The synthesis and chemistry of certain macrolactone pyrrolizidine alkaloids (PAs) will be studied. Members of this subgroup of the PA family, represented structurally by jacobine (1), are found in several species of Senecio, including S. jacobaea (tansy ragwort). This weed, which is indigenous to the Pacific northwest, is the demonstrated source of several acutely hypatotoxic PAs. These alkaloids are known to enter the human food chain via meat consumption, dairy products, and herbal concoctions. Dietary sources of PAs have been shown to cause veno-occlusive disease as well as human fatalities. The principal objective of this research is to devise routes for the total synthesis of macrolactone PAs which will provide a means for elucidating further details of their mechanism of action in vitro and in vivo. Our initial research will focus on jacobine (1). Specific goals for the first grant period will include (i) asymmetric synthesis of retronecine (2), (ii) asymmetric synthesis of jacobinic acid, (iii) isolation and degradation of jacobine and other macrolactone PAs from Senecio species in order to obtain natural necic acids, (iv) reconstitution of PAs from retronecine and the necic acids, (iv) reconstitution of PAs from retronecine and the necic acid, employing novel macrolactonization methodology, (v) incorporation of isotopic labels at strategic locations in both the necine base and necic acid units for tracer studies, and (vi) structural modification of the necic acid segment. A hypothetical mode of action for jacobine is proposed, which envisages the dehydro(pyrrolic) form of the alkaloid as a mono- and/or bis-alkylator, with additional activation provided by the epoxide function in this case. Synthetic approaches have been designed from subunits of 1 which will permit the introduction of 13C/14C and 15N isotopic labels and which will facilitate ongoing studies directed toward elucidating the metabolism and binding of PAs in vivo. These studies are presently being conducted by Professor Donald R. Buhler, Department of Agricultural Chemistry and Environmental Health Sciences Center, Oregon State University, using natural and biosynthetically prepared jacobine. In addition to establishing de novo synthetic routes to PAs, our goals include the synthesis of useful materials for identifying the cellular targets and metabolites of toxic PAs.
