Purines. XXXIII. Syntheses and Cytokinin Activities of Both Enantiomers of 1'-Methylzeatin and Their 9-β-D-Ribofuranosides

The structures and absolute configurations of the cytokinins 1'-methylzeatin and its 9-riboside, both secreted by Pseudomonas syringae pv savastanoi, have been established as [R-(E)]-2-methyl-4-(9H-purin-6-ylamino)-2-penten-1-ol[(1'R)-2] and [R-(E)]-N-(4-hydroxy-1,3-dimethyl-2-butenyl)adenosine [(1''R)-4], respectively, as a result of the chiral syntheses of (1'R)- and (1'S)-1'-methylzeatins [(1'R)-2 and (1'S)-2] and of their 9-β-D-ribofuranosides [(1''R)-4 and (1''S)-4] from D- and L-alanines. These zeatin derivatives were tested for cytokinin activity in the tobacco callus and the lettuce seed germination bioassays. The "natural" enantiomer (1'R)-2 was found to be as active as zeatin (1) itself and more active than its 9-riboside [(1''R)-4]. The "unnatural" enantiomer (1'S)-2 and its 9-riboside [(1''S)-4] were less active than the corresponding natural cytokinins, (1'R)-2 and (1''R)-4,respectively.