Tandem Michael Addition/Amino-Nitrile Cyclization from 2-Formyl-1,4-DHP in the Synthesis of Novel Dihydroindolizine-Based Compounds

Domino Approach to Heterocycles-Based Unsymmetrical Triarylmethanes through Heteroannulation of 2-(2-Enynyl)-pyridines with Enaminones

Herein, we report a copper-catalyzed protocol to access unsymmetrical triarylmethanes containing both indolizine and the chromone scaffolds in the same molecule via a 5-endo-dig cyclization of 2-(2-enynyl)-pyridines followed by reaction with 2-hydroxyaryl enaminones. A variety of 2-hydroxyaryl enaminones and 2-(2-enynyl)-pyridines were subjected to reaction under the optimal reaction conditions, and the respective triarylmethanes were obtained in good to excellent yields.

Smart and concise entry to chiral spiro[cyclopentane-indolizidine]-tetraol diastereomers as a new aza-spirocyclic framework

Synthesis of chiral oxacarba-spiroindolizidine tetraols was achieved by alkene cis-dihydroxylation, diol protection, and lactam carbonyl reduction followed ultimately by acetonide deprotection.

Metal-Free Cascade Approach toward Polysubstituted Indolizines from Chromone-Based Michael Acceptors

An efficient cascade transformation toward indolizine-based molecules has been developed. This process leads to the rapid construction of two C-N bonds and one C-C bond without the need of any metal catalysis. The approach involves easily accessible chromone-based Michael acceptors and propargylamine derivatives as starting materials. This cascade constitutes a novel and very competitive alternative to the well reported strategies using pyridine or pyrrole derivatives for accessing the indolizine ring with substituents at uncommon C-positions.

Enantioselective Formal [4+2] Cycloadditions to 3-Nitroindoles by Trienamine Catalysis: Synthesis of Chiral Dihydrocarbazoles

The first enantioselective formal [4+2] cycloadditions of 3-nitroindoles are presented. By using 3-nitroindoles in combination with an organocatalyst, chiral dihydrocarbazole scaffolds are formed in moderate to good yields (up to 87%) and enantioselectivities (up to 97% ee). The reaction was extended to include enantioselective [4+2] cycloadditions of 3-nitrobenzothiophene. The reaction proceeds through a [4+2] cycloaddition/elimination cascade under mild reaction conditions. Furthermore, a diastereoselective reduction of an enantioenriched cycloadduct is presented. The mechanism of the reaction is discussed based on experimental and computational studies.

Synthesis of dihydrothiophenes or spirocyclic compounds by domino reactions of 1,3-thiazolidinedione

The domino reactions of 1,3-thiazolidinedione, malononitrile, and aromatic aldehydes in the presence of different organic amines were studied. Secondary amines such as pyrrolidine, piperidine, morpholine, and dimethylamine and primary amines such as benzylamine yield dihydrothiophene derivatives through a domino ring-opening/recyclization reaction of 1,3-thiazolidinedione. Bulky diethylamine, diisopropylamine, and 1,4-diazabicyclo[2.2.2]octane give spirocyclohexano-1,3-thiazole through a double Michael addition/spirocyclization reaction.

Multisubstituted N-fused heterocycles via transition metal-catalyzed cycloisomerization protocols

Two complementary protocols for assembly of multisubstituted N-fused heterocycles have been developed. It was demonstrated that 1,3-disubstituted N-fused heterocycles, including indolizines, pyrroloquinoxalines, and pyrrolothiazoles can easily be synthesized via an exceptionally mild and efficient method involving a novel silver-catalyzed cycloizomerization of propargyl-containing heterocycles. Alternatively, 1,2-disubstituted heterocycles can be accessed through the novel cascade transformation involving an alkyne-vinylidene isomerization with concomitant 1,2-shift of hydrogen, silyl, stannyl, or germyl groups. This mild and simple method allows for selective and highly efficient synthesis of indolizines, pyrroloisoquinolines, pyrroloquinoxalines, pyrrolopyrazines, and pyrrolothiazoles.

Pt-catalyzed cyclization/migration of propargylic alcohols for the synthesis of 3(2H)-furanones, pyrrolones, indolizines, and indolizinones

Several heterocycles such as furanones, pyrrolones, and indolizines, which are of pharmacological importance, are easily accessed via the Pt(II)-catalyzed heterocyclization/1,2-migration of propargylic ketols or hydroxy imine derivatives. This method sidesteps the challenges of traditional heteroaromatic oxygenation strategies such as regioselectivity and functional group tolerance in the syntheses of these heterocycles.

Ring Contractions of 3-Azido-4H-quinolizin-4-ones and 3-Azido-4H-azino[1,2 - x]pyrimidin-4-ones: a Novel Approach to 3-Aminoindolizines and their Aza Analogues

Thermal transformations of 3-azido-4H-quinolizin-4-ones 4a,b and 3-azido-4H-azino[1,2–x]pyrimidin-4-ones 4c,d, available from the corresponding heteroarylamines 2a–d, were studied. The reaction products were mostly dependent on the solvent. Thus, heating of 3-azido-1-cyano-4H-quinolizin-4-one (4a) in toluene afforded 2-(pyridin-2-yl)fumaronitrile 5a, whereas 3-amino-1-cyano-4H-quinolizin-4-one (8) was obtained on treatment of 4a in a mixture of toluene and trifluoroacetic anhydride. However, heating of 4a in acetic anhydride and in acetic acid resulted in a ring contraction to produce 3-(diacetylamino)indolizine-1-carbonitrile 6a and 3-(acetylamino)indolizine-1-carbonitrile 7a, respectively. Similarly, ring contractions took place on heating ethyl 3-azido-4-oxo-4H-quinolizin-1-carboxylate 4b and 3-azido-4H-azino[1,2–x]pyrimidin-4-ones 4c,d in acetic anhydride or acetic acid to produce the N-acetylated 3-aminoindolizine derivatives 6b, 7b and 3-aminoimidazo[1,2–x]azine derivatives 6c,d in 30–89% yields. The structures of compounds 5–8 were determined by NMR spectroscopy and X-ray diffraction.

Base- and ligand-free room-temperature synthesis of N-fused heteroaromatic compounds via the transition metal-catalyzed cycloisomerization protocol

A new practical method for the synthesis of N-fused heterocycles via the transition metal-catalyzed cycloisomerization of heterocyles possessing a propagyl group has been developed. This very mild, base- and ligand-free method allows for the synthesis of diverse fused heterocyclic cores in good to excellent yields.

Catalytic Oxidation of 1,4-dihydropyridins by Tetrabutylammonium Periodate in the presence of Manganese Amino Acid Schiff Base

The amino acid Schiff base manganese complex (Tryp–Mn); 1, was prepared with L-tryptophan, salicylaldehyde and Mn(OAc)2.4H2O in methanol. In the presence of 1, 1,4-dihydropyridines (1,4-DHPs) were oxidised by n-Bu4NIO4 to the corresponding pyridine derivatives in high yields. Imidazole was used as an additive to improve the catalytic oxidation.

Pt-catalyzed cyclization/1,2-migration for the synthesis of indolizines, pyrrolones, and indolizinones

Indolizine, pyrrolone, and indolizinone heterocycles are easily accessed via the Pt(II)-catalyzed cycloisomerization or a tandem cyclization/1,2-migration of pyridine propargylic alcohols and derivatives. This method provides an efficient synthesis of highly functionalized heterocycles from readily available substrates. [reaction: see text]