Allylic alcohol, silyl ethers

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... 

A wide variety of silyl ethers can be employed, leading to functionalized homo-allylic alcohols or ethers. This three-component coupling reaction, which generates in a single operation a range of homoallylic ethers, does not require the initial and independent synthesis of the acetal (or ketal) derived from 6. 

Allylic ethers also undergo catalytic ethylmag-nesation with excellent selectivity and in good yield (Table 4.2). However, there are several notable differences between the reactions of allylic ethers and alcohols (I) Zr-catalyzed reactions of allylic ethers afford the anti diastereomers predominantly (vs. the syn isomers observed for alcohols). (2) As the size of the or-alkyl substituent increases, reaction selectivity is also increased, which is also in contrast to the reactions of allylic alcohols. Needless to say, the complementary levels of selectivity observed in reactions of allylic alcohols and ethers represents a useful attribute for applications in organic synthesis. Finally, it is also worth noting that with sterically bulky oxygen substituents, reaction efficiency can suffer significantly allylic silyl ethers (TBS (terl-butyldimethylsilyl)) afford <5% products under identical conditions. 

The oxidation of higher alkenes in organic solvents proceeds under almost neutral conditions, and hence many functional groups such as ester or lac-tone[26,56-59], sulfonate[60], aldehyde[61-63], acetal[60], MOM ether[64], car-bobenzoxy[65], /-allylic alcohol[66], bromide[67,68], tertiary amine[69], and phenylselenide[70] can be tolerated. Partial hydrolysis of THP ether[71] and silyl ethers under certain conditions was reported. Alcohols are oxidized with Pd(II)[72-74] but the oxidation is slower than the oxidation of terminal alkenes and gives no problem when alcohols are used as solvents[75,76]. 

Silyl ethers serve as preeursors of nucleophiles and liberate a nucleophilic alkoxide by desilylation with a chloride anion generated from CCI4 under the reaction conditions described before[124]. Rapid intramolecular stereoselective reaction of an alcohol with a vinyloxirane has been observed in dichloro-methane when an alkoxide is generated by desilylation of the silyl ether 340 with TBAF. The cis- and tru/u-pyranopyran systems 341 and 342 can be prepared selectively from the trans- and c/.y-epoxides 340, respectively. The reaction is applicable to the preparation of 1,2-diol systems[209]. The method is useful for the enantioselective synthesis of the AB ring fragment of gambier-toxin[210]. Similarly, tributyltin alkoxides as nucleophiles are used for the preparation of allyl alkyl ethers[211]. 

Me3SiCH2CH=CH2i TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thiophenol except when CF3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-/-butyldi-methylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. 

Dianion formation from 2-methyl-2-propen-l-ol seems to be highly dependent on reaction conditions. Silylation of the dianion generated using a previously reported method was unsuccessful in our hands. The procedure described here for the metalation of the allylic alcohol is a modification of the one reported for formation of the dianion of 3-methyl-3-buten-l-ol The critical variant appears to be the polarity of the reaction medium. In solvents such as ether and hexane, substantial amounts (15-50%) of the vinyl-silane 3 are observed. Very poor yields of the desired product were obtained in dirnethoxyethane and hexamethylphosphoric triamide, presumably because of the decomposition of these solvents under these conditions. Empirically, the optimal solvent seems to be a mixture of ether and tetrahydrofuran in a ratio (v/v) varying from 1.4 to 2.2 in this case 3 becomes a very minor component. 

ISOC reaction was employed to synthesize substituted tetrahydrofurans 172 fused to isoxazolines (Scheme 21) [44b]. The silyl nitronates 170 resulted via the nitro ethers 169 from base-mediated Michael addition of allyl alcohols 168 to nitro olefins 167. Cycloaddition of 170 followed by elimination of silanol provided 172. Reactions were conducted in stepwise and one-pot tandem fashion (see Table 16). A terminal olefinic Me substituent increased the rate of cycloaddition (Entry 3), while an internal olefinic Me substituent decreased it (Entry 4). 

The formation of ethers such as 1806 by EtsSiH 84b can also be catalyzed by trityl perchlorate to convert, e.g., benzaldehyde in 84% yield into dibenzyl ether 1817 [48]. The combination of methyl phenethyl ketone 1813 with O-silylated 3-phenyl-n-pro-panol 1818, in the presence of trityl perchlorate, leads to the mixed ether 1819 in 68% yield [48] (Scheme 12.15). Instead of trityl perchlorate, the combination of trityl chloride with MesSiH 84a or EtsSiH 84b and sodium tetrakis[3,5-bis-(trifluoro-methyl)phenyl]borane as catalyst reduces carbonyl groups to ethers or olefins [49]. Employing TMSOTf 20 as catalyst gives very high yields of ethers. Thus benzaldehyde reacts with O-silylated allyl alcohol or O-silylated cyclohexanol to give the... 

Asymmetric cyclizahon was also successful in the rhodium-catalyzed hydrosilyla-tion of silyl ethers 57 derived from allyl alcohols. High enanhoselectivity (up to 97%... 

Silyl ethers of vinyl allyl alcohols can also be used in oxy-Cope rearrangements.215 Known as the siloxy-Cope rearrangement, this methodology has been used in... 

Epoxides can also be converted to allylic alcohols using electrophilic reagents. The treatment of epoxides with trialkyl silyl iodides and an organic base gives the silyl ether of the corresponding allylic alcohols.154... 

The rhodium-catalyzed intramolecular hydrosilylation of allylic alcohol derived silyl ethers has been described. Oxidative cleavage of the resulting cyclized hydrosilylation products affords a route to optically active diols (Scheme 28).129,130... 

The combination of PMHS and Pd(PPh3)4 reduces allyl ethers to propene and alcohols.270 The best combination for the reductive cleavage of ethers appears to be Et3SiH/(C6F5)3B, which gives excellent yields of the alcohol (via the silyl ether) and alkane (Eq. 127).145... 

Palladium-catalyzed cross-coupling of 19 with allyl bromide occurs exclusively at the vinylstannane moiety to give 1-ethoxy-1-silyl-1,4-diene 20. The following ether exchange with allyl alcohol causes the Glaisen rearrangement to give an acylsilane derivative 21 (Scheme 64).2... 

Asymmetric cyclization was also successful in the rhodium-catalyzed hydrosilylation of silyl ethers 81 derived from allyl alcohols. High enantioselectivity (up to 97% ee) was observed in the reaction of silyl ethers containing a bulky group on the silicon atom in the presence of a rhodium-BINAP catalyst (Scheme 23).78 The cyclization products 82 were readily converted into 1,3-diols 83 by the oxidation. During studies on this asymmetric hydrosilylation, silylrhodation pathway in the catalytic cycle was demonstrated by a deuterium-labeling experiment.79... 

The reaction tolerates a wide variety of allylic substrates (Scheme 9.38). Allylic alcohols were obtained from silyl ethers by standard methods. 

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