Epoxysilanes reactions

Use of LTMP as base [52] in situ with Me3SiCl allows straightforward access to a variety of synthetically useful a, 3-epoxysilanes 232 at near ambient temperature directly from (enantiopure) terminal epoxides 231 (Scheme 5.55) [81]. This reaction relies on the fact that the hindered lithium amide LTMP is compatible with Me3SiCl under the reaction conditions and that the electrophile trapping of the nonstabilized lithiated epoxide intermediate must be very rapid, since the latter are usually thermally very labile. 

The cyclohexyloxy(dimethyl)silyl unit in 8 serves as a hydroxy surrogate and is converted into an alcohol via the Tamao oxidation after the allylboration reaction. The allylsilane products of asymmetric allylboration reactions of the dimethylphenylsilyl reagent 7 are readily converted into optically active 2-butene-l, 4-diols via epoxidation with dimethyl dioxirane followed by acid-catalyzed Peterson elimination of the intermediate epoxysilane. Although several chiral (Z)-y-alkoxyallylboron reagents were described in Section 1.3.3.3.3.1.4., relatively few applications in double asymmetric reactions with chiral aldehydes have been reported. One notable example involves the matched double asymmetric reaction of the diisopinocampheyl [(Z)-methoxy-2-propenyl]boron reagent with a chiral x/ -dialkoxyaldehyde87. 

Vinylsilanes (Chapter 3) can be readily converted into a/3-epoxysilanes, normally by treatment with mcpba (/). Alternatively, a-chloro-a-lithio-a-trimethylsilanes react efficiently with aldehydes and ketones in a manner reminiscent of the Darzens reaction (2). 

Boron trifluoride etherate (1 mmol) was added dropwise to a stirred solution of the epoxysilane (1 mmol) in dichloromethane (5 ml) at -78 °C, and the mixture was stirred for 5min. The reaction mixture was quenched with saturated sodium hydrogen carbonate solution (1 ml), and allowed to warm gradually to ambient temperature. The organic phase was washed with brine (3 x 5 ml), dried and concentrated. The (Z)-epoxysilane gave the (Z)-silyl enol ether (68%, 96 4(Z) (E)), and the (E)-isomer gave the (E)-silyl enol ether (69%, 95 5 ( ) (Z)). 

Malacria has reported the use of epoxysilanes for intermolecular addition reactions to acrylates, acrylonitrile and vinylsulfones [56]. 

Although early works have reported variable yields and substrate dependence, a marked tendency to stereospecificity has been noticed. For example, the reaction of the trans epoxysilane 176 (Scheme 74) with lithium reagents results, after Si-Li exchange, electrophilic trapping of the resulting lithiooxirane by RLi and Li20 elimination, in the stereoselective formation of an E) alkene in good yield. ... 

However, /3-cleavage is observed in some cases, especially when steric factors are important. The regioselectivity of the reaction of epoxysilane 170 with a-sulphonyl anions derived from 171 (Scheme 9) was found to depend upon the substitution in the anion213. 

Nucleophilic addition to a,P-epoxysilanes is part of a reaction sequence which has been used to effect 1,2-transposition of carbonyl groups (Figure Si5.14). 

Dienes. A new route to (E,Z)-1,3-dienes involves reaction of (Z)-alken-ylcuprates with cis- or traiw-epoxysilanes in the presence of BF3 etherate to give erythro- or f/im>-(3-hydroxysilanes, respectively. These products are converted to either (Z,E)- or (Z,Z)-1,3-dienes by basic or acidic elimination of (CH,),SiOH. 

Nucleophilic substitution of a,/3-epoxysilanes followed by the Peterson elimination is valuable for the stereoselective synthesis of alkenes.3 The reactions with lithium phenylsulfide and diphenylphosphide form alkenyl sulfides and alkenylphosphines, respectively, in a stereospecific manner. 7-Metallo-a,/ -epoxysilanes are isomerized to a-siloxyallylmetals by anionic ring opening and subsequent Brook rearrangement (Equation... 

OL-Alkylidenetetrahydrofuranes. Reaction of epoxysilanes of structure 1 with KH in THF provides a-alkylidenetetrahydrofuranes (2) with >95% retention of stereochemistry. The precursor vinylsilanes can be prepared stcreoselectively from an alkyne by... 

Finally, there has been speculation and recent experimental support for the involvement of a Nazarov-type cyclization in the biosynthesis of c/ s-jasmonic acid ° and marine-derived prostanoids. Radiolabel tracer studies have demonstrated Ae intermediacy of 8-HPETE (104) in the biosynthesis of prostanoid intermediate preclavulone A (lO ). This remarkable conversion was proposed to proceed by formation of allene oxide (105) followed by isomerization to (107) via the 2-oxidocyclopentadienyl cation (106 Scheme 41). To demonstrate the chemical feasibility of this proposal, Corey reported the transformation of epoxysilane (108) to, inter alia, the cyclopentenone (111 Scheme 42). The reaction is presumed to involve formation of the allene oxide (109) followed by isomerization to the 2-oxi-dopentadienylic cation (110). Conrotatory closure of (110) is expected to produce the cis isomer of (111) as observed. 

This epoxysilane rearrangement can also be effected by use of a catalytic amount of MABR. Interestingly, treatment of epoxysilane 143 with 0.2 equiv. MABR facilitated smooth rearrangement at 25 °C to furnish silyl enol ether 144 in 74 % yield reaction of 143 with 2 equiv. MABR at -40 °C gave a-silylaldehyde 145 in 93 % yield (Sch. 116). 

Depending on the reaction conditions, the (E)- or (Z)-isomers of heteroatom-substituted olefins are formed from a, -epoxysilanes (Eq. 106). ... 

On the action of Grignard compounds, a, 3-epoxysilanes undergo rearrangement to a-silylcarbonyl compounds, eo f/tra-lS-hydroxysilanes being formed in a very stereoselective reaction. ... 

Several procedures are available for the preparation of the requisite p-hydroxysi-lanes such as addition of a-silyl carbanions to aldehydes and ketones, reaction of organocuprates with a,P-epoxysilanes, reduction of P-ketosilanes, and addition of organometallic regents to P-ketosilanes. The selection of a particular procedure is dictated by the structure and stereochemistry of the desired alkene. 

In the P-ketosilane approach, the condensation of a-silyl carbanions with carbonyl compounds produces mixtures of diastereomeric P-hydroxysilanes. Therefore, the preparation of stereodefined alkenes via the Peterson reaction hinges on the availability of just one diastereomer. To overcome this shortcoming, procedures have been developed to prepare stereoselectively P-hydroxyalkylsilanes via the P-ketosilane or the epoxysilane routes, as exemplilRed below. 

The reaction of a, P-epoxysilanes with organocuprate reagents proceeds with the regioselective opening of the epoxide ring to form P-hydroxyalkylsilanes, which are then converted to stereodefmed alkenes, as exemplified below. 

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