Chemistry of pyrrolizidine alkaloids revisited—semi-synthetic microwave and continuous-flow approaches toward Crotalaria-alkaloids

Efficient and rapid syntheses of various pyrrolizidine alkaloids via microwave synthetic protocols using monocrotaline from a natural source (Crotalaria spectabilis) are reported. Retronecine, hydroxydanaidal, and acetylated hydroxydanaidal (utilized in the chemical communication system of various insect species) were obtained. In addition, a continuous flow hydrogenolysis/hydrogenation approach to directly access platynecine, retronecanol, and desoxyretronecine was realized.