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Highly diastereo- and enantioselective 1,3-dipolar cycloadditions between functional ketonitrones and β-substituted enals are promoted by MacMillan imidazolidinium organocatalysts. A study of the reaction scope shows that high selectivities are conserved if the N-protecting group or the ester function is varied. However, the reaction is sensitive to steric interactions with the C substituent of the nitrone. In all cases, the reaction proceeds with high exo selectivity. In most cases, a third diastereomer, incompatible with a concerted mechanism, was also observed, albeit in minute amounts. DFT calculations suggest that the cycloaddition proceeds in a nonconcerted fashion by an initial oxa-Michael-type addition of the nitrone to the double bond followed by a cyclization. This mechanism explains the formation of the observed minor diastereomers. In addition, the diastereo- and enantioselectivities of the reaction were shown to be intermediately thermodynamically controlled, and the diastereomeric ratio is modulated by the kinetics of iminium hydrolysis. [ABSTRACT FROM AUTHOR]

1,3-Dipolar cycloadditions of nitrones with pentafluorosulfanyl-substituted acrylic ester and amides afforded new trisubstituted pentafluorosulfanylated isoxazolidines in moderate yields as 4-SF 5 -regioisomers. Theoretical calculations were carried out and the regioselectivity could be explained by the fact that the 5-SF 5 -regioisomer probably undertook a spontaneous degradation in the reaction medium via the loss of pentafluorosulfanyl anion. Diastereoselectivity of the 1,3-dipolar cycloadditions was also assessed according to NMR and X-ray diffraction analysis. [ABSTRACT FROM AUTHOR]

Bicyclic isoxazolidines displaying one or two quaternary stereocenter(s) were formed starting from functional cyclic ketonitrones equipped with a phenyl glycinol chiral auxiliary. The products were engaged in stereocontrolled 1,3-dipolar cycloaddition reactions with a range of electron-rich and electron-poor dipolarophiles. A new reductive removal of the phenyl glycinol chiral auxiliary was introduced and was shown to afford chemoselectively a quaternary isoxazolidine derivative (of oxaprolinol-type) without cleaving the N-O isoxazolidine bond. Keeping the aldehyde function masked as a cyclic pseudo-acetal, the liberated oxy-amine function was shown to be available for a pseudo-peptide coupling with various N-protected amino acids. The isoxazolidine ring was opened by a reductive N-O bond cleavage, giving a pseudo-dipeptide that was C-terminated with an aldehyde function. [ABSTRACT FROM AUTHOR]

Enantiopure vinyl ethers of Stericol® underwent diastereoselective thermal 1,3 dipolar cycloadditions with N-benzyl, N-benzhydryl, and N-PMB aspartate ester nitrones. Chemoselective N-debenzylation of the resulting cycloadducts afforded diastereomerically and enantiomerically pure crystalline NH-isoxazolidine, the absolute configuration of which was established by X-ray crystallography. N-Protection and N-O cleavage of this isoxazolidine gave enantioenriched quaternary aspartate derivatives bearing functionalized side chains. [ABSTRACT FROM AUTHOR]

The article presents information on the synthesis of SF5-substituted isoxazolidines using 1,3-dipolar cycloaddition reactions.

A catalytic 1,3-dipolar cycloaddition between carboalkoxy ketonitrones and methacrolein under the effect of chiral ruthenium Lewis acid (R,R-1) was developed with high regio-, diastereo-, and enantiocontrol. The diastereochemical outcome of the cycloaddition reaction is marked by a significant solvent effect, and a divergent endo or exo control can be tuned by an appropriate choice of both the solvent and the N- and O-substituents of the ketonitrone. A rationale of the solvent effect, based on the computational study of the interactions between the methacrolein-Ru complex and its counteranion (SbF6-), is proposed to explain the selectivities obtained. [ABSTRACT FROM AUTHOR]

In the presence of the Ru catalyst (RUC), ketonitrones of type (I) are found to react with methacrolein with high regio-, diastereo-, and enantioselectivity. [ABSTRACT FROM AUTHOR]

Original acyclic (E)-α,α-dialkylketonitronesbearing a chiral auxiliary on their nitrogen atom were synthesizedand successfully employed for the asymmetric synthesis of α,α-disubstitutedamino acids using regio- and stereocontrolled 1,3-dipolar cycloadditionreactions with vinyl ethers. N-Glycosyl chiral auxiliarieswere found to provide excellent exo- and π-facialstereocontrol. The obtained enantiopure cycloadducts were selectivelytransformed into functional α,α-disubstituted amino acidsand related β-peptides through the highly regioselective openingof an intermediate quaternary anhydride. [ABSTRACT FROM AUTHOR]

Abstract: Highly enantio- and exo-selective 1,3-dipolar cycloadditions of alanine-derived ketonitrones to E-crotonaldehyde could be realized in a good yield by the use of a chiral imidazolidinone salt without the addition of water. The origin of the stereoselectivity in the reaction was discussed and the absolute configuration of the cycloadduct determined unambiguously. [Copyright &y& Elsevier]

The first example of a high facial selective [3 + 2] cycloaddition employing chiral vinyl ethers as dipolarophile and nitrones as dipoles is presented. [ABSTRACT FROM AUTHOR]

The stereoselectivity of the 1,3-dipolar cycloaddition of (E)-ketonitrones (I) bearing a chiral auxiliary on the nitrogen atom with vinyl ethers is studied. [ABSTRACT FROM AUTHOR]