SYNTHESIS, CRYSTAL-STRUCTURE AND CYTOKININ ACTIVITIES OF BETA-SUBSTITUTED 6-STYRYLPURINES

Conformationally restricted (Z)- and (E)-beta-substituted 6-styrylpurines were synthesized by addition reaction to 6-phenylethynylpurine and photoisomerization. The crystalline (Z)- and (E)-beta-chloro-substituted compounds exist in the anti- and syn-forms, respectively. While the (Z)-beta-chloro- and bromo-substituted compounds were almost as active as (E)-6-styrylpurine and N-6-benzyladenine (BA) in the Amaranthus betacyanin assay, they were more active than the latter in the tobacco callus assay. On the other hand, their (E)-isomers showed activities almost comparable to that of kinetin in both assays. The crystal structure, as well as the high cytokinin activity of the conformationally restricted (Z)-isomers strongly suggest that the anti-transoid form is most probably the active conformation of purine cytokinins.