REDOX TROPONIZATION AS A NOVEL METHOD FOR THE SYNTHESIS OF STEREOISOMERIC ESCHENMOSERS OXIMES AND RELATED NON-BENZENOID AROMATIC SYSTEMS

A novel method for the synthesis of the oxime of 4-methyl-2,4,6-cycloh eptatrien-1-one (Eschenmoser's oxime) is proposed. The method involves redox enlargement of the ring of 4-dibromomethyl-4-methyl-2,5-cyclohe xadien-1-one oxime through the action of Ni(PPh(3))(4) in DMF (in the presence of Zn). The product is formed as a mixture of syn- and anti-f orms readily interconverting in solutions. A similar reaction of 4-met hyl-4-trichloromethyl-2,5-cyclohexadien-1-one oxime afforded the dimer of a gem-alpha-centered semiquinoid carbene yimino-2,5-cyclohexadieny l)-1,2-dichloroethylene), together with syn- and anti-isomers of 4-chl oro-5-methyl-2,4,6-cycloheptatrien-1-one oxime, which are readily sepa rable but also quickly interconverting in solutions. For the latter co mpounds, the complete H-1 NMR assignment of the stereoisomeric structu res has been carried out.