Calystegine Alkaloids in the Potato and Other Food Plants

Emerging Issues on Tropane Alkaloid Contamination of Food in Europe

The occurrence of tropane alkaloids (TAs), toxic plant metabolites, in food in Europe was studied to identify those TAs in food most relevant for human health. Information was extracted from the literature and the 2016 study from the European Food Safety Authority. Calystegines were identified as being inherent TAs in foods common in Europe, such as Solanum tuberosum (potato), S. melongena (eggplant, aubergine), Capsicum annuum (bell pepper) and Brassica oleracea (broccoli, Brussels sprouts). In addition, some low-molecular-weight tropanes and Convolvulaceae-type TAs were found inherent to bell pepper. On the other hand, atropine, scopolamine, convolvine, pseudotropine and tropine were identified as emerging TAs resulting from the presence of associated weeds in food. The most relevant food products in this respect are unprocessed and processed cereal-based foods for infants, young children or adults, dry (herbal) teas and canned or frozen vegetables. Overall, the occurrence data on both inherent as well as on associated TAs in foods are still scarce, highlighting the need for monitoring data. It also indicates the urge for food safety authorities to work with farmers, plant breeders and food business operators to prevent the spreading of invasive weeds and to increase awareness.

A short synthetic route to the calystegine alkaloids

An efficient strategy is described for the synthesis of enantiopure calystegine alkaloids. The key step employs a zinc-mediated fragmentation of benzyl-protected methyl 6-iodo-glycosides followed by in situ formation of the benzyl imine and Barbier-type allylation with zinc, magnesium, or indium metal. Stereochemistry in the pivotal allylation is controlled by the choice of the metal. The functionalized 1,8-nonadienes, thus formed, are converted into cycloheptenes by ring-closing olefin metathesis. Regioselective hydroboration and oxidation give the corresponding cycloheptanones, which are deprotected to afford the desired calystegines. Hereby, calystegine B(2), B(3), and B(4) are prepared from D-glucose, D-galactose, and D-mannose, respectively. This route constitutes the shortest synthesis of calystegine B(2) and gives rise to the first total syntheses of calystegine B(3) and B(4).