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Enantiospecific Synthesis of (+)-Nemorensic Acid, a Necic Acid Component of the Macropyrrolizidine Alkaloid, Nemorensine.

A concise enantiospecific synthesis of nemorensic acid 3, a necic acid component of the macropyrrolizidine alkaloid nemorensine 2, isolated from Senecio nemorensis L., is described. Reaction of an epsilon-halo-alpha,beta-unsaturated ester (8), readily accessible from a monoterpene (-)-carvone, with samarium iodide gave a fragmentation product (9), where a carbon-carbon bond cleavage reaction occurred between the gamma and delta positions of the carbonyl group, regioselectively. Deprotection of the silyl group of 9 brought about an intramolecular cyclization to provide tetrahydrofuran derivatives (10), which, upon chemical modification of the side chain, gave nemorensic acid.