Double bonds smallest ring

The irans form, b.p. 143 "C, m.p. — 6 "C is of stereochemical interest since it is the smallest carbocyclic ring capable of accommodating a trans double bond. 

Double bonds are accommodated by rings of all sizes The smallest cycloalkene cyclo propene was first synthesized m 1922 A cyclopropene nng is present m sterculic acid a substance derived from one of the components of the oil present m the seeds of a tree (Sterculia foelida) that grows m the Philippines and Indonesia... 

While it is relatively easy to introduce a cis double bond into a small ring, it is very difficult to introduce a trans double bond. In fact, the smallest trans cycloalkene which has actually been isolated to date is cyclooctene. 

Ring-closing metathesis seems particularly well suited to be combined with Passerini and Ugi reactions, due to the low reactivity of the needed additional olefin functions, which avoid any interference with the MCR reaction. However, some limitations are present. First of all, it is not easy to embed diversity into the two olefinic components, because this leads in most cases to chiral substrates whose obtainment in enantiomerically pure form may not be trivial. Second, some unsaturated substrates, such as enamines, acrolein and p,y-unsaturated aldehydes cannot be used as component for the IMCR, whereas a,p-unsaturated amides are not ideal for RCM processes. Finally, the introduction of the double bond into the isocyanide component is possible only if 9-membered or larger rings are to be synthesized (see below). The smallest ring that has been synthesized to date is the 6-membered one represented by dihydropyridones 167, obtained starting with allylamine and bute-noic acid [133] (Fig. 33). Note that, for the reasons explained earlier, compounds... 

First, a list of unique scaffolds was derived and sorted by complexity. The complexity was calculated from four structural descriptors, namely number of rings in the smallest set of smallest rings, number of heavy atoms, number of bonds and the sum of heavy atomic numbers in the scaffold. Each scaffold or class center in the list was assigned an ID that corresponded to its position in the list. How much a molecule resembled its class center was determined by the number of side-chains attached to the scaffold. Fewer side-chains will give a closer resemblance to the class center. The similarity of a drug with the class center was reflected in the membership value. The membership value was based on the sum of heavy atomic numbers, the number of rotating bonds, the number of one and two nodes and the number of double and triple bonds in a molecule compared with its scaffold. Since the membership value indicated the contribution of rings in the class center for a certain... 

The smallest ring that contains a trans double bond and is still stable enough to be isolated at room temperature is fran -cyclooctene, which is about 11 kcal/mol (46 kJ/mol) more strained than cis-cyclooctene. Cyclobutene, with an untwisted cis double bond, is stable well above room temperature. Pi bonds between orbitals of different sizes are very weak because of poor overlap a good example is the 2p-3p carbon-sulfur double bond (Fig. 2.3). 

So far we have represented cycloalkenes by structural formulas in which the double bonds are of the cis configuration. If the ring is large enough, however, a trans stereoisomer is also possible. The smallest trans cycloalkene that is stable enough to be isolated and stored in a normal way is fran -cyclooctene. 

The smallest stable ring that can contain a trans double bond is cyclooctene—frans-cycloheptene can exist but is very unstable. 

Fully benzenoid hydrocarbons are those for which Clar s formulas contain only sextet rings and anpty rings and no CC double bonds. The smallest fully benzenoid hydrocarbon is triphenylene. 

In these examples, there is no need to identify the stereoisomerism of the double bond when naming each of these compounds, because the stereoisomerism is inferred. For example, the last compound is called cyclohexene, rather than e -cyclohexene. A seven-membered ring containing a trans double bond has been prepared, but this compound (mr r-cycloheptene) is not stable at room temperature. An eight-membered ring is the smallest ring that can accommodate a trans double bond and be stable at room temperature ... 

An eight-membered ring is the smallest size ring that can accommodate a trans double bond. This rule also applies to bridged bicyclic compounds and is called Bredt s rule. 

Of course, the larger the ring, the less problem there is because more atoms are available to span the trans positions of the double bond. In practice, the smallest trans cycloalkene stable at room temperature is /ra r-cyclooctene. Even here, the trans isomer is 11.4 kcal/mol less stable than its cis counterpart (Fig. 3.42). 

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