Silyl enol ethers regiospecific synthesis

Silylated cyanohydrins have also been prepared via silylation of cyanohydrins themselves and by the addition of hydrogen cyanide to silyl enol ethers. Silylated cyanohydrins have proved to be quite useful in a variety of synthetic transformations, including the regiospecific protection of p-quinones, as intermediates in an efficient synthesis of a-aminomethyl alcohols, and for the preparation of ketone cyanohydrins themselves.The silylated cyanohydrins of heteroaromatic aldehydes have found extensive use as... 

Diaryliodonium fluorides can be used as efficient reagents for the arylation of silyl enol ethers [71, 72]. The reaction of silyl enol ethers 95 with o-nitro-phenyliodonium fluoride 96 results in a regiospecific arylation yielding arylke-tones 97 in good yields (Scheme 43) [72]. This arylation has been applied in a new, regiocontrolled synthesis of carbocycle-fused indoles [72]. 

If silyl enol ethers (126) are used instead of enamines, some of these problems can be overcome. The regiospecific synthesis of 5-alkylimidazoles (127) provides an example of the use of these compounds (Scheme 68). In a similar reaction, acylvinylphosphonium salts can be condensed with amidines to give imidazolylphosphonium salts (129), presumably via (128) (80AHC(27)24l). A recent improvement of this method uses, instead of the enamines, the corresponding immonium salts which react with amidines in the presence of triethylamine to give high yields (85-97%) of 2-imidazolines (130). In all cases the trans-imidazoline results (Scheme 68) (80JHC97). 

The so-called silyl enol ethers (enoxyorganylsilanes) are important synthones, e.g, for regiospecific preparation of enolates, aldol condensation, synthesis of a-substituted carbonyl derivatives and for thermal or photochemical cycloaddition. For the preparation of silyl enol ethers the corresponding aldehydes and ketones first have to be enolized and then treated with silylating agents in the presence of a base. Thus, from butanal (608) and Me3SiCl, cis/trans- 1-trimethylsiloxybut-l-ene (609) is obtained (equation 311)347, while 1-trimethylsiloxy-l-phenylethene (610) is the product from acetophenone (90a) (equation 312)347. 

Ito, Y., T. Konoike, and T. Saegusa. Synthesis of 1,4-Diketones by the Reaction of Silyl Enol Ethers with Ag20. A Regiospecific Formation of Silver (I) Enolate Intermediates. J. Amer. Chem. Soc. 97, 649 (1975). 

Regiospecific synthesis of enol silyl ethers can also be achieved from enones either by reductive silylation or by 1,4-addition of the conjugated system. Thus, Li/NHj reduction of the decalone (27) and silylation give the enol silyl ether (281.24 Similarly, addition of lithium dimethylcuprate to cyclohexenone23 followed by silylation gives the enol silyl ether (29). Trimethylsilyl cyanide (30) normally adds 1,2 to conjugated ketones (e.g. carvone, 31). However, in the presence of trialkylaluminum, 1,4-addition takes place to give the enol silyl ether (32 Scheme 9). The same overall transformation can be accomplished by diethylaluminum cyanide and trimethylchlorosilane.26... 

During a synthesis of pederol dibenzoate, a key step required formation of a regiospecific enol silyl ether in a highly oxygenated and base-sensitive substrate. The reaction was uniquely achieved by a rhodium(i)-catalysed hydrosilylation sequence (Scheme 47). ... 

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