Silyl enol ethers photocyclization

Hirano et al. reported on the stereoselective cyclization to give tetralin derivatives using the phenanthrene-p-dicyanobenzene sensitizer system. Pandey independently reported the intramolecular photocyclization of methoxybenzene derivatives bearing silyl enol ether chromophore via their heterodimer radical cations in the presence of 1,4-dicyanonaphthalene gave benzo-annulated cyclic ketones in 70% yields [490] (Scheme 133). 

Indoles substituted at C-3 with butadienyl or styryl groups undergo electrocyclic reactions to give carbazole derivatives. The silyl enol ether (275) can be converted thermally directly into the carbazole (276), as the result of ready elimination of methanol this sequence is crucial for a hyellazole synthesis (Equation (96)) <89CC43>. Several related 3-styrylindole sequences have been developed and differ with respect to the placement of ethyl carbonate groups on the alkene. Both the substrates (277) and (279) undergo photocyclization to the respective benzocarbazoles (278) and (280) as a result of... 

In these photoreactions, silyl groups are activated by an a-heteroa-tom and smoothly removed in reactions of the radical cation intermediates. DCA-sensitized photocyclization of cyclic silyl enol ethers has... 

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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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ ]

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