Asymmetric electrocyclic reactions

Synthesis of cis-(8b,14a)-hexahydro-14H-dibenzo[f,h]oxazolo[3,2-b]isoquinolin-14-ones via photo cascade reaction

A concise method for the synthesis of cis-(8b,14a)-hexahydro-14H-dibenzo[f,h]oxazolo[3,2-b]isoquinolin-14-ones 2 via photo-induced 3-([1,1'-biphenyl]-2-yl)-1-(2-hydroxyethyl)pyridin-2(1H)-ones 1 was developed. Irradiation of 1 in the solution of toluene with a 313 nm UV light in the presence of HCl gave cis-(8b,14a)-9a-α-hexahydro-14H-dibenzo[f,h]oxazolo[3,2-b]isoquinoli n-14-ones and cis-(8b,14a)-9a-β-hexahydro-14H-dibenzo[f,h]oxazolo[3,2-b]isoquinolin-14-ones 2 (2-α and 2-β) in good yields. The protocol simultaneously constructs dearomatized phenanthrene ring and oxindolizidinones ring by photo cascade reaction to achieve high bonding efficiency and high atomic efficiency. Additionally, the antitumor activities of 2 was evaluated and compounds 2b-α, 2b-β, 2j-β and 2 k-α showed similar or better activity compared to the cisplatin against tumor cell lines of Leukemia HL-60, lung cancer A594, liver cancer SMMC-7721 and breast cancer MDA-MB-231.

Asymmetric Photoinduced Excited-State Nazarov Reaction

We report herein the first asymmetric photoinduced excited-state Nazarov reaction of non-aromatic dicyclic divinyl ketones by using hydrogen-bonding catalysis. The enantioselectivity of photoinduced electrocyclization is highly dependent on the structural features of the substrate and its interaction with chiral catalysts. For the simple dicyclic divinyl ketone substrates, there is no discernible selectivity of the hydrogen bond coordination between the thiourea and carbonyl groups of the substrates in the ground state. However, we found that the direction of the electrocyclization was well controlled in each coordination model and the N,N'-dimethylamine motif acts as a base in the regioselective deprotonation process, which leads to the formation of two stereoisomers with high enantioselectivity. Photolysis of dicyclic divinyl ketones bearing a 1,3-dioxolane motif in the presence of bifunctional hybrid peptide-thiourea chiral catalysts gave the tricyclic cis-hydrofluorenones with good enantioselectivity. Mechanistic and DFT studies suggested that the amide and thiourea groups in the bifunctional chiral catalysts play a key role as H-bond donors, which coordinate with both the carbonyl group and the 1,3-dioxolane motif to provide a more favorable chiral species, and control the direction of the electrocyclization. Due to the presence of the rigid 1,3-dioxolane ring, the deprotonation/protonation process occurs regiospecifically with high driving force. This photo-electrocyclization is mild (room temperature and neutral solution), which results a broad reaction scope and functional group tolerance and demonstrates its synthetic potential in organic synthesis.

Catalytic enantioselective synthesis of 2-pyrazolines via one-pot condensation/6π-electrocyclization: 3,5-bis(pentafluorosulfanyl)-phenylthioureas as powerful hydrogen bond donors

A new conjugate-base-stabilized carboxylic acid (CBSCA) containing a 3,5-bis(pentafluorosulfanyl)phenylthiourea functionality catalyses challenging one-pot condensations/6π-electrocyclizations of hydrazines and α,β-unsaturated ketones under mild conditions. Structurally diverse N-aryl 2-pyrazolines are obtained in good yields and enantioselectivities. The superior performance of 3,5-bis(SF5)phenylthioureas over the widely used 3,5-bis(CF3)phenylthioureas is further demonstrated in the Michael addition of dimethyl malonate to nitrostyrene, using a new Takemoto-type catalyst.

Highly Enantioselective 6π Photoelectrocyclizations Engineered by Hydrogen Bonding

Photochemical electrocyclization reactions are valued for both their ability to produce structurally complex molecules and their central role in elucidating fundamental mechanistic principles of photochemistry. We present herein a highly enantioselective 6π photoelectrocyclization catalyzed by a chiral Ir(III) photosensitizer. This transformation was successfully realized by engineering a strong hydrogen-bonding interaction between a pyrazole moiety on the catalyst and a basic imidazolyl ketone on the substrate. To shed light on the origin of stereoinduction, we conducted a comprehensive investigation combining experimental and computational mechanistic studies. Results from density functional theory calculations underscore the crucial role played by the prochirality and the torquoselectivity in the electrocyclization process as well as the steric demand in the subsequent [1,4]-H shift step. Our findings not only offer valuable guidance for developing chiral photocatalysts but also serve as a significant reference for achieving high levels of enantioselectivity in the 6π photoelectrocyclization reaction.

A computational investigation towards substitution effects on 8π electrocyclisation of conjugated 1,3,5,7-octatetraenes

A computational investigation using M06-2X/6-31+G(d) method is reported for the substitution effects on 8π electrocyclisation of conjugated octatetraene. This systematic study describes the mono- and di-substitution effect across the 1,3,5,7-octatetraene skeleton. A general preference of the outward substitution over the inward, at C1 position of the monosubstituted system is observed. However, mesomerically electron donating group (-NH2 and -OH) display an opposite effect with respect to secondary orbital interaction (SOI) between the lone pair on the substituent and the orbital. A comparative evaluation on the computed activation energies for the 1-, 2-, 3-, and 4-monosubstituted system showed an insignificant impact on the rate of the reaction, in contrast to the electrocyclic ring closure of the unsubstituted compound. Computations of disubstituted system are more pronounced, where a remarkable acceleration is observed for 2-NO2-7-NO2 substituted octatetraene at 4.9 kcal mol-1, and a noticeable deceleration for 4-CH3-5-CH3 substituted octatetraene at 25.4 kcal mol-1 from the parent molecule, 17.0 kcal mol-1. A visible accelerated effects are commonly exhibited by the substitution on the terminal double bonds (C1, C2, C7, and C8), that are 1,2-, 1,7-, 1,8-, and 2,7-patterns, in regard to the greater orbital interaction for the new σ-bond formation. Despite the unfavourable steric clashes of the substituents in the 1,8-system, an apparent reduction in the energy barrier up to 7.4 kcal mol-1 is computed for 1-NH2-8-NO2 system from 17.0 kcal mol-1. This is due to the synergistic effect of the electron donor and electron acceptor, enhancing the stability of the transition structure. The electrocyclic ring closure involving vicinal substitution patterns, such as 1,2-, 2,3-, 3,4-, and 4,5-systems are critically dominated by steric crowding between the adjacent functional groups. In certain cases of the 1,2-substituted system, a noticeable accelerated effects are found for 1-NH2-2-NH2-substituted compound (9.7 kcal mol-1) due to an increased in electronic density on the substituted terminal double bond (C1-C2), hence favouring the formation of the new σ-bond.

Expanding the 'aplysinospin cascade' through DNA-templated [2+2] photocycloaddition

Inspired by the unique ability of nucleic acids to template chemical transformations that are otherwise impossible in solution, we embarked on the generalisation of our DNA-templated [2+2] photo-induced homo- and heterodimerization of aplysinopsins. Our process ensures a straightforward access to cyclobutane containing natural products and analogues thereof. Most importantly, this conceptual biomimetic achievement presents interesting arguments to build a biosynthetic scenario.

Synthesis of Pyrrolo[2,1-a]isoquinolines through Cu-Catalyzed Condensation/Addition/Oxidation/Cyclization Cascade

We have developed a copper-catalyzed synthesis of pyrrolo[2,1-a]isoquinolines with terminal alkynes, aldehydes, and tetrahydroisoquinolines. A variety of pyrrolo[2,1-a]isoquinolines have been prepared in 17-69% yield via a condensation/Mannich-type addition/oxidation/cyclization cascade sequence. Modifications through simple chemical transformations provided highly functionalized molecules containing a privileged framework.

Decarboxylative Cyclization of Proline with o-Alkynylbenzaldehyde through an Unexploited 8π-Electrocyclization: A DFT Study

We employed density functional theory (DFT) methods to investigate the most plausible mechanism of cyclization/ring expansion of proline with o-alkynylbenzaldehyde. This one-pot reaction starts with the in situ formation of azomethine ylide, which can undergo three different reaction pathways to form the final product. Two mechanisms are based on nucleophilic addition and 4π-electrocyclization of the azomethine ylide, and our results indicate that the rate-determining step (RDS) of these two cyclizations are 40.1 and 40.2 kcal/mol, respectively. The third novel pathway relies upon 8π-electrocyclization as the key step of this reaction; interestingly, the RDS of this cyclization is ∼20.6 kcal/mol, which shows this route to be most feasible. Thus, we introduce a novel mechanism for the electrocyclization reaction of conjugated azomethine ylides that can help chemists to design and access a new series of compounds.

Stereoselective Synthesis of Atropisomeric Acridinium Salts by the Catalyst-Controlled Cyclization of ortho-Quinone Methide Iminiums

Quinone methides are fundamental intermediates for a wide range of reactions in which catalyst stereocontrol is often achieved by hydrogen bonding. Herein, we describe the feasibility of an intramolecular Friedel-Crafts 6π electrocyclization through ortho-quinone methide iminiums stereocontrolled by a contact ion pair. A disulfonimide catalyst activates racemic trichloroacetimidate substrates and imparts stereocontrol in the cyclization step, providing a new avenue for selective ortho-quinone methide iminium functionalization. A highly stereospecific oxidation readily transforms the enantioenriched acridanes into rotationally restricted acridiniums. Upon ion exchange, the method selectively affords atropisomeric acridinium tetrafluoroborate salts in high yields and an enantioenrichment of up to 93 : 7 e.r. We envision that ion-pairing catalysis over ortho-quinone methide iminiums enables the selective synthesis of a diversity of heterocycles and aniline derivatives with distinct stereogenic units.

Recent Advances in Oxa-6π Electrocyclization Reactivity for the Synthesis of Privileged Natural Product Scaffolds

The stunning advances in understanding the reactivity and selectivity principles of asymmetric pericyclic reactions have had a profound impact on the synthetic planning of complex natural products. Indeed, electrocyclizations, cycloadditions, and sigmatropic rearrangements enable synthetic chemists to craft highly functionalized scaffolds that would not otherwise be possible with a similar atom-, step-, and redox-economy. In this review, selected examples from the last two decades of research (2003–2020) on tandem processes combining oxa-6π electrocyclic reactions are discussed in terms of reactivity challenges, inherent reversibility, and key structural bond formation in the assembly of natural products. A particular emphasis is given to the electrocyclic ring-closures in the tandem processes featuring Knoevenagel-type condensations, Diels–Alder cycloadditions, Stille couplings, and oxidative dearomatizations. The synthetic manifolds reviewed here illustrate how oxa-6π electrocyclizations are intimately linked to the construction of complex natural product scaffolds and have inspired a number of biomimetic syntheses in the laboratory.

Cascade 8π Electrocyclization/Benzannulation to Access Highly Substituted Phenylpyridines

A cascade 8π electrocyclization/benzannulation reaction was developed to obtain the synthetically important highly substituted phenyl-pyridines. This method shows great potential in the rapid and inexpensive application of the scalable and operationally simple production of accessible substrates. On the basis of the resulting phenyl-pyridine products, a new Ru catalyst and bidentate ligand were designed and prepared, further demonstrating its high practicability.

Tandem 6π-Azatriene Electrocyclization of Fused Amino-cyclopentenones: Synthesis of Functionalized Pyrrolo- and Indolo-quinoxalines

A tandem 6π-azacyclization approach for the synthesis of diversified pyrrolo/indolo[1,2-a]quinoxalines from amino-cyclopentenones has been developed. The reaction proceeds through a trifluoroacetic-acid-mediated 6π-electrocyclization and concomitant opening of the cyclopentenone ring. The advantageous features of the developed chemistry include transition-metal-free conditions, operational simplicity, and a broad substrate scope. Further X-ray crystallographic studies confirm the assigned structures of the fused heterocycles.

Synthesis of cis/trans-dihydrochromenones via a photoinduced rearrangement of 4-phenyl-3-aryl/cyclohexenylcoumarins

A concise and environmentally friendly protocol has been developed for the synthesis of cis-dihydrochromenones and trans-dihydrochromenones in EtOH at room temperature. Irradiation of 4-phenyl-3-arylcoumarins in EtOH with 313 nm UV light under an argon atmosphere at room temperature gave cis-4b,15c-dihydro-16H-benzofuro[3',2':7,8]phenanthro[9,10-c]chromen-16-ones and cis-8c,14b-dihydro-9H-benzo[11,12]chryseno[5,6-c]chromen-9-ones in good yields. And an analogous treatment of 4-phenyl-3-alkenylcoumarins as 4-phenyl-3-arylcoumarins provided trans-1,2,3,4,4a,14b-hexahydro-5H-phenanthro[9,10-c]chromen-5-ones. The described photorearrangement proceeded smoothly without the addition of any transition metals and additives. The photorearrangement of 4-phenyl-3-arylcoumarins is believed to proceed via 6π-electrocyclization, a [1,3]-hydrogen shift and keto-enol isomerization.

Total synthesis of endiandric acid J and beilcyclone A from cyclooctatetraene

The endiandric acids are classic targets in natural product synthesis. The spectacular 8π/6π-electrocylisation/intramolecular Diels-Alder (8π/6π/IMDA) reaction cascade at the heart of their biosynthesis has inspired practitioners and students of pericyclic chemistry for nearly forty years. All previous synthetic approaches have sought to prepare a linear tetraene and thereby initiate the cascade. In this communication we demonstrate the use of cyclooctatetraene to rapidly intercept the 8π/6π/IMDA cascade at the cyclooctatriene stage. Endiandric acid J and beilcyclone A are prepared for the first time in six and five steps, respectively. The strategy features a tactical overall anti-vicinal difunctionalisation of cyclooctatetraene through SN2' alkylation of cyclooctatetraene oxide followed by an intriguing tandem Claisen rearrangement/6π-electrocyclisation from the corresponding vinyl ether. This rapidly constructs an advanced bicyclo[4.2.0]octadiene aldehyde intermediate. Olefinations and intramolecular Diels-Alder cycloadditions complete the syntheses. This establishes a short and efficient new path to the endiandric acid natural products. DFT modelling predicts thermal racemisation of bicyclo[4.2.0]octadiene intermediates, dashing hopes of enantioselective synthesis.

Photoelectrocyclization Reactions of Conjugated Cycloalkenones: Scope and Reactivity

Outlined here are studies exploring the scope of the sequential photoelectrocyclization, [1,5]-hydride shift of conjugated bis-aryl cycloalkenone substrates. We have found not only that the cyclization precursors can be synthesized in a modular fashion but also that the cyclization is efficient and amenable to the presence of a range of cycloalkenones and aromatic systems. Among the interesting discoveries from this work is that the electrocyclization intermediate can be competitively captured with protons and that the nature of the excited state (singlet vs triplet) is dependent on aromatic substitution.

Tandem Reaction Approaches to Isoquinolones from 2-Vinylbenzaldehydes and Anilines via Imine Formation-6π-Electrocyclization-Aerobic Oxidation Sequence

Two distinctive transition-metal-promoted aerobic oxidation protocols have been developed for the synthesis of isoquinolones from 2-vinylbenzaldehydes and aniline derivatives. Thus, the one-pot tandem reaction sequence of imine formation, thermal 6π-electrocyclization, followed by either Cu(OAc)₂-mediated or Pd(OAc)₂-catalyzed aerobic oxidation protocol allowed the ready access to isoquinolone derivatives. The control experiments revealed that the 1,2-dihydroisoquinoline intermediates from the 6π-electrocyclization of 1-azatrienes were aerobically oxidized to isoquinolones in the presence of either Cu(OAc)₂ or Pd(OAc)₂ catalyst.

Recent advances in the total synthesis of cyclobutane-containing natural products

Recent developments of strategies on the construction of cyclobutanes and their application in complex natural product synthesis are discussed.

Zinc-Catalyzed Asymmetric Formal [4+3] Annulation of Isoxazoles with Enynol Ethers by 6π Electrocyclization: Stereoselective Access to 2H-Azepines

6π electrocyclization has attracted interest in organic synthesis because of its high stereospecificity and atom economy in the construction of versatile 5-7-membered cycles. However, examples of asymmetric 6π electrocyclization are quite scarce, and have to rely on the use of chiral organocatalysts, and been limited to pentadienyl-anion- and triene-type 6π electrocyclizations. Described herein is a zinc-catalyzed formal [4+3] annulation of isoxazoles with 3-en-1-ynol ethers via 6π electrocyclization, leading to the site-selective synthesis of functionalized 2H-azepines and 4H-azepines in good to excellent yields with broad substrate scope. Moreover, this strategy has also been used to produce chiral 2H-azepines with high enantioselectivities (up to 97:3 e.r.). This protocol not only is the first asymmetric heptatrienyl-cation-type 6π electrocyclization, but also is the first asymmetric reaction of isoxazoles with alkynes and the first asymmetric catalysis based on ynol ethers.

Stereodivergent Photoelectrocyclization Reactions of Bis-aryl Cycloalkenones: Intercepting Photoelectrocyclization Intermediates with Acid

Described here are tandem photoelectrocyclization and [1,5]-hydride shift reactions of heteroaryl-containing bis-aryl cyclohexenone derivatives that give heteroaryl-substituted dihydrophenanthrenes. This Letter demonstrates that electrocyclization intermediates can be trapped with acid when the [1,5]-hydride shift is relatively slow. From a practical perspective, the observation that the acid-mediated reaction gives a divergent stereochemical outcome when compared with the reaction run under neutral conditions makes these transformations powerful.

The 6π-azaelectrocyclization of azatrienes. Synthetic applications in natural products, bioactive heterocycles, and related fields

Covering: 2006 to 2018 The application of the 6π-azaelectrocyclization of azatrienes as a key strategy for the synthesis of natural products, their analogs and related bioactive or biomedically-relevant compounds (from 2006 to date) is comprehensively reviewed. Details about reaction optimization studies, relevant reaction mechanisms and conditions are also discussed.

Domino Self-Sensitized Photooxygenation of Conjugated Dienones for the Synthesis of 1,2,4-Trioxanes

The photochemical behavior of several dienones was studied under aerobic conditions. 2-Allylidene-1,3-cycloalkanediones prepared via Knoevenagel-type condensation between simple readily available 1,3-dicarbonyl substrates and α,β-unsaturated aldehydes afforded 1,2,4-trioxane derivatives upon UVA irradiation in the presence of oxygen. This domino self-sensitized photooxygenation cascade of conjugated carbonyl systems proceeds stereoselectively and involves the formation of two new oxa-cycles, three new bonds (two C-O), and three stereocenters.

Computational chemistry strategies in natural product synthesis

The synthesis of natural products increasingly uses computational chemistry approaches to model and understand molecular phenomena. Calculations are employed to rationalize reaction outcomes, predict how a new system will perform, and inform synthetic design. As a result, new insights into the interactions of fundamental chemical forces have emerged that advance the field of complex small molecule synthesis. This review presents ten examples of computational techniques used in the synthesis of natural products, and discusses the unique perspectives afforded by these quantitative analyses.

Transition-Metal-Free Multicomponent Approach to Stereoenriched Cyclopentyl-isoxazoles through C-C Bond Cleavage

An efficient multicomponent reaction for the synthesis of stereoenriched cyclopentyl-isoxazoles from camphor-derived α-oximes, alkynes, and MeOH is reported. Our method involved a series of cascade transformations, including the in situ generation of an IIII catalyst, which catalyzed the addition of MeOH to a sterically hindered ketone. Oxidation of the oxime, and rearrangement of the α-hydroxyiminium ion generated a nitrile oxide in situ, which, upon [3+2] cycloaddition reaction with an alkyne, delivered the regioselective product. This reaction was very selective for the syn-oxime. This multicomponent approach was also extended to the synthesis of a new glycoconjugate, camphoric ester-isoxazole C-galactoside.

Chiral Brønsted Acid Catalyzed Enantioselective Dehydrative Nazarov-Type Electrocyclization of Aryl and 2-Thienyl Vinyl Alcohols

An efficient chiral Brønsted acid-catalyzed enantioselective dehydrative Nazarov-type electrocyclization (DNE) of electron-rich aryl- and 2-thienyl-β-amino-2-en-1-ols is described. The 4π conrotatory electrocyclization reaction affords access to a wide variety of the corresponding 1 H-indenes and 4 H-cyclopenta[ b]thiophenes in excellent yields of up to 99% and enantiomeric excess (ee) values of up to 99%. Experimental and computational studies based on a proposed intimate contact ion-pair species that is further assisted by hydrogen bonding between the amino group of the substrate cation and chiral catalyst anion provide insight into the observed product enantioselectivities.

Synthesis of Briarane Diterpenoids: Biomimetic Transannular Oxa-6π electrocyclization Induced by a UVA/UVC Photoswitch

A biomimetic synthesis of briareolate ester B (3) from briareolate ester L (1) via the intermediate briareolate ester G (2) has been achieved through a unique transannular oxa-6π electrocyclization induced by UVA light. UVC irradiation of 3 triggered a rapid retro-6π electrocyclization to establish an unprecedented photochromic switch. In the ground state, reaction of 1 led to the formation of a polycyclic γ-spiroketal γ-lactone 5, architecturally related to the ether-bridged cembranoids of the cladiellin class.

Triaryl-Substituted Divinyl Ketones Cyclization: Nazarov Reaction versus Friedel-Crafts Electrophilic Substitution

The acid-catalyzed cyclization of a wide range of triaryl-substituted divinyl ketones has been studied. It was found that the reaction pathway strongly depends on the nature of the aryl substituent at the α-position to the carbonyl group. An electron-rich aromatic substituent promotes the reaction to proceed through the intramolecular Friedel-Crafts electrophilic substitution giving dihydronaphthalene derivatives. In contrast, the presence of an electron-deficient substituent is favorable for the Nazarov 4π-conrotatory cyclization yielding triaryl-substituted cyclopentenones. The electrophilic substitution reaction was applied to thiophene and thiazole derivatives.

Captodative substitution induced acceleration effect towards 4π electrocyclic ring-opening of substituted cyclobutenes

Significance of quantum chemical calculations as a theoretical tool to identify the optimum substitution pattern for a rapid 4π electrocyclic ring-opening of substituted cyclobutenes is presented.

Transition State Gauche Effects Control the Torquoselectivities of the Electrocyclizations of Chiral 1-Azatrienes

Hsung et al. have reported a series of torquoselective electrocyclizations of chiral 1-azahexa-1E,3Z,5E-trienes that yield functionalized dihydropyridines. To understand the origins of the torquoselectivities of these azaelectrocyclizations, we modeled these electrocyclic ring closures using the M06-2X density functional. A new stereochemical model that rationalizes the observed 1,2 stereoinduction emerges from these computations. This model is an improvement and generalization of the "inside-alkoxy" model used to rationalize stereoselectivities of the 1,3-dipolar cycloaddition of chiral allyl ethers and emphasizes a stabilizing hyperconjugative effect, which we have termed a transition state gauche effect. This stereoelectronic effect controls the conformational preferences at the electrocyclization transition states, and only in one of the allowed disrotatory electrocyclization transition states is the ideal stereoelectronic arrangement achieved without the introduction of a steric clash. Computational experiments confirm the role of this effect as a stereodeterminant since substrates with electropositive groups and electronegative groups have different conformational preferences at the transition state and undergo ring closure with divergent stereochemical outcomes. This predicted reversal of stereoselectivity for the ring closures of several silyl substituted azatrienes have been demonstrated experimentally.

4π electrocyclisation in domino processes: contemporary trends and synthetic applications towards natural products

Recent most instructive and reliable literature reports which deal with domino processes involving conrotatory 4π electrocyclic reactions, along with a precise mechanistic insight and latest synthetic applications towards biologically active natural products are concisely reviewed. To inspire further research in this domain, a real insight into the overarching emerging themes and potential imminent prospects is also provided.

Synthesis of ent-ketorfanol via a C-H alkenylation/torquoselective 6π electrocyclization cascade

The asymmetric synthesis of ent-ketorfanol from simple and commercially available precursors is reported. A Rh(I) -catalyzed intramolecular CH alkenylation/torquoselective 6π electrocyclization cascade provides a fused bicyclic 1,2-dihydropyridine as a key intermediate. Computational studies were performed to understand the high torquoselectivity of the key 6π electrocyclization. The computational results demonstrate that a conformational effect is responsible for the observed selectivity. The ketone functionality and final ring are introduced in a single step by a redox-neutral acid-catalyzed rearrangement of a vicinal diol to give the requisite carbonyl, followed by intramolecular Friedel-Crafts alkylation.

Efficient catalytic enantioselective Nazarov cyclizations of divinyl ketoesters

An efficient catalytic enantioselective Nazarov cyclization of divinyl ketoesters is developed using a BOX/Cu(ii) catalyst, affording various optically active cyclopent-2-enone esters.

No acid required: 4π and 6π electrocyclization reactions of dienyl diketones for the synthesis of cyclopentenones and 2H-Pyrans

The 1,6-conjugate addition of nucleophiles to dienyl diketones produces either cyclopentenone or 2H-pyran products with high selectivity through either Nazarov (4π) or 6π electrocyclization, respectively. The outcome of the reaction is dependent upon the nature of the nucleophile used. Nucleophiles that are anionic or easily deprotonated exclusively produce cyclopentenones via Nazarov cyclization, whereas the neutral nucleophile DABCO promotes 6π cyclization to afford 2H-pyrans. Experimental evidence is presented for both retro-4π and -6π electrocyclization in these systems, lending support to the bifurcated mechanistic hypothesis proposed for these cyclizations.