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In the study of glycosidases, a class of activity-based probes (ABPs), that are carbocyclic mimics of natural carbohydrates and can covalently bind the enzyme, have proven to be useful tools. This type of ABP has however not yet been reported for sialidases, glycosidases involved in various important biological processes in both health and disease, which hydrolyse terminal sialic acids. Here we present our study towards the synthesis of a carbocyclic sialic acid suitable for conversion into ABPs. We developed a route starting from a chiral furanone that includes a key early stage nitrone [3+2] cycloaddition to install most of the chiral centres present in N-acetylneuraminic acid. The final stereocentre is installed via a Barbier alkylation, after which a ring closing metathesis forms the pivotal carbocyclic intermediate. Due to challenges in the final stretch, we were not able to convert this intermediate into an N-acetylneuraminic acid ABP. However, the work presented here still represents a versatile route to potential future carbocyclic sialic acid derivatives

BACKGROUND: Floral timing is a carefully regulated process, in which the plant determines the optimal moment to switch from the vegetative to reproductive phase. While there are numerous genes known that control flowering time, little information is available on chemical compounds that are able to influence this process. We aimed to discover novel compounds that are able to induce flowering in the model plant Arabidopsis. For this purpose we developed a plant-based screening platform that can be used in a chemical genomics study.RESULTS: Here we describe the set-up of the screening platform and various issues and pitfalls that need to be addressed in order to perform a chemical genomics screening on Arabidopsis plantlets. We describe the choice for a molecular marker, in combination with a sensitive reporter that's active in plants and is sufficiently sensitive for detection. In this particular screen, the firefly Luciferase marker was used, fused to the regulatory sequences of the floral meristem identity geneAPETALA1 (AP1), which is an early marker for flowering. Using this screening platform almost 9000 compounds were screened, in triplicate, in 96-well plates at a concentration of 25 µM. One of the identified potential flowering inducing compounds was studied in more detail and named Flowering1 (F1). F1 turned out to be an analogue of the plant hormone Salicylic acid (SA) and appeared to be more potent than SA in the induction of flowering. The effect could be confirmed by watering Arabidopsis plants with SA or F1, in which F1 gave a significant reduction in time to flowering in comparison to SA treatment or the control.CONCLUSIONS: In this study a chemical genomics screening platform was developed to discover compounds that can induce flowering in Arabidopsis. This platform was used successfully, to identify a compound that can speed-up flowering in Arabidopsis.

Chapter 1 provides a perspective of synthetic organic chemistry as a discipline involved in the design, synthesis and evaluation of complex molecules. The reader is introduced with a brief history of synthetic organic chemistry, all the while dealing with different aspects of synthetic organic chemistry. These aspects include design, synthesis and evaluation of complex molecules, which are described with representative examples. For instance, chapter 1 described different strategies to design antibiotics like penicillin. Furthermore, efforts at the interface of chemistry and biology has led to the emerging of a new subdisciplines such as chemical biology. Although this discipline is relatively new, the scientific community has witnessed many breakthrough discoveries and some of these discoveries are highlighted in this chapter. After the general introduction presented in chapter 1, the following chapters of this thesis focus on the design, synthesis and evaluation of small molecular probes towards the study of proteins and glycans in plant and mammalian cells. Most of these probes are complex glycomimetic small molecules that in many instances are prepared with a nitrone-olefin [3+2] cycloaddition as a key step. In chapter 2, a novel nitrone is presented for the nitrone-olefin [3+2] cycloaddition reaction. This reaction is a powerful tool in synthetic organic chemistry for the synthesis of a wide range of complex molecules. The versatile nature of the reaction is illustrated by the synthesis of several classes of natural product such as vitamins and alkaloids – all complex molecules containing multiple neighboring chiral centers – through the nitrone-olefin [3+2] cycloaddition. However, most nitrones that show good regio- and stereoselectivity are limited in their synthetic versatility, as subsequent synthetic modification of the cycloaddition products are limited. The nitrone described in chapter 2 is a novel masked aldehyde-containing nitrone. This nitrone is prep