Steroid-like ring skeletons by cyclohexadiene annulation to enamines with alkynylcarbene complexes of chromium and tungsten via pyran-2-ylidene complexes

A highly regioselective cyclohexadiene annulation to the C=C(N) bond of an enamine is achieved in two steps involving the condensation of a 1-alkynylcarbene complex (CO)(5)M=C(OEt)C=CPh 1 (M = Cr, W) with an enolizable carbonyl compound (2,4-pentanedione, 3-oxobutyric acid ester, 2-tetralones, and 1,3-cyclopentanedione) to give a pyran-2-ylidene complex 3, 5, and 17, which on subsequent reaction with cyclic enamines 6, 9, 13, and 16, generate a 5-amino-1,3-cyclohexadiene by elimination of M(CO)(6). Thus, bicyclic ring skeletons 7 and 10, steroid-like molecules 8, 18, and 19, and tetracyclic compounds 11, 12, and 15 are obtained mostly in good chemical yields and under mild conditions. Side reactions, such as base-induced self-condensation of 3 to give aryl pyran-2-ylidene complexes 20 become efficiently suppressed in hydrocarbon solvents.