Stereocontrolled Synthesis of Angularly Fused Tricyclic Ring Systems by Means of 1-Metalla-1,3,5-hexatrienes (M=Cr, W)

An efficient pathway for the stereocontrolled synthesis of functionalized, angularly fused tricyclic ring systems from readily available (1-alkynyl)carbene complexes [(OC)(5)M=C(OEt)C(triple bond)CR] (M=Cr, W; R=Ph, c-C(6)H(9)) is described. The synthesis involves the formation of a 1-metalla-1,3,5-hexatriene from the (1-alkynyl)carbene tungsten complex [(OC)(5)W=C(OEt)C(triple bond)C-c-C(6)H(9)] and a secondary amine, and its thermally induced pi-cyclization to a tetrahydroindene, which undergoes a spontaneous isomerization to another tetrahydroindene. Condensation of these tetrahydroindenes with pyran-2-ylidene complexes derived from (1-alkynyl)carbene complexes [(OC)(5)M=C(OEt)C(triple bond)CPh] (M=Cr, W) proceeds smoothly giving angularly fused tricyclic ring systems, rearrangement of which may generate spiro(cyclopentane-1,1-indanes) as side products. The synthesis is highly versatile and can be applied to the formation of various ring systems, such as steroid-type ring skeletons.