A general procedure for the synthesis of isoxazolidin-5-ones

Imbuing an Old Heterocycle with the Power of Modern Catalysis: An Isoxazolidin-5-one Story

Isoxazolidin-5-ones have been regarded as β-amino acid surrogates owing to their labile N-O bond. While many efforts have been devoted to the catalytic enantioselective synthesis of the core of this heterocycle, its further transformation has been less explored, especially in the context of catalysis. This review summarizes the author's research on the development of catalytic reactions using isoxazolidin-5-ones as substrates. Asymmetric catalysis has proven effective for C-C bond formation at the carbonyl α-carbon. Catalytic asymmetric allylation and direct Mannich-type reactions have been developed. Further, the resulting products have been readily converted into the corresponding quaternary β^2,2-amino acids. Moreover, isoxazolidin-5-ones have been identified as alkyl nitrene precursors in the presence of a suitable metal catalyst. The generated metallonitrene undergoes either the electrophilic amination of the aromatic ring or aliphatic C-H insertion, affording a series of cyclic β-amino acids. A remarkable difference in chemoselectivity between rhodium and copper alkyl nitrenes has also been demonstrated, highlighting the unique nature of the underexplored reactive intermediates. The various linear and cyclic β-amino acids obtained through the study are likely to find great utility in a broad range of chemical sciences.

Synthesis of Unprotected Spirocyclic β-Prolines and β-Homoprolines by Rh-Catalyzed C-H Insertion

A series of unprotected spirocyclic β-prolines and β-homoprolines are prepared by Rh-catalyzed C-H insertion. The key intermediate, a Rh nitrenoid, is generated by the N-O bond cleavage of a substituted isoxazolidin-5-one. The reaction proceeds on a gram scale with a catalyst loading of as little as 0.1 mol %, affording spirocyclic β-amino acids that are otherwise difficult to obtain. The building blocks prepared in this work will likely find applications in medicinal chemistry.

On the Nitrogen Inversion of Isoxazolidin-5-ones

The nitrogen inversion energies of a series of N-substituted isoxazolidin-5-ones were studied by density functional theory. The transition state energy was found to strongly correlate with the s-character of the lone pair of electrons on the nitrogen in the ground state. Although the activation energy trends for isxazolidin-5-ones and isoxazolidines are similar, their conformational equilibria are slightly different and the isoxazolidin-5-one inversion energies are generally higher.

Lewis Base Assisted Lithium Brønsted Base Catalysis: A New Entry for Catalytic Asymmetric Synthesis of β2,2-Amino Acids

A new catalytic system comprising chiral Ag complex and Li aryloxide/bisphosphine oxide is developed for the synthesis of β^2,2-amino acids via direct asymmetric Mannich-type reaction of 4-subsituted isoxazolidin-5-ones. The Mannich adduct is a direct precursor of β-peptidic compounds otherwise difficult to obtain.

Catalytic asymmetric aza-Michael addition of fumaric monoacids with multifunctional thiourea/boronic acids

Chiral multifunctional thiourea/boronic acid catalysts enabled the synthesis of N-hydroxyaspartate derivatives applicable for KAHA amidation through acid-β selective aza-Michael addition.

Synthesis of 3-alkylbenzoxazolones from N-alkyl-N-arylhydroxylamines by contiguous O-trichloroacetylation, trichloroacetoxy ortho-shift, and cyclization sequence

Benzoxazolone pharmacophore is present in clinical pharmaceuticals, drug candidates, and many compounds having a wide spectrum of biological activities. The methods available for the synthesis of benzoxazolones have limited diversity due to problems in accessibility and air-sensitivity of diversely substituted o-aminophenols from which they are generally prepared by cyclocarbonylation with phosgene or its equivalents. The present paper describes a mild method for the synthesis of 3-alkylbenzoxazolones from easily accessible and air-stable nitroarenes. Nitroarenes were converted to N-alkyl-N-arylhydroxylamines in two steps involving partial reduction to arylhydroxylamines followed by selective N-alkylation. Treatment of N-alkyl-N-arylhydroxylamines with trichloroacetyl chloride and triethylamine afforded 3-alkylbenzoxazolones generally in good yields through an uninterrupted three-step sequence involving O-trichloroacetylation, N→C(ortho) trichloroacetoxy shift, and cyclization in a single pot at ambient temperatures. The present method is mild, wide in scope, economical, and regioselective. Many sensitive groups like alkyl and aryl esters, amide, cyano, and the carbon-carbon double bond survive the reaction.

Organocatalytic asymmetric 5-hydroxyisoxazolidine synthesis: a highly enantioselective route to beta-amino acids

The highly chemo- and enantioselective organocatalytic tandem reaction between N-protected hydroxyl amines and alpha,beta-unsaturated aldehydes is presented; the reaction provides access to 5-hydroxyisoxazolidines and beta-amino acids in high yields and with 90-99% ee.

Expedient synthesis of substituted (S)-N-(α-methylbenzyl)aziridines

We report for the first time that after O-acylation the conjugate addition products of (S)-N-(alpha-methylbenzyl)hydroxylamine undergo an efficient diastereoselective 3-exo-tet ring-closure reaction affording 2- and 2,3-disubstituted-N-alkylaziridines in good to excellent yields.

A divergent chemoenzymatic route to an intermediate for nucleoside analogues

Two regioisomeric isoxazoline monoacetates 1 and 2 were synthesized from the corresponding diacetate 3 via PPL or PLE catalyzed hydrolysis. With both the enzymes, the initial regioselectivity (approximately 3-4:1) was offset by an intramolecular acyl transfer. In addition to a non-enzymatic catalysis for the acyl transfer, preliminary experiments do suggest a definite but minor role of enzyme for this intramolecular acyl transfer. Compounds 1 and 2 may serve as intermediates for nucleoside analogues.

Stereoselective synthesis of beta-amino acids

A convenient method for the stereoselective syntheses of beta-amino acids with alpha-substitutions has been developed. This synthetic route involves the preparation of isoxazolidinones through hydroxylamine addition to unsaturated esters and subsequent hydrogenation. This procedure is also useful for the stereoselective syntheses of alpha-deuterated beta-amino acids.

?2-amino acids?syntheses, occurrence in natural products, and components of ?-peptides1,2

Although they are less abundant than their alpha-analogues, beta-amino acids occur in nature both in free form and bound to peptides. Oligomers composed exclusively of beta-amino acids (so-called beta-peptides) might be the most thoroughly investigated peptidomimetics. Beside the facts that they are stable to metabolism, exhibit slow microbial degradation, and are inherently stable to proteases and peptidases, they fold into well-ordered secondary structures consisting of helices, turns, and sheets. In this respect, the most intriguing effects have been observed when beta2-amino acids are present in the beta-peptide backbone. This review gives an overview of the occurrence and importance of beta2-amino acids in nature, placing emphasis on the metabolic pathways of beta-aminoisobutyric acid (beta-Aib) and the appearance of beta2-amino acids as secondary metabolites or as components of more complex natural products, such as peptides, depsipeptides, lactones, and alkaloids. In addition, a compilation of the syntheses of both achiral and chiral beta2-amino acids is presented. While there are numerous routes to achiral beta2-amino acids, their EPC synthesis is currently the subject of many investigations. These include the diastereoselective alkylation and Mannich-type reactions of cyclic- or acyclic beta-homoglycine derivatives containing chiral auxiliaries, the Curtius degradation, the employment of transition-metal catalyzed reactions such as enantioselective hydrogenations, reductions, C-H insertions, and Michael-type additions, and the resolution of rac. beta2-amino acids, as well as several miscellaneous methods. In the last part of the review, the importance of beta2-amino acids in the formation of beta-peptide secondary structures is discussed.

Efficient synthesis of enantiomerically pure beta2-amino acids via chiral isoxazolidinones

We report a practical and scalable synthetic route for the preparation of alpha-substituted beta-amino acids (beta(2)-amino acids). Michael addition of a chiral hydroxylamine, derived from alpha-methylbenzylamine, to an alpha-alkylacrylate followed by cyclization gives a diastereomeric mixture of alpha-substituted isoxazolidinones. These diastereomers are separable by column chromatography. Subsequent hydrogenation of the purified isoxazolidinones followed by Fmoc protection affords enantiomerically pure Fmoc-beta(2)-amino acids, which are useful for beta-peptide synthesis. This route provides access to both enantiomers of a protected beta(2)-amino acid.

Reaction between N-alkylhydroxylamines and chiral enoate esters: more experimental evidence for a cycloaddition-like process, a rationale based on DFT theoretical calculations, and stereoselective synthesis of new enantiopure beta-amino acids

The reactions between N-benzyl- and N-methylhydroxylamine and chiral enoate esters, derived from D-glyceraldehyde and (-)-verbenone, respectively, have been investigated. Theoretical calculations show that the most favorable mechanism involves the concerted cycloaddition of the hydroxylamine to the substrate. This result is in good agreement with the stereospecificity observed when the trisubstituted olefins are used. The open-chain adducts have been isolated when the processes are carried out at low temperatures and for short reaction times. These compounds evolve to the corresponding isoxazolidinones on standing at room temperature or under acid catalysis. The high pi-facial diastereoselection has been rationalized on the basis of steric effects induced by the dioxolane ring for D-glyceraldehyde derivatives or by the cyclobutane gem-dimethyl substitution for esters prepared from (-)-verbenone. As an application of these reactions, new beta-amino acids have been synthesized in a highly efficient and stereocontrolled manner.

N-Benzylhydroxylamine addition to beta-aryl enoates. Enantioselective synthesis of beta-aryl-beta-amino acid precursors

Chiral Lewis acid catalyzed N-benzylhydroxylamine addition to pyrrolidinone-derived enoates afforded beta-aryl-beta-amino acid derivatives in high enantiomeric purity with moderate to very good chemical efficiency.

An Efficient Route to beta-D-Isoxazolidinyl Nucleosides via Diastereoselective Michael Addition of Hydroxylamine to Unsaturated Esters

Enantioselective syntheses of beta-D-isoxazolidinyl pyrimidine and purine nucleosides are described. Michael addition of N-methylhydroxylamine to alpha,beta-unsaturated esters was investigated. Both E- and Z-esters 10E and 10Z produced the same intermediates which were cyclized to isoxazolidin-5-ones 8 with high diastereoselectivity. The major isoxazolidin-5-one 8a was reduced and acetylated to acetate 11 for the preparation of nucleosides. The coupling reaction of acetate 11 with silylated thymine, uracil, and N(4)-benzoylcytosine using TMSOTf as a Lewis acid yielded the corresponding nucleoside derivatives. The related purine analogue was produced by the BF(3).Et(2)O-catalyzed condensation of acetate 11 with silylated 6-chloropurine. The predominant formation of the cis isomers for both pyrimidine and purine analogues was unexpected and the reaction mechanism was investigated. The nucleoside intermediates were converted to the corresponding 1,2-diols, which were latter oxidized and reduced to the desired monoalcohol products such as 14, 16, 19, and 24.