Complex Ring Species

The GeCNS ring resulted similarly10- from a dichlorogermylene reaction with an adamantylimidethione. In the ring, angles are close to 90°, Ge-C = 200.8 pm and Ge-N = 181.3 pm. 

The five-membered SnOSnC=C ring (lib) is formed105 by the oxidation of the Sn—Sn bond of the SnSnCC ring (11a) discussed later. 

A spiro-Sn completing two stannacyclopentadiene rings109 has twisted ring structures due to the bulky substituents Sn-C 215-216.5 pm and the ring angles at Sn are 85° and the non-ring ones, 117°. 

The earlier structures1,2,3 5 of sesquisulphides and selenides, (RE Xf,. were of the adamantane or P4O6 type. The alternative arrangement was found for RE = t-BuGe, 

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The term "macromolecules" implies that the subject matter includes chain molecules that may be built up of repeat units, as well as more complex ring, branched, or crosslinked species (for example, see Organo-Oxo-Element Macromolecules Related to Sol-Gel Processes, and contributions by Murray (p. 408) or Seyferth (p. 143)). 

A series of enterobactin analogs has been prepared as structural analogs to Fe(III) complexes. These species have facile oxidation processes (Ef = —0.53 to —l.OOV versus Cp2Fe/DMF) and are based on three catecholato ligands joined to a central aromatic ring (Fig. 7) or nitrogen atom by amide bridges [52]. The parent [V(cat)3] (cat = 1,2-catecholato)... 

Arenes are attractive building blocks for the synthesis of complex carbocyclic species. In addition to being inexpensive and available in a variety of substitution patterns, they possess ring structures composed entirely of unsaturated carbons. Because of this unsaturation, they have the potential for extensive functionalization. However, the realization of this potential requires synthetic methods that overcome the energetic barrier associated with aromatic stabilization. 

The different types of polycatenated species can be enumerated and classified on the basis of the increasing dimensionality of the component motifs (OD, ID or 2D) [3]. Finite (OD) motifs, containing closed circuits, can give, in principle, catenation into infinite periodic arrays (from ID up to 3D), though no real example has as yet been characterized. Examples of polycatenated species containing many molecular rings have been described within complex organic species [54], proteins [66] and synthetic DNA assemblies [67]. 

Carbonyl ylides, most often in the form of isomunchnones (formed by decomposition of diketo diazo compounds in the presence of rhodium (II) acetate, and subsequent cyclization of the intermediate rhodium carbenoid species) are by far the most studied 1,3-dipolar cycloaddition partners for indole derivatives. These cycloadditions have been employed in elegant examples of complex ring construction en route to a number of polycyclic indole-containing natural products. Preliminary work by Pirrung [54, 55] (Scheme 23) on simple intermolecular cycloadditions was followed shortly by the utilization of intramolecular examples by Padwa, Boger and others. 

Biosynthesis The biosynthesis of the L. a. has not been completely elucidated. Since Lycopodium species cannot or can only be cultivated with difficulty, tracer experiments must be performed at the natural locations. It is certain that the biogenetic building blocks for the complex ring systems are lysine and acetate. Pharmacological activity The L. a. are moderately toxic. Some L. a. are purported to be potent inhibitors of acetylcholine esterase. Huperzine A has attracted attention on account of its stimulating effect on learning and memory behavior in laboratory animals. In Chinese traditional medicine various Lycopodium species are used to treat skin diseases and as a tonic. 

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