Cycloadditions. 50. Multipath reactions between intramolecularly formed oxazolium salts and nucleophiles
作者:Alfred Hassner、Bilha Fischer
DOI:10.1021/jo00037a023
日期:1992.5
Reaction of 2-(4'-bromobutyl)-5-ethoxyoxazole (1) with nucleophiles led either to S(N)2 substitution products or to products with a piperidine skeleton. The latter were shown to arise from an intramolecular ring closure to an oxazolium salt 7, which was faster in the presence of a catalytic amount of NaI and in a polar solvent and for which NMR evidence is presented. The further transformation of 7 to 3-6 apparently involves addition of nucleophiles to 7 to produce 4-oxazoline 8 which opens to azomethine ylide 9. Neutralization of the latter occurred either via a proton shift, an alkyl shift, or via trapping by a dipolarophile (electron poor or electron rich). FMO calculations explain the preferred regiochemistry observed during trapping of ylide 9b.