Aziridinium from N,N-Dibenzyl Serine Methyl Ester: Synthesis of Enantiomerically Pure β-Amino and α,β-Diamino Esters

2,3-Diaminopropanols Obtained from d-Serine as Intermediates in the Synthesis of Protected 2,3-l-Diaminopropanoic Acid (l-Dap) Methyl Esters

A synthetic strategy for the preparation of two orthogonally protected methyl esters of the non-proteinogenic amino acid 2,3-l-diaminopropanoic acid (l-Dap) was developed. In these structures, the base-labile protecting group 9-fluorenylmethyloxycarbonyl (Fmoc) was paired to the p-toluensulfonyl (tosyl, Ts) or acid-labile tert-butyloxycarbonyl (Boc) moieties. The synthetic approach to protected l-Dap methyl esters uses appropriately masked 2,3-diaminopropanols, which are obtained via reductive amination of an aldehyde prepared from the commercial amino acid N^α-Fmoc-O-tert-butyl-d-serine, used as the starting material. Reductive amination is carried out with primary amines and sulfonamides, and the process is assisted by the Lewis acid Ti(OⁱPr)₄. The required carboxyl group is installed by oxidizing the alcoholic function of 2,3-diaminopropanols bearing the tosyl or benzyl protecting group on the 3-NH₂ site. The procedure can easily be applied using the crude product obtained after each step, minimizing the need for chromatographic purifications. Chirality of the carbon atom of the starting d-serine template is preserved throughout all synthetic steps.

Enantio- and Diastereoselective Organocatalytic Conjugate Additions of Nitroalkanes to Enone Diesters

Enantio- and diastereoselective conjugate addition reactions between nitroethane or nitropropane and enone diesters are reported. A bifunctional triaryliminophosphorane catalyzed the addition reaction with consistently excellent stereoselectivities and yields across a wide range of substrates. By the use of the gem-diester functional handle present in the adducts, local desymmetrization via diastereotopic group discrimination was demonstrated, and a polyfunctionalized lactam with three contiguous stereocenters was synthesized.

Synthetic and theoretical investigation on the one-pot halogenation of β-amino alcohols and nucleophilic ring opening of aziridinium ions

Aziridinium ions are useful reactive intermediates for the synthesis of enantiomerically enriched building blocks. However, N,N-dialkyl aziridinium ions are relatively underutilized in the synthesis of optically active molecules as compared to other three-membered ring cogeners, aziridines and epoxides. The characterization of both optically active aziridinium ions and secondary β-halo amines as the precursor molecules of aziridinium ions has been scarcely reported and is often unclear. In this paper, we report for the first time the preparation and experimental and theoretical characterization of optically active aziridinium ions and secondary β-halo amines. Optically active secondary N,N-substituted β-halo amines were efficiently synthesized from N,N-substituted alaninol via formation and ring opening at the more hindered carbon of aziridinium ions by halides. Optically active β-halo amines and aziridinium ions were characterized by NMR and computational analyses. The structure of an optically active β-chloro amine was confirmed via X-ray crystallographic analysis. The aziridinium ions derived from N,N-dibenzyl alaniol remained stable only for several hours, which was long enough for analyses of NMR and optical activity. The stereospecific ring opening of aziridinium ions by halides was computationally studied using DFT and highly-accurate DLPNO-CCSD(T) methods. The highly regioselective and stereoselective ring opening of aziridinium ions was applied for efficient one-pot conversion of β-alaninols to enantiomerically enriched β-amino alcohols, β-amino nitriles, and vicinal diamine derivatives.

Nucleophilic radiofluorination at room temperature via aziridinium intermediates

2-Fluoroamines were synthesized at RT from alcohols with different sources of nucleophilic [¹⁸F]- or [¹⁹F]-fluoride.

Memory of chirality (MOC) concept in imino-aldol reaction: enantioselective synthesis of α,β-diamino esters and aziridines

A simple strategy for the synthesis of chiral α,β-diamino- and α-amino,β-hydroxy ester derivatives in high yields with moderate to high ee has been developed via asymmetric imino-aldol and aldol reactions, respectively, starting from protected aminoesters employing memory of chirality concept for chiral induction. This strategy has been extended for the enantioselective synthesis of aziridines (ee up to 92%). The absolute configuration of the imino-aldol adducts has been determined. The stereochemical outcome of the products has been explained by a suitable mechanism and supported by computational studies.

Synthesis of enantiopure substituted piperidines via an aziridinium ring expansion

Herein we report a novel methodology for the asymmetric synthesis of 3-substituted piperidines from readily available chiral building blocks. This method, which features a novel irreversible dihydropyrole-tetrahydropyridine ring expansion, allows the introduction of a large variety of substituents at the 3-position and permits substitution at the 2- and 6-position giving mono-, di-, or trisubstituted piperidines with high diastereocontrol.

Assessing the rates of ring-opening of aziridinium and azetidinium ions: a dramatic ring size effect

Rates for the ring-opening of aziridinium and azetidinium ions by DMAP were measured. The four-membered ring appears to be ca. 17,000 times less reactive compared to the three-membered ring but is still highly relevant from a synthetic viewpoint. The electrophilicity of these strained ammonium ions is measured for the first time.

Rearrangement of beta-amino alcohols and application to the synthesis of biologically active compounds

Beta-amino alcohols derived from natural amino acids have been used extensively as a powerful source of chirality. Transformation of the hydroxy group of these beta-amino alcohols into a good leaving group, by using trifluoroacetic anhydride, led to rearranged beta-amino alcohols in good yields and with high enantiomeric excesses. This rearrangement has allowed the transformation of substituted prolinols to substituted 3-hydroxypiperidines and linear beta-amino alcohols, issued from natural amino acids, to rearranged beta-amino alcohols.

Rearrangement of beta-amino alcohols via aziridiniums: a review

This tutorial review focuses on the rearrangement of beta-amino alcohols via aziridinium intermediates. It covers the literature from 1947 to January 2009 (55 references). The rearrangement of beta-amino alcohols can be performed by activation of the hydroxy group followed by the addition of nucleophiles (Nu). In most examples, an aziridinium intermediate is involved in the rearrangement. The ratio of amines resulting from the attack of nucleophiles at either the C-1 or C-2 position of the aziridinium intermediate, depends on the nature of the nucleophiles and the R(2) substituent. In some cases, solvent as well as temperature can influence the ratio of amines.

Highly enantioselective synthesis of linear beta-amino alcohols

Beta-amino alcohols derived from alpha-amino acids have been extensively used as a powerful source of chirality. Transforming the alcohol moiety into a good leaving group has allowed the rearrangement of these beta-amino alcohols and the introduction of a large number of nucleophiles through the anchimeric participation of the nitrogen atom. An overview on the recent progress realized on the rearrangement of these beta-amino alcohols in the presence of (CF(3)CO)(2)O and H(2)SO(4) is reported.

A new strategy for the synthesis of 1,4-benzodiazepine derivatives based on the tandem N-alkylation-ring opening-cyclization reactions of methyl 1-arylaziridine-2-carboxylates with N-[2-bromomethyl(phenyl)]trifluoroacetamides

A new method for the synthesis of novel 1,4-benzodiazepine derivatives has been established from a one-pot reaction of methyl 1-arylaziridine-2-carboxylates with N-[2-bromomethyl(aryl)]trifluoroacetamides. The reaction proceeds through the N-benzylation and highly regioselective ring-opening reaction of aziridine by bromide anion followed by Et3N-mediated intramolecular nucleophilic displacement of the bromide by the amide nitrogen. The easy availability of starting materials, simple and convenient synthetic procedure, and formation of functionalized 1,4-benzodiazepine scaffold ready for further chemical manipulations render this strategy useful in synthetic and medicinal chemistry.

Highly enantioselective synthesis of beta-amino alcohols: a catalytic version

Highly enantioselective rearrangement of beta-amino alcohols was realized by using a catalytic amount of trifluoroacetic anhydride.

Novel Synthesis of 3,4-Diaminobutanenitriles and 4-Amino-2-butenenitriles from 2-(Cyanomethyl)aziridines through Intermediate Aziridinium Salts: An Experimental and Theoretical Approach

1-Arylmethyl-2-(cyanomethyl)aziridines were transformed into 4-(N,N-bis(arylmethyl)amino)-3-(pyrrolidin-1-yl)butanenitriles and 4-(N,N-bis(arylmethyl)amino)-2-butenenitriles via 4-(N,N-bis(arylmethyl)amino)-3-bromobutanenitriles in high yields and purity. The key steps involve the unprecedented regiospecific ring opening of intermediate 2-(cyanomethyl)aziridinium salts by bromide and pyrrolidine in acetonitrile, exclusively at the substituted aziridine carbon atom. The results were rationalized on the basis of ab initio calculations.

Nitrile Biotransformations for the Efficient Synthesis of Highly Enantiopure 1-Arylaziridine-2-carboxylic Acid Derivatives and Their Stereoselective Ring-Opening Reactions

Catalyzed by the Rhodococcus erythropolis AJ270 whole cell catalyst under very mild conditions, biotransformations of racemic 1-arylaziridine-2-carbonitriles proceeded efficiently and enantioselectively to produce highly enantiopure S-1-arylaziridine-2-carboxamides and R-1-arylaziridine-2-carboxylic acids in excellent yields. Although the nitrile hydratase exhibits no selectivity against all nitrile substrates, the amidase is highly R-enantioselective towards 1-arylaziridine-2-carboxamides. When treated with benzyl bromide, 1-phenylaziridine-2S-carboxamide underwent a highly regioselective and enantiospecific ring-opening reaction to afford an almost quantitative yield of R-beta-[(benzyl)phenylamino]-alpha-bromopropanamide (C-2 attack) and R-alpha-[(benzyl)phenylamino]-beta-bromopropanamide (C-3 attack) in a 10.5:1 ratio. Further treatment of the resulting ring-opening products with an N-nucleophilic reagent such as amine and azide led to, through most probably the aziridinium intermediate, the formation of S-alpha-substituted-beta-[(benzyl)phenylamino]propanamides in good chemical yields with high enantiomeric purity.

Highly Enantioselective Synthesis of β-Amino Alcohols

[reaction: see text] N,N-Dialkyl-beta-amino alcohols derived from alpha-amino acids can be rearranged enantiospecifically by using TFAA/Et3N/NaOH to give 1,2-amino alcohols with enantiomeric excess up to 99%.