Other Iminium-Activated Cascade Reactions

SCHEME 1.85 Catalytic enantioselective hydride-transferring closure. 

Other iminium-activated cascade reactions, including [4-e3] cycloaddition, Robinson annulations, and reductive Michael cyclization, were also developed to furnish useful synthons [133]. 

---

Iminium-activated hetero-[3+3] reactions Other iminium-activated cascade reactions... 

Two C-C Bond-Forming Events In 2008, Frechet and coworkers described an impressive asymmetric cascade reaction promoted by soluble star polymers with core-confined catalytic entities [10]. The encapsulation of catalysts into soluble star polymers allowed the use of incompatible catalysts and prevented undesired interactions between these catalytic systems. The organocascade corresponded to a nucleophilic addition of Af-methylindole to a,p-unsaturated aldehydes followed by a Michael addition of the adduct to methylvinylketone (MVK) in the presence of H-bonding additive (Scheme 12.5). Each catalyst - imidazolidinone 8 for the nucleophilic addition and diphenylprolinol methyl ether 9 for the Michael addition - or their combination cannot mediate both reaction steps. In particular, p-toluenesulfonic acid (p-TSA) diminished the ability of the chiral pyrrolidine 9 to effect enamine activation. Therefore, p-TSA and 9 were encapsulated in the core of star polymers, which cannot penetrate each other. Imidazolidone 8 was added to the acid star polymer and diffused to the core to form the salt, which allowed the iminium activation and catalyzed the first step. The second step was catalyzed by the pyrrolidine star polymer in presence of the H-bonding additive 10, which... 

One main advantage of aminocatalysts is their ability to promote several activation modes (iminium-enamine) or their high compatibility with other systems allowing for their applications in cascades reactions. 

---