Nature's inventory of halogenation catalysts: oxidative strategies predominate.

Halogenated molecules are widely distributed in the biosphere. A variety of halogenated aromatic and aliphatic compounds are man-made and can be harmful to the environment due to their slow rates of biodegradation.1 Halogenated organic molecules can also arise as natural products in which the carbon-halogen bonds are generated enzymatically. To date, more than 4500 halogenated natural products have been discovered,2 though it is likely to be a substantially incomplete inventory. Enzymatic incorporation of halogens during natural product assembly alters physical properties, including electronic and steric effects that can be consequential for determining the affinity and selectivity of interactions with biological targets. Many of the genes encoding such halogen-incorporating enzymes are embedded in specific biosynthetic gene clusters, enabling coordinate regulation to activate these secondary metabolite pathways. Biological halogenation occurs on a diverse array of organic scaffolds, from terpenes to polyketides to nonribosomal peptides. Within these scaffolds, halogen atoms are incorporated on aliphatic carbons, olefinic centers, and a wide variety of aromatic and heterocyclic rings. A recent inventory of halogen-containing natural products indicated 2300 organochlorines, 2100 organobromines, 120 organoiodines, and 30 organofluorines.2 The larger representation of chlorinated and brominated metabolites probably reflects the abundance of chloride and bromide ions in microenvironments of terrestrial and marine producer organisms. A brief list of some chlorinated, brominated, and iodinated metabolites is noted below. Among the chlorinated natural products of therapeutic interest are vancomycin (1), rebeccamycin (2), chlortetracycline (3), and chloramphenicol (4; Figure 1A).3-6 The natural products 1-3 carry chlorine groups on aromatic rings while chloramphenicol has a 2,2-dichloroacetyl group. As shown in Figure 1B, the cyanobacterial metabolite barbamide (5) has a trichloromethyl substituent and syringomycin E (6), produced by the phytotoxic Pseudomonas syringae, is monochlorinated on the terminal methyl of a threonyl residue.7,8 In the biocontrol agents pyrrolnitrin (7) and pyoluteorin (8; Figure 1C), also produced by Pseudomonas strains, pyrrole rings are monoand dichlorinated, respectively.9,10 Brominated metabolites are typically produced by marine microorganisms, and three such molecules are shown in Figure 2. The bromoterpene snyderol comes in three isomeric forms (9-11; R, â, and γ), reflecting three modes of quenching an intermediate carbocation.11,12 Convolutamine A (12) has a tribromoanisole ring presumably reflecting three tandem bromination steps during maturation.13 The terpenoid product laurallene (13) has two C-Br bonds, most notably the terminal bromoallene group.14 Among iodinated natural products, calicheamicin (14), produced by a soil actinomycete, and tetraiodothyronine (15; thyroxine; T4), synthesized in humans (Figure 3), show * To whom correspondence should be addressed. Telephone: 617-432-1715. Fax: 617-432-0438. E-mail: [email protected]. † Harvard Medical School. ‡ Current address: Department of Biological Sciences, Research and Development, Boehringer Ingelheim (Canada) Ltd, Laval, Quebec H7S 2G5, Canada. § Department of Chemistry, Harvey Mudd College, Claremont, California 91711. | Current address: Department of Medicinal Chemistry, College of Pharmacy and the Life Sciences Institute, University of Michigan, Ann Arbor, MI 48109. 3364 Chem. Rev. 2006, 106, 3364−3378