Nos lO/ll

Abstract: The aryl groups are easily converted to the carboxyl group by oxidation with ruthenium tetroxide. Utilizing this transformation as a key step, several phytosiderophores (mugineic acid (l), 3-epi-hydroxymugineic acid (2), 2'-hydroxynicotianamine (3), and distichonic acid A (4)) and polyoxamic acid (S), the side chain moiety of the antifungal antibiotics polyoxins, have been efficiently and stereoselectively synthesized Reactivity and water solubility of the carboxyl group sometimes preclude the efficient and convenient synthesis of some natural products having the carboxyl functions. Since the carboxyl function will be easily formed by the oxidation of the aryl groups with ruthenium tetroxide, the temporary use of the aryl groups as a stable and non-reactive substitute for the carboxylic acid during the synthesis will be quite effective. -CO2H ) R u O~ Ar E C02H ( Ar Utilizing this methodology, we have succeeded in the efficient, convenient, and stereoselective synthesis of some natural products containing hydroxy amino acid functions (1-3), which are shown below. Mugineic acid (l), 3-epi-hydroxymugineic acid (2), 2'-hydroxynicotianamine (3), and distichonic acid A (4) are the phytosiderophores while polyoxamic acid (5) is the side chain moiety of the antifungal antibiotics polyoxins. Phytosiderophores The phytosiderophores produced in plants promote uptake and transport of iron required for the chlorophyll biosynthesis. Importance of the phytosiderophores in plant physiology as well as their unique amino acid structures have led us to synthesize them in an efficient manner suitable for the large scale production. In general, these phytosiderophores are composed of three parts, each of which is connected through the nitrogen atom. Thus, the three fragments should be synthesized first, and then the coupling of each fragment will follow to construct the whole molecule. 215

Nicotianamine Chelates Both Fe(III) and Fe(II). Implications for Metal Transport in Plants

Nicotianamine (NA) occurs in all plants and chelates metal cations, including Fe(II), but reportedly not Fe(III). However, a comparison of the Fe(II) and Zn(II) affinity constants of NA and various Fe(III)-chelating aminocarboxylates suggested that NA should chelate Fe(III). High-voltage electrophoresis of the FeNA complex formed in the presence of Fe(III) showed that the complex had a net charge of 0, consistent with the hexadentate chelation of Fe(III). Measurement of the affinity constant for Fe(III) yielded a value of 10(20.6), which is greater than that for the association of NA with Fe(II) (10(12.8)). However, capillary electrophoresis showed that in the presence of Fe(II) and Fe(III), NA preferentially chelates Fe(II), indicating that the Fe(II)NA complex is kinetically stable under aerobic conditions. Furthermore, Fe complexes of NA are relatively poor Fenton reagents, as measured by their ability to mediate H(2)O(2)-dependent oxidation of deoxyribose. This suggests that NA will have an important role in scavenging Fe and protecting the cell from oxidative damage. The pH dependence of metal ion chelation by NA and a typical phytosiderophore, 2′-deoxymugineic acid, indicated that although both have the ability to chelate Fe, when both are present, 2′-deoxymugineic acid dominates the chelation process at acidic pH values, whereas NA dominates at alkaline pH values. The consequences for the role of NA in the long-distance transport of metals in the xylem and phloem are discussed

A Synthesis of All Stereoisomers of Tenuecyclamide A Employing a Fluorous-Fmoc Strategy

A concise liquid-phase combinatorial synthesis of all stereoisomers of Tenuecyclamide A was achieved using a mixture of d-/l-alanine with each stereoisomer encoded by a different f-Fmoc tag. The synthetic strategy using f-Fmoc reagents as the protecting group for amino acids has been demonstrated to be a useful method for diverse polypeptide analogue synthesis