The molecular design and chemical synthesis of novel dienediyne systems related to the neocarzinostatin chromophore (1), which is the labile heart of an antitumor antibiotic, neocarzinostatin, and their chemical and DNA cleaving properties are described. The monocyclic dienediynes 6-8, which are high simplified analogs of the neocarzinostatin chromophore (1), were effectively synthesized from xylitol (13) in a short step. The synthesis includes the conversion of the ketoaldehyde 24 into the highly strained 10-membered ring keto-enediyne 25 by a simple intramolecular aldol condensation using lithium hydroxide as the key step. The dienediyne 8 possessing acetoxy groups as leaving groups at propargylic positions was smoothly cycloaromatized by methyl thioglycolate in the presence of triethylamine in methanol to give two benzenoides 28 and 29 through radical pathways. The addition of pyrrolidine to 8 in ethanol also afforded the benzenoid 38 via a pathway similar to that for 29. Furthermore, it was clearly found that the dienediyne 8 effectively cleaved DNA without any additive and the DNA cleaving activities significantly increased in the presence of the thiol, methyl thioglycolate.