Liriogerphines E–U, further unique sesquiterpene-alkaloid hybrids from the rare Chinese tulip tree.

Seventeen undescribed sesquiterpene-alkaloid hybrids (liriogerphines E–U, 1 – 17) were isolated and identified during a further phytochemical investigation on the branches and leaves of Chinese tulip tree (Liriodendron chinense), a rare medicinal and ornamental plant endemic to China. These unique heterodimers are conjugates of germacranolide-type sesquiterpenoids with structurally diverse alkaloids [i.e., aporphine- (1 – 15), proaporphine- (16), and benzyltetrahydroisoquinoline-type (17)] via the formation of a C–N bond. The previously undescribed structures were elucidated by comprehensive spectroscopic data analyses and electronic circular dichroism calculations. Such a class of sesquiterpene-alkaloid hybrids presumably biosynthesized via an aza-Michael addition is quite rare from terrestrial plants. In particular, the sesquiterpene-benzyltetrahydroisoquinoline hybrid skeleton has never been reported until the present study. All the isolates were evaluated for their cytotoxic effects against a small panel of leukemia cell lines (Raji, Jeko-1, Daudi, Jurkat, MV-4-11 and HL-60), and some of them exhibited considerable activities. Seventeen previously undescribed hybrids (liriogerphines E–U, 1 – 17), conjugating by germacranolide-type sesquiterpenoids and structurally diverse alkaloids, were isolated and identified during a further phytochemical investigation on the brunches and leaves of Chinese tulip tree (Liriodendron chinense). These sesquiterpene-alkaloid hybrids were quite rare in terrestrial plants and some of them exhibited significant anti-leukemia activities. [Display omitted] • 17 undescribed sesquiterpene-alkaloid hybrids were isolated from Chinese tulip tree. • The structures were established by spectroscopic analysis and chemical calculations. • The hybrids presumably formed by aza-Michael addition were quite rare in nature. • Liriogerphine U is the first example of sesquiterpene-benzylisoquinoline hybrid. • Some isolates exhibited considerable cytotoxicities against six leukemia cell lines. [ABSTRACT FROM AUTHOR]