Stereodivergent approaches to the synthesis of isoxazolidine analogues of alpha-amino acid nucleosides. Total synthesis of isoxazolidinyl deoxypolyoxin C and uracil polyoxin C

Broadening the reaction scope of unprotected aldoses via their corresponding nitrones: 1,3-dipolar cycloadditions with alkenes

Condensation reactions of unprotected tetroses and pentoses with hydroxylamines afforded nitrones, which were easily converted to densely functionalized isoxazolidines in the presence of electron-poor alkenes. The 1,3-dipolar cycloaddition occurred with good facial discrimination of the chiral nitrone but under rather low endo/exo control. Stereochemistry of isomers was ascertained by chemical correlation with known derivatives from the literature. Microwave activation appeared as the most efficient reaction mode, affording the expected adducts within several minutes whereas hours were needed under standard heating. Alternatively, the transformation proved also possible under high pressure conditions by using a hand pump system, avoiding any energy source. Although water could not be used as the solvent, leading to hydrolysis of the nitrone substrate, a large variety of organic solvents proved efficient. The method has potential use in the preparation of non-ionic carbohydrate-based amphiphiles.

Alkenylazaarenes as dipolarophiles in 1,3-dipolar cycloaddition of nitrones: regioselectivity-switchable and highly diastereoselective synthesis of multisubstituted isoxazolidines

Metal free 1,3-dipolar cycloaddition between alkenylazaarenes and nitrones has been developed for the synthesis of 4-substituted isoxazolidines and 5-substituted isoxazolidines in good yields and selectivities.

New mechanistic interpretations for nitrone reactivity

The reactivity of nitrones in cycloadditions and related reactions is revisited by introducing a topological perspective.

Isoxazolidine: A Privileged Scaffold for Organic and Medicinal Chemistry

The isoxazolidine ring represents one of the privileged structures in medicinal chemistry, and there have been an increasing number of studies on isoxazolidine and isoxazolidine-containing compounds. Optimization of the 1,3-dipolar cycloaddition (1,3-DC), original methods including electrophilic or palladium-mediated cyclization of unsaturated hydroxylamine, has been developed to obtain isoxazolidines. Novel reactions involving the isoxazolidine ring have been highlighted to accomplish total synthesis or to obtain bioactive compounds, one of the most significant examples being probably the thermic ring contraction applied to the total synthesis of (±)-Gelsemoxonine. The unique isoxazolidine scaffold also exhibits an impressive potential as a mimic of nucleosides, carbohydrates, PNA, amino acids, and steroid analogs. This review aims to be a comprehensive and general summary of the different isoxazolidine syntheses, their use as starting building blocks for the preparation of natural compounds, and their main biological activities.

Pyrimidine as constituent of natural biologically active compounds

This review describes the various manifestations of the pyrimidine system (alkylated, glycosylated, benzo-annelated.). These comprise pyrimidine nucleosides as well as alkaloids and antibiotics--some of them have been discovered and isolated from natural sources already long time ago, others have been reported very recently. A short overview on pyrimidine syntheses (prebiotic synthesis, biosynthesis, and metabolism) is given. The biological activities of most of the pyrimidine analogs are briefly described, and, in some cases, syntheses are formulated.

Reaction of nitrones with silyl ketene acetals: a DFT study

Theoretical calculations at the DFT level indicate that the reaction of a nitrone with a silyl ketene acetal proceeds, contrary to previous suggestions, through a classical 1,3-dipolar cycloaddition, followed by a silyl group transfer step to give the open-chain product. Introduction of a diffuse basis set is necessary to describe properly nitrones. The influence of a Lewis acid has been studied.

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.

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.

Synthesis of a new class of 5'-functionalized adenosines using a rh(ii)-catalyzed 1,3-dipolar cycloaddition

[reaction: see text] Chemically protected adenosine was functionalized at the 5' position to generate novel dipolarophiles and mesoionic dipoles. These species were found to undergo facile 1,3-dipolar cycloaddition to afford a new series of adenosine derivatives that contain a point of diversification at the 5' position of adenosine.