Theoretical insight into the regioselective ring-expansions of bicyclic aziridinium ions

Synthetic Applications of Aziridinium Ions

Nonactivated aziridine with an electron-donating group at the ring nitrogen should be activated to an aziridinium ion prior to being converted to cyclic and acyclic nitrogen-containing molecules. This review describes ways to generate aziridinium ions and their utilization for synthetic purposes. Specifically, the intra- and intermolecular formation of aziridinium ions with proper electrophiles are classified, and their regio- and stereoselective transformations with nucleophiles are described on the basis of recent developments.

Lewis acid-mediated synthesis of mono- and tris-indole adducts from chiral aziridines

Lewis acid-mediated regio- and stereoselective nucleophilic addition of 2- or 3-substituted indoles to non-activated aziridine-2-carboxaldehydes in dioxane afforded 2-(indol-3-ylhydroxymethyl)aziridines whose ring opening with various nucleophiles rendered multi-substituted tryptamine derivatives. The reaction of the same aziridine-2-carboxaldehyde with three moles of indole in dichloromethane yielded tris-indole adducts β-(3,3'-bisindolyl)methyl (BIM) tryptamines from sequential steps including nucleophilic addition to aldehyde, Michael type Friedel-Crafts alkylation of the mono-adduct followed by regio- and stereoselective ring-opening of the aziridine ring.

N-(1-Phenylethyl)aziridine-2-carboxylate esters in the synthesis of biologically relevant compounds

Since Garner’s aldehyde has several drawbacks, first of all is prone to racemization, alternative three-carbon chirons would be of great value in enantioselective syntheses of natural compounds and/or drugs. This review article summarizes applications of N-(1-phenylethyl)aziridine-2-carboxylates, -carbaldehydes and -methanols in syntheses of approved drugs and potential medications as well as of natural products mostly alkaloids but also sphingoids and ceramides and their 1- and 3-deoxy analogues and several hydroxy amino acids and their precursors. Designed strategies provided new procedures to several drugs and alternative approaches to natural products and proved efficiency of a 2-substituted N-(1-phenylethyl)aziridine framework as chiron bearing a chiral auxiliary.

Alkylative Ring-Opening of Bicyclic Aziridinium Ion and Its Application for Alkaloid Synthesis

Alkylative ring-opening of bicyclic aziridinium ion generated from 4-hydroxybutylaziridine with organocopper reagent was achieved successfully to afford 2-alkylsubstituted piperidine in high or moderate yield. This method allowed carbon-carbon bond formation of "non-activated" aziridine via aziridinium ion ring-opening in regio- and stereo-selective manner for the first time. This newly developed reaction was applied for an efficient synthesis of alkaloid with the representative example of conine and epiquinamide.

Theoretical investigation of the regioselective ring opening of 2-methylaziridine. Lewis acid effect

The formation of substituted 1,2-diamines via the regiospecific nucleophilic ring opening of 2-methylaziridine with methylamine was performed by nucleophilic attack at aziridine carbon atoms. A detailed theoretical study was investigated by density functional theory (DFT) at the B3LYP level and second order Moller Plesset perturbation theory (MP2) by using the 6-311G(d,p) basis set. The third Grimme correction term (D3) was used to take into account weak interactions. Solvent effects were computed in methanol and dimethylsulfoxide using the polarizable continuum model (PCM). Emphasis was placed on the ring opening mechanisms of neutral aziridines and aziridinium ions obtained through N-complexation with the BF₃ Lewis acid. Moreover, the effect of substituent groups on the regioselectivity of the ring opening was investigated. The nucleophilic attack was carried out via two pathways (frontside attack M1 and backside attack M2) where activation barriers proved the preference for ring opening through the backside attack at the C3 aziridine carbon atom. The obtained results showed that the frontside attack with methylamine takes place along a concerted mechanism that leads to formation of products through one transition state. However, the backside attack is carried via a stepwise process in which the methylamine attack takes place in an S_N2 fashion where the leaving group is the ring nitrogen. It first conduces a ring opening considered as the rate-determining step followed by formation of a zwitterionic intermediate. This latter undergoes a rotation to allow the proton transfer step and finally leads to formation of the thermodynamic products.