From Epoxysilanes

Treatment of a,jS-epoxysilanes with magnesium bromide etherate leads to specific cleavage of the a-bond j3-oxosilanes (4) are obtained after prolonged reaction. y-Oxosilanes (6) are available from unstable jS,Y-epoxysilanes which are formed by base treatment of the corresponding halohydrins (5).  

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From Epoxysilanes.—Epoxysilane rearrangements continue to provide useful routes to carbonyl compounds. For instance, homologation of a ketone to the corresponding aldehyde using lithium trimethylsilyldiazomethane (2) involves the intermediate formation and the subsequent hydrolysis of an a,/3-epoxysilane (3). Furthermore, it has been shown that pyrolysis of a,/8-epoxysilanes yields silyl enol ethers which, on hydrolysis, would give carbonyl compounds structurally isomeric to those which would be obtained by direct hydrolysis (Scheme 7). ... 

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. 

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.)...

Enamines. In the presence of alumina (Woelm 200 neutral) cw-a,p-epoxy-silanes can be opened by pyrrolidine or morpholine to give a-amino-p-hydroxy-silanes, from which the pure as-enamine can be obtained by KH-induced syn p-elimination (equation I). The isomeric trans-a,p-epoxysilane is inert to this ring... 

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. 

Hodgson, D. M. Norsikian, S. L. M. First direct deprotonation-electrophile trapping of simple epoxides synthesis of a,/f-epoxysilanes from terminal epoxides. Org. Lett. 2001, 3, 461—463. 

When H2O is added to a,p-epoxysilanes in the presence of acid, the a,P-epoxysilanes are converted into aldehydes or ketones in which the carbonyl group is formed from the carbon atom which originally carried the silyl group (Equations Si5.20 and Si5.21). 

Note that generation of an a,p-epoxysilane from an aldehyde followed by addition of water in the presence of acid constitutes the transformation of RCHO to RCH2CHO. 

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... 

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

Table 9 Epoxysilanes from the Condensation of o-Chloro-a-trimethylsilyl carbanion (73) and Carbonyl...

An optically active silyl-stabilised lithiated epoxide has been used in the synthesis of (-F)-cerulenin 111 (Scheme 21) [73]. Starting from the epoxysilane 107, obtained in high enantiomeric purity via Sharpless epoxidation of 106, deprotonation by s-BuLi TMEDA was followed by electrophile trapping using (4E,7E)-nonadienal 109 to allow a concise preparation of 110, which is a key intermediate in the (-F)-cerulenin synthesis. 

A similar transformation (Scheme 12) can be achieved by allowing the carbanion derived from a-chloroethyltrimethylsilane to react with an aldehyde or ketone to give the a/3-epoxysilane, which is readily hydrolysed to the methyl ketone. Similarly chloromethyltrimethylsilane provides aldehydes through homologation of ketones and aldehydes. Direct conversion of the intermediate epoxysilanes into the corresponding aldehyde protected as the ketal or dithioketal is also described. ... 

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