Pyridinium p-toluene sulfonate

Trans-acetalisations are often performed in DMF at elevated temperatures under reduced pressure or a stream of nitrogen. These conditions result in removal of the methanol that is formed in the reaction and, as a. result, the benzylidenation is driven to completion. It has been reported that the use of chloroform as a solvent gives higher yields and, in this case, pyridinium p-toluene sulfonate (PPTS) may be used as a catalyst. 

Salt 1-Methoxy- pyridinium iodide 1-Ethoxy- pyridinium iodide 1-Butoxy- pyridinium p-toluene- sulfonate... 

The 1,3-anti -reduction of (3-hydroxy ketones was utilized in the total synthesis of (+)-roxaticin (14), a pentaene macrolide isolated from streptomycete X-149944 (Scheme 4.2e). The (3-hydroxy ketone 15 underwent 1,3-anti -reduction to afford the diol 16 in 99% yield and greater than 95 5 diastereoselectivity. The resulting diol was then protected as a cyclopentylidene ketal (17) by using cyclopentylidene dimethyl ketal and pyridinium p-toluene sulfonate (PPTS). 

Pyridinium p-toluen sulfonate, EtOH, 22-55°, 1.2-2 h, 80-92% yield.27 These conditions were used to reii iove cleanly a TBDMS group in the presence of a TBDPS group. 

Our final example is a base-labile 4-(phenylsulfonyl)methyl-l,3-dioxolane protecting group for aldehydes and ketones.4 Protection is carried out by the reaction of diol 17,1 (obtained by dihydroxylation of ally phenyl sulfone) with a carbonyl compound in the presence of pyridinium p-toluene sulfonate [Scheme 2.17], Cleavage is accomplished by treatment with DBU. /erf-Butyldimethylsilyl ethers, p-toluenesulfonate esters, tetrahydropyranyl ethers, carboxylic esters and benzoates are well tolerated. A disadvantage to the use of 17.1 is the introduc-... 

The DTBMS ester was prepared (THF, DTBMSOTf, Et N, rt) to protect an ester so that a lactone could be reduced to an aldehyde. The ester is cleaved with aq. HF/THF or BtUtNE in wet THF. A THP derivative can be deprotected (pyridinium p-toluene-sulfonate, warm ethanol) in the presence of a DTBMS ester. ... 

This zinc-promoted reaction has been used with a variety of carbonyl compounds. Thus, the Luche conditions were applied in a synthesis of (-1-)-muscarine using an aldehyde derived from ethyl lactate [109]. Allyl halide condensation onto a-ketoamides of proline benzyl ester gave good diastereoselec-tivity when performed in the presence of zinc dust and pyridinium p-toluene-sulfonate in a water/THF mixture. In this way, a-hydroxy ketones were obtained with good enantioselectivity after removal of the chiral auxiliary [110]. Reactions of allyl bromide under the Luche conditions with y-aldo esters afforded y-hydroxy esters, which were converted in a one-pot reaction to y-allyl-y-butyro-lactones (Scheme 22) [111]. 

The catalysts of this ring opening polymerisation reactions are pyridinium salts (for example N-benzyl pyridinium p-toluene sulfonate [75]), p-toluene sulfonic acid [75], stannium or titanium compounds [68-74] etc. Other cyclic polymerisable cyclic carbonates are ethylene carbonate and propylene carbonate [68-74]. 

Such is the lability of TMS ethers, that they can usually be removed selectively in the presence of other members of the silyl ether family. For example, in Car-reira s synthesis of Zaragozic Acid C, a tertiary TMS ether was removed in the presence of a primary /err-butyldimethylsilyl (TBS) ether with chloroacetic acid in methanol [ heme 4.2] moreover, tertiary TMS ethers can be removed in the presence of primary triisopropyl (TIPS) ethers using pyridinium p-toluene-sulfonate in methanol [Scheme 4.3]. ... 

Triol 3. A solution of 1 (330 mg 0,84 mmol) in THF (5 mL) was treated at -78"C with MeLi (1.04 mL 1.62 mmol) in Et . After 15 min a sat. solution of NaHCOs was added, followed by extraction with tBuOMe. After evaporation of the solvent, the residue was treated with H2O2 (0.3 mL of 85%) and a catalytic amount of pyridinium p-toluene-sulfonate in THF (4 mL). After 20 min the mixture was extracted with petroleum ether and the crude hydroperoxide 2 was dissolved in THF (3 mL). EtaN (0.34 mL), p-nitrobenzenesulfonylchloride (197 mg 0.888 mmol) were added and after 29 min the mixture was diluted with tBuOMe and washed with NaHCOs sol. The solvent was exchanged with THF, EtaN (0.78 mL 5.7 mmol), AC2O (0.4 mL 4 mmol) and a catalytic amount of DMAP were added. After 2 h, work up and chromatography (silica gel, tBuOMeipetroleum ether 1 10) afforded 303 mg of 3 (80%). 

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