Synthetic and photochemical studies of N-arenesulfonyl amino acids

Abstract

Near-UV irradiation of N-arenesulfonyl amino acids in aqueous solution in the presence of a water-soluble 1,5-dialkoxynaphthalene as light absorber and single electron source results in cleavage of the sulfonamide with very sub-stoichiometric release of the intact amino acid because of concurrent decarboxylation during photocleavage. In compounds with a single carboxylate group ortho to the sulfonamide this decarboxylation is significantly suppressed, presumably because the additional carboxylate is a preferred target for the oxidative decarboxylation. However, release of free amino acid is at best 30% of the converted starting material and the yield is not further improved if the sulfonamide is flanked by two carboxylate groups. The data suggest that fragmentation of the initially generated sulfonamide radical anion occurs by two pathways, only one of which can be intercepted by an ortho-carboxylate. Synthetic routes to the 2,6-disubstituted arenesulfonamides are based on ortho-directed metalation reactions followed by quenching with SO 2 and efficient conversion of the resulting arenesulfinates to arenesulfonyl chlorides.