<unk>  <unk>  or pyran-2-one) is an unsaturated cyclic chemical compound with the molecular formula C5H4O2.  It is isomeric with<unk> .<unk>  is used in organic synthesis as a building block for more complex chemical structures because it may participate in a variety of cycloaddition reactions to form bicyclic lactones.  For example, it readily undergoes Diels-Alder reactions with alkynes producing, upon loss of carbon dioxide, substituted benzenes.. The Gogte Synthesis (1938) is a method for the alkylation of certain<unk> s with acid chlorides.<unk>  also forms the the core structure of a variety of natural organic compounds.  For example, the coumarins are an important class of compounds which are benzo-fused derivatives of<unk> .<unk> 
Coumarin<unk> 
Coumarin
2-Pyrone
Styramate
Metirosine
Metyrosine
Panthenol
Monobenzone
Tetrahydrozoline
Allyl chloride