[Zurück]

M. Draskovits, C. Stanetty, H. Kalaus, M.D. Mihovilovic:
"The N-Heterocyclic Carbene Controlled Dehomologation of Aldoses";
Poster: 29th International Carbohydrate Symposium, Lissabon; 14.07.2018 - 19.07.2018; in: "Book of Abstracts", (2018), S. 489.

Although being Nature´s biggest chiral pool, there is only a small set of standard carbohydrates, which are readily available (commercially and synthetically). In our research, we focus on the development of new synthetic methodology for the interconversion of these abundant sugars to more exotic ones by utilizing and targeting the aldehyde moiety. In our current study, we investigate the organocatalytic anomeric activation with N-Heterocyclic Carbenes (NHCs) as promising highly carbonyl selective reagent (see Figure 1).
Building up on the recently published sacrificial degradation of aldoses1, we set out to develop a method, which intercepts the degradation, thus allowing the isolation of the desired, shortened sugars. To understand this new and complex transformation in detail, which consists of retro-benzoin and Stetter-type reactions, we first set out to establish a fast and robust analytic system. This should give rise to a fast and thorough screening for the optimal reaction conditions, catalyst and substrate structure.
Our method of choice for the quantitative analysis of the obtained complex mixtures of reducing sugars was a combination of a solid phase extraction followed by derivatization and analysis via GC.2 With this analytical method in hand, we gathered crucial insights in this attractive transformation. A time-resolved screening was carried out, the relative stereochemistry in the starting material was investigated and several different catalysts were tested for their usability in this reaction. We discovered a crucial follow-up reaction, which can be suppressed or even enhanced by the right choice of catalyst. Furthermore, several parameters, which need to be controlled closely to allow a fast and clean conversion, were also identified. Overall, we were able to establish a successful and very selective interaction between a NHC and the aldehyde function of an aldose, which should expand this transformation from achiral aliphatic and aromatic aldehydes to the realm of carbohydrates.

https://publik.tuwien.ac.at/files/publik_273443.pdf