Silylated allylic alcohol

Schemes 28 and 29 illustrate Curran s synthesis of ( )-hirsutene [( )-1]. Luche reduction58 of 2-methylcyclopentenone (137), followed by acetylation of the resulting allylic alcohol, furnishes allylic acetate 138. Although only one allylic acetate stereoisomer is illustrated in Scheme 28, compound 138 is, of course, produced in racemic form. By way of the powerful Ireland ester enolate Clai-sen rearrangement,59 compound 138 can be transformed to y,S-unsaturated tm-butyldimethylsilyl ester 140 via the silyl ketene acetal intermediate 139. In 140, the silyl ester function and the methyl-substituted ring double bond occupy neighboring regions of space, a circumstance that favors a phenylselenolactonization reac-...

On reacting aldehydes such as propionic aldehyde, however, with a 1 1-mixture of a silylated allyl- or benzyl alcohol such as 589 and phenylthiotrimethylsilane 584 in the presence of TMSOTf 20 0,S-acetals such as 591 are obtained, via the probable intermediate 590, in high yields [63]. The a-ketoamide 592 is converted by methylthiotrimethylsilane 593/F3B-OEt2 into the bisthioketal 594 in 81% yield [145]. Ethylenethioketals such as 595 are cleaved by MesSiBr 16 or Me3SiI 17 in... 

The diacetal 629, prepared from the carbonyl compound and O-silylated allylic alcohols in the presence of TMSOTf 20, reacts with ( )-l-trimethylsilyl-2,4-penta-diene 630, in the presence of TMSOTf 20 in CH2CI2 at -78°C, to afford 60% 631 this undergoes Diels-Alder-cyclization at 170 °C in toluene to give a substituted... 

Benzaldehyde dimethyl acetal 121 reacts, for example, with the silylated allylic alcohol 645, in the presence of SnCl2-MeCOCl, via an intermediate analogous to 641, to the 3-methylenetetrahydrofuran 646 and methoxytrimethylsilane 13 a [182], whereas benzaldehyde dimethyl acetal 121 reacts with the silylated homoallylalco-hol 640 in the presence of TMSOTf 20 to afford exclusively the ds 4-vinyltetrahy-drofuran 647 and 13 a [183]. A related cyclization of an a-acetoxy urethane 648 containing an allyltrimethylsilane moiety gives the 3-vinylpyrrohdine 649 in 88% yield and trimethylsilyl acetate 142 [184, 185]. Likewise, methyl 2-formylamido-2-trimethylsilyloxypropionate reacts with allyltrimethylsilane 82 or other allyltri-methylsilanes to give methyl 2-formamido-2-aUyl-propionate and some d -unsatu-rated amino acid esters and HMDSO 7 [186] (Scheme 5.56). 

C-Substitution Reactions of Silylated Allyl or Benzyl Alcohols... 

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... 

Et3SiH/Ni(cod)2 brings about the reaction of an aldehyde and an alkyne to provide the silylated allyl alcohol (Eq. 190).350 The reaction also occurs in an intramolecular mode. 

Lithiation of the vinylstannane moiety of 22 with BunLi followed by the reaction with PhCHO gives (Z)-7-silyl allylic alcohol 23 (Scheme 65).261 The subsequent Cu(i)-mediated cross-coupling with allyl chloride affords (Z)-allylic alcohol 24 with the (Z)-stereochemistry retained. 

Comparison of rates of Brook rearrangement of jS-substituted a-silyl allyl alcohols as a means of estimating the a-carbanion stabilizing ability of heteroatom substituents suggests that PhS is much more stabilizing than MesSi. ... 

Ab initio calculations also confirm that the use of an allyl magnesium alkoxide in place of the alcohol functionality will lead to high or complete stereoselectivity (138). When homoallylic alcohols are used, the Kanemasa protocol afforded the respective isoxazolines with poor stereoselectivity ( 55 45) in the case of terminal aUcenes, but with very high diastereoselectivity (up to 96 4) in the reaction of cis-1,2-disubstituted olefins (136). Extension of this concept to the reaction of a-silyl allyl alcohols also proved feasible and produced the syn (threo) adducts as nearly pure diastereomers (>94 6) (137). Thus, the normal stereoselectivity of the cycloaddition to the Morita-Baylis-Hillman adducts (anti > syn, see above) can be reversed by prior addition of a Grignard reagent (176,177). Both this reversal... 

The influence of both the steric and electronic properties of the silyl group on the rate of epoxidation have been examined experimentally [104], Two rate effects were considered. First, the overall rate of epoxidation of the silyl allylic alcohols was found to be one-fifth to one-sixth that of the similar carbon analogs. This rate difference was attributed to electronic differences between the silicon and carbon substituents. Second, the increase in k[el to 700 for silyl allylic alcohols compared with carbon analogs (e.g., 104 for entry 3, Table 6A.8) was attributed to the steric effect of the large trimethylsilyl group. As expected, when abulky (-butyl group was placed at C-3, k[e] increased to 300 [104],... 

Reaction of /-silyl allylic alcohols 85 with Et2Zn and CH2I2 in the presence of (+)-diethyl tartrate as a chiral auxiliary affords the corresponding cyclopropylmethyl... 

The regioselective reaction of /ran.v-2-(/-butyldimethylsilyl)-3-vinyloxirane (13) with primary, secondary, and tertiary butyllithium has proceeded in a S 2 fashion and allowed the formation of a-silylated allylic alcohols with diastereomeric ratios of over (g) 7 1 in favour of the (Z)-alkenes (Scheme ll).72 A study of the effect of temperature, time, addition of salt, and polarity of the solvent on the diastereoselectivity of the reaction has been described. 

Oxygenation of vinyls Hanes ally He alcohols. Singlet oxygen undergoes the ene reaction with vinylsilanes to give, after reduction with sodium borohydride, -silylated allylic alcohols. These products are desilylated by tetra-n-butylam-monium fluoride with preservation of the geometry of the double bond. 

Photooxygenation of vinylsilanes affords, after reduction, /3-silyl allylic alcohols regioselective-ly in good yields i4a c. Moreover, because of the steric demand of the silyl group, the titanium-catalyzed oxyfunctionalization described leads in high yield to (R, S )-epoxy alcohols80. 

Panek et al. introduced the synthesis of optically active crotylsilanes by Claisen rearrangement of allyl vinyl ethers derived from homochiral y-silylated allyl alcohols (Scheme 10.141) [393]. These allylsilanes have been used for highly diastereo-and enantioselective allylation of aldehydes and acetals [334], which enables efficient syntheses of complex natural products [394]. 

Vinylsilanes — silyl epoxy alcohols. Photooxygenation of vinylsilancs in the presence of Ti(0-(-Pr)4 affords silyl epoxy alcohols with high regio- and diastcreosclcctivity. The conversion involves as the first step an enc reaction with O2 to provide a /3-silyl allylic alcohol, which then undergoes epoxidation. 

Interestingly, a-silylated allyl alcohols are transformed into a-silyl-(J-hydroxy ketones on oxidation with ( -PrO)4Ti-/-BuOOH. 

Carbonylation. Insertion of CO 1 from unsaturated amines as well lyl)ethynes are realized in the presenc a-Siloxy aldehydes. The concur aldehydes by R jSiH and CO proceeds ai complex Rh(cod>2BF4 catalyzes hydros ( )-y-silyl allylic alcohols. ... 

Admixture of alkynylstannanes, aldehydes, trimethylsilyl chloride, and InClj in acetonitrile at room temperature results in the formation of propargyl silyl ethers. In the synthesis of homoallylic alcohols, simple allylic halides can be used to form the tin halides in situ in water. ... 

---