Enantioselective Total Synthesis of Natural Isoflavans: Asymmetric Transfer Hydrogenation/Deoxygenation of Isoflavanones with Dynamic Kinetic Resolution

Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

Carpe diene! Europium-catalyzed [3,3] and [5,5] rearrangements of aryl-pentadienyl ethers

A general protocol for the europium-catalyzed rearrangement of aryl-pentadienyl-ethers is described. The mode of rearrangement and product formation in this reaction was solely determined by the aryl substituent para to the phenol. If the para-position is occupied by a substituent, the substrate undergoes a [3,3] rearrangement to the ortho-position to form a prochiral branched diene. In turn, a free para-position in the starting material allows the reaction to proceed via a [5,5] rearrangement and leads to a linear conjugated diene product. The severely underdeveloped and synthetically valuable [5,5] rearrangement was investigated in terms of scope and mechanism.

Synthesis and Bioactivity of Isoflavones from Ficus carica and Some Non-Natural Analogues

Ficucaricone D (1) and its 4'-demethyl congener 2 are isoflavones isolated from fruits of Ficus carica that share a 5,7-dimethoxy-6-prenyl-substituted A-ring. Both natural products were, for the first time, obtained by chemical synthesis in six steps, starting from 2,4,6-trihydroxyacetophenone. Key steps are a microwave-promoted tandem sequence of Claisen- and Cope-rearrangements to install the 6-prenyl substituent and a Suzuki-Miyaura cross coupling for installing the B-ring. By using various boronic acids, non-natural analogues become conveniently available. All compounds were tested for cytotoxicity against drug-sensitive and drug-resistant human leukemia cell lines, but were found to be inactive. The compounds were also tested for antimicrobial activities against a panel of eight Gram-negative and two Gram-positive bacterial strains. Addition of the efflux pump inhibitor phenylalanine-arginine-β-naphthylamide (PAβN) significantly improved the antibiotic activity in most cases, with MIC values as low as 2.5 μM and activity improvement factors as high as 128-fold.

The Suzuki-Miyaura Cross-Coupling-Claisen Rearrangement-Cross-Metathesis Approach to Prenylated Isoflavones

Isoflavones were synthesized via Suzuki-Miyaura coupling of 3-iodochromones and para-methoxybenzene- and para-phenolboronic acid. In our hands, conditions commonly used for similar cross couplings turned out to be unsuccessful or difficult to reproduce, for example, due to the unplanned partial cleavage of MOM-protecting groups. Using Pd(dba)₂ as a precatalyst and tricyclohexylphosphine as an activating ligand, reliable cross-coupling conditions were identified. In all cases, notably higher yields of isoflavones were obtained with para-phenolboronic acid than with para-methoxybenzene boronic acid. This observation and the commercial availability of para-phenolboronic acid suggest that for the synthesis of the important 3'-prenyl- or 3',5'-diprenylisoflavone substitution pattern a synthetic route that introduces the prenyl substituents after the Pd-catalyzed cross-coupling step, thereby avoiding laborious and protecting-group-intensive multistep syntheses of C-prenylated arene boronic acids, is advantageous.

Dynamic Kinetic Resolution of β-Substituted α-Diketones via Asymmetric Transfer Hydrogenation

Developing innovative dynamic kinetic resolution (DKR) modes and achieving the highly regio- and enantioselective semihydrogenation of unsymmetrical α-diketones are two formidable challenges in the field of contemporary asymmetric (transfer) hydrogenation. In this work, we report the highly regio- and stereoselective asymmetric semi-transfer hydrogenation of unsymmetrical α-diketones through a unique DKR mode, which features the reduction of the carbonyl group distal from the labile stereocenter, while the proximal carbonyl remains untouched. Moreover, the protocol affords a variety of enantioenriched acyclic ketones with α-hydroxy-α'-C(sp²)-functional groups, which represent a new product class that has not been furnished in known arts. The utilities of the products have been demonstrated in a series of further transformations including the rapid synthesis of drug molecules. Density functional theory calculations and plenty of control experiments have also been conducted to gain more mechanistic insights into the highly selective semihydrogenation.

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 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.

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).

Antifungal Activity of Chemical Constituents from Piper pesaresanum C. DC. and Derivatives against Phytopathogen Fungi of Cocoa

In this study, the antifungal potential of chemical constituents from Piper pesaresanum and some synthesized derivatives was determined against three phytopathogenic fungi associated with the cocoa crop. The methodology included the phytochemical study on the aerial part of P. pesaresanum, the synthesis of some derivatives and the evaluation of the antifungal activity against the fungi Moniliophthora roreri, Fusarium solani and Phytophthora sp. The chemical study allowed the isolation of three benzoic acid derivatives (1-3), one dihydrochalcone (4) and a mixture of sterols (5-7). Seven derivatives (8-14) were synthesized from the main constituents, of which compounds 9, 10, 12 and 14 are reported for the first time. Benzoic acid derivatives showed strong antifungal activity against M. roreri, of which 11 (3.0 ± 0.8 µM) was the most active compound with an IC₅₀ lower compared with positive control Mancozeb^® (4.9 ± 0.4 µM). Dihydrochalcones and acid derivatives were active against F. solani and Phytophthora sp., of which 3 (32.5 ± 3.3 µM) and 4 (26.7 ± 5.3 µM) were the most active compounds, respectively. The preliminary structure-activity relationship allowed us to establish that prenylated chains and the carboxyl group are important in the antifungal activity of benzoic acid derivatives. Likewise, a positive influence of the carbonyl group on the antifungal activity for dihydrochalcones was deduced.

Synthesis of 4-amino-5-allenylisoxazoles via gold(I)-catalysed propargyl aza-Claisen rearrangement

Propargyl aza-Claisen rearrangement of 4-propargylaminoisoxazoles 1 proceeded in the presence of cationic gold(i) catalysts to give 4-amino-5-allenylisoxazoles 2 in good to high yields. The silyl group at the terminal alkyne and a cationic gold(i) catalyst bearing a sterically bulky ligand are essential for the generation of isolable allene intermediates. The N-protection of the generated 4-amino-5-allenylisoxazoles 2 allowed the isolation of 5-allenylisoxazoles 4 that have never been synthesized. N-Propargyl aniline 5 was successfully converted to the corresponding ortho-allenyl aniline 6 under the current reaction conditions.

Asymmetric one-pot transformation of isoflavones to pterocarpans and its application in phytoalexin synthesis

Phytoalexins have attracted much attention due to their health-promoting effects and their vital role in plant health during the last years. Especially the 6a-hydroxypterocarpans glyceollin I and glyceollin II, which may be isolated from stressed soy plants, possess a broad spectrum of bioactivities such as anticancer activity and beneficial contributions against western diseases by anti-oxidative and anti-cholesterolemic effects. Aiming for a catalytic asymmetric access to these natural products, we establish the asymmetric syntheses of the natural isoflavonoids (−)-variabilin, (−)-homopterocarpin, (−)-medicarpin, (−)-3,9-dihydroxypterocarpan, and (−)-vestitol by means of an asymmetric transfer hydrogenation (ATH) reaction. We successfully adapt this pathway to the first catalytic asymmetric total synthesis of (−)-glyceollin I and (−)-glyceollin II. This eight-step synthesis features an efficient one-pot transformation of a 2′-hydroxyl-substituted isoflavone to a virtually enantiopure pterocarpan by means of an ATH and a regioselective benzylic oxidation under aerobic conditions to afford the susceptible 6a-hydroxypterocarpan skeleton.

Concise total syntheses of 6a-hydroxypterocarpans are sought after due to their broad spectrum of bioactivities. Here, the authors report the asymmetric syntheses of several natural isoflavonoids, including (−)-glyceollin I and (−)-glyceollin II, by means of an asymmetric transfer hydrogenation (ATH) reaction.

Pummerer Synthesis of Chromanes Reveals a Competition between Cyclization and Reductive Chlorination

The competition between an unprecedented reductive chlorination and the Pummerer reaction was studied and applied to the synthesis of benzofused oxygen heterocycles including 3-aminochromanes and in the intramolecular chlorination of activated aromatic rings. The use of (COCl)₂ as a Pummerer activator showed substantial activity, producing α-chlorinated sulfides that can undergo Pummerer-Friedel-Crafts cyclization. If the aromatic ring has electron-donating groups in position three, then the reaction follows a different pathway, yielding the reductive chlorination products, where the chlorine atom comes from a sulfonium salt.

Enantioselective Synthesis of Homoisoflavanones by Asymmetric Transfer Hydrogenation and Their Biological Evaluation for Antiangiogenic Activity

Neovascular eye diseases are a major cause of blindness. Excessive angiogenesis is a feature of several conditions, including wet age-related macular degeneration, proliferative diabetic retinopathy, and retinopathy of prematurity. Development of novel antiangiogenic small molecules for the treatment of neovascular eye disease is essential to provide new therapeutic leads for these diseases. We have previously reported the therapeutic potential of anti-angiogenic homoisoflavanone derivatives with efficacy in retinal and choroidal neovascularization models, although these are racemic compounds due to the C3-stereogenic center in the molecules. This work presents asymmetric synthesis and structural determination of anti-angiogenic homoisoflavanones and pharmacological characterization of the stereoisomers. We describe an enantioselective synthesis of homoisoflavanones by virtue of ruthenium-catalyzed asymmetric transfer hydrogenation accompanying dynamic kinetic resolution, providing a basis for the further development of these compounds into novel experimental therapeutics for neovascular eye diseases.