Silyl enol ethers photocycloadditions

Photocycloadditions of silyl enol ethers 362 with aromatic aldehydes 363 have been used to prepare substituted oxiranes with high diastereoselectivities favoring the isomer... 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

The degree of asymmetric induction in the photocycloaddition reaction can be quite high with substrates containing a stereogenic center. Winkler, Scott and Williard have reported that irradiation of 1-tryptophan-derived vinylogous amide 107 led to the isolation of ketoimine 108 in 91% yield as a single diastereomer . Closure to the tetracyclic portion of the aspidosperma ring system 109 was achieved in two steps by formation of the silyl enol ether with LDA and r-butyl dimethylsilyl triflate followed by treatment with tetrabutylammonium fluoride (TBFA). Conversion of 109 to 110 with >97% optical purity was then achieved. As 110 is an intermediate in Biichi s synthesis of vindorosine, the sequence outlined in Scheme 26 represents a formal total synthesis of vindorosine. 

He found that this is also true in the [2 + 2] photocycloaddition between 1,1-disubstituted silyl enol ethers (e.g., 75a) [83a] and benzaldehyde, as in Scheme 43. The regiochemistry is largely determined on the basis of the most stable biradical, which, before closing, arranges itself so that the bulkiest substituents are trans to each other. Hence, in the cases of 1,1-disubstituted and trisubstituted silyl enol ethers, sterics is the determining factor in the stereoselectivity of the Patem6-Buchi reaction. 

Mizuno, Pac, and co-workers reported photo-Michael reactions via the 2n+2n photocycloaddition-hydrolysis sequences of silyl enol ethers with electron-deficient alkenes such as acrylonitrile, methyl acrylate, and 1-cyanonaphthalene. - Similarly, regioselective 2jr+27l-photocycloaddition of 1-trimethylsiloxynaphthalene with... 

The photochemical reactivity of P-ketoesters is different form that of P-diketones. Irradiation of a P-ketoester in the presence of an alkene produces oxetane via the ketone carbonyl instead of the desired cyclobutane ring system. Therefore, it is necessary to covalently lock the ketoesters as the enol tautomers. To this end, silyl enol ethers, 129 and 132a, and enol acetates, 130 and 132b, were prepared, but these substrates still fail to undergo the desired intramolecular [2 + 2] photocycloaddition with olefins. The only new products observed in these reactions result from the photo-Fries rearrangement of the cyclic enol acetate (130 to 131) and cis-trans isomerization of both acyclic substrates 132a/b. However, tetronates are appropriate substrates for both intermolecular and intramolecular photocycloadditions with olefins. In addition, enol acetates and silyl enol ethers of p-keto esters are known to undergo [2 + 2] photoaddition with cyclic enones (vide infra). 

Photoinduced reactions of cyclic a-diketones with different alkenes takes place via [2 + 2], [4 + 2] or [4 + 4] photocycloaddition pathways. Photoaddition of electron deficient silyl ketene acetals to 2-, 3- and 4-acetylpyridine generates oxetanes as major products. The reaction is favoured in non polar solvents. The photoreaction between silyl enol ethers and henzil affords [2 + 2] cycloaddition products, while in the case of 9,10-phenanthrenequinone [4 + 2] cycloacidition predominates. Photocycloaddition of p-henzoquinones to hicyclopropylidene affords spirooxetanes (21) as the primacy products further irradiation leads to rearranged spiro[4.5]deca-6,9-diene-2,8-diones. With 9,10-anthraqui-none, in addition to the spirooxetane, a spiro[indan-l,l -phthalan]-3 -one is also obtained. ... 

The induced diastereoselectivity in a Paterno-Biichi reaction resulting from a stereogenic center in the alkene part was recently described by Bach and co-workers in the photocycloaddition of chiral silylenol ethers 67 with benzaldehyde 18. The substituents, R, at the stereogenic center were varied in order to evaluate the influence of steric bulk and possible electronic effects. In accord with the 1,3-aUyhc strain model, the facial diastereoselectivity was at a maximum with large (R = t-Bu, SiMejPh) and polar (R = OMe) substituents at the y-position of silyl enol ether (diastereomeric ratio of oxetanes 68 > 95 5). 

The [2+2]photocycloaddition of 5-arylfuran-2,3-diones (496) to trimethylsilyl-oxyethylenes (495) occurs with high regio- and stereoselectivity to efficiently yield polyfunctionalised cyclobutanes (497). ° A convenient four step stereoselective synthesis of ( + )-norasteriscanolide (500) has used the [2+2]photoadduct (499) from 2-cyclopentenone and the trimethylsilyl enol ether (498) to assemble the 5/8 ring system present in many terpenoids. 2-Naphthaldehyde undergoes regio-and stereoselective [2+2]photoadditon to the cyclic ketene silyl acetals (501) to... 

The Patemo-Buchi reaction is the photocycloaddition of an alkene with an aldehyde or ketone to form oxetanes. This transformation has been shown to proceed through a biradical intermediate, and up to three new stereocenters can be formed as a result of this reaction. A general mechanism for the reaction between an aldehyde and a chiral enol silyl ether is shown in Eq. (13.7) [18]. Allylic 1,3-strain is cited as the control element in reactions of this type, and diastereomeric ratios of >95 5 are reported for products 30 containing four contiguous stereocenters. Examples of photocyclizations of amino acid derivatives proceeding through biradical intermediates have been repotted [19]. 

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