Silanes 0-hydroxy, elimination reactions

Scheme 9 demonstrates the further synthetic application of the thus obtained N,0-acetals. Substitution of the alkoxy or acyloxy group by nucleophiles like enol ethers, enol esters, enamines, other electron-rich olefins, CH-acidic compounds, electron-rich aromatics, isocyanides, trimethylsilyl cyanide, organometallics, vinyl and allyl silanes, hydroxy functions, or trialkylphosphites either catalyzed by Lewis acids or proton acids leads to the product of the amidoalkylation reaction (path a). In the presence of stereocenters as control elements, diasteroselective amidoalkylation reactions can be performed as shown in a large number of examples. On the other side, as Nyberg showed for the first time [196], elimination with formation of enecarbamates [208] and enamides [196,208,209] followed by reaction with electrophiles or nucleophiles (path b) also is possible. 

The Peterson Reaction allows the preparation of alkenes from a-silylcarbanions. The intermediate p-hydroxy silane may be isolated, and the elimination step - the Peterson Elimination - can be performed later. As the outcome of acid or base-induced elimination is different, the Peterson Olefination offers the possibility of improving the yield of the desired alkene stereoisomer by careful separation of the two diastereomeric p-hydroxy silanes and subsequently performing two different eliminations. 

The rearrangement regularly takes place with retention of configuration at the carbon atom to which the silyl group is attached, and it is compatible with ketone and ester groups. In the case of p-hydroxy-silanes, no Peterson-type elimination occurs. Some representative reactions are shown in Scheme 31. 

In addition to the examples in Table 1, the Peterson methylenation has been used in several interesting natural product syntheses, as the examples in equation (2M6) indicate. Danishefsky and coworkers used the Peterson reaction in an approach to mitomycins (4 equation 2). This application demonstrated the use of unique elimination conditions. The hydroxy silane intermediate was stable to direct Peterson elimination. Therefore, the removal of the silyl protecting group and the elimination of the silyloxy group were carried out with DDQ in quantitative yield. 

Reaction of the -hydroxy selenides with phosphorus triiodide or A. A -carbonyl-diimidazole leads to elimination of both the hydroxy and selanyl moieties, and, as with the / -hydroxy silanes, to formation of alkylidenecyclopropanes (see Section 5.2.2.7.3.). 

The conversion of tertiary 8-hydroxy cyclopropylsilanes into (alkenylcyclopropyl)silanes by acid-catalyzed dehydration has been described in Section 5.2.2.2. It is also possible to obtain alkylidenecyclopropanes by elimination of both the hydroxy and silyl groups in a kind of Peterson olefination reaction. ... 

There is a silicon version of the Wittig reaction, known as the Peterson reac-tion. Reaction of an aldehyde or ketone with an a-silyl carbanion forms a -hydroxy silane, from which elimination of trialkylsilanol, RsSiOH, provides... 

This reaction was first reported by Peterson in 1968. It is a two-step synthesis of olefin involving the addition of an a-silyl carbanion to a carbonyl compound to form y3-silyl alcohol (or /3-hydroxy silane) and the elimination of silol. Therefore, this reaction is generally known as the Peterson olefination. In addition, this reaction is also referred to as the Peterson reaction " and Peterson elimination. Occasionally, the Peterson olefination is also called the Peterson alkenation or Peterson alkenylation. ... 

Three groups of workers have reported the use of the addition of allyltrimethyl-silyl anion to aldehydes, giving P-hydroxy-silanes, and their subsequent elimination to give dienes. Yamamoto et al. have reported the diastereoselective synthesis of the threo-o coho (209) by the boron- or aluminium-mediated reaction of (208), and of the erythro-dXcohols (210) by using the tin-mediated reaction of (208). A similar approach, using the titanium-catalysed addition... 

In an extension to previous work, a stereoselective synthesis of trisubstituted alkenes has been reported through reaction of /3-keto-silanes with alkyl-lithium reagents followed by elimination of the hydroxy and silyl groups. 5y -Elimination is encouraged under basic conditions, a/7n -elimination on acid treatment of the -hydroxysilanes. 

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