Oxazolopiperidone lactams

Although outside the specific realm of oxindole synthesis, an interesting intramolecular route into a spiroindoline scaffold has been reported by Amat, Bosch, and coworkers who unexpectedly observed the Lewis-acid catalyzed cyclization of tryptophan derived oxazolopiperidone lactams 120 in the presence of triethyl silane [73]. In the event, spiroindoline 122 was obtained as a single stereoisomer in 86% optimized yield (Scheme 32). It was demonstrated that the hydroxymethyl group of the ring-opened oxazolidine was important for directing the stereochemical outcome of the transformation as removal of the ethyl side chain of the lactam ring did not diminish the selectivity of the spirocyclization. 

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