Hydroxy leaving groups

The formation of 69 suggests that even the p-toluenesulfonate ion can undergo the regioselective nucleophilic attack at the 5 position, as the 1-hydroxy leaving group departs from 39 upon protonation. 

In contrast to phosphorus esters, sulfur esters are usually cleaved at the carbon-oxygen bond with carbon-fluorine bond formation Cleavage of esteri nf methanesulfonic acid, p-toluenesidfonic acid, and especially trifluoromethane-sulfonic acid (tnflic acid) by fluoride ion is the most widely used method for the conversion of hydroxy compounds to fluoro derivatives Potassium fluoride, triethylamine trihydrofluoride, and tetrabutylammonium fluoride are common sources of the fluoride ion For the cleavage of a variety of alkyl mesylates and tosylates with potassium fluoride, polyethylene glycol 400 is a solvent of choice, the yields are limited by solvolysis of the leaving group by the solvent, but this phenomenon is controlled by bulky substituents, either in the sulfonic acid part or in the alcohol part of the ester [42] (equation 29)... 

As reagents concentrated sulfuric acid, hydrochloric acid, liquid sulfur dioxide, thionyl chloride, phosphorus pentachloride, zinc oxide" and even silica gel can be used. Reagents like phosphorus pentachloride (as well as thionyl chloride and others) first convert the hydroxy group of the oxime 1 into a good leaving group ... 

The stereochemical course of the Beckmann rearrangement often allows for the prediction of the reaction product to be obtained in general the substituent R anti to either the hydroxy or the leaving group will migrate ... 

In the initial step one hydroxy group is protonated, and thus converted into a good leaving group—i.e. water. Subsequent loss of water from the molecule proceeds in such a way that the more stable carbenium ion species 2 is formed. The next step is a 1,2-shift of a group R to the tertiary carbenium center to give a hydroxycarbenium ion species 4 ... 

This reaction illustrates the striking difference in behavior between carboxylic esters on the one hand and aldehydes and ketones on the other. When a carbanion such as an enolate ion is added to the carbonyl group of an aldehyde or ketone (16-41), the H or R is not lost, since these groups are much poorer leaving groups than OR. Instead the intermediate similar to 146 adds a proton at the oxygen to give a hydroxy compound. 

Because aromatic purines and purine nucleosides and free purines such as hypo-xanthine and guanine 242 are readily silylated-aminated [64] (cf Scheme 4.24), it is obvious that 6-membered hydroxy-N-heterocycles are analogously silylated-aminated, with reactivity in the order given in Scheme 4.25 [73] X=OTf is the best leaving group and X=NHSiMe3 (cf the transamination as discussed in Section 4.2.4) is the weakest. 

Polar C=Y double bonds (Y = NR, O, S) with electrophilic carbon have been added to suifinic acids under formation of sulfones. As in the preceding section one must distinguish between carbonyl groups and their derivatives on the one hand, and carboxylic acids (possessing leaving groups at the electrophilic carbon) on the other. Aldehydes " of sufficient reactivity—especially mono-substituted glyoxals - —and their aryl or arylsulfonyl imines have been added to suifinic acids (in a reversible equilibrium) to yield a-hydroxy or a-amino sulfones the latter could also be obtained from the former in the presence of primary amines (equation 26). 

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