O-BODIPYs as fluorescent labels for sugars: glucose, xylose and ribose

Sugar-O-BODIPY conjugates, in which the fluorescent dye O-BODIPY is linked to sugars directly through B–O–C bonds, have been synthesized via an acid-catalyzed transesterification procedure. Conjugates of glucose, xylose and ribose with O-BODIPYs bearing meso-p-tolyl and meso-mesityl substituents were prepared. The products were characterized using multi-dimensional 1H, 13C and 11B NMR spectroscopy, including an application of the 1H–11B HMBC experiment used for the first time in sugar-boronate chemistry. Binding to the furanose ring form was preferred in most conjugates, and DFT studies revealed non-ideal geometries for the pyranose conjugates, causing them to be disfavoured. A correlation between the 11B NMR chemical shift and the O–B–O bond angle was observed, allowing boron bound to 1,2-, 1,3- or 1,4-diols and the 1,2-diol site on furanose or pyranose to be discriminated. Measurement of the photophysical properties showed that the highly fluorescent mesityl-O-BODIPY conjugates retained their absorption and emission properties, and novel 1 : 3 glucose-O-BODIPY conjugates were identified, which demonstrated unique optical properties that were attributed to charge transfer.