Recent application of oxa-Michael reaction in complex natural product synthesis

Recent Advances in the Synthesis of Diarylheptanoids

In the quest for synthesizing biologically important natural products, medicinal chemists embark on an endless journey. This review focuses on the reports published towards the syntheses of diarylheptanoids, classifying them into linear, tetrahydropyran, diarylether, and biphenyl categories. The synthesis methods for each class from 2013 to 2023 are discussed, providing a comprehensive overview of the advancements in the field. Representative natural product examples are highlighted for each category. The review emphasizes the importance of diarylheptanoids in the realms of chemistry and medicine, showcasing their potential as valuable compounds for medicinal and synthetic chemists.

Synthesis of meta-Aminophenol Derivatives via Cu-Catalyzed [1,3]-Rearrangement-Oxa-Michael Addition Cascade Reactions

Cu-catalyzed reactions of N-alkoxy-2-methylanilines and alcohols in the presence of catalytic amounts of IPrCuBr and AgSbF₆ afforded the corresponding meta-aminophenol derivatives in good to high yields. These reactions proceed via a [1,3]-rearrangement, in which the alkoxy group migrates from the nitrogen atom to the methyl-substituted ortho position, followed by an oxa-Michael reaction of the resulting ortho-quinol imine intermediate.

Poly(ether)s derived from oxa-Michael polymerization: a comprehensive review

Poly(ether)s represent an important class of polymers and are typically formed by ring-opening polymerization, Williamson ether synthesis, or self-condensation of alcohols. The oxa-Michael reaction presents another method to form poly(ether)s with additional functional groups in the polymer backbone starting from di- or triols and electron deficient olefins such as acrylates, sulfones, or acrylamides. However, research on oxa-Michael polymerization is still limited. Herein, we outline the principles of the oxa-Michael polymerization and focus on the synthesis and preparation of poly(ether-sulfone)s, poly(ether-ester)s, poly(ether)s, and poly(ether-amide)s. Further, challenges as well as future perspectives of the oxa-Michael polymerization are discussed.

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Stereodivergent Assembly of 2,6-cis- and -trans-Tetrahydropyrans via Base-Mediated Oxa-Michael Cyclization: The Key Role of the TMEDA Additive

The stereodivergent synthesis of cis- and trans-2,6-disubstituted tetrahydropyrans (THPs) via sodium hexamethyldisilazide-promoted oxa-Michael cyclization of (E)-ζ-hydroxy α,β-unsaturated esters is presented. The cyclization affords the kinetically favored trans-THPs with high stereoselectivity (dr up to 93:7) at a low temperature (-78 °C), while the room-temperature reaction does not produce the thermodynamically preferred cis-THPs as major products and occurs with poor stereocontrol. The addition of tetramethylethylenediamine (TMEDA) significantly improves the stereochemical outcome of the room-temperature cyclization and allows attaining high cis-selectivity (dr up to 99:1). The remarkable effect of TMEDA indicates that the sodium cation plays an important role in controlling the stereoselectivity of the thermodynamically driven process, that is, complexation of the cation with the cyclization products results in diminished selectivity. DFT calculations support this conclusion, indicating a greater difference in Gibbs energies of sodium-free cis- and trans-enolates compared to the respective sodium chelate complexes. The synthetic utility of the method has been demonstrated by the formal syntheses of (+)-Neopeltolide and (-)-Diospongin B and the total synthesis of (-)-Diospongin A.

Mechanistic Studies Yield Improved Protocols for Base-Catalyzed Anti-Markovnikov Alcohol Addition Reactions

The catalytic anti-Markovnikov addition of alcohols to simple alkenes is a longstanding synthetic challenge. We recently disclosed the use of organic superbase catalysis for the nucleophilic addition of alcohols to activated styrene derivatives. This article describes mechanistic studies on this reversible reaction, including thermodynamic and kinetic profiling as well as computational modeling. Our findings show the negative entropy of addition is counterbalanced by an enthalpy that is most favored in nonpolar solvents. However, a large negative alcohol rate order under these conditions indicates excess alcohol sequesters the active alkoxide ion pairs, slowing the reaction rate. These observations led to an unexpected solution to a thermodynamically challenging reaction: use of less alcohol enables faster addition, which in turn allows for lower reaction temperatures to counteract Le Chatelier's principle. Thus, our original method has been improved with new protocols that do not require excess alcohol stoichiometry, enable an expanded alkene substrate scope, and allow for the use of more practical catalyst systems. The generality of this insight for other challenging hydroetherification reactions is also demonstrated through new alkenol cyclization and oxa-Michael addition reactions.

Tandem Macrolactone Synthesis: Total Synthesis of (-)-Exiguolide by a Macrocyclization/Transannular Pyran Cyclization Strategy

Tetrahydropyran-containing macrolactones were synthesized by integrating Meyer-Schuster rearrangement, macrocyclic ring-closing metathesis, and transannular oxa-Michael addition under gold and ruthenium catalysis. Single-step access to a variety of 14- to 20-membered macrolactones containing a tetrahydropyran ring was possible from readily available linear precursors in good yields and with moderate to excellent diastereoselectivity. A 13-step synthesis of (-)-exiguolide, an anticancer marine macrolide, showcased the feasibility of our tandem reaction sequence for macrolactone synthesis and also demonstrated the power of transannular reactions for rapid assembly of the tetrahydropyran rings of the target natural product.

Synthesis of Phenolic Coumestans via a Sequential Dehydrogenation/Oxa-Michael Addition Reaction of 2',4'-Dihydroxyl-3-arylcoumarins

A facile and practical approach for the synthesis of natural coumestans and derivatives starting from 2',4'-dihydroxyl-3-arylcoumarins has been developed. The process involved a seqential intramolecular dehydrogenation/oxa-Micheal reaction efficiently promoted by 1,8-diazabicyclo[5.4.0]undec-7-ene at 40 °C under metal- and ligand-free conditions with good functional group compatibility.

Stereodivergent Synthesis of Enantioenriched 2,3-Disubstituted Dihydrobenzofurans via a One-Pot C-H Functionalization/Oxa-Michael Addition Cascade

A one-pot rhodium-catalyzed C-H functionalization/organocatalyzed oxa-Michael addition cascade reaction has been developed. This methodology enables the stereodivergent synthesis of diverse 2,3-disubstituted dihydrobenzofurans with broad functional group compatibility in good yields with high levels of stereoselectivity under exceptionally mild conditions. The full complement of stereoisomers of chiral 2,3-disubstituted dihydrobenzofurans and 3,4-disubstituted isochromans could be accessed at will by appropriate permutations of the two chiral catalysts. The current work provides a rare example of two chiral catalysts independently controlling two contiguous stereogenic centers subsequently via a two-step reaction in a single operation.

Stereoselective Synthesis of the Southern Hemisphere Acyclic Domain of Neaumycin B

A stereocontrolled synthetic entry to the southern hemisphere C3-C17 acyclic domain of neaumycin B, a highly potent cytotoxic macrolide natural product, has been developed. The present synthesis is based on (i) a tandem olefin cross-metathesis/hemiacetalization/intramolecular oxa-Michael addition, (ii) a regioselective reductive acetal opening for differential protection of the C14 hydroxy group, (iii) a Horner-Wadsworth-Emmons reaction for the stereoselective formation of the C8-C9 olefin, and (iv) a Corey-Bakshi-Shibata asymmetric reduction to create the C7 stereogenic center.

Synthetic efforts on the road to marine natural products bearing 4-O-2,3,4,6-tetrasubstituted THPs: an update

Scientific literature is inundated with secondary metabolites from marine sources. In this ocean of natural products, the presence of recurring patterns has traditionally led scientists to unravel the biosynthetic mechanisms that naturally yield these products, as well as to imitate Nature to prepare them in the laboratory, especially when promising bioactivities and stimulating molecular architectures are involucrate. For instance, natural products containing multisubstituted oxygenated rings and macrocyclic lactones are recurrently selected as targets for developing total syntheses. Thus, in the last decades a noteworthy number of synthetic works regarding miyakolide, madeirolide A and representative compounds of polycavernosides, lasonolides and clavosolides have come to fruition. Up to now, these families of macrolides are the only marine natural products bearing a tetrasubstituted tetrahydropyran ring with carbon substituents at positions 2, 3 and 6, as well as an oxygen at position 4. Their splendid structures have received the attention of the synthetic community, up to the point of starring in dozens of articles, and even some reviews. This work covers all the synthetic studies towards miyakolide and madeirolide A, as well as the synthetic efforts performed after the previous specialised reviews about lasonolide A, polycavernoside A and clavosolides, published in 2006, 2007 and 2016, respectively. In total, this review summarises 22 articles in which these marine natural products with 4-O-2,3,4,6-tetrasubstituted tetrahydropyrans have the leading role.

Brønsted acid-catalyzed dynamic kinetic resolution of in situ formed acyclic N,O-hemiaminals: cascade synthesis of chiral cyclic N,O-aminals

A H2O controlled dynamic kinetic resolution was involved in a Brønsted acid-catalyzed acyclic N,O-hemiaminal formation/oxa-Michael reaction cascade, leading to highly enantioenriched cis-2,6-disubstituted tetrahydropyrans bearing an exo amide group.

The oxa-Michael reaction in the synthesis of 5- and 6-membered oxygen-containing heterocycles

In this review, we provide an updated account on the recent advances and applications of oxa-Michael reaction in the synthesis 5- and 6-membered monocyclic oxygen-containing heterocyclic compounds published in the literature since 2013 to date.

Benzene-1,3-diol derivatives as the inhibitors of butyrylcholinesterase: An emergent target of Alzheimer’s disease

Molecular docking is a powerful and significant approach for the identification of lead molecules on the basis of virtual screening. With it a large number of compounds can be tested and based on the scoring function and ranking, the conclusion can be made about how the selected compounds can inhibit the targeted protein/receptor. Considering the importance of selective inhibitors of cholinesterase in the treatment of Alzheimer disease, this research is focused on the determination of the mechanism of binding interactions of few benzene-1,3-diol derivatives within the active site of both acetyl-choline-sterase (AChE) and butyrylcholinesterase (BChE). All the selective ligands were found to have a greater binding affinity with the BChE when compared to that of AChE, by an average value of ~?28.4 and ~?12.5 kJ/mol, respectively. The results suggested that the identified inhibitors can be used as the lead com-pounds for the development of novel inhibitors of the targeted enzymes against some specific diseases, thus opening the possibility of new therapeutic strategies.

A Vinyl Cyclopropane Ring Expansion and Iridium-Catalyzed Hydrogen Borrowing Cascade

A vinyl cyclopropane rearrangement embedded in an iridium-catalyzed hydrogen borrowing reaction enabled the formation of substituted stereo-defined cyclopentanes from Ph* methyl ketone and cyclopropyl alcohols. Mechanistic studies provide evidence for the ring-expansion reaction being the result of a cascade based on oxidation of the cyclopropyl alcohols, followed by aldol condensation with the pentamethyl phenyl-substituted ketone to form an enone containing the vinyl cyclopropane. Subsequent single electron transfer (SET) to this system initiates a rearrangement, and the catalytic cycle is completed by reduction of the new enone. This process allows for the efficient formation of diversely substituted cyclopentanes as well as the construction of complex bicyclic carbon skeletons containing up to four contiguous stereocentres, all with high diastereoselectivity.

Synthesis of the 8,19-Epoxysteroid Eurysterol A

We report the first chemical synthesis of eurysterol A, a cytotoxic and antifungal marine steroidal sulfate with a unique C8−C19 oxy‐bridged cholestane skeleton. After C19 hydroxylation of cholesteryl acetate, used as an inexpensive commercial starting material, the challenging oxidative functionalization of ring B was achieved by two different routes to set up a 5α‐hydroxy‐7‐en‐6‐one moiety. As a key step, an intramolecular oxa‐Michael addition was exploited to close the oxy‐bridge (8β,19‐epoxy unit). DFT calculations show this reversible transformation being exergonic by about −30 kJ mol⁻¹. Along the optimized (scalable) synthetic sequence, the target natural product was obtained in only 11 steps in 5 % overall yield. In addition, an access to (isomeric) 7β,19‐epoxy steroids with a previously unknown pentacyclic ring system was discovered.

KOtBu-Catalyzed Michael Addition Reactions Under Mild and Solvent-Free Conditions

Designed transition metal complexes predominantly catalyze Michael addition reactions. Inorganic and organic base-catalyzed Michael addition reactions have been reported. However, known base-catalyzed reactions suffer from the requirement of solvents, additives, high pressure and also side-reactions. Herein, we demonstrate a mild and environmentally friendly strategy of readily available KO^t Bu-catalyzed Michael addition reactions. This simple inorganic base efficiently catalyzes the Michael addition of underexplored acrylonitriles, esters and amides with (oxa-, aza-, and thia-) heteroatom nucleophiles. This catalytic process proceeds under solvent-free conditions and at room temperature. Notably, this protocol offers an easy operational procedure, broad substrate scope with excellent selectivity, reaction scalability and excellent TON (>9900). Preliminary mechanistic studies revealed that the reaction follows an ionic mechanism. Formal synthesis of promazine is demonstrated using this catalytic protocol.

A molecular electron density theory (MEDT) study of the role of halogens (X2 = F2, Cl2, Br2 and I2) on the aza-Michael-addition reactions

The energy profile for gas phase F₂-catalyzed nucleophilic attack of PYR R1 on the β-conjugated carbon of MA R2 marked.

How an early or late transition state impacts the stereoselectivity of tetrahydropyran formation by intramolecular oxa-Michael addition

The intramolecular oxa-Michael addition giving tetrahydropyrans has been examined experimentally using both acidic and basic catalysis. With acidic catalysis, the diequatorial product is exclusively obtained in a kinetically controlled reaction in all cases. Under basic conditions at low temperature, the reaction is again under kinetic control, but formation of the axial-equatorial isomer is generally favoured with an (E)-Michael acceptor, although isomerisation to the diequatorial isomer is observed at higher temperatures. Computationally, it is found that the acid catalysed reaction has a late transition state and the kinetic favouring of the diequatorial isomer has a steric explanation. In contrast, under strongly basic conditions, an early transition state is found. Electrostatic effects are likely to be the main contributor to the stereoselectivity for the (E)-isomer and steric interactions for the (Z)-isomer.

How Dihalogens Catalyze Michael Addition Reactions

We have quantum chemically analyzed the catalytic effect of dihalogen molecules (X2 =F2 , Cl2 , Br2 , and I2 ) on the aza-Michael addition of pyrrolidine and methyl acrylate using relativistic density functional theory and coupled-cluster theory. Our state-of-the-art computations reveal that activation barriers systematically decrease as one goes to heavier dihalogens, from 9.4 kcal mol-1 for F2 to 5.7 kcal mol-1 for I2 . Activation strain and bonding analyses identify an unexpected physical factor that controls the computed reactivity trends, namely, Pauli repulsion between the nucleophile and Michael acceptor. Thus, dihalogens do not accelerate Michael additions by the commonly accepted mechanism of an enhanced donor-acceptor [HOMO(nucleophile)-LUMO(Michael acceptor)] interaction, but instead through a diminished Pauli repulsion between the lone-pair of the nucleophile and the Michael acceptor's π-electron system.

A Unified Synthetic Approach to Optically Pure Curvularin-Type Metabolites

A unified and concise approach to the synthesis of nine curvularin-type metabolites and two analogues has been developed with few steps and high yields. Among them, sumalactones A-D were synthesized for the first time. The key steps in this approach included esterification, Friedel-Crafts acylation, and ring-closing metathesis (or cross metathesis).

Total Synthesis of (-)-Indoxamycins A and B

The concise total syntheses of (-)-indoxamycins A and B is reported. The chemistry features a seven-step preparation of a highly congested [5.5.6] tricyclic advanced common intermediate from a readily available R-carvone derivative. Key steps involve a Pauson-Khand reaction for the rapid construction of a basic scaffold bearing a quaternary carbon, a copper-catalyzed Michael addition for the introduction of another adjacent all-carbon quaternary stereocenter, and a tandem retro-oxa-Michael addition/1,2-addition/oxa-Michael addition for the installation of a trisubstituted olefin side chain. This synthetic strategy allows for easy access to both enantiomers of this family of natural products and their analogues from cost-effective starting material through straightforward chemical transformations.

Chemoselective α,β-Dehydrogenation of Saturated Amides

We report a method for the selective α,β-dehydrogenation of amides in the presence of other carbonyl moieties under mild conditions. Our strategy relies on electrophilic activation coupled to in situ selective selenium-mediated dehydrogenation. The α,β-unsaturated products were obtained in moderate to excellent yields, and their synthetic versatility was demonstrated by a range of transformations. Mechanistic experiments suggest formation of an electrophilic SeIV species.

An Enantioconvergent and Concise Synthesis of Lasonolide A

Efficient access to medicinally significant natural products is an essential basis for the development of pharmaceuticals. The limited availability of marine natural products impedes broad biological evaluation. Despite several elegant syntheses of (-)-lasonolide A having been reported, a practical synthesis of this potent anticancer polyketide remains elusive. Based on the application of borane as a traceless protecting group and the development of an unprecedented bissulfone reagent for Julia olefination, (-)-lasonolide A was assembled in an enantioconvergent manner through the application of stereoselective hydroboration, allylation, and oxidation. This concise route may provide a realistic solution for accessing derivatives and analogues.

Recent Advances in Metal-Catalyzed Asymmetric 1,4-Conjugate Addition (ACA) of Nonorganometallic Nucleophiles

The metal-catalyzed asymmetric conjugate addition (ACA) reaction has emerged as a general and powerful approach for the construction of optically active compounds and is among the most significant and useful reactions in synthetic organic chemistry. In recent years, great progress has been made in this area with the use of various chiral metal complexes based on different chiral ligands. This review provides comprehensive and critical information on the enantioselective 1,4-conjugate addition of nonorganometallic (soft) nucleophiles and their importance in synthetic applications. The literature is covered from the last 10 years, and a number of examples from before 2007 are included as background information. The review is divided into multiple parts according to the type of nucleophile involved in the reaction (such as C-, B-, O-, N-, S-, P-, and Si-centered nucleophiles) and metal catalyst systems used.

Total Syntheses of Thailanstatins A-C, Spliceostatin D, and Analogues Thereof. Stereodivergent Synthesis of Tetrasubstituted Dihydro- and Tetrahydropyrans and Design, Synthesis, Biological Evaluation, and Discovery of Potent Antitumor Agents

Efficient and selective total syntheses of spliceosome modulating natural products thailanstatins A-C and spliceostatin D are reported. A number of stereoselective methods for the construction of various tetrasubstituted dihydro- and tetrahydropyrans were developed as a prerequisite for the syntheses of these naturally occurring molecules and variations thereof. The pyran-forming reactions utilize a Heck/Saegusa-Ito cascade sequence to generate hydroxy α,β,γ,δ-unsaturated aldehyde precursors followed by a catalyst-controlled oxa-Michael cyclization to furnish tetrasubstituted dihydropyrans with high stereocontrol. Subsequent optimized homogeneous or heterogeneous hydrogenations of these dihydropyran systems afford their tetrahydropyran counterparts, also in a highly stereoselective manner. The synthesized thailanstatins and related analogues were biologically evaluated for their cytotoxic properties, leading to the identification of a number of compounds with exceptionally potent antitumor activities suitable for further development as potential antibody-drug conjugate payloads, single drugs, or drug combinations for cancer therapies. Important structure-activity relationships within the thailanstatin family and structurally related compounds are discussed and are expected to be path-pointing for future studies.