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COMMUNICATIONSIn conclusion. we have demonstrated that the oxide anion accelerated retro-Diels-Alder reaction can be employed for the facile generation of cyclic enediynes from 1,5-diyne precursors. The use of 9-BOM-anthracene (2) to mask the double bond of the enediyne offers the advantages of excellent stability towards a variety of synthetic transformations and convenient removal when desired. Although the enediyne precursor is highly stable under acidic or neutral conditions, treatment with base leads to rapid cycloreversion and development of the enediyne. This methodology offers significant scope for the generation of new cyclic enediynes. Precursors such as 11 have potential as novel prodrugs when conjugated to appropriate delivery systems.

It has been shown that a nickel(O) complex of bis(trimethylsilyl)butadiyne, (PhsP^Nif?/2-Me3SiC=CC=CSiMe3), readily reacts with the titanocene Cp2Ti(MesSiC=CSiMe3) and zirconocene Cp2Zr(THF)(Me3SiC=CSiMe3) complexes to form the heterobimetallic, doubly acetylide bridged complexes Cp2M(«-?7* 1:?72-C=CSiMe3)Ni(PPh3)(JM-?71:?72-C=CSiMe3) (M = Ti (1), M = Zr (2)). The structures of these complexes have been established by X-ray crystal structure analysis. Two , -bridging acetylide units are -bonded to different metals and -bonded to the second metal. Compound 1 in solution at 303 K is highly fluxional. An NMR study showed that at 190 K an equilibrium exists between one isomer with two nonequivalent and another isomer with two equivalent acetylide units. The cleavage of the central C-C single bond of the butadiyne was not observed in the reaction of the unsymmetrically substituted butadiyne PhC=CC=CSiMe3 with titanocene "Cp2Ti" generator Cp2Ti(Me3SiC=CSiMe3). The product is a bridging tetradehydro-(l-3-?/):(2-4-?7)-trans,transbutadiene unit (zigzag butadiyne) between two titanium centers in 0 2 {µ-(1-3-?7):(2-4-?7Hrans,irans-PhC=CC=CSiMe3}TiCp2 (5). If the phenyl(trimethylsilyl)butadiyne is complexed by nickel(O), the reaction with the titanocene generated from Cp2Ti(Me3SiC=CSiMe3) yields the heterobimetallic complex Cp2Ti(/i-?;1:?/2-C=CSiMe3)(w-?;1:?72-C=CPh)Ni(PPh3) (6). Both acetylide units are -bonded to the titanium atom and -bonded to the nickel atom, giving a "tweezerlike" structure.

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ZUSCHRIFTEN [5] Kronenether mit einer zentralen Arylboronsiureester-Einheit sindabhdngig vom Anionzu einer mono-oder ditopen Bindung von Kaliumsalzen fahig.

The activation and cleavage of the central C–C bond of substituted butadiynes has been combined for the first time with a subsequent coupling reaction of the resulting alkynyl units: In the presence of an equimolar amount of “Cp2Ti”, the butadiynes 1 and 2 afford after workup the asymmetrically substituted diyne 3. This photocatalyzed metathesis also proceeds in the reverse direction.