6-dimethylaminopurine (6-DMAP) (85; Scheme 17) and
isopentenyladenine (86; Scheme 17), from Castanea species,
showed weak inhibition of cyclin-dependent kinase 1 (CDK1)/
cyclin B, which led to the search for other purine-derived
compounds.257 Another plant secondary metabolite originally
isolated from the cotyledons of the radish, and subsequently
named olomucine (87; Scheme 17), demonstrated an improved
efficacy (IC50 ) 7 μM) and selectivity for cyclin
dependent kinases (CDKs) and, to some extent, MAP
kinases, by direct competition with ATP. Olomucine, which
earlier had been synthesized,258 disproved the existing dogma
that no specific kinase inhibitors could be found for ATPbinding
sites since they would be swamped by the presence
of excess of ATP