The Synthetic Routes of 8-Hydroxy-1H-quinolin-2-one

The synthesis of 8-hydroxy-1H-quinolin-2-one, commonly referred to as quinoline-8-carboxaldehyde, is an important step in the chemical industry.
This molecule finds extensive use in various applications, including the synthesis of pharmaceuticals, agrochemicals, and other specialty chemicals.

There are several synthetic routes available for the preparation of quinoline-8-carboxaldehyde.
Some of the most commonly used methods are outlined below:

1. Reaction of tropontone with quinoline
   Tropontone is a precursor to quinoline-8-carboxaldehyde and can be synthesized by several methods.
   One common synthesis method involves the reaction of tropone with quinoline in the presence of an acid catalyst.
   The reaction proceeds through a series of steps, including the formation of a beta-hydroxyethylidine intermediate, followed by elimination of water and dehydration to form tropontone.
   The product can then be hydrolyzed to form quinoline-8-carboxaldehyde.
   
2. Reduction of quinolone-5-carboxylic acid
   Quinolone-5-carboxylic acid can be reduced to form quinoline-8-carboxaldehyde.
   The reduction can be carried out using various reagents, including lithium aluminum hydride (LiAlH4), hydrogen gas, and borane-pyridine complexes.
   The reduction process involves the removal of carbon dioxide from the carboxylic acid to form the aldehyde.
   
3. Oxidation of anthranilic acid
   Anthranilic acid can be oxidized to form quinoline-8-carboxaldehyde.
   The oxidation can be carried out using various reagents, including chromic acid, lead dioxide, and potassium permanganate.
   The oxidation process involves the addition of oxygen to the benzene ring of anthranilic acid to form the carboxaldehyde.
   
4. Reaction of 2-aminotoluene with hydrazine
   2-Aminotoluene can be converted to quinoline-8-carboxaldehyde by reaction with hydrazine.
   The reaction proceeds through a series of steps, including the formation of a diazonium salt intermediate, followed by reduction to form the carboxaldehyde.
   
5. Reaction of 2-chlorotoluene with sodium hydroxide
   2-Chlorotoluene can be converted to quinoline-8-carboxaldehyde by reaction with sodium hydroxide.
   The reaction involves the substitution of chlorine with sodium hydroxide, followed by oxidation to form the carboxaldehyde.
   

In conclusion, the synthesis of quinoline-8-carboxaldehyde is an important step in the chemical industry, and there are several synthetic routes available for its preparation.
The method chosen depends on several factors, including the starting materials, the desired yield, and the cost of the reaction.
The reaction can be carried out using various reagents and conditions, and the product can be purified and isolated using various methods.