An attempt to trap radical intermediates during the monoamine oxidase (MAO) catalyzed oxidation of amines by intramolecular cyclization with an activated alkene that is built into the substrate was unsuccessful. (E)-2- (Iodoethenyl)benzylamine (3a) was shown to be a reversible inhibitor of MAO B, but the corresponding Z isomer (3b) was a good substrate. By GC-MS analysis, the expected radical trapping product, isoquinoline (5), was observed as one of two products in 1:1 ratio. However, NMR analysis prior to GC-MS analysis showed no evidence of isoquinoline, suggesting that the isoquinoline was generated during gas chromatography. As a model for this GC- dependent reaction, the corresponding aldehyde, (Z)-2- (iodoethenyl)benzaldehyde (14), was treated with ammonia, and the product was analyzed by GC-MS; isoquinoline was detected. Likewise, the reaction of 14 with methylamine also produced isoquinoline by GC-MS analysis (but not by NMR analysis). These results are explained by electrocyclization of the corresponding imines at the elevated temperatures in the GC (Schemes 7 and 8). Substrate 3b and aldehyde 14 were recovered when the enzyme reaction products were chromatographed, although they were not detected by GC-MS. These products could arise via hydrolytic decomposition of the corresponding imine (17, Scheme 9). GC-MS analysis of 17 produced isoquinoline and (Z)-2- (iodoethenyl)benzyl iodide (18) in a 1:1 ratio; these are the two products observed in a 1:1 ratio after incubation of MAO with 3b and apparently arise from thermally induced electrocyclization and iodide ion cleavage (Scheme 9).
ASJC Scopus subject areas
- Organic Chemistry