Phenazines are redox-active bacterial secondary metabolites that participate in important biological processes such as the generation of toxic reactive oxygen species and the reduction of environmental iron. Their biosynthesis from chorismic acid depends on enzymes encoded by the phz operon, but many details of the pathway remain unclear. It previously was shown that phenazine biosynthesis involves the symmetrical head-to-tail double condensation of two identical amino-cyclohexenone molecules to a tricyclic phenazine precursor. While this key step can proceed spontaneously in vitro, we show here that it is catalyzed by PhzA/B, a small dimeric protein of the Delta(5)-3-ketosteroid isomerase/nuclear transport factor 2 family, and we reason that this catalysis is required in vivo. Crystal structures in complex with analogues of the substrate and product suggest that PhzA/B accelerates double imine formation by orienting two substrate molecules and by neutralizing the negative charge of tetrahedral intermediates through protonation. HPLC-coupled NMR reveals that the condensation product rearranges further, which is probably important to prevent back-hydrolysis, and may also be catalyzed within the active site of PhzA/B. The rearranged tricyclic product subsequently undergoes oxidative decarboxylation in a metal-independent reaction involving molecular oxygen. This conversion does not seem to require enzymatic catalysis, explaining why phenazine-1-carboxylic acid is a major product even in strains that use phenazine-1,6-dicarboxylic acid as a precursor of strain-specific phenazine derivatives.

The intramolecularly coordinated triorganotin hexafluorophosphate {4-t-Bu-2,6-[P(O)(Oi-Pr)2]2C6H2}SnPh2 +PF6 - (3a) was prepared by reaction of the [4+2]-coordinated tetraorganotin compound {4-t-Bu-2,6-[P(O)(Oi-Pr)2]2C6H2}SnPh3 (2a) with Ph3C+PF6 - and shown to react under intramolecular cyclization with bromide and fluoride ion, respectively, as well as with water to give the intramolecularly coordinated benzoxaphosphastannole [1(P),3(Sn)-SnPh2OP(O)(Oi-Pr)-6-t-Bu-4-P(O)(Oi-Pr)2]C6H2 (4a). Analoguously, the in situ-generated intramolecularly coordinated triorganosiliconium hexafluorophosphate {4-t-Bu-2,6-[P(O)(OEt)2]2C6H2}SiPh2 +PF6 - (6) reacts with water to give the corresponding intramolecularly coordinated benzoxaphosphasilole [1(P),3(Si)-P(O)(OEt)OSiPh2-6-t-Bu-4-P(O)(OEt)2]C6H2 (7). Isotope-labeling experiments with H2 18O in combination with electrospray mass spectrometric studies reveal that in the case of the organotin compound 3a the cyclization exclusively proceeds via nucleophilic attack of water at the POC-carbon. In contrast, two pathways account for the formation of the benzoxaphosphasilole 7, that is, attack of water at the POC-carbon as well as at phosphorus. The latter pathway either is in contrast to the axial entry−axial departure principle of nucleophilic substitution at phosphorus or indicates Berry pseudorotation involving a five-membered chelate ring. The molecular structure of 3a was determined by single-crystal X-ray diffraction analysis.

Keywords: Ditopic complexation / Host-guest chemistry / NMR spectroscopy / X-ray crystallography / TinThe syntheses of the organotin-substituted crown ethers m-(Ph 2 XSnCH 2 CH 2 )-15-benzocrown-5 (2, X = Ph; 3, X = I; 4, X = Cl; 5, X = NCS) and the salt [m-(Ph 3 SnCH 2 CH 2 )-15-benzocrown-5·Na] + SCN -(2a) are reported. Compounds 2a and 4 (as its aqua complex 4·H 2 O) are characterized by single-crystal X-ray diffraction sudies. Multinuclear NMR spectroscopy and electrospray mass spectrometry reveal the triorganotin

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