Thermal cyclizations of protonated poly-unsaturated aldehydes

Abstract

. 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 ...

Model Studies on the Pattern of Volatiles Generated in Mixtures of Amino Acids, Lipid-Oxidation-Derived Aldehydes, and Glucose

“…Subsequently, rearrangement of the double bonds leads to the corresponding 2-alkyl-2-cyclopenten-1-one. The intermediacy of 2-alkyl-3-cyclopenten-1-one and its transformation to the thermodynamically more stable 2-alkyl-2-cyclopenten-1-one has been described before . Correspondingly, 2-methyl-2-cyclopenten-1-one was detected in some of the ( E )-2-hexenal model systems (up to 0.5%).…”

“…Thus, in addition to the degradation of amino acids induced by lipid oxidation products, which has been reported before by Hidalgo and co-workers, , amino-acid-induced degradations and further reactions of lipid oxidation products may be of considerable importance in thermally processed foods as well. In addition, most of the volatiles identified in this study have been reported as volatile constituents of thermally processed lipid-rich food products, ,, indicating that the mechanisms explained play a role in the formation of food volatiles.…”

“…The intermediacy of 2-alkyl-3-cyclopenten-1-one and its transformation to the thermodynamically more stable 2-alkyl-2-cyclopenten-1-one has been described before. 21 Correspondingly, 2-methyl-2-cyclopenten-1-one was detected in some of the (E)-2-hexenal model systems (up to 0.5%). In this case, dehydrogenation of 2-hexenal to 2,4-hexadienal is required in a first step.…”

Multicomponent Domino Synthesis of Cyclopenta[b]furan-2-ones

“…This reaction, in an analogous fashion to the classic Nazarov transformation, involves the initial electrophilic activation of a linearly conjugated triene-carbonyl compound 8 followed by a 4πe – electrocyclization at the end of the polyene chain (Scheme , eq 1) . Depending on substrate structure, the brand new cyclopentenyl cation 10 can then lead to a cyclopentadiene product via elimination , or afford bicyclic systems such as cyclopenta­[ b ]­furans and [3.1.0]­bicyclohexenes through intramolecular nucleophilic trapping processes. − Based on this synthetic strategy, we envisaged a simple and conceptually different domino protocol for the synthesis of substituted cyclopenta­[ b ]­furan-2-ones (Scheme , eq 2). The Knoevenagel condensation between dicarbonyl component Meldrum’s acid 11 and a substituted dienal 12 could afford a suitable polyunsaturated substrate for an interrupted VIN reaction providing cyclopenta­[ b ]­furan intermediate 13 as product.…”