1.3- Dioxolanes acetals

It was thought that the acetal might have reacted with peroxy reagents, forming explosive peroxides. It was suggested that this could also occur with compounds such as tetrahydrofuranyl acetals, 1,3-dioxolanes, and methoxymethyl ethers. 

Deprotection of acetals.1 Dialkyl acetals are cleaved to the carbonyl compound when refluxed in aqueous DMSO. Deprotection of cyclic acetals (1,3-dioxolanes, 1,3-dioxanes) requires more drastic conditions, such as refluxing aqueous DMSO in combination with a cosolvent such as ethanol or 2-butanone. 

All the approaches described have been used to prepare functional polymers by cationic ring-opening polymerization. From this point of view, groups of monomers that have been investigated most are cyclic ethers (tetrahydrofuran), cyclic acetals (1,3-dioxolane), cyclic imines (N-f-butylaziridine), and oxazolines, i.e., these monomers for which the living conditions can be approached. 

Thus, in sequential polymerization of two different cyclic acetals, 1,3-dioxolane and 1,3-dioxepane, the sequential polymerization (i.e., polymerization of added second monomer initiated by active species of the first monomer polymerization) may be easily achieved as evidenced by increase of molecular weight [130]. The isolated polymer is not a block copolymer, however, having nearly statistical distribution of both types... 

In many instances in cationic ring-opening polymerization, all the reaction steps, however, are reversible. The final composition of copolymer (in equilibrium) is governed then by thermodynamics. Thermodynamic approaches have been developed [305] and recently reviewed [306]. Such thermodynamic approach has been used to analyze the copolymerization of pairs of cyclic acetals (1,3-dioxolane with 1,3-dioxepane and... 

Diols Only vicinal cw-OH groups of cyclic 1,2-diols readily form acetals. " Acetal exchange is the most common method for preparing isopropylidene acetals (1,3-dioxolanes). 

Starting from hydroxybenzaldehydes, several workers prepared independently phenylcarbamates the phenyl group of which contains cyclic acetal (1,3-dioxolane) and cyclic mercaptal (1,3-dithiolane) groups (Nikles et al., 1966 Nikles, 1969 Durden and Weiden, 1969 Bachmann and Legge, 1968). Of the former, 2-(l,3-dioxolan-2-yl)-phenyl N-methylcarbamate, dioxocarb (22), and of the latter, 2-(l,3-dithiolan-2-yl)-phenyl N-methylcarbamate, (23) attained practical importance. 

Polymerizability of 5-membered cyclic acetals (1,3-dioxolanes) have been studied in much greater detail. 1,3-Dioxolanes adopt the stable envelope conformation. One of the carbon atoms in the ethylene group is located at the tip of the flap, giving a dihedral angle between the cis- neighbouring hydrogens of 35°47). 

Of the cationically-polymerizable cyclic acetals, 1-3-dioxolan has received the most attention. By means of an ion-trapping technique, quantitative measurements of the concentration of active centres have been made and correlated with initiator incorporation to confirm the living characteristics of the linear polymers under certain conditions. The oxycarbenium ion nature of these propagating centres has been verified by C-n.m.r. spectroscopy. Methyl substitution alters the polymerizability of the homologous 1,3-dioxepanes significantly and steric hindrance in the initiator can influence both the homopolymerization and copolymerization of cyclic acetals. ... 

The effects of the endbiting observed in polymerization of cyclic acetals can be explained as follows (Scheme 5). As an example of cyclic acetals, 1,3-dioxolane was chosen. 

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