The Direct Observation of a Stereospecific Pentadienyl – Cyclopentenyl Cation Cyclization

Chiu, N. W. K.; Sorensen, Ted S.

doi:10.1139/v73-415

Cited by 14 publications

(2 citation statements)

References 6 publications

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“…The characteristic reaction of this important class of carbenium ions is ring closure of a pentadienyl fragment to form a substituted cyclopentenyl cation. This cyclization reaction has been shown to proceed via a stereoselective 4-n electron electrocyclic ring closure (4,5). These reactions are generally facile, occurring, for example, at very low temperatures when the ions are generated as long-lived species in strong acid media.…”

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Thermal cyclizations of protonated poly-unsaturated aldehydes

Elia¹,

Childs²,

Shaw³

1992

Can. J. Chem.

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. Protonated 2,4-hexadienal (IH) and 2,4,6-octatrienal (2H), prepared by protonation of the analogous aldehydes in FS03H, isomerized to give cyclized products 3H and 4H at 30°C and -20°C respectively. The rate constants for the cyclization of 1H were measured in both FS03H and CF3S03H. It was found that the rate constant for isomerization decreased when CF3S03H was used as the reaction medium. It is suggested that the thermal cyclizations of 1H and 2H involve diprotonated species with protonation occurring on oxygen. 1H and 2H underwent photoisomerization at -78°C in FS03H to yield 5H and 7H respectively. It was found that 5H underwent a cyclization reaction at -40°C to give 6H, which subsequently rearranged to give 3H at -10°C. However, 7H was found to isomerize to 2H at -50°C. The mechanisms of the thermal isomerization are discussed. GEORGE R. ELIA, RONALD F. CHILDS et GARY S. SHAW. Can J . Chem 70, 2065Chem 70, (1992. L'hexa-2,4-diknan (IH) et l'octa-2,4,6-triknal (2H protonts, prCpares par protonation des aldehydes correspondants par le FS03H, s'isomCrisent pour donner les produits cycliques 3H et 4H, a 30°C et -20°C respectivement. On a mesurC les constantes de vitesse pour la cyclisation du produit l H , tant dans le FS03H que dans le CF3S03H. On a trouvC que la constante de vitesse pour 1'isomCrisation diminue lorsqu'on utilise le CF3S03H comme milieu rkactionnel. I1 est suggCrC que les cyclisations thermiques des composes 1H et 2H impliquent des espkces diprotonCes dans lesquelles la protonation se fait sur l'oxygkne. Dans le FS03H, 2 -78"C, les composCs 1H et 2H subissent des photo-isomkrisations qui conduisent respectivement aux composCs 5H et 7H. On a trouvC que le composC 5H subit une reaction de cyclisation i -40°C qui conduit au produit 6H qui se transpose subsCquemment pour donner le compose 3H a -10°C. Toutefois, on a trouvC que, a -50"C, le composC 7H s'isomkrise en composC 2H. On discute des mecanismes des isomCrisations thkrmiques.[Traduit par la redaction]The chemistry of polyenyl carbocations has been investigated extensively (1 -3). The characteristic reaction of this important class of carbenium ions is ring closure of a pentadienyl fragment to form a substituted cyclopentenyl cation. This cyclization reaction has been shown to proceed via a stereoselective 4-n electron electrocyclic ring closure (4, 5). These reactions are generally facile, occurring, for example, at very low temperatures when the ions are generated as long-lived species in strong acid media.There have been fewer studies reported on the chemistry on the hydroxy-or alkoxy-substituted polyenyl carbenium ions, despite the ease by which these ions can be prepared by the protonation of unsaturated carbonyl compounds and their derivatives (6). One important example is the cyclization of protonated cross-conjugated dienones to form protonated cyclopentenones, a reaction that forms the key step in what has become known as the Nazarov reaction (7). This cationic cyclization reaction parallels the cyclization reactions observed ...

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confidence: 99%

Thermal cyclizations of protonated poly-unsaturated aldehydes

Elia¹,

Childs²,

Shaw³

1992

Can. J. Chem.

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“…The formation of cyclopentenyl cations from pentadienyl cations were well documented . Compound 3 involved the establishment of intermediaries γ‐lactones 5 via a cyclopentenyl zwitterion, and one of these, 5c could be isolated (Scheme ).…”

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confidence: 99%

Novel π_2s+π_2a Electrocyclization of Triethylenic‐Malonic Acids Exemplified for a One‐Pot Synthesis of New γ‐Dilactones cis‐Fused with a Cyclopentene

Dillenschneider

Dufossé²,

Fouillaud³

et al. 2015

Journal of Heterocyclic Chem

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A new, easy and rapid synthesis of γ-dilactones is cis-fused with a cyclopentenic ring via cyclization of 7-chlorotriethylenic-malonic acids. The key step implicates an intramolecular cyclization to a cyclopentenyl cation, according to an electrocyclic π 2s + π 2a conrotatory process. This cyclopentenyl cation led to unstable γ-lactones intermediates that are rearrange to more stable isomers. δ-lactones (6Z and 6E-(3-chlorobut-2-en-2-yl)-5-methyl-3,6-dihydro-2H-pyran-2-one) were obtained as secondary products. Mechanistic pathways were considered. The structures of the newly synthesized compounds were established by elemental and spectral data.

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The N azarov Cyclization

1994

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The Nazarov cyclization is named after the eminent Russian chemist I. N. Nazarov (1900–1957). In the course of an extensive study on the formation of allyl vinyl ketones by the mercuric ion and acid‐catalyzed hydration of dienynes, Nazarov and his co‐workers discovered a secondary reaction to form 2‐cyclopentenones. Nazarov initially formulated a direct acid‐catalyzed closure of the allyl vinyl ketones and demonstrated the preparation of 2‐cyclopentenones from these precursors in dozens of cases. However in 1952, Braude and Coles suggested the intermediacy of carbocations and demonstrated that the formation of 2‐cyclopentenones actually proceeds via the α,α′‐divinyl ketones. This fact, together with further mechanistic clarification, has led to the current formulation of the Nazarov cyclization as the acid‐catalyzed closure of divinyl ketones to 2‐cyclopentenones. This process was already documented in 1903. Treatment of dibenzylideneacetone with concentrated sulfuric acid and acetic anhydride followed by mild alkaline hydrolysis afforded a ketol, the correct structure of which was finally proposed in 1955. Other examples of acid‐catalyzed cyclizations of divinyl ketones exist in the early literature, as do reactions which must have proceeded via divinyl ketones. A broader definition of the Nazarov cyclization includes a wide variety of precursors that under specific reaction conditions also produce 2‐cyclopentenones via divinyl ketones or their functional equivalents. A case in point is the formation of a cyclopentenone by treatment of either divinyl ketone or tetrahydropyrone with ethanolic hydrochloric acid. It is the structural variety of precursors that lends versatility to the Nazarov cyclization and which also serves as the organizational framework for this chapter. To facilitate presentation, the reaction is divided into six categories defined by the structure of the precursor: (1) cyclization of divinyl and allyl vinyl ketones, (2) cyclization of silylated (stannylated) divinyl ketones, (3) in situ generation/cyclization of divinyl ketones or equivalents, (4) generation/cyclization of divinyl ketone equivalents by solvolysis, (5) alkyne‐based precursors of divinyl ketones, and (6) coupling reactions to form and cyclize divinyl ketones. The logic of this sequence follows from the order of decreasing structural similarity of the precursors to divinyl ketones. This is intended to be comprehensive in the coverage of cyclizations which produce 2‐cyclopentenones. The related electrocyclic closure of less oxidized pentadienylic cations is not covered. The reaction was most recently reviewed in 1991. Prior to that, the Nazarov cyclization had been reviewed in 1983 and also in the context of pentannulation.

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The Direct Observation of a Stereospecific Pentadienyl – Cyclopentenyl Cation Cyclization

Cited by 14 publications

References 6 publications

Thermal cyclizations of protonated poly-unsaturated aldehydes

Thermal cyclizations of protonated poly-unsaturated aldehydes

Novel π_2s+π_2a Electrocyclization of Triethylenic‐Malonic Acids Exemplified for a One‐Pot Synthesis of New γ‐Dilactones cis‐Fused with a Cyclopentene

The N azarov Cyclization

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The Direct Observation of a Stereospecific Pentadienyl – Cyclopentenyl Cation Cyclization

Cited by 14 publications

References 6 publications

Thermal cyclizations of protonated poly-unsaturated aldehydes

Thermal cyclizations of protonated poly-unsaturated aldehydes

Novel π2s+π2a Electrocyclization of Triethylenic‐Malonic Acids Exemplified for a One‐Pot Synthesis of New γ‐Dilactones cis‐Fused with a Cyclopentene

The N azarov Cyclization

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Novel π_2s+π_2a Electrocyclization of Triethylenic‐Malonic Acids Exemplified for a One‐Pot Synthesis of New γ‐Dilactones cis‐Fused with a Cyclopentene