Intramolecular ether bonds

With regard to the type of thermally induced cross-links, the following conclusions may be drawn from our results Formation of C—C bonds can be definitely excluded due to the cleavage observed by aqueous acid. Intramolecular ether bonds within the glucose unit might explain the change in cuprammonium viscosity, but they should be cleaved also by aqueous alkali. The most probable conclusion is, according to our opinion, a formation of intercatenary acetal bonds (12). 

As for the biaryl ether containing macrocycles, an array of bioactive macrocycles with an endo aryl-aryl bond exist in nature. A new palladium catalyzed reaction has been recently developed in which bis(pinacolato)diborane(4) mediated the process to reach such a structural motif. The reaction consists of a domino process involving a Miyaura s arylboronic ester synthesis and an intramolecular Suzuki-coupling. Synthesis of a bicyclic A-B-O-C ring system of RP-66453 273, a neurotensine receptor antagonist, with an endo aryl-aryl and an endo aryl-aryl ether bond was described (Scheme 53).141... 

Vinyl radicals also add to carbon-carbon double bonds intramolecularly to give 2,6-cw-disubstituted cyclic ethers (Equation (5)).41 In the tin hydride-mediated cyclization of the substrates including alkynes, alkyl radicals attack to carbon-carbon triple bonds leading to uco-alkylidene allylic alcohols (Equation (6)).42 The coupling reaction between alkyl radicals may afford cyclization products. Thus, the reduction of 1,3-diiodopropane derivatives with a tin hydride provides substituted cyclopropanes.4... 

MDI is derived from the breakage reaction of the urethane linkage drawn in Scheme 12.1c, the polyester product are formed by intramolecular rearrangement of ether bonds, eliminating cyclic oligomers, and by radical breakage of the polyester chain to partly unsaturated linear oligoethers. 

The catalytic system [A] based on RuCl(Cp)(tris(p-fluorophenyl)phosphine)2 (5 mol%), tris(p-fluorophenyl)phosphine (20 mol%), (BU4NPF6, 15 mol%) and N-hydroxysuccinimide sodium salt (50 mol%) led to the selective transformation of pent-4-yn-l-ols into cyclic enol ethers via intramolecular anh-Markovnikov addition of the hydroxy group to the terminal carbon of the triple bond [21],... 

Cularine alkaloMSi A small group of benzylisoquino-line alkaloids that have an intramolecular ether bond between the A ring of the isoquinoline unit and the 1-benzyl group (oxepine ring). C.a. occur especially in Fumariaceae and in particular in Corydalis, Sarcocap-nos, Ceratocapnos, and Dicentra species. They exhibit anesthetic and hypotensive activities... 

In general, dehydration and depolymerization have been considered as the two main processes associated with the degradation mechanism of polysaccharides [22], In the case of starch, ether bonds and unsaturated stmctures are formed via thermal condensation between hydroxyl groups of starch chains, which eliminates water and other small molecnles, and by dehydration of hydroxyl groups in the glucose ring. It was observed that water was the main prodnct of decomposition of modified starch at a temperatnre below 300°C, formed by inter-molecnlar or intramolecular condensation of starch hydroxyls [23,24],... 

A copper-catalyzed enantioselective aziridination of aryl cinnamyl ethers and intramolecular arylation of the in situ generated tethered aziridine was developed by Hajra and Sinha. This method provides an easy access of N-sulfonyl-protected fra s-3-amino-4-arylchromans with high regio-, diastereo- (dr > 99 1), and enantioselectivity (ee up to 95%) (Scheme 8.113). Three C(sp )-H bonds were activated in this transformation under this Cu(0Tf)2/Cu(C10 )2-6H20 catalyst system. The major side reaction is the C-H insertion of nitrene at the O-CH2 unit followed by oxidative cleavage [188]. 

A particular pathway for the synthesis of cyclic alkenyl ethers by intramolecular C-0 bond formation consisted in the reaction of y-acetylenic aldehydes in the... 

The intramolecular carbopalladation (or insertion) of the triple bond in dimethyl 4-pentynylmalonate (215) with Pd—H species and malonate anion as shown by 216 proceeds in the presence of f-BuOK and 18-crown ether, affording the methylenecyclopentane derivatives 217 and 218, the amounts of which depend on the reaction conditions. The Pd—H species may be formed... 

Next in what amounts to an intramolecular Williamson ether synthesis the alkoxide oxygen attacks the carbon that bears the halide leaving group giving an epoxide As m other nucleophilic substitutions the nucleophile approaches carbon from the side oppo site the bond to the leaving group... 

Many of the crown ether syntheses with which we are concerned in this book are one form or another of the Williamson ether synthesis. Although the simplest example of such a reaction would involve an co-haloethylene glycol oligomer which undergoes intramolecular cyclization, it is more common for two new bonds to be formed in crown syntheses. An early example of the formation of a crown by a double-Williamson can be found in Dale s synthesis of 18-crown-6. The rather obvious chemical steps are shown in Eq. (2.1). 

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