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2004
Alkaloids
U 0600
50- 170
Enantioselective Synthesis of (+)-Anatoxin-a via Enyne Metathesis. — The title
alkaloid (VIII) is prepared by a series of nine chemical operations and in 27% overall
yield from D-methyl pyroglutamate (I). The route involves a new method for the diastereoselective synthesis of cis-2,5-disubstituted pyrrolidines bearing an unsaturated
side chain by application of BF3·OEt2 and Ph 3SiH to effect the selective reduction of
∆1,2-pyrrolinium ions derived from carbamates like compound (II). A further improvement of this method is demonstrated by application of catalytic amounts of tris(pentafluorophenyl)borane in the cyclization/reduction of carbamate (IX). The bridged bicyclic framework of the title alkaloid is derived from an intramolecular enyne metathesis reaction of compound (IV). The transformation of the isopropenyl group in compound (V) into the desired acetyl group is achieved by selective dihydroxylation of the
less substituted C=C double bond to give diol (VI), followed by cleavage of the C—C
bond. — (BRENNEMAN, J. B.; MACHAUER, R.; MARTIN*, S. F.; Tetrahedron 60
(2004) 34, 7301-7314; Dep. Chem. Biochem., Univ. Tex., Austin, TX 78712, USA;
Eng.) — Klein
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