Elimination from sulfonates

The elimination from sulfonates of secondary alcohols is frequently easier than more direct methods applied to the free alcohols. As with the latter, there are the possibilities of isomeric olefin formation and rearrangement reactions. In addition, displacement and hydrolysis may occur, but these side reactions can usually be suppressed. 

Ester eliminations are normally one of two types, base catalyzed or pyrolytic. The usual choice for base catalyzed j5-elimination is a sulfonate ester, generally the tosylate or mesylate. The traditional conditions for elimination are treatment with refluxing collidine or other pyridine base, and rearrangement may occur. Alternative conditions include treatment with variously prepared aluminas, amide-metal halide-carbonate combinations, and recently, the use of DMSO either alone or in the presence of potassium -butoxide. 

The use of mesyl chloride for the dehydration of C-11 alcohols has already been mentioned, and mesylates can certainly be intermediates at least in the a-series. The preference for a coplanar trans arrangement is demonstrated by the well-known elimination reactions of tosylates of epimeric 20-alcohols (ref. 185, p. 616), although this does not restrict the usefulness of the reaction, and in some cases (sulfonates of 1 la-alcohols, for example) cw-elimination occurs (ref. 216, p. 293 ref. 224, 225, 226). 

A -Oleflns in the 5a-series are frequently formed by elimination from the tosylates of 3j5-alcohols. Contamination with A -compound is common, and puriflcation via a derivative (e.g. dibromide) may be necessary (see page 343). A 2-methyl substituent increases the selectivity  

It has been claimed that the elimination of tosylates of 3a-alcohols in 5jS-series gives 3-oleflns with high selectivity. However, the homogeneity of these products is questionable, in view of recent findings concerning the ehmination of 3-chloro compounds (see below) and Fieser s results with the elimination of methyl lithocholate tosylate (ref. 232, cf. ref. 233). Neutral alumina may also be used to effect elimination of tosylates of 3j5-alcohols if the alumina is pretreated with potassium hydroxide the inverted alcohol is the predominant product. 

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Olefin formation by elimination from sulfonate esters, 331... 

It should be mentioned here that if no other leaving group is present, sulfonyl can act as its own leaving group in hydroxide- or alkoxide-catalyzed elimination from sulfones. Carbanion formation is not involved in this but the promotion of the ionization of a C—H bond by the sulfonyl group is seen at the /1-carbon rather than the a-carbon, e.g. equation 21. 

Scheme 69 Elimination from sulfone linkers and incorporation of nucleophiles...

The simplest monomer, ethylenesulfonic acid, is made by elimination from sodium hydroxyethyl sulfonate and polyphosphoric acid. Ethylenesulfonic acid is readily polymerized alone or can be incorporated as a copolymer using such monomers as acrylamide, aHyl acrylamide, sodium acrylate, acrylonitrile, methylacrylic acid, and vinyl acetate (222). Styrene and isobutene fail to copolymerize with ethylene sulfonic acid. 

Ehminations of HX to give double bonds offer considerable scope for selectivity and choice of reaction conditions. The dehydration of alcohols is the most common example of this class and may be achieved directly or through intermediate derivatives. In most cases, such derivatives are transient species formed in situ, but sometimes e.g. sulfonates, certain other esters and halides) they are isolated and characterized. Eliminations from jS-substituted ketones are very facile. The dehydration of jS-hydroxy ketones has been covered in section V. 

Nitroalkenes react with lithium dianions of carboxylic acids or with hthium enolates at -100 °C, and subsequent treatment of the Michael adducts with aqueous acid gives y-keto acids or esters in a one-pot operation, respectively (Eq. 4.52).66 The sequence of Michael addition to nitroalkenes and Nef reaction (Section 6.1) provides a useful tool for organic synthesis. For example, the addition of carbanions derived from sulfones to nitroalkenes followed by the Nef reaction and elimination of the sulfonyl group gives a,P-unsaturated ketones (Eq. 4.53).67... 

Tin radical-induced elimination from P-nitro sulfones or P-nitro sulfides proceeds in a stereoselective way to give anti elimination products.153 When diastereomers of P-nitro sulfones can be separated, each diastereomer gives ( )- and (Z)-alkenes selectively (Eq. 7.111). Such... 

When an oral suspension of albendazole was given via the drinking water to laying hens, both the parent compound and the sulfoxide and sulfone metabolites were rapidly eliminated from the tissues. At 96 h after treatment, liver con-... 

Table 4.1 gives the chemoselectivity of E2 eliminations from representative bromides of the type Rprim—Br, Rsec—Br, and Rten—Br. The fraction of E2 product increases in this sequence from 1 to 79 and to 100% and allows for the following generalization E2 eliminations with sterically unhindered bases can be carried out chemoselectively (i.e., without a competing SN2 reaction) only starting from tertiary alkyl halides and sulfonates. To obtain an E2 product from primary alkyl halides and sulfonates at all or to obtain an E2 product from secondary alkyl halides and sulfonates exclusively, one must change the base (see Section 4.4.2). 

The synthetic utility of this base (1) was demonstrated in the preparation of vinyl iodides in high yields from simple ketohydrazones and iodine (Table), a process that normally gives mixtures of vinyl iodides and geminal diiodides if less hindered bases are employed.6 This base has also been used in the elimination of sulfonic acids from the corresponding sulfonates, the alkylation of compounds containing active methylene groups, the conversion of hydrazones to vinyl selenides, and the preparation of esters from sterically hindered acids.4 5... 

To systematize the nomenclature of these processes, the eliminations of SO2 and SO3 are termed desulfination (from sulfinate) and desulfonation (from sulfonate), respectively. 

Reductive metallation must now be performed at room temperature for the electron transfer to proceed at a reasonable rate and under Barbier conditions. At this temperature, the intermediate reagent is stable enough for a carbon-carbon bond formation in the presence of the carbonyl compound (Barbier conditions) because without the latter, it would -eliminate or protonate. This simple procedure is mild enough to be extended to the synthesis of a carbon-linked disaccharide, as shown in Figure 17, for the preparation of the derivative of the C-linked mimic 24 of the a-D-marmopyranosyl(l->2)-D-glucopyra-noside from sulfone 22 and aldehyde 23. ... 

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