The acquisition of water and nutrients by plant roots is a fundamental aspect of agriculture and strongly depends on root architecture. Root branching and expansion of the root system is achieved through the development of lateral roots and is to a large extent controlled by the plant hormone auxin. However, the pleiotropic effects of auxin or auxin-like molecules on root systems complicate the study of lateral root development. Here we describe a small-molecule screen in Arabidopsis thaliana that identified naxillin as what is to our knowledge the first non-auxin-like molecule that promotes root branching. By using naxillin as a chemical tool, we identified a new function for root cap-specific conversion of the auxin precursor indole-3-butyric acid into the active auxin indole-3-acetic acid and uncovered the involvement of the root cap in root branching. Delivery of an auxin precursor in peripheral tissues such as the root cap might represent an important mechanism shaping root architecture.

Experimental results on the electrical characteristics of Hg-alkanethiol∕arenethiol-Au molecular junctions are used to develop a physical model for the hysteretic negative-differential resistance (NDR) for these, and possibly other, metal-molecule-metal junctions. The dependence of the room-temperature current-voltage characteristic on sweep direction and sweep rate is examined. Based on several specific electronic behaviors, it is concluded that the NDR is caused by slow charge capture (reduction or oxidation) during the forward sweep and the resultant effect on tunneling. The implications of this model on potential electronic applications are discussed.

Roxaticin has been synthesized from polyol tetraacetonide 5, which was prepared by a threefold convergent route. Each of the optically pure building blocks (2, 3, and 4) was prepared using a Noyori asymmetric hydrogenation. Sequential a Iky lation of dibromide 3 with cyanohydrin acetonides 2 and 4 followed by stereoselective reductive decyanation gave tetraacetonide 5. The initial approach to roxaticin using a 1 -methylcyclopropyl ether in a key protection step was unsuccessful due to the instability of the polyene chain to oxidative deprotection. A 1,3-benzodithiolan-2-yl (BDT) ether performed well in a model study and was used in the roxaticin system. Protection of the roxaticin precursor as a BDT ether followed by elaboration of the polyene using Wollenberg's method gave a tetraenal. The macrocyclic ring was closed using an intramolecular Horner-Emmons Wittig reaction, and acid-catalyzed deprotection completed the synthesis of roxaticin. Our synthesis of roxaticin illustrates a first generation approach to the highly convergent synthesis of polyene macrolide antibiotics that should ultimately be useful for preparing stereochemical and structural analogs.Polyene macrolide antibiotics such as amphotericin B are important in the treatment of systemic fungal infections.2 Several

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Clathculins A and B represent a new class of vic-diamine alkaloids containing a PA2 unit as the basic structure. We report the first total syntheses of 1 and 2, which confirm the assigned structure of each. Dependence of their NMR spectroscopic behavior as a function of protonation state has been observed.