Epoxides aryl, alkylation with

Vinyl epoxides are highly useful synthetic intermediates. The epoxidation of dienes using Mn-salen type catalysts typically occurs at the civ-olefin. Epoxidations of dienes with sugar-derived dioxiranes have previously been reported to react at the trans-olefin of a diene. A new oxazolidinone-sugar dioxirane, 9, has been shown to epoxidize the civ-olefin of a diene <06AG(I)4475>. A variety of substitution on the diene is tolerated in the epoxidation, including aryl, alkyl and even an additional olefin. All of these substitutions provided moderate yields of the mono-epoxide with good enantioselectivity. 

R" may be alkyl or aryl. For dialkyl ethers, the reaction does not end as indicated above, since R OH is rapidly converted to R OR by the sulfonic acid (reaction 0-16), which in turn is further cleaved to R 0S02R" so that the product is a mixture of the two sulfonates. For aryl alkyl ethers, cleavage always takes place to give the phenol, which is not converted to the aryl ether under these conditions. Ethers can also be cleaved in a similar manner by mixed anhydrides of sulfonic and carboxylic acids733 (prepared as in 0-33). p-Hydroxy alkyl perchlorates734 and sulfonates can be obtained from epoxides.735 Epoxides and oxetanes give dinitrates when treated with N2Os,736 e.g.,... 

Chiral (salen)Mn(III)Cl complexes are useful catalysts for the asymmetric epoxidation of isolated bonds. Jacobsen et al. used these catalysts for the asymmetric oxidation of aryl alkyl sulfides with unbuffered 30% hydrogen peroxide in acetonitrile [74]. The catalytic activity of these complexes was high (2-3 mol %), but the maximum enantioselectivity achieved was rather modest (68% ee for methyl o-bromophenyl sulfoxide). The chiral salen ligands used for the catalysts were based on 23 (Scheme 6C.9) bearing substituents at the ortho and meta positions of the phenol moiety. Because the structures of these ligands can easily be modified, substantia] improvements may well be made by changing the steric and electronic properties of the substituents. Katsuki et al. reported that cationic chiral (salen)Mn(III) complexes 24 and 25 were excellent catalysts (1 mol %) for the oxidation of sulfides with iodosylbenzene, which achieved excellent enantioselectivity [75,76]. The best result in this catalyst system was given by complex 24 in the formation of orthonitrophenyl methyl sulfoxide that was isolated in 94% yield and 94% ee [76]. 

A regioselective and highly syn-stereoselective catalyst-free intermolecular alkylation of aryl borates with aryl epoxides under mild, neutral conditions has been reported.27 The reaction of /ra .s-stilbene oxide with tri(3,5-dimethylphenyl)borate gave a 38% yield (>95% syn) of the C-alkylated product (13), easily separated from (g) the O-alkylated product(s). Triflic anhydride has been used to activate enones to nucleophilic attack by electron-rich arenes in the presence of a sterically hindered base.28 Resorcinol dimethyl ether, for example, reacted with cyclohex-2-en-l-one to... 

Finally, but certainly not least, we note that the enzyme chloroperoxidase (CPO), catalyzes the highly enantioselective (>98% ee) sulfoxidation of a range of substituted thioanisoles [308]. In contrast to the epoxidation of alkenes, where turnover frequencies were low (see above), in the case of sulfoxidation of thioa-nisole a turnover frequency of around 16 s-1 and a total turnover number of 125000 could be observed. A selection of data is represented in Fig. 4.112. Besides aryl alkyl sulfides, also dialkylsulfides could be oxidized with reasonable enantioselectivities [27]. 

In 2003, Fiirstner s group reported applications of this methodology to total syntheses of some natural products, and to the stereospecific Sn2 reaction of alkyl and aryl Grignards with optically enriched propargyl epoxides to yield alkyl- and aryl-substituted 2,3-allenols with high stereochemical fidelity (equation 29). ... 

The cyclization in Step B is an improvement of Butler s procedure for the synthesis of which employs less convenient reagents, KNH and l-bromo-3-chloroacetone acetal. Beside the acetals derived from neopentyl glycol, those derived from ethanol, 1,3-propanediol and 2,4-pentanediol have been synthesized by the present method. The second part of Step B involves the formation and the electrophilic trapping of cyclopropenyl anion 2, which is the key element of the present preparations. Step B provides a simple route to substituted cyclopropenones, but the reaction is limited to alkylation with alkyl halides. The use of lithiated and zincated cyclopropenone acetal, on the other hand, is more general and permits the reaction with a variety of electrophiles alkyl, aryl and vinyl halides, Me3SiCl, Bu3SnCl, aldehydes, ketones, and epoxides. Repetition of the lithiation/alkylation sequence provides disubstituted cyclopropenone acetals. 

Suitable cleaving agents include organic halides such as primary alkyl bromides, iodides or sulfo-nates " ° allylic," - allenic and propargylic bromides vinylic or aryl halides with nickel or palladium promoters " alkyl chloroformates epoxides and even cyanogen or cyanogen bromide. Typical examples of such couplings are depicted in equations (55)-(57). 

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