Furan-2[5H]-ones (?a,s-butenolides): preparation and biological significance. Heterocyclic systems are one of the most versatile areas in organic chemistry. The five members oxacyclic system of furan-2[5H]-ones, commonly named as ?a,s-butenolides, are of scientific interest since many examples include important bioactivity. During last decades ?a,s-butenolides have been highly important synthetic targets, originating new developments in their construction. A short compendium about different synthetic methodologies focused into the ?a,s-butenolide ring formation is presented, discussing also the biological importance of these promising pharmaceutical entities.