Abstract
The total synthesis of the entitled four alkaloids, which were oxindole alkaloids of Uncaria species and Mitragyna species, has been completed through several steps starting from the condensation of 2-hydroxytryptamine hydrochloride (IV) with 3, 3-dimethoxy-propionaldehyde (V). The condensation product (XV) was submitted to the Michael addition with methyl vinyl ketone, followed by acid treatment to give the α, β-unsaturated ketones (XIVa, b) as two stereoisomers. The isomer (XIVb) was condensed with methyl malonate to yield the ester (XXb), which was hydrogenated with Adams' catalyst to XXIb. The compound (XXIb) was treated with dil. sulfuric acid to give the lactones (XXIIa, b) as two spiro-iso-mers, the isomer (XXIIa) of which in turn, was heated with the aminal ester, then stirred with 5% MeOH-HCl, refluxed with aq. dioxane and heated with PPA to afford (±)-for-mosanine (IIIa) and (±)-isoformosanine (IIIb). Similarly, the lactone (XXVa) which was obtained from (XXb) by reduction with NaBH4 and hydrolysis with acid, gave (±)-mitraphylline (IIa) and (±)-isomitraphylline (IIb).
