Triple bonds smallest ring

The smallest unstrained cyclic triple bond is found in cyclononyne. Cyclooctyne has been isolated," but its heat of hydrogenation shows that it is considerably strained. There have been a few compounds isolated with triple bonds in seven-membered rings. 3,3,7,7-Tetramethylcycloheptyne (lOS) dimerizes within 1 h at room temperature,but the thia derivative 106, in which the C—S bonds are longer than the corresponding C—C bonds in 105, is indefinitely stable even at 140°C. ° Cycloheptyne itself has not been isolated, though its transient existence... 

Problem 15.40 Show steps in the synthesis of cyclooctyne, the smallest ring with a triple bond, from C.H,OOC(CH,),COOC,H, ... 

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... 

Like trans cycloalkenes, cycloalkynes with small rings are unstable. The carbon chain must be long enough to connect the two ends of the triple bond without introducing too much strain. Cyclooctyne is the smallest isolated cycloalkyne, though it decomposes upon standing at room temperature after a short time. 

The average bond strength of a triple bond is 200 kcal/mol (837 kJ/mol) therefore each pi bond is worth about 59 kcal/mol (247 kJ/mol). If placed in a ring, the triple bond can be bent from colinearity and become weaker and much more reactive. Cyclooctyne is the smallest ring that contains a triple bond and is still stable enough to be isolated. 

This method was used to make the smallest cyclic acetylenes—even an eight-mem bered ring (40) can have a triple bond in it. The a-diketone (41) is best made by an acyloin reaction from diester (42). ... 

Double and triple bonds can be contained in rings if the ring size is large enough to accommodate the strain incurred by the required bond angles. The smallest trans cycloalkene stable at room temperature is /rawr-cyclooctene. The smallest stable cycloalkyne is cyclooctyne. 

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