2-Allyl ethers

A feature specific to allyl protection is the possibility of introducing the allyl group indirectly via the allyl carbonate by Pd-mediated decarboxylation. This has 

Allylation conditions are similar to those used for benzylation to a certain extent. Williamson s etherification using allyl bromides and NaH smoothly produces the allyl ethers [85], NaH can be replaced with BaO when base-sensitive moieties are present in the molecule [86, 87], The use of allyl imidates or TMSOTf-catalyzed Et3SiH-reductive allylation of a TMS ether using allyl aldehydes also provides the 

SCHEME 2.4 Allyl and substituted allyl ether formations. 

SCHEME 2.5 Cleavage conditions of allyl and substituted allyl ethers. 

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You have already seen that a carbon-heteroatom bond is easy to make, since we used such bonds as natural places for disconnections (frames 234 ft). It is good strategy therefore to make a carbon-heteroatom bond and then to transform it into a carbon-earbon bond. The Claisen rearrangement is one way to do this an ortho allyl phenol (B) made from an allyl ether (A) ... 

Allyllic ether 53 is oxidized regioselectively to the /3-alkoxy ketone 54, which is converted into the a,/i-unsaturated ketone 55 and used for annulation[99]. The ester of homoallylic alcohol 56 is oxidized mainlv to the 7-acetoxy ketone 57[99]. 

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

The a-bromo-7-lactone 901 undergoes smooth coupling with the acetonyltin reagent 902 to afford the o-acetonyl-7-butyrolactone 903[763j. The o-chloro ether 904, which has no possibility of //-elimination after oxidative addition, reacts with vinylstannane to give the allyl ether 905, The o -bromo ether 906 is also used for the intramolecular alkyne insertion and transmetallation with allylstannane to give 907[764],... 

The cyclic 2,4-dienoate 184, formed by the Pd-catalyzed cyclization of the 1,6-enyne 183, reacted with 154 to form the azulene derivative 185[118], The 3-methylenepyrrolidine 188 is formed by the reaction of the Zn reagent 186 with the chiral imine 187 with high diastereomeric excess. The structure of the allylic ethers is important for obtaining high diastereoselectivity[l 19],... 

Allylamines are difficult to cleave with Pd catalysts. Therefore, amines are protected as carbamates, but not as allylamines. Also, allyl ethers used for the protection of alcohols cannot be cleaved smoothly, hence alcohols are protected as carbonates. In other words, amines and alcohols are protected by an allyloxycarbonyl (AOC or Alloc) group. 

A method for protecting ketones and aldehydes is the formation of oximes, but sometimes further protection of the oximes is required. For this purpose, the oximes can be protected as allyl ethers. The oxime ethers ean be eleaved with triethylammonium formate in boiling dioxane[444]. The allyl ether of oximes is eleaved under mild conditions without attaeking the aeetal group in 677. 

Q The mechanism of the Claisen rearrangement of other allylic ethers of phenol is analogous to that of allyl phenyl ether What is the product of the Claisen rearrangement of C6H50CH2CH CHCH3 /... 

The mechanism of thermolysis and photolysis of ethers of 3-hydroxy-1,2-benzisoxazole has also been studied. Heating of the allyl ether (43) gave minor amounts of (44) and two benzoxazoles. Photolysis of (45) in methanol gave a benzisoxazole and an iminoester, via intermediate (46). Thermolysis at 600 °C gave a benzoxazole, a benzoxazolone and cyano-phenol (Scheme 16) (71DIS(D)4483). 

A slereoselectrve base catalyzed [2,3] sigmatropic rearrangement of allyl ethers to honwallylic alcohols (stereoselectrve)... 

NBS, CH3CN, H2O, 62-90% yield.The POM group has been selectively removed in the presence of an ethoxy ethyl ether, TBDMS ether, benzyl ether, p-methoxybenzyl ether, an acetate, and an allyl ether. Because the hydrolysis of a pentenyl 2-acetoxyglycoside was so much slower than a pentenyl 2-benzyloxyglycoside, the 2-benzyl derivative could be cleaved selectively in the presence of the 2-acetoxy derivative. The POM group is stable to 75% AcOH, but is cleaved by 5% HCl. 

Pyridinium p-toluenesulfonate, r-BuOH or 2-butanone, heat, 80-99% yield. This method also cleaves the MOM ether and has the advantage that it cleanly cleaves allylic ethers that could not be cleaved by Corey s original procedure. 

The use of allyl ethers for the protection of alcohols is common in carbohydrate literature because allyl ethers are generally compatible with the various methods... 

Allyl carbonates have been converted to allyl ethers. In the case below acid-... 

Allyl ethers are isomerized by (Ph3P)3RhCl, and r-BuOK/DMSO in the following order ... 

Lithium diphenyphosphide (THF, 25°, 2 h HCl, H2O, 87% yield) selectively cleaves an aryl methyl ether in the presence of an ethyl ether.It also cleaves a phenyl benzyl ether and a phenyl allyl ether to phenol in 88% and 78% yield, respectively. ... 

Allyl ethers can be prepared by reaction of a phenol and the allyl bromide in the presence of base. Several reagents have been used to effect their cleavage ... 

NaAlH2(0CH2CH20CH3)2, PhCH3, reflux, 10 h, 62% yield/ An aryl allyl ether is selectively cleaved by this reagent (which also cleaves aryl benzyl ethers) in the presence of an A-allylamide. 

The section on the cleavage of allyl ethers of alcohols should also be consulted. 

This has led to chemical modification of the polyesters, in particular the introduction of allyl ether groups into the resins. Amongst the monomers figuring prominently in the literature are allyl glyceryl ether I, trimethylolpropane diallyl ether II (1,1-diallyloxypropanol) and pentaerythritol triallyl ether III (2,2,2-trial-lyloxyethanol), as shown in Figure 25.32. 

Allyl amine Allyl cyanide Allyl ether Allyl halide Amines... 

An interesting feature of the synthesis is the use of allyl as a two-carbon extension unit. This has been used in the stereospecific synthesis of dicyclohexano-18-crown-6 (see Eq. 3.13) and by Cram for formation of an aldehyde unit (see Eq. 3.55). In the present case, mannitol bis-acetonide was converted into its allyl ether which was ozonized (reductive workup) to afford the bis-ethyleneoxy derivative. The latter two groups were tosylated and the derivative was allowed to react with its precursor to afford the chiral crown. The entire process is shown below in Eq. (3.59). 

Perfluoroallyl fluorosulfate is prepared by the treatment oiperfluoropropene with sulfur tnoxide m the presence of boron catalysts [2, 3, 4, 5, 6, 7] (equation 2) Perfluoroisopropyl allyl ether reacts similarly to give 58% polyfluoroallyl fluorosulfate in a cis/trans ratio of 6 4 [S] Sultones are the exclusive products without catalyst. Polyfluoroolefins such as 2-hydropentafluoropropylene [9], (2,3-dichloropropyl)tri-fluoroethylene [70], perfluoropropene [2, i], perfluoroisopropyl alkenyl ethers [S], and acyclic polyfluoroallyl ethers [77] undergo sulfur trioxidation to regioselectively produce the corresponding P-sultones in high yield... 

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