Acetoxy group, participation

Furthermore, the rate constant for the reaction with the fra s-2-acetoxy compound was found to be nearly 10 greater than that of the cis isomer and five times greater than that of cyclohexyl tosylate itself. This evidence supported the view that the acetoxy group participates in the rate-limiting step of the reaction, not in a subsequent step after formation of the intermediate carbocation. The ionization of the tosylate is said therefore to be assisted... 

The effects of the anomeric configuration and the presence of acetoxy-groups at C-2 on the photocyclization of 3-oxobutyl glycopyranosides (Scheme 6) have been studied. j3-Compounds react preferentially, and C-2 acetoxy-groups participate in the reactions.A further photochemical reaction provides means of making L-ribofuranosides from D-mannoside derivatives via ketonic intermediates (Scheme C-Glycosidic ketones behave similarly (see below). 

A classic example of neighboring-group participation involves the solvolysis of compounds in which an acetoxy substituent is present next to a carbon that is undergoing nucleophilic substitution. For example, the rates of solvolysis of the cis and trans isomers of 2-acetoxycyclohexyl p-toluenesulfonate differ by a factor of about 670, the trans compound being the more reactive one ... 

Cl-to-C2 bond, the Cl-acetoxy group of ketose acetates can be expected to participate in replacement reactions at the anomeric center. The retention of configuration obtaining in the mercaptolysis of /3-D-fructopyranose pentaacetate (CXIV) suggests that the 1,2-cyclic ion (CXV) is an intermediate in the reaction. 

Using tetrafluoroboric acid, several unsaturated acids and nitriles were thus made reactive and were converted into acetoxy lactones [3]. Tosyloxy-, phosphoryloxy-and iodomethyl lactones as well as cyclic ethers resulting from analogous neighbouring group participation will be discussed in connection with other hypervalent iodine reagents. 

Participation of a 5-acetoxy group in the chromophore of the open form. 

Alcohols protected as methyl ethers can be retrieved by reaction with tribromo-borane. Activation of the methyl ether by co-ordination of the Lewis acidic tri-bromoborane followed by nucleophilic cleavage of the O—Me bond (with concomitant formation of bromomethane) is typical behaviour. However, the cleavage took a different course with 39.1 [Scheme L39] instead of the O—Me bond being cleaved, the alternative C—O bond cleaved owing to participation of the remote acetoxy group.72 The formation of bromide 39.4 with retention of configuration is circumstantial evidence implicating dioxonium ion intermediate 393. 

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