Stereoselective Diels-Alder Reaction Research Articles

Highly Stereoselective Diels-Alder-Based Strategy for the Synthesis of 3-epi-Formicin A and 1-epi-Formicin B.

The first enantioselective approach based on a highly stereoselective Diels-Alder reaction for the synthesis of 3-epi-formicin A and 1-epi-formicin B with rare N-acetylcysteamine-containing indenone thioesters is reported. The strategy utilizes a key Diels-Alder reaction to form the core hydrindane system with three contiguous stereocenters in very high levels of diastereo- and regioselectivity and one-pot oxidation/isomerization/dehydrogenation. The scope of this method was tested with different substrates to give cycloadducts in a highly diastereoselective manner.

Total syntheses of Tetrodotoxin and 9-epiTetrodotoxin

Tetrodotoxin and congeners are specific voltage-gated sodium channel blockers that exhibit remarkable anesthetic and analgesic effects. Here, we present a scalable asymmetric syntheses of Tetrodotoxin and 9-epiTetrodotoxin from the abundant chemical feedstock furfuryl alcohol. The optically pure cyclohexane skeleton is assembled via a stereoselective Diels-Alder reaction. The dense heteroatom substituents are established sequentially by a series of functional group interconversions on highly oxygenated cyclohexane frameworks, including a chemoselective cyclic anhydride opening, and a decarboxylative hydroxylation. An innovative SmI2-mediated concurrent fragmentation, an oxo-bridge ring opening and ester reduction followed by an Upjohn dihydroxylation deliver the highly oxidized skeleton. Ruthenium-catalyzed oxidative alkyne cleavage and formation of the hemiaminal and orthoester under acidic conditions enable the rapid assembly of Tetrodotoxin, anhydro-Tetrodotoxin, 9-epiTetrodotoxin, and 9-epi lactone-Tetrodotoxin.

Highly Stereoselective Diels–Alder Reactions Catalyzed by Diboronate Complexes**

AbstractA highly enantioselective catalytic system for exo‐Diels–Alder reactions was developed based on the newly discovered bispyrrolidine diboronates (BPDB). Activated by various Lewis or Brønsted acids, BPDB can catalyze highly stereoselective asymmetric exo‐Diels–Alder reactions of monocarbonyl‐based dienophiles. When 1,2‐dicarbonyl‐based dienophiles are used, the catalyst can sterically distinguish between the two binding sites, which leads to highly regioselective asymmetric Diels–Alder reactions. BPDB can be prepared as crystalline solids on a large scale and are stable under ambient condition. Single‐crystal X‐ray analysis of the structure for acid‐activated BPDB indicated that its activation involves cleavage of a labile B←N bond.

Highly Stereoselective Diels-Alder Reactions Catalyzed by Diboronate Complexes.

A highly enantioselective catalytic system for exo-Diels-Alder reactions was developed based on the newly discovered bispyrrolidine diboronates (BPDBs). When activated various Lewis or Brønsted acids, BPDBs could catalyze highly stereoselective asymmetric exo-Diels-Alder reactions of monocarbonyl-based dienophiles. When 1,2-dicarbonyl-based dienophiles are used, the catalyst can sterically distinguish between the two binding sites, which leads to highly regioselective asymmetric Diels-Alder reactions. BPDBs can be prepared as crystalline solids on a large scale and are stable under ambient conditions. Single-crystal X-ray analysis of the structure for acid-activated BPDB indicated that activation involves breakage of a labile N→B bond.

Exploring Stereospecificity, Regioselectivity, and Stereoselectivity in Diels-Alder Reactions: Insights from HMO Method, FMO Theory, and DFT Calculations

Exploring Stereospecificity, Regioselectivity, and Stereoselectivity in Diels-Alder Reactions: Insights from HMO Method, FMO Theory, and DFT Calculations

Enantioselective fullerene functionalization through stereochemical information transfer from a self-assembled cage.

The regioselective functionalization of C60 remains challenging, while the enantioselective functionalization of C60 is difficult to explore due to the need for complex chiral tethers or arduous chromatography. Metal-organic cages have served as masks to effect the regioselective functionalization of C60. However, it is difficult to control the stereochemistry of the resulting fullerene adducts through this method. Here we report a means of defining up to six stereocentres on C60, achieving enantioselective fullerene functionalization. This method involves the use of a metal-organic cage built from a chiral formylpyridine. Fullerenes hosted within the cavity of the cage can be converted into a series of C60 adducts through chemo-, regio- and stereo-selective Diels-Alder reactions with the edges of the cage. The chiral formylpyridine ultimately dictates the stereochemistry of these chiral fullerene adducts without being incorporated into them. Such chiral fullerene adducts may become useful in devices requiring circularly polarized light manipulation.

Catalytic mechanism and endo-to-exo selectivity reversion of an octalin-forming natural Diels-Alderase.

We have previously reported the identification of CghA, a proposed Diels-Alderase responsible for the formation of the bicyclic octalin core of the fungal secondary metabolite Sch210972. Here we show the crystal structure of the CghA-product complex at a resolution of 2.0 Å. Our result provides the second structural determination of eukaryotic Diels-Alderases and adds yet another fold to the family of proteins reported to catalyse [4 + 2] cycloaddition reactions. Site-directed mutagenesis-coupled kinetic characterization and computational analyses allowed us to identify key catalytic residues and propose a possible catalytic mechanism. Most interestingly, we were able to rationally engineer CghA such that the mutant was able to catalyse preferentially the formation of the energetically disfavoured exo adduct. This work expands our knowledge and understanding of the emerging and potentially widespread class of natural enzymes capable of catalysing stereoselective Diels-Alder reactions and paves the way towards developing enzymes potentially useful in various bio/synthetic applications.

Total Synthesis of (−)‐Lepadin F based on a Stereoselective Diels–Alder Reaction Controlled by a Ketolactone‐type Dienophile

An efficient approach to the type III lepadin alkaloids (lepadins F and G) has been developed through a key Diels-Alder reaction, in which a novel ketolactone-type dienophile with chiral diol unit is employed to generate the desirable all-cis-trisubstituted cyclohexene with excellent regio- and stereoselectivity control. The subsequent selective sulfonylation of the diol unit followed by SN 2 cyclization under hydrogenation conditions could construct the substituted piperidine ring. By using this approach, (-)-lepadin F is synthesized from ethyl l-lactate for the first time.

Highly selective Diels-Alder and Heck arylation reactions in a divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles from 2-formylpyrrole.

A highly regio-, chemo- and stereoselective divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles is herein described, starting from 2-formylpyrrole and employing Diels–Alder and Heck arylation reactions. 3-(N-Benzyl-2-pyrrolyl)acrylates and 4-(pyrrol-2-yl)butenones underwent a highly endo-Diels–Alder cycloaddition with maleimides to furnish octahydropyrrolo[3,4-e]indoles, which served as precursors in the regioselective synthesis of aza-polycyclic skeletons via an intramolecular Heck arylation reaction. Through the latter reaction, the 3-(N-benzyl-2-pyrrolyl)acrylates give rise to 3-(pyrrolo[2,1-a]isoindol-3-yl)acrylates. A further oxidative aromatization of the polycyclic intermediates provides the corresponding polycyclic pyrrolo-isoindoles and isoindolo-pyrrolo-indoles. A theoretical study on the stereoselective Diels–Alder reactions, carried out by calculating the endo/exo transition states, revealed the assistance of non-covalent interactions in governing the endo stereocontrol.

Site-Selective N-Methylation of 5,15-Diazaporphyrins: Reactive Cationic Porphyrinoids that Provide Isoporphyrin Analogues.

N-Alkylation significantly changes the electronic and optical properties, as well as the reactivity of nitrogen-containing π-conjugated molecules. In this study, it is found that treating 5,15-diazaporphyrins with methyl triflate selectively affords the corresponding N-methyl-5,15-diazaporphyrinium cations in good yield. N-Methylation substantially alters the electronic properties and reactivity of diazaporphyrins. The electron-accepting properties of the N-methyl-5,15-diazaporphyrinium cations are enhanced due to their lowered LUMO level. Stabilization of the LUMO energy enables regio- and stereoselective Diels-Alder reactions of the cationic diazaporphyrin with cyclopentadiene. N-Methylation also enhances the acidity of the inner NH protons, and thus, allows facile deprotonation to provide nitrogen-substituted isoporphyrin analogues with only one NH group in the central cavity.

Highly Diastereoselective Palladium-Catalyzed Oxidative Cascade Carbonylative Carbocyclization of Enallenols.

A palladium-catalyzed oxidative cascade carbonylative carbocyclization of enallenols was developed. Under mild reaction conditions, a range of cis-fused [5,5] bicyclic γ-lactones and γ-lactams with a 1,3-diene motif were obtained in good yields with high diastereoselectivity. The obtained lactone/lactam products are viable substrates for a stereoselective Diels-Alder reaction with N-phenylmaleimide, providing polycyclic compounds with increased molecular complexity.

Synthesis and Reaction of ortho-Benzoquinone Monohemiaminals.

The preparation and reactions of ortho-benzoquinone monohemiaminals are described. The oxidative dearomatization of phenols bearing amino alcohol groups induced N-cyclization to afford ortho-benzoquinone monohemiaminals. The N-cyclization stereoselectively affords the product when a chiral amino alcohol is used as the substituent. The chiral ortho-benzoquinone monohemiaminal undergoes stereoselective Diels-Alder reactions with electron-deficient alkenes, as expected, confirming the promising utility of ortho-benzoquinone monohemiaminals.

The Role of Supramolecular Intermediates in the Potential Energy Surface of the Diels-Alder Reaction

The potential energy surface of four stereoselective Diels-Alder reactions was studied, namely: cyclopentadiene-maleic anhydride, furan-maleic anhydride, the dimerization of cyclopentadiene, and cyclopentadiene-cyclopropene. For completeness, we also studied the reaction between ethylene and 2-hydroxy-6-methyl-1,4-benzoquinone, a [5+2] cycloaddition reaction. For all cases at least a stationary state of supramolecular nature a van der Waals complex, was determined. These stationary states are complexes formed by the interaction between the reagents, minima located in the paths between the non-interacting molecules and the transition states. The existence of these complexes makes it necessary to reconsider the role of Secondary Orbital Interactions in the selectivity of these reactions. As it is the case with other complexes, the stability of these supramolecular intermediates depends on electrostatic phenomena such as dispersion forces. The observation of [5+2] intramolecular complexes in solution is important since up to now, this kind of van der Waals complexes had only been described in the gas phase.

Stereoselective Synthesis of (+)-Annuionone A and (-)-Annuionone B.

A stereoselective synthetic approach was utilized to synthesize enantiopure annuionones A (1b) and B (2b), two ionone-type norsesquiterpenoids that both bear a 6-oxabicyclo[3.2.1]octane framework and possess allelopathic activity. A stereoselective Diels-Alder reaction based on chiral trisubstituted dienophile 20 was employed to obtain the optically active polysubstituted cyclohexane core of both natural products. Using this approach, (+)-annuionone A (1b) and (-)-annuionone B (2b) were synthesized from lactol (+)-15 in 10% overall yield.

Regioselectivity and stereoselectivity of Diels-Alder reaction: a DFT study on the functionalization of organic semiconductor crystals

Diels–Alder (DA) reaction is one of the most commonly tools in functionalizing organic semiconductor crystals. The DA reactions of two organic semiconductors, i.e. tetracene () and rubrene (), to several dienophiles ( to ) were performed experimentally recently. But the kinetics and mechanism of stereoselectivity and regioselectivity remain unknown. In the current study, all related 20 DA reactions (totally 32 possible pathways) were investigated by density functional theory. It was found that the reaction of and a–b position of is the most favorable one. The c–d position of is more reactive than its a–b position when combines with , but is less reactive when combines with to . The endo and exo pathways have similar activation barriers in each reaction. The rate coefficients were calculated using the canonical variational transition state theory and their Arrhenius expressions were fitted. The theoretical conclusion agrees with the experimental observations and is of general importance for similar reactions. Solvent has a slight effect on these reactions. Copyright © 2016 John Wiley & Sons, Ltd.

ChemInform Abstract: Enantioselective Organocatalytic Diels—Alder Trapping of Photochemically Generated Hydroxy‐o‐quinodimethanes.

The photoenolization/Diels-Alder strategy offers straightforward access to synthetically valuable benzannulated carbocyclic products. This historical light-triggered process has never before succumbed to efforts to develop an enantioselective catalytic approach. Herein, we demonstrate how asymmetric organocatalysis provides simple yet effective catalytic tools to intercept photochemically generated hydroxy-o-quinodimethanes with high stereoselectivity. We used a chiral organic catalyst, derived from natural cinchona alkaloids, to activate maleimides toward highly stereoselective Diels-Alder reactions. An unconventional mechanism of stereocontrol is operative, wherein the organocatalyst is actively involved in both the photochemical pathway, by leveraging the formation of the reactive photoenol, and the stereoselectivity-defining event.

Enantioselective Organocatalytic Diels-Alder Trapping of Photochemically Generated Hydroxy-o-Quinodimethanes.

The photoenolization/Diels–Alder strategy offers straightforward access to synthetically valuable benzannulated carbocyclic products. This historical light‐triggered process has never before succumbed to efforts to develop an enantioselective catalytic approach. Herein, we demonstrate how asymmetric organocatalysis provides simple yet effective catalytic tools to intercept photochemically generated hydroxy‐o‐quinodimethanes with high stereoselectivity. We used a chiral organic catalyst, derived from natural cinchona alkaloids, to activate maleimides toward highly stereoselective Diels–Alder reactions. An unconventional mechanism of stereocontrol is operative, wherein the organocatalyst is actively involved in both the photochemical pathway, by leveraging the formation of the reactive photoenol, and the stereoselectivity‐defining event.

Enantioselective Organocatalytic Diels–Alder Trapping of Photochemically Generated Hydroxy‐o‐Quinodimethanes

AbstractThe photoenolization/Diels–Alder strategy offers straightforward access to synthetically valuable benzannulated carbocyclic products. This historical light‐triggered process has never before succumbed to efforts to develop an enantioselective catalytic approach. Herein, we demonstrate how asymmetric organocatalysis provides simple yet effective catalytic tools to intercept photochemically generated hydroxy‐o‐quinodimethanes with high stereoselectivity. We used a chiral organic catalyst, derived from natural cinchona alkaloids, to activate maleimides toward highly stereoselective Diels–Alder reactions. An unconventional mechanism of stereocontrol is operative, wherein the organocatalyst is actively involved in both the photochemical pathway, by leveraging the formation of the reactive photoenol, and the stereoselectivity‐defining event.

ChemInform Abstract: A Ring‐Closing Enyne Metathesis Approach to Functionalized Semicyclic Dienes: The Total Synthesis of (‐)‐Tetrangomycin.

The angucycline antibiotic (–)-tetrangomycin was synthesized in 13 steps (11 % overall yield) by using a stereoselective Diels–Alder reaction between a naphthoquinone and a semicyclic diene to construct the benz[a]anthraquinone ring system. The diene intermediates were synthesized through a ring-closing enyne metathesis reaction. The tertiary alcohol at C-3 was installed by an asymmetric dihydroxylation reaction. The relative stereochemistry of the dienes was verified by the NMR analyses of the cycloadducts that were obtained from their reaction with N-phenylmaleimide. Selective aromatization of the B ring was achieved under oxidative conditions.

Theoretical study on the Diels-Alder reaction of bromo-substituted 2H-pyrane-2-ones and some substituent vinyls

A DFT study of the reactivity, regio- and stereoselectivity of Diels-Alder reaction between 3-bromo, 5-bromo, and 3,5-dibromo-2(H)-pyran-2-ones and some weakly activated and unactivated alkenes has been carried out using density functional theory (DFT). Four possible reaction channels, which are related to the formation of meta- and para- and endo- and exo-cycloadducts have been explored and characterized. The energy and natural bond orbital analysis shows that the meta-regioselectivity on the exo pathway is preferred and follows an asynchronous concerted mechanism with a polar nature in all Diels-Alder cycloadditions. Moreover, the activation free energies of Diels-Alder cycloadditions of 3,5-dibromo-2(H)-pyran-2one are lower than 3- bromo-2(H)-pyran-2one and 5-bromo-2(H)-pyran-2one, which are in line with experimental observation. DFT-based reactivity indices clearly predict the regiochemistry of the isolated cycloadducts.