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)₅M=C(OEt)C≡CR] (M = Cr, W; R = Ph, c-C₆H₉) is described. The synthesis involves the formation of a 1-metalla-1,3,5-hexatriene from the (1-alkynyl)carbene tungsten complex [(OC)₅W=C(OEt)C≡Cc-C₆H₉] and a secondary amine, and its thermally induced π-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)₅M=C(OEt)C≡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.