Ethers oxidative ring-formation

A variety of cyclic ethers, 410, have been obtained via both, solution-phase and polymer-supported methods in the [3 + 2] cycloadditions of nitrile oxides to alkenes and dienes to give isoxazolines (Scheme 1.50). Both simple and substituted dienes have been found suitable for polymer-supported formation of cyclic ethers of ring sizes five through seven (449). 

Methoxypyrazine-2-one Af-oxides 341 were prepared from oxime derivatives 340 by one-pot cyclization in the presence of DCC/DME with subsequent interaction with Me2S04/K2C03 and NaOH (equation 148) . Similar intramolecular cyclizatiou of ester and oxime O-ethers groups iu the presence of lithium arylthiolate also leads to pyrazin-2-one ring formation . ... 

Of these products the main one was the carbinol LXII. A similar oxide intermediate explains the formation of benzylmethylcarbinol from a-bromopropiophenone. In addition to the ethers corresponding to these carbinols, there was also obtained the monoether of a glycol. This may have been formed from the intermediate oxide (LVI) by the addition of isopropyl alcohol to the oxide ring, or by diipct reaction of the bromo-hydrin (LV) with aluminum isopropoxide. Curiously enough, when the reduction was carried out at 33°, a-bromoisobutyrophenone gave a bromohydrocarbon as the main product. This was shown to be LXIII accompanied by some of its allylic isomer (LXIV). 

Ring systems containing cyclohexadienones spiro-linked to a cyclic ether can be constructed by oxidative C—O bond connection in the intramolecular mode. Five-membered ring formation is well documented, and the reactions may be efficient. For example, the linked bisnaphthols (230), (231) and (232)... 

There are many examples of such reactivity and some of these have been reviewed by Roth and coworkers, a research group that is extremely active in this area. An example that is typical of the processes encountered involves the cyclization of the diene geraniol (1). In this case the sensitizer is 9,10-dicyanoanthracene (DCA) and the reactions are carried out in methylene chloride. The authors state that a contact radical-ion parr is involved, i.e. the radical cation of the diene is in close proximity to the radical anion of the DCA. Reaction within this yields the cyclopentane derivatives 2 and 3 in the yields shown. The ring formation is the result of a five centre CC cyclization within the radical cation of 1. When a more powerful oxidant such as p-dicyanobenzene is used as the sensitizer in acetonitrile as solvent, separated radical-ion pairs are involved. This leads to intramolecular trapping and the formation of the bicyclic ethers 4 and 5 . The bicyclic ether incorporates an aryl group by reaction of the radical cation of the diene with the radical anion of the sensitizer (DCB). This type of reactivity is referred to later. Other naturally occurring compounds such as (/fj-f-bj-a-terpineol (6) and (R)-(- -)-limonene (7)... 

The small contribution of macrocyclization in the polymerization of THF is due to two effects The formation of smaller cyclic oligomers like dimer and trimer is thermodynamically unfavorable (in contrast to the formation of e.g. 1,4-dioxane in the polymerization of ethylene oxide) because 10- and 15-membered cyclic ethers are strained. It is also kinetically hampered since the rate of chain transfer to polymer which leads to ring formation is low due to the lower basicity of the polymer units than that of THF. 

Two diastereoisomeric allylic silyl ethers 133 and 134 were first investigated and in both cases, the desired radical cyclization products were not observed. In the case of 133, cyclization proceeded exclusively in a 6-endo mode, affording diol 135 after oxidative cleavage of the silyl linker. The preference for 6-ring formation was attributed to the conformational rigidity of the allylic system and the less hindered nature of the distal sp -... 

Another method enables conversion of sUyl ethers 62 into the corresponding cycloheptanones 63 (Scheme 19) [16, 17]. In this case, a tandem oxidative ring expansion and addition promoted by HTIB (Phl(OH)OTs) occurs through the formation of a cycloheptenone intermediate. The authors also describe the formation of a bicyclic cyclooctanone (n = 2) in this process. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

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