Asymmetric organocatalysts cycloaddition reactions

As discussed above, the vast synthetic potential of cinchona alkaloids and their derivatives in the asymmetric cycloadditions has been well demonstrated over the past few years. Cinchona-based organocatalysts possess diverse chiral skeletons and are multifunctional. There is no doubt that the further development of cinchona-based organocatalysts and their catalyzed cycloaddition reactions will continue to provide exciting results in the near future. 

A review of formal aza-Diels-Alder reactions of imines with e-rich dienes and enones, in the presence of Lewis acids/Brpnsted acids/organocatalysts, has been presented. Bifunctional A-acyl aminophosphine catalysts (75) are effective asymmetric organocatalysts in the hetero-4- -2-cycloaddition of a-substituted allenoates with tosylaldimines to produce optically active tetrahydropyridines. The Brpnsted acid-catalysed aza-Diels-Alder reactions of cyclopentadiene with iminoacetates possessing two chiral auxiliaries yielded 2-azabicyclo[2.2.1]hept-5-ene cycloadducts with high exo-selectivities. ... 

On the other hand, Hayashi et al. have reported the highly enantio-selective formal [3 + 3] cycloaddition of a,p-unsaturated aldehydes with ene-carbamates catalysed by diphenylprolinol silyl ether as an organocatalyst. This reaction consisted of four consecutive reactions including an asymmetric ene reaction, an isomerisation from an imine into an enecarbamate, a hydrolysis and a hemiacetal formation in one pot to afford synthetically important chiral piperidine derivatives with excellent enantioselectivities of up to 99% ee, good yields and moderate to good diastereoselectivities, as shown in Scheme 6.22. 

On the other hand, modified-imidazoHdinones endowed with imidazole (125) [149] or a fluorous tag in the side chain (126) [150] proved to be active in the Diels-Alder cycloaddition in the presence of 5 vol.% water (Figure 24.40). The highly efficient organocatalyst 127 promoted, in the presence ofp-(trifluoromethyl) benzoic acid, the asymmetric Diels-Alder reaction of cyclohexenones with aromatic nitroolefins in seawater and brine with excellent chemo-, regjo-, and stereoselectivities [151], It has been postulated that the cychzation was involved in a concerted addition process and that seawater and brine significantly promoted the... 

The greater part of this chapter is concerned with the Diels-Alder and hetero-Diels-Alder reaction. The asymmetric version of both of these reactions can be catalysed with metal-based Lewis acids and also organocatalysts. The catalytic asymmetric 1,3-dipolar cycloaddition of nitrones and azomethine ylides is also discussed. Again, most success in this area has been achieved using metal-based Lewis acids and the use of organocatalysts is begiiming to be explored. This chapter concludes with a brief account of recent research into the asymmetric [2+2]-cycloaddition, catalysed by enantiomerically pure Lewis acids and amine bases, and also the Pauson-Khand [2- -2- -l] cycloaddition mediated by titanium, rhodium and iridium complexes. 

High enantioselectivites in the aza-Michael reaction have been achieved using alternate organocatalysts, and the addition of benzotriazole to nitroolefins occurs with up to 94% ee using bifimctional catalysts such as (11.64). Guerin and Miller have developed an alternate approach to the enantioselective introduction of triazoles based on the asymmetric conjugate addition of azide followed by a 1,3-dipolar cycloaddition of the product with an alkyne. In this approach, the addition of hydrazoic acid to Michael acceptors such as (11.133) proceeds with good ee in the presence of the dipeptide (11.134). ... 

Abstract Cycloadditions have been for a long time one of the most useful reactions in organic synthesis. Recently, the ability to promote the reactions by organocatalysts further expands the realm of its synthetic apphcation. This review aims to highhght the recent advances in this area with particular emphasis on the asymmetric cycloaddition promoted by organocatalysts. 

Asymmetric 1,3-dipolar cycloaddition (1,3-DC) reaction is an important strategy for the construction of highly substituted chiral heterocycles [1-3], which are the core structural elements prevalent in many natural products, pharmaceuticals, and organocatalysts [4-6]. Considerable attention has been paid to azomethine ylide-involved 1,3-DC in last several years, and the substrate scope has largely been expanded [7, 8]. 

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