Acetals Hosomi-Sakurai allylation

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 Hosomi-Sakurai allylation of acetals is successfully performed by catalytic use of silicon Lewis acids (la) [7b], MesSil [53], Me3SiN(S02P)2 [46], Me3Si0S02P [54], and (lb) [14]. The (la)-catalyzed allylation is very valuable for diastereo- and diastereoface-selective carbon-carbon bond formation [2, 50]. As expected from the Si chemical shifts, (lb) shows higher activity than (la), and Me3SiN(S02P)2 (Scheme 9.12). 

The reaction of carbonyls with alcohols or TMS ethers under catalysis by (la) or Mes Sil provides an efficient, rapid access to oxocarbenium ions. The oxocarbenium ions generated in situ have frequently been utilized for the Hosomi-Sakurai allylation (Scheme 9.14) [2, 56]. This tandem acetalization-allylation reaction is quite convenient to the synthesis of homoallyl ethers. 

Allylation of aldehydes or ketones using allylsilanes, known as the Hosomi-Sakurai reaction, is a useful method for obtaining homoallylic alcohols. TiIV compounds have been successfully applied to this reaction (Scheme 21) 80 Besides aldehydes and ketones, acylsilanes, 0,0-acetals, and A-,(7-acetals can be employed.81-83 1,4-Addition of an allyl group to an a,/ -unsaturated ketone has been also reported.84... 

The allylation of a carbonyl compound, or equivalent thereof (aldehydes, ketones, acetals, ketals, enones, acid chlorides, epoxides, etc.), performed with an allylsilane 39 promoted by a Lewis acid (TiCl, SnCl, BF3-OEt2, AICI3, EtjAlCl, etc.) is known as Hosomi-Sakurai reaction (Scheme 12.11, Eq. 1) [68, 69]. This reaction was first described in 1976, allowing the synthesis of homoallylic alcohols 43 from aldehydes and ketones 40 [70]. A year later, the authors extended the method to ketals 41, which provide homoallylic ethers 44 (Scheme 12.11, Eq. 2) [71], and to a,p-unsaturated ketones 42 (Scheme 12.11, Eq. 3) [72]. 

Protic-acid-catalyzed Michael additions (59) are subject to most of the limitations of base-catalyzed Michael additions (regioselectivity and stereoselectivity of enol generation, polyaddition, etc.), and hence, the stereochemistry has been little studied (60). At low temperatures silyl and stannyl enol ethers,+ ketene acetals, and allyl species are unreactive to all but the most reactive activated olefins. However, it was discovered by Mukaiyama and co-workers that enol ethers and ketene acetals react with a,/f-unsaturated carbonyl compounds in the presence of certain Lewis acids (4,61,62). Sakurai, Hosomi, and co-workers found that allylsilanes behave similarly (5,63,64). 

As a result of the preliminary examples described in Sect. 3.1 and the quest for automation techniques in solution-phase synthesis, various examples of continuous flow processes appeared in the literature lately which utilized solid-phase-bound chemical catalysts. In a simple example, Yamamoto and coworkers studied the use of super Bronsted acids loaded on polystyrene beads 5 for use in a single-pass column system (Fig. 4) [30]. It was shown that these columns are suited for the acetylation of alcohols, acetalization of carbonyl compounds, Sakurai-Hosomi allylation reactions, and Mukaiyama aldol reactions. 

The catalytic activities of polystyrene-bound tetrafluorophenylbis(triflyl)methane in several reactions (direct esterification, Friedel-Crafts acylation, acetalization, the Mukaiyama aldol reaction, and Sakurai-Hosomi allylation) were superior to those of Nafion SAC-13 which is perfiuororesinsulfonic acid (Table 2.3). 

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