Cyclic bridged

The effect of C ,C -disubstituted amino acids (aaAAs) on peptide secondary structure has been studied in recent years.2a d While longer side-chain C ,C -di-n-alkyl amino acids promote extended peptide conformation,23 alicyclic aaAAs, in which the Ca carbon forms a cyclic bridge with itself, such a 1-aminocyclopentane-l-carboxylic acid (Ac5c) and 1-aminocyclohexane-l-carboxylic acid (Ac6c), have helix-forming characteristics similar to those of 1 -aminoisobutyric acid (Aib).2ax... 

The carbocations so far studied are called classical carbocations in which the positive charge is localized on one carbon atom or delocalized by resonance involving an unshared pair of electrons or a double or triple bond in the allylic positions (resonance in phenols or aniline). In a non-classical carbocation the positive charged is delocalized by double or triple bond that is not in the allylic position or by a single bond. These carbocations are cyclic, bridged ions and possess a three centre bond in which three atoms share two electrons. The examples are 7-norbomenyl cation, norbomyl cation and cyclopropylmethyl cation. 

Instead of vinylene bridges and their aza-analogues the heterocyclic end groups of a Weitz system can be linked by iso- or heterocyclic rr-ring systems. The remarkable variability of these cyclic bridges is demonstrated by the following examples. 

S.C. LCPs Flexible Cyclic Bridging Cyclic Functional Spacer ... 

The open carbocation is replaced by a cyclic bridged ion having Br partially bonded to each C (bromonium ion). In this way the stereochemical differences of the starting materials are retained in the intermediate. In the second step, the nucleophile attacks the side opposite the bridging group to yield the anti addition product. 

Cyclic nitrone (52) has a cyclic bridge and cyclic enol ether dipolarophile thus all three elements are present in rings (Scheme 13).24 Models show that direct cyclization of (52) is not possible because of the relative positions of the dipole and dipolarophile. However, a pentacyclic cage isoxazolidine was formed by heating (52) presumably epimerization at the phenyl-bearing bridgehead carbon preceded cycloaddition. 

Many substrates containing a cyclic bridge have been investigated in intramolecular nitrile oxide cycloadditions. Typical examples include cyclization of the nitrile oxides derived from nitro compounds (91) and (92 Scheme 26).44 MM2 calculations are consistent with these results a chair-like transition state led to formation of the 5,6-fused product (93). Cyclization of (92) occurred through a more flexible... 

Tetrahedral O (acyclic, bridging) Tetrahedral O (cyclic, terminal) Tetrahedral O (cyclic, bridging) Octahedral O or OH (terminal) Pentacoordinate O (terminal)... 

Cardenolides, cyclic bridged cardiac glycosides and bufadienolides are... 

A non-classical carbocation is a cyclic, bridged structure with delocalized a-bond in 3-centre-2-electron bonds of bridged system. 

Cyclic bridged enamines or fused iminium ions were similarly prepared and successively reduced to the corresponding amines, e.g. 32-34139. The formation of regioisomeric products 33 and 34, not separable by chromatography, is due to the rearrangement of the intermediate carbonium... 

Stabilizing PP, as was a new rubber grafted maleic anhydride HAS system. Cyclic bridged amines are also effective co-synergists with HAS, as are lactones. °° The polymer matrix has been found to influence the excited state properties of UV absorbers, while novel effective adducts of HAS and 2-hydroxyphenylbenzotriazoles have been made. ... 

Bearing in mind these requirements many classes of unconjugated dienes have been investigated. These include alicyclic, monocyclic, alkenyl substituted monocyclic, fused-ring cyclic, bridged-ring bicy-clic and tricyclic dienes. Of all the specific dienes investigated three have reached commercial status ... 

Figure 37 Bridged dendrimer types (a) linear bridged, (b) radial bridged, (c) macro-cyclic bridged.

The cyclic bridge structure (Fig. 2) proposed for this compound on the basis of infrared evidence has been confirmed by B and H nuclear... 

The predominant ion present is Pb4(OH)J, which is likely to have a cyclic bridged structure similar to that of Sn4(OH)J. ... 

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