Catalysis tetrahydropyranylation

The involvement of transition-metal allenylidene complexes in homogeneous catalysis was reported for the first time by B. M. Trost and co-workers in 1992 (Scheme 35) [293-295]. The catalytic reactions allowed the preparation of a wide variety of tetrahydropyranyl and furanyl p,y-unsaturated ketones starting from hydroxy-functionalized alkynols and allylic alcohols, the key step in the catalytic... 

Several authors reported the use of ionic liquids containing protonic acid in catalysis (118-120). For example, strong Bronsted acidity in ionic liquids has been reported to successfully catalyze tetrahydropyranylation of alcohols (120). Tetra-hydropyranylation is one of the most widely used processes for the protection of alcohols and phenols in multi-step syntheses. Although the control experiments with the ionic liquids showed negligible activity in the absence of the added acids, high yields of product were obtained with the ionic liquid catalysts TPPTS or TPP.HBr-[BMIM]PF6. By rapid extraction of the product from the acidic ionic liquid phase by diethyl ether, the reaction medium was successfully reused for 22 cycles without an appreciable activity loss. A gradual loss of the catalyst and a reduced volume of the ionic liquid were noted, however, as a consequence of transfer to the extraction solvent. 

O S-Orthoesters are much more stable to acid hydrolysis than their 0.0,0 analogues. Nevertheless they can be unleashed under essentially neutral conditions using mercury(II) catalysis [Scheme 2.116]241 or silver(I) catalysis [Scheme 2 117].144 Note that in both cases the labile ethoxyethyl and tetrahydropyranyl (THP) groups survived. 

Tetrahydropyranylation of hypoxanthine in DMSO obcurs with acidic catalysis and 2,3-dihydropyran, to produce l,9-bis(tetrahydropyran-2-yl)hypoxanthine (105) and the corresponding 1,7 isomer (106) (66JOC268s). Alkylation studies of 2-oxodihydropurine have been limited and no data are available for the 8-oxo isomer. Methylation of the 2-oxo derivative with methyl iodide in alkaline solution at 80 °C leads to the 7-methyl, then the 3,7-dimethyl derivatives. 

Problem 3.7. Dihydropyran (DHP) reacts with alcohols under acid catalysis to give tetrahydropyranyl (THP) ethers. The alcohols can be released again by treating the THP ether with MeOH and catalytic acid. Thus, the THP group acts as a protecting group for alcohols. Draw mechanisms for the formation and cleavage of the THP ether. 

S-, N-Protective derivatives. The reagent reacts under mild acid catalysis with primary and secondary alcohols, and with phenols, to form tetrahydropyranyl ethers, which are stable to bases, to RMgX, to L1AIH4, to acetic anhydride in pyridine, and to oxidation. When synthetic operations at another site in the molecule have... 

Propargylic alcohols, even though tertiary (1), react with dihydropyrane under catalysis from p-toluenesulfonic acid to give tetrahydropyranyl derivatives (2), which are stable to base but hydrolyzed readily by aqueous acid or magnesium sulfate. ... 

The hydrolysis of p-nitrophenyl tetrahydropyranyl ether does indeed exhibit general acid catalysis with a Bronsted a of 0.5. ... 

Electrophilic catalysis of the departure of halogens in the century-old Koenigs-Knorr reaction is implicit in the use of heavy metal bases such as silver oxide and mercuric cyanide, but the first demonstration of electrophilic catalysis in water (in the hydrolysis of the p-glucoside of 8-hydroxyquinoline by first-row transition metals (Cu Np > C")) was by Clark and Hay in 1973. The observations were expanded to the more conveniently followed (because more labile) benzaldehyde methylacetals or tetrahydropyranyl derivatives of 8-hydroxyquinoline, whose hydrolysis is now known to give solvent-equilibrated oxocarbenium ions (Figure 3.19). Surprisingly, however, the observation of electrophilic catalysis of glycoside hydrolysis itself was not picked up by paper... 

Aryl but not alkyl tetrahydropyranyl acetals show general acid catalysis, for the same reason [13] but aryl methyl acetals do not, because the methoxymethyl car-benium ion is not sufficiently stable. (This situation can lead to enforced general acid catalysis, when the specific acid catalyzed reaction requires nucleophilic assistance if the nucleophile is the conjugate base of the general acid this will be observed as general acid catalysis.) At the other extreme, sufficient stabilization of the carbenium ion can have the same effect, as shown by the observation of general acid catalysis of tropolone diethyl acetal 2.1 (Scheme 2.10) [14]. And even... 

Kloetstra, R van Bekkum, H Catalysis of the tetrahydropyranylation of alcohols and phenols by the H-MCM-41 mesoporous molecular sieve. J. Chem. Research (S) 1995, 26-27. 

Miscellaneous Reactions. In addition to the key reactions above, DDQ has been used for the oxidative removal of chromium, iron, and manganese from their complexes with arenes and for the oxidative formation of imidazoles and thiadia-zoles from acyclic precursors. Catal)ftic amounts of DDQ also offer a mild method for the oxidative regeneration of carbonyl compounds from acetals, which contrasts with their formation from diazo compounds on treatment with DDQ and methanol in nonpolar solvents. DDQ also provides effective catalysis for the tetrahydropyranylation of alcohols. Furthermore, the oxidation of chiral esters or amides of arylacetic acid by DDQ in acetic acid provides a mild procedure for the synthesis of chiral a-acetoxy derivatives, although the diastereoselectivity achieved so far is only 65-67%. ... 

Under acid catalysis, an alcohol reacts with dihydropyran to give the tetrahydropyranyl derivative (called a THP ether ) of the alcohol. 

Cyclohepta-amylose has been converted into the phosphates (21)—(23), which were examined as catalysts for the hydrolysis of 4-nitrophenyl tetrahydropyranyl ether at low pH and for the exchange of 4-t-butylphenacyl alcohol (tritiated in the a-methylene group) at high pH. All three isomers, as the phosphate dianions, were effective catalysts for the latter reaction in which the functional group assists enolization of the bound ketone. Only the 3-phosphate (22) showed net catalysis of the hydrolysis of the ether, which was bound and hydrolysed with the assistance of a monoanionic phosphoric acid group. 

The synthesis of shell crosslinked knedel (SCK) micelles has been reported. Various applications, in areas as diverse as solubilisation, catalysis, fillers, coatings and delivery, have been proposed for these nanoparticles. However, in all studies the micelle cores are based on PS or polyisoprene and are therefore permanently hydrophobic. The synthesis of two new classes of SCK micelles with hydrophilic micelle cores are reported. Successftil shell crosslinking relies on. selective quatemisation of the A block, which comprises 2-(dimethylamino)ethyl methacrylate (DMAEMA) residues. The B block comprises 2-(N-morpholino)ethyl methacrylate (MEM A) and forms the micelle core. The second class is zwitterionic SCK micelles, prepared from precursor DMAEMA-2-tetrahydropyranyl methacrylate diblock copolymers. Depending on the synthetic route employed, two types of zwitterionic SCK micelles can be obtained Type I micelles, with anionic cores and cationic coronas, and Type II micelles, with cationic cores and anionic coronas. These zwitterionic SCK micelles exhibit isoelectric points in aqueous solution. 14 refs. 

These ethers (and methoxymethyl) have not found extensive use in phenol chemistry as protecting groups possibly because their lability towards aqueous acid is so much greater than that of the conventional alkyl or benzyl group. One distinction which may be of advantage is that they are formed in the absence of base. Thus they are generated by addition of the phenol to an olefin, isobutylene or dihydropyran, under acid catalysis [12, 18]. The tetrahydropyranyl ether of 2-methyl-4-acetoxy-a-naphthol is thus formed at room temperature in ethyl acetate saturated with hydrogen chloride [18]. 

Carboxylic acids add to olefinic linkages under acid catalysis to form esters and the method has been employed to prepare tetrahydropyranyl (reagent dihydropyran) [52] and t-butyl (reagent wo-butene) [53, 54, 55] esters. The little used isopropylidene malonate (15) is similarly produced from iso-propenyl acetate and malonic acid in the presence of sulphuric acid [56, 57]. 

Partial tetrahydropyranylation of meso-erythritol and sorbitol made the products suitable for alkylation using phase transfer catalysis the advantages of a dimethylsulphoxide-hydrogen chloride system for selective tetrahydropyranylation were outlined. 

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