Organocatalytic Enantioselective Aziridination of α,β-Unsaturated Aldehydes

Organocatalytic asymmetric remote aziridination of 2,4-dienals

Highly regio- and stereoselective remote aziridination of 2,4-dienals has been developed, based on a vinylogous iminium-ion-dienamine catalytic cascade reaction. Transformations of the aziridine products into enantioenriched motifs are also demonstrated. Furthermore, the reaction concept is extended to include enantioselective 1,6-addition of thiols.

Enantioselective aziridination of cyclic enals facilitated by the fluorine-iminium ion gauche effect

The enantioselective, organocatalytic aziridination of small, medium and macro-cyclic enals is reported using (S)-2-(fluorodiphenyl methyl)-pyrrolidine. Central to the reaction design is the reversible formation of a β-fluoroiminium ion intermediate, which is pre-organised on account of the fluorine-iminium ion gauche effect. This conformational effect positions the fluorine substituent synclinal-endo to the electropositive nitrogen centre thus benefiting from favourable stereoelectronic and electrostatic interactions (σC-H →σC-F *; F(δ-…︁) N(+) ). Consequently, one of the shielding groups on the fluorine-bearing carbon atom is positioned above the π-system, forming the basis of an enantioinduction strategy. Treatment of this intermediate with a "nitrene" source furnished a series of novel, optically active aziridines (e.r. up to 99.5:0.5). Further derivatisation of the product aziridines gives facile access to various amino acid derivatives, including β-fluoroamino acids. Crystallographic analyses of both the aziridines and their derivatives are disclosed.

Bifunctional cinchona alkaloid-squaramide-catalyzed highly enantioselective aza-Michael addition of indolines to α,β-unsaturated ketones

An enantioselective aza-Michael addition of indolines to α,β-unsaturated ketones was achieved using a bifunctional cinchona alkaloid-derived chiral squaramide derivative. Various β-indolinyl ketone derivatives were obtained in good to excellent yields and with high enantioselectivity. DDQ or MnO2 oxidation of indoline derivatives provided convenient access to various enantioenriched N-substituted indole derivatives.

Catalytic asymmetric diaziridination

The first catalytic enantio- and diastereoselective synthesis of diaziridines is presented. Aziridination of N-tosyl aldimines applying modified hydroxylamine under asymmetric phase-transfer catalysis furnished optically active diaziridines. The diaziridines are formed as a single diastereomer in up to 96% ee, containing two orthogonal N-protecting groups, which can be deprotected selectively.

Synthesis and ring openings of cinnamate-derived N-unfunctionalised aziridines

tert-Butyl cinnamates are aziridinated with high trans-selectivity by an N–N ylide generated in situ from N-methylmorpholine and O-diphenylphosphinyl hydroxylamine. The resulting N-unfunctionalised aziridines are shown to be versatile synthetic building blocks that undergo highly selective ring-opening reactions with a wide range of nucleophiles.

Organocatalytic cascade aza-Michael/hemiacetal reaction between disubstituted hydrazines and α,β-unsaturated aldehydes: Highly diastereo- and enantioselective synthesis of pyrazolidine derivatives

The catalytic synthesis of nitrogen-containing heterocycles is of great importance to medicinal and synthetic chemists, and also a challenge for modern chemical methodology. In this paper, we report the synthesis of pyrazolidine derivatives through a domino aza-Michael/hemiacetal sequence with chiral or achiral secondary amines as organocatalysts. Thus, a series of achiral pyrazolidine derivatives were obtained with good yields (up to 90%) and high diastereoselectivities (>20:1) with pyrrolidine as an organocatalyst, and enantioenriched pyrazolidines are also achieved with good results (up to 86% yield, >10/1 regioselectivity, >20:1 dr, 99% ee) in the presence of (S)-diphenylprolinol trimethylsilyl ether catalyst.

Noncovalent organocatalytic synthesis of enantioenriched terminal aziridines with a quaternary stereogenic center

A high-yielding and enantioselective access to novel N-Boc terminal aziridines, bearing a quaternary stereogenic center, has been developed via an aza-Michael initiated ring-closure (aza-MIRC) reaction of α-acyl acrylates with an N-tosyloxy tert-butyl carbamate catalyzed by a chiral amino thiourea. The feasibility of the aziridine regioselective ring-opening to valuable α,α-disubstituted α-amino acid esters has been demonstrated.

The diarylprolinol silyl ether system: a general organocatalyst

The past few decades have witnessed some of the most important and revolutionizing advances in the field of asymmetric catalysis. Chemists no longer rely solely on natural sources as the starting point of their synthetic strategy, as in chiral pool or auxiliary-based synthesis. Instead, naturally occurring chiral motifs are selected and, either unchanged or after modification, used in substoichiometric amounts as chiral catalysts or ligands. In this way, they effectively transfer their chirality to prochiral substrates, thereby rapidly amplifying and diversifying the arsenal of useful chiral building blocks available to the synthetic community. A long-standing goal in the pursuit of new catalytic systems is the discovery of general catalysts. Ideally, such catalytic systems should be capable of promoting a large number of enantioselective reactions, via multiple modes of activation, with good substrate tolerance and high stereoselectivity. In this Account, we describe the synthetic usefulness, efficiency, selectivity, and robustness of the diarylprolinol silyl ether system as the catalyst in various reactions of aldehydes. Based on the diarylprolinol silyl ether system, several studies on enamine-mediated transformations of saturated aldehydes have resulted in the introduction of different functionalities into the α-position of aldehydes in a highly stereoselective manner. This HOMO-activation concept was later extended to include α,β-unsaturated aldehydes, which after condensation with the aminocatalyst generate a dienamine species capable of undergoing stereoselective Diels-Alder-type reactions. As a result, the effective functionalization of the γ-position of the aldehyde is achieved. Recently, the activation principle was further developed to include 2,4-dienals, which form trienamine intermediates upon condensation with the aminocatalyst. The trienamines effectively react with carbon-centered dienophiles, forming aldehyde products having up to four contiguous stereocenters. Because of the concerted nature of the reaction and the efficient catalyst shielding of the β-position, the stereoinduction is achieved at the remote ε-position of the original aldehyde. Complementary to the enamine-mediated activations, α,β-unsaturated aldehydes can also be efficiently functionalized by applying the diarylprolinol silyl ether system via conjugate addition through iminium-ion-mediated processes, that is, LUMO-activation. In such reactions, the aminocatalyst not only effectively shields one of the enantiotopic faces of the enal, it also ensures excellent chemoselectivity, affording 1,4-adducts as the only products. Several different carbon and heteroatom nucleophiles can be added in a highly stereoselective fashion. The ability of the catalysts to participate in various enamine- and iminium-ion-mediated processes also makes them ideal for the sequential addition of nucleophiles and electrophiles in a cascade manner. These cascade reactions thereby afford access to products having at least two stereocenters. In the years to come, the diarylprolinol silyl ether catalysts will probably maintain their prominent position as general catalysts in the field of aminocatalysis. Moreover, recent efforts devoted to mechanistic studies might soon engender further advances with this versatile catalytic system, particularly in the areas of activation modes, catalyst loadings, and industrial applications.

Selectivity in the addition reactions of organometallic reagents to aziridine-2-carboxaldehydes: the effects of protecting groups and substitution patterns

Good to excellent stereoselectivity has been found in the addition reactions of Grignard and organozinc reagents to N-protected aziridine-2-carboxaldehydes. Specifically, high syn selectivity was obtained with benzyl-protected cis, tert-butyloxycarbonyl-protected trans, and tosyl-protected 2,3-disubstituted aziridine-2-carboxaldehydes. Furthermore, rate and selectivity effects of ring substituents, temperature, solvent, and Lewis acid and base modifiers were studied. The diastereomeric preference of addition is dominated by the substrate aziridines' substitution pattern and especially the electronic character and conformational preferences of the nitrogen protecting groups. To help rationalize the observed stereochemical outcomes, conformational and electronic structural analyses of a series of model systems representing the various substitution patterns have been explored by density functional calculations at the B3LYP/6-31G* level of theory with the SM8 solvation model to account for solvent effects.

Secondary and primary amine catalysts for iminium catalysis

Formation of iminium ions from the condensation of chiral secondary or primary amines with alpha,beta-unsaturated aldehydes or ketones can be used as an effective platform for the acceleration of a wide variety of catalytic asymmetric cycloaddition and conjugate addition reactions. The reversible formation of the active iminium ion species simulates the pi-electronics and equilibrium dynamics traditionally associated with Lewis acid activation of alpha,beta-unsaturated carbonyl compounds lowering the energy level of the LUMO associated with the pi-system and activating subsequent reaction. Importantly, these iminium ion catalysed processes offer the opportunity to conduct reactions in the presence of both moisture and air greatly adding to the practicality and general applicability of the chemistry described. Proposed catalytic cycles and transition state models for the induction of asymmetry provide reliable and robust predictive tools for the outcome of reactions and high functional group tolerance suggests this class of transformation will have broad application in the arena of synthetic organic chemistry as the area matures. This review describes the rapid expansion of iminium ion catalysis over recent years from its conceptual introduction to the development of a whole new arsenal of highly practical and effective methods with which to approach challenging and fundamental bond construction processes.