S. Nishikawa et al., SYNTHESIS, CRYSTAL-STRUCTURE AND CYTOKININ ACTIVITIES OF BETA-SUBSTITUTED 6-STYRYLPURINES, Phytochemistry, 37(4), 1994, pp. 915-919
Conformationally restricted (Z)- and (E)-beta-substituted 6-styrylpuri
nes were synthesized by addition reaction to 6-phenylethynylpurine and
photoisomerization. The crystalline (Z)- and (E)-beta-chloro-substitu
ted compounds exist in the anti- and syn-forms, respectively. While th
e (Z)-beta-chloro- and bromo-substituted compounds were almost as acti
ve 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 struc
ture, as well as the high cytokinin activity of the conformationally r
estricted (Z)-isomers strongly suggest that the anti-transoid form is
most probably the active conformation of purine cytokinins.