Allyl Allenyl Ethers

Allenes undergo the Ciaisen rearrangement at temperatures as low as 70 °C [34]. The rearrangement of 22 yields only the Z-isomer of 23 (Eq. 3.1.26). Examination of molecular models reveals that the chair-hke transition state C leading to the -isomer of 23 suffers destabilizing steric interactions from the bulky terUbutyl group (Eq. 3.1.27). These interactions are absent from the diastereomeric transition state B that affords the Z-isomer. 

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Claisen rearrangement. Allyl allenyl ethers resulting from isomerization of allyl propargyl ethers with t-BuOK are ready to undergo Claisen rearrangement. A route to a-substituted acroleins is established. 

The high-valent metal species required for activation of an alkyne has also been generated by the oxidative addition to an allylic or propargylic system. For example, with an allyl aryl ether as the substrate, this type of reaction achieves a cycloisomerization that occurs through an 0- to C-allyl migration (Equation (92)) 323,324 similarly, (9-propargyl derivatives lead to a mixture of allenyl and propargyl products (Equation (93)).325,326... 

In this approach, the glycosyl donor and the glycosyl acceptor are linked by the 2-OH of the donor and the free OH of the acceptor. It is one of the most predictable and reliable methods for achieving 1,2-cis stereocontrol. Acetals, mixed p-methoxybenzylacetals and silicon tethering have been widely used as well as iodonium mediated tethering acetals derived from vinyl, allyl and allenyl ethers. These methodologies have been revised.6,76... 

The competition between a propargylic ether and a teriary propargyhc amine provided an allenyl ether rather than an allenylamine [182], The reaction was also successful with propargyl allyl ethers [232]. An additional ester group in the propargylic position is tolerated [233], and consequently the reaction also works with esters of propargyhc alcohols [234—236]. In the past 4years, several derivatives of carbohydrates were converted successfully [217, 237-241] two examples are the isomeriza-tions of enantiomerically pure 98 [242] and 100 [217, 243] (Scheme 1.43). 

As mentioned before, allenes can be formed by prototropic rearrangement of alkynes or, if an appropriate hydrogen is present in the allylic position, by direct elimination. The bromovinyl ether yields, in a trans elimination, the alkyne ether (Scheme 39), but the other isomer, where trans elimination is not possible, gives both the alkynyl ether and the allenyl ether (Scheme 40). This corresponds to the problem of Hofmann and Zaitsev orientation in alkene synthesis. 

Aldol condensation, 111, 249 Aldoximes, 41, 42, 383 Alkali acetylides, 166 Alkamines, 261 Alkenyl bromides, 254 Alkyl azides, 403, 404 9-Alkylbicyclo[3.3.1JnonanoI-9, 31 Alkyl bromides, 254 Alkyl fluorides, 136 Alkyl halides, 133, 378 5-Alkyloxazolidones, 122 Alkylsulfenes, 103 Alkynes, 141, 248, 249 Alkynes, trimerization, 23 Allene, 388 AHenes, 274-276, 465 Allenyl ethers, 337 Allyl alcohol, 415 Allyl alcohols, 4)5 Allylailenes, 52 Allylamines, 326 Allyl bromide, 324 Allylcarbinyl chloride, 463 Allyl chloride, 109 Allyl ethers, 158... 

Various reactions of pronucleophiles with allenes has been reviewed by Yamamoto and Radhakrishnan [18]. Regioselectivity in these reactions is controlled by steric and electronic effects. The attack at C-3 is a main path, but several exceptional cases have been reported. Due to electronic bias, the hydrocarbonation of alkoxy- or phenoxyallenes 70 gives allylic ethers as the C-1 adduct 72 either exclusively or predominantly via 7r-allylpalladium 71 [19]. Trost expanded the regioselective formation of allylic ether to an asymmetric reaction. Addition of the Meldrum s acid 74 to the allenyl ether 73 catalyzed by palladium trifluoroacetate and (5, 5)-XIII-l as a chiral ligand in the presence of trifluoroacetic acid afforded the the allylic ether 75 with 99 % ee in 75 % yield [20]. 

Carbohydrate allenyl ethers have been prepared via the corresponding propargyl ethos,and sucrose 0-octadienyl ethers have been synthesized by a novel telomerisation of butadiene with sucrose in the presence of Pd(acac>2 and PhsP. Radical bromination of allyl ethers using MBS in CCU in the presence of isopropylidene acetals, acetates and benzoates has allowed the selective removal of the allyl ethers with a hydrolytic work-up. ... 

Allenyl Silyl enol ethers, 86 Allyl alcohol trimethylsilyl ether, 84 Allyl carbonates, 114-15 9 Allyl-ay 2 octalone, 34-5 2-Allyl-2 methylcyclohexanone, 106 (Allyldimethylsilyl)methyl chloride, 58, 59 (AUyldimethylsilyl)methylmagnesium chloride, 59... 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

Propargyl dianion (QF I ). This anion can be prepared by dilithiation of allene with BuLi in 1 1 ether/hexane. Use of THF (- 50°) or BuLi/TMEDA results in a mixture of propargylide and allenyl anions. The anion couples readily with alkyl and allyl halides to give terminal alkynes. The intermediate lithium acetylide can also react with various electrophiles.3 Example ... 

Rhodium(i)-catalyzed ene-allene carbocyclization strategy is suggested for the formation of seven-membered heterocycles, azepines and oxepines. In particular, treatment of an allenyl allyl ether with a catalytic quantity of chlorodi(carbonyl)rhodium dimer affords 4-alkylidene-5-alkyl-2,3,4,5-tetrahydrooxepines (Equation 28) in 40-55% yields <20040L2161>. 

The addition of an allyl alcohol to racemic allenyl sulfoxides results in vinyl ethers with the sulfinyl moiety at C-1 that undergo sigmatropic rearrangements upon distillation to produce 2,4-dienones after ehmination of sulfenic acid. In one example, an isomeric vinyl ether was obtained with a sulfinyl methyl substituent at C-2 that gave rise to a sulfinyl enone upon rearrangement [138]. In related work, the addition-elimination of an allyl alkoxide to a functionalized vinyl sulfoxide results in a sulfinyl enol ether that rearranges with loss of sulfenic acid to the unsaturated ester [139-141] (Scheme 21). 

The intermolecular coupling of allenes 123 and enones 124 selectively afforded dienones 125 in 53-81% yields (Scheme 4.45) [93]. As a catalyst precursor, [CpRuCl(cod)] was employed with CeCl3 7H20 and an alkynol 126 as activators. The proposed reaction mechanism involves the regioselective oxidative cyclization of the two components on a cationic ruthenium center, leading to the ruthenacyclopentane intermediate 127. When allenyl alcohols 128 were employed under otherwise identical conditions, the final products were cyclic ethers 129 (Scheme 4.46) [94]. As a catalyst precursor, the cationic ruthenium complex 68 can be used in the absence of the alkynol 126. The ether ring was considered to be formed directly via the ruthenacyclopentane 130 or alternatively through its Jt-allyl form 131. 

DienonesJ A new synthesis of 2,4-dienones (4) involves reaction of an allylic alcohol (1) with an allenyl phenyl sulfoxide (2) in benzene (ice bath). The resulting enol ether (3) is treated with this base, which induces Claisen rearrangement and elimination of benzenesulfenic acid, with formation of the dienone 4 (equation I). 

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