Elimination toluenesulfonate

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

Conversion to p-toluenesulfonate esters (Section 8.14) Alcohols react with p-toluenesulfonyl chloride to give p-toluenesulfonate esters. Sulfonate esters are reactive substrates for nucleophilic substitution and elimination reactions. The p-toluenesulfonate group is often abbreviated —OTs. 

The reaction of benzenesulfonic acid with sodium hydroxide (first entry in Table 24.3) proceeds by the addition-elimination mechanism of nucleophilic aromatic substitution (Section 23.6). Hydroxide replaces sulfite ion (S03 ) at the carbon atom that bear s the leaving group. Thus, p-toluenesulfonic acid is converted exclusively to p-cresol by an analogous reaction ... 

The ketoxime derivatives, required as starting materials, can be prepared from the appropriate aromatic, aliphatic or heterocyclic ketone. Aldoximes (where R is H) do not undergo the rearrangement reaction, but rather an elimination of toluenesulfonic acid to yield a nitrile. With ketoxime tosylates a Beckmann rearrangement may be observed as a side-reaction. 

Benzo-annulated dihydrooxepinones are readily accessible (see Houben-Weyl, Vol. 6/4, pp453) and can be converted to the corresponding hydroxy derivatives by reduction of the ketone function. The elimination of water from 10,ll-dihydrodibenz[fc,/]oxepin-10-ol to give 1 was accomplished with /t-toluenesulfonic acid.165-167... 

Alternatively, the hydroxy group can be converted to the tosyl ester168 or replaced by chlorine108,169 followed by a base-catalyzed elimination. 1-Benzoxepin-5-(4//)-one is reduced with cerium(III) chloride/sodium borohydride to the hydroxy derivative. After conversion to the p-toluenesulfonate, the double bond is formed in 2 upon treatment with potassium tert-pen-toxide.168... 

The synthesis of 1-benzothiepin (7) has also been achieved, in 80% yield, by the reaction of 4,5-dihydro-l-benzothiepin-5-ol (6) with potassium hydride and -toluenesulfonyl chloride and subsequent elimination of p-toluenesulfonic acid with lerf-pentylpotassium 22... 

Triethylamine addition must be carried out slowly to avoid base-promoted elimination of p-toluenesulfonate in the final product. 

The preparation of imines, enamines, nitroalkenes and N-sulfonylimines proceeds via the azeotropic removal of water from the intermediate in reactions that are normally catalyzed by p-toluenesulfonic acid, titanium(IV) chloride, or montmorillonite K 10 clay. A Dean-Stark apparatus is traditionally used which requires a large excess of aromatic hydrocarbons such as benzene or toluene for azeotropic water elimination. 

Substituted furan formation by an indirect cyclization of 1,4-dicarbonyl derivatives has also been adopted as a key step in the synthesis of 3-oxa-guaianolides. Although 1,4-dicarbonyl compounds have been traditionally considered as the direct precursors for furans, treatment of 1,4-dicarbonyl compounds having a tertiary acetoxy group with p-toluenesulfonic acid leads to only 11% yield of an alkenylfurans as derived from a cyclization/acetoxy-elimination route. The following scheme shows an alternative multi-step conversion of the 1,4-dicarbonyl that leads to a more acceptable yield of the acetoxyfuran . 

Several procedures were developed for the transformation (223—>-224). The most commonly used procedures are based on treatment of the nitroso acetal with benzene saturated with HC1 (204, 205) and by the addition of a catalytic amount of para -toluenesulfonic acid or trifluoroacetic acid (206). The introduction of branched radicals at the silicon atom decelerates elimination of silanol. 

Elimination of water to afford a higher level of unsaturation was reported in some cases as shown in Scheme 30. Thus, the hydroxy compound 249, when treated with sodium hydroxide, gave 250 <1994ZOR774>, whereas treatment of 251 with />-toluenesulfonic acid gave rise to formation of 252 <2004KGS71>. 

Oxidation of evermicose (122) with bromine yielded a mixture of y- and <5-lactones, which was directly acetylated. Refluxing the acetate in benzene solution in the presence ofp-toluenesulfonic acid gave (176) a mixture of the unsaturated lactones 131 and 132. In related work, Ganguly and Saksena (177) obtained an enonolactone by oxidation of D-nogalose with Jones reagent, followed by -elimination promoted by piperidine. Similarly, L-no-galose gave the enantiomeric lactone. 

The cydization can also be carried out on a-tosylaminoallenes, in which case the choice of reaction conditions determines whether the product is the N-tosyl-3-pyrro-line or whether elimination of toluenesulfonic acid acid gives the pyrrole. For example, in the presence of catalytic silver nitrate, allene 141 (Eq. 13.47) rearranges to N-tosylpyrroline 142 in excellent yield, whereas when 141 is treated with potassium tert-butoxide in DMSO, pyrrole 143 is formed in 71% yield [54]. Warming the lithium salt of 141 in DMSO also leads to 143. The rearrangement of 141 to 143 may be mechanistically related to the conversion of 130 to 131 (Eq. 13.42). 

A long series on stereochemistry has continued in a smdy of the acetolysis of triterpenoid / -toluenesulfonates in the presence of NaOAc. Both substitution and elimination products were formed. Substitution could be accounted for by bimolecular processes (5 n2 on carbon, SaN on sulfur). Some confirmation of this was obtained by kinetic studies. 

The preparation of the requisite y-keto-p-toluenesulfonate rac-35 as homo-Favorskii precursor commenced with commercially available 2,5-dihy-droanisole (36) that was protected and epoxidized to acetal rac-31 (Scheme 11). Regioselective opening of the epoxide with p-chlorophenylse-lenide followed by sequential oxidation to the selenoxide and thermal elimination generated an allylic alcohol that was protected to give pivaloate rac-38. 

An intramolecular cycloaddition brought about by reaction with acetyl chloride and di-isopropylamine converted the imine 377, presumably via 378, to a mixture of the carbazole 379 and the tetracycle 380 prolonged heating alone or brief treatment of the latter with p-toluenesulfonic acid caused elimination and the formation of 379. ... 

For 3-methoxydihydrobenzoxazine 276, trifluoroacetic acid (TEA) in dichloromethane at 0°C was used <1987M273>. />-Toluenesulfonic acid (PTSA) is suitable also for the cyclization stage, but the reaction must be followed carefully to avoid the alcohol elimination. The 3-methoxydihydrobenzoxazines 277 and 278 were prepared using PTSA in toluene at 75 °C <1986H(24)3483>. 

Secondary and tertiary (3-hydroxy amides can be cyclized to oxazolines in the presence of strong acids such as methanesulfonic acid or p-toluenesulfonic acid. For tertiary (3-hydroxy amides, elimination to the enamide can often be a competing... 

The procedure described below is quite general and uses finely, freshly machine--powdered KOH, which is added to a solution of the primary or secondary (acetylenic) alcohol and a 10-15% molar excess of tosyl chloride in Et20, kept around 0 C The excess of tosyl chloride is destroyed during the reaction of the excess of KOH. Side- and subsequent reactions ("saponification of the ester by KOH and 1,2-elimination of p-toluenesulfonic acid from thtfester) can be suppressed by keeping the temperature of the reaction mixture below 5 C. This procedure can be carried out within 2 h and generally gives excellent (often almost quantitative) yields of the tosylates. Purification of acetylenic tosylates by distillation, which is risky because of the limited thermal stability of the esters, is not necessary because the... 

Base-catalyzed ring expansion has heen observed vit i cyclo propyInictliy p-toluenesulfonate and bromide.205 Treatment with potassium rert-butoxide in dimethyl sulfoxide induced competing y- and -elimination yielding cyclobutene (14) and methylenecyclopropane (15), respectively. An initial a-elimination was ruled out by deuterium-labeling studies. 

The elimination of methanol, ethanol, or acetic acid is useful for the preparation of 4f/-pyrans, provided that the products exhibit sufficient stability. Thus the thermolyses of 2-ethoxy- and 2-acetoxy-2, 3-dihydro-4//-pyrans 265 undoubtedly led to unsubstituted 4//-pyran (5),18,293 but only when R = Ac was it possible to separate the unstable product 5 from reaction mixtures by GLC in 15 to 30% yields.18 Analogously, 25% of air-sensitive 2-methoxy-2//-pyran (267) was obtained on heating 266 with aluminum tri-butoxide under a nitrogen atmosphere at 155°C.33 A general technique for the preparation of condensed 4//-pyrans from their 2-ethoxy-2,3-dihydro derivatives is based on the elimination of ethanol in the presence of p-toluenesulfonic acid or polyphosphoric acid at decreased pressures293 to give... 

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