Leaving group tosylate

Step 4 The 1° alcohol is converted to a good leaving group (tosylate) for the ensuing displacement reaction (Step 5). 

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. 

Treatment with sodium gave the epoxide (S)-epi-chlorohydrin (19).Reaction of 17 with base, however, took advantage of the presence of the leaving group (tosylate) to form a hydroxy oxirane. Upon treatment of... 

FIGURE 21.55 If the leaving group (tosylate ion) re-adds to the cycUc ion to re-form starting material, racemization results. 

Section 8 14 Nucleophilic substitution can occur with leaving groups other than halide Alkyl p toluenesulfonates (tosylates) which are prepared from alcohols by reaction with p toulenesulfonyl chloride are often used... 

One of the more important approaches to 1-azirines involves a similar base-induced cycloelimination reaction of a suitably functionalized ketone derivative (route c. Scheme 1). This reaction is analogous to route (b) (Scheme 1) used for the synthesis of aziridines wherein displacement of the leaving group at nitrogen is initiated by a -carbanionic center. An example of this cycloelimination involves the Neber rearrangement of oxime tosylate esters (357 X = OTs) to 1-azirines and subsequently to a-aminoketones (358) (71AHC-(13)45). The reaction has been demonstrated to be configurationally indiscriminate both syn and anti ketoxime tosylate esters afforded the same product mixture of a-aminoketones... 

The preparation of esters can be classified into two main categories (1) carboxy-late activation with a good leaving group and (2) nucleophilic displacement of a caiboxylate on an alkyl halide or sulfonate. The latter approach is generally not suitable for the preparation of esters if the halide or tosylate is sterically hindered, but there has been some success with simple secondaiy halides and tosylates (ROTs, DMF, K2CO3, 69-93% yield). ... 

Trifluoromethanesulfonate (triflate) ion is an exceptionally good leaving grov. It can be used for nucleophilic substitution reactions on unreactive substrates. Acetolysis of cyclopropyl triflate, for example, occurs 10 times faster than acetolysis of cyclopropyl tosylate. Table 5.11 gives a conqiarison of the triftate group with some other common leaving groups. 

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)... 

The propensity of isocyanides to undergo nucleophilic a-additions at the terminal carbon, together with the presence of an activated methylene and a potential leaving group (i.e. tosyl), led van Leusen to suggest the following reaction path ... 

In general, the reaction can be performed between 0-60°C with the majority of the reactions being run at room temperature. The reactivity of the hydrazones with either the acyl or tosyl leaving group with thionyl chloride depends on the substrate. However, the acylated hydrazones generally provide gaseous by-products where as the tosyl chloride reaction products have to be separated from the reaction mixture. 

Perhaps the single most important reaction of enolate ions is their alkylation by treatment with an alkyl halide or tosylate, thereby forming a new C-C bond and joining two smaller pieces into one larger molecule. Alkylation occurs when the nucleophilic enolate ion reacts with the electrophilic alkyl halide in an SN2 reaction and displaces the leaving group by backside attack. 

Alkylation reactions are subject to the same constraints that affect all Sn2 reactions (Section 11.3). Thus, the leaving group X in the alkylating agent R—X can be chloride, bromide, iodide, or tosylate. The alkyl group R should be primary or methyl, and preferably should be allylic or benzylic. Secondary halides react poorly, and tertiary halides don t react at all because a competing E2 elimination of HX occurs instead. Vinylic and aryl halides are also unreactive because backside approach is sterically prevented. 

Tosylate (Section 11.1) A p-toluenesulfonate ester useful as a leaving group in nucleophilic substitution reactions. 

As shown, an asymmetric carboxylic-sulfonic acid anhydride is formed, but the cellulose attack occurs on the C = O group, since a nucleophilic attack on sulfur is slow, and the tosylate moiety is a much better leaving group than the carboxylate group [193]. Similar to other acylation reactions, there is a large preference for tosylation at the 5 position, and cellulose tosylates... 

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