Epoxysilane

Commonly employed anion-stabilizing groups are those containing silicon (Table 5.4, Entries 1-5). Magnus et al. reported that epoxysilane 147 could be deproto-nated with t-BuLi, and that the lithiated epoxide 148 thus generated could be trapped with allyl bromide to give epoxysilane 149 in a synthetically useful yield (Scheme 5.34) [55], Iodomethane (88%) and chlorotrimethylsilane (60%) could also be trapped. 

Molander and Mautner demonstrated that deprotonation of cis-a, 3-epoxysilane 150 with s-BuLi/TMEDA was complete in 10 minutes, whereas the corresponding trows-isomer 150 required 4 hours [56]. Similarly, treatment with butyraldehyde was more efficient with cis-151 (Scheme 5.35), which could also be trapped with a wide variety of other carbonyl-containing electrophiles. The results demonstrated that lithiated epoxides cis- and trons-151 were configurationally stable at -116 °C for periods of up to 4 hours. Only in the case of cis-151 (t-butyl = n-octyl) was the lithiated epoxysilane found to be configurationally unstable. 

During studies directed towards the synthesis of spatol, Salomon and Murthi studied epoxysilane 153 bearing a temporary tether (Scheme 5.36), which circumvented the potential problem of configurational instability described above [57]. 

In their synthesis of (+)-cerulenin, Mani and Townsend employed lithiated epoxysilane 157, which they trapped with (4E,7 )-nonadienal to give a 77% yield of 158, which was further manipulated to give the natural product (Scheme 5.37) [58], as-ot, 3-Epoxy-Y,S-vinylsilanes 159 are regioselectively lithiated at the a-silyl position, and can subsequently be stereo selectively trapped with a range of electrophiles to give a-substituted epoxyvinylsilanes 160, which can in turn be isomerized to a-silyl-P-vinylketones 161 (Scheme 5.38) [59]. 

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. 

Metalated epoxides are a special class of a-alkoxy organometallic reagent. Unstabilized oxiranyl anions, however, tend to undergo a-elimination. On the other hand, attempts to metalate simple unfunctionalized epoxides may lead to nucleophilic ring opening. The anion-stabilizing capability of a trimethylsilyl substituent overcomes these problems. Epoxysilanes 22 were... 

Addition of the metalated civ-epoxysilane to acyclic or cyclic ketones generally proceeded with higher diastereoselectivity than the addition to aldehydes. A low diastcreomeric ratio (60 40) was only observed upon addition to 4-methylcyclohexanone13. 

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. 

The above mechanism involves a-opening of the epoxysilane, followed by a 1,2-elimination of a /3-hydroxysilanc (Peterson olefination, Chapter 10). However, it has recently been shown that aj8-dihydroxysilanes, particularly t-butyldimethylsilyl species, undergo an acid-catalysed sila-pinacol rearrangement to produce /J-aldehydo- and /i-kctosilancs (5) ... 

Recently it has been shown (4) that cr/3-dihydroxysilanes. particularly t-butyldimethyl species, undergo an acid-catalysed silapinacol rearrangement to /3-aldehydo- and / -ketosilanes, in most respectable yields. The implications of this rearrangement on the acid-catalysed rearrangement of u/i-epoxysilanes to carbonyl compounds are discussed in Chapter 4. 

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). 

Cyclohexanone (5.7 mmol) was added to a solution of the reagent (6.2 mmol) at -78 to -50°C. After 0.5 h at this temperature, it was allowed to warm to ambient temperature over 3 h. The mixture was poured into dilute HC1 (25ml, 0.5m), extracted with dichloromethane (3 x 30ml), and the organic extracts dried and concentrated, to give the epoxysilane (4.7mmol, 83%). 

To a stirred solution of the epoxysilane (1 mmol) in THF/water (3 ml, 4 1) was added perchloric acid (0.1 ml, 70%), and stirring was continued for 4 h at ambient temperature. The mixture was then poured into water (20ml), and extracted with dichloromethane (3 x 20 ml). Drying and concentration gave cyclohexane carboxaldehyde (0.71 mmol, 71%). 

Geometrically defined a/ -epoxysilanes have been shown (6) to undergo a highly stereoselective rearrangement to silyl enol ethers (see also Chapter 15). This rearrangement is catalysed by boron trifluoride etherate, and seems to involved-opening of the epoxysilane, as shown ... 

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)). 

Enolate generation, 106-7 Enolate trapping, 99-101 Enones, 34-5 Epoxidation, 21-3 a/3-Epoxysilanes, 21-4, 78 -Ethoxy acylsilane, 110 1-Ethoxy-l-trimethylsilyloxycyclo-propane,133 Ethyl bromoacetate, 123 Ethyl 2-chloropropanoate, 133 Ethyl glycinate, 87,88-9 Ethyl m-nitrobenzene, 137 Ethyl irimethylsilylacetate. 71, 123-4, 134 Ethyllithium, 66... 

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

Table 1. Scratch and abrading tests of different coatings composition (mole-%) MeO s 20 epoxysilane 50 SiO 30...

Call et al. (2001) of the Pacific Northwest National Laboratory in Richland, WA, also studied the immobilization of unmodified oligonucleotides. Amine-modified and unmodified oligonucleotides could be attached to epoxysilane slides (covalent attachment) or acid-washed slides (noncovalent attachment) under the same conditions by printing in an alkaline-sodium... 

The PDMS microwell array is mounted onto a glass slide and activated using an epoxysilane to which the protein may be attached. The wells were about 1.4 mm in diameter and 300 pm deep allowing a volume of approximately 300 nL. The microwell protein chip should be widely applicable. 

Benters et al. (2002) of the University of Bremen in Germany utilized starburst dendrimers to create polymeric surface coatings for glass substrates in an effort to increase sensitivity for DNA microarrays (Figure 3.18). Ami-nosilane or epoxysilane chemistries were applied to glass substrates. The... 

Taylor etal. (2003) at Virginia Commonwealth University examined three commonly used glass slide surface chemistries (poly-L-lysine, epoxysilane, and aminopropylsilane) and a dendrimer structure (DAB) similar to that described by Benters et al. (2002) see Figure 3.21. Slides derivatized... 

BSA-blocked slides exhibifed reduced backgrounds over fhose blocked with succinic anhydride. However, succinic anhydride-blocked epoxy slides appeared to be less variable. Microarrays created on unblocked epoxysilane glass exhibited the greatest hybridization S B (signal to background noise) efficiency. 

Figure 3.21 Abbreviated surface chemistry structures PLL, epoxysilane, APS, and DAB. (From Taylor, S. et al.. Nucleic Acid Res., 31(16), 1-19, 2003. With permission.)...

Soak slides in 24 8 1 v/v epoxysilane solution comprising anhydrous xylene, glycidoxypropyl trimethoxysilane, and N,N-diisopropyl eth-ylamine at 80°C for 5 hr. 

Terminal oxiranes have recently been deprotonated by i-BuLi at —90°C in the presence of various diamine ligands. The resulting lithiooxirane could be trapped by TMSCl to give trans epoxysilanes in good yields (Scheme 51). 

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. ... 

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