Ethers, cleavage crown

Epoxides, summary of chemistry, 290 Equatorial bond, 168 Equilibrium and free energy, 36 Equivalent H s, 50 Erythro form, 82, 85 Esters, formation, 338 inorganic, 262, 272, 276 reduction, 261 Ethers, cleavage, 282 crown, 286 cyclic, 285 silyl, 286... 

The analogous ether cleavage reaction of the crown-5 analog of 40 rendered its synthesis from the aryl bromide and magnesium next to impossible isolation of crystalline 2-bromomagnesio-I,3-xylyl-18-crown-5 (41), formed in 16% yield, from the crude reaction mixture was not feasible. Therefore, pure 41 had to be obtained by treatment of the aryl bromide with /j-butyllithium, followed by the addition of 1 equiv magnesium bromide. The crystal structure of 41 contained two independent residues (41a and 41b) and proved to be very irregular the conformation of its crown... 

Reactions.—New methods for the synthesis and cleavage of benzyl and trityl ethers, and for the cleavage of allyl ethers have been discussed in an earlier section (Protection of Alcohols). Primary and secondary alkyl methyl ethers have been demethylated by the combination boron tribromide-sodium iodide-15-crown-5 The reagent methyltrichlorosilane-sodium iodide is a new combination for regioselective ether cleavage for example aliphatic methyl ethers undergo predominant demethylation to alcohols (for primary or secondary alkyl groups) or iodides (in tertiary cases). 

The principal variations on the normal crown synthesis methods were applied in preparing mixed crowns such as those shown in Eq. (3.55) and in forming isomers of the dibinaphthyl-22-crown-6 systems. The latter has been discussed in Sect. 3.5 (see Eq. 3.21) . The binaphthyl unit was prepared to receive a non-naphthyl unit as shown in Eq. (3.57). Binaphthol was allowed to react with the tetrahydropyranyl ether or 2-chloroethoxyethanol. Cleavage of the THP protecting group followed by tosyla-tion of the free hydroxyl afforded a two-armed binaphthyl unit which could serve as an electrophile in the cyclization with catechol. Obviously, the reaction could be accomplished in the opposite direction, beginning with catechol". ... 

A one-pot conversion of benzyl alcohols to benzyl fluorides by treatment of the alcohols with a combination of methanesulfonyl fluoride, cesium fluoride and 18-crown 6 ether in tetrahydrofuran has been repotted The reaction involves mesylation of the alcohols followed by cleavage of the resultant mesyl esters with a fluoride ion The reaction has been extended also to certain heterocycles bearing the N hydroxymethyl group [43] (equation 31)... 

Cleavage of mesyl or tosyl esters with K F in the presence of 18-crown-6 ether [46] or Kryptofix 222 [47] provides a reliable method for the preparation of F-labeled biologically active compounds... 

Importantly, the tethers in these bisadduds can be readily removed, making them true templating units. This point is illustrated by the cleavage of the crown ether tether in conjugate 24 to give untethered trans-1 fundionalized fullerenes that are of interest for use as further molecular scaffoldings (Scheme 5).156 60 611... 

Si—Siexo av. 2.352/2.409 A) reacts with KCg in diethyl ether by reductive cleavage of an exocyclic Si-Si bond. In the crystal, K(18-crown-6 R 3Si4 shows a separated ion pair and the anion R 3Si4 has a significantly distorted tetrahedran skeleton. 

Desired chemical functionality may also be introduced into SA monolayers via in situ cleavage of unsaturated surfactants. Crown-ether-complexed potassium permanganate was shown to break the carbon-carbon double bonds in the substrate-immobilized, unsaturated surfactants and to convert them to carboxyl groups. Slow lateral propagation of this bond breaking resulted in the formation of shorter monolayers with exposed terminal moieties (Fig. 22) [195]. 

The findings that, both in ester and amide cleavage, an alkaline-earth metal ion is still catalytically active when complexed with a crown ether, and that a fraction of the binding energy made available by coordinative interactions with the polyether chain can be translated into catalysis, provide the basis for the construction of supramolecular catalysts capable of esterase and amidase activity. 

Breccia, P., Cacciapaglia, R.. Mandolini. L. and Scorsini, C. (1998) Alkaline-earth metal complexes of thiol pendant crown ethers as turnover catalysts of ester cleavage. J. Chem. Soc., Perkin Trans., 2, 1257. 

Cleavage of aliphatic methyl ethersMethyl ethers can be cleaved in moderate lo high yield by addition of BBr3 to a solution of the ether and the Nal complex with I he crown ether in CH2C12. All three components are necessary for satisfactory results. The cleavage of primary and secondary methyl ethers proceeds satisfactorily, hut yields are low with tertiary ethers. 

McPhee, M. M. Kern, J. T. Hoster, B. C. Kerwin, S. M, (2000) Propargylic sulfone-armed lariat crown ethers alkali metal ion-regulated DNA cleavage agents Bioorg. Chem. 8, 98-118. 

McPhee, M. M. Kerwin, S. M, (2001) Synthesis, DNA cleavage, and cytotoxicity of a series of bis(propargylic) sulfone crown ethers Bioorg. Med. Chem. 9, 2809-2818. 

A high molecular weight 3-PL is obtained by the homogeneous polymerization of (5-PL in THF using metallic potassium and 18,6-crown ether. The initiation step is believed to involve the cleavage of the o-bond between the -CH2-CH2- of (5-PL instead of the acyl-oxygen or alkyl-oxygen bond (Scheme 10). 

Electrides made with crown ethers and oxa-based cryptands are generally unstable above -40 °C, since the ether linkages are vulnerable to electron capture and reductive cleavage. Using a specifically designed pentacyclic tripiperazine cryptand TripPip222 [molecular structure displayed... 

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