioactive flavonoids with a high enantiomeric ratio. Additionally, the employment of the Diels-Alder reaction based on the stereodivergent reaction on a racemic mixture strategy or using catalyst complexes to synthesise pure enantiomers of flavonoids was reported. Furthermore, biomimetic pathways displayed another approach as illustrated by the asymmetric coupling of 2-hydroxychalcones driven by visible light. Recently, an asymmetric transfer hydrogen-dynamic kinetic resolution was also applied to synthesise (R,R)-cis-alcohols which, in turn, would be used as building blocks for the stereoselective synthesis of flavonoids.

Ru(II)-Catalyzed Asymmetric Transfer Hydrogenation of Chalcones in Water: Application to the Enantioselective Synthesis of Flavans BW683C and Tephrowatsin E

The oxo-tethered-Ru(II) precatalyst promoted the one-pot C═C/C═O reduction of chalcones using sodium formate as the hydrogen source in water through asymmetric transfer hydrogenation. Twenty-seven 1,3-diarylpropan-1-ols were obtained in good to excellent yields (up to 96%) and enantiomeric purities (up to 98:2). Our data suggested that the enones are first reduced to the corresponding dihydrochalcones (1,4-selectivity) and then into 1,3-diarylpropan-1-ols (C═O reduction). The stereoelectronic effects of electron-donating and electron-withdrawing groups at the ortho, meta and para positions of both aromatic rings were evaluated. The 2-OH group at the B ring was well tolerated, allowing a straightforward enantioselective synthesis of two flavans through the Mitsunobu cyclization, the antiviral (S)-BW683C and the natural flavan (S)-tephrowatsin E.

Heterocycle-containing Noyori-Ikariya catalysts for asymmetric transfer hydrogenation of ketones

The synthesis of a range of N-(heterocyclesulfonyl)-functionalised Noyori-Ikariya catalysts is described. The complexes were prepared through a short sequence from C2-symmetric 1,2-diphenylethylene-1,2-diamine (DPEN) and were characterised by a range of methods including X-ray crystallography. The complexes were active catalysts for the asymmetric transfer hydrogenation (ATH) of a range of acetophenone derivatives, giving products of high ee in most cases, with notably good results for ortho-substituted acetophenones.

Asymmetric transfer hydrogenation of boronic acid pinacol ester (Bpin)-containing acetophenones

A series of Bpin-containing acetophenone derivatives were reduced by asymmetric transfer hydrogenation (ATH), using Noyori-Ikariya catalysts, with formic acid/triethylamine, to alcohols in high ee when the Bpin is in the para- or meta-position. Substrates containing ortho-Bpin groups were reduced in lower ee, with formation of a cyclic boron-containing group. The products were converted to substituted derivatives using Pd-catalysed coupling reactions. The results represent the first examples of ATH of Bpin-containing ketones.

Base-Catalyzed Traceless Silylation and Deoxygenative Cyclization of Chalcones to Cyclopropanes

Development of simple synthetic methods from readily available compounds to complex products is of utmost interest in modern synthesis. Catalytic synthesis of cyclopropanes is important for diverse chemical applications. We present a method for the transformation of readily accessible α,β-unsaturated ketones (chalcones) to cyclopropanes. A simple base, KOH, catalyzed the selective reduction of the enone carbonyl functionality, dehydrogenative silylation, and deoxygenative cyclization of chalcones to provide the cyclopropane products. Chalcones with extended conjugation and 4-chromanone-based substrates also provided the corresponding cyclopropanes. One-pot synthesis of cyclopropanes directly from industrial feedstock compounds such as ketones and aldehydes is also demonstrated using catalytic KOH for both intermolecular condensation and dehydrogenative silylation-deoxygenative intramolecular cyclization cascade.

Stereoselective Iridium-N,P-Catalyzed Double Hydrogenation of Conjugated Enones to Saturated Alcohols

Asymmetric hydrogenation of prochiral substrates such as ketones and olefins constitutes an important instrument for the construction of stereogenic centers, and a multitude of catalytic systems have been developed for this purpose. However, due to the different nature of the π-system, the hydrogenation of olefins and ketones is normally catalyzed by different metal complexes. Herein, a study on the effect of additives on the Ir-N,P-catalyzed hydrogenation of enones is described. The combination of benzamide and the development of a reactive catalyst unlocked a novel reactivity mode of Crabtree-type complexes toward C=O bond hydrogenation. The role of benzamide is suggested to extend the lifetime of the dihydridic iridium intermediate, which is prone to undergo irreversible trimerization, deactivating the catalyst. This unique reactivity is then coupled with C=C bond hydrogenation for the facile installation of two contiguous stereogenic centers in high yield and stereoselectivity (up to 99% ee, 99/1 d.r.) resulting in a highly stereoselective reduction of enones.

Asymmetric Hydrogenation of Racemic Allylic Alcohols via an Isomerization-Dynamic Kinetic Resolution Cascade

Prochiral racemic allylic alcohols are converted to enantioenriched chiral alcohols bearing adjacent stereocenters catalyzed by a diamine diphosphine Ru complex in the presence of tBuOK. The protocol features a broad substrate scope (56 examples) and high diastereo- and enantioselectivities (up to >99:1 dr, >99% ee) and could be applied to the synthesis of enantioenriched chromane and indane compounds. Mechanistic studies suggest that the reaction proceeds via tBuOK-promoted allylic alcohol isomerization followed by Ru-catalyzed hydrogenative dynamic kinetic resolution.

Asymmetric hydrogenation and transfer hydrogenation in the enantioselective synthesis of flavonoids

In this review, we explore the applications of Asymmetric Hydrogenation (AH) and Asymmetric Transfer Hydrogenation (ATH) in the total synthesis of natural flavonoids and their analogues, highlighting the limitations and opportunities in the field.

Ir/f-Ampha complex catalyzed asymmetric sequential hydrogenation of enones: a general access to chiral alcohols with two contiguous chiral centers

A general and highly efficient method for asymmetric sequential hydrogenation of α,β-unsaturated ketones has been developed by using an iridium/f-Ampha complex as the catalyst, furnishing corresponding chiral alcohols with two contiguous stereocenters in high yields with excellent diastereo- and enantioselectivities (up to 99% yield, >20 : 1 dr and >99% ee). Control experiments indicated that the C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C and CO bonds of the enones were hydrogenated sequentially, and the final stereoselectivities were determined by the dynamic kinetic resolution of ketones. Moreover, DFT calculations revealed that an outer sphere pathway was involved in both reduction of CC and CO bonds of enones. The synthetic utility of this method was demonstrated by a gram-scale reaction with very low catalyst loading (S/C = 20 000) and a concise synthetic route to key chiral intermediates of the antiasthmatic drug CP-199,330.

A general and efficient method for asymmetric sequential hydrogenation of α,β-unsaturated ketones has been developed. A dynamic kinetic resolution and an outer sphere pathway were involved in this transformation.

Asymmetric Transfer Hydrogenation of α-Keto Amides; Highly Enantioselective Formation of Malic Acid Diamides and α-Hydroxyamides

The asymmetric transfer hydrogenation (ATH) of α-keto-1,4-diamides using a tethered Ru/TsDPEN catalyst was achieved in high ee. Studies on derivatives identified the structural elements which lead to the highest enantioselectivities in the products. The α-keto-amide reduction products have been converted to a range of synthetically valuable derivatives.

Ru(II)-Catalyzed Asymmetric Transfer Hydrogenation of 3-Fluorochromanone Derivatives to Access Enantioenriched cis-3-Fluorochroman-4-ols through Dynamic Kinetic Resolution

Enantioenriched cis-3-fluoro-chroman-4-ol derivatives were conveniently prepared by the ruthenium-catalyzed asymmetric transfer hydrogenation of a new family of 3-fluoro-chromanones through a dynamic kinetic resolution process. The reaction proceeded under mild conditions using a low catalyst loading and HCO₂H/Et₃N (1:1) as the hydrogen source, affording the reduced fluorinated alcohols in good yields (80-96%), high diastereomeric ratios (up to 99:1 dr), and excellent enantioselectivities (up to >99% ee).