The Structure of the Alkaloid Juliprosopine from Prosopis juliflora A. Dc. A new alkaloid, juliprosopine, has been isolated from Prosopis juliflora A. Dc. Leguminosae, for which structure 1 is proposed. The piperidine moiety could be elucidated on the basis of spectroscopic data (MS., 1H‐NMR., 13C‐NMR.) of juliprosopine and the derivatives 2–4. The central hexahydroindolizine part and its substitution pattern has been determined mainly on the bases of the Hofmann‐degradation product 11, which yielded the hydrogenation products 13 and 14. By ozonolysis 11 was converted to 16 and 18. It is proposed that juliprosopine was built up in nature by two ‘monomeric’ Prosopis alkaloids and one dihydropyrrol unit. Copyright © 1980 Verlag GmbH & Co. KGaA, Weinheim