Tobacco bioassays, cytokinin activities

The observations that cytokinins have activity in the tobacco bioassay detectable at concentrations as low as 10 H M, and that the promotion of such activity can depend on structural parameters... 

Figure 1. Cytokinin activities in the tobacco bioassay of 6-(3-methyl-2-butenyl-amino)purine (3), 6-isopentylaminopurine, and four (3-methyl-)7-alkylaminopura-zolo[4,3-d]pyrimidine (11-14)...

The accumulated assay results obtained with the 4-alkyl-2-methylpyrrolo[2,3-d]pyrimidines are of special interest. Can-pounds 25 and 26 exhibited significant cytokinin activity in all assays utilized. On the other hand, compound 27 was a potent antagonist in the tobacco bioassay but had little activity in the other two assay systems, while 29 acted as an anticytokinin in the tobacco bioassay and a cytokinin in the lettuce seed germination assay, but was without activity in the Amaranthus test system. 

A tremendous upsurge in synthetic activity in this field is evident. A stereoselective synthesis of cis-zeatin (22), whose geometrical isomer was first isolated from Zea mays and shown to be a stimulant of cell division in plant tissue cultures, has been reported (Scheme 2). The Diels-Alder reaction of 1-chloro-l-nitro-socyclohexane (19) with isoprene gave the dihydro-l,2-oxazine hydrochloride (20) in moderate yield. Liberation of the base of (20) followed by zinc-acetic acid reduction gave the required amino-alcohol (21), which upon treatment with 6-chloropurine provided a separable mixture of cis-zeatin (22) and trans-zeatin. Standard tobacco callus bioassay for cytokinin activity showed that the natural trans-zeatin is at least 50 times more active than the synthetic cis-isomer (22). 

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