合成路线1

The reduction of (R)-malic acid (I) with BH3/Me2S and NaBH4 gives 1,2(R),4-butane-triol (II), which is treated with benzaldehyde dimethylacetal (III) and Ts-OH to yield the 1,3-dioxane (IV). The oxidation of the hydroxymethyl group of (IV) by means of oxalyl chloride in DMSO/dichloromethane affords the carbaldehyde (V), which is submitted to a Wittig condensation with allyltriphenylphosphonium bromide (VI) and potassium tert-butoxide to provide the butadienyl derivative (VII). The reductive cleavage of the benzylidene acetal (VII) by means of DIBAL, followed by Z/E photoisomerization of the resulting diene gives (E)-3(R)-(benzyloxy)-4,6-heptadien-1-ol (VIII). The reaction of the OH group of (VIII) with TsCl and pyridine yields the corresponding tosylate (IX), which is treated with NaCN in DMSO to afford (E)-4(R)-(benzyloxy)-5 ,7-octadienonitrile (X). The hydrolysis of the CN group of (X) with NaOH in methanol/water provides the carboxylic acid (XI), which is esterified with diazomethane to afford the methyl ester (XII). The reaction of (XII) with hydroxylamine and KOH in methanol provides the hydroxamic acid (XIII), which is oxidized with tetrapropylammonium periodate in water to generate an acylnitroso intermediate (XIV), which undergoes a (4+2) cycloaddition to give the trans-1,2-oxazinolactam (XV) along with some cis isomer that is separated by chromatography. The reductive cleavage of the N-O bond of (XV) by means of sodium amalgam yields the allyl alcohol (XVI), which is silylated with Tbdps-Cl and imidazole affording the silyl ether (XVII). The catalytic dihydroxylation of the double bond of (XVII) by means of OsO4 and NMO provides the diol (XVIII) along with some diastereoisomer that is separated by chromatography. The protection of the OH groups of (XVIII) by reaction of 2,2-dimethoxypropane (XIX) and PPTS gives the acetonide (XX). The reduction of the lactone group of (XX) with LiAlH4 in THF proceeds with simultaneous desilylation yielding the piperidino-alcohol (XXI), which is cyclized by means of CBr4, PPh3 and TEA to provide the indolizidine derivative (XXII). The hydrogenolytic removal of the benzyl group of (XXII) with H2 over PdCl2 gives the alcohol (XXIII), which is finally submitted to hydrolysis of its acetonide group by means of 6N HCl in THF to afford the target swainsonine.