Iron-mediated and -catalyzed metalative cyclization of electron- withdrawing-group-substituted alkynes and alkenes with grignard reagents

摘要

Treatment of ethyl (E)-5,5-bis[(benzyloxy)methyl]-8-(N,N-diethylcarbamoyl)- 2-octen-7-ynoate with an iron reagent generated from FeCl_2 and tBuMgCl in a ratio of 1:4 (abbreviated as FeCl_2/4 tBuMgCl) afforded ethyl [4,4-bis[(benzyloxy)methyl]-2-[(E)-(N,N-diethylcarbamoyl)methylene] cyclopent-1-yl]acetate in good yield. Deuteriolysis of an identical reaction mixture afforded the bis-deuterated product ethyl [4,4-bis[(benzyloxy)methyl]-2- [(E)-(N,N-diethylcarbamoyl)deuteriomethylene]cyclopent-1-yl]deuterioacetate, thus confirming the existence of the corresponding dimetalated intermediate. The latter intermediate can react with halogens or aldehydes to facilitate further synthetic transformations. The amount of FeCl_2 was reduced to catalytic levels (10 mol% relative to enyne), and catalytic cyclizations of this sort proceeded with yields comparable to those of the aforementioned stoichiometric reactions. The cyclization of diethyl (E,E)-2,7-nonadienedioate with a stoichiometric amount of FeCl_2/4 tBuMgCl, followed by the addition of sBuOH as a proton source, afforded a mixture of 2-(ethoxycarbonyl)- 3-bicyclo[3.3.0]octanone and its enol form in good yield. The use of aldehyde or ketone in place of sBuOH afforded 2-(ethoxycarbonyl)-3-bicyclo[3.3.0]octanone, which has an additional hydroxyalkyl side chain. Additionally, the metalation of a carbon-carbon unsaturated bond in N,N-diethyl-5,5-bis[(benzyloxy)methyl]-7,8- epoxy-2-octynamide or (E)-3,3-dimethyl-6-(N,N-diethylcarbamoyl)-5-hexenyl p-toluenesulfonate with FeCl_2/4 tBuMgCl or FeCl_2/4 PhMgBr was followed by an intramolecular alkylation with an epoxide or alkyl p-toluenesulfonate to afford 5,5-bis[(benzyloxy)methyl]-3-[(E)-(N,N- diethylcarbamoyl)methylene]-1-cyclohexanol or N,N-diethyl(3,3- dimethylcyclopentyl)acetamide after hydrolysis. In both cases, the remaining metalated portion α to the amide group was confirmed by deuteriolysis and could be utilized for an alkylation with methyl iodide.