SAKURAI-HOSOMI Allylation

Substituted allylsilanes are subject to ene reaction with aldehydes and a,/3-unsaturated carbonyl compounds in the presence of a Lewis acid. The Et2AlCl-promoted reaction of /3-siloxymethyl-substituted allylsilane 27 with aldehydes gives more functionalized allylsilanes (Equation (37)).148 The use of TiCU instead of Et2AlCl leads to the Hosomi-Sakurai allylation. Catalytic enantioselective carbonyl-ene reactions of methallylsilanes have been achieved by using chiral Ti and A1 complexes.149,150... 

Since the discovery of the Hosomi-Sakurai allylation reaction [332] much attention... 

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]. 

S )-(+)-4-Phenyloxazolidinone (28) was acylated with crotonoyl chloride to give crotonamide 29 (Scheme 5). Diastereoselective Hosomi-Sakurai allylation of 29 with allyltrimethylsilane in the presence of TiCLi afforded compound 30 in a sufficient yield with a diastereomeric ratio of ca. 8 1. Next, the direct conversion of the oxazolidinone 30 into the Weinreb amide 31 was achieved using A,(9-dimethyl-hydroxylamine. 

Other references related to the Hosomi-Sakurai allylation are cited in the literature. ... 

The same group also reported the synthesis of Paraconic acids, using a Hosomi-Sakurai allylation with 1 combined with a retro-aldol/lactonization cascade (eq 40). ... 

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). 

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). 

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. 

In the pioneering works by Hosomi and Sakurai, a stoichiometric or substoichio-metric amount of TiCU, a conventional Lewis acid, was used for the carbonyl allylation with allylsilanes [106]. Davis and coworkers found that TMS borates such as Me3Si[BX(OTf)3] (X = OTf, Cl) enables an efficient, catalytic Hosomi-Sakurai allylation of aldehydes although (la) and Me3SiI show low catalytic activities (Scheme 9.41) [18, 107]. Ishihara and Yamamoto have demonstrated the utility of (lb) as Lewis acid catalyst of the carbonyl allylation as well as the Mukaiyama aldol reaction [15]. The silicon Lewis acid (lb), generated in situ by the reaction of allyltrimethylsilane with HNTf2, efficiently promotes the allylation of aldehydes and ketones with a loading of 0.5 mol% (Scheme 9.41). 

Yadav and coworkers have reported that MesSil, prepared in situ from al-lyltrimethylsilane and h, serves as an effective catalyst of the Hosomi-Sakurai allylation in MeCN (Scheme 9.41) [110]. This result stands in sharp contrast with the low activity of Me3SiI in CH2CI2 [107]. 

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 introduction of the allylic silane moiety required for the intermolec-ular Hosomi-Sakurai reaction is depicted in Scheme 16. Following the formation of the enol triflate 97, a Stille coupling provided excess to the allylic alcohol 98 [51]. The allylic alcohol (98) was endowed with a phosphate leaving group for the subsequent allylic substitution. Utilizing a trimethylsilyl cuprate as nucleophile for the 5 2 reaction, the allylic phosphate was converted into the allylic silane 89. A useful substrate-induced diastereoselectivity in favour of (14i )-89 was encountered at small scale but decreased significantly upon up-scaling. 

The Hosomi Sakurai Reaction involves the Lewis acid-promoted allylation of various electrophiles with allyltrimethysilane. Activation by Lewis acids is critical for an efficient allylation to take place. 

A one-pot synthesis of 2,3>S-trisubstituted tetrahydrofurans by a double Hosomi-Sakurai reaction has been described. The product was obtained without the contamination of any regio- or stereoisomers. This remarkable selectivity has been explained by the difference in reactivity between the allylic starting material and the allylic silane formed in situ and between that of the two aldehydes employed (Scheme 80) <2004AGE1417>. 

In 1976 we reported that aldehydes and ketones are efficiently allylated with allyl-trimethylsilane in the presence of a substoichiometric amount of I iLf, [332]. Subsequently, BU4NF, a fluoride ion source, was found to be an effective catalyst of this allylation reaction [333]. After these initial reports of the Hosomi-Sakurai reaction, several allylsilanes, including highly functionalized compounds, were used for regio- and stereoselective allylation of a variety of carbon electrophiles [6, 13, 14, 334]. In the nineteen-eighties, some Lewis acids (TMSOTf [335], TMSI [336]... 

The Hosomi-Sakurai reaction is a powerful method for conjugate allylation of a,fi-unsaturated ketones [425], In the presence of TiCl4 the allylation occurs smoothly at the y-position of allylsilanes and the / -position of a, -unsaturated ketones. This highly regioselective process has been widely used for introduction of functionalized carbon chains and construction of carbocycles in natural product synthesis [6, 426]. When TBAF is used as catalyst, both conjugate addition and 1,2-addition occur competitively [333]. The fluoride ion-catalyzed procedure is, however, effec-... 

Sakurai allylation reaction (Hosomi-Sakurai reaction)... 

The Hosomi-Sakurai-Prins reaction of the easily available enal 142 and allylsilane 143 was performed in the presence of Lewis acids to give the 4-methylene-tetrahydropyran with poor cis-trans selectivity ( 2 1), however, the pro tic acid provided only cis-144, which was converted into the aldehyde 145. The Takai iodoalkenylation, followed by desilylation and Sharpless asymmetric epoxidation, provided 146 with a 4 1 ratio of /Z, which was separable via the treatment of TBAE After protection of the primary alcohol, the alkenyllithium derived from the iodoalkene, reacted with aldehyde 147 to form 148, which was converted into epoxy-carboxylic add 149 in five steps. The key macrocyclization was performed by the treatment of 149 with Ti(Oi-Pr)4 [73] under high diluted conditions (2 mM) at 75 °C to provide the macrolactone 150 in moderate yield with 30% of the starting material recovery. After desilylation, the chemoselective oxidation of the allyl alcohol with 4-acetylamino-2,2,6,6-tetramethylpiperidine- 1-oxoammonium tetraflu-oroborate, followed by oxidative cleavage of the C20-C21 diol, produced (-)-dactyloUde (Scheme 30). 

The aldehyde 218 possessing 2,6-frans-tetrahydropyran, was synthesized as shown in Scheme 48. /3-Keto ester 220 was reduced by Noyori hydrogenation [97] to give 6-hydroxy ester 221 in 94% ee, which was converted into iodide 222. Asymmetric alkylation using Myers chiral auxiliary [98] with 222, followed by acid treatment, furnished 5-lactone 223 with high stereoselectivity. Reductive acetylation, axial allylation by the Hosomi-Sakurai reaction, and ozonolysis completed the synthesis of 218. 

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]. 

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