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22.7.2.1.2.35.1 Variation 1: Reactions with Diols

DOI: 10.1055/sos-SD-022-00806

Lebel, H.; Grenon, M.Science of Synthesis, (200522729.

The transorthoesterification of ortho esters with diols is commonly used to protect proximal hydroxy groups and to allow further transformations to be carried out. The exchange reactions are usually performed in the presence of an acid catalyst and, although a plethora of different acids can be used, some are more often selected than others. For instance, 4-toluenesulfonic acid is the most popular, and it is used with many common solvents of varying polarity, including benzene, toluene, dichloromethane, dimethylformamide, dimethyl sulfoxide, and acetonitrile, either at room temperature or when heated at reflux. Scheme 65 shows a representative example where this acid acts to promote the transorthoesterification of the corticosteroid 168 to give the corresponding cyclic ­ortho esters 169 in good to excellent yields (Scheme 65).[‌267‌,‌268‌] An alternative reagent, ­pyridinium 4-toluenesulfonate, is also employed under similar conditions. Benzoic acid, a considerably weaker acid than 4-toluenesulfonic acid, is used occasionally as a catalyst for the protection of 1,2-diols by converting them into the corresponding cyclic ortho esters.[‌269‌,‌270‌] Typical conditions require heating the 1,2-diols and an ortho ester (usually trimethyl or triethyl orthoacetate) in the presence of a small amount of the acid; in many cases the ortho ester also acts as a solvent or cosolvent. Strong inorganic acids, such as hydrochloric acid or sulfuric acid, have also found widespread use as catalysts for the transorthoesterification process, as illustrated by the conversions of enantiopure 1,2-diols into the chiral 1,3-dioxolane ortho esters 170 (Scheme 65).[‌271‌]

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