Exo-selective Diels-Alder Reaction Research Articles

Exo-Selective Diels-Alder Reactions.

The Diels-Alder reaction stands as one of the most pivotal transformations in organic chemistry. Its efficiency, marked by the formation of two carbon-carbon bonds and up to four new stereocenters in a single step, underscores its versatility and indispensability in synthesizing natural products and pharmaceuticals. The most significant stereoselectivity feature is the "endo rule". While this rule underpins the predictability of the stereochemical outcomes, it also underscores the challenges in achieving the opposite diastereoselectivity, making the exo-Diels-Alder reactions often considered outliers. This review delves into recent examples of exo-Diels-Alder reactions, shedding light on the factors inverting the intrinsic tendency. We explore the roles of steric, electrostatic, and orbital interactions, as well as thermodynamic equilibriums in influencing exo/endo selectivity. Furthermore, we illustrate strategies to manipulate these factors, employing approaches such as bulky substituents, s-cis conformations, transient structural constraints, and innovative control physics. Through these analyses, our aim is to provide a comprehensive understanding of how to predict and design exo-Diels-Alder reactions, paving the way for new diastereoselective catalyst systems and expanding the chemical scope of Diels-Alder reactions.

Intermolecular exo-selective Diels–Alder reaction catalysed by dual-functional Brønsted acid: conformational restriction of transition states by hydrogen bonds as the key interaction

An exo-selective Diels–Alder reaction was predicted by a computational approach and was experimentally confirmed by the combined use of a dual-functional acid catalyst, such as phosphoric acid, and the conformationally restricted dienylcarbamate.

Synthesis of the spiroimine fragment of portimines A and B.

Portimines A and B are spirocyclic imine natural products, which display remarkable anticancer, anti-HIV, and antifouling activities. Herein, we report a facile synthesis of the spirocyclic core of portimines A and B. Our strategy utilized a scalable Diels-Alder addition of a 2-bromo-1,3-butadiene to a symmetrical malonate dienophile, coupled with a diastereoselective lactonization of the resulting malonate that enabled differentiation of the two carbonyl groups. This approach overcame issues encountered in previous studies focused on the use of exo selective Diels-Alder reactions, by programming formation of the key stereodiad of the spiroimine fragment into the diastereoselective lactonization event, rather than the cycloaddition step. Elaboration of the key lactone intermediate afforded a functionalized spirolactam fragment as a useful intermediate en route to the portimines. Importantly, a key alcohol intermediate could be resolved by enzymatic resolution, thereby providing an asymmetric route to the spiroimine fragment of portimines A and B.

Inducing high exo selectivity in Diels–Alder reaction by dimethylborane substituent: a DFT study

In this work, the role of Lewis acid–base (LAB) interaction on the stereoselectivity of the Diels–Alder (DA) reaction has been studied by DFT in gas and solution (dichloromethane) phases. The calculations were performed at the B3LYP/6-311G++ (d, p) level. Two different series of DA reactions were investigated: (1)—three mono-substituted cyclopentadienes + dimethyl(vinyl)borane; (2)—five α,β-unsaturated carbonyl compounds + cyclopenta-2,4-dien-1-yldimethylborane. The reacting diene and dienophile pairs were chosen to restrict LAB interaction to the exo reaction pathway. It was found that in some of the examined cases, the favorable LAB interaction is so strong that it can lead to a completely exo-selective DA reaction. Furthermore, a novel multistep synthetic method was hypothesized for preparing exo cycloadduct with near 100% stereoselectivity. Our results can open up new avenues toward the rational design of exo-selective DA reactions for synthesizing novel bioorganic compounds.

Intermolecular Enzyme-Catalyzed endo- and exo-Selective Diels–Alder Reaction

Intermolecular Enzyme-Catalyzed endo- and exo-Selective Diels–Alder Reaction

Understanding exo-selective Diels-Alder reactions involving Fischer-type carbene complexes.

The factors controlling the selectivity of the Diels-Alder cycloaddition reactions involving Fischer-type carbene complexes and cyclopentadiene have been explored computationally by means of density functional theory calculations. To this end, the influence of the substituents directly attached to the carbene ligand on the endo : exo ratio has been compared to the available experimental data and quantitatively analysed in detail by means of the combination of the activation strain model of reactivity and energy decomposition analysis methods. The insight gained in this computational study may be important for the rational design of exo-selective Diels-Alder reactions.

Synthesis directed towards trans-clerodanes employing an exo-selective Diels–Alder reaction as a key-step

A potential intermediate in the synthesis of several trans-clerodane natural products has been constructed employing a Diels–Alder reaction as a key-step. Two epimeric exo-adducts were formed in a 4:3 ratio in an EtAlCl2-mediated cycloaddition of O-silylated 2-vinylcyclohex-2-enol and N-tigloylisoxazolidinone. Both isomers were converted to the trans-clerodane intermediate in four steps; reductive removal of the oxazolidinone, followed by O-benzylation, removal of the silyl protecting group, and, finally, a Dess–Martin oxidation of the deprotected alcohol to the corresponding ketone. It was possible to transform both isomers from the cycloaddition into the final target, but the major isomer was converted at greater yields. An interesting discovery made during the work was that the desilylation demanded significantly different conditions depending on which isomer was deprotected. The fact that the cycloaddition not only resulted in an excellent exo-selectivity, but also that the alkene to a large extent approached from the least hindered side, opens the possibility for enantioselective synthesis of the target compound from (R) or (S) diene starting materials in the future.

Janus Face of the Steric Effect in a Lewis Acid Catalyst with Size-Exclusion Design: Steric Repulsion and Steric Attraction in the Catalytic Exo-Selective Diels–Alder Reaction

An exo-selective catalytic Diels–Alder reaction was developed using a Lewis acid catalyst with size-exclusion structural design. Exploiting the steric effect, especially the steric attraction, the Lewis acid catalyst was able to reroute the reaction along a higher-energy pathway. The experimental findings were also supported by theoretical calculations. This catalyst development allows an easy and practical access to highly complex and pharmaceutically relevant compounds.

A Negishi cross-coupling reaction enables the total synthesis of (+)-stachyflin

We present a full account on the development of the total synthesis of the antiviral meroterpenoid (+)-stachyflin. The decalin subunit is rapidly accessed by an exo-selective Diels–Alder reaction, whereas the isonindolinone was synthesized via a highly efficient and practical de novo route starting from dimedone. A challenging sp2–sp3 Negishi cross-coupling reaction enabled construction of the crucial C15–C16 bond that connects the arene with the decalin subunit. For the final installation of the cis-decalin framework, a Lewis acid-catalyzed cyclization was applied.

Stereo-controlled synthesis of polyheterocycles via the diene-transmissive hetero-Diels-Alder reaction of β,γ-unsaturated α-keto esters.

We describe the stereoselective synthesis of polyring-fused heterocyclic compounds based on diene-transmissive hetero-Diels-Alder reactions utilizing β,γ-unsaturated α-keto esters. This protocol involves the initial endo- or exo-selective Diels-Alder (DA) reactions with electron-rich dienophiles, methylenation of the ester carbonyl groups with the Tebbe reagent, and a stereoselective second DA reaction with electron-deficient dienophiles. The use of enantioselective DA reactions in the initial reaction enables access to chiral polyring-fused heterocyclic compounds with multiple chiral centres.

Exo-Selective Diels-Alder Reaction of 2,4-Dienals and Nitroalkenes

Correction to: exo-Selective Diels-Alder Reaction of 2,4-Dienals and NitroalkenesSynfacts 2011; 2011(10): 1134-1134DOI: 10.1055/s-0030-1261122

The Catalytic Asymmetric Diels–Alder Reactions and Post‐cycloaddition Reductive Transpositions of 1‐Hydrazinodienes

The Catalytic Asymmetric Diels–Alder Reactions and Post‐cycloaddition Reductive Transpositions of 1‐Hydrazinodienes

Azido-Coronatine: A Useful Platform for “Click Chemistry”-Mediated Probe Synthesis for Bioorganic Studies

We report on the development of azide-coronatine as a useful platform for azide alkyne cycloaddition ("click chemistry")-mediated synthesis of molecular probes. (+)-Azido-coronatine was synthesized in 10 steps with 11% yield using improved synthesis of coronafacic acid, in which the highly exo-selective Diels-Alder reaction (endo:exo > 1:25) is the key step. Azido coronatine was as effective as the original coronatine in a stomatal opening assay, and was easily modified to a fluorescein isothiocyanate (FITC)-labeled probe with high yield.

ChemInform Abstract: Enantioselective Synthesis of 2-Alkyl-5-methylene-1,3-dioxolan-4-ones and Exo-Selective Diels-Alder Reactions with Cyclopentadiene.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform Abstract: exo-Selective Diels-Alder Reactions Mediated by Et2AlCl in the Presence of H2O.

Abstract Diels-Alder reactions of 4-triisopropylsilyloxy-2H-thiopyran with N-(2-propenoyl)-2-oxazolidones are promoted by Lewis acids. Unusually high exo selectivity is observed when Et2AlCl in the presence of H2O is used as the Lewis acid. The involvement of the sulfur atom with the bidentate Lewis acid dichlorodiethyl-μ-oxodialuminum is postulated.

Total syntheses of coronatines by exo-selective Diels–Alder reaction and their biological activities on stomatal opening

The natural phytotoxin coronatine, which is composed of two individual parts, coronafacic acid and coronamic acid, exhibits various promising biological activities similar to jasmonic acid. Interestingly, coronatine induces stomatal opening involving the swelling of guard cells in which jasmonic acid is not involved as an endogenous regulator. We established syntheses of four stereoisomers of coronatine employing the exo-selective Diels–Alder reaction as a key step. Remarkable differences in stomatal opening activity were observed between enantiomers of coronatine. This result strongly suggests that the stereo structure of coronatine is very important for its stomatal opening activity. In addition, SAR studies suggested that coronatine operates as a molecular mimic of jasmonyl-L-isoleucine in plant guard cells.

Exo- and Enantioselective Diels-Alder Reactions: Pyrazolidinone Auxiliaries as a Means to Enhanced exo Selectivity

Achiral pyrazolidinone auxiliaries have been investigated in exo- and enantioselective Diels-Alder reactions. The effect of pyrazolidinone N-1 substitution and chiral ligand identity were studied in Yb(OTf)3-catalyzed exo-selective Diels-Alder reactions. An appropriate choice of pyrazolidinone auxiliary and Yb(OTf)3/Pybox chiral Lewis acid led to moderate exo selectivity and high enantioselectivity for the exo-cycloadduct.

Total Synthesis of Pinnatoxin A

A familiar ring: A ruthenium-catalyzed cycloisomerization of an enyne, synthesized by an exo-selective Diels–Alder reaction with an α-methylene lactone as the dienophile, proved to be highly effective for the construction of the 27-membered carbocycle of pinnatoxin A. The total synthesis was completed upon the formation of the seven-membered cyclic imine by self-catalyzed dehydration.

Synthesis of the first fluorinated cantharidin analogues

Fluorinated cantharidin analogues ( 4– 7 ) have been synthesized for the first time. The key step in each synthesis is an exo-selective Diels–Alder reaction of furan with the appropriate fluorinated maleic anhydride. 17O NMR measurements of 5– 7 and the corresponding non-fluorinated parent compounds were undertaken to show the influence of the fluorine substituent(s) on the electronic properties of the oxygen atoms.

ChemInform Abstract: High, Exoselective Diels—Alder Reaction in 5.0 M Lithium Perchlorate in Diethyl Ether Medium: Efficient Synthesis of Novel Heterocyclic Derivatives Containing a Spirobicyclo[2.2.1]heptane System.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.