Transition metal catalyzed glycosylation reactions – an overview
Carbohydrates are a large class of natural products that play key roles in a number of biological processes such as in cellular communication or disease progression. Carbohydrates are also used as vaccines and pharmaceuticals. Their synthesis through glycosylation reactions is challenging, and often stoichiometric amounts of promoters are required. Transition metal catalyzed glycosylation reactions are far less common, but can have advantages with respect to reaction conditions and selectivity. The review intends to approach the topic from the catalysis and carbohydrate perspective to encourage researchers from both the fields to perform research in the area. The article covers the basics in glycosylation and catalysis chemistry. The catalysts for the reaction can be roughly divided into two groups. In one group, the catalysts serve as Lewis acids. In the other group, the catalysts play a higher sophisticated role, are involved in all elementary steps of the mechanism and remain coordinated to the substrate throughout the whole catalytic cycle. Based on selected examples, the main trends in transition metal catalyzed glycosylation reactions are explained. Lewis acid catalysts tend to require a somewhat higher catalyst load compared to other organometallic catalysts. The reaction conditions such as the temperature and time depend in many cases on the leaving group employed. An outlook is also presented. The article is not meant to be comprehensive; it outlines the most common transition metal catalyzed processes with the intention to bring the catalysis and carbohydrate communities together and to inspire research activities in both areas.