Homoallylic ethers Allyltrimethylsilane

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... 

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). 

A three-component one-pot procedure was also developed. This method involves stirring the aldehyde, trialkylorthoformate, and allyltrimethylsilane in CH3CN in the presence of a catalytic amount of Bi(OTf)3. The homoallyl ethers were again obtained in moderate to good yields with a more simple and straightforward procedure (Scheme 11). 

Homoallyl ethers. Trimethylsilyl triflate catalyzes a reaction between dimethyl ketals and allyltrimethylsilane to form homoallyl ethers. Allylation is not possible with the parent ketones. 

Homoallylic ethers Reaction of aldehydes with allyltrimethylsilane and Cl2Ti(OR)2 (prepared from LiOR and TiCl4) results in homoallylic ethers in 50-95% yield. 

Selective cleavage of 2-methoxyethyl hemiacetats. 2-Methoxyethyl hemiacetals react with allyltrimethylsilane in the presence of TiCl4 (1 equiv.) to give a homoallyl ether with selective cleavage of the 2-methoxyethoxy group (cf, 7,370-371).8 Examples ... 

Employing a silyl ether instead of 38 provided a connective assembly of homoallylic ethers. This three-component reaction leads to the formation of homoallylic ethers 45 via activation of carbonyl 6 by Lewis acid 17. The in situ generated oxo-nium cation 43 can then be trapped by the nucleophilic silyl ether 42 affording 44. The new species can then react with allyltrimethylsilane 1, to form the desired ether 45 with subsequent regeneration of the catalyst and loss of TMSOTMS 47 (Scheme 13.16). 

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. 

The asymmetric allylation of unfunctionalized aliphatic ketones has also been described (Scheme 10-23) [43]. Simple aliphatic ketones are treated with a mixture of the trimethylsilyl ether of norpseudoephedrine (58), two equivalents of allyltrimethylsilane, and a catalytic amount of triflic acid. The homoallylic ethers... 

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]. 

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. 

In a recent study, the reaction of allyltrimethylsilane with acetals to produce homoallylic ethers with allyl inversion, which is known to be mediated by various Lewis acids such as TiCU (1,179), has been performed with catalytic amounts of trimethylsilyl trifluoromethanesulphonate at low temperatures, according to... 

Most of the methods for the Hosomi-Sakurai allylation using silicon and other Lewis acids require the use of chlorinated organic solvents such as CH2CI2 and often inconveniently low temperatures. A recent study has demonstrated that the (la)-catalyzed allylation with allyltrimethylsilane (10) in ionic liquids proceeds smoothly at room temperature to afford the corresponding homoallyl ethers in good yield [55]. Since ionic liquids can easily be recovered and recycled, they are environmentally friendly alternatives to CH2CI2 (Scheme 9.13). 

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]... 

Nevertheless, the use of chirally modified Lewis acids as catalysts for enantioselective aminoalkylation reactions proved to be an extraordinary fertile research area [3b-d, 16]. Meanwhile, numerous publications demonstrate their exceptional potential for the activation and chiral modification of Mannich reagents (generally imino compounds). In this way, not only HCN or its synthetic equivalents but also various other nucleophiles could be ami-noalkylated asymmetrically (e.g., trimethylsilyl enol ethers derived from esters or ketones, alkenes, allyltributylstannane, allyltrimethylsilanes, and ketones). This way efficient routes for the enantioselective synthesis of a variety of valuable synthetic building blocks were created (e.g., a-amino nitriles, a- or //-amino acid derivatives, homoallylic amines or //-amino ketones) [3b-d]. 

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. 

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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