Allyl aryl ethers, preparation

The Pd-catalyzed electro-cleavage of the C—O bond of allyl aryl ether proceeds smoothly in a DMF-Bu4NBp4-(Mg)-(Stainless Steel) system, giving depro-tected products in 73 99% yield [437]. The sp-sp intermolecular coupling reaction with the Pd water-soluble catalyst prepared in situ from Pd(II) acetate and sul-fonated triphenylphosphine in an MeCN-H2O system yields diynes in 45 65% yields [438]. Similarly, the sp -sp coupling of 2-iodophenols or 2-iodoanilines and terminal alkynes followed by intramolecular cyclization gives indol and furan... 

Similar to the formation of allylmagnesium chloride (25), the oxidative addition of allyl halides to transition metal complexes generates allylmetal complexes 26. However, in the latter case, a 7i-bond is formed by the donation of 7i-electrons of the double bond, and resonance of the n-allvl and 7i-allyl bonds in 26 generates the 7i-allyl complex 27 or (/ -allyl complex. The carbon-carbon bond in the 7i-allyl complexes has the same distance as that in benzene. Allyl Grignard reagent 25 is prepared by the reaction of allyl halide with Mg metal. However, the 7i-allyl complexes of transition metals are prepared by the oxidative addition of not only allylic halides, but also esters of allylic alcohols (carboxylates, carbonates, phosphates), allyl aryl ethers and allyl nitro compounds. Typically, the 7i-allylpalladium complex 28 is formed by the oxidative addition of allyl acetate to Pd(0) complex. 

The usefulness of the Claisen rearrangement in synthetic work depends on the following facts. The allyl aryl ethers, such as phenyl allyl ether (LX), can be prepared easily in high yields and can be transformed readily in good yields to the 2-allylphenols (LXI). The reaction thus... 

Fig. 14.46. Preparation of an allyl aryl ether and subsequent Claisen rearrangement. (The rearrangement is named after the German chemist Ludwig Claisen. Basically, the name is pronounced the German way but it is known that the family preferred the pronunciation /klaezn/.)...

Allylic chlorides, e.g., allyl, methallyl, and crotyl chlorides, are very reactive and are employed in the synthesis of unsaturaled ethers Besides the usual coupling of the sodium alcoholate and halide in alcohol solutions other conditions have been described, including reaction of the alcohol and unsaturated halide in the presence of potassium carbonate or sodium hydroxide in acetone or water. The combination of anhydrous potassium carbonate and acetone is widely used in the preparation of allyl aryl ethers the reaction is aided by the addition of finely powdered potassium iodide. ... 

The extent of C-alkylation as a side reaction in etherification varies about 1% of allyl 2-allylphenyl ether is formed when phenol is used in the acetone and potassium carbonate method with allyl bromide with cinnamyl bromide or 7,7-dimethylallyl bromide the extent of C-alkylation is greater. A complicated mixture of C- and 0-alkylation products results from the treatment of phenol with 4-bromo-2-hexene and 4-chloro-2-hexene. 4-Hexenylresorcinol has been obtained in about 40% yield from the reaction of I-bromo-2-hexene, resorcinol, and potassium carbonate in boiling acetone. An appreciable amoimt of C-allQrlation occurs when 2,6-dimethylphenol is treated with allyl bromide and sodium ethoxide in ethanol. Since, in general, the ampunt of C-alkjda-tion is greatly increased by carrying out the alkylation on the sodium salt of the phenol in benzene, this method is unsuitable for the preparation of allyl aryl ethers. 

Of greater preparative value is the rearrangement of allyl aryl ethers to allyl-phenols, also discovered by Claisen ... 

The most common preparative method to prepare the aryl allyl ether is the Williamson s ether synthesis [la,b]. Typically, aryl allyl ethers can be obtained from phenol derivatives and allylic halide under basic conditions (KjCOj) in refluxing acetone. This method is convenient for the preparation of simple allyl aryl ethers. However, some side reactions such as a competitive C-allylation (Sn2 type reaction) often accompany the formation of undesired byproducts. Mitsunobu reaction of phenol derivatives with allylic alcohols instead of allylic halides can be used under mild conditions [13]. In particular, when the allyl halide is unstable, this procedure is effective instead of the Williamson s ether synthesis. This method is also useful for the preparation of chiral allyl aryl ether from chiral allylic alcohol with inversion at the chiral center. Palladium catalyzed O-allylation of phenols is also applicable, but sometimes a lack of site-selectivity with unsymmetrical allylic carbonate [14] may be a problematic issue. 

The pioneering work by Guibe and Saint M Leux described the Pd-catalyzed extrusion of CO2 from an alkyl allyl carbonate to afford the corresponding etheri (Scheme 31). The alkoxide formed in the mechanistic cycle (Scheme 1), in the absence of any pronucleophile, acts itself as the nucleophile, attacking the cationic rj -allylpalladium complex. Applications to the preparation of allyl aryl ethers were reported later. The formation of allyl aryl ethers occnrs with overall retention of configuration. ... 

Condensation of resin bound ketones with hydrazines followed by cyclizative cleavage was used in a synthesis of pyrazolinones [67]. Samarium iodide induced radical cyclization of allyl aryl ethers led to benzofuran derivatives [68]. Kurth et al. have reported a two reaction iterative process for the construction of polyisoxazolines [69] and polyisoxazoles [70]. An intramolecular Heck reaction has been employed to prepare 2-oxindole derivatives [71]. 

Two approaches for the synthesis of allyl(alkyl)- and allyl(aryl)tin halides are thermolysis of halo(alkyl)tin ethers derived from tertiary homoallylic alcohols, and transmetalation of other allylstannanes. For example, dibutyl(-2-propenyl)tin chloride has been prepared by healing dibutyl(di-2-propenyl)stannane with dibutyltin dichloride42, and by thermolysis of mixtures of 2,3-dimethyl-5-hexen-3-ol or 2-methyl-4-penten-2-ol and tetrabutyl-l,3-dichlorodistannox-ane39. Alternatively dibutyltin dichloride and (dibutyl)(dimethoxy)tin were mixed to provide (dibutyl)(methoxy)tin chloride which was heated with 2,2,3-trimethyl-5-hexen-3-ol40. 

Not only an aryl group—as in Figure 14.46—but also an alkenyl group can participate in the Claisen rearrangement of allyl ethers (Figure 14.47). Allyl enol ethers are the substrates in this case. Figure 14.47 shows how this kind of allyl alkenyl ether (D) can be prepared from an... 

The thermal [3,3]-sigmatropic rearrangement of allyl vinyl ethers, first reported by Claisen in 1912, allows the preparation of y,6-unsaturated carbonyl compounds. This reaction is called [3,3]-sigmatropic rearrangement because the new a-bond has a 3,3 relationship to the former a-bond. The aromatic variant of the Claisen rearrangement, in which an aryl allyl ether (5) is converted thermally to the corresponding y,6-unsaturated carbonyl compound (6), is followed by a re-aromatization providing an ortho-di y phenol (7). 

In 2002, Trost and Tang reported the chiral total synthesis of (-)-codeine in short reaction steps using a palladium-catalyzed asymmetric allylic alkylation (AAA) [53] as the key transformation [54], In 2005, a detailed full account of their synthesis was published [55]. The key features of their synthesis are (1) a preparation of an aryl ether with high optical purity by the Pd-catalyzed AAA reaction, (2) the intramolecular Heck reaction to generate the A-C-E benzofuran skeleton, (3) the second intramolecular Heck reaction of Z-vinyl bromide providing the phenan-throfuran core, and (4) the intramolecular hydroamination for the construction of D-ring by the action of LDA and visible light. 

Fission of aryl ethers can also be effected by metallic sodium or potassium. For instance, diphenyl ether, which is one of the most resistant ethers, is decomposed by liquid potassium-sodium alloy at room temperature.38 Weber and Sowa have cleaved 4,4 -disubstituted diphenyl ethers into benzene and phenol derivatives by sodium in liquid ammonia.39 Diallyl ether is converted by sodium dust at 35° into allylsodium and sodium allyl oxide, which provides a suitable method for preparation of allylsodium.3 7a Anisole, phenetole, benzyl phenyl ether, and diphenyl ether give the phenols in 90% yield, without by-products, when boiled with sodium or potassium in pyridine.40... 

Other Alkyl and Aryl Ethers - A number of 3-0-aminoalkyl-l,2-0-isopro-pylidene-a-D-gluco-or xylo-furanoses have been synthesized and evaluated for antiviral activity. Some 6-0-alkyl-a-D-glycopyranosides have been prepared by displacement of 6-0-tosylates with alkoxides, while the regioselectivity of 3- or 5-0-ether formation on l,2-0-isopropylidene-4,6-di-0-benzyl-/n> o-inositol has been shown to depend on the nature of the 0-alkylating agent. A facile synthesis of methyl 2-or 3-0-allyl-5-0-benzyl-p-D-ribofuranoside has been described. ... 

Danishefsky et al. developed a stereospedfic route for the synthesis of deoxy analog of mitomycin via an aromatic Claisen rearrangement [84]. The aUyl aryl ether 112 was prepared from allylic alcohols and phenol derivative via the Mitsu-nobu reaction. The aromatic Claisen rearrangement of 1,3-disubstituted aUyhc ether 112 proceeded under thermal conditions (N,N-dimethylaniline reflux) to afford the ortho rearrangement product 113 in 80% yield. 

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