text: It was originally hypothesized that salinosporamide B was a biosynthetic precursor to salinosporamide A due to their structural similarities.
  It was thought that the halogenation of the unactivated methyl group was catalyzed by a non-heme iron halogenase. Recent work using C-labeled feeding experiments reveal distinct biosynthetic origins of salinosporamide A and B.
  While they share the biosynthetic precursors acetate and presumed β-hydroxycyclohex-2'-enylalanine (3), they differ in the origin of the four-carbon building block that gives rise to their structural differences involving the halogen atom. A hybrid polyketide synthase-nonribosomal peptide synthetase (PKS-NRPS) pathway is most likely the biosynthetic mechanism in which acetyl-CoA and butyrate-derived ethylmalonyl-CoA condense to yield the β-ketothioester (4), which then reacts with (3) to generate the linear precursor (5).