Large Ring Systems

The number of possible conformations rises dramatically with increasing ring size. Therefore, large ring systems carmot be handled by the methods applied to small rings. However, in rigid polymacrocyclic structures for example, an overall... 

Structural isomerization of three-membered rings with exocyclic unsaturation (Scheme 21) has been found with aziridineimines (Section 5.04.3.13) and diaziridineimines (Section 5.08.3.2.4). This involves an acyclic intermediate, while large ring systems prefer to isomerize through bicyclic ones. The 1,2- to 1,3-diazepine conversion is an example (Scheme 22) (Section 5.18.2.2). 

Oxidation of thiirane and thiirene oxides to the dioxides is the best method to obtain the sulfones. Indeed, in the acyclic, or large-ring systems, the sequence sulfide- sulfoxide - sulfone is by far the easiest method to prepare sulfoxides and sulfones. The situation is different in the three-membered ring series Thus, oxidation of thiiranes to the oxides by either perbenzoic acid or m-chloroperbenzoic acid under mild conditions affords the corresponding thiirane sulfoxides in almost quantitative yield - . However, further... 

Dimetallic large-ring systems incorporating bridging small... 

From biochemical characterization of this lead compound, we discover that three chemical groups make up the pharmacophore. The first group, shown in black, must contain a large ring system. Crystallographic analysis reveals that both single and... 

Medium-sized and large ring systems often show complicated conformational interconversions involving pseudorotations in one or even more conformational families. This makes stereochemical assignments in diastereomers rather difficult. Thus, very few systematic studies have been published. The situation is improved if such rings are embedded in polycyclic systems, or if they contain double bonds, which leads to restricted conformational mobility. An example is the differentiation of diastereomeric 2,3-dihydro-lf/-benzo[6]azepines 1 on the basis of y-gauche effects and on d(13C) and 3/H H values640. 

Even more interesting turned out to be the Sml2 -promoted intramolecular Reformatsky reaction. It opens a route to medium and large ring systems, as shown by the examples reported in Table l79a e. 

Chapter 3.5 attempts to give an overview of the reactivity of small or large ring systems treated individually in CHEC and CHEC-II. The great diversity of these systems presents a serious problem of organization. In structuring this chapter on reactivity, the nature of the reaction and the distance of the site of attack from the heteroatoms were used. 

SMALL AND LARGE RING SYSTEMS 1.17.4.1 Four-membered Rings... 

The contrasting reluctance of the three-membered ring -it system to take on four electrons is illustrated by the very low acidity of triphenyl cyclopropene (54), estimated to be roughly 18 powers of ten less than that of triphenylmethane.33 A number of ions and hetero-atom large ring systems are also known.34... 

The intramolecular iron-catalyzed Alder-ene reaction of enynes in the carbocy-clization reaction was recently reported by Furstner et al. (Scheme 9.8) [20], A low-valent cyclopentadienyliron catalyst, specifically the [CpFe(C2H4)2][Li(tmeda)] complex, is a reactive catalyst for enyne cydoisomerization reactions. Bicyclic products, also incorporating large ring systems, are thereby accessible, and the Thorpe-Ingold effect seems to be helpful for these types of reactions. 

Reactions of this type are also implicated in the isomerization of large ring systems through bicyclic ones, e.g., the 1,2- to 1,3-diazepine conversion (Scheme 8). 

Diphosphines with long chains between the P atoms, for example, Bu2P(CH2)5 gPBu2, can span trans positions in square complexes or give large ring systems, (10-LXXI) and (10-LXXII), respectively. A similar situation has also been found in an octahedral complex,86 where the trans isomer has been shown to be more stable than the cis. 

Examples often occur in large ring systems, but it is often possible to redraw the ring to avoid a reentrant bond. 

Such isomerizations can only be observed in open-chain structures and, of course, medium-large ring systems. The mode of irradiation is important, as illustrated in the representative example of equation (41), i.e the direct and sensitized irradiation of the 1,4-diene (35). While direct photolysis leads to the expected di-ir-methane product, sensitized irradiation brings about cis-trans isomerization before cycli-zation, resulting in products with inverted stereochemistry at the double bond. ... 

Although large ring systems pose interesting structural and stereochemical problems, few heterocycles have been reported that contain a metal or metalloid heteroatom. Those that have been prepared are generally obtained by an indirect route, and only derivatives containing an arsenic or a silicon atom have been reported. 

Rings containing seven to twelve carbon atoms are also subject to torsional strain, and hence these compounds, too, arc less stable than cyclohexane scale models also reveal serious crowding of hydrogens inside these rings. Only quite large ring systems seem to be as stable as cyclohexane. 

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