Electron impact ion fragmentation pathways of peracetylated C-glycoside ketones derived from cyclic 1,3-diketones.

Monosaccharide C-glycoside ketones have been synthesized by aqueous-based Knoevenagel condensation of isotopically labeled and unlabeled aldoses with cyclic diketones, 5,5-dimethyl-1,3-cyclohexanedione (dimedone) and 1,3-cyclohexanedione (1,3-CHD). The reaction products and their corresponding acetylated analogs produce characteristic molecular adduct ions by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS). Analysis of the peracetylated C-glycosides by electron ionization (EI) gas chromatography/mass spectrometry (GC/MS) revealed diagnostic fragment ions that have been used to deduce the EI fragmentation pathways and the structure of each C-glycoside ketone. Characteristic gluco- and ribo-specific ions were observed at m/z 350 and 278, respectively. Ions common to both carbohydrate fragmentation pathways were observed at m/z 193 and 169 for the dimedone-C-glycosides, and m/z 165 and 141 for the 1,3-CHD-C-glycosides. Ions with m/z 169 and 141 retain the anomeric carbon (carbon-1) of the original sugar, while m/z 193 and 165 are shown to retain carbons-1, 2, and 3.