The Diels-Alder reactions of quinone imine ketals: a versatile synthesis of highly substituted 5-methoxyindoles.

[reaction: see text]. N-benzoylated quinone imine ketals undergo smooth cycloadditions in a [4 + 2] sense to yield the expected cycloadducts. The crude cycloadducts, when subjected to a short series of simple transformations, produce synthetically useful quantities of 5-methoxyindoles in excellent overall yields.

Cyclopentannulation of 3-alkylindoles: a synthesis of a tetracyclic subunit of the kopsane alkaloids.

Indoles which bear an alkyl substituent in the 3-position undergo a [3 + 2] annulation reaction when treated with 1,1-cyclopropane diesters in the presence of Yb(OTf)(3) resulting in 2,3-cyclopentanoindolines. Typically, the reactions are performed at elevated temperatures or at ultrahigh pressures. In cases where steric crowding is an issue, ultrahigh pressures are required. In reactions involving substituted cyclopropanes, significant regio- and diastereocontrol was observed. When the substituent was aromatic or olefinic, the reactions took place at ambient temperature and pressure. The applicability of this methodology to the preparation of a key tetracyclic subunit of the kopsane alkaloids was demonstrated.