Asymmetric Synthesis of α-Alkylidene-β-hydroxy-γ-butyrolactones via Enantioselective Tandem Michael–Aldol Reaction

Proposal for structure revision of pinofuranoxin A through total syntheses of stereoisomers

The relative configuration of the epoxide functionality in pinofuranoxin A (1), α-alkylidene-β-hydroxy-γ-methyl-γ-butyrolactone with trans-epoxy side chain isolated by Evidente et al. in 2021, was revised by DFT-based spectral reinvestigations and stereo-controlled synthesis. The present investigation demonstrates the difficulty of the configurational elucidation of the stereogenic centers on the conformationally flexible acyclic side-chains. Sharpless's enantioselective epoxidations and dihydroxylations were quite effective in the reinvestigations of the configurations. As our syntheses made all diastereomers available, these would be quite effective in the next structure-biological activity relationship studies.

Non-volatile constituents from Monimiaceae, Siparunaceae and Atherospermataceae plant species and their bioactivities: An up-date covering 2000-2021

The Monimiaceae, Siparunaceae, and Atherospermataceae, formerly included in the broad ''old'' Monimiaceae family, have long been known for their uses in traditional medicine and have proven to be rich sources of chemically diverse specialized metabolites with numerous potent biological and therapeutical properties. The progress made recently has expanded their phytochemistry and pharmacology albeit to different extents. This review focuses on the non-volatile constituents isolated from the three plant families during the last two decades and their emerging therapeutic potential. Based on the data collected from multiple databases without statistical analysis, approximately 93 components, of which 35 undescribed compounds including γ-lactones, alkaloids, terpenoids, flavonoids, and homogentisic acid derivatives, have been reported. Moreover, diverse biological activities of pure isolated compounds such as anticancer, antioxidant, antiparasitic, antiviral, and antibacterial activities have been evidenced. Besides offering new important perspectives for different diseases' management, the chemical and biological diversities among the isolated compounds, open promising avenues of research and contribute to renewed interest in these families needing further studies. This review provides an updated overview of their potential as sources of leads for drug discovery, while also highlighting ongoing challenges and future research opportunities.

Total synthesis of proposed elgonene C and its (4R,5R)-diastereomer

The total synthesis of the proposed elgonene C (1) and its (4R,5R)-diastereomer (1a) has been achieved using a second-generation oxazaborolidinium ion-catalysed Diels-Alder reaction, Sharpless asymmetric dihydroxylation, and a Ni-catalysed cross-carboxyl coupling reaction via redox-active ester (RAE) formation as key reactions. The spectral and analytical data for our synthetic compounds 1 and 1a do not match the isolation data provided by Stadler et al. which indicates that structural revision is required for the proposed elgonene C.

Enantioselective Cyclopropanation/[1,5]-Hydrogen Shift to Access Rauhut-Currier Product

A Michael addition initiated cyclopropanation/[1,5]-hydrogen shift has been developed for the enantioselective synthesis of Rauhut-Currier products. The reaction of α-alkyl diazoesters and in situ generated o-quinone methides proceeds in the presence of chiral oxazaborolidinium ion, providing Z-stereocontrolled Rauhut-Currier products in high yields (up to 96%) with excellent Z/E selectivities (>20:1) and enantioselectivities (up to >99% ee). The synthetic utility was illustrated by conversion of the product to 3,4-dihydrocoumarins with two adjacent chiral stereocenters.

Enantioselective Carbonyl 1,2- or 1,4-Addition Reactions of Nucleophilic Silyl and Diazo Compounds Catalyzed by the Chiral Oxazaborolidinium Ion

Boron Lewis acid catalysis has a long history and has become one of the most powerful methods for organic synthesis. In addition to achiral boron catalysts such as BX₃ (X = F, Cl, Br) and B(C₆F₅)₃, chiral boron catalysts are also significant synthetic tools used by organic chemists in academic laboratories and industry. Since first reported by Corey et al. in 2002 ( Corey et al. J. Am. Chem. Soc. 2002 , 124 , 3808 ), the chiral oxazaborolidinium ion (COBI), an activated form of proline-derived oxazaborolidine, has been used as a strong Lewis acid catalyst. Although the early examples of asymmetric synthesis through COBI-catalyzed nucleophilic 1,2- or 1,4-carbonyl additions were reported in 2004-2006, Diels-Alder and cycloaddition reactions of various carbonyl compounds were mostly developed over the next several years to afford enantioenriched cyclized products. The power of COBI in catalyzing carbonyl 1,2- or 1,4-addition reactions triggered our interest in developing asymmetric synthetic methodologies to generate versatile enantiomerically enriched compounds. In this Account, we summarize our recent studies on COBI-catalyzed asymmetric nucleophilic carbonyl addition and tandem reactions. Logical mechanistic explanations of asymmetric COBI catalysis are also discussed. The proton-activated COBI catalyst, which can activate various carbonyl compounds such as aldehydes, ketones, acroleins, and enones through Lewis acid-base interactions and synergistic hydrogen bonds, facilitates asymmetric 1,2- or 1,4-carbonyl additions of nucleophiles. Nucleophiles bearing trialkylsilyl groups successfully reacted with aromatic, aliphatic, and α,β-unsaturated aldehydes through 1,2-addition reactions resulting in chiral β-hydroxy esters. In addition, efficient asymmetric hydrosilylation of ketones was achieved with a TfOH-activated COBI catalyst. Optically active β-keto esters and all-carbon quaternary aldehydes were synthesized successfully through asymmetric 1,2-addition of diazo compounds and tandem H- or C-migration, respectively. In some cases, epoxide products were obtained as side products via the Darzens reaction pathway. Solvent and π-π interactions played important roles in favoring C-migration over H-migration. Nucleophilic 1,4-addition of diazo compounds and chemoselective ring-closure afforded an efficient approach to cyclopropanes, and their tandem rearrangements provided four- and seven-membered cyclic compounds with excellent stereoselectivity. After a Michael addition of diazo compounds, the selective β-hydride shift pathway afforded the β-substituted cyclic enones with high diastereo- and enantioselectivity. The presence of π-bond(s) in the substituents at the α-position of the diazo compound hindered the β-hydride shift pathway and, as a result, favored the cyclopropanation pathway. While there still remain challenges to be overcome, these results further understanding of COBI catalysis and open a window for future development of new asymmetric synthetic methods using carbonyl addition and tandem reactions.

Total Synthesis of 3-epi-Juruenolide C

In this study, the total synthesis of 3-epi-juruenolide C is achieved in 10 steps (longest linear sequence) starting from ethyl (2E,4S,5S)-4,5-dihydroxy-2-hexenoate. The synthetic highlights of our approach include one-pot regioselective bromination, intramolecular carbonylation using bis(triphenylphosphine)dicarbonylnickel, and face-selective hydrogenation using a homogeneous Wilkinson's catalyst.

Lewis Acid-Promoted Enantioselective Dearomative Spirocyclizations of Allenes

A chiral oxazaborolidine combined with SnCl₄ has been found to promote the dearomative spirocyclization of electron-rich benzyl allenyl ketones. The reaction outcome is sensitive to the nature of activating acid, which was rationalized using hard-soft acid-base (HSAB) theory. The spirocyclic product was obtained with up to 72% ee, which is the best result reported to date for these substrates. The formation of cross-conjugated or conjugated products is readily controlled by changing the oxygen-protecting groups.

Marliolide inhibits skin carcinogenesis by activating NRF2/ARE to induce heme oxygenase-1

Heme oxygenase-1 (HO-1) catalyzes the enzymatic degradation of heme to produce three anti-oxidant molecules: carbon monoxide (CO), ferrous ion (Fe²⁺), and biliverdin. Induction of HO-1 is currently considered as a feasible strategy to treat oxidative stress-related diseases. In the present study, we identified marliolide as a novel inducer of HO-1 in human normal keratinocyte HaCaT cells. Mechanism-based studies demonstrated that the induction of HO-1 by marliolide occurred through activation of NRF2/ARE via direct binding of marliolide to KEAP1. Structure-activity relationship revealed chemical moieties of marliolide critical for induction of HO-1, which renders a support for Michael reaction as a potential mechanism of action. Finally, we observed that marliolide significantly inhibited the papilloma formation in DMBA/TPA-induced mouse skin carcinogenesis model and this event was closely associated with lowering the formation of 8-OH-G and 4-HNE in vivo. Together, our study provides the first evidence that marliolide might be effective against oxidative stress-related skin disorders.

Pd-Catalyzed Site-Selective Mono-allylic Substitution and Bis-arylation by Directed Allylic C-H Activation: Synthesis of anti-γ-(Aryl,Styryl)-β-hydroxy Acids and Highly Substituted Tetrahydrofurans

An efficient palladium-catalyzed site-selective arylation of γ-vinyl-γ-lactone by aryl boronic acid has been developed. γ-Vinyl-γ-lactone 1a has been contemplated as allyl electrophile donor for allylic arylation via π-allyl palladium intermediate using 1.5 equiv of aryl boronic acid 2. Using 3.0 equiv of the latter resulted in mono-arylation by allylic substitution and subsequent site-selective second arylation by directed allylic C-H activation giving stereoselectively anti-γ-(aryl,styryl)-β-hydroxy acids. Presence of O₂ was crucial for the second arylation via Pd(II) catalysis. Thus, a good synergy of dual catalysis by Pd(0) and Pd(II) was observed. This methodology has been elaborated to synthesize highly substituted tetrahydrofurans including aryl-Hagen's gland lactone analogues via intramolecular iodoetherification.

Ruthenium pincer-catalyzed synthesis of substituted γ-butyrolactones using hydrogen autotransfer methodology

The ruthenium pincer-catalyzed synthesis of γ-butyrolactones from 1,2-diols and malonates using borrowing-hydrogen methodology is reported. This regioselective domino-process takes place through catalytic C-C bond formation, followed by intramolecular transesterification. Herein, we show the Ru-MACHO-BH complex as a valuable catalyst in hydrogen autotransfer reactions.

Rapid access to bicyclic δ-lactones via carbene-catalyzed activation and cascade reaction of unsaturated carboxylic esters

The carbene-catalyzed activation of α,β-unsaturated ester initiates a well-controlled cascade process for access to iridoid-type multi-cyclic lactones with high stereo-selectivities.

Donor-acceptor substituted cyclopropane to butanolide and butenolide natural products: enantiospecific first total synthesis of (+)-hydroxyancepsenolide

An oxygen substituted donor-acceptor cyclopropane (DAC) is used as a common intermediate in the enantiospecific collective total synthesis of butanolide- and butenolide-based natural products like (+)-juruenolide C and D, (+)-blastmycinone, (+)-antimycinone, and (+)-ancepsenolide. Enantiospecific first total syntheses of (+)-hydroxyancepsenolide and its acetate are achieved confirming their absolute stereochemistry.

Iron-catalyzed aerobic oxidation of allylic alcohols: the issue of C═C bond isomerization

An aerobic oxidation of allylic alcohols using Fe(NO3)3·9H2O/TEMPO/NaCl as catalysts under atmospheric pressure of oxygen at room temperature was developed. This eco-friendly and mild protocol provides a convenient pathway to the synthesis of stereodefined α,β-unsaturated enals or enones with the retention of the C-C double-bond configuration.