Diels-Alder cycloaddition stereoselectivity

Keywords metal-catalyzed Diels-Alder cycloaddition, stereoselectivity... 

Anionic Diels-Alder reactions have been studied less extensively with the interest having been focused mainly on the cycloaddition of enolates of a,/l-unsaturated ketones with electron-poor olefins [24] (Equations 1.8 and 1.9). These reactions are fast and stereoselective and can be regarded as a sequential double Michael condensation, but a mechanism involving a Diels-Alder cycloaddition seems to be preferred [24b,f, 25]. 

Enhanced stereoselectivity in aqueous intramolecular hetero-Diels-Alder cycloaddition of chiral acyinitroso compounds [17c, d, 99]... 

Keywords regio- and stereoselective silicon-tethered Diels-Alder cycloadditions, synthesis of branched sugars and linear and polycyclic hydrocarbons... 

Lopez J. C., Lukacs G. Pyranose-Derived Dienes and Conjugated Enals. Preparation and Diels-Alder Cycloaddition Reactions ACS Symp. Ser. 1992 494 33-49 Keywords carbohydrate, befera-Diels-Alder reactions, stereoselectivity... 

Although the use of rihozymes and DNA-zymes is hmited to the field of nucleic acid transformations, with small exceptions only that concern their use as catalysts in the Diels-Alder cycloaddition, a strong interest to use them in the stereoselective synthesis of heteroatom compounds can be expected. 

A similar transformation occurs as a critical step in the total synthesis of (+)-estrone by a Diels-Alder cycloaddition-cycloreversion pathway (Eq. 80).227 It is worth noting that in this reaction the conjugated double bond is stereoselectively reduced while both an isolated double bond and a ketone carbonyl are preserved. 

Collins and co-workers have also reported on an enantioselective catalytic Diels—Alder cycloaddition, in which zirconocene and titanocene bis(triflate) complexes were used as catalysts [104], The influence of the solvent polarity on the observed levels of stereoselectivity is noteworthy. For example, as shown in Scheme 6.34, with 108 as the catalyst, whereas in CH2C12 (1 mol% catalyst) the endo product was formed with 30% ee (30 1 endoxxo, 88% yield), in CH3N02 solution (5 mol% catalyst) the enantioselectivity was increased to 89% (7 1 endoxxo, 85% yield). Extensive 1H and 19F NMR studies further indicated that a mixture of metallocene—dienophile complexes was present in both solutions (-6 1 in CH2C12 and -2 1 in CH3N02, as shown in Scheme 6.34), and that most probably it was the minor complex isomer that was more reactive and led to the observed major enantiomer. For example, whereas nOe experiments led to ca. 5 % enhancement of the CpH proton signals of the same ring when Hb in the minor complex was irradiated, no enhancements were observed upon irradiation of Ha in the major complex. 

Another example, in which the piperidine cycle is generated de novo, exploits a hetero Diels-Alder cycloaddition of 1 -/r-tolylsulfinyl-1,3-penta-diene 91 with benzylnitrosoformate, that generates an oxazine 92 with complete regioselectivity and 7i-facial diastereoselectivity.69 Osmilation of the double bond inserts stereoselectively two hydroxyl groups on the oxazine skeleton, protection and catalytic hydrogenation finally afforded the enantiomerically pure imino sugars 94 (Fig. 38). 

Scheme 11 Regio- and stereoselectivity issues of the model hetero-Diels-Alder cycloaddition...

Table 9-5 Regio and Stereoselectivity in Diels-Alder Cycloadditions of Substituted Cyclopentadienes with Acrylonitrile ... 

A 1-thiabuta-1,3-diene (104) undergoes highly stereoselective hetero-Diels-Alder cycloadditions with chiral A -acryloyloxazoIidinones.180... 

An AMI semiempirical method was used to investigate the Diels-Alder cycloaddition reactions of vinyl sulfenes with buta-1,3-dienes.156 The reactivity and stereoselectivity of vinyl boranes have been reviewed.157 Aromatic methyleneamines undergo reverse-electron-demand Diels-Alder reactions with cyclopentadiene, norbom-ene, and vinyl sulfides.158... 

DFT calculations on the intramolecular Diels-Alder reaction of penta-l,3-dienyl acrylates predict stereoselectivities that are in good agreement with the experimen- (g) tal results.85 Another DFT study at the B3LYP/6-31G(d) level of the intramolecular Diels-Alder cycloaddition of 5-vinylcyclohexa-1,3-dienes has been reported. Reaction rates are influenced by dienophile twisting and substituent effects.86 The intramolecular dehydro-Diels-Alder reactions of ynamides (79) provides a new synthesis of benzo[fc]-, tetrahydrobenzo[fc]-, naphtho[l,2-/j -, and dibenzo[a,c]carbazoles... 

Another application of camphor-based chiral auxiliaries is the use in stereoselective Diels-Alder cycloadditions (Chapter 26), especially for the construction of quaternary carbon centers. One example, shown in Scheme 5.12, uses a camphor-derived lactam 30 as the auxiliary.54... 

Unsaturated sugar derivatives can undergo a wide range of transformations, in particular, when an enone is involved.31 Examples include cyclopropanations, Diels-Alder cycloadditions and photohydroxymethyl-ation. Many of these reactions proceed with high regio- and stereoselectivity (Scheme 3.6c). 

Another special example in the use of enaminones of the aminomethylene type as dienes in the Diels-Alder cycloaddition is the stereoselective reaction with several dienophiles to yield substituted cyclohexenes279 (equation 204). 

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