New methods for reductive free-radical cyclizations of α-bromoacetals to 2-alkoxytetrahydrofurans with activated chromium(II)-acetate
作者:Claus Hackmann、Hans J. Schäfer
DOI:10.1016/s0040-4020(01)81285-0
日期:1993.5
free-radical cyclization of α-bromoacetals 4 to 2-alkoxytetrahydrofurans 5 in 54–93% yield induced by activated chromium(II)-acetate. Four new methods have been developed in order to activate the transition metal. Two of them require only catalytic amounts of chromium(II)-acetate, because it can be regenerated in situ chemically or electrochemically. The diastereoselectivity of the cyclization depends on
Dimeric Metal Complexes as Mediators for Radical C?C Bond-Forming Reactions
作者:Bernd Giese、Gebhard Thoma
DOI:10.1002/hlca.19910740523
日期:1991.8.7
(2) in the presence of alkyl halides leads to C-centered radicals which can be trappedby alkenes and yields saturated and/or unsaturated addition products. Carbon radicals are generated via halogen abstraction by the initially formed metal-centered radicals resulting from homolysis of the metal-metal bond of dimericmediators 1 and 2. No reaction occurs using octacarbonyldicobalt (3).
Catalytic amounts of cobalt complexes 2 and 3 are capable of generating alkyl radicals from the corresponding bromides under mild readion conditions, allowing the selective preparation of either saturated or unsaturatedradicalcyclization products.
Homolytic carbocyclization by use of a heterogeneous supported organotin catalyst. A new synthetic route to 2-alkoxytetrahydrofurans and .gamma.-butyrolactones