Compounds having the general formula (I): wherein X is S, Se or NH, and stereoisomers thereof, and de-O-sulfonated analogues of all of the foregoing, but excluding naturally occurring kotalanol and de-O-sulfonated kotalanol, and methods for synthesizing same. The compounds are useful as glycosidase inhibitors, and may be used in the treatment of diabetes. The synthetic compounds may also be used as standards in the calibration or grading of natural or herbal remedies produced from natural sources of glycosidase inhibitors such as kotalanol.