1.3- Dioxolanes s. a. Acetals

The most commonly used protected derivatives of aldehydes and ketones are 1,3-dioxolanes and 1,3-oxathiolanes. They are obtained from the carbonyl compounds and 1,2-ethanediol or 2-mercaptoethanol, respectively, in aprotic solvents and in the presence of catalysts, e.g. BF, (L.F. Fieser, 1954 G.E. Wilson, Jr., 1968), and water scavengers, e.g. orthoesters (P. Doyle. 1965). Acid-catalyzed exchange dioxolanation with dioxolanes of low boiling ketones, e.g. acetone, which are distilled during the reaction, can also be applied (H. J. Dauben, Jr., 1954). Selective monoketalization of diketones is often used with good success (C. Mercier, 1973). Even from diketones with two keto groups of very similar reactivity monoketals may be obtained by repeated acid-catalyzed equilibration (W.S. Johnson, 1962 A.G. Hortmann, 1969). Most aldehydes are easily converted into acetals. The ketalization of ketones is more difficult for sterical reasons and often requires long reaction times at elevated temperatures. a, -Unsaturated ketones react more slowly than saturated ketones. 2-Mercaptoethanol is more reactive than 1,2-ethanediol (J. Romo, 1951 C. Djerassi, 1952 G.E. Wilson, Jr., 1968). 

A review discusses the condensation of aldehydes and ketones with glycerol to give 1,3-dioxanes and 1,3-dioxolanes. The chemistry of 0 0 and 0 S acetals has been reviewed, and a recent monograph discusses this area of protective groups in a didactic sense. ... 

The acid-catalyzed reaction of 1,3,2-dioxathiolane 2-oxide (16) with carbonyl compounds leads to 1,3-dioxolanes (149), i.e. cyclic acetals of glycol <66HC(2i-i)i. The ring transformation occurs exclusively with S—O bond fission and a mechanism involving a seven-membered intermediate (148) has been postulated. However, in the presence of traces of water the intermediate formation of glycol and its reaction with the carbonyl compound cannot entirely be ruled out (66BCJ1785). 

A number of measurements were made also for cyclic acetals, mostly 1,3-dioxolane. AF though the measured kp value can be lower than the actual value (e.g. if a too high concentration of the active q>e(%s is taken for calculations), it is much more difficult to find reasons why the measured value could have been higher than the real value of kp. Thus, the vMues mealy equal to ko 10 mole" 1 s... 

Related Reagents. (/ )- ,l -Bi-2,2 -naphthotitanium Dichloride (/ )- ,T-Bi-2,2 -naphthotitanium Diisopropoxide Dihydroquinine Acetate (15,9.S)-l,9-Bis [(r-butyl)dimethylsilyloxy]-methyl -5-cyanosemicorr in 2,2-Dimethyl-a,a,a, a -tetraphenyl-1,3-dioxolane-4,5-dimethanolatotitanium Diiso-... 

Cyclic acetals having the 4,4 -bis (1,3-dioxolane) system exhibit a simple fragmentation pattern in m.s. The main fragmentation routes are shown in Scheme 26. The most important of these routes (namely, a) is that involving cleavage of the bond connecting the dioxolane rings. 

Dioxolans. Some of the most interesting work in this series has come from Hoffmann s group in London. The debromination of aa -dibromo-ketones, e.g. (202), with a zinc-copper couple in dimethylformamide or dimethylacetamide gives 2-dimethylamino-4-methylene-l, 3-dioxolans (203). These compounds can be regarded formally as 1,3-dipolar adducts of dimethyl amides with oxyallyl (204), or as 1-amino-acetals. Their great... 

With a further shift in the direction of still more advanced breaking of the bond within active species this borderline S 2 mechanism could eventually convert into the S l mechanism. This should be promoted by the presence of a stabilizing group located close to the carbenium ion (like in the polymerization of cyclic acetals) and/or high ring strain (like in the three-membered rings). Indeed, contribution of the SnI mechanism in both cases has been postulated for polymerization of 1,3-dioxolane and isobutylene oxide (2,2-dimethyloxirane) but there is still no clear-cut evidence for its operation. This is probably the reason for high kinetic poiymerizabiiity of these monomers. 

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