Oxidation of the C-H bond in acetals

In 1971, it was discovered that ozone reacts in a completely specific fashion with the acetal function derived from an aldehyde to give the correspon-ing ester and alcohol (114). This reaction proceeds via the insertion of ozone into the C-H bond of the acetal forming a hydrotrioxide intermediate (156) which breaks down to yield the reaction products, the ester, the alcohol and singlet oxygen (115, 116), The hydrotrioxide intermediate 156 can be detected at low temperature (115). 

In the course of a study on the generality of this reaction (117), it was found that the rates at which different acetals are oxidized vary considerably and this led to the proposal that the reaction is controlled by stereo- 

A study (11B, 119) on the reactivity of a series of appropriate chemical models did confirm this postulate. Conformers A, C, and F of an acetal function (Fig. 1, p. 4) possess on each oxygen atom an electron pair oriented anti peri planar to the C — H bond, conformers B and D have only one oxygen with an electron pair properly oriented whereas conformer has none. Thus, conformers D, and E should be inert and conformers A, C, and F should be reactive towards ozone. 

Conformational1y rigid e-glycosides were found to be reactive towards ozone. At least one of the rotamers i, 2 3 B-glycoside (Fig. 9) which corresponds to conformers A, and respectively of an acetal function should therefore be reactive. Compounds and 158 are rigid model compounds for rotamers Ei and E3. These two compounds react with ozone thus, conformers A and are reactive conformers. On the other hand, the unreac-tive compound 159 is a rigid model for rotaraer 3 thus, conformer B is not reactive, as predicted. 

Conformationally rigid n-glycosides were found to be inert toward ozone. Rotamers A-, 3. and A3 of an n-glycoside which correspond to conformers D, B , and E respectively of an acetal function are. consequently unreactive as predicted. Compound 160 can be considered a rigid model for the most stable rotamer (conformer ) of an n-glycoside and it was found unreac- 

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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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