Pyridinium tosylate

Ethyl vinyl ether, pyridinium tosylate (PPTS), CH2CI2, it, 0.5 h. ... 

Pyridinium tosylate, -PrOH, 80-85% yield. An acetonide was not affected by these conditions. 

Pyridinium tosylate (PPTS), acetone, H2O, heat, 100% yield... 

A similar pre-orientation involving unsaturated carbon chains was operative on generating twelve-membered enediyne 23 and arenediyne lactams 24 [7]. The seco methylesters 21 and 22 were cleaved with LiOH, the corresponding carboxylic acids underwent cyclizations after activation with 2-fluoro-pyridinium tosylate 25 [8]. Dimerization products were found as by-products (<10%). It should be pointed out, that the lactamization succeeded in a single step in about 75% yield by treating the seco-methylesters 21 and 22 with Me3Al in refluxing methylene chloride. Obviously, the latter route was more convenient (Scheme 5). 

Dipolar cycloaddition of 5-aminofuro[3,2-f]pyridinium tosylate 140 and the alkynyl ester 141 yields furo[3,2-f]pyrazolo[l,5- ]pyridine 142 in moderate yield (Equation 33) <1999CCC539>. 

Alkenyloxazolidines.1 In the presence of pyridinium tosylate or BF, eth-erate, the N-protected norephedrine (1) cyclizes with the a,(3-unsaturated acetal 2 to give the 2-alkenyloxazolidine 3 as the major product. Cuprates add to 3 from the si face with high selectivity to give adducts (4), which are readily converted to (S)-3-alkylsuccinaldehydes in high enantiomeric purity. 

Aminofuro[3,2- ]pyridinium tosylates can be prepared from furopyridines by treatment with 0-(4-methylbenzene-sulfonyl)hydroxylamine. Treatment of the tosylate salt with base, such as potassium carbonate, leads to a pyridinium A -imide, 50. Compound 50 readily undergoes 1,3-dipolar cycloaddition reactions <1999CCC539>. 

Chabardes developed a process for the preparation of vitamin A and its intermediates, from cyclogeranylsulfone and Cio aldehyde-acetals [30]. For example, chlorocitral reacted with ethylene glycol, HC(OMe)3 and pyridinium tosylate to provide the chloroacetal (40%), as a mixture of two isomers. Reaction of this allylchloride with A-methylmorpholine oxide (NMO) and Nal furnished the aldehyde, as a mixture of four isomers. These compounds underwent condensation with P-cyclogeranylsulfone. Further chlorination of the sulfone-alkoxide salts, led to a mixture of sulfone-chloride acetals and their products of hydrolysis in 45-50% yield. Double elimination of the chloride and the sulfone, followed by hydrolysis with pyridinium tosylate (PPTS) gave retinal, as a mixture of all E and 13Z isomers (78/22). The overall yield from the chloroacetal was 18%. In another one-pot example, retinal was obtained in 52% yield from the aldehyde, and was then isomerised and reduced to retinol (all E 95.5, 13Z 4, 9Z 0.5) Fig. (8). 

Some heterocycles can be linked to supports as tetrahydropyranyl derivatives. Attachment of indoles, purines, or tetrazoles (Table 3.29) has been achieved by treatment of a support-bound dihydropyran with the heterocycle in the presence of catalytic amounts of pyridinium tosylate [487], camphorsulfonic acid [539], or TFA [540] in DCE at 60-80 °C for 16-24 h. Indole-derived orthoesters, such as that in Entry 7 (Table 3.29), can be prepared by heating the indole with triethyl orthoformate (160 °C, 24 h) followed by acid-catalyzed reaction of the resulting orthoester with a resin-bound diol [541,542], As illustrated by Entry 8 (Table 3.29), indoles can also be linked to the Wang resin or related supports as carbamates. Cleavage by TFA is, how-... 

Acetic acid,127 pyridinium trifluoroacetate (PTFA)121 or pyridinium tosylate (PPTS)128 are often added in order to speed up PDC oxidations. Acetic acid, which is described as superior127a and very easy to remove, is used most often. Although this precludes the advantages of using an almost neutral PDC medium, it provides a very useful substantial acceleration of the oxidations. The combined employment of molecular sieves and an acid can provide a synergistic accelerating effect.127a... 

Very occasionally, solvents other than benzene, such as toluene,23 CH2CI224 or DME,25 have been used. It must be mentioned that the use of polar solvents tends to promote the formation of methylthiomethyl ethers in oxidations with activated DM SO.26 So far, pyridinium trifluoroacetate27 is the acid most commonly used, while phosphoric28 and dichloroacetic acid18 are being used less often. Acids rarely used include pyridinium tosylate,29 pyridinium phosphate30 and pyridinium chloride,31 which are normally employed in the presence of excess of pyridine. 

Dihydroxy-6a-methyl-4-pregnene-3,20-dione Pyridinium tosylate Trimethyl orthoformate... 

Hydrolysis of alkylated products and carbonyl compound adducts derived from a-lithiated DHF and DHP with 2 M HC1 in THF at room temperature gave y- and 5-hydroxy ketones, respectively824,865 (Scheme 162). Jones oxidation generated keto acids866,887 and when the R substituent bears an hydroxy group, cyclization occurred in the presence of pyridinium tosylate (PPTS) in CH2C12 or HC1 in ether to provide spiroketals875,883,894,901. 

Methoxybutadienes. These dienes can be obtained by alkylation of the anion of methoxyallene followed by isomerization to the diene with pyridinium tosylate. Other acid catalysts are not useful. The (E)-isomer of the diene is formed predominantly or exclusively. 

In cases where readily oxidized functional groiqis are also present in either the substrate or the product of the reaction, overoxidation can occur. However, this can sometimes be advantageous. For example, in the total synthesis of ( )-steiepolide Trost and Chung effected the deprotection of the MPM ether (40) with DDQ in moist dichloromethane. Under the conditions of the reaction, furtiier oxidation of the allylic alcohol took place to afford the final product (equation 30). However, separate treatment of the allylic alcohol with PDC in dichloromethane was found to be more effective. Similarly, treatment of the ether (41) with pyridinium tosylate followed by excess DDQ afforded the Ireland alcohol (42), a key intermediate for the synthesis of tirandamycin A (equation 31). Propargylic alcohols, however, would appear to be less susceptible to oxidation. ... 

Acetalization. Pyridinium tosylate catalyzes the reaction of ketones with cthanediol to form ketals (90-95% yield) it also catalyzes the transacetalization of the ketals with acetone (90-95% yield). 

To a solution of 1 mmol of (-)-/V-bcnzyloxycarbonylnorcphcdrinc in 10 mL of dry benzene is added 0.3 mmol of pyridinium tosylate and 1.1 mmol of methyl 4 4-dimethoxy-2-butenoate. The mixture is refluxed for 12 h with a bypassed dropping funnel filled with 4 A molecular sieves placed between the flask and the reflux condenser. The mixture is cooled, 10 mL of Et20 arc added, the filtrate washed with 5% aq NaHCOj and sat. aq NaCl and finally dried over Na2S04. The mixture is then filtered and the solvent evaporated in vacuo. GC analysis of the crude product gives a d.r. of 94 6. Chromatographic purification gives the 2-alkenyloxazolidine 1 in 88% yield. [a]D —81.6 (c = 1, CHCf,). 

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