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.