Formation of Lactones from Diols

The oxidation of 1,4- and 1,5-diols with many oxidants leads to intermediate hydroxycarbonyl compounds that equilibrate with lactols, which are transformed in situ into lactones. This side reaction is very uncommon during Swern oxidations, due to the sequential nature of alcohol activation versus base-induced transformation of the activated alcohol into a carbonyl compound. Thus, during the oxidation of a diol, normally when the first alcohol is transformed into an aldehyde or ketone, the second alcohol is already protected by activation, resulting in the impossibility of formation of a lactol that could lead to a lactone. 

An eventful oxidation of a 1,4-diol into a dialdehyde occurs, with no interference by the  

However, when one of the alcohols from the diol is a tertiary one— which, therefore, is difficult to protect by activation—formation of lactones is possible.247 

This is a rare case in which a 1,5-diol is transformed into a lactone by a Swern oxidation. The oxidation of the primary alcohol into an aldehyde is followed by the formation of a lactol by attack of the tertiary alcohol. At this point, in spite of the presence of Et3N, enough activated DMSO is present for the activation of the hydroxy group in the lactol and 

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