[reaction: see text] Suitably substituted heteroaromatic compounds such as indole and pyrrole derivatives are excellent acceptor units for intramolecular couplings of samarium ketyls. They furnish highly functionalized indole derivatives with very good diastereoselectivities additionally. Intermediate samarium enolates can be trapped by electrophiles, allowing efficient tandem reactions.

Here we summarise our results for SmI 2 -induced 5-exo-trig to 8-exo-trig reductive cyclisations of suitably substituted indole and pyrrole derivatives. All precursors were easily prepared by simple N-alkylation or N-acylation of indole and pyrrole derivatives with the corresponding iodo alkanones, acid chlorides or lactones. The SmI 2 -induced cyclisations in most cases provided tri-and tetracyclic derivatives, even in the absence of HMPA, in good to very good yields and with excellent diastereoselectivities. Extensive investigations of the reaction conditions revealed that in the presence of dif-

Analogous applications of samarium diiodide as a promoter for ketyl cyclizations led to the discovery of several new and synthetically interesting reactions. Whereas samarium ketyl cyclizations with alkyne and allene units could be expected -though they have not been known before -the unplanned cyclization of the ketyl towards an aromatic ring uncovered a new mode of reaction. The resulting hexahydronaphthalene derivatives were formed in excellent diastereoselectivities and are synthetically very valuable building blocks. This new method was systematically investigated and extended to polycyclic compounds and those with heteroatoms in the spacer between the carbonyl group and the aromatic ring. The cyclizations involving indole and pyrrole derivatives are most promising for the stereoselective construction of alkaloid skeletons.

This report describes the development of a first and second generation approach towards the synthesis of the ABCEG pentacyclic core structure of Strychnos alkaloids. First, we discuss a sequential approach applying a series of functional group transformations to prepare suitable precursors for cyclization reactions. These include attempts of samarium diiodide-induced cyclizations or a Barbier-type reaction of a transient lithium organyl, which successfully led to a tetracyclic key building block earlier used for the synthesis of strychnine. Secondly, we account our first steps towards the development of an atom-economical samarium diiodide-induced cascade reaction using "dimeric" indolyl ketones as cyclization precursors. In this context, we discuss plausible mechanisms for the samarium diiodide-induced cascade reaction as well as transformations of the obtained tetracyclic dihydroindoline derivatives.