Synthesis, structure and chiroptical properties of isoxazolidin-5-ones

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.

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

The synthesis of new nucleoside analogues is currently of high interest. We report here full details of a study leading to the synthesis of novel isoxazolidinyl analogues of alpha-amino acid nucleosides. Three different synthetic approaches starting from L-serine have been evaluated for the construction of the isoxazolidine ring. These approaches consisted of Michael addition of N-benzylhydroxylamine to alpha,beta-unsaturated esters, nucleophilic addition of silyl ketene acetals to nitrones and 1, 3-dipolar cycloaddition of nitrones with vinyl acetate. Both Michael addition and nucleophilic addition of enolates could be carried out with stereocontrol at the newly formed stereogenic carbon. The stereocontrol observed in these reactions arises from the protecting group arrangement in the L-serine-derived substrates. Thus, whereas compounds having a diprotected nitrogen led to syn adducts, compounds having a monoprotected nitrogen gave rise to anti adducts. On the other hand, substrates having either a diprotected or monoprotected nitrogen atom led to anti adducts through the cycloaddition route. So, by choosing the appropriate route, isoxazolidinyl analogues having either syn or anti configuration with respect to the glycine unit can be prepared in enantiomerically pure form. The stereoselective synthesis of isoxazolidinyl analogues of deoxypolyoxin C and uracil polyoxin C in both D and L enantiomeric forms using these techniques has been achieved in good yields.