Cyclic Sesquiterpene−Flavanone [4+2] Hybrids, Syzygioblanes A−C, Found in an Indonesian Traditional Medicine, “Jampu Salo” (Syzygium oblanceolatum)

A plant used in an Indonesian traditional herbal
medicine as a diabetes treatment and known locally as “Jampu
Salo” was collected on Sulawesi Island, Indonesia. It was identified
as Syzygium oblanceolatum (C. B. Rob.) Merr. (Myrtaceae) and
found for the first time in Sulawesi; it was previously reported only
in the eastern Philippines and Borneo. A phytochemical study of S.
oblanceolatum led to the isolation of three unprecedented
meroterpenoids, syzygioblanes A−C (1−3, respectively). These
compounds might be biosynthesized through [4+2] cycloaddition
of various germacrane-based cyclic sesquiterpenoids with the
flavone desmethoxymatteucinol to form a spiro skeleton. The
unique and complex structures were elucidated by microcrystal
electron diffraction analysis in addition to general analytical
techniques such as high-resolution mass spectrometry, various nuclear magnetic resonance methods, and infrared spectroscopy.
Synchrotron X-ray diffraction and calculations of electronic circular dichroism spectra helped to determine the absolute
configurations. The newly isolated compounds exhibited collateral sensitivity to more strongly inhibit the growth of a multidrug
resistant tumor cell line compared to a chemosensitive tumor cell line.