Iodine-catalyzed aerobic oxidative formal [4+2] annulation for the construction of polyfunctionalized pyridines

Ketones as strategic building blocks for the synthesis of natural product-inspired compounds

Natural product-inspired compound collections serve as excellent sources for the identification of new bioactive compounds to treat disease. However, such compounds must necessarily be more structurally-enriched than traditional screening compounds, therefore inventive synthetic strategies and reliable methods are needed to prepare them. Amongst the various possible starting materials that could be considered for the synthesis of natural product-inspired compounds, ketones can be especially valuable due to the vast variety of complexity-building synthetic transformations that they can take part in, their high prevalence as commercial building blocks, and relative ease of synthesis. With a view towards developing a unified synthetic strategy for the preparation of next generation bioactive compound collections, this review considers whether ketones could serve as general precursors in this regard, and summarises the opulence of synthetic transformations available for the annulation of natural product ring-systems to ketone starting materials.

TMSOTf-mediated Kröhnke pyridine synthesis using HMDS as the nitrogen source under microwave irradiation

An efficient protocol for the preparation of pyridine skeletons has been successfully developed involving the TMSOTf/HMDS (trifluoromethanesulfonic acid/hexamethyldisilane) system for the intermolecular cyclization of chalcones under MW (microwave) irradiation conditions. This method provides a facile approach to synthesize 2,4,6-triaryl or 3-benzyl-2,4,6-triarylpyridines in good to excellent yields. Interestingly, the 2,6-diazabicyclo[2.2.2]oct-2-ene core was obtained by changing the acid additive to Sn(OTf)₂, and the desired product was also confirmed using X-ray single-crystal diffraction analysis.

A Kröhnke pyridine synthetic route towards functionalized TAPs and 3-benzyl TAPs has been established using TMSOTf/HMDS under microwave irradiation conditions.

Pd(II)-Catalyzed Synthesis of Alicyclic[b]-Fused Pyridines via C(sp2)-H Activation of α,β-Unsaturated N-Acetyl Hydrazones with Vinyl Azides

A new synthetic protocol for alicyclic[b]-fused pyridines with complete regioselectivity from α,β-unsaturated N-acetyl hydrazones and vinyl azides via Pd(II)-catalyzed C-H activation/cyclization/aromatization strategy has been described. A series of five- to eight-membered alicyclic[b]-fused pyridines were prepared in a one-step manner with wide substrate scope and good functional group tolerance.

An Isoxazole Strategy for the Synthesis of Fully Substituted Nicotinates

A four-step quasi one-pot procedure for the preparation of fully substituted nicotinates from ketone enamines and 4-methylideneisoxazol-5-ones has been developed. The reaction sequence involves (1) reaction of 4-methylideneisoxazol-5-ones with ketone enamines with the formation of isoxazole-5-ols, (2) their O-methylation with diazomethane, (3) hydrogenative cleavage of the O-N bond in 5-methoxyisoxazoles under action of Mo(CO)₆/H₂O and simultaneous isomerization and condensation of the formed enamines, with the formation of dihydropyridines, and (4) aromatization of the latter.

Revisiting applications of molecular iodine in organic synthesis

Molecular iodine contributes significantly to organic transformations in synthetic organic chemistry. It works effectively due to its mild Lewis acidic character, ability as an oxidizing agent, good moisture stability, and easy availability.

Synthesis and application of chitosan supported vanadium oxo in the synthesis of 1,4-dihydropyridines and 2,4,6-triarylpyridines via anomeric based oxidation

1,4-Dihydropyridines and 2,4,6-triarylpyridines were synthesized via anomeric based oxidation using chitosan supported vanadium oxo.

One-pot synthesis of 2,4,6-triarylpyridines from β-nitrostyrenes, substituted salicylic aldehydes and ammonium acetate

The diversity-oriented synthesis of 2,4,6-trisubstituted pyridines via K₂CO₃-promoted multicomponent reactions of the β-nitrostyrenes, available substituted salicylic aldehydes and ammonium acetate have been developed.