Novel Ent-Kaurene Glycosides with Eight Glycosyl Units from Stevia rebaudiana

Several steviol glycosides from Stevia rebaudiana are associated with extremely potent sweetness and are sought-after as low-calorie alternatives to sucrose. With the exception of a few nine sugar glycosides, steviol glycosides described in the scientific literatures contain less than a total of seven glycosyl units. We present here two new steviol glycosides, rebaudioside VIIIa (1) and VIIIb (2) containing eight sugar units isolated from S. rebaudiana Bertoni. The complete structure elucidation of these ent-kaurene diterpene glycosides with eight sugar units was completed using 1D- and 2D-NMR spectroscopy experiments (H-1, C-13, H-1-H-1 COSY, H-1-C-13 HSQC-DEPT, H-1-C-13 HMBC, 1D TOCSY, and H-1-H-1 ROESY) and mass spectral data. Both compounds have four glycosyl units each attached to the C-13 and C-19 positions of aglycone core but vary in the glycosylation pattern and number of glucose and rhamnose units. This report therefore expands the catalog of highly glycosylated components from S. rebaudiana.