1.2- dioxolanyl radical

In addition, 2-substituted chiral succinic acid derivatives can be obtained via diastereoselective radical addition of 2-dioxolanyl radicals to ftimaric acid diamides with a de up to 98% (Scheme 19) [25],... 

A radical cyclization has been achieved from a 1,3-dioxolanyl-thiocarbonate containing an alkyne group in an appropriate position (Equation 49) <2003TA2961>. The stereocontrol between cis- and trans-ivscd tricycles was 1.5-4.2 1. The products were similar to those depicted in Equation (36). 

Reactions of 1,4-dimethoxynaphthalene and its 2-chloro, 2-bromo, and 2-(1,3-dioxolanyl) derivatives with BAIB/TMSX (X = C1, Br) combinations in dichloromethane result in acetoxylation, monohalogenation, or dihalogenation of the more activated ring (Scheme 26) [79]. Specific outcomes depend on the naphthalene derivative and reaction conditions. It is interesting that the 2-(1,3-dioxolanyl) derivative undergoes ipso-bromination with BAIB/TMSBr, and that this mode of reactivity was not observed with 2-(l,3-dioxolanyl)-l,4-di-methoxybenzene. These reactions are mechanistically diverse. Evidence was presented that bromination occurs after the formation of molecular bromine, and that chlorination probably follows a radical pathway involving the homo-... 

Some preliminary results from the reviewers laboratory (293) may shed light on the mechanism of the ethylene and carbon dioxide formation. The liquid phase photolysis (185 nm) of 2,2-dimethy1-1,3-dioxolane affords large quantities of ethylene and some carbon dioxide. Carbon dioxide is also formed if di-t-butyl-peroxlde is photolyzed in 2-methyl-l,3-dloxolane. Thus it seems probable that in the primary process the l,3-dioxolanyl-2-radical is formed. The latter appears to be prone to fragmentation and further eliminates ethylene to give rise to a 0=CR-0 radical, a likely precursor of the carbon dioxide. This sequence is not possible in the open chain acetals. In agreement with this, carbon dioxide is not a product in the photolysis (185 nm) of liquid formaldehyde and acetaldehyde dimethyl acetal (103a,b). 

Finally, the stereoselectivity of the alkylation of dioxolanyl radicals was studied. Interestingly, the reactions became more selective when the reactivity of the radical trap was enhanced (Scheme 18) [40]. This is clearly contrary to the reactivity-selectivity principle and is best explained by the position of the transition state. 

The reaction in Entry 12 is related and uses thioglycolate esters as a chain transfer agent. In this particular reaction involving an electron-rich alkene, the yield is only 8% in the absence of the thioglycolate. Entry 13 is another example of the addition of an acyl radical to relatively electrophilic alkene. Entry 14, involving the addition of formamide was done with acetone photosensitization. The 2-dioxolanyl radical involved in Entry 15 would be expected to be nucleophilic in character and higher yields were obtained with diethyl maleate than with typical terminal alkenes. The addition of 1,3-dioxolane to various enones has been done using benzophenone sensitization.The radicals in Entries 17 and 18 are electrophilic in character. Entries 19 and 20 are examples of thiol additions. 

Ziegler FE, Harran PG (1993) Intramolecular addition of a dioxolanyl radical to the indole nucleus preparation of enantiomerically pure, oxygenated perhydro-3H-pyirolo[l, 2-a] indoles. J Org Chem 58 2768... 

Photochemical addition of dioxolanyl radicals to -unsaturated ketones gives the monoprotected 1,4-dione derivatives 13 <01T10319> and the platinum catalysed domino... 

The presence of a double bond two carbons away from the peroxyl radical enables attack of the peroxyl radical on this double bond to form an endoperoxide and a carbon-centered radical [11-13]. The 1,2-dioxolanyl radical generated by 5-exo cycliza-tion is a key intermediate in the formation of secondary products such as serial cyclic endoperoxides, isoprostanes, and diepoxides. 

The 1,2-dioxolanyl radical can react with a second molecnle of oxygen. If an additional double bond is present in the appropriate position, the resulting peroxyl radical can once again react with this double bond giving rise to serial cyclic endoperoxides [14-16],... 

Alternatively, the 1,2-dioxolanyl radical can react with the double bond adjacent to the endoperoxide to create a highly unstable cyclopentane ring [13,17], The bicy-clic endoperoxides created in this reaction are analogous to prostaglandin H2 formed enzymatically by cyclooxygenases the resulting products, therefore, have been given the trivial name isoprostanes [18],... 

Exo cyclization of the 1,2-dioxolanyl radical leads to bicyclic endoperoxides and isoprostanes... 

A fourth reaction option for the 1,2-dioxolanyl radical is the intramolecular radical substitution (Sgi) of the peroxide by the carbon-centered radical [19,20], This reaction can subsequently form diepoxyalcohols that also have been formed in the reaction of mutant cyclooxygenase-2 [21], These diepoxyalcohols have been proposed as alternative precursors for isofuran and isopyran compounds [6,22],... 

Irradiation of 1,3-dioxolanes in the presence of a sensitizer such as benzophenone (PhjCO), leads to the abstraction of an acetal hydrogen atom (by the triplet state ketone) to generate 1,3-dioxolanyl radicals, together with the resonance-stabilized diphenyl ketyl radical (Scheme 16). These nucleophilic 1,3-diox-olanyl radicals have been shown to add to a variety of alkenes to form adduct radicals, which can then accept a hydrogen atom from the diphenyl ketyl radical to give the desired alkylated product. The diastereoselective addition of 1,3-dioxolanyl radicals to alkenes bearing chiral auxiliaries has also recently been reported, while the photochemical alkylation of diketene with 2-undecyl-l,3-dioxolane has been utilized as a key step in the synthesis of (-)-tetrahydrolipstatin (Scheme 17). ... 

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