1.3- Dioxolanes s. a. Acetals cyclic

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... 

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. 

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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