(-)-Swainsonine has been synthesized enantioselectively via the stereoselective iodoamination of trichloro-acctimidate derived fron (Z)-olefinic allylic alcohol 97a. Allylic alcohol 96 prepared from lactol 90 and phosphonium salt 91 was converted into 97a. Reaction of 97a with trichloroacetonitrile was followed by cyclization with iodine monobromide to afford trans-oxazoline 102a. After deprotection of silyl group of 102a, the sequential oxidation and lactonization with silver carbonate afforded 115. Deprotection of 115 and the following protection with benzyl chloroformate gave 123. Conversion of 123 into 130b was carried out by its sulfonation and protection as acetonide in sequence to furnish 130b. Hydrogenation of 130b with 10% Pd/C gave 133. (-)-Swainsonine 1 was obtained from 130b by the sequential treatment of borane and conc.HCI.
Total synthesis of pyrrolizidines, (-)-platynecine 2 and (-)-hadinecine 4 has been attained employing the iodocyclization of the imidate of allylic alcohols 150a and 154a. 150a and 154a prepared from D-malic acid and phosphonate 141 were converted into the corresponding imidates and then cyclized with iodine to afford 158. Reduction of 158 with zinc followed by acidic deprotection gave 168. After the double cyclization of 168 with tetrachloromethane and triphenylphosphine, hydroboration of 136 furnished (-)-platynecine 2 and osmylation of 136 afforded (-)-hadinecine 4.
For the synthesis of herquline the required intermediate 182 was synthesized via the oxidative cyclization of 205 from L-tyrosine with iodobenzene diacetate and sodium bicarbonate in methanol to afford 206a. 206a was hydrogenated with 10% Pd/C in ethylacetate, protected with 1, 3-propanediol and then reduced via xanthate to provide 214a.
