Palladium-Catalyzed Direct α-Ketoesterification of 8-Methylquinoline Derivatives with α-Ketoacids via Dehydrogenation Coupling Reaction

Transition Metal-Catalyzed C-H Activation/Functionalization of 8-Methylquinolines

8-Methylquinoline is regarded as an ideal substrate to participate in diversely C(sp3)-H functionalization reactions. The presence of the chelating nitrogen atom enables 8-methylquinoline to easily form cyclometallated complexes with various transition metals, leading to the selective synthesis of functionalized quinolines. Considering the great importance of quinoline cores in medicinal chemistry, in this review article, we have covered the publications related to the C-H activation and functionalization of 8-methylquinoline under transition metal catalysis during the last decade.

Light Induced Diastereoselective Ketoesterification To Access 6,5-Fused Tetrahydrobenzofuranones in Batch and Continuous Flow Conditions

Ketoesterification stands as a pivotal technique in organic synthesis, particularly due to its essential role in the construction of numerous natural products and bioactive compounds. In this study, we have successfully accomplished a visible-light-induced cyclization and diastereoselective direct ketoesterification of cyclohexadienones, facilitating access to cis 6,5-fused tetrahydrobenzofuranone derivatives. The utilization of TEMPO radical quenching experiments has provided insights, suggesting an ionic mechanism underlying this methodology. Additionally, the regioselective addition of 2-oxo-2-phenylacetate to the least hindered side in a cis-selective fashion makes this protocol more appealing toward natural product development. Incorporation of a continuous flow reaction into the batch protocol has notably bolstered the efficiency and reaction rate. Furthermore, the demonstration of gram-scale reactions in the flow setup and synthetic utility with NaOH underscore the scalability and practical applicability of this approach.

Switchable, Reagent-Controlled C(sp3)-H Selective Iodination and Acetoxylation of 8-Methylquinolines

An efficient Pd-catalyzed C(sp³)-H selective iodination of 8-methylquinolines is reported herein for the first time. Because of the versatility of organic iodides, the method offers a facile access to various C8-substituted quinolines. By slightly switching the reaction conditions, an efficient C(sp³)-H acetoxylation of 8-methylquinolines has also been enabled. Both approaches feature mild reaction conditions, good tolerance of functional groups, and a broad substrate scope.

Pd(II)-Catalyzed Arylation/Oxidation of Benzylic C-H of 8-Methylquinolines: Access to 8-Benzoylquinolines

An efficient access to 8-benzoylquinoline was developed by a sequential arylation/oxidation of 8-methylquinolines with aryl iodides in the presence of Pd(OAc)₂. This transformation demonstrates good tolerance of a wide range of functional groups on aryl iodides, providing good to excellent yields of 8-benzoylquinolines.

Recent trends in catalytic sp3 C-H functionalization of heterocycles

Heterocycles are a ubiquitous substructure in organic small molecules designed for use in materials and medicines. Recent work in catalysis has focused on enabling access to new heterocycle structures by sp³ C-H functionalization on alkyl side-chain substituents-especially at the heterobenzylic position-with more than two hundred manuscripts published just within the last ten years. Rather than describing in detail each of these reports, in this mini-review we attempt to highlight gaps in existing techniques. A semi-quantitative overview of ongoing work strongly suggests that several specific heterocycle types and bond formations outside of C-C, C-N, and C-O have been almost completely overlooked.