22 CHEMICAL CONVERSION OF OXINDOLIC ALKALOIDS TO INDOLIC ALKALOIDS
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Though the general methods of oxidative conversion of 2, 3-disubstituted indolic alkaloids to 3, 3-disubstituted oxindoles are well known, the reverse changes have been realized only in a few, rather simple oxindoles by use of strictly controlled amounts of LiAlH_4. In our structural study of gardneramine (12), NaBH_4 in acetic acid was found to be a good reducing reagent of iminoether to dihydro derivative (13). When this procedure was applied to the iminoethers derived from uncarines C and E, isorhynchophylline and yohimbine, the corresponding 3-monosubstituted indoles 22, 24 and 25 were formed. Obviously, two step reduction took place through an iminium intermediate (A) as shown in chart 5. Then the indoles, thus obtained, were subjected to the oxidative cyclization using Hg(OAc)_2 to form natural indolic alkaloids. Thus, tetrahydroalstonine (28) and akuammigine (29) were formed in approximately equal amounts from 22. Hirsutine (31), though in poor yield, was obtained from 24. Compound (25) afforded pseudoyohimbine as a main product along with yohimbine (1).
