1.3- Dioxoles photolysis

Oxiranes have been made by extrusion of nitrogen from oxadiazolenes (Scheme 81) (B-73MI50502), by flash thermolysis of dioxazolones (Scheme 82) (81JA5414), and (cc-lactones) by photolysis of l,2-dioxole-3,5-diones (Scheme 83) (73JOC2269). 

A photodimerization approach has been applied here. Thus photolysis of 4-(4-chlorophenyl)-thiazoline-2-thione forms the head-to-head dimer (37) <86CC1030>. Under radical conditions 1,3-dioxole forms two geometrical isomers of cyclobutabis-l,3-dioxolane <85CC1088>. 

Diels-Alder cycloaddition of 1,3-dioxole (7) with 3-tosyl-2-pyranone has been used in natural product synthesis <90T4573>, and l,3-dioxol-2-one (8) undergoes an unusual meta cycloaddition upon photolysis in benzonitrile or anisole to give mixtures of regio- and stereoisomers of the tetracyclic adducts (92) where the CN or OMe group can be at any of the three positions shown... 

The [3 + 2] photocycloaddition (Scheme 6.79) usually involves the ground-state alkene and the Si excited state of an electron-donor substituted benzene derivative, often via an exciplex intermediate.807,809 811,816 The discrimination between the ortho- and metacycloaddition pathways is dependent on the electron donor acceptor properties of the reaction partners and the position and character of the reactants substituents.807 The reaction typically produces many regio- and stereoisomers however, a suitable structure modification can reduce their number. Intermolecular and intramolecular versions of the reaction are presented in Scheme 6.88 (a) photolysis of the mixture of anisole and 1,3-dioxole (226) leads to the formation of two stereoisomers, exo- and endo-221, in mediocre ( 50%) chemical yields 830 (b) four different isomers are obtained in the intramolecular photocycloaddition of an anisole derivative 228. 831... 

In some cases the addition of bromide to alkene radical cations is reversible. For example, the addition of bromide to the p-methyl-4-methoxystyrene radical cation occurs reversibly, as demonstrated by the formation of the radical cation when the P-bromo radical is generated independently by photolysis of l-(4-methoxyphenyl)-l,2-dibtomopropane (Eq. 18). An equilibrium constant of 2 x 10 M has been measured for the loss of bromide from this radical in acetonitrile. The apparent lack of reactivity of 1,3-dioxole radical cations with bromide ion in water has also been explained on the basis of reversible addition with rapid loss of bromide from the product radical. However, on the basis of the solvent effects noted above, it is also possible that the lack of reactivity in water is a solvent effect since decreases in reactivity of 4 to 5 orders of magnitude have been observed for reactions of bromide ion with styrene radical cations in largely aqueous solvent mixtures. - ... 

Synthesis of Ketene. Photolysis (270 nm) of 2-thiono-l,3-dioxol-4-ene results in the quantitative formation of ketene via Wolff rearrangement of the corresponding a-ketocarbenes (eq 2). Substituted dioxolenes can also be used in this fashion to generate substituted ketenes (eq 3). This serves as a convenient alternative to the use of a-diazo ketones in the generation of ketenes. 

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