1.3- Dioxolanes, formation

ZnBr2, CH2CI2, 25°, 2-10 h, 90% yield. When a MEM-protected diol was cleaved using ZnBt2 EtOAc, 1,3-dioxolane formation occurred, but this can be prevented by the use of in situ-prepared TMSI. ... 

Procedure for 1,3-dioxolane formation with ethyl acetoacetate by azeotropic removal of water.128a Ethyl acetoacetate (30 g, 0.23 mol), ethane-1,2-diol (16g, 0.248 mol), a crystal of toluene-p-sulphonic acid and benzene (50 ml) (CAUTION) were placed in a round-bottomed flask fitted with a Dean and Stark water separator (Fig. 2.31(a)) and a reflux condenser. The reaction mixture was heated until no more water collected. The product was fractionally distilled under reduced pressure to give the cyclic acetal (35 g, 87%), b.p. 99.5-101 °C/17-18 mmHg. 

A case of anomalous 1,3-dioxolane formation has been reported when the sesquiterpene lactone parthenin 9 is treated under Baylis-Hillman conditions with aromatic aldehydes and affords products of type 10 <07TL955>. A computational study of copper-catalysed carbonyl... 

Dioxolanes, formation by acetylation of carbohydrates using 2-alk-oxypropenes 81 H(16)l587. 

Hydroxyethyl esters can be prepared from the free acid by reaction with ethylene carbonate. During an attempt to protect the ketone group of a keto-acid by 1,3-dioxolane formation, it has been found that mixed carboxylate sulphonate esters [e.g. (121) from ethylene glycol and tosic acid] can easily be formed extensions of this reaction show it to have a general applicability. In the presence of fluoride ions, carboxylic acids become sufficiently nucleophilic to attack alkyl halides. Thus, reaction between an acid and 1,3-dibromopropane leads to diesters (122). Molecular sieves have been recommended as suitable absorbants... 

Scandium triflate is also an effective catalyst for dioxolane formation.246... 

Unfortunately, the comparison is not reliable because the temperature was not measured in the domestic oven. Dioxolane formation by acid-catalyzed exchange between 2,2-dimethyl-l,3-dioxolane (DMD) and a ketone in a inert solvent, or simply in excess DMD, requires 4 to 7h under classical conditions [63], This reaction is readily achieved under microwave irradiation in high yields in 4 to 30 min (Scheme 8.43). 

The ozonolyses of enol ethers has been reviewed <91MI 4l6-0l>. The relative dipolarophilicity of certain species to attack by carbonyl oxides has been investigated and, in general, the order of reactivity is aldehydes > enol ethers > esters ss ketones. It is apparent that enol ethers are very reactive towards carbonyl oxides, so much so that 1,2-dioxolane formation can be a major reaction pathway (especially for formaldehyde-O-oxide) <85JOC3365>. 

Dioxolane formation is not observed in allyl silanes as a result of silicons diminished ability in comparison to tin to stabilize a P positive charge. Nevertheless, the propensity of the carbon-silicon bond to donate electron density to electron deficient sites leads to population of a perpendicular conformation (Sch. 10) and ultimately to an unusually high yield of the sterically less stable Z-allylic hydroperoxide (e.g., 17). 

Before looking at the answer below, try to write a mechanism for this reaction. If you need it, use the mechanism we gave for the formation of acyclic acetals, acid-catalysed dioxolane formation... 

HFA than of the imine. Attack on the metal atom of one molecule of HFA and imine is a prerequisite for the formation of five-membered rings. The lack of electron density in the HFA adduct inhibits ring expansion. According to these observations the displacement of triphenylphosphane by the more nucleophilic methyldiphenylphosphane promotes dioxolane formation 40). Platinaoxazolidines are available in both ways. No structural assignment has been given for the mixed compounds 182a, b 16). 

More sterically demanding ketones can influence the selectivity of dioxolane formation. For example, butane-1,2,4-triol reacts with 3-pentanone43 44 or 3,3-dimethoxypentane45 46 to give a mixture of the dioxolane 2SA and dioxane 28 5 in a ratio of 45 1 respectively [Scheme 3.28]. Enhanced preference for the dioxolane isomer is also observed in the reaction of 2-0-p-methoxybenzyl-t-threitol (29 1) with 3-pentanone [Scheme 3.29].47 48... 

H0(CH2)20H, CsHsN-TsOH, CsHs, reflux, 1-3h, 90-95% yield." This is a commonly used, mild and general method for dioxolane formation. 

Figure 6.83 Proposed mechanism for the reaction catalyzed by AlnA, AlnB, Aln6, and Aln4. (i) Attachment of ribose onto prealnumycin. (ii) Dioxolane formation, (iii) Conversion of diox-olane into a dioxane unit.

The benzylamines 46 are obtained from (2-cyanobenzyl)aryl ketones 49, the products of a Claisen condensation of 2-cyanotoluenes and benzoates, by dioxolane formation with glycol and reduction of the CN-group with B2H6. 

Lu, C.-D., Chen, Z.-Y, Liu, H., Hu, W.-H., Mi, A.-Q. (2004). Highly chemoselective 2,4,5-triaryl-l, 3-dioxolane formation from intermolecular 1,3-dipolar addition of carbonyl yhde with aryl aldehydes. Organic Letters, 6, 3071-3074. 

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