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