Tetrahydropyranyl cation

Such exploded transition states, in which the reaction centre carries a pronounced positive charge, are the commonest type of nucleophilic displacements at acetal centres. The reactions are unambiguously bimolecular, but in quantitative measures of transition state structure they differ only slightly from those of true 5 nI reactions. Thus, the value of for the hydrolysis of aryloxytetrahydropyrans, in which the solvent-equilibrated tetrahydropyranyl cation is a true intermediate, is —1.18, modestly but significantly more negative than the value of —0.82 obtained for hydrolysis of methoxymethyl esters of the type CH30CH20C0R. ... 

The reactions of nucleophiles with oxygen-substituted radical cations seem to be slower than the reactions of closed-shell molecules. In trifluoroethanol-acetonitrile-water mixtures the tetrahydropyranyl 2,3-radical cation " reacts with water at ambient temperature with a rate constant of 1.0 X 10" M This extrapolates to a lifetime in pure water of 18 ns, whereas that of the tetrahydropyranyl cation, " probably similar to that of the... 

This formal [2h-2h-2] alkyne/alkene/carbonyl cycloaddition proceeds through the opening of the cyclopropyl carbene intermediate 1-4 by the carbonyl group to form oxonium cation 1-5, which undergoes nucleophilic attack by the vinylgold intermediates in a Prins-type cyclization to give tetrahydropyranyl cation 1-6. 

The mechanism for this reaction is analogous to its intramolecular version [Ref. 223 in Chap. 1], and starts with the formation of the cyclopropyl carbene I-ll (Scheme 2.6). Direct attack of the aldehyde to the cyclopropyl gold(I) intermediate 1-11 leads to the oxonium cation 1-20, which suffers Prins-type cycUzation to give tetrahydropyranyl cation 1-21. Intermediate 1-21 can undergo metalation to yield bicycle 1-16 or can evolve by a fragmentation reaction to form 1,3-diene 1-17. 

Unsaturated tetrahydropyran derivatives have received only cursory attention in the literature as heterocyclic monomers. 2,3-Dihydropyran and several of its substituted derivatives apparently undergo cationic polymerization in a manner typical of vinyl ethers (72MI11103), while tetrahydropyranyl esters of methacrylic acid (123) are fairly typical free radically polymerizable monomers (Scheme 35) (74MI11105). The THP group was used in this study as a protecting group for the acid functionality, and it was found that deprotection of polymers (124) could be accomplished under extremely mild conditions. 

The reaction of dehydrolinalool acetate (274) with the homocuprate of olivetol bis(tetrahydropyranyl ether) (273) was expected to yield cannabichromene by analogy with the reaction with 3-acetoxy-3-methylbut-l-yne (79JCS(Pl)20l). However, the major product was 3,4-m-A1,2-tetrahydrocannabinol (275), which could readily be purified, thereby providing a valuable route to this substance. Its formation is thought to involve the generation of an allene which proceeds to the chroman via an allylic cation and trans addition to the isopropylidene group (Scheme 69). 

The use of a mixture of a strongly acidic cation-exchange resin and methanol to remove a tetrahydropyranyl protecting group offers a very mild method of deblocking that does not require the use of a base during the workup. 

In the early 1970s, Barton et al. published the results of their woik on the oxidation of acetals and ethers by hy de transfer. o They observed that substituted benzyl ethers and benzyloxy carbonates, on brief exposure to trityl tetrafluoroborate in dichloromethane at 0 C followed by aqueous work-up, afforded go( yields of the parent alcohols together with the corresponding bennldehydes. Undw the same conditions, the tetrahydropyranyl ether of cholesterol was also efficiently deprotected. A mechanism was proposed which involved an initial hydrogen abstraction, followed by quenching of the resulting stabilized cation by water (Scheme 6). 

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) ... 

Alder, R. W., Harvey, J. N., Oakley, M. T. Aromatic4-Tetrahydropyranyl and 4-Quinuclidinyl Cations. Linking Prins with Cope and Grob. J. Am. Chem. Soc. 2002, 124, 4960-4961. 

According to this scheme, A-(or-tetrahydropyranyl) aziridine is prepared and polymerized cationically to give the poly(iminoether) which is then hydrolyzed with aqueous acid and finally neutralized with alkali. 

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. 

Scheme 11.23 Synthetic outcomes guided by the weakening of bonds adjacent to radical cations (DCE = dichloroethane, THP = tetrahydropyranyl, PhMe = toluene).

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