To design a heterocyclic compound using pyridine as a starting material and incorporating at least one nitrogen atom in the heterocyclic ring, we can synthesize a compound called 2,3-dihydropyrido[1,2-a]pyrimidin-4-one. This compound has a fused pyrimidine ring with pyridine, and the heterocyclic ring contains two nitrogen atoms.Here is the synthesis of 2,3-dihydropyrido[1,2-a]pyrimidin-4-one from pyridine:1. Bromination of pyridine:Reagents: N-bromosuccinimide NBS , benzoyl peroxide BPO, as a radical initiator Product: 2-bromopyridineMechanism: The NBS reacts with BPO to generate a bromine radical. This bromine radical then reacts with the pyridine at the 2-position to form 2-bromopyridine.2. Formation of imine:Reagents: Ammonia NH3 , methanol MeOH, as a solvent Product: 2- pyridin-2-yl ethanamineMechanism: The 2-bromopyridine reacts with ammonia through nucleophilic substitution, replacing the bromine atom with an amino group -NH2 to form 2- pyridin-2-yl ethanamine.3. Intramolecular cyclization and dehydration:Reagents: Phosphorus oxychloride POCl3 Product: 2,3-dihydropyrido[1,2-a]pyrimidin-4-oneMechanism: The phosphorus oxychloride acts as a dehydrating agent, promoting the intramolecular cyclization of 2- pyridin-2-yl ethanamine. The amino group attacks the pyridine ring at the 3-position, forming a new nitrogen-nitrogen bond. The dehydration step removes a water molecule, leading to the formation of the fused pyrimidine ring and the final product, 2,3-dihydropyrido[1,2-a]pyrimidin-4-one.In summary, the synthesis of 2,3-dihydropyrido[1,2-a]pyrimidin-4-one from pyridine involves three main steps: bromination, formation of imine, and intramolecular cyclization with dehydration. The reagents used in each step play crucial roles in promoting the desired reactions and driving the synthesis towards the target heterocyclic compound.