Reactions with Allylic Ethers

Nitroalkylation Reactions Reactions with Allylic Ethers Wittig Reactions with Lactols... 

FIGURE 7.13 Example of reactions with allylic ethers. 

Allylic halides couple effectively with vinyltin reagents to provide the corresponding 1,4-dienes in high yields.2 Like nickel- and palladium-catalyzed Grignard reactions with allylic ethers, this reaction also proceeds with net inversion of configuration at the allylic center. Similarly, vinyl epoxides react... 

Another notable difference between the Zr-catalyzed ethylmagnesations of allylic ethers and alcohols is the effect of solvent Lewis basicity on reaction selectivity. Thus, as iUustrated in Scheme 3.79, whereas reactions with allylic ethers are entirely insensitive to variations in solvent structure, those of allylic alcohols are strongly influenced. These observations led Hoveyda and coworkers to conclude that for allylic alcohols (allyhc alkoxides after rapid deprotonation by the Grignard reagent) there is chelation between the Lewis basic heteroatom and a metal center (Zr or Mg) this association, which gives rise to transition state organization and high diastereocon-trol, is altered in the presence of Lewis basic THF, with diminution in selectivity. 

Another notable difference between the Zr-catalyzed ethylmagnesations of allylic ethers and alcohols is the influence of solvent Lewis basicity on reaction selectivity. Thus, as illustrated in Table 4.3, whereas reactions with allylic ethers are entirely insensitive to variations in solvent structure, those of allylic alcohols are strongly... 

Reaction with allylic ethers. Alkenes are obtained by reaction of allylic ethers with Grignard reagents catalyzed by cuprous chloride or bromide. Direct replacement of the ether group takes place as the major reaction when the ether is primary. Secondary and tertiary allylic ethers react mainly with rearrangement. 

When a bidentate phosphine such as DEPE [DEPE = 1,2-bis(diethyl-phosphino)ethane] is employed in the reaction with allyl ether, cationic complex [Ru(j7 -C3H5)(depe)]+[OPh] is obtained (Eq. 3.17) [55b]. 

In certain cases the reaction may proceed by a concerted mechanism. With allyl ethers a concerted [2,3]-sigmatropic rearrangement via a five-membered six-electron transition state is possible " ... 

Scheme 40 Zr-catalyzed reaction of allyl ethers with EtMgBr to produce 3-ethyl-1 -alkenes.

Scheme 44 Oxidative addition reactions of Cp2Zr derivatives with allylic ethers.

By taking advantage of the C(2) activation, 2-allyloxy-3-iodopyridine (173) was prepared by an SNAr displacement of 2-chloro-3-iodopyridine with sodium allyloxide [137]. 2-Chloro-3-iodopyridine was prepared by orrto-lithiation of 2-chloropyridine followed by iodine quench. The intramolecular Heck reaction of allyl ether 173 under Jeffery s ligand-free conditions resulted in 3-methylfuro[2,3-6]pyridine (174). 

Sulfur dioxide does not homopolymerize but does participate in a rather wide variety of free radical copolymerizations with unsaturated monomers. The resulting polysulfones have been known for quite a long time. Solonina ( f ) obtained a white solid from the reaction of SO2 with allyl ethers in 1898, but such products were not recognized as copolymers until the work of Marvel and Staudinger in the 1930 s. 

A series of pyrido[2,3-rf pyrimidine-2,4-diones bearing substituents at C-5 and/or C-6 were synthesized using palladium-catalyzed coupling of uracil derivative 417 with vinyl substrates or allyl ethers to give the regioisomeric mixtures of 418/419 and 420/421, respectively. The ratio of the isomeric structures was dependent on the substituent R. In the case of the reaction with -butyl vinyl ether, only the product 419 was obtained. However, the reactions with acrylonitrile, ethyl acrylate, 2-trifluoromethylstyrene, and 3-nitrostyrene afforded only 418. Also, reaction with allyl phenyl ether gave only 420. The key intermediate 417 was prepared by the reaction of 6-amino-l-methyluracil with DMF-DMA (DMA = dimethylacetamide), followed by N-benzylation with benzyl chloride and vinyl iodination with iV-iodosuccinimide (NIS) (Scheme 15) <2001BML611>. 

Ally lie y benzylic, or tertiary bromides (iodides). Allylic, benzylic, or tertiary alcohols are converted into bromides or iodides by reaction with BF3 etherate and (C2H5)4NBr or (C2H5)4NI in CH2C12 in 60-80% yield. NaBr or Nal can replace (C2H5)4NX in the case of allylic or benzylic alcohols.12 The combination of BF3 etherate and (C2H5)4NX is useful for cleavage of ethers.13... 

The great majority of experimental variants discussed for benzylation (see Sect. 2.1) has also been applied for the preparation of allyl ethers. Among them, the alkylation with allyl bromide and sodium hydride in a dipolar aprotic solvent is most frequently used for complete allylation. Reaction with the methylsulfinyl carbanion in DMSO to form an alkoxide, followed by the reaction with allyl bromide provides a convenient high-yield route to 2,3,6-tri-O-allyl-amylose [227], With the limited amount of reagent, 35% of methyl 2-0-allyl-3,6-dideoxy-a-D-xy/o-hexopyranoside was synthesized from the corresponding glycoside [228]. The 2-allyl ether was the major product (43 % yield) of the reaction of methyl 4,6-0-benzylidene-a-D-glucopyranoside with allyl bromide and 1.1 equiv. of sodium hydride in benzene [71]. 

As reported in the literature synthesis, the starting phenol may be converted to the corresponding allyl ether by reaction with allyl bromide in the presence of base. This step was accomplished in 80% yield. Heating the allyl ether yields the o-allyl phenol. 

The reaction with enol ethers, enamines, and electron-rich alkenes, without allylic hydrogens of proper orientation, to yield 1,2-dioxetanes... 

Benzyl methyl ether or allyl methyl ethers can be selectively metalated at the benzylic/allylic position by treatment with BuLi or sBuLi in THF at -40 °C to -80 C, and the resulting organolithium compounds react with primary and secondary alkyl halides, epoxides, aldehydes, or other electrophiles to yield the expected products [187, 252, 253]. With allyl ethers mixtures of a- and y-alkylated products can result [254], but transmetalation of the lithiated allyl ethers with indium yields y-metalated enol ethers, which are attacked by electrophiles at the a position (Scheme 5.29). Ethers with ft hydrogen usually undergo rapid elimination when treated with strong bases, and cannot be readily C-alkylated (last reaction, Scheme 5.29). Metalation of benzyl ethers at room temperature can also lead to metalation of the arene [255] (Section 5.3.11) or to Wittig rearrangement [256]. Epoxides have been lithiated and silylated by treatment with sBuLi at -90 °C in the presence of a diamine and a silyl chloride [257]. 

The displacement reactions with the ethers having aldehyde groups in the positions ortho or para to the ether linkage are similar, although they do not go as smoothly and the temperatures required seem to be higher. Thus allyl 2-formyl-4-allyl-6-methoxyphenyl ether (XXXIX) gives XL in 60% yield when heated at 170-285°. t... 

Ketene, dichloroketene, and related compounds react with allylic ethers, sulfides, and selenides in a [3.3] sigmatropic rearrangement. This interesting reaction allows the synthesis of a number of medium and large ring compounds by expansion [72] [73] [74]. Dichloroketene has been prepared in situ by slow addi-... 

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