General procedure for silyl enol ethers

As discussed in Chapter 9, various nucleophiles can be introduced at the ortho position of nitroarenes via the VNS process. This provides a useful strategy for the synthesis of indoles. One of the most attractive and general methods of indoles and indolinones would be the reductive cyclization of a-nitroaryl carbonyl compounds (Eq. 10.54). The VNS and related reactions afford a-nitroaryl carbonyl compounds by a simple procedure. For example, alkylation of 4-fluoronitrobenzene with a lactone silyl enol ether followed by reductive cyclization leads to tryptophols (Eq. 10.55).73... 

The most direct procedure for the synthesis of a-haloacyl silanes is electrophilic halo-genation of enolates or enol ethers of acyl silanes. This has been achieved with the silyl enol ethers using bromine at low temperatures, but the reaction suffers from the general... 

This procedure illustrates a general method for the preparation of crossed aldols. The aldol reaction between various silyl enol ethers and carbonyl compounds proceeds smoothly according to the same procedure (see Table I). Sllyl enol ethers react with aldehydes at -78°C, and with ketones near 0°C. Note that the aldol reaction of sllyl enol ethers with ketones affords good yields of crossed aldols which are generally difficult to prepare using lithium or boron enolates. Lewis acids such as tin tetrachloride and boron trifluoride etherate also promote the reaction however, titanium tetrachloride is generally the most effective catalyst. 

Addition of Silyl Nitronates to Enals Catalyzed by Quaternary Ammonium Bifluoride General Procedure for the Isolation of Enol Silyl Ether [34] (p. 128)... 

Substituted cycloheptenones 25 can be prepared by thermolysis of neat cyclopropyl silyl enol ethers 23 under an inert atmosphere, followed by hydrolysis of the resulting crude silyl enol ethers 24. This procedure is quite general for low molecular weight compounds. The enol ethers 23 are generated from either the cis- or trani-isomers of 22 and rearrange in excellent yields, albeit at different temperatures. 

General procedure for the enantioselective protonation of silyl enol ether ... 

Diethyl [(2-tetrahydropyranyloxy)methyl]phosphonate is useful in the Wlttig-Horner synthesis of enol ethers, which are intermediates in one-carbon homologations of carbonyl compounds. This procedure is an adaptation of a general method for making dialkyl hydroxymethylphosphonates. An 0-tetra-hydropyranyl derivative also has been made from dibutyl hydroxymethyl -phosphonate, and diethyl hydroxymethylphosphonate has been O-silylated with tert-butylchiorodimethylsilane and imidazole. Another useful congener in this series has been prepared by an Arbuzov reaction of methoxyethoxymethyl (HEM) chloride and triethyl phosphite. 

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