Stereocontrolled Cyclic Nitrone Cycloaddition Strategy for the Synthesis of Pyrrolizidine and Indolizidine Alkaloids

Short synthesis, X-ray and conformational analysis of a cyclic peracetylated l-sorbose-derived nitrone, a useful intermediate towards N–O-containing d-gluco-iminosugars

A crystalline peracetylated nitrone underwent regio- and stereoselective 1,3-dipolar cycloaddition with alkynes, affording d-gluco-configured isoxazolines as a new class of iminosugars.

Facile selective synthesis of 2-methyl-5-amino-1,2,4-oxadiazolium bromides as further targets for nucleophilic additions

The reaction of aminonitrones with isocyanides and Br₂ gives 2-methyl-5-amino-1,2,4-oxadiazolium bromides, which are convenient precursors for other heterocycles and ureides.

Cyclopentadienones via a tandem C-cyclopropylnitrone cyclization-cycloreversion sequence

Aldonitrones derived from spiro[2.4]hepta-4,6-diene-1-carbaldehyde and its benzo analog undergo a tandem uncatalyzed intramolecular cyclopropane-nitrone cyclization-5,6-dihydro-1,2-oxazine cycloreversion to give cyclopentadienones. Similarly, the NH-nitrone generated in situ from spiro[cyclopropane-1,1'-indene]carbaldehyde oxime leads to benzocyclopentadienone (1H-inden-1-one) by the same mechanism. DFT calculations are in favor of a concerted yet highly asynchronous pathway for the cyclizations. Control experiments with the dihydro and tetrahydro derivatives show that the spirocyclopentadiene unit is essential for the success of the reaction, invoking spiroconjugative effects for increased cyclopropane reactivity.

Polyhydroxylated Quinolizidine Iminosugars as Nanomolar Selective Inhibitors of α-Glucosidases

Polyhydroxylated quinolizidines bearing a hydroxymethyl group at the ring junction were synthesized from a readily available l-sorbose-derived ketonitrone. Evaluated as glycoside hydrolase inhibitors, these quinolizidines revealed to be potent and selective α-glucosidase inhibitors. Quinolizidine 9a is the first quinolizidine-scaffolded iminosugar exhibiting nanomolar inhibition of a glycoenzyme.

Stereoselective Synthesis of Isoxazolidines via Copper-Catalyzed Alkene Diamination

A convenient copper-catalyzed intramolecular/intermolecular alkene diamination reaction to synthesize 3-aminomethyl-functionalized isoxazolidines under mild reaction conditions and with generally high levels of diastereoselectivity was achieved. This reaction demonstrates that previously underutilized unsaturated carbamates are good [Cu]-catalyzed diamination substrates. Sulfonamides, anilines, benzamide, morpholine, and piperidine can serve as the external amine source. This relatively broad amine range is attributed to the mild reaction conditions. Reduction of the N-O bond could also be achieved, revealing the corresponding 3,4-diamino-1-alcohols efficiently.

Utilization of electron-donating α,β-unsaturated oximes: regioselective inverse 1,3-dipolar cycloaddition of nitrones

The cycloaddition of nitrones with α,β-unsaturated carbonyl compounds (enones) afforded predominantly 4-acylisoxazolidines, whereas the cycloaddition of the corresponding oximes afforded 5-iminoisoxazolidines. This inverse regioselection is due to HOMO activation by the oxime functionality.

A molecular electron density theory study of the [3 + 2] cycloaddition reaction of nitrones with ketenes

The zw-type 32CA reactions of nitrones with ketenes are controlled by the nucleophilic character of the nitrone and the electrophilic character of the ketene. They are chemo- and regio-selective and the use of electrophilic ketenes changes the mechanism from one-step to two-step.

Synthesis of dendrobatid alkaloid (+)-167B and (+)-209D and the investigation of diastereoselectivity using DFT calculations

The synthesis of dendrobatid alkaloid (+)-167B and (+)-209D has been developed on the basis of the effective preparation of chiral tropinone 7-azabicyclo[3.2.1]octan-6-ol.

New mechanistic interpretations for nitrone reactivity

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

Synthesis of Aminoindolizidines through the Chemoselective and Diastereoselective Catalytic Hydrogenation of Indolizines

Indolizidines are bioactive heterocyclic compounds of great potential normally prepared following multistep routes. However, to the best of our knowledge, the synthesis of 1-aminoindolizidines has never been reported. Herein, 1-(dialkylamino)-3-substituted indolizidines have been straightforwardly synthesized using an atom-economic protocol that involves a copper-catalyzed three-component synthesis of indolizines followed by heterogeneous catalytic hydrogenation. The latter was found to be chemoselective using platinum(IV) oxide as the catalyst at 3.7 atm, providing the aminoindolizidines in modest-to-high yields (35-95%) and high diastereoselectivity (92:8 to >99:1). It has been experimentally demonstrated that the hydrogenation occurs through the intermediate 5,6,7,8-tetrahydroindolizine, which contains a pyrrole moiety. Moreover, the diastereomerically pure 1-(dibenzylamino)-3-substituted indolizidines could be further transformed into the corresponding monobenzylated or fully debenzylated aminoindolizidines by selective hydrogenolysis catalyzed by Pt/C or Pd/C, respectively, under ambient conditions.

Three-component oxysulfenylation reaction: two simple and convenient approaches to β-alkoxy sulfides

An unprecedented method for the synthesis of β-alkoxy sulfides via a NaI/HBr-mediated three-component oxysulfenylation reaction of alkenes with arylsulfinic acids and alcohols is reported. Furthermore, I2-promoted oxysulfenylation of alkenes using sodium arylsulfinates instead of arylsulfinic acids to synthesise various β-alkoxy sulfides is also described.

First total synthesis of (+)-broussonetine W: glycosidase inhibition of natural product & analogs

Broussonetine W and its 11 analogues have been first synthesized from cyclic nitrones and assayed as potential gycosidase inhibitors.

Visible light active CdS nanorods: one-pot synthesis of aldonitrones

A new CdS nanorod was synthesized and it was used in the one-pot synthesis of aldonitrones under blue LED irradiation.

New developments of ketonitrones in organic synthesis

Various vinyl ketonitrones are prepared using new strategies and were applied to synthesize heterocycles in organic synthesis.

Diastereoselective Synthesis of Fluorine-Containing Pyrrolizidines via Triphenylcarbenium Tetrafluoroborate-Promoted Carbofluorination of N-3-Arylpropargylpyrrolidine-Tethered Tertiary Allylic Alcohols

Inexpensive and air stable triphenylcarbenium tetrafluoroborate efficiently promoted the carbofluorination of N-arylpropargylpyrrolidines bearing a tertiary allylic alcohol tether at the 2-position of the pyrrolidine ring to provide 1-isobutenyl-2-(fluoro(phenyl)methylenylhexahydro-1H-pyrrolizidines in a stereoselective fashion. When subjected to bis(trifluoromethane)sulfonamide, the same substrates underwent cycloisomerization reaction within minutes to generate 1-isobutenyl-2-benzoylhexahydro-1H-pyrrolizidines with excellent stereoselectivity.

Oxidation of N,N-Disubstituted Hydroxylamines to Nitrones with Hypervalent Iodine Reagents

Hypervalent iodine compounds are viable reagents for the oxidation of N,N-disubstituted hydroxylamines to the corresponding nitrones, with IBX performing best. The procedure is very simple and user-friendly and affords the target compounds with high efficiency and regioselectivity, highlighting IBX as the reagent of choice for preparation of aldonitrones from nonsymmetric hydroxylamines. Evidence for a mechanism involving nitrogen to iodine coordination has been collected.

Dehydrogenative TEMPO-Mediated Formation of Unstable Nitrones: Easy Access to N-Carbamoyl Isoxazolines

N-carbamoyl nitrones represent an important class of reagents for the synthesis of a variety of natural and biologically active compounds. These compounds are generally converted into valuable 4-isoxazolines upon cyclization reaction with dipolarophiles. However, these types of N-protected nitrones are highly unstable, which limits their synthesis, storage and practical use, enforcing alternative lengthy or elaborated synthetic routes. In this work, a 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO)-mediated formal "dehydrogenation" of N-protected benzyl-, allyl- and alkyl-substituted hydroxylamines followed by in situ trapping of the generated unstable nitrones into N-carbamoyl 4-isoxazolines is presented. A plausible mechanism is also proposed, in which the dipolarophile shows an important assistant role in the generation of the active nitrone intermediate. This simple protocol avoids the problematic isolation of N-carbamoyl protected nitrones, providing new synthetic possibilities in isoxazoline chemistry.

Synthesis of tricyclic indolizidines from ethyl isocyanoacetate

Tricyclic indolizidines were synthesized in good yields from commercially available ethyl isocyanoacetate by a novel sequential alkylation, thiol-mediated radical cyclization, N-alkylation, and microwave-assisted Pauson-Khand reaction.