Tetrahydropyranyl ethers cleaved

The tetrahydropyranyl ether, prepared from a phenol and dihydropyran (HCl/ EtOAc, 25°, 24 h), is cleaved by aqueous oxalic acid (MeOH, 50-90°, 1-2 h). ... 

The cyclohexylidene ketal, prepared from a catechol and cyclohexanone (AI2O3/ TsOH, CH2CI2, reflux, 36 h), is stable to metalation conditions (RX/BuLi) that cleave aiyl methyl ethers. The ketal is cleaved by acidic hydrolysis (coned. HCl/ EtOH, reflux, 1.5 h, 20°, 12 h) it is stable to milder acidic hydrolysis that cleaves tetrahydropyranyl ethers (1 AHCl/EtOH, reflux, 5 h, 91% yield). ... 

The /-propyldimethylsilyl ester is prepared from a carboxylic acid and the silyl chloride (Et3N, 0°). It is cleaved at pH 4.5 by conditions that do not cleave a tetrahydropyranyl ether (HOAc-NaOAc, acetone-H20, 0°, 45 min - 25°, 30 min, 91% yield). ... 

The tetrahydropyranyl ether, prepared from a phenol and dihydropyran (HCl/EtOAc, 25°, 24 h) is cleaved by aqueous oxalic acid (MeOH, 50-90°, 1-2 h). Tonsil, Mexican Bentonite earth, HSZ Zeolite, and H3[PW,204o] have also been used for the tetrahydropyranylation of phenols. The use of [Ru(ACN)3(triphos)](OTf)2 in acetone selectively removes the THP group from a phenol in the presence of an alkyl THP group. Ketals of acetophenones are also cleaved. ... 

Dihydropyran is of value as a protecting group for alcohols and phenols, and to a lesser extent amines, carboxylic acids and thiols (B-67MI22403, B-81MI22404). The resulting tetrahydropyranyl ethers (736) are stable to base, but are readily cleaved under acidic conditions (Scheme 284). 

Detritylation Trityl ethers are cleaved by reaction with formic acid in ether in high yield in 7-45 minutes. Under these conditions isopropylidene and benzyl-idene acetals and r-butyldimethylsilyl ethers are not affected, but tetrahydropyranyl ethers are partially cleaved. 

The mechanism of the formation of the tetrahydropyranyl ether (see Figure 23.1) is an acid-catalyzed addition of the alcohol to the double bond of the dihydropyran and is quite similar to the acid-catalyzed hydration of an alkene described in Section 11.3. Dihydropyran is especially reactive toward such an addition because the oxygen helps stabilize the carbocation that is initially produced in the reaction. The tetrahydropyranyl ether is inert toward bases and nucleophiles and serves to protect the alcohol from reagents with these properties. Although normal ethers are difficult to cleave, a tetrahydropyranyl ether is actually an acetal, and as such, it is readily cleaved under acidic conditions. (The mechanism for this cleavage is the reverse of that for acetal formation, shown in Figure 18.5 on page 776.)... 

Hydrogenolysis of the phosphate phenol ether 311.1 [Scheme 4.311] released a phosphoric diester whose inherent acidity was sufficient to cleave the 4-methoxy-tetrahydropyranyl ether function in situ. The desired product 31L2 was recovered as the sodium salt generated with the aid of an ion exchange resin. 

Cleavage of tetrahydropyranyl ethers. Tetrahydropyranyl ethers are cleaved to the alcohol by dimethylaluminum chloride or methylaluminum dichloride in high yield at temperatures of -25 to 25°, conditions that do not affect t-butyldimethylsilyl ethers. MOM and MEM ethers are converted into ethyl ethers by a methyl transfer reaction. 

Tetrahydropyranyl ethers are stable to bases and the protection is removed by acid-catalyzed hydrolysis. For example, geraniol (1.60) is protected as geraniol tetrahydropyranyl ether (1.80) in the presence of pyridinium p-toluenesulfonate (PPTS) reagent. These ethers are cleaved with PPTS in warm ethanoP (Scheme 1.19). 

The tricyclic alcohol (130) is an intermediate in the synthesis of rimuene. Hydroboronation of its tetrahydropyranyl ether and then oxidation with iodine and lead tetra-acetate afforded the 6—18 ether which could be cleaved and oxidized to a keto-acid. Such derivatives might form suitable intermediates for the synthesis of the rosane lactones. O-MethyI-14-methyl podocarpic acid has been synthesized "" by a conventional ring a + ring c route. 

The rearrangement which involves the intermediacy of the same cyclopropylcarbinyl cation (138) is completed in few minutes as monitored by t.l.c. Upon treatment with 10 mol % of pyridinium p-toluenesulphonate (PPTS) in ethanol at 55°C in order to cleave the protective group, some tetrahydropyranyl ethers of conjugated dienols (140) or enones (141) have undergone total C3 - C4 ring enlargement (equation 102) ... 

Cyclic acetals react faster their reactions require less than 2 h at -78 °C [111]. Tetrahydropyranyl ethers are cleaved to hydroxy esters (equations 370 and 371) [111]. 

The reaction of ozone with tetrahydropyranyl ether is similar to the reaction of ozone with an acetal. Since the hydrotrioxide cleaves to an oxy anion, the control elements that influence the chemistry of hemi-orthoesters will also control the chemistry of such hydro trioxides. The relative orientation of the hydrotrioxide functional group is therefore not important. However, the steric interactions need to be considered in arriving at the predominant conformers 120a, 120b, and 120c. 

Ethers, Esters, and Related Derivatives of Alcohols.—5a-Cholestanyl methyl ether has been cleaved inter alia) and converted into 5a-cholestanol by successive treatment with trimethylsilyl iodide and water.Pyridinium toluene-p-sulphonate has been reported as an efficient and mild catalyst for the conversion of alcohols into their tetrahydropyranyl ethers.Bile acids were efficiently performylated by treatment with 90% HCO2H-HCIO4. Selective base-catalysed... 

Deprotection of ethers. Both tetrahydropyranyl ethers and allyl ethers are cleaved... 

De-O-protection. Both tetrahydropyranyl ethers and p-methoxybenzyl ethers are said to be cleaved with this reagent. However, the method does not seem to have any advantage over existing procedures. 

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