Multipath Reactions between Intramolecularly Formed Oxazolium Salts and Nucleophiles

Reaction of 2-(4′-bromobutyl)-5-ethoxyoxazole (1) with nucleophiles led either to S_N2 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 nucelophiles 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.