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Synthesis, Reactions and Medicinal Uses of Furan

Palladium and charcoal are used for decarboxylation to form furan from furfural in the vapor phase.

Furan

Synthesis

Palladium and charcoal are used for decarboxylation to form furan from furfural in the vapor phase.


Copper-catalyzed oxidation can be used to convert 1,3-butadiene to furan.



Paal-Knorr synthesis - A cyclization of 1,4-diketones followed by dehydration produces furans under non-aqueous acidic conditions.


Fiest – Binary synthesis - An ammonia or pyridine base converts a α - halo ketone (or diketone) to a β - ketone (or diketone).



Allenyl ketones are converted into furans when heated in H3CN with silver nitrate or silver boron tetrafluoride.



Ring expansion - Upon being exposed to sulfuric acid and mercury sulfate, alkynic oxiranes undergo ring expansion, resulting in furans.



From other heterocycles - The cycloaddition of oxazoles with acetylenic dienophiles is a Diels-Alder reaction. Nitrile is lost in the reaction and furan is formed.


From ring contraction - Aqueous hydrogen peroxide and perchloric acid oxidize pyrylium salts to form 2-acylfurans through ring contraction.





Reactions

Protonation - An electron-withdrawing substituent gives furans stability against acid. During protonation, electron-releasing substituents on furan generate reactive electrophiles that activate polymerization and lead to ring-opening reactions.


Mercuration - Mercuration is an easy process for furan.

Reduction - A simple furan cannot be reduced to a tetrahydrofuran without opening its ring. It is possible to decompose furoic acid into dihydrofurans.

Electrophilic Substitution

Nitration - Furan is nitrated at a low temperature with acetyl nitrate, a mild nitrating agent.


Sulphonation - Furan can be sulfonated with sulfur trioxide, pyridine, or dioxane at room temperature to form 2,5-disubstituted furan.

Halogenation - At room temperature, furan reacts strongly with bromine and chlorine, forming polyhalogenated products. Therefore, mono-chloro and mono-bromo furans must be produced under milder conditions. When DMF or dioxane is brominated at –5°C, a 2-bromofuran is formed.

Furans substituted with electron-withdrawing substituents at position-2 give 5-Bromo derivatives.



Alkylation - Friedel-Crafts alkylation does not occur with furan because of its acid sensitivity. A mild catalyst, such as phosphoric or boron trifluoride, can be used to alkylate furan at position-2 with alkene.



Acylation - The acylation of furans with acids anhydrides or acids halides normally requires a mild catalyst, such as phosphoric acid or boron trifluoride. It is not necessary to use a catalyst to acylate trifluoroacetic anhydride.


Condensation with aldehydes and ketones - Reactions involving furan and aldehydes produce oligomers. Furan and acetone are condensed to form macrocycles.

Reactions with diazonium salts - In place of an azo compound, 2-phenyl furan is formed from furan when it reacts with benzene diazonium salt.


Reactions with nucleophile reagents - Nucleophiles react more strongly with halofurans than simple furans. Halofurans are more reactive when substituted with electron-withdrawing groups (nitro, carboxy, or carboalkoxy).


Oxidation - If sodium hypochlorite, hydrogen peroxide, or meta chloroperbenzoic acid is used to treat furan, ring opening occurs.


Medicinal Uses

Ascorbic acid (vitamin C), nitrofurazone (antibacterial), and ranitidine (antiulcer) all contain furan as scaffolds.
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