Silyl enol ethers photochemical cycloaddition

The sensitized photochemically-induced [2 + 2] cycloadditions of silyl enol ethers with acrylonitrile or methyl acrylate have been found, because of the regiospecificity of the... 

Diels-Alder reactionsBoth 1,4-dicyanonaphthalene (DCN) and 2,6,9,10-te-tracyanoanthracene (TCA) have been used as sensitizers to effect photochemical [4 + 2]cycloadditions of electron-rich dienes and electron-rich dienophiles, which do not normally undergo thermal cycloadditions. These cycloadditions are known as triplex Diels-Alder reactions because they are postulated to involve as an intermediate a three-membered complex of sensitizer, dienophile, and diene. This reaction is useful for synthesis of bicyclo[2.2.2]octenes from some silyl enol ethers, alkenes, or arylalkynes. 

The photochemical cycloaddition of a carbonyl, generally from an aldehyde or ketone, and an alkene is called the Patemd-Buchi reaction This [2 + 2]-cycloaddition gives an oxetane (213) and the reaction is believed to proceed via a diradical intermediate. Silyl enol ethers react with aldehydes under nonphoto-chemical conditions using ZnCl2 at 25°C or SnCl4 at —78°C. ... 

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. 

Abe and co-workers revealed that the photochemical [2+2] cycloaddition (Pa-terno-Biichi reaction) of silyl enol ethers 86 of selenol esters to aromatic aldehydes affords the 3-selanyl-3-siloxyoxetanes 87 (trans cis=65 35-85 15) regio-selectively in good to high yields (Eq. 57) [110]. 

Total syntheses of racemic muscarine and allomuscarine were achieved by Pirrung and DeAmicis 11), who used a new approach to formation of the stereoselectively substituted tetrahydrofuran nucleus (Scheme 8). The key step in this procedure was the stereospecific photochemical ring expansion of a cyclobutanone derivative. The cycloaddition of silyl enol ether with methylchloroketene generated by zinc dechlorination of 2,2-dichloropropionyl chloride followed by reductive removal of the halogen produced the methylsiloxycyclobutanone intermediate. Irradiation of the... 

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