Subject reaction with allylsilanes

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... 

Allylsilane 59 was obtained by initial reaction of the succinimide 55 with the corresponding Grig-nard alkylsilane and a subsequent simple sequence of reactions. This allylsilane was readily cyclized in formic acid at room temperature to 60, which was subjected to a Wacker oxidation (Tsuji 1984) to afford the dihydroanatoxin-a derivative 28. The nitrogen deprotection step with iodotrimethylsilane (Melching et al. 1986) funushedthe ( )-dihydroanatoxin-a, which was reprotected with oc according to the previously discussed Rapoport methodology (Sardina et al. 1989) to furnish the racemic ( )-anatoxin-a. The overall yield of this synthesis was 7% from the succinimide. 

When an allylsilane containing a C(3) substituent reacts with an aldehyde, two new stereogenic centers are established. The reaction of ( )- and (Z)-2-butenyI-trialkylsilanes with aldehydes was first reported by Hayashi and Kumada in 1983 [28]. In these studies either ( )- and (Z)-2-butenyltrimethylsilane or ( )- and (Z)-cinnamyltrimethylsilane combined with various aldehydes in the presence of TiCl4 (Scheme 10-9). The ( )-2-butenylsilane and ( )-cinnamylsilane both afford >95 f of the syn diastereomer upon reaction with either propanal, isobutyraldehyde, or pivalaldehyde. When the Z-silanes were subjected to the same reaction conditions much lower selectivities were observed (65-72% syn selectivity). An acyclic transition structure with an antiperiplanar arrangement of double bonds was proposed to account for the diastereoselectivity observed in these reactions. A transition stmcture which includes a synclinal arrangement of double bonds may be necessary to explain the lower selectivities observed with the Z-allylsilanes. 

A stereoselective construction of 1,3-diol systems is based on the reaction of lactol acetates with allylsilanes or silyl enol ethers. Formation of the product is subject to 1,3-asymmetric induction by one or more substituents in the ring. Note that BFj OEt2 is not a suitable catalyst. 

Most interestingly, allylsilanes, stannanes, and silyl enolethers function as nucleophiles in the reactions described here and are subject to the same stereochemical preferences as alcohols [148, 149]. Thus, unselective reactions are observed with a per-O-benzyl mannopyranosyl donor, p-selective couplings are seen with a 4,6-0-benzylidene protected mannopyranosyl donor, and ot-selective reactions with the corresponding glucopyranosyl donor (Scheme 46). 

Only recently a selective crossed metathesis between terminal alkenes and terminal alkynes has been described using the same catalyst.6 Allyltrimethylsilane proved to be a suitable alkene component for this reaction. Therefore, the concept of immobilizing terminal olefins onto polymer-supported allylsilane was extended to the binding of terminal alkynes. A series of structurally diverse terminal alkynes was reacted with 1 in the presence of catalytic amounts of Ru.7 The resulting polymer-bound dienes 3 are subject to protodesilylation (1.5% TFA) via a conjugate mechanism resulting in the formation of products of type 6 (Table 13.3). Mixtures of E- and Z-isomers (E/Z = 8 1 -1 1) are formed. The identity of the dominating E-isomer was established by NOE analysis. 

The methodology was adapted to the synthesis of a simplified fragment of bryostatin. As in the previous example, an aldehyde (130) was reacted with (2-chloromethyl)allylstannane 131 using BINOL/Ti(Oi-Pr)4 catalysis to obtain 132. In parallel, aldehyde 133 was subjected to a Keck reaction employing the silylated stannane 134. The resulting allylsilane 135... 

Soon after the first report of the aldol reaction of silyl enol ethers was disclosed, allylsilanes were reported to show similar reactivity toward aldehydes and ketones when activated by a stoichiometric amount of TiCU (Scheme 3-85). This synthetically important reaction has subsequently become the subject of many synthetic chemists and was improved extensively using various kinds of Lewis acid catalysts. Acyclic transition states are proposed to explain diastereoselectivities of the reaction depending on a Lewis acid and reaction conditions. Particularly, synclinal orientation of reactants is suggested to be more preferable rather than an antiperiplanar one particularly for ( )-allylsilanes based on molecular model studies (Scheme 3-86). High diastereoselectivity observed in the reaction of chiral allylsilanes with aldehydes is understood in terms of this transition state model which is based on the Felkin-type induction (Scheme 3-87). ... 

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