Flexible cyclic compounds

Acyclic diketones 120 (and sufficiently flexible cyclic compounds) possessing a hydrogen atom at C-4 of the 1,2-diketo system undergo intramolecular reactions via their lowest triplet states to form cyclobutanolones 122 in high chemical and quantum yields 1). The proposal that excited states of diones assume, whenever possible, a pla-... 

In cyclic compounds, there are much greater restrictions on conformational flexibility. In six-membered rings, the antiperiplanar requirement for E2 elimination is satisfied when both the leaving group and the adjacent H atom are axial. Compounds in which such a conformation is readily achievable undergo E2 elimination much more readily than those in which it is not. For example, in menthyl chloride, the C-Cl bond is not antiperiplanar to any adjacent C-H bond in the lowest energy conformation, and E2 elimination is therefore much slower than it is in the diastereomer neomenthyl chloride, in which the C-Cl bond is antiperiplanar to two C-H bonds in the lowest energy conformation. Moreover, two C-H bonds are antiperiplanar to the C-Cl bond in the reactive conformation of neomenthyl chloride, so two products are obtained upon E2 elimination, whereas only one C-H bond is antiperiplanar to the C-Cl bond in the reactive conformation of menthyl chloride, so only one product is obtained. 

There is good stereoselectivity with cyclic chiral alcohols such as 109 (>100 1) but the more flexible achiral compounds such as 111 are not so good. In both cases the OH group delivers the Si-H from the same face but 111 can adopt a different conformation.19... 

Ishihara developed a highly diastereo- and enantioselective direct Man-nich-type reaction of aldimines with 1,3-dicarbonyl compounds using chiral lithium binaphtholate salts as effective Lewis-acid-Bronsted-base catalysts (Scheme 2.5). ° The stereoselectivity of the Mannich products anti-S and syn-7 ) was reversed when the nucleophile was changed from acyclic 1,3-dicarbonyl compound 4 to cyclic compound 6. The molecular flexibility and acidity of the nucleophiles 4 and 6 would be the major factor in differentiating the two reaction pathways. 

It has been postulated that intramolecular rather than intermolecular chain-transfer contributes the predominant reaction in a solution of living poly(methyl-phenylsiloxane), and poly(dimethylsiloxane) with trimethylsilyl end-blocked groups. Occurrence of the intramolecular reaction, leading to formation of cyclic compounds of various sizes, was ascribed to the flexibility of siloxane chains. 

Both arrangements allow n overlap of indpient parallel 2p orbitals as the C bonds are broken. The more common anti periplanar geometry corresponds to the staggered anticonformation, which is easily achieved in conformationally flexible molecules. The syn periplanar geometry corresponds to an eclipsed conformation, which is important only in some rigid, cyclic compounds. 

The formation of macrocycles may also concern the rigid block. Rozbs [37] prepared liquid crystalline copolymers with flexible blocks of aliphatic polyethers and rigid blocks of aromatic polyesters. They were prepared by polytransesterification of aliphatic diols with aromatic diesters derived from biphenyl or frans-stilbene. Scheme 36 shows the formation of cycles when the diester is dimethyl-2,2 -biphenyldicarboxylate. In this case, the respective location of the ester groups is responsible for the formation of the cyclic compound. 

An interesting observation should be made concerning the dependence of the physical properties on molecular cyclicity, since it will have a significant effect on the formulation of electrolytes for lithium ion cells. While all of the ethers, cyclic or acyclic, demonstrate similar moderate dielectric constants (2—7) and low viscosities (0.3—0.6 cP), cyclic and acyclic esters behave like two entirely different kinds of compounds in terms of dielectric constant and viscosity that is, all cyclic esters are uniformly polar (c = 40—90) and rather viscous rj = 1.7—2.0 cP), and all acyclic esters are weakly polar ( = 3—6) and fluid (77 = 0.4—0.7 cP). The origin for the effect of molecular cyclicity on the dielectric constant has been attributed to the intramolecular strain of the cyclic structures that favors the conformation of better alignment of molecular dipoles, while the more flexible and open structure of linear carbonates results in the mutual cancellation of these dipoles. 

Steric control elements are also important for the diastereoselectivity in alkylations of mono-cyclic cyclohexanone enolates. However, electronic control becomes more evident in these systems compared to monocyclic cyclopentanone enolates The flexibility of the six-membered ring system, and the large number of possible ring conformations, makes predictions of the diastereoselectivity difficult. In general, one may conclude that the diastereoselectivity in alkylations of enolates derived from monocyclic cyclohexanones is not as high as in alkylations of cyclopentanone enolates. The syntheses of compounds 21-27 demonstrate the effect of substitution in each position of the six-membered ring49,61 -7°. 

Chls and all tetrapyrroles are heteroaromatic compounds and the aromatic character of the underlying tetrapyrrole moiety and the reactivity of the functional groups in the side chains govern their chemistry. Three different classes of tetrapyrroles, differentiated by their oxidation level, occur in nature porphyrins (11, e.g. hemes), chlorins (12, e.g. chls) and bacteriochlorins (13, e.g. bchls). As a cyclic tetrapyrrole with a fused five-membered ring, the overall reactivity of chi is that of a standard phytochlorin 7. Such compounds are capable of coordinating almost any known metal with the core nitrogen atoms. Together with the conformational flexibility of the macrocycle and the variability of its side chains, this accounts for their unique role in photosynthesis and applications ... 

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