Readily available biphenyl derivatives containing an alkyne unit at one of their ortho-positions are converted into substituted phenanthrenes on exposure to catalytic amounts of either PtCl2, AuCl, AuCl3, GaCl3 or InCl3 in toluene. This 6-endo-dig cyclization likely proceeds through initial pi-complexation of the alkyne unit followed by interception of the resulting eta2-metal species by the adjacent arene ring. The reaction is inherently modular, allowing for substantial structural variations and for the incorporation of substituents at any site of the phenanthrene product. Moreover, it is readily extended to the heterocyclic series as exemplified by the preparation of benzoindoles, benzocarbazoles, naphthothiophenes, as well as bridgehead nitrogen heterocycles such as pyrrolo[1,2-a]quinolines. Depending on the chosen catalyst, biaryls bearing halo-alkyne units can either be converted into the corresponding 10-halo-phenanthrenes or into the isomeric 9-halo-phenanthrenes; in the latter case, the concomitant 1,2-halide shift is best explained by assuming a metal vinylidene species as the reactive intermediate. The scope of this novel method for the preparation of polycyclic arenes is illustrated by the total synthesis of a series of polyoxygenated phenanthrenes that are close relatives of the anticancer agent combretastatin A-4, as well as by the total synthesis of the aporphine alkaloid O-methyl-dehydroisopiline and its naturally occurring symmetrical dimer.

Synthesis of Phenanthrenes and Polycyclic Heteroarenes by Transition‐Metal Catalyzed Cycloisomerization Reactions

Green plants, within certain limitations, can adapt to a wide variety of unfavourable conditions such as drought, temperature changes, light variations, infectious attacks, air pollution and soil contamination. Depending on the strength of the individual impact(s), fluent or abrupt changes in visible or measurable stress symptoms indicate the deviation from normal metabolic conditions. Most of the visible or measurable symptoms are connected with altered oxygen metabolism principally concerning the transition from mostly heterolytic (two-electron transition) to increased homolytic (one-electron transition) processes. Homolytic reactions within metabolic sequences create, however, free radicals and have to be counteracted by the increase in radical-scavenging processes or compounds, thus warranting reaction sequences under metabolic control. At later states of stress episodes, the above control is gradually lost and more or less chaotic radical processes take over. Finally, cellular decompartmentalisations induce lytic and necrotic processes which are visible as the collapse of darkening cells or tissues. Every episode during this process is governed by a more or less denned balance between pro- and antioxidative capacities, including photosynthetic (strongly under metabolic and oxygen-detoxifying control) and photodynamic (only controlled by scavenger- and/or quencher-availability) reactions. This (theoretical) sequence of events in most cases can only be characterised punctually by strongly defined (analytical) indicator reactions (ESR) and is certainly species- and organ-specific.

Mechanisms of oxygen activation during plant stress

Indole-3-butyric acid (IBA) was recently identified by GC/MS analysis as an endogenous constituent of various plants. Plant tissues contained 9 ng g−1 fresh weight of free IBA and 37 ng g−1 fresh weight of total IBA, compared to 26 ng g−1 and 52 ng g−1 fresh weight of free and total indole-3-acetic acid (IAA), respectively. IBA level was found to increase during plant development, but never reached the level of IAA. It is generally assumed that the greater ability of IBA as compared with IAA to promote rooting is due to its relatively higher stability. Indeed, the concentrations of IAA and IBA in autoclaved medium were reduced by 40% and 20%, respectively, compared with filter sterilized controls. In liquid medium, IAA was more sensitive than IBA to non-biological degradation. However, in all plant tissues tested, both auxins were found to be metabolized rapidly and conjugated at the same rate with amino acids or sugar.

Studies of auxin transport showed that IAA was transported faster than IBA. The velocities of some of the auxins tested were 7. 5 mm h−1 for IAA, 6. 7 mm h−1 for naphthaleneacetic acid (NAA) and only 3. 2 mm h−1 for IBA. Like IAA, IBA was transported predominantly in a basipetal direction (polar transport). After application of 3H-IBA to cuttings of various plants, most of the label remained in the bases of the cuttings. Easy-to-root cultivars were found to absorb more of the auxin and transport more of it to the leaves.

It has been postulated that easy-to-root, as opposed to the difficult-to-root cultivars, have the ability to hydrolyze auxin conjugates at the appropriate time to release free auxin which may promote root initiation. This theory is supported by reports on increased levels of free auxin in the bases of cuttings prior to rooting. The auxin conjugate probably acts as a ‘slow-release’ hormone in the tissues. Easy-to-root cultivars were also able to convert IBA to IAA which accumulated in the cutting bases prior to rooting. IAA conjugates, but not IBA conjugates, were subject to oxidation, and thus deactivation. The efficiency of the two auxins in root induction therefore seems to depend on the stability of their conjugates. The higher rooting promotion of IBA was also ascribed to the fact that its level remained elevated longer than that of IAA, even though IBA was metabolized in the tissue.

IAA was converted to IBA by seedlings of corn and Arabidopsis. The Km value for IBA formation was low (approximately 20 μM), indicating high affinity for the substrate. That means that small amounts of IAA (only a fraction of the total IAA in the plant tissues) can be converted to IBA. It was suggested that IBA is formed by the acetylation of IAA with acetyl-CoA in the carboxyl position via a biosynthetic pathway analogous to the primary steps of fatty acid biosynthesis, where acetyl moieties are transferred to an acceptor molecule. Incubation of the soluble enzyme fraction from Arabidopsis with 3H-IBA, IBA and UDP-glucose resulted in a product that was identified tentatively as IBA glucose (IBGIc). IBGIc was detected only during the first 30 min of incubation, showing that it might be converted rapidly to another conjugate.

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Indole-3-butyric acid in plants : occurrence, synthesis, metabolism and transport

Biosynthesis and Metabolism of Plant Hormones

Prologos 1 Biochemical and Molecular Mechanisms of Ethylene Synthesis 2 Perception and Signal Transduction Pathways 3 Growth and Development 4 Ethylene and Senescence of Plant Organs 5 Stress Ethylene 6 Biotechnological Control of Ethylene Index of Authors Index of Keywords

Biology and Biotechnology of the Plant Hormone Ethylene II

Regulation of enzymic oxidation of indole-3-acetic acid by phenols: Structure-activity relationships

Effects of orchinol and related phenanthrenes on the enzymic degradation of indole-3-acetic acid

Glyphosate herbicides are used for weed control in crops. It has been found that the desired crop plants can be protected from the herbicidal effect by providing a pretreatment so that the crop plants have assimilated a phenolic compound found to give a safening effect.

Tsung T. Lee

Papers

Regulation of enzymic oxidation of indole-3-acetic acid by phenols: Structure-activity relationships

Effects of orchinol and related phenanthrenes on the enzymic degradation of indole-3-acetic acid

Phenolic safeners for glyphosate herbicides

Effect of 3,4-dihydroxyacetophenone and some related phenols on the peroxidase-catalysed oxidation of indole-3-acetic acid

Inhibition of enzymic oxidation of indole-3-acetic acid by metabolites of the insecticide carbofuran