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The rapid development of natural product chemistry has led to the isolation of a wide varieLty of secondary metabolites, which, in many cases, was shown to be of glycosidic nature. In particular our knowledge of the chemistry, occurrence and biogenesis of glycosides has grown substantially during the last two decades [1, 2]. Most of this effort has been directed toward the large group of the 0-glycosides as well as the Sand N-glycosides. Far less effort has been given to the C-glycosides due to analytical difficulties. C-Glycosides are a special type of glycoside since the aglycone is directly attached to carbon 1 of a pyranose ring of a sugar while 0-glycosides possess a hemiacetal linkage. It is more correct to regard them as C-glycosyls or anhydropolyols as proposed by BANDYUKOVA and YUGIN [3]. Since the original term C-glycoside is well established, it should be maintained. One of the main characteristics of C-glycosides is resistance towards acid hydrolysis. Even after prolonged acid treatment the sugar residue, which is attached by a carbon-carbon-bond, remains attached to the aglycone. Cleavage of the C-C-linkage is achieved only under drastic conditions. When oxidizing reagents are used partial degradation of the sugar residue is likely [4]. Classification of C-glycosides is usually based on the aglycone [5, 6]. Five aromatic ring systems are known to occur C-glycosylated (Fig. 1). The largest group, the C-glycosyl flavonoids, has been intensively studied by many authors and was the subject of two recent review articles [3, 7]. Xanthone C-glycosides appear to be biogenetically related to C-glycosyl flavonoid compounds. Another group, C-glycosylated chromone derivatives, are relatively rare. Aromatic three-ring systems, notably the anthrones, are C-linked to glucose. C-Glycosyl derivatives of gallic acid Fig. 1. Basic structures of C-glycoside aglycones.

Chemistry, Occurrence and Biosynthesis ofC-Glycosyl Compounds in Plants