An efficient and mild bismuth triflate-catalysed three-component Mannich-type reaction

In the presence of a catalytic amount of Bi(OTf)(3).4H(2)O, aldehydes together with amines react with silyl enolates to afford the corresponding Mannich-type adducts smoothly. A wide variety of silyl enolates derived from ketones, as well as esters and thioesters, react rapidly to afford the beta-amino ketones or the beta-amino esters in high yields (up to 94%).     

Catalytic asymmetric synthesis of alpha- and beta-amino phosphonic acid derivatives

Catalytic asymmetric reactions provide one of the most powerful and economical synthetic approaches to a variety of enantiomerically enriched compounds. As being analogues of the corresponding alpha- and beta-amino acids, optically active alpha- and beta-amino phosphonic acid derivatives have found widespread use in medicinal chemistry and the pharmaceutical sciences. Using catalytic amounts of chiral materials, asymmetric synthesis of enantiomerically enriched alpha- and beta-amino phosphonates has been a subject of growing interest. This tutorial review contains a compilation of the catalytic asymmetric synthetic methods developed to date and highlights their utility for obtaining these target compounds.     

CuI-catalyzed coupling reaction of beta-amino acids or esters with aryl halides at temperature lower than that employed in the normal Ullmann reaction. Facile synthesis of SB-214857

[reaction: see text] The CuI-catalyzed coupling reaction of aryl halides with beta-amino acids or beta-amino esters is completed at 100 degrees C in 48 h, which indicates that the structure of the beta-amino acid has an accelerating effect for the Ullmann-type aryl amination reaction. This coupling reaction can be used to prepare enantiopure N-aryl beta-amino acids. An efficient synthetic route to SB214857, a potent GPIIb/IIIa receptor antagonist, is developed using this method.     

Synthesis of beta-amino acids based on oxidative cleavage of dihydropyridone derivatives

[reaction: see text] A new method for the synthesis of beta-amino acids based on 2,3-dihydropyridones as starting materials is presented. Conversions of 2,3-dihydropyridones with NaIO4 and subsequently with base gave the corresponding beta-amino acids in a one-pot procedure. The reactions have been monitored by 1H NMR indicating that the beta-amino acids were formed in quantitative yields mostly. This method appears to be of broad scope, as 2-substituted 2,3-dihydropyridones are easily accessible via N-acyliminium ions generated from 4-methoxypyridine.     

Copper(I)-Fesulphos Lewis Acid catalysts for enantioselective Mannich-type reaction of N-sulfonyl imines

[reaction: see text] Copper(I) complexes of Fesulphos ligands are efficient chiral Lewis acid catalysts in the Mannich-type addition of silyl enol ethers of ketones, esters, and thioesters to N-(2-thienyl)sulfonyl aldimines. The corresponding optically active beta-amino carbonyl derivatives were obtained in good yields (58-91%) and with moderate to good enantioselectivity (61-93% ee). Removal of the N-activating group was achieved under mild conditions by simple treatment of the products with Mg in methanol.     

Residue requirements for helical folding in short alpha/beta-peptides: crystallographic characterization of the 11-helix in an optimized sequence

Design of functional foldamers requires knowledge of the conformational propensities of constituent residues. Here, we explore the effects of variations in both alpha-amino acid and beta-amino acid substitution on alpha/beta-peptide helicity. We also report the first X-ray crystal structure of a helical alpha/beta-peptide. We conclude that a certain amount of conformational preorganization in alpha/beta-peptides (via the inclusion of constrained beta-amino acids or alpha,alpha-disubstituted alpha-amino acids) is needed to promote helical folding; acyclic beta-amino acids and beta-branched alpha-amino acids are tolerated to only a limited extent.     

Synthesis of conformationally constrained C-glycosyl alpha- and beta-amino acids and sugar-carbamino sugar hybrids via Diels-Alder reaction

[structure: see text] Sugar-derived dienes undergo Diels-Alder reactions with methyl alpha-nitro acrylate and ethyl beta-nitro acrylate to form the corresponding cycloadducts which have been converted into conformationally constrained C-glycosyl alpha- and beta-amino acids. Further, these beta-amino acids are converted into sugar-carbamino sugar hybrid molecules.     

Highly effective chiral ortho-substituted BINAPO ligands (o-BINAPO): applications in Ru-catalyzed asymmetric hydrogenations of beta-aryl-substituted beta-(acylamino)acrylates and beta-keto esters

A novel family of chiral ortho-substituted BINAPO ligands (o-BINAPO) were synthesized from BINOL, and their Ru complexes were highly efficient catalysts for asymmetric hydrogenation of beta-aryl-substituted beta-(acylamino)acrylates and beta-aryl-substituted beta-keto esters. The Ru-bisphosphinite catalysts can tolerate an E/Z mixture of beta-aryl-substituted beta-(acylamino)acrylates. These highly enantioselective hydrogenations provide a useful way to prepare beta-aryl-substituted beta-amino acids and beta-hydroxyl acids.     

Recent developments in the catalytic asymmetric synthesis of alpha- and beta-amino acids

The stereoselective synthesis of amino acids is of great importance for the construction of optically active natural products and pharmaceuticals. Apart from enzymes, a broad repertoire of chiral reagents, auxiliaries, and catalysts can be used for the formation of amino acids. Asymmetric reactions using catalytic amounts of chiral molecules provide efficient methods for the generation of optically active proteinogenic and nonproteinogenic amino acids. This minireview collects recent work on catalytic asymmetric synthesis of alpha- and beta-amino acids.     

Synthesis of alpha-substituted beta-amino acids using pseudoephedrine as a chiral auxiliary

[reaction: see text] beta-Amino acids are becoming increasingly attractive as intermediates in the synthesis of a variety of molecular structures. However, few methods are available for the synthesis of alpha-substituted beta-amino acids that are both readily scalable and highly stereoselective. Herein we report a new method for synthesizing alpha-substituted beta-amino acids that satisfies both of these requirements using enantiomerically pure pseudoephedrine as a chiral auxiliary.     

Synthesis of alpha-trifluoromethyl-beta-lactams and esters of beta-amino acids via 1,3-dipolar cycloaddition of nitrones to fluoroalkenes

Nitrones derived from aromatic or aliphatic aldehydes or ketones react with hexafluoropropene (HFP) or 2H-pentafluoropropene (PFP) to give the respective fluorinated isoxazolidine derivatives in good yields with complete regioselectivity and moderate diastereoselectivity. Catalytic hydrogenolysis of the N-O bond under ambient pressure and temperature leads to fluorides of beta-amino acids that undergo cyclization to alpha-trifluoromethylated beta-lactams or, under acidic conditions, form esters of alpha-trifluoromethylated beta-amino acids.     

Preparation of protected syn-alpha,beta-dialkyl beta-amino acids that contain polar side chain functionality

We report the synthesis of syn-alpha,beta-dialkyl beta-amino acid derivatives suitably protected for solid-phase synthesis that give rise to residues containing positively charged lysine-like side chains. These amino acids, as well as syn-alpha,beta-dialkyl beta-amino acids that contain diverse hydrophobic side chains, are prepared in good de and ee. The key step in this route involves Davies's protocol for the conjugate addition of a chiral lithium amide to alpha,beta-unsaturated tert-butyl esters (Davies, S. G.; Ichihara, O.; Walters, I. A. S. J. Chem. Soc., Perkin Trans. 1 1994, 9, 1141). syn-alpha,beta-Dialkyl beta-amino acids are interesting building blocks because of their sheet-forming propensity and because of their presence in bioactive compounds.     

α-Oxoketene dithioacetals: Versatile substrates for 1,3-carbonyl transpositions

A two step procedure for the conversion of α-oxoketene dithioacetals to β-substituted α, β-unsaturated thioesters or acids is described. The overall transformation represents a 1,3-carbonyl transposition in which the original ketone carbonyl emerges as the carbonyl of an acid or thioester. The resulting thioesters and acids constitute versatile acylating reagents in accord with established procedures.     

Asymmetric synthesis of syn-alpha-substituted beta-amino ketones by using sulfinimines and prochiral Weinreb amide enolates

Syn-alpha-substituted beta-amino Weinreb amides are new chiral building blocks for asymmetric synthesis of syn-alpha-substituted beta-amino acids, aldehydes, and ketones and are prepared by addition of prochiral lithium enolates of Weinreb amides to sulfinimines (N-sulfinyl imines).     

The first aminoacylase-catalyzed enantioselective synthesis of aromatic beta-amino acids

The first aminoacylase-catalyzed enantioselective synthesis of aromatic beta-amino acids is reported. The presence of an N-chloroacetyl group as acyl group in the substrate as well as the use of porcine kidney acylase I as a suitable enzyme component are prerequisites for this resolution process whereby optically active beta-amino acids are formed with high enantioselectivities of >98% ee.     

Mass spectra of some dithiocarboxylic acid esters

The mass spectra of a variety of aliphatic esters of aromatic dithiocarboxylic acids are reported; their breakdown patterns are compared with those of the corresponding oxygen esters, O-alkyl thioesters and S-alkyl thioesters. Small quantities of O-alkyl thioesters present in alkyl dithioesters can be detected by the appearance of the peak [ArC?O]⁺.     

Treatment of N,N-dibenzylamino alcohols with sulfonyl chloride leads to rearranged beta-chloro amines, precursors to beta-amino acids, and not to tetrahydroisoquinolines

[reaction: see text] Very recently, the unexpected formation of 3-substituted 1,2,3,4-tetrahydroisoquinolines starting from N,N-dibenzyl-protected beta-amino alcohols was reported. The authors claimed that treatment with tosyl chlorides induced intramolecular Friedel-Crafts alkylation. Reexamination of the reactions in our laboratory clearly proved rearranged chloro amines instead of the initially assumed tetrahydoisoquinoline structures. The chloro amines investigated can be employed as highly useful intermediates for an EPC synthesis of beta-amino nitriles and beta-amino acids.     

Amino acid-derived enaminones: a study in ring formation providing valuable asymmetric synthons

A new reaction for the preparation of enaminones has been discovered. This method employs beta-amino acids as starting materials to allow diversification as well as incorporation of chirality. The beta-amino acids, once converted to ynones, are readily cyclized to the desired six-membered enaminone via a two-step, one-pot protocol. Although disguised as a 6-endo-dig cyclization, the reagents employed in the transformation play a direct role in bond making and bond breaking, thus changing the mode of addition.     

A concise approach to structurally diverse beta-amino acids

We have demonstrated that the high yields and selectivities of 1,3-dipolar cycloadditions can be translated into facile stereoselective syntheses of a diverse array of beta-amino acids, key components of bioactive natural products, beta-lactams, and peptidomimetics. Simply by selecting different combinations of three readily available starting materials (an oxime, a chiral allylic alcohol, and a nucleophile), we used the reaction sequence to prepare four different beta-amino acid structural types with a variety of substitution patterns in good overall yield. Of particular note is the use of this approach to prepare highly substituted beta-amino acids not readily accessible by previously reported methodologies. This will pave the way for future studies of the structure and function of this important class of molecules.     

Differentiation of diastereomeric cyclic beta-amino acids by varying the neutral reagent in ion/molecule reactions studied by electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry

Stereochemical differentiation of diasteromeric pairs of cis- and trans-2-aminocyclohexane-, -2-amino-4-cyclohexene-, and -2-aminocyclopentanecarboxylic acids was investigated with host-guest complexes where tetraethyl resorcarene was the host molecule. Diastereoselectivity was evaluated by ion/molecule reactions and collision-induced dissociation with electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICRMS). The effect of varying the neutral reagent (n-propylamine, i-propylamine, diethylamine, and triethylamine) in ion/molecule reactions was evaluated. Both steric interactions and proton affinity of the neutral reagents influenced the reaction rates. High proton affinity of the neutral reagent apparently had a twofold effect. If the proton affinity of the neutral reagent was too high, the reaction tended to become too exothermic and part of the host-guest complex decomposed instead of transforming to a new host-guest complex, effecting a decrease in the reaction rate. The remaining portion of the host-guest complexes meanwhile reacted very fast with the neutral reagent due to high proton affinity causing an increase in the reaction rate. n-Propylamine and i-propylamine proved to be the best neutral reagents, providing clear diastereoselectivity for beta-amino acids in ion/molecule reactions. Interestingly, diastereoselectivity was better for flexible cyclohexane beta-amino acids (2 and 3) than for more rigid cyclopentane beta-amino acids (6 and 7). The results of ab initio and hybrid density functional theory calculations on the structures of the host-guest complexes of saturated beta-amino acids were in good agreement with the experimental results.