Catalytic Asymmetric [4 + 2] Cycloaddition of ortho-Alkenyl Naphthols/Phenols with ortho-Quinone Methides: Highly Stereoselective Synthesis of Chiral 2,3,4-Trisubstituted Chromans

o-Quinone methides in natural product synthesis: an update

In recent years, ortho-quinone methides have emerged as indispensable reactive species for enhancing synthetic efficiency in accessing various bioactive natural products. Importantly, the emulation of nature's patterns and strategies has yielded numerous refined synthetic pathways for the construction of intricate molecules. o-Quinone methides (o-QMs) have demonstrated remarkable versatility in organic synthesis, especially in strategies guided by biomimetic logic. This review aims to delve into bio-inspired strategies employed over the past five years in the total synthesis of natural products involving ortho-quinone methides.

Carbon-nitrogen transmutation in polycyclic arenol skeletons to access N-heteroarenes

Developing skeletal editing tools is not a trivial task, and realizing the corresponding single-atom transmutation in a ring system without altering the ring size is even more challenging. Here, we introduce a skeletal editing strategy that enables polycyclic arenols, a highly prevalent motif in bioactive molecules, to be readily converted into N-heteroarenes through carbon-nitrogen transmutation. The reaction features selective nitrogen insertion into the C-C bond of the arenol frameworks by azidative dearomatization and aryl migration, followed by ring-opening, and ring-closing (ANRORC) to achieve carbon-to-nitrogen transmutation in the aromatic framework of the arenol. Using widely available arenols as N-heteroarene precursors, this alternative approach allows the streamlined assembly of complex polycyclic heteroaromatics with broad functional group tolerance. Finally, pertinent transformations of the products, including synthesis complex biheteroarene skeletons, were conducted and exhibited significant potential in materials chemistry.

Catalytic Enantioselective [4+2] Cycloadditions of Salicylaldehyde Acetals with Enol Ethers

A readily accessible conjugate-base-stabilized carboxylic acid (CBSCA) catalyst facilitates highly enantioselective [4+2] cycloaddition reactions of salicylaldehyde-derived acetals and cyclic enol ethers, resulting in the formation of polycyclic chromanes with oxygenation in the 2- and 4-positions. Stereochemically more complex products can be obtained from racemic enol ethers. Spirocyclic products are also accessible.

Construction of cyclopenta[b]dihydronaphthofurans via TsOH-catalyzed consecutive biscyclization of dithioallylic alcohols and 1-styrylnaphthols

An efficient TsOH-catalyzed consecutive biscyclization cascade reaction of dithioallylic alcohols with 1-styrylnaphthols is demonstrated for the concise construction of pharmaceutically important cyclopenta[b]dihydrobenzofuran scaffolds. This process involved an acid-catalyzed (3+2) cycloaddition followed by an intramolecular nucleophilic addition, providing cyclopenta[b]dihydronaphthofurans bearing a tetra- or fully substituted cyclopentane core in good yields with exclusive diastereoselectivities (>20 : 1 d.r.).

Divergent cyclodimerizations of styrylnaphthols under aerobic visible-light irradiation and Brønsted acid catalysis

Dimeric cyclization reactions show great potential to rapidly form highly substituted complex cyclic molecules from simple starting materials. However, such an appealing process is often hampered by the lack of selectivity. Herein we report two divergent cyclodimerization reactions of 1-styrylnaphthalen-2-ol derivatives under simple and very mild reaction conditions. A stereoselective visible light-induced oxidative (1 + 1 + 4 + 4) homodimerization gave rise to highly substituted 1,5-dioxocanes in moderate yields. This transformation harnessed singlet oxygen as a safe and mild oxidant under photocatalyst-free reaction conditions. Additionally, we demonstrated that the same substrates undergo a (4 + 2) heterodimerization under Brønsted-acid catalysis to produce chromane derivatives featuring 3 contiguous tertiary stereocenters in good to high yields with excellent diastereoselectivities.

Functionalized Chromans from ortho-Quinone Methides and Arylallenes

ortho-Quinone methides (o-QMs) underwent formal [4 + 2]-cycloaddition reactions with arylallenes regioselectively at the styrenyl olefin to furnish the corresponding 3-methylene-2-arylchromans in moderate to good yields (up to 88%). When R ≠ H, the reactions also proceeded with moderate stereoselectivity (up to 5:1) which was governed by the nature of the R group. The 3-methylene-2-arylchromans could serve as common intermediates for further functionalization including epoxidation, oxidative cleavage/Baeyer-Villiger oxidation, Riley oxidation, acid-catalyzed rearrangement, and Pd-catalyzed cross-coupling reactions to furnish the corresponding derivatives in moderate to good yields.

Asymmetric Synthesis of Chromans Through Bifunctional Enamine-Metal Lewis Acid Catalysis

Cooperative enamine-metal Lewis acid catalysis has emerged as a powerful tool to construct carbon-carbon and carbon-heteroatom bond forming reactions. A concise synthetic method for asymmetric synthesis of chromans from cyclohexanones and salicylaldehydes has been developed to afford tricyclic chromans containing three consecutive stereogenic centers in good yields (up to 87 %) and stereoselectivity (up to 99 % ee and 11 : 1 : 1 dr). This difficult organic transformation was achieved through bifunctional enamine-metal Lewis acid catalysis. It is believed that the strong activation of the salicylaldehydes through chelating to the metal Lewis acid and the bifunctional nature of the catalyst accounts for the high yields and enantioselectivity of the reaction. The absolute configurations of the chroman products were established through X-ray crystallography. DFT calculations were conducted to understand the mechanism and stereoselectivity of this reaction.

Asymmetric Brønsted Acid Catalyzed Cycloadditions of ortho-Quinone Methides and Related Compounds

This review summarizes recent developments in the area of Brønsted acid catalyzed, enantioselective cycloadditions of ortho-quinone methides, ortho-quinone methide imines as well as heterocyclic indole- and pyrrole-based methides. In a straightforward and single-step transformation complex polycyclic N- and O-heterocyclic scaffolds are accessible, with typically good yields and excellent stereocontrol, from simple benzyl and heterobenzyl alcohols upon acid-catalyzed dehydration. The transient precursors are hydrogen-bonded to a chiral Brønsted acid which controls the enantioselectivity of the process.

1 Introduction

2 Cycloadditions of ortho-Quinone Methides

2.1 Brønsted Acid Catalyzed Processes

2.2 Cooperative Brønsted Acid/Transition-Metal-Catalyzed Processes

3 Cycloadditions of ortho-Quinone Methide Imines

4 Cycloadditions of Indolyl-3-methides

5 Cycloadditions of Indolyl-2-methides

5.1 Brønsted Acid Catalyzed Processes

5.2 Cooperative Brønsted Acid/Transition-Metal-Catalyzed Processes

6 Cycloadditions of Pyrrolyl-2-methides

7 Cycloadditions of Pyrrolyl-3-methides

8 Conclusions

Organocatalyzed Oxa-Diels–Alder Reactions: Recent Progress

The oxa-Diels–Alder reaction is a straightforward, atom-economical process for the construction of six-membered oxacycles, which are privileged structures due to their very common occurrence in several pharmaceuticals and natural products. As with many other asymmetric transformations, organocatalysis provides an elegant pathway to their synthesis via [4+2] annulation under mild reaction conditions. The oxa-Diels–Alder reaction utilizes either an α,β-unsaturated carbonyl as an oxa-diene with a suitable dienophile or a simple carbonyl as a dienophile with other dienes. A range of organocatalysts has been explored in the past decade to execute this strategy. The catalysts induce stereoselectivities via two basic reactivities: (1) The formation of chiral intermediates, or (2) selectively activating suitable reactants via a transition state. The present short review compiles organocatalyzed asymmetric oxa-Diels–Alder reactions published over the last ten years, along with detailed discussions on mechanistic approaches.

1 Introduction

2 Catalysis through Covalent Activation

2.1 N-Heterocyclic Carbenes

2.2 Amines

2.3 Isothiourea Catalysis

2.4 Phosphines

3 Catalysis through Non-Covalent Activation

3.1 Bifunctional Amines

3.2 Brønsted Acids

3.3 Guanidines

4 Multicatalysis through Both Covalent and Non-Covalent Activation

5 Conclusion

Palladium-Catalyzed [4+4] Cycloadditions for Highly Diastereo- and Enantioselective Synthesis of Functionalized Benzo[b]oxocines

Asymmetric cycloaddition reactions represent a powerful strategy for building up complex molecular architectures, especially those with medium-sized rings. Herein, we disclose a highly diastereo- and enantioselective cycloaddition strategy that involves...

Organocatalytic Asymmetric Synthesis of Biologically Relevant 3,3- Dihydroxyphenyloxindoles via para-Quinone Methides Derived from Isatins

The first asymmetric synthetic approach to biologically relevant 3,3-diphenyloloxindoles was developed using para-quinone methides derived from isatins and phenols. Chiral phosphoric acid efficiently catalyzed the reaction and delivered 3,3-diphenyloloxindoles under mild conditions with up to an equivalent yield and excellent enantioselectivity (up to >99% ee). The chirality was maintained in further synthesis.

Asymmetric syntheses of spiro[benzofuro-cyclopenta[1,2-b]indole-indoline] scaffolds via consecutive cyclization

An efficient method to construct enantioenriched spiro[benzofuro-cyclopenta[1,2-b]indole-indoline] scaffolds via consecutive cyclization is described here. The new scaffolds possess five successive chiral stereogenic centers and two spiroheterocycles. A range of highly enantioenriched scaffolds has been obtained with up to 93% yield, 99% ee and >19 : 1 d.r. catalyzed by Brønsted acid catalysts.

Lewis-Acid-Mediated Intramolecular Trifluoromethylthiolation of Alkenes with Phenols: Access to SCF3-Containing Chromane and Dihydrobenzofuran Compounds

A Lewis-acid-mediated intramolecular trifluoromethylthiolation of alkenes with phenols that can offer direct access to SCF₃-containing chromane and dihydrobenzofuran compounds was disclosed for the first time. Numerous SCF₃-containing chromanes were obtained in moderate to good yields using γ-substituted 2-allyphenols as substrates. Meanwhile, various SCF₃-containing dihydrobenzofurans with oxa-quaternary centers were also delivered in moderate to good yields using β-substituted 2-allyphenols as substrates.

Asymmetric organocatalytic double 1,6-addition: rapid access to chiral chromans with molecular complexity

We have developed a highly δ-selective organocatalytic asymmetric double 1,6-addition protocol by tethering ortho-hydroxyphenyl substituted p-QMs onto the linear unbiased 2,4-dienals towards the formation of synthetically enriched chiral chromans.