Reductive cyclization indolizidine from

Pyrrolizidine was also prepared directly from the previously discussed pyrrolidinepropanol intermediate (3) (Section II,B) by treatment with Raney nickel in 49% yield. This reductive cyclization gave a lower yield of pyrrolizidine than that obtained with indolizidine or quinolizidine under similar conditions. It is suggested that this is due to steric effects in the formation of the presumed iminium ion intermediate (31), which could not be formed from pyrrolizidine even under forcing conditions (but see Section II,G). 

Ring transformation of the mannopyranoside derivative 61, obtained from noncarbohydrate, to 1 has been developed (Scheme 7).225.226 Radical cyclization of the thiocarbonylim-idazolo derivative of 61 gave 62, which upon oxidation and reduction afforded 63 (30%) and 64 (55%). The latter was benzylated to give 65, which was converted into (fi)-oxime 67 via 66. Beckmann rearrangement of 67 followed by desilylation furnished 68. Cyclization of 68 to indolizidine skeleton followed by debenzylation, reduction of the lactam with BMS and hydrolysis afforded (-)-swainsonine (1). 

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