Formation and hydrolysis of the acetal function

Ue have presented experimental results which demonstrate the influence of stereoelectronic effects on the configuration and the conformation of the acetal function. A pertinent question which follows is whether or not these stereoelectronic effects play a similar role in the formation and in the hydrolysis of this functional group. 

The formation or the hydrolysis of an acetal function proceeds by the mechanism described in Fig, 16 in which oxonium ions and hemiacetals occur as intermediates. It has also been established (76) that the rate determining step in acetal hydrolysis is generally the cleavage of the C —0 + bond of the protonated acetal 100 to form the oxonium ion 111. This ion is then rapidly hydrated to yield the protonated hemiacetal 112 which can give the aldehyde product after appropriate proton transfers. It is pertinent therefore to find out if stereoelectronic effects influence the rate determining step (110- 111) of this hydrolysis reaction. 

We will first examine what might occur when stereoelectronic effects are taken into account. An analysis of the conformations D, A, and (Fig. 17) of dimethoxymethane which have two, one, and zero anomeric effects respec- 

Bond lengths collected from experimental data in X-ray crystal-structure determination of methyl pyranosides (11) and other products (77) strongly support the above discussion. Oxygen atoms with an electron pair antiperi-planar to a C — 0 R bond definitely have shorter C—0 bonds whereas the C—0 R bond is longer than normal. Interestingly, theoretical calculations have been carried out (11, 7B) and the results are consistent with the above discussion and experimental observations. 

Consequently, by protonation of the 0 R oxygen, the C-0 R bond length should increase to a larger extent in conformer A (- 113) than in conformer C ( 114) because the non-protonated oxygen of 113 has an electron pair anti peri planar to the C —0 1 bond which is not the case with 114. With conformer D, protonation of the electron pair of the 0 R oxygen which is anti peri planar to the C—OR bond (-115) should lead to a situation similar to that of 113. 

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Conformation of acetals Conformation of mono and dithioacetals Conformation of 1,3-oxazines and 1,3-diazines Formation and hydrolysis of the acetal function Hydride transfer to cyclic oxonium ion Oxidation of the C —H bond in acetals REFERENCES... 

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