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. 

Dihydrotriazoles were prepared diastereoselectively by cycloaddition of aryl azides to enol ethers and, under controlled conditions, enamines6-7. In only a few cases were the aziridines prepared by photochemical decomposition of thus formed dihydrotriazoles 84-85. 

Photochemical [2 + 2] Cycloadditions. Photochemical [2 + 2] cycloadditions between alkenes and chiral phenylgly-oxylate derivatives of 3-hydroxyisobomeol show minimal diastereoselectivity (16% de). Better results are obtained in [2 + 2] cycloadditions between chiral enol ethers and Dichloroketene (eq 7). After ring expansion and expulsion of the auxiliary (Diazomethane, Chromium ll) Perchlorate),... 

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. ... 

Electron-rich alkenes (e.g. enol ethers) react with electron-deficient dienophiles in [2-1-2] cycloaddition reactions giving four-membered rings. According to the Woodward-Hoffmann rules the reactions are forbidden in a 25-1-25 pathway and thus have to proceed stepwise via dipolar or diradical intermediates. The thermochemically allowed 25-I-25 attack is sterically hindered and thus probably extremely rare." Photochemically, however, 25-1-25 additions are allowed and are of preparative interest. 

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... 

In contrast with the photochemical cycloaddition reaction of two alkenes, the [2+2] cycloaddition of a ketene and an alkene occurs under thermal conditions. The ketene is formed typically from an acid chloride and a mild base such as EtsN, or from an a-halo-acid chloride and zinc. Cycloaddition with an alkene occurs stereospecifically, such that the geometry of the alkene is maintained in the cyclobutanone product. The regioselectivity is governed by the polarization of the alkene, with the more electron-rich end of the alkene forming a bond to the electron-deficient central carbon atom of the ketene. Thus, the product from cycloaddition of dimethylketene with the enol ether Z-171 is the cyclobutanone m-172, whereas with -171, the isomer trans-lll is formed (3.116). ... 

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