Transformation of indole alkaloids—I : Conversion of oxindole alkaloids into indole alkaloids

Chemical conversion of some natural oxindoles (pteropodine, isopteropodine and isorhynchophylline) into the corresponding indole alkaloids has been made by way of a sequence of reactions which include formation of iminoethers of the natural oxindoles with Meerwein's reagent, reduction of the iminoethers to 2,3-seco-indoles and cyclization of 2,3-seco-indoles to the desired natural indole alkaloids. Sodium borohydride in acetic acid was found to be a specific reagent for the reduction of oxindole-iminoethers to 2,3-seco-indoles which were the key intermediates in these transformations. Yohimbine-oxindole iminoether was similarly converted to yohimbine and pseudoyohimbine. A number of by-products were obtained and their structures were elucidated.