Some recent applications of Fischer carbenemetal complexes in organic synthesis

{[2-(Dialkylamino)ethenyl]ethoxycarbene}chromium complexes 4 have been made available from lithiated terminal alkynes, hexacarbonylchromium, triethyloxonium tetrafluoroborate and secondary amines in a one-pot operation, in good to excellent yields. Reactions of these complexes with alkynes afford 5-dialkylamino-3-ethoxycyclopentadienes 8 with excellent chemoselectivity. From cyclopentadienes of type 8, angular and linear triquinanes, di- and triannelated benzene derivatives 24/25, steroid-like skeletons 30/31, and hexacycles 32/33 can be obtained with great facility. In addition, otherwise not easily accessible cyclopenta[b]pyrans 42/43 and novel spiro[4.4]nonatrienes 52/53 can be prepared in single operational steps from complexes 4 and terminal alkynes via [3 + 2 + 2 + 1] and [3 + 2 + 2 + 2] cocyclizations incorporating two and three alkyne units, respectively. Upon heating simple Fischer carbene complexes of type 2 with methylenecyclopropanes 64, cyclopentenones 65 are formed by formal [4 + 1] cycloadditions. New carbenemetal complexes which have different chemical reactivities can be formed in situ by transmetallation from the corresponding carbenechromium complexes. Various cyclopentenone, cyclopentene and cycloheptanone derivatives are easily accessible from these new carbenemetal (nickel and rhodium) complexes and an alkyne or an allene.