Six– and seven–membered heterocycles with two heteroatoms – pyrimidine, diazepine and their derivatives.
Pyrimidine- nucleus is found in many biologically important compounds. Pyrimidine is a six-membered aromatic heterocycle. Its electronic structure is similar to that of benzene and pyridine.
Pyrimidine derivatives.
Pyrimidine and purine, described later, are heterocyclic fundamental structural components of a major class of biomolecules - nucleic acids. Nucleic acids, deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), are the chemical carriers of a cell’s genetic information.
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) consist of two very long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds.
The pyrimidine bases are cytosine, uracil, and thymine.
Cytosine is a pyrimidine substituted with an amino group at position C(4) and a keto group at position C(2) (at neutral pH). Cytidine triphosphate (CTP), it can act as a co-factor to enzymes and can transfer a phosphate in conversion of adenosine diphosphate (ADP) into adenosine triphosphate (ATP). In DNA and RNA, cytosine is paired with guanine using three hydrogen bonds.
Uracil has pyrimidine core substituted with two keto groups at C (2) and C (4). All three of the tautomeric structures shown are aromatic. Uracil is found in RNA only. Its base pairs with adenine using two hydrogen bonds (dashed lines) and replaces thymine during DNA transcription.
Thymine is one of the four nucleobases in DNA. Thymine is like uracil having one additional methyl group at C (5), therefore sometimes referred to as a 5-methyluracil. The most stable in physiological conditions is the bis-lactam tautomer with pyrimidine core substituted with two keto groups at C(2) and C(4).All three tautomeric forms are Aromatic systems. Thymine pairs with adenine through two hydrogen bonds (dashed lines) which stabilize the nucleic acid structure.
Notice! – all oxygenated pyrimidines in the nucleobases exist in keto forms.
Barbituric acid.
described as 2,4,6-trihydroxypyrimidine (6- hydroxy uracil) it exists in bis-lactam-keto form.
Barbituric acid is an odorless powder soluble in water.
Barbituric acid is the parent compound of barbiturate drugs but barbituric acid itself is not pharmacologically active. Barbiturates are classified as sedative-hypnotic agents. They are drugs that act as CNS depressants (Decreasing the responsiveness of central nervous system and promoting sleep).
Long-acting barbiturates such as Luminal and Mephobarbital when taken at bedtime, they help treat insomnia (so called “sleeping pills”), and when taken during the day they have sedative effects that can aid in the treatment of tension and anxiety.