Construction of Spiro[pyrrolidine‐3,3′‐oxindoles] − Recent Applications to the Synthesis of Oxindole Alkaloids
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J. S. Bindra in:The Alkaloids(Ed.: R. H. F. Manske) Academic Press New York 1973 vol. 14 pp. 84−121.
R. T. Brown in:Heterocyclic Compounds(Ed.: J. E. Saxon) Wiley Interscience New York 1983 vol. 25 part 4 pp. 85−97.
For a detailed discussion of the mechanism see:
Casaschi A., 1993, Gazz. Chim. Ital., 123, 137
The ring expansion also proceeds in the presence of MgBr2·Et2O but is considerably slower and in the presence of Mg(OTf)2 no reaction takes place. Additionally N‐benzyl‐3‐(2‐iodoethyl)oxindole was obtained as a byproduct in one of the ring expansion reactions. This compound is the keto form of enole63after aqueous workup.
The ring closure of iminium compound64to60is a disfavored 5‐endo‐trigcyclization process; see:
For other examples of 5‐endo‐trigcyclization processes see:
The use of the 2‐naphthylsulfonyl group as nitrogen protective group led to improved diastereoselectivity in some cases.
We could establish that the ring‐expansion reaction can also be performed with other monoactivated cyclopropanes. A lead result was published; see ref.[66]This discovery has been pursued by other research groups:
The ring expansion of62with tosyl isocyanate and the transformations of pyrrolidinone67into compounds68−72were undertaken by A. Lerchner during his diploma studies: A. Lerchner Diploma Thesis ETH Zürich 1999.
Angenot L., 1978, Plant. Med. Phytother., 12, 123
Only a single‐intermediate purification was required.