Sandrine Py

SmI2-Mediated Cross-Coupling of Nitrones with β-Silyl Acrylates: Synthesis of (+)-Australine

The SmI(2)-mediated cross-coupling of nitrones with β-silyl-α,β-unsaturated esters, followed by zinc reduction, allows an efficient and highly diastereoselective preparation of β-silyl lactams, which are precursors of β-hydroxy lactams through Tamao-Fleming oxidation. By applying the method to a cyclic, carbohydrate-derived nitrone, a new synthesis of (+)-australine has been realized in only 11 steps and in 21% overall yield from L-xylose.

A short and convenient synthesis of 1-deoxymannojirimycin and N-oxy analogues from D-fructose.

Ketonitrone 8 was prepared from D-fructose as an inexpensive starting material and was used in a stereoselective synthesis of 1-deoxymannojirimycin (DMJ, 4), of its previously unknown N-hydroxy analogue 15, and of the polyhydroxylated ketonitrone 14. The latter were assayed as potential glycosidase inhibitors on a panel of 13 selected purified enzymes. Disappointingly, the polyhydroxylated nitrone 14 inhibited none of these enzymes. However, N-hydroxy-DMJ (15) exhibited a moderate and non-selective activity toward the snail beta-mannosidase EC 3.2.1.25.

A direct and versatile access to alpha,alpha-disubstituted 2-pyrrolidinylmethanols by SmI2-mediated reductive coupling.

Various alpha,alpha-disubstituted 2-pyrrolidinylmethanols are efficiently prepared in a single step from ketones using a SmI2-mediated cross-coupling with 1-pyrroline N-oxide. The N-hydroxy-alpha,alpha-diphenylprolinol is also easily prepared and resolved.

Diastereoselective addition of terminal alkynes to chiral nitrones: asymmetric synthesis of propargylic N-hydroxylamines

Abstract 1,3-Asymmetric induction in the Et2Zn-catalyzed addition of terminal alkynes was studied with nitrones bearing a chiral auxiliary on their nitrogen atom. The obtained propargylic N-hydroxylamines were generally isolated in good yields and with satisfactory to excellent diastereoselectivities.

cis-Stereoselective SmI2-promoted reductive coupling of keto-nitrones: first synthesis of 1-epitrehazolamine

An expeditious synthesis of 1-epitrehazolamine is presented from readily available 2,3,4,6-tetra-O-benzyl-D-glucose. The key step involves a samarium diiodide-promoted reductive cyclization of a masked keto-nitrone to form a five-membered ring aminocyclitol. The excellent cis selectivity observed in this nitrone-ketone reductive coupling contrasts surprisingly with the trans selectivity of ketone-oxime reductive couplings.

Novel polyhydroxylated cyclic nitrones and N-hydroxypyrrolidines through BCl3-mediated deprotection.

A general method to prepare a new class of carbohydrate-derived, enantiomerically pure polyhydroxypyrroline N-oxides from their alkoxy (protected) derivatives is presented. Boron trichloride is shown to cleave efficiently benzyl ethers and ketals without affecting the imine N-oxide functionality of nitrones. The same reagent (BCl3) also allowed the efficient synthesis of a polyhydroxylated N-hydroxypyrrolidine, giving access to a novel class of N-hydroxyiminosugars.

Hydroxymethyl-Branched Polyhydroxylated Indolizidines: Novel Selective α-Glucosidase Inhibitors

α,α-Disubstituted piperidines and conformationally constrained polyhydroxylated indolizidines bearing a hydroxymethyl substituent in position 8a were synthesized from a readily available l-sorbose-derived ketonitrone. Diastereoselective vinylation under two sets of complementary conditions allowed access to both configurations of the newly formed quaternary stereocenter. Subsequent N-allylation and ring-closing metathesis afforded 8a-branched indolizidines in high yield. The newly prepared iminosugars demonstrated highly potent inhibition of α-glucosidases. Most interestingly, compound 9b exhibits very high selectivity toward this class of enzymes, with an unusual mode of binding.

Enantioselective Ruthenium-Catalyzed 1,3-Dipolar Cycloadditions between C-Carboalkoxy Ketonitrones and Methacrolein: Solvent Effect on Reaction Selectivity and Its Rational

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.

Taxamycin studies: Synthesis of taxoid-calicheamicin hybrids

Abstract The synthesis of the bicyclic enediyne compounds 14–18 is described. The bicyclo[7.3.1]trideca-4,9-dien-2,6-diynes (21) are calicheamicin-taxoid mimics, that possess the Taxotere® side chain and an enediyne core. Cyclization of the iodo-aldehyde 13 [CrCl2(THF)2] afforded the bicyclic alcohol 14 as a single diastereomer (76%) and allylic oxidation (SeO2) followed by reaction with the oxazolidine intermediate 19, resolution and hydrolysis provided the taxamycins 21.

Asymmetric synthesis of α,α-disubstituted amino acids by cycloaddition of (E)-ketonitrones with vinyl ethers.

Original acyclic (E)-α,α-dialkylketonitrones bearing a chiral auxiliary on their nitrogen atom were synthesized and successfully employed for the asymmetric synthesis of α,α-disubstituted amino acids using regio- and stereocontrolled 1,3-dipolar cycloaddition reactions with vinyl ethers. N-Glycosyl chiral auxiliaries were found to provide excellent exo- and π-facial stereocontrol. The obtained enantiopure cycloadducts were selectively transformed into functional α,α-disubstituted amino acids and related β-peptides through the highly regioselective opening of an intermediate quaternary anhydride.