Cyclopropyl ring

The molecule decomposes by elimination of CF, which should occur with equal probabilities from each ring when energy is randomized. However, at pressures in excess of 100 Torr there is a measurable increase in the fraction of decomposition in the ring that was initially excited. From an analysis of the relative product yield versus pressure, it was deduced that energy flows between the two cyclopropyl rings with a rate of only 3x10 s In a related set of experiments Rabinovitch et al [116] studied the series of chemically activated fliioroalkyl cyclopropanes ... 

Structures III and IV that have different spin-pairing schemes are expected to be higher in energy than type-I because of the strain introduced by the cyclopropyl rings. They may be anchors for secondary conical intersections around the most symmetric one. 

The most frequently encountered examples of cyclopropyl ring opening reactions in the steroid field are usually associated with angular or side chain methylation sequences. In fact, isotope labeling of the C-19 angular methyl group is the only reported application of this reaction for deuteration or tritiation pui poses. 

Reductive opening of the cyclopropyl ring in 9j5,19-cycloandrostan-ll-one (234) has been achieved by treatment with a large excess of sodium in iso-propanol-OD. Analysis of the product for isotopic purity after oxidation to the corresponding ketone and base-catalyzed back exchange of the 9a-deuterium [(235) (236)] shows 19% do and 10% 62 isotopic impurities. The 10% 62 product is probably due to incomplete back exchange. 

Cleavage of cyclopropyl rings in the presence of an acid or base is illustrated by tlie tritiation of the 19-angular methyl group in 5, 19-cycloandrostane-3, 17-dione [(239) (240) R = T]. This reaction is carried out by heating... 

Deuteration by cyclopropyl ring opening Ijas been used also for the stmcture elucidation of a photolysis product (241) of cholesta-3,5-diene. This bicyclobutyl derivative is very reactive and readily hydrolyzed by aqueous dioxane. When the ring opening is carried out in the presence of deuterium oxide, a deuterium is incorporated in the 4y -position of the resulting 6jS-hydroxy-3a,5a-cyclocholestane (242). ... 

The strength of the carbon-carbon bond adjacent to a difluoromethylene group IS hardly affected by the presence of fluorine on the cyclopropyl ring When heated, 2,2 difluoromethylenecyclopropane undergoes methylene-cyclopropane rearrangement [/2 ] Under kinetic control, 2,2 difluoro 1 methylenecyclopropane and (difluoromethylene)cyclopropane are formed in a 2 1 ratio, although the latter IS slightly more stable [129] (equation 27)... 

Opening of the cyclopropyl ring by the cyclopropylcarbinyl-liomoallyl arrangement, a scheme developed earlier for prosta-... 

A further example of the importance of this type of stereoelectronic effect is seen in the reactions of /-butoxy radicals with spiro[2,n]alkanes (22) where it is found that hydrogens from the position a- to the cyclopropyl ring arc specifically abstracted. This can be attributed to the favorable overlap of the breaking C-H bond with the cyclopropyl cr bonds.120131 No such specificity is seen with bicyclo[n, 1,0]alkanes (23) where geometric constraints prevent overlap. 

Cyclopropyl as a Neighboring Group On page 180, we saw that the properties of a cyclopropane ring are in some ways similar to those of a double bond. Therefore it is not surprising that a suitably placed cyclopropyl ring can... 

In some cases, conjugate addition has been performed on systems where a double bond is conjugated with a cyclopropyl ring. An example is ... 

The i yn-preference of 59a is concluded to be attributable to the fused cyclopropyl ring, based on the observation that the bicyclo[2.2.2]octene (59b) fused with a... 

Interest in presqualene pyrophosphate continues and it is claimed that the structure (88) assigned to a squalene precursor is incorrect. Presqualene pyrophosphate has been shown to contain a cyclopropyl ring (89), and both (89) and its parent alcohol have been synthesised. -Mechanisms for the conversion of (89) into squalene have been pub-lished. -... 

In a recent study, we showed that the more flexible pyrido[l,2-a]indole-based cyclopropyl quinone methide is not subject to the stereoelectronic effect.47 Scheme 7.17 shows an electrostatic potential map of the protonated cyclopropyl quinone methide with arrows indicating the two possible nucleophilic attack sites on the electron-deficient (blue-colored) cyclopropyl ring. The 13C label allows both nucleophile attack products, the pyrido[l,2-a]indole and azepino [l,2-a]indole, to be distinguished without isolation. The site of nucleophilic is under steric control with pyrido [1,2-a]indole ring formation favored by large nucleophiles. 

FTIR, 1H NMR and 13C NMR analysis support the polyacetal structure containing intact cyclopropyl rings cited by previous workers (5,6)... 

Earlier in this section we represented -cleavage as an opening of the cyclopropyl ring to yield a biradical. However, an alternative possibility exists/681 For example, of the intermediates in the following,... 

The second large group of reactions is characterised by the elaboration of the exocyclic double bond. This chemistry is particularly interesting because it allows the incorporation of a cyclopropyl ring in a more complex molecule, to take advantage of its peculiar feature as a reactive three-carbon functional group. 

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