RESUMO
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,ß-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.
RESUMO
The transition-metal-catalyzed [2 + 2 + 2]-cycloaddition of alkynes has become a powerful atom-economical strategy for aromatic ring construction. Unfortunately, the control of the stereo-, regio-, and chemoselectivity of these processes is usually challenging, and these reactions can potentially lead to complex unuseful mixtures. While solid-phase chemistry has proven to be a successful tool for decreasing the number of cycloadducts formed and for facilitating the purification step, an integral use of the outcoming products in this complex reaction is described herein. By using an immobilized monoalkyne, the transition-metal-catalyzed [2 + 2 + 2]-cycloaddition with soluble 1,6-diyne-esters led to the simultaneous preparation of soluble and solid-supported phthalides, showing a new way to benefit from solid-phase synthetic methodologies.
RESUMO
The gold-catalyzed intermolecular hydroalkylation of olefins with ß-ketoesters represents a conceptually attractive and useful synthetic tool; however, it has been scarcely applied, remaining a challenge for chemists. The aim of the current study was to investigate the addition of these 1,3-diketo-compounds to alkenes under gold catalysis conditions, in order to establish the electronic and steric effects of the alkenyl substrates in the reaction outcome. The screening of different catalyst systems and diverse olefins enabled defining the alkenyl requirements and the best reaction conditions to efficiently achieve the coupled products.
Assuntos
Alcenos/química , Ésteres/química , Ouro/química , Cetonas/química , Catálise , Estrutura Molecular , Espectroscopia de Prótons por Ressonância Magnética , EstereoisomerismoRESUMO
The synthesis of biologically active alkaloid benzosimuline, isolated from the shrub Zanthoxylum simulans, is reported. Key transformation involves an oxa-6π electrocyclic ring-opening/hetero-Diels-Alder pericyclic cascade. Although the last aromatization step proved to be cumbersome, this work unfolds a unique route to access interesting molecules from simple precursors.
Assuntos
Alcaloides/síntese química , Benzopiranos/química , Reação de Cicloadição , Análise Espectral/métodos , Estereoisomerismo , Zanthoxylum/químicaRESUMO
The UVA (350 nm) irradiation of an α-pyran in the presence of oxygen led to the unexpected formation of a tetraoxygenated compound whose structure could not be unambiguously determined on the basis of conventional (1)H-(13)C correlated experiments. 1,1-ADEQUATE (adequate double quantum transfer experiment) and 1,n-ADEQUATE combined with computer-assisted structure elucidation software led to two structural possibilities involving the formation of either an epoxide or an oxetane. Residual dipolar couplings allowed not only the identification of the compound as a spiroepoxide but also the determination of its relative configuration. To account for its formation, we propose a bisepoxide intermediate that, as opposed to most α,ß-epoxyketones under irradiation, undergoes O-Cß cleavage probably due to the presence of an extra oxygen substituent in the ß position. 1,2-Acyl migration would then proceed stereoselectively to the final product obtained as a single diastereomer.
RESUMO
Conjugated trienones and isomeric 2H-pyrans were found to engage in a novel cycloisomerization cascade toward cyclopenta[b]furan derivatives. Knoevenagel chemistry and pericyclic reactions meet again to expand the polyene-carbonyl manifold.
RESUMO
Iron(III) chloride-catalyzed cyclopentannulation of π-conjugated 1,3-dicarbonyl compounds is described. An interrupted vinylogous iso-Nazarov reaction of trienones was established in which cyclopenta[b]furan derivatives are obtained as single diastereomers.
Assuntos
Ciclopentanos/síntese química , Furanos/síntese química , Catálise , Ciclização , Ciclopentanos/química , Furanos/química , Estrutura Molecular , EstereoisomerismoRESUMO
A gold-catalyzed cyclization of immobilized 2-alkynylanilines was developed as the key step in the synthetic sequence for the preparation of 2-substituted indoles. These results demonstrate the potential of the unexplored combination of gold catalysis and solid-phase organic synthesis.
Assuntos
Ouro/química , Indóis/síntese química , Catálise , Ciclização , Estrutura MolecularRESUMO
A new cascade pathway viable for Knoevenagel chemistry that involves the coupling between 1,3-dicarbonyl systems and α,ß,γ,δ-unsaturated aldehydes has been developed. The process comprises the combination of a classic aldol-type condensation and a rare spontaneous metal-free cycloisomerization, representing a convergent and innovative approach for the stereoselective synthesis of cyclopenta[b]furan-type derivatives. The scope and limitations with respect to both reaction partners and mechanistic features were investigated. Meaningfully, our study provides valuable guidance concerning the structural and electronic effects controlling the reactivity of conjugated polyene carbonyl systems.
Assuntos
Aldeídos/química , Furanos/química , Ciclização , Estrutura Molecular , EstereoisomerismoRESUMO
A new approach to the synthesis of cyclopenta[b]benzofuran derivatives via reaction of 1,3-dicarbonyl compounds with α,ß,γ,δ-unsaturated aldehydes is described. The constitution and configuration of the new products have been firmly established by means of residual dipolar couplings (RDCs) and ab initio (13)C NMR chemical shift predictions.
Assuntos
Benzofuranos/síntese química , Isótopos de Carbono , Biologia Computacional , Ciclização , Espectroscopia de Ressonância Magnética , Modelos Moleculares , Estrutura Molecular , EstereoisomerismoRESUMO
A simple one-pot method to prepare dioxabicyclo[2.2.1] heptane derivatives, from readily available 1,2,4-trioxane frameworks, under catalytic hydrogenation conditions over a platinum surface is reported. The overall transformation involves the hydrogenation of the double bond and a ring contraction rearrangement that presumably proceeds via a hydrogenolytic cleavage of the O-O bond and subsequent intramolecular ketalization. The strategy was successfully applied to the synthesis of a Riesling acetal.