Other Diels-Alder Reactions

Tridentate bis(oxazolinyl)pyridinyl rhodium and ruthenium pincer complexes are useful as catalysts for hydrosilylations and cyclopropanations. These NNN-type inorganic pincer complexes are not as stable, however, as phosphine or salen-type pincer complexes. On the other hand, an organometallic tridentate bis(oxazolinyl) phenyl NCN-type complex is stable. These optically active NCN-type pincer complexes act as efficient catalysts for enantioselective hetero Diels-Alder reactions of Danishefsky s diene with glyoxylates [26]. 

Asymmetric hetero Diels-Alder reactions of l-methoxy-3-[(t-butyldimethylsilyl) oxy]-1,3-butadiene (Danishefsky s diene) with n-butyl glyoxylate are catalyzed highly enantioselectively with cA(endo)-diastereoselectivity, for example, by chiral bis(oxazolinyl)phenylrhodium aqua dichloride 8.43, as shown in Eq. (8.8) [27]. 

Other metal-catalyzed reactions with five-membered pincers include asymmetric aldol-type condensations [26, 28-30], cyclopropanations [31], enantioselective allylations [32], reductive eUminations [33], transfer hydrogenations [28-30, 34], hydroaminations [28], and polymerizations [26, 28, 35]. 

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These results can be extended to other Diels-Alder reactions. In view of the stmctures of most dienes and dienophiles a spatial separation of these compounds upon binding to micelles can be expected for the majority of Diels-Alder reactions. This arrangement most likely explains the unexpectedly small influence of micelles on the rates of Diels-Alder reactions as reported in the literature. 

Some other Diels Alder reactions have been investigated in subcritical water [79] and some of them are illustrated in Scheme 6.33. The cycloadditions are fast and occur with good yields. In the absence of solvent, the reagents tend... 

A synthesis of the antitumor agent elliptidne has utilized the indolyl-substituted oxazole (351) as a key intermediate (77JOC2039). Diels-Alder reaction of (351) with acrylonitrile in acetic acid afforded a pyridinecarbonitrile (352) which was reacted with methyllithium, and the ketimine salt was hydrolyzed and cyclized to ellipticine (353 Scheme 76). Other Diels-Alder reactions of this type, particularly intramolecular cycloadditions of oxazoles with alkenic dienophiles should provide rapid access to a variety of alkaloid systems. 

Recently, the reaction of masked ortho-benzoquinone [92] with C60 was tested [93]. The [4+2] cycloaddition reaction of such electron-deficient dienes with fullerenes resulted in the formation of highly functionalized bicyclo [2.2.2] octenone-fused fullerenes. The reactants were generated in situ by the oxidation of the readily available 2-methoxy phenols with hypervalent iodine agents. For the several different masked ortho-benzoquinones that were tested, it was found that the yield of the cycloadducts depends on the nature of the starting materials and the reaction conditions. Other Diels-Alder reactions of such electron-deficient dienes with electron-poor fullerenes involved tropones [94], 1,3-butadienes substituted with electron-withdrawing groups [95], and 2-pyrone [96]. 

Like other Diels-Alder reactions, reactions involving 2-trimethylsilyloxy-buta-1,3-dienes proceed under good stereochemical control. In the examples depicted in Equation Si3.12 and Si3.13, the dienophiles react with the dienes through conventional endo transition states to give exclusively endo adducts. 

Nakayama and co-workers established that carbonyl-stabilized sulfur ylide (83) reacted with elemental selenium to afford 1,3-oxaselenole (85) in good yield (85TL2201). The reaction was presumed to proceed through the intermediacy of selenal (84). Subsequently, support for this mechanism has come from the isolation of adduct 86 when the reaction is performed in the presence of dimethylbutadiene (87TL4423). Other Diels-Alder reactions of intermediate a-oxoseleno-aldehydes and -ketones were also found... 

This ratio is comparable to the endo ratio of other Diels-Alder reactions in aqueous media. 

Other Diels-Alder reactions reported in Organic Syntheses include... 

In 2000, Nakano et al. developed N-P type ligand, chiral phosphinooxazoline (POZ) (59) and found that the Pd complexes (61a-c) of ligand (59) works as an effective catalyst of asymmetric ally lie alkylation (Schemes 16.17 and 16.18) [19]. Recently, they have reported that cationic Pd- and Pt-POZ (59) catalysts are effective in Diels-Alder reactions on various substrates. The cationic Pd-POZ (59) complex (61c) provided particularly excellent enantioselectivity up to 98% ee in the Diels-Alder reactions of cyclopentadiene with acryloyl (15a), crotonoyl (15b), and fumaroyl-oxazolidinones (15c) (Scheme 16.18). The Pd and Pt complexes with POZ ligands that they explored have a characteristic structure therefore, they should prove useful not only for other Diels-Alder reactions but also for other asymmetric processes. 

Although commercially available, 1-trimethylsilyloxy-1,3-butadiene (1) can be easily prepared by silylation of crotonaldehyde. It has often been used as a reactive diene in Diels-Alder reactions. For example, reaction with dimethyl acetylenedicarboxylate (2) affords the cyclohexadiene diester (3) in 68% yield. This initial Diels-Alder adduct can be converted into two different aromatic products by the proper choice of conditions namely, thermal elimination affords the phthalate (4), while oxidation produces the phenol (5), hoth in good yield (eq 1). Reaction with methyl 3-nitroacrylate (6) followed hy hydrolysis of the initial adduct (7) and elimination of the -nitro group leads to the cyclohexadienol (8) (eq 2). Many other Diels-Alder reactions of this type have been carried out using (1), as shown in Table 1. In general, the endo adduct is favored, especially at lower temperatures. 

The reaction of the azirine 75 with furan proved exceptional in that it was slow at room temperature and it eventually gave only the exo cycloadduct 82 (Scheme 6.33). This is consistent with other Diels-Alder reactions of furan from which the thermodynamic product, the exo adduct, is isolated because the cycloaddition is reversible. 

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