Ana J. Royo

SUPPORTED CHIRAL AMINO ALCOHOLS AND DIOLS FUNCTIONALIZED WITH ALUMINIUM AND TITANIUM AS CATALYSTS OF DIELS-ALDER REACTION

Several chiral amino alcohols are supported on chloromethylated polystyrene-divinylbenzene and treated with AlEtCl2. The supported catalysts are far more active than the corresponding homogeneous which is explained by site isolation. The catalyst derived from (S)-prolinol leads to a 14% e.e. Tartaric acid supported on polystyrene-divinylbenzene by means of two ester linkages is treated with aluminium and titanium derivatives. The aluminium catalyst yields a 13% e.e. and the titanium one can be recovered and reused. The different behaviour of homogeneous and heterogeneous systems is discussed.

Silica and alumina modified by Lewis acids as catalysts in Diels-Alder reactions of carbonyl-containing dienophiles

Abstract Several aluminium and titanium derivatives are supported on silica and alumina. Some of the solids obtained contain strong Lewis acids sites and are efficient catalysts in Diels-Alder reactions of carbonyl-containing dienophiles. In cyclopentadiene reactions, high conversions and endo/exo selectivities are obtained in short reaction times, without excess of diene and with a small amount of catalyst at room temperature. With less reactive dienes, temperature, diene proportion and reaction times must be increased. However, high regio and endo/exo selectivities are obtained. The catalyst obtained by treatment of silica with diethyl aluminium chloride retains a great deal of its catalytic activity when recovered or stored in the open air for one month.

The use of heterogeneous catalysis in Diels-Alder reactions of N-acetyl-α,β-dehydroalaninates

Abstract Several solids have been tested as heterogeneous catalysts in the Diels-Alder reactions of the relatively unreactive dienophiles methyl ( 1a ) and (−)-menthyl ( 1b ) α,β-dehydroalaninates with cyclopentadiene, which is the key step in the asymmetric synthesis of cycloaliphatic α-amino acids. Reactions were carried out in toluene and in the absence of a solvent, the latter method leading to higher percentages of conversion. Silica gels modified with Lewis acids are the best catalysts for the reaction of the non-chiral dienophile 1a , but they are comletely inefficient in the reaction of the chiral dienophile 1b , probably due to diffusional limitations. In the case of the chiral dienophile 1b , silica gel is the best catalyst. Both exo/endo and diastereofacial selectivities are very similar to those obtained using homogeneous catalysts. The complete diastereofacial selectivity obtained in exo cycloadducts constitutes the best asymmetric induction described to date in asymmetric Diels-Alder reactions with heterogeneous catalysts.

Silica and alumina modified by Lewis acids as catalysts in Diels-Alder reactions of chiral acrylates

Abstract Silica and alumina, modified by treatment with AlClEt 2 and TiCl 4 , are tested as catalysts in the reactions of cyclopentadiene ( 1 ) with (-)-menthyl acrylate ( 2a ), (R)-O-acryloyl pantolactone ( 2b ) and (S)-O-acryloyl lactate ( 2c ). Silica-modified systems are the most efficient heterogeneous catalysts described to date for this kind of reaction, leading to selectivities similar to those obtained with aluminium homogeneous catalysts. Whereas silica· AlClEt 2 is better for the reaction of 2a and 2c , silica· TiCl 4 is more suitable for the reaction of 2b .

TADDOL-TiCl2 catalyzed Diels-Alder reactions: unexpected influence of the substituents in the 2-position of the dioxolane ring on the stereoselectivity

Abstract Several α,α,α′,α′-tetra(3,5-dimethylphenyl)-1,3-dioxolane-4,5-dimethanols have been synthesized, and their TiCl 2 derivatives used as catalysts in the reaction of cyclopentadiene with ( E )-2-butenoyl-1,3-oxazolidin-2-one. The substituents in the 2-position of the dioxolane ring exert a decisive influence on the extent and direction of the asymmetric induction and, consequently any possible enantiomer of the cycloadducts can be preferentially obtained by changing these substituents. Molecular mechanics calculations of the structures and relative energies of the possible dienophile-catalyst complexes offer a tentative explanation for the results described.

Chiral lewis acids supported on silica gel and alumina, and their use as catalysts in Diels-Alder reactions of methacrolein and bromoacrolein

Abstract Several derivatives of ( S )-tyrosine are supported on silica gel through the phenolic oxygen atom. The Lewis acids obtained by treatment of these solids with BH 3 are able to promote the reactions of methacrolein and bromoacrolein with cyclopentadiene, but they do not provide asymmetric induction. ( S )-Prolinol is supported on silica gel and alumina through the nitrogen atom, and the solids obtained by treatment with AlEtCl 2 catalyse the reaction between methacrolein and cyclopentadiene, leading to 8% enantiomeric excess (ee). The best results in these reactions (until 31% ee) are obtained by using (−)-menthol-aluminium Lewis acids supported on silica gel and alumina through the aluminium atom. The supported catalysts are compared with the analogous homogeneous Lewis acids.

Diels-Alder reactions of α-amino acid precursors by heterogeneous catalysis: Thermal vs. microwave activation

Abstract The Diels-Alder reactions of methyl and (−)-menthyl 2 acetamidoacrylates with cyclopentadiene, a key step in the synthesis of cyclic α-amino acids, are carried out in the presence of heterogeneous catalysts. Given the low reactivity of these dienophiles the best results are obtained when the reactions are carried out in the absence of a solvent. A close relationship between the relative catalytic activity of the different solids and the nature of the dienophile exists. The selectivities obtained are similar to those described for the reactions catalyzed by homogeneous Lewis acids, in particular a complete diastereofacial selectivity is obtained. Microwave activation increases the reaction rate without decreasing the selectivity. However, it is necessary to choose a suitable input power in order to avoid the decomposition of the 2-acetamidoacrylate.

Fluorinated alcohols as solvents for diels-alder reactions of chiral acrylates

Abstract Fluorinated alcohols increase the rates and endo/exo selectivities of the Diels-Alder reactions of cyclopentadiene with (−)-menthyl and (−)-8-phenylmenthyl acrylates. The use of fluorinated alcohols also increases the diastereofacial selectivity in the reactions of (−)-menthyl acrylate, but decreases it when the dienophile is (−)-8-phenylmenthyl acrylate. These facts are accounted for by the conformational preferences of the dienophile in non-HBD solvents and by the tendency of fluorinated alcohols to shift the conformational equilibrium towards the s-trans conformation.