Pyrrolizidine Alkaloids from Cynoglossum creticum. Synthesis of the Pyrrolizidine Alkaloids Echinatine, Rinderine, and Analogues

Reinvestigation of Cynoglossum creticum led to the isolation of the previously reported echinatine 1 and heliosupine 2 as well as rinderine 3, 7-angelylheliotridine 4 and a new alkaloid, cynoglossamine 5. The structures have been determined by spectral means (ir, ms, 1H-13C HETCOR nmr), comparison with literature data and authentic samples, and/or syntheses. In addition, 1 and all three of its isomers 3, 6, and 7 and other semisynthetic analogues (8-13) were prepared and characterized.