Magnesium complex Diels—Alder reactions

Fujisawa et al. [Ill] have reported that the magnesiiun complex prepared from chiral 2-[2-[(tolylsulfonyl)amino]phenyl]-4-phenyl-l,3-oxazoline 81 and methyl-magnesium iodide was efficient, in a stoechiometric amount, for promoting the enantioselective Diels-Alder reaction of 3-alkenoyl-l,3-oxazohdin-2-one with cyclopentadiene (Scheme 45) leading exclusively to the endo adducts in up to 92% ee. The use of 10 mol% of the complex led to an important decrease in enantioselectivity of the product (51% ee). 

The Diels-Alder reaction has been performed with a range of Lewis acids. Copper complexes are the most successfully used, but other metals such as iron, magnesium, palladium, nickel or ytterbium have proved to be efficient to catalyse this reaction. 

Perhaps the most attractive method of introducing enantioselectivity into the Diels-Alder reaction is to use a chiral catalyst in the form of a Lewis acidic metal complex. In recent years, this area has shown the greatest progress, with the introduction of many excellent catalytic processes. Quite a number of ligand-metal combinations have been evaluated for their potential as chiral catalysts in Diels-Alder reactions. The most commonly used metals are boron, titanium, and aluminum. Copper, magnesium, and lanthanides have also been used in asymmetric catalytic Diels-Alder reactions. 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

The Diels-Alder reaction of ethyl 2-benzoylacrylate (450) with cyclopentadiene was effectively catalyzed by magnesium(II) complexes of bis(oxazolidine) 448 and oxazolidine 449 (equation 134). When the catalysts were prepared in refluxing acetonitrile, adduct 451 was obtained with virtually complete endo selectivity for the ethoxycarbonyl group and up to 87% ee282. 

In 1993, Evans and co-workers examined phe-box 6, /-pr-box 45, and bu-box 3 ligands in the Diels-Alder reaction of cyclopentadiene 68 and 3-acryloyl-l,3-oxazolidin-2-one 69 using a weak Lewis acid such as copper(II) triflate." The results are summarized in Table 9.9. The reaction was carried out between —50 and —78 °C for 3-18 h and achieved selectivities of up to 98 2 (endo/exo) with an endo ee of >98% (using bu-box 3). Interestingly, the enantiomer produced in these reactions was the (25) configuration, compared to the (2K) isomer obtained with iron(III) and magnesium(II) as reported by Corey. This observed stereochemistry was explained by the chelation model of the copper(II) complex 74 (Fig. 9.23)... 

The Diels-Alder reaction between cyclopentadiene and acryloyloxazolidinone is enantioselectively catalysed by the bis(oxazoline)-magnesium perchlorate complex.214... 

Metal-bis(oxazoline) complexes were widely used as effective catalysts for enantioselective Diels-Alder reactions. Two research groups could achieve excellent diastereo- and enantio-selectivity for the reaction of cyclopentadiene (54) and the acrylamide 55 (Fig. 9) [27]. Yet the decisive feature is only recognizable when both studies are analyzed together. In both cases the endo products are obtained in high selectiv-ities using either the magnesium- or the copper-containing catalyst. However, despite the same... 

The magnesium and copper bis-oxazoline catalysts used in the asymmetric Diels-Alder reaction (see Section 8.1) display high levels of selectivity in the nitrone cyclo addition with bidentate, electron-deficient dipolarophiles. The magnesium bis-oxazoline catalysts effect moderate to good ee in the cycloaddition with oxazolidines such as (8.51), while copper bis-oxazoline complexes show good selectivity in the addition to bidentate pyrazolidinones rather than oxazo-lidinones and also a-hydroxyenones such as (8.75) used as substrates in the copper bis-oxazoline-catalysed Diels-Alder reaction. ... 

The asymmetric magnesium-catalysed hetero-Diels-Alder reaction of Danishefsl s diene 6 with aldehydes 7 has been reported by Ding et al. A chiral Mg(ii)-binaphthonate complex was selected as the most efficient catalyst to give a variety of 2-substituted 2,3-dihydro-4iL-pyran-4-ones 8 in high yields and with excellent enantiomeric excesses (Scheme 3.2). ... 

Diels-Alder reaction between N-acryloyloxazoIidinone and cyclopentadiene for the case of the magnesium-based bis(oxazoline) complexes [65]. Poor enantiomeric excesses and conversion were observed for the analogous copper-based systems. 

This particular reaction has been reported recently by Lukacs in the context of complex molecules synthesis by Diels-Alder cycloaddition [19]. A typical example is given in the experimental section. Note that the reaction of 5 with vinyl magnesium bromide gave a mixture of 4 and 6 because of epimerization of the axial methyl group at C-2 owing to the basicity of the reagents [20]. 

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