Carbohydrates are a fascinating part of Nature’s chiral pool and are provided in renewable fashion in great variety and abundance. It is a burden for extensive use by organic chemists, that only a small fraction of carbohydrates is readily available and consequently studied in great detail. Synthetic glyco-chemistry is challenging due to the structural density of both functional groups and stereochemistry. Adding to this complexity, an equilibrium exists between cyclic hemiacetals and the open chain carbonyl form (minor). Graphical Abstract It is this neglected molecular minority, with its intrinsically reactive aldehyde moiety, which is in the focus of our research. An operationally simple UV-based assay was developed to efficiently capture this relevant pre-equilibrium.1 This tool was utilized in the studied elongation of the carbohydrates via indium mediated acyloxyallylation (Fig. bottom)2 as well as the organocatalytic anomeric activation of reducing sugars by N-heterocyclic carbenes towards a controlled dehomologation method (Fig. top).3