Cycloadditions chiral

Keywords carbonyl compounds, chiral dienophiles, chiral dienes, chiral catalysts, intramolecular cycloadditions, chiral Lewis acids... 

Togni A., Pastor S. D. Cooperativity of Chirality in Homogeneous Catalysis The Gold(I)-Catalyzed Aldol Reaction and the Vanadium(IV)-Catalyzed Hetero-Diels-Alder Cycloaddition Chirality 1991 3 331-340... 

In the area of [3 + 2]-cycloadditions (1,3-dipolar cycloadditions), chiral silver catalysts have been utilized extensively for the enantioselective formation of five-membered rings from prochiral substrates. For example, Zhang and co-workers360 have reported the highly enantioselective Ag(i)-catalyzed [3 + 2]-cycloaddition of azomethine ylides to electron-deficient alkenes. Thus, reaction of ct-imino esters 442 with dimethyl maleate in the presence of catalytic amounts of silver(i) acetate and the chiral bisferrocenyl amide phosphine 443 provided the chiral pyrrolidines 444 with high stereoselectivities and chemical yields (Scheme 131). Only the endo-products were isolated in all cases. 

Vasella, A, Stereoselektivitat und Reaktivitat bei der 1,3-dipolaren Cycloaddition chiraler N-(Alkoxyalkyl)nitrone, Helv. Chim. Acta, 60, 1273-1294, 1977. 

Asymmetric dipolar cycloadditions. Chiral pyrazolines are accessible through 1,3-dipolar cycloaddition of trimethylsilyidiazomethane to (V-(2-alkenoyl)borane-10-2-sultams. 

Dipoles are the three-atom components of [3+2]-cydoadditions. These are 4n electrons species, so [3+2]-cycloadditions are thermally allowed according to the rules of orbital symmetry [624g], The interactions between the frontier orbitals of the two partners are of prime importance, and concerted reactions take place in a supra-suprafacial fashion. When the 1,3-dipoles are not symmetrical, their reactions with unsymmetrical dipolarophiles may give regioisomers. Lack of regioselectivity limits some types of cycloadditions. Chirality can be introduced other on the dipole or on the dipolarophile. 

Intramolecular versions of the ene reaction using Lewis acids can be applied to nonconjugated dienes. Thus, 2.8-dienoic acid derivatives lead to cyclohexane systems27. Side products in this conversion stem from intramolecular hetero-Diels-Alder cycloaddition. Chiral Lewis acids, such as titanium alkoxidcs modified with tartaric acid derived chiral diols, lead to asymmetric induction with up to 98% ee27,88. 

Induced Diastereoselection in Nitrone Cycloadditions Chiral Nitrones... 

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