FURAN The least aromatic 5-membered ring Reaction with electrophiles - Protonation Conc. H 2 SO 4 Lewis acids (i.e. AlCl 3 ) Decomp. Reaction with electrophiles - Nitration Cannot use conc. HNO 3 / H 2 SO 4 Halogenation Reaction with electrophiles - Acylation Alkylation Generally not practical (polyalkylation, polymerisation) Condensation with Aldehydes and Ketones C.f. Reaction with electrophiles - Condensation with imines / iminium ions Reaction with oxidating agents Reaction with nucleophiles Some ex. on furans activated with -NO 2 group Metallation and further react. Pd-cat couplings c.f. Heteroaryl-Heck Cycloadditions Furanes as diene - one of the first DA examples Furan reacts with many dienophiles (alkenes, alkynes, allenes) exo isolated (termodyn favoured) endo (kinetic prod.) With 1 O 2 Furan as dienophile (only intramolec. ex) Photochemical cycloaddition Furyl-CH 2 -X Oxyfurans Isol. marine sponges cytotoxic Aminofurans -Aminoform -Unstable Synthesis of Furans Strategy a 1 Paal Knorr (1,4-dikacarbonyl) Strategy a - pyrroles Paal Knorr Carbonyl condensations Strategy b Pd-cat. Cyclisations etc. Cycloadditions