Homoallylic alcohols Allyltrimethylsilane

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]... 

Si. rra(pentafluorophenyl)boron was found to be an efficient, air-stable, and water-tolerant Lewis-acid catalyst for the allylation reaction of allylsilanes with aldehydes.167 Sc(OTf)3-catalyzed allylations of hydrates of a-keto aldehydes, glyoxylates and activated aromatic aldehydes with allyltrimethylsilane in H2O-CH3CN were examined. a-Keto and a-ester homoallylic alcohols and aromatic homoallylic alcohols were obtained in good to excellent yields.168 Allylation reactions of carbonyl compounds such as aldehydes and reactive ketones using allyltrimethoxysilane in aqueous media proceeded smoothly in the presence of 5 mol% of a CdF2-terpyridine complex (Eq. 8.71).169... 

Allyltributyltin, 10 Vinyltrimethylsilane, 343 Homoallylic ethers Allyltrimethylsilane, 11 Crotyltrimethylsilane, 86 Trityl perchlorate, 339 Homopropargylic alcohols Allenylboronic acid, 36 D imethoxy [ 1 -trimethylsilyl-1,2-buta-dienyljborane, 218 Lithium acetylide, 44 Triisopropoxy[l-(trimethylsilyl)-l,2-buta-dienyl]titanium, 218 Hydroperoxides Oxygen, singlet, 228 Hydroxy acids a-Hydroxy acids... 

CM of allyltrimethylsilane with homoallylic alcohols containing both anti-al-lylic and syn-allylic substituents has been reported to proceed effectively and display enhanced E selectivity [72,73], The reaction has been proposed to be a step in the synthetic route leading to vinylcyclopropanes (Eqs. 40,41). 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

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. 

Evidence for a closed transition-state model was gathered on the basis of the diastereoselectivity in reactions of pentacoordinate allylic silicates. Bis(l,2-benzenediolato)allylsilicates la and lb, which can be prepared via the reactions of E- and Z-crotyltrichlorosilanes with dilithium catecholate, react with aromatic aldehydes to give the corresponding homoallylic alcohols 2 in high yields3 (Scheme 3.2a). Unlike allyltrimethylsilane,4 the allylation reactions of... 

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 %). 

Similar reactions, but not accompanied by the silyl migration, have also been reported. When a bulky allylsilane is reacted with benzoylformates, the product obtained is an oxetane, as shown in Eq. (100) [270]. If this reaction is executed with allyltrimethylsilane, the expected homoallyl alcohol is produced in good yield. When, moreover, interception of the /3-silyl cation with a neighboring nucleophile is faster than desilylation, even the trimethylsilyl group is preserved in the product (Eq. 101) [271]. 

Taddei and coworkers reported that chiral allyltrimethylsilanes containing an optically active ligand derived fiom (-)-myrtenal attached to silicon (96) underwent enantioselective addition reactions with achiral aldehydes (Scheme 46) to give, after acid hydrolysis, optically active homoallyl alcohols (98). A variety of Lewis acids were examined to optimize enantiomeric excess, and TiCU was found to be the most effective catalyst. The results of the TiCU-promoted additions are reported in Table 12. ... 

Achiral allylic silanes and chiral aldehydes. The stereochemical course of reaction for simple allylsilanes is largely governed by Cram (Felkin-Anh) [29] or chelation control [lOh]. Two examples which illustrate this phenomenon are shown in Scheme 10-10 [30]. In the first example, the reaction of 2-phenylpropanal 19 with allyltrimethylsilane 20 affords the homoallylic alcohols 22 and 24 with almost no selectivity. However, methallylsilane 21 afforded a much higher syn selectivity under the influence of BF3-OEt2. The reaction of the n-alkoxy aldehyde... 

The use of C2-symmetric 1,2- and 1,3-diols as chiral auxiliaries is a reliable method for asymmetric allylation of acetals [382]. Acyclic acetals derived from homochiral 1-phenylethanol undergo the Hosomi-Sakurai allylation with high diastereoselectivity [383]. Tietze et al. have, on the other hand, reported that the TMSOTf-catalyzed successive acetalization-allylation reaction of aliphatic aldehydes with homochiral silyl ethers 123 and allyltrimethylsilane gives the corresponding homoallyl ethers with complete diastereocontrol these ethers can be readily converted into enantiomerically pure homoallyl alcohols without epimerization (Scheme 10.135) [384]. This method is applicable to asymmetric allylation of methyl ketones [385]. 

The allylation and cyanation of aldehydes and ketones are mediated by BiCl3 and BiBr3 [174, 175]. When a chiral bismuth(lll) catalyst is used for cyanation, cyanohydrins are obtained in up to 72% ee (Scheme 14.85) [175]. The Bi(OTf)3-promoted intramolecular Sakurai cyclization of homoallylic alcohols is involved as a key step in the stereoselective synthesis of polysubstituted tetrahydropyrans (Scheme 14.86) [176]. In the presence of the BiCl3-xMl binary catalyst, allyltrimethylsilane [177] and silyl enolates [178] are acylated to give aUyl ketones and /l-dikelories, respectively. 

Yamamoto et al. [16] envisaged that acyloxy-boranes might behave as mixed anhydrides because of the electronegative trivalent boron atom and could serve as effective asymmetric catalysts in selected reactions. In the presence of 20 mole% of chiral acyloxy borane (CAB) complex 7 prepared from 2R,3R)-2-0-(2,6-diisopropoxybenzoyl)tartaric acid and BHs THF, various allyltrimethylsilanes react with achiral aldehydes to afford the corresponding homoallylic alcohols in good yield and high enantio- and diastereoselectivity (Scheme 3). 

Allylation. The catalyzed addition of an allyl group to aldehydes by using allyltrimethylsilane, and a similar reaction on imines with allyltributylstannane, proceed at room temperature or below. Homoallylic alcohols and amines are formed, respectively. 

Preparation of homoallylic alcohols was achieved by reacting aromatic aldehydes with allyltrimethylsilane in the presence of 20mol% Pr-proazaphosphatarene base. Lower... 

Modified Sakurai reaction.- The original reaction involved the TiCU-catalyzed addition of allyltrimethylsilane to aldehydes and ketones or the acetals and ketals to form homoallylic alcohols or ethers (7,370-371). Marko et al. have extended this reaction to a synthesis of homoallylic ethers by a Lewis acid catalyzed reaction of allyltrimethylsilane with a carbonyl compound and a trimethylsilyl ether. 

The reaction of allyltrimethylsilane with acetals to give homoallylic ethers, previously conducted with stoicheiometric amounts of Lewis acids, has now been shown to proceed in high yield under very mild conditions with catalytic quantities of trimethylsilyl trifluoromethanesulphonate. No reaction could be achieved with aldehydes (or ketones) in place of the acetals. By contrast, 3-chloroallyltrimethylsilane reacts with aromatic and aliphatic aldehydes, though not with ketones, in the presence of Lewis acids, to give methyl ethers of homoallylic alcohols (e.g. Scheme 67), a process involving an unusual methyl group transfer. 

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). 

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