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

Au-catalyzed ring-opening reactions of 2-(1-alkynyl-cyclopropyl)pyridines with nucleophiles

A novel method for the C-C bond cleavage of cyclopropanes was developed by gold-catalyzed cycloisomerization of 2-(1-alkynyl-cyclopropyl)pyridine with nucleophiles, which provides efficient access to structurally diverse indolizines under mild conditions. A series of N-, C- and O-based nucleophiles were involved in this reaction to afford the corresponding indolizines in modest to excellent yields.

One-pot synthesis of highly substituted N-fused heteroaromatic bicycles from azole aldehydes

An efficient route to substituted N-fused aromatic heterocycles, including indolizines, imidazo[1,2-a]pyridines, and imidazo[1,5-a]pyridines from azole aldehydes, is reported. Wittig olefination of the aldehydes with fumaronitrile and triethylphosphine affords predominantly E-alkenes that undergo rapid cyclization upon treatment with a mild base. Substituent control of the 1-, 2-, and 3-positions of the resulting heteroaromatic bicycles is shown. Alternatively, the isolable E-alkene undergoes selective alkylation with electrophiles, followed by in situ annulation to indolizines additionally substituted at the 6-position.

Palladium-catalyzed direct and regioselective C–H acyloxylation of indolizines

A direct and highly regioselective C1-acyloxylation of indolizines was developed via palladium-catalyzed C–H functionalization. In this reaction, the regioselectivity was achieved in the absence of a directing group.

Palladium-catalyzed highly efficient synthesis of functionalized indolizines via cross-coupling/cycloisomerization cascade

Pd-catalyzed cross-coupling/cycloisomerization of 3-(2-pyridyl) propargyl carbonates with organoboronic acids provides a straightforward route for the synthesis of 1,3-disubstituted indolizines.

Indolizine synthesis via Cu-catalyzed cyclization of 2-(2-enynyl)pyridines with nucleophiles

Cu-catalyzed cyclization of 2-(2-enynyl)pyridines with various nucleophiles was developed, providing efficient access to indolizine derivatives.

A copper-catalyzed domino reaction to construct functionalized indolizinones

An efficient one-step copper-catalyzed domino reaction to 3,8α-disubstituted indolizinone derivatives has been developed for the first time, in which cheap and readily available pyridine ketones and terminal alkynes are the starting materials.

Silver-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides: divergent synthesis of indolizines and pyrroles

Divergent syntheses of indolizines and 2,4-disubstituted pyrroles by the silver-catalyzed cyclization of 2-pyridyl alkynyl carbinols with isocyanides are reported.

(11aS)-1,5,11,11a-Tetra-hydro-1-benzo-thieno[3,2-f]indolizin-3(2H)-one

The absolute configuration of the title compound, C₁₄H₁₃NOS, was assigned from the synthesis and confirmed by the structure determination. There are two independent mol­ecules in the asymmetric unit. The central six-membered ring of the indolizine moiety adopts an envelope conformation, with the greatest deviations from the mean planes being 0.569 (3) and 0.561 (3) Å for the indolizine bridgehead C atoms of the two mol­ecules. The benzothieno ring attached to the indolizine ring system is planar to within 0.015 (3) Å in both mol­ecules. In the crystal, weak C—H⋯O and C—H⋯π inter­actions lead to the formation of a three-dimensional framework structure.

Protic-solvent-mediated cycloisomerization of quinoline and isoquinoline propargylic alcohols: syntheses of (±)-3-demethoxyerythratidinone and (±)-cocculidine

Putting the "benz" in indolizinones: A cycloisomerization approach to benz[g]indolizinones and benz[e]indolizinones provides the first general route to these unique azacycles (see example). The utility of the benzindolizinone products was demonstrated by the application of this method to the total synthesis of the Erythrina alkaloids 3-demethoxyerythratidinone and cocculidine.

5''-Benzyl-idene-1''-methyl-1'-phenyl-1',2',3',5',6',7',8',8a'-octa-hydro-dispiro[acenaphthyl-ene-1,3'-indolizine-2',3''-piperidine]-2,4''(1H)-dione

In the title compound, C(37)H(34)N(2)O(2), the pyridinone ring adopts a half-chair conformation. In the octa-hydro-indolizine fused-ring system, the piperidine ring is in a chair conformation and the pyrrole ring is twisted about the N-C(piperidine) bond. The mol-ecular structure features a weak intra-molecular C-H⋯O inter-action.

ORGANOCOPPER-MEDIATED TWO-COMPONENT SN2'-SUBSTITUTION CASCADE TOWARDS N-FUSED HETEROCYCLES

Organocuprates efficiently undergo reaction with heterocyclic propargyl mesylates at low temperature to produce N-fused heterocycles. The copper reagent plays a "double duty" in this cascade transformation, which proceeds through an S_N2'-substitution followed by a consequent cycloisomerization step.

Indolizinones as synthetic scaffolds: fundamental reactivity and the relay of stereochemical information

Indolizinones are under-explored N-heterocycles that react with exquisite chemo- and stereoselectivity. An exploration of the fundamental reactivity of these azabicycles demonstrates the potential to relay stereochemical information from the ring-fusion to newly formed stereocenters on the bicyclic core. The indolizinone diene undergoes selective hydrogenation and readily participates in Diels-Alder cycloadditions as well as ene reactions. The vinylogous amide embedded in the five-membered ring is resistant to reaction when the diene is in place. However, removal of the diene allows for diastereoselective hydrogenation of, and 1,4-additions to, the vinylogous amide. These fundamental reactions with indolizinones have provided a structurally diverse array of products that hold promise in the context of natural product synthesis.

Pyrrole-based scaffolds for turn mimics

Two amino acid derived synthons were combined to give homopropargylic amines 2. Platinum dichloride was used to cyclize these intermediates into pyrroles 3 which collapsed to the target secondary structure mimics 1 on treatment with base. Side chains of these compounds overlay with an idealized type 1 β-turn and with an inverse γ-turn.

On the validity of Au-vinylidenes in the gold-catalyzed 1,2-migratory cycloisomerization of skipped propargylpyridines

A mechanism of the Au-catalyzed cycloisomerization of propargylpyridines has been investigated. Both DFT computational and experimental results strongly support generation of a Au-carbene via a cyclization/proton transfer sequence over the previously proposed path involving a Au-vinylidene intermediate. For the β-Si-substituted Au-carbene (G = SiR(3)), a 1,2-Si migration was shown to be kinetically favored over a 1,2-H shift. This study highlights the importance of alternative pathways that could explain reactivities commonly attributed to an alkyne-vinylidene isomerization in Au catalysis.

Palladium-catalyzed intramolecular carbopalladation/cyclization cascade: access to polycyclic N-fused heterocycles

An efficient palladium-catalyzed intramolecular carbopalladation/cyclization cascade toward tetra- and pentacyclic N-fused heterocycles has been developed. This transformation proceeds via the palladium-catalyzed coupling of aryl halides with internal propargylic esters or ethers followed by the 5-endo-dig cyclization leading to polycyclic pyrroloheterocycles in moderate to excellent yields.

Two-component approach toward a fully substituted N-fused pyrrole ring

An efficient two-component palladium-catalyzed arylation/cyclization cascade approach toward a variety of N-fused pyrroloheterocycles has been developed. This transformation proceeds via the palladium-catalyzed coupling of aryl halides with propargylic esters or ethers followed by the 5-endo-dig cyclization leading to highly functionalized pyrroloheterocycles in good to excellent yield.