Diterpenoids are important molecules that play significant roles in many biological processes. In plants, acyclic diterpene phytol is a representative side chain of chlorophyll, and tetracyclic diterpene gibberellins ubiquitously serve as plant hormones regulating growth and development. Decoration of diterpene core structures by oxidation and glycosylation are usually key steps to confer final bioactivity of diterpenoid compounds. In contrast to the extensively studied oxidases involved in gibberellin biosynthesis, diterpenoid-related glycosyltransferases have been scarcely documented, possibly due to the relative rarity of diterpene glycosides in the plant kingdom. Only a few UDPdependent glycosyltransferases (UGTs) have been characterized to participate in diterpene glycoside biosynthesis (Richman et al., 2005;Nagatoshi et al., 2012).Rubusoside is an important ent-kaurene-type diterpenoid glycoside mainly produced by two edible and medicinal plants of Rubus suavissimus (also called ''Chinese sweet tea'') and Angelica keiskei (also called ''Ashitaba'' in Japanese) (Tanaka et al., 1981;Zhou et al., 2012). The leaves of both plants have a long history of being collected by local people to make herbal tea due to their natural sweetness or putative therapeutic activities.Rubusoside is approximately 60 times as sweet as sucrose (Tanaka et al., 1981) and has less bitter aftertaste. Recently, it was also reported as the first molecule that inhibited both human fructose transporter GLUT5 and glucose transporter GLUT1 (Thompson et al., 2015). The biosynthesis of rubusoside is proposed to resemble that of the steviol glycosides (SGs) occurring in Stevia rebaudiana (Ceunen and Geuns, 2013). Steviol generated by oxidation of the ent-kaurene skeleton serves as a common aglycone for all SGs compounds. Further glycosylation via UGT-mediating decorations produce rubusoside and related glycosides (Supplemental Figure 1).

Five new acylated aminooligosaccharides (1–5), together with one known related analogue (6), were isolated from Streptomyces sp. HO1518. Their structure was identified by extensive spectroscopic analysis, including 1D and 2D NMR data and high resolution electrospray ionization mass spectrometry (HRESIMS), and by comparison with those reported in the literature. All of the new compounds showed more promising porcine pancreatic α-amylase (PPA) inhibitory activities than the clinical drug acarbose, indicating them as potential pharmaceutical drug leads toward type II diabetes.

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