Ethers, allyl silyl

A combination of a Pd-catalyzed arylation of a ketone followed by intramolecular cyclization of the formed enolate with an allylic silyl ether moiety in one of the substrates led to the direct formation of a 1-vinyl-lH-isochromene, as described by Wills and coworkers [164]. 

Kibayashi and coworkers have used enantiometrically pure allylic silyl ethers obtained from amino acids in cycloaddition with nitrones (Eq. 8.49).71 Cyclic nitrone reacts with a chiral allyl ether to give selectively the exo and erythro isomer (de 90%). Optically active alkaloids containing a piperidine ring such as (+)-monomorine,71c (+)-coniine,71a and (-)-oncinotine71b have been prepared from the addition product. 

The stereoselective isomerization of allyl silyl ethers to (E)- or (Z)-silyl enol ethers was carried out in the presence of a cationic iridium(i) catalyst. The complex, prepared in situ by treating [Ir(cod)2]PFf,/2PPi3 with hydrogen was... 

Several classes of silyl ethers have been shown to be excellent substrates for the C-H insertion chemistry of donor/ acceptor-substituted carbenoids.81 Effective C-H insertions predominantly occur at methylene sites. Primary sites are not sufficiently activated electronically while tertiary sites are sterically too crowded. Rl -DOSP -catalyzed functionalization of the allyl silyl ether 3 resulted in a highly diastereoselective transformation, leading to the formation of the /3-hydroxyester 4 in 94% yield and 82% ee (Equation (17)).81 This example illustrates the regioselectivity of this chemistry, because 3 contains two allylic sites but only the methylene site adjacent to the siloxy group was functionalized. Even better substrates are the commercially available tetraalkoxysilanes such as 5, because with these substrates, the high diastereoselectivity was retained while the enantioselectivity was increased (Equation (18)).81... 

The Alder-ene cyclization of allylic silyl ethers represents a clever use of cycloisomerization chemistry, as the enol ether products can be easily unmasked to yield aldehydes. Palladium-catalyzed cycloisomerization of 1,6- and 1,7-enynes containing an allylic oxygen most often gives rise to 1,3-dienes (see Section 10.12.4.1). However, enynes of type 63 underwent facile Alder-ene cyclization to the corresponding five- or six-membered rings (Equation (40)) using both [CpRu(MeCN)3]PF6 41 and the Cp analog ([Cp Ru(MeCN)3]PF6, 64).53... 

Extending the reaction to acyclic trans-allyl silyl ethers 220 results in the highly diaster-eoselective formation of syn-aldol products 221 (Eq. 30) [143]. Even higher stereoselectivity can be achieved with the tetraalkoxysilanes 222, where both the diastereo- and en-antioselectivity for the formation of 223 are exceptional (Eq. 31) [144]. C-H insertion into tetraethoxysilane 222 generates the syn-aldol product 223 in 70% yield (>94% de) with 95% enantiomeric excess. 

Various bicyclic and polycyclic compounds are produced by intramolecular reactions[127]. In the syntheses of the decalin systems 157 [38] and 158 [128], cis ring junctions are selectively generated. In the formation of 158, allylic silyl ether remains intact. A bridged bicyclo[3.3.1]nonane ring 159 was constructed... 

It was later found that the anti selectivities could be greatly improved by using sUyl ethers and the more reactive Shi s reagent (equation 68). The transition structure in which the C—O bond eclipses the C=C bond was proposed to account for the sense of induction and is based on the work of Gung and coworkers . It should also be mentioned that Z-aUcyl-substituted allylic silyl ethers led to the anti isomer as well, and their Z-aryl-substituted analogues led to the syn isomer. 

Isomerization of siloxycydopropane 3 to the allylic silyl ether in the presence of Znl2 has close mechanistic similarity to the homoenolate formation, Eq. (11) [26]. The initial zwitterionic intermediate after ring cleavage undergoes a hydride shift rather than elimination of the silyl group. 

Use of remote allylic silyl ethers, such as 72b,c, rather than of allylic alcohol 72a in intramolecular oxymercurations leads to a higher 1,3-syM-diastereoselectivity (equation 34)47. The overall yields for syn-Ti and anti-14 range between 85-93%. The best selectivity reaches 7 1 involving the (t-BujPl Si group (entry 36 of Table 1). 

An excellent synthesis of a,/J-unsaturated acyl silanes from allyl silyl ethers is shown in Scheme 4020. This simple two-step procedure hinges on the Wittig rearrangement127 128,... 

Wittig rearrangement of allyl silyl ethers, followed by Simmons-Smith cyclopropana-tion and Collins oxidation, produces a-cyclopropyl acyl silanes, e.g. 22, in 10-85% yields (Scheme 55)85. 

Benzylic silyl ethers couple with allylsilanes in the presence of trityl tetrakis [3,5-bis(trifluoromethyl)phenyl]borate (TFPB) catalyst leading to carbon-carbon bond formation (equation 77). The corresponding Z11CI2-catalyzed reactions with an allyl silyl ether lead to a mixture of regioisomers145,146. 

The ozonolysis of substituted-allyl silyl ethers or allyl esters followed by treatment with bases or Ph3P give the corresponding a-silyloxy ketones or a-acyloxy ketones.116 The reaction is proposed to proceed via an ene-diol rearrangement of the corresponding a-silyloxyaldehyde or a-acyloxyaldehydes intermediates. 

Os04-catalyzed dihydroxylation of mono allylic silyl ethers shows slight diaste-reoselectivity, but dihydroxylation of a bis-allylic silyl ether such as 1 provides essentially only one product (2), and a second dihydroxylation also provides a single isomer (3).9... 

Good results have been achieved in the isomerization of meso-ene-dialkyl ethers and their O-silylated derivatives [12]. The allyl silyl ether 32 reacts in the presence of a Rh-BINAP catalyst to give the chiral enolsilyl ether 33 (Scheme 10). On treatment with fluoride, cyclic ketones 34 result in excellent yield and enantioselecti-... 

Eq. 3.21 shows the introduction of a hydroxymethyl group at the (3-position of allylic alcohol with the silicon-tethered radical approach developed by Stork [78-83]. Allyl silyl ether (64) was first prepared by the treatment of allylic alcohol (63) with dimethylbromomethylsiliyl chloride, and subsequently treated with Bu3SnH/AIBN, and finally treated with hydrogen peroxide in the presence of KF, to give a 1,3-diol derivative (66), as shown in eq. 3.21. 

Allyl silyl ethers. (3-Hydroxyvinylsilanes (I), readily available by hydromagne-siation of silylalkynes (10, 130), are stable to sulfuric acid or to sodium hydride in THE. 

In a short known reaction sequence, enal 250 was obtained from commercially available material 184). With methylamine and magnesium sulfate imine 251 was formed and combined with acyl chloride 252 185) (>4 steps). The use of low temperatures for this acylation led exclusively to the less substituted dienamide 253. The desired basic skeleton of dendrobine 254 was obtained by cyclizing 253 at 180°C in an acceptable 50% yield, Adduct 254 was accompanied by small amounts of the exo-adduct. Epoxidation led exclusively to exo-epoxide 255, which by means of trimethylsilyltriflate was converted into the allylic silyl ether. Acid treatment liberated the hydroxy group and subsequent oxidation of alcohol 256 led to enone 163, an intermediate of Inubushi s dendrobine synthesis and thus concluded this formal synthesis. The intermediate 163 was obtained from commercially not available materials in seven steps in 22.5% overall yield. To reach ( )-dendrobine according to Inubushi et al. would afford six additional steps, reducing the overall yield to 0.4% without including the preparation of the starting materials from commercially available compounds. 

Yang s chiral ketones 75 have also been used as catalysts in the kinetic resolution of acyclic secondary allyl silyl ethers <2001JOC4619>. Dioxiranes generated in situ from dehydrocholic acid derivatives 122 and Oxone have been used in the asymmetric epoxidations of cinnamic acid derivatives with product ee s up to 95% <2001TA1113, 2002JOC5802> and unfunctionalized olefins (up to 98% ee) <2006T4482>. 

Kinetic Resolution of Acyclic Secondary Allylic Silyl Ethers. (/ )- can catalyze kinetic resolution of acyclic secondary allylic silyl ethers (eq 5). When racemic silyl ethers (7a-7e) are submitted to the optimized epoxidation conditions, the recovered starting materials are found to be enriched in the (5)-enantiomers, and the resulting epoxides (8a-8e) are single diastereomers ery-... 

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