Intramolecular asymmetric propargylation of esters and imides: C-H functionalisation enables stereocontrolled access to UCS1025A

We report an iridium-catalysed stereoselective cyclisation of aryl alkynes with tethered esters and imides to give silyl-protected O,O- and N,O-acetals. The optimised conditions afford chiral 5- and 6-membered heterocycles in excellent ee and moderate to high dr and can be isolated chromatographically as single diastereomers. A variety of esters, including acetates, formates, benzoates, and pivalates, can be employed to give functionalised tetrahydrofuran derivatives. Additionally, phthalimide- and maleimide-containing substrates give stereodefined nitrogen-containing bi- and tricyclic fused-ring products with potential applications in natural product synthesis, as illustrated by the synthesis of a known intermediate en route to furopyrrolizidine alkaloid UCS1025A.

Imidazole-catalyzed construction of bridged bicyclo [3.3.1] ketals via formal [3 + 3]-cycloaddition of naphthols and 2-hydroxyl chromene derivatives

Bridged bicyclo[3.3.1]nonanes having the O,O-ketal subunit are widely found in medicinal compounds. While several acid-catalyzed approaches have been reported for the construction of ketals, this study discloses an imidazole-catalyzed modular construction of bicyclo[3.3.1]nonanes from naphthols/phenols and 3-acyl-2-hydroxyl chromenes. The proposed reaction pathway follows a formal [3 + 3] cycloaddition of phenols and an oxonium dipolar intermediate. The reaction is efficient regarding scalability, environmentally benign conditions, and resulting in the products in good to excellent yields.

Brønsted Acid-Catalyzed Cascade Ring-Opening/Cyclization of 3-Ethoxy Cyclobutanones to Access 2,8-Dioxabicyclo[3.3.1]nonane Derivatives

A cascade ring opening of 3-ethoxy cyclobutanones followed by a double cyclization strategy has been developed via Brønsted acid catalysis. A range of 2,8-dioxabicyclo[3.3.1]nonanes are obtained from various substituted naphthols in a one-pot and open flux manner. Additionally, a 15-membered macrocycle has been synthesized by ring closing metathesis as a synthetic application.

One-pot synthesis of benzofused 8-oxabicyclo[3.3.1]nonanes via GaCl3-mediated cyclocondensation of o-allylbenzaldehydes and 1,3-dicarbonyl synthons

GaCl3-mediated one-pot cyclocondensation of o-allylbenzaldehydes and 1,3-dicarbonyls generates benzofused 8-oxabicyclo[3.3.1]nonanes in moderate to good yields in refluxing MeNO2 under easy-operational conditions. A plausible mechanism is proposed and discussed here. In the overall reaction process, various metal chloride-promoted conditions were investigated for one-pot cyclocondensation.

Total Synthesis of (+)-Peniciketal B

An efficient synthesis of (+)-peniciketal B has been accomplished in 15 steps from the commercially available materials atraric acid, acryloyl chloride, and (+)-homoallylic alcohol. A convergent synthetic approach that is quite concise for constructing either "hemisphere" of (+)-peniciketal B with a common intermediate is employed that relies on a cascade intermolecular FeCl3-mediated "inner sphere" Michael-type reaction/double cyclization of an α,β-unsaturated ketone and substituted phenol to build the benzo-fused 2,8-dioxabicyclo[3.3.1]nonane with excellent diastereoselectivity. The generality of the transformation was also demonstrated by the broad scope of substrates that would be potential candidates for natural product synthesis and medicinal chemistry. Benzannulated [6,6]spiroketal was installed by a late-stage acid-catalyzed spiroketalization.

Different routes for the construction of biologically active diversely functionalized bicyclo[3.3.1]nonanes: an exploration of new perspectives for anticancer chemotherapeutics

Cancer is the second most high-morbidity disease throughout the world. From ancient days, natural products have been known to possess several biological activities, and research on natural products is one of the most enticing areas where scientists are engrossed in the extraction of valuable compounds from various plants to isolate many life-saving medicines, along with their other applications. It has been noticed that the bicyclo[3.3.1]nonane moiety is predominant in most biologically active natural products owing to its exceptional characteristics compared to others. Many derivatives of bicyclo[3.3.1]nonane are attractive to researchers for use in asymmetric catalysis or as potent anticancer entities along with their successful applications as ion receptors, metallocycles, and molecular tweezers. Therefore, this review article discusses several miscellaneous synthetic routes for the construction of bicyclo[3.3.1]nonanes and their heteroanalogues in association with the delineation of their anticancer activities with few selective compounds.

Enantioselective transannular reactions by palladium-catalysed conjugate addition of aryl boronic acids

A palladium-catalyzed asymmeric conjugate addition of aryl boronic acids to medium-sized cycloalkenones followed by intramolecular aldol trapping is reported. The use of in situ formed [Pd/(QuinoxP*)] as the catalyst enables the synthesis of arylbicyclic scaffolds in good yields and with excellent stereocontrol (up to 7 : 1 dr, up to 99% ee). The reaction is applicable to a range of medium size ketoenone substrates and funcionalized aryl boronic acids, including heterocyclic compounds.

Chiral Squaramide Catalyzed Asymmetric Spiroketalization toward Aromatic [6,5] Spiroketals

Herein is disclosed an efficient enantio- and diastereoselective spiroketalization of aromatic ketone tethered to ortho-homoformyl and enone moiety via in situ enol formation using quinine derived squaramide organocatalyst to access aromatic [6,5] spiroketals with complete atom economy. Furthermore, aromatic spiroketals undergo Brønsted acid catalyzed Piancatelli type rearrangement to provide dihydronaphtho[1,2-b]furans with retention of the enantioselectivities.

Acid-catalysed iterative generation of o-quinone methides for the synthesis of dioxabicyclo[3.3.1]nonanes: total synthesis of myristicyclins A-B

We report a practical and efficient method for the synthesis of bioactive flavanoids relying on the strategic use of o-quinine methide (o-QM) intermediates. This involves Brønsted acid-catalysed iterative generation of o-QMs/[4+2] cycloaddition/intermolecular Michael addition/cyclative acetalization in a cascade sequence for the synthesis of dioxabicyclo[3.3.1]nonanes. The 'one-pot', controlled cascade sequence successfully provided the shortest route amenable for gram scale synthesis of natural products (±)-myristicyclins A-B.

Controllable Synthesis of Two Isomers 4H-Chromene and 2,8-Dioxabicyclo[3.3.1]nonane Derivatives under Catalyst-Free Conditions

A one-pot method for the selective synthesis of two isomers 4H-chromene and 2,8-dioxabicyclo[3.3.1]nonane derivatives was developed without a catalyst and using EtOH/H₂O (4:1, v/v) as the solvent. The reaction was conducted under mild conditions, with forming multiple chemical bonds in one pot and high atom economy, and only a stoichiometric amount of H₂O is produced as the byproduct. Its selectivity was controlled by thermodynamics and kinetics, and the reasons for the transformation of the two structures are also discussed.

Copper-catalyzed enantioselective synthesis of bridged bicyclic ketals from 1,1-disubstituted-4-methylene-1,6-hexanediols and related alkenols

Bridged bicyclic ketals display a range of bioactivities. Their catalytic enantioselective synthesis from acyclic 1,1-disubstituted alkene diols is disclosed. This reaction combines asymmetric catalysis with a distal radical migration. Alkynes and arenes undergo the group transfer.

Highly enantioselective addition of aliphatic aldehydes to 2-hydroxychalcone enabled by cooperative organocatalysts

Herein, we developed an enantioselective addition of aliphatic aldehydes to 2-hydroxychalcone promoted by cooperative organocatalysts, giving access to hybrid flavonoids in excellent enantioselectivities. This reaction took advantage of cycloisomerization of 2-hydroxychalcone to form a transient flavylium under the irradiation of 24 W CFL, which was trapped by the in situ generated chiral enamine intermediate. The synergistic action of chiral phosphoric acid secured the excellent outcome of this reaction by ion-pairing with the transient flavylium.

Synthesis of A-Type Proanthocyanidins and Their Analogues: A Comprehensive Review

Proanthocyanidins (PACs) are oligomers or polymers composed of units of flavanols. A-type PACs are a subclass of PACs characterized by the presence of at least a double linkage between two consecutive monomers of flavanol. These A-type PACs are found in some fruits and spices and possess potential health benefits as a result of their interesting biological activities, and consequently, their isolation and synthesis have given rise to great interest in the past. This review summarizes the synthetic efforts made to obtain both naturally occurring A-type PACs and their structurally simplified analogues. Most of the synthetic protocols reported involve the addition of a π-nucleophilic molecule over a molecule with two electrophilic carbons, such as a chalcone, a flavylium salt, or a flavanol derivative, among others. Synthesis of A-type PACs remains an issue at a very early stage of development compared to that of PACs with single linkages between monomers (B-type PACs), but the advances that are taking place in the last few years point to a significant development of the subject in the near future.

An asymmetric multicatalytic reaction sequence of 2-hydroxycinnamaldehydes and enolic 1,3-dicarbonyl compounds to construct bridged bicyclic acetals

2-Hydroxycinnamaldehydes and cyclic 1,3-dicarbonyl nucleophiles were used in an asymmetric organocatalyzed reaction sequence to construct bridged bicyclic acetals via a multicatalytic process involving iminium catalysis and anion-binding catalysis.

Pd(II)-Catalyzed Enantioselective Desymmetrization of Polycyclic Cyclohexenediones: Conjugate Addition versus Oxidative Heck

Pd(II)-catalyzed desymmetrization of polycyclic cyclohexenediones has been achieved with high enantio- and diastereoselectivities. Up to five contiguous stereocenters are desymmetrized, while simultaneously, an additional stereocenter is created by the enantioselective conjugate addition. Surprisingly, the conjugate addition products dominate even under typical oxidative Heck conditions, and these observations may provide some insight into the competition between the two related reactions.

Intramolecular Benzannulation of 3-Sulfonyl-2-benzylchromen-4-ones: Synthesis of Sulfonyl Dibenzooxabicyclo[3.3.1]nonanes

In this work, a concise route for the synthesis of sulfonyl dibenzo-oxabicyclo[3.3.1]nonanes by a two-step route is described, including (i) NaBH₄/LiCl-mediated reduction of 3-sulfonyl-2-benzylchromen-4-ones and (ii) sequential BF₃·OEt₂-mediated intramolecular annulation of the resulting 3-sulfonyl-2-benzylchroman-4-ols. A plausible mechanism is proposed and discussed herein. This protocol provides a highly effective stereocontrolled aryl-hydroxyl Friedel-Crafts-type cross-coupling to construct the tetra- or pentacyclic bridged framework. The use of various reaction conditions is investigated for an efficient transformation.

A bio-inspired synthesis of hybrid flavonoids from 2-hydroxychalcone driven by visible light

An efficient protocol for the synthesis of hybrid flavonoids was achieved via a bio-inspired tandem reaction of 2-hydroxychalcone with various nucleophiles.

An unexpected multi-component one-pot cascade reaction to access furanobenzodihydropyran-fused polycyclic heterocycles

An unexpected multi-component one-pot cascade reaction of hydroxyketones with ortho-hydroxychalcones has been developed to afford an array of new furanobenzopyran-fused polycyclic compounds with complex molecular architectures.

Organocatalytic Asymmetric Synthesis of Bridged Acetals with Spirooxindole Skeleton

The first highly diastereo- and enantioselective synthesis of bridged O, O-acetals embedded with spirooxindoles has been developed. Dioxindoles and 2-hydroxy cinnamaldehydes were employed as the reaction partners in this method. The desired products were obtained via diaryl prolinol TBS ether catalyzed Michael reaction followed by acetal formation with TFA.

Palladium(ii)-catalyzed γ-selective hydroarylation of alkenyl carbonyl compounds with arylboronic acids

A catalytic γ-selective syn-hydroarylation of alkenyl carbonyl compounds using arylboronic acids has been developed using a substrate directivity approach with a palladium(ii) catalyst. This method tolerates a wide range of functionalized (hetero)arylboronic acids and a variety of substitution patterns on the alkene. Preliminary mechanistic studies suggest that transmetalation is rate-limiting.

Pd(ii)-Catalyzed asymmetric 1,6-conjugate addition of arylboronic acids to Meldrum's acid-derived dienes

A Pd(ii)-catalyzed asymmetric 1,6-conjugate addition of arylboronic acids to Meldrum's acid-derived dienes has been realized using a new pyridine–oxazoline ligand bearing an extended arm.

Metal-free diastereoselective construction of bridged ketal spirooxindoles via a Michael addition-inspired sequence

A TfOH-catalyzed highly diastereoselective Michael addition/ketalization sequence of 3-hydroxyoxindoles and ortho-hydroxychalcones was developed, leading to biologically important bridged ketal spirooxindoles in moderate to excellent yields.

Biomimetic Approach to the Catalytic Enantioselective Synthesis of Flavonoids

Herein is reported the direct asymmetric addition of phenol nucleophiles to benzopyrylium salts as a means to produce enantioenriched flavonoid-like compounds. This enantioselective C-C bond construction was achieved through a chiral anion phase-transfer strategy that mimics the proposed biosynthesis of this structurally diverse set of natural products. The utility of this methodology was demonstrated in enantioselective synthesis of a 2,8-dioxabicyclo[3.3.1]nonane and a 2,4-diarylbenzopyran.

The first catalytic asymmetric thioacetalization by chiral phosphoric acid catalysis

We report here the first catalytic asymmetric thioacetalization of salicylaldehyde and dithiol. Chiral phosphoric acid STRIP C5 is identified as a powerful catalyst for this reaction to afford various chiral dithioacetals in high to excellent yields and enantioselectivities under mild conditions.