1.4- Dioxanes, formation

Bu2SnO, toluene, reflux, 4 h Pd(Ph3P)4, THE, CH2=CHCH(OAc)2, rt, 1 h, 80-89% yield. In pyranoside protection, selectivity for 1,3-dioxane formation is generally observed, but dioxolanes are often formed. 

The major 1,3-dioxane could be separated by column chromatography but the diastereoisomeric 1,3-dioxolane derivatives could not be separated [Scheme 3.89],39 The strong preference for 1,3-dioxane formation is underscored in the benzylidenation of L-gulonolactone depicted in Scheme 3.90. Two different products formed depending on the reaction conditions, but in neither case was a 1,3-dioxolane ge tie rated.165... 

The less nucleophilic acetals copolymerize with vinyl compounds more readily. Perhaps, in these systems the alkoxycarbenium ions (... —OCH ) that coexist in equilibrium with oxonium ions facilitate copolymerization with vinyl compounds. Styrene copolymerizes with trioxane 51,52) and tetraoxane53). The latter system yields polytrioxane and trioxane-styrene copolymer together with 1,4-phenyl-1,3-dioxane. It was formed in 25% yield in ethylene dichloride at 30 °C after 1 hr using [BF3 OEtj] = 10-2 mol l-1, [styrene], = [tetraoxane = 0.5 mol l-1. The proposed mechanism of 4-phenyl- 1,3-dioxane formation is shown below (cf. also Chap. 7) ... 

Like all acetalizations, 1,3-dioxane formation is reversible it finds wide appHcation for establishing the 1,3-dioxane moiety as protective group for carbonyl compounds, as well as for systems with a 1,3-diol structural element. 

Experimental [9] and theoretical data [10], [11] clearly demonstrate that the presence of formaldehyde oligomers (FO) is a prerequisite for the 4-alkyl-1,3-dioxanes formation, whereas FO are much more reactive than... 

FIGURE 10.3 Potential energy surface of 1,3-dioxane formation by the Prins reaction. 

Due to the analysis of the calculated data of the transition states structure of 4-alkyl-1,3-dioxane formation from formaldehyde oligomers and alkenes, it is found that 1,3-dioxane structures are formed in the result of direct isomerization of 7i-cation on the first stage. A free o-cation formation is not observed here. 

It is shown lhat interaction of formaldehyde oligomers with alkenes in gas phase or nonpolar solvents, accompanied by 1,3-dioxane formation must occur as a pseudo synchronous syn-addition. Only formaldehyde oligomers addition to ethylene can be considered as a synchronous interaction. 

The calculations show that the formation of hydrogenated pyrans can be carried out by FD and alkenes reaction accompanied by 1,3-dioxanes formation as well. However, the activation energies of these reactions are higher lhan the ones of 1,3-dioxanes formation... 

The role of formaldehyde dimer in 0-containing heterocycles formation by the Prins reaction have been investigated. It was shown that the 1,3-di-oxanes, hydrogenated pyrans and oxetanes can be can be obtained from formaldehyde dimers and alkenes in the gas phase. The activation energy of these reactions is different. It is lower for 1,3-dioxanes formation, and higher for oxetanes formation. Thus formation of 1,3-dioxanes happens in the conditions of kinetic control. Opposite, the hydrogenated pyrans formation happens in the conditions of thermodynamic control... 

Ejqierimental [4] and theoretical data [5], [6] clearly demonstrate that the presence of formaldehyde oligomers (FO) is a prerequisite for the 1,3-dioxanes formation, whereas FO are much more reactive than the monomer. It is assmned that the FO addition at the double bond can be described as sequential, pseudo synchronous or concert [7], The implementation of the two mechanisms corresponds to the experimental data on a stereoselective Prins reaction for a series of cycloalkenes [8],... 

In Fig. 11.2, the potential energy surface section corresponding to 1,3-dioxanes formation from FD and alkenes is shown ... 

TABLE 11.1 Energy Parameters of the Key Stages of 1,3-Dioxanes Formation From Alkenes and FD, kJ/mol... 

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