ENANTIOSELECTIVE TOTAL SYNTHESES OF INDOLIZIDINE ALKALOIDS (-)-205A AND (-)-235B

Enantioselective total syntheses of indolizidine alkaloids (-)-205A and (-)-235B are described. The syntheses proceed via a common late-stage intermediate, alpha-aminonitrile 1. Absolute stereochemical control over the C8 and C8a stereocenters in these materials was achieved by a stereoselective crotylation reaction between chiral acyliminium ion (R)-3b and crotylmagnesium chloride. The selectivity of this reaction, which produced the (future)-8R,8aS configuration was complementary to the result obtained by crotylation of acyliminium ion (S)-3a with trans-crotyltrimethylsilane, which produced predominantly an adduct with the (future)-8S,8aS configuration. This latter crotyl lactam was converted to two additional diastereomers of alkaloid 205A. Comparison of the H-1 and C-13 NMR and optical rotation values of the four synthetic diastereomers of 205A with literature values supported the proposed assignment of the absolute and relative configuration of (-)-205A. The C-13 spectrum of synthetic (5R,8R,8aS)-235B was identical with that of natural 235B and supported the proposed assignment of relative configuration of the alkaloid. The optical rotation differed in sign and magnitude from the published value. Revised values of the optical rotations of (-)-205A and (-)-235B are suggested. This work constitutes the first enantioselective syntheses of 205A and 235B, which were prepared in 15 and 14 steps, respectively, from succinic anhydride, in an average overall yield of 17%.