The Photoisomerization Behavior of the (4‐Hydroxycinnamoyl)‐spermidines

The photoisomerization behavior of three mono[( E )‐3‐(4‐hydroxyphenyl)prop‐2‐enoyl]spermidines, 1, 2 , and 3 , and three bis[( E )‐3‐(4‐hydroxyphenyl)prop‐2‐enoyl]spermidines, 4, 5 , and 6 , are investigated. The synthetic product ( E )‐ 1 could be almost quantitatively (> 96%) converted into its isomer ( Z )‐ 1 under UV light irradiation. In the cases of ( E )‐ 2 and ( E )‐ 3 , a mixture of ( E )/( Z ) ca. 1:2 was obtained, when the same conditions were applied. The comparison of their UV spectra provides the possible explanation for these different behaviors. Furthermore, it was noticed that the ( Z ) → ( E ) isomerization of the CC bond took place during the purification by reverse‐phase high‐performance liquid chromatography (RP‐HPLC), and the ( E )/( Z )‐mixture is thus inseparable. The same feature could be observed during the isolation of the ( Z , Z )‐ N , N ′‐bis[3‐(4‐hydroxyphenyl)prop‐2‐enoyl]‐spermidines, ( Z , Z )‐ 4 , ( Z , Z )‐ 5 , and ( Z , Z )‐ 6 . Nevertheless, the fractions of ( Z , Z )‐ 5 and ( Z , Z )‐ 6 were in almost pure state collected, and their 1 ‐NMR spectra are presented.