A concise review on some synthetic routes and applications of pyridine scaffold compounds

Article history: Received March20, 2021 Received in revised form May12, 2021 Accepted July 6, 2021 Available online July 6,2021 Different methods for the synthesis of pyridine derivatives as well as the chemical reactivity profiles and structures of these substances are reviewed. The utility of these compounds as precursors is emphasized in the synthesis of many heterocycles that are pharmacologically active organic compounds and agrochemicals. This review results from a literature survey containing some synthetic methods and applications of pyridine derivatives. © 2021 by the authors; licensee Growing Science, Canada.


Introduction
Pyridine derivatives are an important class of azaheterocycles found in many natural products, active pharmaceuticals and functional materials. 1-8 Synthetic routes appeared from the latter half of the 19 th century although pyridine derivatives were of little commercial importance for decades and required quantities could be obtained from coal tar distillation.
Pyridines came to prominence in the 1930s with the recognition of the importance of niacin 1 for the prevention of dermatitis and dementia. In the 1940s a new major application was discovered for 2vinylpyridine 2 as a constituent in latex. Demand for 2-picoline 3 for latex production outstretched its availability from coal tar sources and so researchers at Reilly industries developed an industrial synthesis of 2-and 4-picolines 3, 4 by vapour phase catalytic reactions. The demand for pyridine and its derivatives has further increased over the last 50 years by the discovery of many bioactive pyridinecontaining compounds (Fig. 1). 9 Many pyridine-based alkaloid natural products are derivatives of nicotinic acid 5. 11 Nicotine 6 is formed by the incorporation of a pyrrolidine moiety derived from L-ornithine onto the molecular frame work of nicotinic acid. Like nicotine, similar alkaloids, including anabasine 7, ricinine 8, and arecoline 9, all originate from nicotinic acid (Fig. 2). 11

Fig. 2. Structure of compounds 5-9.
A large number of reports concerning the chemistry and applications of numerous pyridines as well as their condensed derivatives have been published during the last five decades, 12-15 and a very lengthily review will be required to cover them. So, this work will be focused only on special interesting aspects related the synthesis and applications of pyridine derivatives.

From other ring system.
Pyrylium salts, for example, 84, were efficiently converted into the 2,4,6-trisubstituted pyridine 85, as shown by Balaban in 1969 (Fig. 26)  Transformation of 2-amino-4H-pyrans 86 into the corresponding 2-pyridinones 65 was achieved when the pyrans were allowed to react with nitrosylsulfuric acid in acetic acid solution. The reaction can be understood by assuming the formation of an open chain intermediate 87 due to the nucleophilic attack of water to the protonated pyran ring followed by cyclization and spontaneous dehydrogenation to furnish the product 88 (Fig. 27). 50 Fig. 27. Synthsis of compound 88.

Applications of pyridine derivatives
Thieno[2,3-b]pyridines systems -as an example of pyridine derivatives-are proved to be an interesting class of heterocycles. Most of them are reported to possess anticipated biological activities.

Conclusions
Pyridine, a six membered nitrogen bearing heterocyclic scaffold, can be found in a number of pharmacologically efficient structures. There has been an increasing attention in the development of bioactive compounds, bearing the heterocyclic, pyridine. The data studied in this review obviously determine the great synthetic potential of pyridine scaffold. This recommends that pyridine scaffold can be principally encouraging synthons in synthesis of novel greatly effective pharmaceuticals.