3-COMPONENT INTRAMOLECULAR 2-ALKYNE ANNULATIONS OF FISCHER CARBENE COMPLEXES - NEW STRATEGIES FOR STEROID-SYNTHESIS

The two-alkyne annulations of a variety of chromium and tungsten carbene complexes are examined in which both of the alkynes are tethered to the carbene complex. The reactions of complexes in which the two alkynes and the first alkyne and the carbene carbon are connected by methylene groups were investigated for their potential in the construction of tricyclic phenols. This methodology was screened for a number of different ring sizes, and it was found that the efficiency of phenol formation depended on the nature of the metal, the solvent, and the sizes of the rings produced. In the reactions of chromium complexes, for certain ring sizes an unprecedented lactone product was also observed which resulted from the incorporation of the two alkynes as well as two carbon monoxide ligands from the metal. A new strategy for entry to the tetracyclic ring system of steroids is developed that involves a tandem coupling of a Diels-Alder reaction of carbene complexes with the double-intramolecular two-alkyne annulation. In a model system where the A and C rings are aromatic, the feasibility of this strategy was demonstrated in a reaction where all four rings of the steroid ring system can be generated in a one-pot reaction. In a second generation of this strategy, it was demonstrated that the tandem coupling of these reactions can also lead to the generation of nonaromatic steroidal ring systems.