Ruthenium-NHC-catalyzed asymmetric hydrogenation of indolizines: access to indolizidine alkaloids.

Indolizidine alkaloids constitute a very important class of natural products. These metabolites derived from lysine can be isolated from diverse sources, like fungi, bacteria, higher plants, invertebrates, and vertebrates. Their core structure is characterized by fused sixand five-membered rings, with a bridgehead nitrogen atom, and is found in 25–30% of all naturally occurring alkaloids. The most important types of indolizidines are plant-derived polyhydroxylated indolizidines, like swainsonine and castanospermine, which are glycosidase inhibitors with activity against HIV, as well as alkylindolizidines, like monomorine (3), which is isolated from ants (Monomorium pharaonis) and from the skin of amphibians (Melanophryniscus stelzneri), and is believed to serve in their defense against predators (Scheme 1).