Lewis add catalyzed Diels-Alder reactions

For recent examples of asymmetric Lewis add-catalyzed Diels—Alder reactions of ketone dienophiles, see (a) D.H. Ryu, T.W. Lee, E.J. Corey,... 

Hubbard and Miller87 used a Lewis acid catalyzed Diels-Alder reaction between y.y-disubstituted o. /i-unsaluralcd esters and cyclopentadiene in their approach toward oligomeric cyclopentanoids. In order for the reaction to proceed, they needed to add trimethylaluminum as a desiccant prior to addition of the Lewis acid catalyst aluminum trichloride. The endo/exo selectivity of the reaction with 97, depicted in equation 29, increased from 98/99 = 75/25 to 88/12 when the reaction temperature was dropped from room temperature to —20 °C. 

The carbo-Diels-Alder reaction of acrolein with butadiene (Scheme 8.1) has been the standard reaction studied by theoretical calculations in order to investigate the influence of Lewis acids on the reaction course and several papers deal with this reaction. As an extension of an ab-initio study of the carbo-Diels-Alder reaction of butadiene with acrolein [5], Houk et al. investigated the transition-state structures and the origins of selectivity of Lewis acid-catalyzed carbo-Diels-Alder reactions [6]. Four different transition-state structures were considered (Fig. 8.4). Acrolein can add either endo (N) or exo (X), in either s-cis (C) or s-trans (T), and the Lewis acid coordinates to the carbonyl in the molecular plane, either syn or anti to the alkene. 

Wada E., Yasuoka H., Pei W., Chin U., Kanemasa S. Lewis Add-Catalyzed Stereoselective Hetero Diels-Alder Reactions of (E)-l-Phenylsulfonyl-3-Alken-2-Ones With Vinyl Ethers. Synthetically Equivalent to Stereoselective Michael Type... 

On the other hand, rare-earth trifluoromethanesulfonates (rare earth triflate, RE(OTf)3) have been found to work efficiently as Lewis acids even in aqueous media or in the presence of amines [4], A catalytic amount of RE(OTf)3 enables several synthetically useful reactions, for example aldol, Michael, allylation, Mannich, Diels-Alder reactions, etc., to proceed. It has also been demonstrated that a small amount of RE(OTf)3 is enough to complete the reactions and that RE(OTf)3 can easily be recovered from the reaction mixture and can be reused. A key to accomplishing the catalytic processes was assumed to be the equilibrium between Lewis acids and Lewis bases, for example water, carbonyl compounds, and amines, etc. A similar equilibrium was expected between Lewis adds and aromatic ketones, and, thus, RE(OTf)3-catalyzed Friedel-Crafts acylation was investigated [5]. 

Lewis adds are frequently employed in catalyzing the Diels-Alder reaction. Particularly fasdnating is the observation of the different regiochemistry arising from monodentate and bidentate chelation of 2-methoxy-5-methyl-l,4-benzoquinone with boron trifluoride and stannic chloride, respectively [177]. 

A complete coverage of stereoselective [4 + 2] cycloaddition reactions is given in Section D. 1.6,1.1. Besides the uncatalyzed Diels-Alder reaction, catalyzed [4 + 2] cycloaddition methods have been used, either with main group or transition metal derived Lewis adds ( hard acid catalysis"12 -14), or with low-valent transition metal complexes ( soft acid catalysis"12-14). In this section, only transition metal catalyzed versions are described. Numerous related surveys are available63-76. 

This stabilization of a transition state related to tihe uncatalyzed process can also occur in reactions catalyzed by transition metal complexes, such as those catalyzed by Lewis acids. For example, Diels-Alder reactions catalyzed by transition metal complexes sometimes ocau" by mechanisms related to the concerted [4+2] mechanism of tiie imcatalyzed process. In tiiis case, the catalyst changes the electronic properties of the substrate bound to the Lewis add in a fashion that reduces the barrier for tihe [4+2] cycloaddition. Figure 14.3 shows the transition state proposed for enantioselective Etiels-Alder reactions catalyzed by copper complexes. The transition state structure is proposed on tiie basis of the calculated structure of the Lewis add complex formed between tiie copper-bisoxazoline fragment and the acrylate. 

Aube and Golden (dJ) have demonstrated the utility of a combined intramolecular Diels-Alder/intramolecular Schmidt reaction to construct the azepinoindole intermediate 110 (Scheme 5). The Lewis add-catalyzed tandem intramolecular Diels-Alder/Schmidt reaction required azidodiene 109, which was prepared after five steps as a 85 15 mixture of diastereoisomers from diene 108 via a modified Julia olefination between aldehyde... 

Boron Lewis Add Catalyzed Enantioselective Diels-Alder Reaction... 

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