Acetals allyltrimethylsilane

Allyl ethers and acetates. Allyltrimethylsilanes and allyltributyltins on treatment with TTFA (1.2 equiv.) in alcohols or acetic acid give the corresponding ethers or acetates in moderate to high yield. 

Initial attempts to effect addition of allyltrimethylsilane with compound 132 under conditions developed in Kishi s C-glycoside work were plagued by epimerization of the acetal center subsequent to the coupling event, providing 133 as a 1 1 mixture of diastereomers (Eq. 21). Boons reported that acetal... 

In the presence of catalytic amounts of TMSOTf 20 methyl pyruvate reacts with N,N-bis(trimethylsilyl)formamide 22c in CCI4 to give the silylated 0,N-acetal 433 in 83% yield ]44] this, with allyltrimethylsilane 82 and excess TMSOTf 20, after 30 h at 78 °C in CH2CI2, affords 72% of the methyl ester of 2-allyl-N-formylala-... 

Phenylpropanal 583 reacts with phenylthiotrimethylsilane 584 in the presence of TiCl4, via the 0,S-acetal 585, to give the S,S-acetal 586 [144]. Conducting the reaction in the presence of allyltrimethylsilane 82 and SnCl furnishes the allylic sulfides 587 and 588 in 3 1 ratio and 56% yield [144] (Scheme 5.45). 

Dimethyl acetals of aldehydes and ketones, for example benzaldehyde dimethyl acetal 121, and hemiacetals, react with allyltrimethylsilane 82 at -78 °C in CH2CI2, in the presence of TMSOTf 20 [169], trimethyhodosilane TIS 17 [159, 170],... 

Additions of silylated ketene acetals to lactones such as valerolactone in the presence of triphenylmethyl perchlorate in combination with either allyltrimethylsilane 82, trimethylsilyl cyanide 18, or triethylsilane 84b, to afford substituted cyclic ethers in high yields have already been discussed in Section 4.8. Aldehydes or ketones such as cyclohexanone condense in a modified Sakurai-cyclization with the silylated homoallylic alcohol 640 in the presence of TMSOTf 20, via 641, to give unsaturated cyclic spiro ethers 642 and HMDSO 7, whereas the 0,0-diethyllactone acetal 643 gives, with 640, the spiroacetal 644 and ethoxytrimethylsilane 13b [176-181]... 

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). 

The weaker Lewis add TMSOTf 20 as catalyst gives, after 2 h at 0°C in CH2CI2, a 20 80 mixture of 805 and 806 in only 23% yield (Scheme 6.8). But this yield will probably increase either on longer reaction time at 0°C or on shorter reaction time at 25 °C On replacing one of the methyl groups in 804 by an acetylene substituent the resulting enyne adds allyltrimethylsilane 82 or anisole in the presence of TMSOTf 20 to give allenes [18]. Substituted allyltrimethylsilanes such as 808 react with the allylic silylether 807 after 70 h at 25 °C in 62% yield to a 41 59 mixture of 809 and 810 as well as 7 [17]. Closely related additions of 82 to allylic ethers or O-acetates are discussed in Refs. 17a-c. 

TMSOTf 20 (0.11 g, 0.5 mmol) is added to a solution of a-phenyl-N-methylni-trone 976a (0.68 g, 5.0 mmol) and allyltrimethylsilane 82 (0.69 g, 6 mmol) in dry CH2CI2 (5 mL). After 36 h at room temperature the reaction mixture is quenched with 3 M aqueous HCl and stirred for 1 h. After neutralization with 3 M NaOH, extraction with Et20 (2x25 mL), and evaporation the residue is purified by flash chromatography on silica gel with hexane-ethyl acetate (7 3) to give 0.763 g (86%) 978a as a crystaUine solid, m.p. 93-95°C [70] (Scheme 7.59). 

Jung and Maderna have reported the microwave-assisted allylation of acetals with allyltrimethylsilane in the presence of copper(I) bromide as promoter (Scheme 6.77b) [162]. Stoichiometric amounts of copper(I) bromide are required and the reaction works best with aromatic acetals free from strongly electron-withdrawing substituents on the aromatic ring. 

For the addition of ethylene, EtOAc as solvent was particularly advantageous and gave 418 in 60% yield (Scheme 6.86). The monosubstituted ethylenes 1-hexene, vinylcyclohexane, allyltrimethylsilane, allyl alcohol, ethyl vinyl ether, vinyl acetate and N-vinyl-2-pyrrolidone furnished [2 + 2]-cycloadducts of the type 419 in yields of 54—100%. Mixtures of [2 + 2]-cycloadducts of the types 419 and 420 were formed with vinylcyclopropane, styrene and derivatives substituted at the phenyl group, acrylonitrile, methyl acrylate and phenyl vinyl thioether (yields of 56-76%), in which the diastereomers 419 predominated up to a ratio of 2.5 1 except in the case of the styrenes, where this ratio was 1 1. The Hammett p value for the addition of the styrenes to 417 turned out to be -0.54, suggesting that there is little charge separation in the transition state [155]. In the case of 6, the p value was determined as +0.79 (see Section 6.3.1) and indicates a slight polarization in the opposite direction. This astounding variety of substrates for 417 is contrasted by only a few monosubstituted ethylenes whose addition products with 417 could not be observed or were formed in only small amounts phenyl vinyl ether, vinyl bromide, (perfluorobutyl)-ethylene, phenyl vinyl sulfoxide and sulfone, methyl vinyl ketone and the vinylpyri-dines. 

Iodine is known to catalyze the condensation of aldehydes, benzyl carbamate and allyltrimethylsilane to homoallylic amines. However, in this case the involvement of an in sitn prepared [MejSi] species was suggested to be the active catalyst [235], An iodine catalyzed acetalization of carbonyl compounds was reported, where the active catalyst was believed to be hydroiodic acid [236],... 

The first procedure is a one-pot sequential process in which the acetal is formed from the aldehyde in a Bi(OTf)3-catalyzed reaction with trialkylorthoformate and the corresponding alcohol. After the aldehyde was consumed, the remaining alcohol was removed under reduced pressure. Acetonitrile, allyltrimethylsilane, and additional Bi(OTf)3 were added and the desired homoallyl ethers were obtained in moderate to good yields (Scheme 10). 

Competition experiments have been carried out to determine the relative reactivities of 23 alkyl chlorides toward allyltrimethylsilane in the presence of ZnCh. The kre scale has been found to span 11 orders of magnitude from the least reactive 1-adamantyl chloride to the most reactive bis(p-methoxyphenyl)methyl chloride384. By contrast, analogous acetals RCH(OMe)2 exhibited very little differences in reactivity385. 

Again there are numerous reports on the synthesis of methylenefurans. The reaction of acetals with 2-(trimethylsilyloxymethyl)allyltrimethylsilane 71 under the influence of tin(II)halide and acetyl halide afforded 2-suhstituted 4-methylenetetrahydrofurans in good yields. 

Tetrahydropyrans bearing halides at C4 can be accessed by a modified Taddei-Ricci reaction. The reaction involves condensation of allyltrimethylsilane, aldehydes and a cyclic acetal 371 in the presence of a Lewis acid to afford all ry -tetrahydropyrans 372 as a single diastereomer. The reaction proceeds via anion mediated ring closure of oxonium ion intermediate 373 (Scheme 89). This methodology was successfully applied to the synthesis of a model compound bearing all the structural motifs present in the eastern subunit of okadaic acid <1997TL2895>. 

Allyltriorganosilanes react with activated C-N double bonds such as iminium salts and Lewis acid-coordinated imines at the y-position to give homoallylamines.14,118 For example, in the presence of BF3, iV-acylimines generated in situ by the reaction of aldehydes or acetals with carbamates are efficiently allylated with allyltrimethylsilanes (Equation (26)).119,119a,12° The use of homochiral crotylsilanes such as 20 leads to highly diastereo- and enantioselective synthesis of homoallylamines (Equation (27)). a Allenylation of the iV-acylimines can be performed with propargylsi lanes. 

Aldehydes, ketones, and acetals react with allyltrimethylsilane in the presence of a catalytic amount of BiX3 (X = C1, Br, OTf) to give homoallyl alcohols or homoallyl alkyl ethers (Equation (52)).91-93 The BiX3-catalyzed allylation of aldehydes and sequential intramolecular etherification of the resulting homoallylic silyl ethers are involved in the stereoselective synthesis of polysubstituted tetrahydropyrans (Equation (53)).94,95 Similarly, these Lewis acids catalyze the cyanation of aldehydes and ketones with cyanotrimethylsilane. When a chiral bismuth(m) catalyst is used in the cyanation, cyanohydrines are obtained in up to 72% ee (Equation (54)). a-Aminonitriles are prepared directly from aldehydes, amines, and cyanotrimethysilane by the BiCl3-catalyzed Strecker-type reaction. 

Acetaliziation and aUylation of carbonyl compounds Iodotrimethylsilane is an effective catalyst for acetalization with an alkoxysilane. Addition of allyltrimethylsilane to this reaction results in allylation of the acetal to form a homoallyl ether. 

Chiral homoaXlylic alcohols. The chiral acetals 2 formed from an aldehyde and 1, undergo titanium-catalyzed coupling with allyltrimethylsilane with marked stereoselectivity. Highest stereoselectivity is usually obtained with the mixed catalyst TiCl4-Ti(0-/-Pr)4 (6 5). Cleavage of the chiral auxiliary, effected by oxidation to the ketone followed by P-elimination, provides optically active alcohols (4) with —95% ee (equation I).1... 

A detailed study of the mechanism and origin of stereoselectivity in reactions of allyltrimethylsilane with dioxane acetals has been published. ... 

Aromatic acetals can be converted in 63-87 % yield to a-allylbenzylbromides by use of allyltrimethylsilane and a catalytic amount of SnBr2 in combination with excess acetyl bromide (Eq. 14) [23]. This reaction works equally well with SnCl2 and acetyl chloride to yield the corresponding a-allylbenzyl chlorides. The Friedel-Crafts allyla-tion of anisole with ally lie bromides catalyzed by SnBr2 has also been reported (Eq. 15) [24]. 

Propargylic ethers are allylated by allyltrimethylsilane in the presence of the blend SnCl4-ZnCl2 (Eq. 86) [122], Thus acetals can be transformed to 1,5-enynes in one pot with sequential nucleophilic additions. The blend also catalyzes the allylation of aldehydes by allyltrimethylsilane, yielding homoallylic alcohols in good yields (61-74 %). 

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