Ene reaction of Diels–Alder adducts of levoglucosenone and 1,3-dienes with acetaldehyde

Diels–Alder adducts of levoglucosenone with isoprene, butadiene and piperylene in the presence of AlCl₃ smoothly react with acetaldehyde or benzaldehyde to give products of the ene reaction, the hydroxy group of the primary intermediates participating in the formation of semiketal moiety. The yields of the reaction products depend both on the Lewis acid used (AlCl , BF ·Et O, ZnBr , SnCl or EtAlCl₂) and on the nature of the substrate.     

Enantioselective Synthesis of cis-Decalins by Merging the Birch Reduction and Inverse-Electron-Demand Diels–Alder Reaction

Herein, we show that the combination of the Birch reduction of readily available anisole derivatives and the catalytic asymmetric inverse-electron-demand Diels–Alder reaction of 2-pyrones can serve as a powerful platform for the diverse synthesis of synthetically important cis-decalin scaffolds. Enabled by a well-modified chiral bis(oxazoline) ligand/Cu^II complex, a wide range of polysubstituted cis-decalin scaffolds with up to six contiguous stereocenters were generated efficiently. The synthetic potential of this method is demonstrated by the concise synthesis of the sesquiterpene (+)-occidentalol and a key intermediate for seven triterpenes. Mechanistic studies suggest the 1,3-cyclohexadienes formed in situ are the key intermediates, and efficient kinetic resolution occurs when C2- and/or C3-substituted 1,4-cyclohexadienes are utilized as substrates. DFT calculations elucidated that the Diels–Alder reaction proceeds in a stepwise fashion and revealed the origins of the stereoselectivities.     

Regio- and Diastereoselective Synthesis of Cyclohexadienylborons via an Intermolecular Diels–Alder Reaction of Alkenyl MIDA Boronates with 2-Pyrones

An efficient synthesis of highly functionalized cyclohexadienylborons via an inverse electron-demand Diels–Alder reaction/CO₂ extrusion of alkenyl MIDA boronates with 2-pyrones is outlined. By controlling the reaction temperature, the corresponding C(sp³)-rich bicyclolactones could also be readily formed. The exo-selective reactions feature good functional-group tolerance, broad substrate scope, and excellent regio- and diastereoselectivity. Oxidation of the cyclohexadienylborons in a one-pot procedure led to the construction of aromatic boronates bearing valuable functional groups. Synthetic transformations of the C−B bond were demonstrated.     

Enantioselective Synthesis of cis-Decalin Derivatives by the Inverse-Electron-Demand Diels–Alder Reaction of 2-Pyrones

A novel strategy for the synthesis of cis-decalins by an ytterbium-catalyzed asymmetric inverse-electron-demand Diels–Alder reaction of 2-pyrones and silyl cyclohexadienol ethers is reported here. A broad range of synthetically important cis-decalin derivatives with multiple contiguous stereogenic centers and functionalities are obtained in good yields and stereoselectivities. A full set of diastereomeric substituted cis-decalin motifs are readily accessible by tuning the absolute configurations of substituted silyl cyclohexadienol ethers (R or S) as well as the ligands (R or S). The synthetic potential is showcased by the enantioselective total synthesis of 4-amorphen-11-ol, and further demonstrated by the first total synthesis of cis-crotonin.     

Organocatalytic Enantioselective Diels–Alder Reaction of 2-Trifluoroacetamido-1,3-dienes with α,β-Unsaturated Ketones

We report herein the first examples of chiral phosphoric acid-catalyzed enantioselective Diels–Alder reactions between 2-trifluoroacetamido-1,3-dienes 1 and α,β-unsaturated carbonyl compounds 2 . Polysubstituted 1-acetamido cyclohexenes 3 were formed in high yields with excellent diastereo- and enantioselectivities. The reaction proceeds through a stepwise process as shown by deuterium labelling experiments. A catalytic enantioselective three-component reaction of 1 , 2 and ortho-hydroxybenzhydryl alcohols 4 was subsequently developed furnishing the densely functionalized hexahydroxanthenes 5 in a highly stereoselective manner. This multicomponent reaction generates four chemical bonds with concurrent creation of five contiguous stereocenters.     

Enantioselective Diels–Alder reactions with left-handed G-quadruplex DNA-based catalysts

Since the discovery of left-handed G-quadruplex(L-G4)structure formed by natural DNA,there has been a growing interest in its potential functions.This study uti...     

Generation of Selenoaldehydes via Retro Diels–Alder Reaction and Their Behavior in the Reaction with Some Enophiles

Abstract

The [4, 2] cycloadducts of selenoaldehydes and anthracene regenerate selenoaldehydes in situ quantitatively under neutral conditions via thermal retro Diels–Alder reaction. The reactions of selenoaldehydes generated by this method with 2-silyloxy-1,3-butadiene, 2-methoxyfuran, and 5-ethoxyoxazoles are described.     

Re-Evaluation of a Fulvene-Based Self-Replicating Diels–Alder Reaction System

We re-evaluate our claim of a high diastereoselectivity in the self-relicating Diels–Alder reaction between maleimide 1 and fulvene 3 . It was shown that the system has a diastereoselectivity of 1.8:1 for NN-4 : NX-4 , which is contrary to the 16:1 ratio claimed by Dieckmann et al. The analysis of ¹H NMR monitoring of the reaction revealed that both replicators show sigmoidal growth which is typical for auto-catalytic systems.     

Reaction of the levoglucosenone Diels–Alder adducts with acetaldehyde under the McMurry conditions

Reactions of acetaldehyde with Diels–Alder adducts of levoglucosenone with butadiene, isoprene, and cyclohexa-diene assisted by low-valence titanium afford products of acetaldehyde addition to the acetal center with opening of the 1,6-anhydro bridge. In the case of the cyclopentadiene adduct, the reaction gives the product of addition of the ethyl substituent to the acetal center while the 1,6-anhydro bridge remains unchanged.     

Redox Modulation of the Reactivity and Regioselectivity in Diels–Alder Reaction of Metallofullerene La@C82 with Cyclopentadiene

Modulation of the reactivity of metallofullerenes is critical for production of metallofullerene derivatives with desired properties and functionalities. In this study, we investigate the effects of reduction and oxidation on the reactivity and regioselectivity in Diels–Alder reaction of metallofullerene La@C₈₂ by means of density functional theory calculations. Because of the enhanced electron-deficiency characteristic upon oxidation, the oxidized metallofullerene exhibits higher thermodynamic and kinetic reactivity as compared with neutral La@C₈₂. The regioselectivity in the reaction of La@C₈₂ with cylcopentadiene is remarkably changed after oxidation of the metallofullerene, which is explained in terms of the changes in the geometrical structure and the electronic structure of the metallofullerene. Quantitative analysis based on the activation-strain model demonstrates that the low activation energy barrier for the reaction of the cation La@C₈₂⁺ with cyclopentadiene originates from small strain energy and large interaction energy between the reactants. Energy decomposition analysis on the transition states of the reactions reveals that the exchange-repulsion interaction energy is one of the critical factors that determine the kinetic reactivity of the metallofullerene. This study not only provides new theoretical insights on how to modulate the reactivity of metallofullerenes, but also offers guideline for future experimental synthesis of new metallofullerene derivatives.     

Self-Accelerating Diels–Alder Reaction for Preparing Polymers of Intrinsic Microporosity

It is a formidable challenge in polycondensation to simultaneously construct multiple covalent bonds to prepare double-stranded polymers of intrinsic microporosity (PIMs) with fused multicyclic linkages. To the best of our knowledge, this is the first study to develop a self-accelerating Diels–Alder reaction for successfully preparing double-stranded PIMs with fused multicyclic backbone structures. A self-accelerating Diels–Alder reaction was developed based on the [4+2] cycloaddition of sym-dibenzo-1,5-cyclooctadiene-3,7-diyne (DIBOD) and ortho-quinone compounds. In this reaction, the cycloaddition of ortho-quinone with the first alkyne of DIBOD activates the second alkyne, which reacts with ortho-quinone at a rate constant 192 times larger than that of the original alkyne. Using this self-accelerating reaction to polymerize DIBOD and spirocyclic/cyclic difunctional ortho-quinone monomers, a novel stoichiometric imbalance-promoted step-growth polymerization method was developed to prepare PIMs. The resultant PIMs possess intrinsic ultramicropores with pore sizes between 0.45 to 0.7 nm, high specific surface areas above 646 m² g⁻¹, and good H₂ separation performance.     

Tandem intramolecular Diels–Alder/retro-Diels–Alder cycloaddition of 2H-chromen-2-one as dienes with the expulsion of CO_2

To study the intramolecular Diels-Alder cycloadditon of 2H-chromen-2-one as a diene, a series of chiral N-allyl-N-benzylamides that contain a 2H-chromen-2-one moiety were designed for the synthesis of benzo[f]isoindol-1-ones via an intramolecular Diels-Alder and a subsequent retro-Diels-Alder cycloaddition with the expulsion of CO₂. Both the yield (80%-89%) and absolute stereocontrol of the tandem reaction were high when an electron-withdrawing group was attached to the dienophile. The double bond in the styrene substructure remained in the products could be further derivatized by dihydroxylation.     

Yb(OTf)3‐Catalyzed Intermolecular Imino Diels–Alder Reaction of 2‐Azetidinone‐Tethered Aryl Imines as Azadienes

Yb(OTf)₃ is an efficient catalyst for the intermolecular imino Diels–Alder reaction of aldimines derived from 2‐azetidinone‐tethered aryl imines and electron‐rich dienophiles to afford the quinoline‐β‐lactams.     

Enantioselective Diels–Alder reactions of chiral racemic acyclic dienes with (SS)-2-(p-tolylsulfinyl)-1,4-naphthoquinone

Enantiopure sulfinylnaphthoquinone (+)-1 reacted with racemic acyclic dienes 2a–f bearing a stereogenic allylic center, through a tandem cycloaddition/pyrolytic sulfenic acid elimination, to afford enantio-enriched compounds 4a–f and 5a–f with good like:unlike selectivities (ca. 75:25) and good enantiomeric excesses (68–82%), arising from the partial kinetic resolution of the racemic dienes.     

Rh(I)-Catalyzed Enantioselective Hydrogenation of α-Substituted Ethenylphosphonic Acids

A class of chiral Rh(I) catalysts containing monodentate phosphorous acid diesters tautomerized from the corresponding secondary phosphine oxides was discovered by serendipitous hydrolysis of phosphoramidite ligands. The evolved catalysts demonstrated unprecedented enantioselectivities (98-99% ee) and high catalytic activities (as low as 0.01 mol% catalyst loading) in asymmetric hydrogenations of a wide variety of α-aryl-/alkyl-substituted ethenylphosphonic acids, providing a facile approach to the corresponding enantiopure phosphonic acids with significant biological importance.     

Probing the Regioselectivity with Encapsulated H2: Diels–Alder Reaction of an Open-Cage C60 Derivative with Anthracene

The regioselectivity in the Diels–Alder reaction of an unsymmetrical open-cage C₆₀ derivative with anthracene was studied. By using an encapsulated H₂ molecule as a magnetic probe, the product population was successfully evaluated in detail, indicating the formation of approximately ten compounds as major components. The nucleus-independent chemical shift (NICS) calculations showed a close resemblance to the observed ¹H NMR spectrum, which allowed for a facile characterization of the products. Theoretical studies on the formation of all 29 possible anthracene adducts were also performed. The results indicated that the regioselectivity is strongly governed by steric factors, values of the frontier orbital coefficients, and thermodynamic stabilities. Single-crystal X-ray analysis of the dominant compound revealed the supramolecular architecture between the anthracene moiety and the π-sphere of a neighboring molecule.     

Construction of the Benzomesembrine Skeleton: Palladium(0)-Catalyzed Intermolecular Arylative Dearomatization of α-Naphthols and Subsequent Aza-Michael Reaction

A novel palladium(0)-catalyzed intermolecular arylative dearomatization of α-naphthols and subsequent aza-Michael reaction is described. Two adjacent stereocenters were constructed efficiently through consecutive arylative dearomatization and Michael addition reactions. By utilizing this method, structurally diverse benzomesembrine derivatives were synthesized with excellent yields and chemoselectivity. The benzomesembrine products were shown to undergo versatile functional-group transformations.     

Synthesis of Functionalized Δ3 Phosphabenzenes by Means of Diels Alder Reaction with Phosphaalkenes

Abstract

Diels Alder reaction provides an easy and general access to functionalized Δ³ phosphabenzenes after aromatization with the possibilities of introducing the functional group either with the diene or the phosphaalkene.     

Synthesis of natural product inulavosin via Ga(OTf)3-Catalyzed Hetero Diels–Alder Dimerization of salicyl alcohol derivative

Inulavosin, a natural melanogenesis inhibitor, has been synthesized smoothly from readily available and inexpensive starting materials by using a Ga(OTf)₃-catalyzed room temperature hetero Diels–Alder dimerization of salicyl alcohol derivative and a regioselective phenol monobromination as the key steps.