When the Nine-Membered Enediynes Play Hide and Seek

The lack of stability of the 9-membered enediynes not associated with an apoprotein may explain the low number of isolated natural compounds containing this core. To overcome such a problem, particular attention should be paid during the process of extraction and isolation of secondary metabolites, especially from microorganisms such as actinomycetes in order to identify the non-cycloaromatized derivatives.