Synthesis of pyrroles, indolizines, and furans via metal-catalyzed cycloisomerization approaches.

Synthesis of pyrroles, indolizines, and furans via metal-catalyzed cycloisomerization approaches

A Novel Cu-Assisted Cycloisomerization of Alkynyl Imines: Efficient Synthesis of Pyrroles and Pyrrole-Containing Heterocycles

A Novel Cu-Assisted Cycloisomerization of Alkynyl Imines:  Efficient Synthesis of Pyrroles and Pyrrole-Containing Heterocycle

1,2-Halogen Migration in Haloallenyl Ketones: Regiodivergent Synthesis of Halofurans

Selective 1,2-iodine, bromine, and chlorine migration in haloallenyl ketones in the presence of Au catalyst has been demonstrated. It was found that, depending on the nature of the Au catalyst used, either selective bromine migration or hydrogen shift occurs, leading to the formation of 3- or 2-bromofurans, respectively. Halirenium intermediate was proposed for the unusual 1,2-halogen migration. This cascade transformation allows for mild and efficient synthesis of various types of 3-halofurans

A Novel Cu-Assisted Cycloisomerization of Alkynyl Imines: Efficient Synthesis of Pyrroles and Pyrrole-Containing Heterocycles

A Novel Cu-Assisted Cycloisomerization of Alkynyl Imines:  Efficient Synthesis of Pyrroles and Pyrrole-Containing Heterocycle

Mechanistically Diverse Copper-, Silver-, and Gold-Catalyzed Acyloxy and Phosphatyloxy Migrations: Efficient Synthesis of Heterocycles via Cascade Migration/Cycloisomerization Approach

A set of cycloisomerization methodologies of alkynyl ketones and imines with concurrent acyloxy, phosphatyloxy, or sulfonyloxy group migration, which allow for the efficient synthesis of multisubstituted furans and N-fused heterocycles, has been developed. Investigation of the reaction course by way of employing 17O-labeled substrates allowed for elucidation of the mechanisms behind these diverse transformations. It was found that, while the phosphatyloxy migration in conjugated alkynyl imines in their cycloisomerization to N-fused pyrroles proceeded via a [3,3]-sigmatropic rearrangement, the analogous cycloisomerization of skipped alkynyl ketones proceeds through two consecutive 1,2-migrations, resulting in an apparent 1,3-shift, followed by a subsequent 1,2-migration through competitive oxirenium and dioxolenylium pathways. Investigations of the 1,2-acyloxy migration of conjugated alkynyl ketones en route to furans demonstrated the involvement of a dioxolenylium intermediate. The mechanism of cycloisomerization of skipped alkynyl ketones containing an acyloxy group was found to be catalyst dependent; Lewis and Brønsted acid catalysts caused an ionization/SN1‘ isomerization to the allene, followed by cycloisomerization to the furan, whereas transition metal catalysts evoked a Rautenstrauch-type mechanistic pathway. Furthermore, control experiments in the cycloisomerization of skipped alkynyl ketones under transition metal catalysis revealed that, indeed, these reactions were catalyzed by transition metal complexes as opposed to Brønsted acids resulting from hydrolysis of these catalysts with eventual water. Further synthetic utility of the obtained phosphatyloxy-substituted heterocycles was demonstrated through their efficient employment in the Kumada cross-coupling reaction with various Grignard reagents

Palladium-Catalyzed Arylation and Heteroarylation of Indolizines

A highly effective protocol for palladium-catalyzed selective arylation and heteroarylation of indolizines at C-3 has been developed. Mechanistic studies unambiguously support an electrophilic substitution pathway for this transformation

Synthesis and Evaluation of Fluorinated Aporphines: Potential Positron Emission Tomography Ligands for D₂ Receptors

The 2-fluoroalkoxy-substituted catechol-aporphines 6, 8a−f and 11-mono-hydroxyaporphines 11a−e were synthesized and found to have high in vitro affinity and selectivity for the dopamine D2 receptors. The catechol aporphines, 8b and 8d, and the monohydroxy aporphines, 11a−d, were identified as candidates for development as potential PET ligands

Metal-Catalyzed 1,2-Shift of Diverse Migrating Groups in Allenyl Systems as a New Paradigm toward Densely Functionalized Heterocycles

A general, mild, and efficient 1,2-migration/cycloisomerization methodology toward multisubstituted 3-thio-, seleno-, halo-, aryl-, and alkyl-furans and pyrroles, as well as fused heterocycles, valuable building blocks for synthetic chemistry, has been developed. Moreover, regiodivergent conditions have been identified for C-4 bromo- and thio-substituted allenones and alkynones for the assembly of regioisomeric 2-hetero substituted furans selectively. It was demonstrated that, depending on reaction conditions, ambident substrates can be selectively transformed into furan products, as well as undergo selective 6-exo-dig or Nazarov cyclizations. Our mechanistic investigations have revealed that the transformation proceeds via allenylcarbonyl or allenylimine intermediates followed by 1,2-group migration to the allenyl sp carbon during cycloisomerization. It was found that 1,2-migration of chalcogens and halogens predominantly proceeds via formation of irenium intermediates. Analogous intermediate can also be proposed for 1,2-aryl shift. Furthermore, it was shown that the cycloisomerization cascade can be catalyzed by Brønsted acids, albeit less efficiently, and commonly observed reactivity of Lewis acid catalysts cannot be attributed to the eventual formation of proton. Undoubtedly, thermally induced or Lewis acid-catalyzed transformations proceed via intramolecular Michael addition or activation of the enone moiety pathways, whereas certain carbophilic metals trigger carbenoid/oxonium type pathway. However, a facile cycloisomerization in the presence of cationic complexes, as well as observed migratory aptitude in the cycloisomerization of unsymmetrically disubstituted aryl- and alkylallenes, strongly supports electrophilic nature for this transformation. Full mechanistic details, as well as the scope of this transformation, are discussed