Toluenesulfonyl leaving group

The synthesis of other anhydro sugar derivatives is usually accomplished by an internal displacement of a leaving group with inversion, as has already been emphasized for those anhydrides in which the anhydride bridge engages the anomeric position. Three such compounds that have been used in polymerizations deserve specific mention 5,6-anhydro-l,2-0-isopropylidene-a-D-glucofuranose,12 13 its 3-methyl ether,11 and 3,5-anhydro-l,2-0-isopropylidene-a-D-xylo-furanose,14 all of which were synthesized by p-toluenesulfonylation of the primary hydroxyl group of the parent, isopropylidene derivative, followed by treatment with base. 

At lower temperatures, a tosylate is formed from the reaction of p-toluenesulfonyl chloride and an alcohol. The new bond is formed between the toluenesulfonyl group and the oxygen of the alcohol. At higher temperatures, the chloride anion can displace the -OTos group, which is an excellent leaving group, to form an organochloride. 

The first step in the synthesis of saccharin is an electrophilic substitution reaction, like the first step of the benzocaine synthesis, but this time we want the orf/ro-substituted product. Chloro-sulfonic acid gives a mixtiue of ortho and para products—it is impossible to find conditions that completely avoid forming the para-toluenesulfonyl chloride. However, you may recognize an old friend here—the by-product is, of course, TsCl. You may have wondered why we always use TsCl and not PhS02Cl to make OH into a leaving group now you know. 

Abbreviations are often used for monovalent groups that commonly appear in organic compounds. Some of these abbreviations are shown in Table 1.1. Aryl may be phenyl, a substituted phenyl, or a heteroaromatic group like furyl, pyridyl, or pyrrolyl. Tosyl is shorthand for p-toluenesulfonyl, mesyl is shorthand for methanesul louyi, and triflyl is shorthand for trifluoromethanesulfonvl. TsO, MsO-, and TI O are abbreviations for the common leaving groups tosylate, mesylate, and triflate, respectively. 

Phenylmethanesulfonyl fluoride was chosen for the first step because the phenylmethanesulfonyl group is a better leaving group in the SN2 displacement of the second step than the p-toluenesulfonyl group,... 

Alkyl chlorides, bromides and iodides can be formed by the reaction of alcohols with p-toluenesulfonyl chloride (or tosyl chloride, abbreviated as TsCl) in the presence of a nitrogen base (e.g. triethylamine or pyridine). The OH group is converted into a tosylate (abbreviated as ROTs), which can be displaced on reaction with CF, BF or I-. The stable tosylate anion is an excellent leaving group (SN1 or SN2 mechanism depending on the nature of the alkyl group, R). 

In the system at hand, diosgenin (7-2) is first converted to its 3-toluenesulfonate 10-1 by reaction with /i-toluenesulfonyl chloride (Scheme 2.10). Solvolysis of this compound under weakly acidic conditions leads to displacement of the excellent leaving group / -toluenesulfonate and formation of the cyclopropane-containing derivative 10-2. The newly formed hydroxyl group is next oxidized with chromium trioxide to give the 6-ketone... 

It is fairly easy to convert an alcohol into a tosylate by treating it with para-toluenesulfonyl chloride, so this would be an alternative way of converting the poor OH leaving group into a good leaving group. 

For primary and secondary alcohols, the hydroxyl is best made into a leaving group for elimination reactions by sulfonylation with para-toluenesulfonyl chloride (tosyl chloride, TsCl) or methanesulfonyl chloride (mesyl chloride, MeS02Cl or MsCl). 

Much more widely used carbene precursors are tosylhydrazones, which are readily prepared from aldehydes or ketones by reaction with 4-toluenesulfonyl hydrazide. Tosylhydrazones produce transient diazo compounds by base-catalyzed elimination of toluenesulfinate. The diazo compound is not normally isolated, and decomposes to the carbene on heating. The whole process is known as the Bamford-Stevens reaction. The leaving group is not the famihar tosylate (toluene-p-sulfonate, p.CH3-C, H4S03 , TsO ), but the less famihar toluene-p-sulfinate (Ts , P.CH3-C5H4S02 ) (Scheme 5.7). 

Nucleophilic substitution can occur with leaving groups other than halide. Alkyl p-toluenesulfonates (tosylates), which are prepared from alcohols by reaction with p-toluenesulfonyl chloride, are often used. 

The hydroxyl group of an alcohol is a poor leaving group and so sulfonation by treatment with the appropriate sulfonyl chloride provides a valuable method of activating the hydroxyl group towards nucleophilic substitution. The tosylate derivatives (17, R = / -MeCeH4) obtained by treatment of the alcohol with / -toluenesulfonyl chloride (tosyl chloride) in pyridine are widely used for the activation of the hydroxyl group. 

Alcohols react with />-toluenesulfonyl chloride to give useful synthetic intermediates called toluene-sulfonates, or tosylates. The/>-toluenesulfonate ion, a weak base, is an excellent leaving group. Methyl, primary, and secondary tosylates undergo S displacement reactions with nucleophiles. They also react with strong bases in E2 reactions. 

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