The thermal 1,3-dipolar cycloaddition of unactivated azomethine ylides derived from allylamine and aromatic or heteroaromatic aldehydes with maleimides and 1,1- and 1,2-bis(phenylsulfonyl)ethylene affords endo-2,5- trans cycloadducts in moderate to good yields. DFT calculations provide evidence that the diastereoselectivity observed depends on the isomerization between S- and W-ylides according to Curtin-Hammett's principle. DFT calculations also explain the different diastereomeric ratio observed for 2-pyridyl and 2-thienyl derivatives in which the isomerization is not possible due to the competitiveness between isomerization barrier and the rate-limiting step (ylide formation barrier). This methodology is applied to the diastereoselective synthesis of a tricyclic thrombin inhibitor.

1,3-Dipolar cycloadditions (1,3-DC) between imino esters (as precursors of N-metallated azomethine ylides) and π-deficient alkenes are promoted by cooperative asymmetric Lewis acid/Brønsted base catalysis. The components of these catalytic pairs are silver salts derived from enantiopure commercially available BINOL-based phosphoric acids and Cinchona alkaloids. Chiral phosphoric silver(I) salts promote HOMO raising of in situ formed 1,3-dipoles, whereas protonated cinchona alkaloids generate a LUMO lowering for the dipolarophiles resulting in a global acceleration of the 1,3-DC. The best results were obtained with BINOL-derived silver phosphate and hydrocinchonine. Matching between both cooperative metallo- and organocatalyst results in an enhanced enantiomeric excess, superior to that reached by both separate components. NOESY experiments and DFT calculations are compatible with a non-covalent interaction (hydrogen bond) between both catalysts, which results in close contacts and mutually coupled chiral environments.

Diastereoselective multicomponent reactions of enantioenriched 4-nitroprolinates, obtained by enantiocatalyzed 1,3-dipolar cycloaddition (1,3-DC) of imino esters and nitroalkenes, with α,β-unsaturated aldehydes and electrophilic alkenes proceed with total periselectivity depending on the structure of the aldehyde employed. This process evolves through a [3 + 2] 1,3-DC when cinnamaldehyde is used in the presence of an azomethine ylide, giving the corresponding highly substituted pyrrolizidines with endo selectivity. However, in the case of the α,β-unsaturated aldehyde, which contains a hydrogen atom at the γ position, an amine-aldehyde-dienophile (AAD) [4 + 2] cycloaddition takes place by formation of an intermediate 1-amino-1,3-diene, affording highly functionalized cyclohexenes with high endo diastereoselectivity. This AAD process only occurred when a nitro group is bonded to the 4-position of the initial enantiomerically enriched pyrrolidine ring. DFT calculations have been carried out with the aim of explaining this different behavior between pyrrolidines with or without a nitro group, demonstrating the strong nitro-group-dependent periselectivity. The results of these computational studies also support the experimentally obtained absolute configuration of the final adducts.

No abstract

The enantioselective formation of nitroprolinates followed by diastereoselective Michael‐type addition onto a second unit of the nitro alkene is studied. The reaction occurred in a one pot‐sequential process controlled by the chiral phosphoramidite⋅silver benzoate complex. The origin of the high diastereoselectivity is studied by DFT computational analysis where the crucial effect of the benzoic acid is justified. The employment of this strategy to the preparation of pyrazine‐2‐ones is also surveyed, as well as the preparation of diketopiperazines from enantiomerically enriched exo‐prolinates using conventional N‐acylation‐amination‐cyclization steps.